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99. Antagonism and synergism in the action of electrolytes on the coagulation process
Mutual coagulation occurs when two colloids with different charge signs are mixed. Each colloid can be considered as an electrolyte, in which one ion is normal, and the other has a huge mass. It follows that a colloid with positively charged particles will play the role of a coagulating electrolyte for a sol with negative particles, and vice versa. Naturally, the most complete coagulation occurs at some optimal ratio of colloidal solutions corresponding to the mutual neutralization of particles. With an excess of one of the colloids, partial coagulation will occur, or the system will remain stable with the sign of the charge of the excess colloid (recharge). The results of coagulation of colloidal solutions by mixtures of electrolytes turn out to be different. There are three cases here:
1) the phenomenon of additivity;
2) ion antagonism;
3) ion synergy.
In works Yu. M. Glazman, E. Matievich and other authors studied a more complex, but very important case for practice - coagulation with a mixture of electrolytes.
additive the effect is that the coagulation capacity in the mixture is added arithmetically according to the mixing rule. In the case of additive action, if one electrolyte is added from 1/2 to 1 sol, then to achieve coagulation it is necessary to add from 2/2. An additive effect is often observed, especially when coagulating with mixtures of electrolytes with dominant ions of the same valence.
It has long been known that, along with the additive coagulating action of two counterions, there are cases of antagonism and synergism in their action, which are very important not only for many technological processes, but also for understanding the patterns of the effect of ions on organs and tissues of a living organism, in which biologically active ions often act as antagonists or synergists.
The coagulating action of one electrolyte begins in the presence of another, which is a phenomenon that is observed in mixtures of different ions (for example, Al 3+ and R +), as well as during the coagulation of a negative sol. The reasons for the deviation from additivity can be an electrostatic decrease in the activity of an ion in a mixture of electrolytes and complex formation.
When coagulating with mixtures of electrolytes, in some cases, synergism of ions is observed (the opposite effect of the phenomenon of antagonism, i.e., when the coagulating effect of one electrolyte increases in the presence of another). At a low concentration of electrolytes, colloidal solutions undergo coagulation. With the additional creation of adsorption layers with enhanced structural and mechanical properties on the surface of colloidal particles, it is possible to significantly increase the stability of solutions against electromagnetic coagulation. These layers can prevent coagulation by electrolytes. Such stabilization of the sol with respect to electrons by adding a small amount of a solution of macromolecular compounds (gelatin, agar-agar, egg albumin, etc.) is called protection.
Protective sols are very resistant to electrolytes. For example, colloidal solutions of silver, which are protected by protein substances and are used as drugs (protargal, collargal), become insensitive to electrolytes and can be evaporated to dryness. After treatment with water, the dry residue is again converted into a sol. However, different substances have different protective effects. The amount of substance sufficient to prevent the coagulation of one or another sol, under certain standard conditions, serves as a measure of the protective action. For example, the "Golden Number" of gelatin is 0.01, which means that 0.01 mg of it protects 10 ml. Gold sol from coagulation with 1 ml of 10% NaCl solution The “golden number” of egg albumin is 2.5, starch is 20. Similarly, you can evaluate the “Silver number”, “Sulfur number”, etc.
100. Coagulation of strongly and weakly charged sols
In the course of the development of colloid chemistry, many theories have arisen that attempt to relate the stability of hydrophobic sols (in particular, the coagulating effect of electrolytes) to various system parameters and phenomena arising from the interaction of a dispersed phase with a dispersion medium. The most successful was the modern theory of stability, which bears the name of Soviet scientists and is designated as the DLVO theory (B. V. Deryagina, L. D. Landau, E. Verweya, Ya. Overbeck). According to the DLVO theory, an increase in the electrolyte concentration in a dispersion medium leads to a decrease in the thickness of the diffuse layer. The thickness of the diffuse layer decreases to the size at which the forces of molecular attraction begin to act. As a result, there is a loss of aggregative, and then kinetic stability. The DLVO physical theory of coagulation is the first quantitative theory. It can be used to calculate the coagulation threshold. As a consequence, the Schulze-Hardy rule follows from this theory.
"Law of the sixth degree" Deryagin Z 6 Z 6 , establishes the dependence of the coagulation threshold or coagulating ability ( V k = 1/Sk) on the charge of the ion. Quantities V k for one-, two- and three charged counterions correlate with each other as 1:64:729 in accordance with the Schulze-Hardy rule.
If coagulation occurs as a result of short-range interaction of particles, then such systems are unstable, and coagulation proceeds in most cases irreversibly, since the depth of the first minimum is usually greater than kT. The decrease in the barrier height can be caused by specific adsorption. Therefore, we can talk about two types of coagulation: concentration and adsorption.
It should be noted that a comparison of the considered simple theory with experience for z > 2 is not possible, since this version of the theory does not take into account ψ 1 (с) for multiply charged counterions, concerning both the magnitude and sign of ψ 1 .
With further development of the DLVO theory for the mutual fixation of particles in the second minimum, it is possible to arrive at a value of the exponent equal to 3.5–2.5. This is confirmed by imaginary experimental data on further interaction.
All collaborations are based on the DLVO theory, which establishes a relationship between the properties of the electric layer and the stability of disperse systems. In these works, more complex cases are considered (for example, taking into account the adsorption of ions), and, consequently, the change in ψ 1 that comes to the phenomenon of coagulation zones.
The idea of the electrical nature or repulsion becomes more legitimate when a connection is established between the coagulation zones and the nature of changes in ψ 1 in electrolyte solutions with multiply charged counterions. With the same charge of particles of the disperse phase of the same composition, it seems obvious that they should repel each other electrostatically.
Consequently, in the framework of a qualitative consideration, repulsive forces arise when the diffuse layer is deformed, and in order for the particles to approach each other, it is necessary to overcome the barrier that is the higher, the higher ψ 1 , and the further it lags behind the surface, the greater the thickness of the diffuse layer.
For multivalent counterions, the values of ψ 1 decrease with increasing concentration much faster, which explains the Schulze-Hardy rule.
101. Flocculation, heterocoagulation (definitions, examples)
flocculation- a type of coagulation, which leads to the formation of loose, flaky coagulates - floccules.
In many cases, the dependence of stability, expressed in terms of some quantitative characteristic, for example c to, on the amount of added "protective" colloid (HMC), passes through a clearly defined minimum. In other words, the stability is reduced when the IUD is added in an amount insufficient for a protective effect. This phenomenon, which is especially characteristic of linear macromolecules bearing polar groups at both ends of the chain (for example, polyvinyl alcohols), is currently explained by the fact that a long polymer molecule attaches at both ends to two different particles of the dispersed phase, holding them together with a hydrocarbon "bridge".
Quantitative interpretation of the phenomenon of flocculation, carried out in the theory La Mera based on views I. Langmuir , showed that the probability of adsorption by the other end on the second particle for molecules already adsorbed on the first particle will be the greater, the greater the number of these molecules and the greater the fraction of the free surface. Consequently, the minimum stability corresponds to half filling of the surface layer with macromolecules.
This phenomenon (flocculation), due to the comparative cheapness of flocculants, is widely used for sedimentation of suspensions, sols, and especially for the purposes of purification of natural and waste waters.
heterocoagulation– interaction between particles different in composition or size. The concept of heterocoagulation is general; it includes, as a special case, the interaction of two identical bodies considered by us.
An example of heterocoagulation is mutual coagulation oppositely charged particles. In this case, the electrostatic forces change sign and become attractive forces. The absence of an energy barrier leads to rapid coagulation at any values With.
This process is widely used for the practical destruction of dispersed systems, which is especially important in connection with the problem of purification of natural and industrial waters. So, at waterworks, before water enters the sand filters, Al 2 (SO 4) 3 or FeCl 3 is added to it; positively charged sols of hydrates of Fe or Al oxides, formed as a result of hydrolysis, cause rapid coagulation of suspended negatively charged soil particles. The phenomenon of mutual coagulation of sols is of great importance in a number of natural and technological processes. Mutual coagulation is common in nature (for example, when mixing sea and river water). Coagulation of colloids of river water occurs as follows. Seawater salt ions are adsorbed on charged colloidal particles of river water. As a result of adsorption, the particles are discharged, combined into large aggregates and deposited. That is why a lot of silt gradually accumulates at the bottom, and later islands and shallows form. This is how the deltas of many of our rivers were formed.
Application of the DLVO theory to the processes of heterocoagulation shows that in some cases, not only U ter, but also U a changes sign. The nature of the London forces in these cases does not change, they are always forces of attraction. An essential role in the process of fixation of adsorbed colloids is played by their coagulation caused by opposite charges of the adsorbed particles and the surface of the adsorbent.
L. A. Kulsky found that it is not colloidal impurities of water that undergo coagulation, but hydroxides formed during the hydrolysis of the coagulant. The purification of water itself does not occur as a result of coagulation, but due to the adsorption of colloidal impurities on the surface of hydroxides. The coagulation of aluminum hydroxide particles and their associated precipitation from water occur under the action of electrolytes dissolved in water.
102. Influence of electrolytes on the electrokinetic potential. Coagulation zone
Value ζ -potential is determined by the total content of electrolytes in the solution. An increase in concentration entails a reduction in the thickness of the diffuse layer and, therefore, is accompanied by a decrease in the electrokinetic potential. It depends not only on the concentration of ions, but also on their valency, and counterions, i.e., ions whose charge is opposite to the charge of particles, play a particularly important role. Particularly strong influence on ζ -potential is exerted by monovalent complex organic ions (dyes, alkaloids, etc.), the effect of which is commensurate with the effect on the potential of divalent inorganic ions.
Experience shows that hydrogen and hydroxide ions, high valence ions (AI 3+, Fe 3+, PO 3-, citrate ions, etc.), as well as complex organic ions of alkaloids, dyes, are not only able to greatly reduce ζ -potential, but also at a certain concentration cause a change in its sign.
During coagulation, the particles should approach at such a distance that the energy of mutual attraction would be greater than the energy of thermal (Brownian) motion, which moves the particles away from each other. The necessary approach is prevented by the electrostatic repulsion that occurs when the ionic shells of the diffuse layer come into contact. When an electrolyte is introduced into a colloidal solution, two independent processes occur.
First- exchange adsorption of ions in the outer diffuse shell, i.e., the exchange of ions of the diffuse layer for the dominant ions of the introduced electrolyte; this explains their passion for coagulum.
Second process– compression of this diffuse layer, as a result of which some of its ions pass into the inner (Helmholtz) part of the electrical double layer. Due to the reduction in the thickness of the diffuse layer, colloidal particles acquire the possibility of closer approach without repulsive forces between them; at some sufficiently small distance, the forces of mutual attraction are able to cause adhesion, coagulation of particles.
The compression of the electrical double layer can be judged from the incidence ζ -potential, which is usually observed as the electrolyte is added. Its drop is not in itself the cause of coagulation, but serves as an indicator of changes occurring in the structure of the electrical double layer. Connection ζ -potential with coagulation is well manifested in the appearance of irregular rows or zones of coagulation and can be considered as an example. Ions of tri- and tetravalent metals, as well as large organic cations, when added to a negative sol in increasing quantities, behave quite differently. Initially, upon reaching the coagulation threshold, they, like other coagulating ions, cause coagulation of the sol (the first coagulation zone). Then, in a new portion of the sol at a higher electrolyte concentration, coagulation does not occur (stability zone). Further, at an even higher electrolyte concentration, coagulation occurs again (second coagulation zone). In the second stability zone, as is easy to establish by electrophoresis, colloidal particles no longer have a negative charge, but a positive one. Obviously, highly adsorbed highly charged cations and large organic cations can enter the Helmholtz part of the double layer in superequivalent amounts. Due to this, the anions accompanying them enter the diffuse part of the double layer, which changes the sign ζ -potential.
This phenomenon has been named coagulation zones, which consists in the appearance of a second stability zone after the coagulation zone with increasing electrolyte concentration. In this second zone, the particle charge turns out to be opposite in sign to the charge in the initial stability zone. With further growth With at some new critical value from "to the second zone of coagulation occurs.
103. Kinetics of fast coagulation. Smoluchowski's theory
In a narrow range of concentrations, there is a rapid increase v up to a certain value, which does not change with further increase With. In accordance with this, three clearly demarcated zones can be distinguished: stability, slow coagulation (with a threshold sk m) and fast coagulation (with a threshold ck b).
Because with growth With the height of the energy barrier U decreases, we can explain the observed regularity by the fact that at c = ccm there is a certain probability that the “hottest” (T ≥ U) particles; Further, this probability increases and for c > ck b reaches the limiting value – unity. In other words, in this region the barrier is reduced to such an extent that all particles overcome it and the number of effective collisions leading to the connection of particles no longer changes. This number depends only on the particle concentration v and their speed.
The region of fast coagulation is defined as the region in which all impacts are effective.
The calculation of v for this region is greatly simplified, since it reduces to counting the number of collisions. However, many difficulties arise here, since one has to take into account collisions not only of primary particles, but also of more complex ones formed in the process of coagulation. This task was brilliantly solved M. Smoluchovsky (1916), who proposed a quantitative interpretation of the kinetics of fast coagulation based on the consideration of the Brownian motion (diffusion) of particles.
Process speed v is a function of the concentration v and the intensity of the Brownian motion, characterized by the diffusion coefficient D.
The kinetics of coagulation was developed by M. Smoluchowski in relation to the simplest case of homogeneous spherical particles. When a known electrolyte concentration corresponding to the stability limit is reached, the initial single particles, colliding, form double particles; they, in turn, colliding with each other or with primary particles, form more and more complex (five, six, etc.) aggregates. If we denote by p 1, p 2, p 3, ... the concentration of particles consisting of one, two, three initial ones, then the total number of all particles after the start of coagulation is Σp = p 1 + p 2 + p 3 + ...
Since with each combination of two particles one is formed (halving occurs), the coagulation process formally proceeds as a bimolecular reaction, i.e., the total number of particles decreases with time according to the second-order reaction kinetics equation:
Where k is the coagulation rate constant, which depends on the particle diffusion rate constant and on the radius of the attraction sphere.
Smoluchowski's theory repeatedly subjected to experimental verification. Values v(process speed) and ξ , (coagulation period) is determined experimentally: either in a direct way - by counting the number of particles per unit volume by the ultramicroscopic method at various points in time, with the construction of curves v-t, or by the light scattering method using the Rayleigh formula. Values v are found by the tangent of the slope of the tangent to the curve, the values ξ - by the tangent of the slope of the straight line in coordinates. It should be noted that for an approximate estimate v And from to often the time elapsed from the onset of exposure to a coagulating agent to the onset of a noticeable clouding of the solution is used, as well as the ratio of the optical density (or light scattering) of the sol at a given standard time point (for example, 1 or 24 hours from the start) to the initial optical density. This method is commonly referred to as turbidimetric or nephelometric. Experimental confirmation of the theory of fast coagulation is an excellent proof of the correctness of the basic ideas of the theory of diffusion and Brownian motion.
104. Kinetics of coagulation. Reversibility of the coagulation process. Peptization
The theory developed by the Soviet physicist and chemist N. A. Fuchs originally for aerosol coagulation, it takes into account the interaction of particles by introducing the energy barrier value into the kinetic equations.
Where W- coagulation deceleration coefficient or random factor, showing how many times the process speed decreases compared to fast coagulation.
It can be seen from the equation that coagulation slows down sharply with an increase in the height of the energy barrier u, expressed in units kT, as well as with an increase in the thickness of the diffuse layer (braking at the "far" approaches) and with a decrease in the radius of the particle.
Theory shows a linear relationship W from With, confirmed experimentally. The physical meaning of the result corresponds to the fact that the coagulation rate in the force field is greater than in the case of fast coagulation in the absence of the field. Consequently, the effect of energy parameters on the kinetics of the process is described by the theory of slow coagulation.
slow coagulation can be explained by the incomplete efficiency of collisions due to the existence of an energy barrier.
Reversibility of the coagulation process– the ability of coagulated systems to peptization.
Precipitates that fall out during coagulation have a different structure. Some of them are dense, compact, which indicates close contact of particles, and coagulation is irreversible. Other coagulates occupy a large volume and have a loose, openwork structure. The particles in them remain isolated, separated by thin layers of liquid and compressed electrical layers. It can be assumed that, by increasing the degree of diffusion of the electrical double layer, one can again transfer the coagulate to the sol state. Indeed, in some cases, getting rid of the electrolyte-coagulator by washing the precipitate, it is possible to induce a process that is the reverse of coagulation - peptization (transition of a coagel into a sol).
Peptization- this is the disaggregation of particles, the violation of the connection between them, their separation from each other. Peptization is all the more probable, the more lyophilized the initial sol and the less time has passed since the moment of coagulation, since over time, during short-range interaction, gradual coalescence of particles occurs with a decrease in dispersion and surface energy. In this case, coagulation becomes irreversible, peptization is excluded. How to practice peptization depends on the causes of coagulation. Indeed, peptization will be possible if the coagulate is washed from the electrolyte with water (using decantation, filtration or dialysis). For example, by washing it is possible to peptize fresh precipitates of silicon dioxide, tin dioxide, metal sulfides, and sulfur, especially coagulated with singly charged ions. An example of peptization with a pure liquid is the peptization of clay under the action of water. When interacting with water, ion-solvation layers appear on the surface of clay particles, which weaken the bond between clay particles; as a result, a fairly stable suspension of clay in water is formed. Peptization proceeds more easily with the addition of a small amount of a peptizing agent, which makes it possible to restore the structure of the electrical double layer. Peptizers are potential-forming electrolytes. The soils are water-permeable, highly swellable, structureless, in a word, peptized. The washing action of soap is also related to the peptization process. Fatty acid ions are adsorbed on the surface of “dirt” particles, thereby tearing them off the contaminated surface and converting them into a sol state - they peptize; the flow of water and foam bubbles removes the sol from the object.
When studying the coagulation of sols, many theories arose, with the help of which they tried to explain all the observed patterns at the qualitative and quantitative levels.
So, in 1908, G. Freindlich formulated the main provisions adsorption theory of coagulation observed when electrolytes are added to the sol. According to this theory, the aggregation of colloidal particles occurs due to the adsorption of counterions by the surface of the granule and a decrease in the value of its zetta potential. However, this theory was of limited use, since took into account only the effect of electrolytes and could not explain those facts in which the adhesion of particles was associated only with changes in the diffuse layer of the micelle, while the value of the ζ potential of the granule remained unchanged.
Later, G. Muller developed electrostatic theory, which already proceeded from the fact that the introduction of an electrolyte into a sol does not change the total charge in the electrical double layer of the particle, but causes compression (reduction in size) of the diffuse layer. This leads to a decrease in the stability of the system.
Adsorption, electrostatic and a number of other theories of coagulation could not explain all the observed experimental facts, but they played a positive role in the development of ideas about the stability of colloidal systems. Their most important provisions have become an integral part of the modern theory of stability, which is in good agreement with the behavior of typical lyophobic disperse systems.
This theory was developed in 1937-1943. independently of each other B.V. Deryagin and L.D. Landau in the USSR and E. Verwey and J. T. Overbeck in Holland. In accordance with the first letters of the names of the authors, the theory is called DLFO.
According to this theory, colloidal particles in solution due to Brownian motion can freely approach each other until they touch their liquid diffuse shells or layers. In this case, no interaction forces arise between them. For further approach, the particles must deform their diffuse shells so that their mutual overlapping (or penetration into each other) occurs. But liquids are poorly compressible, and in response to deformation, so-called disjoining pressure force that impede the implementation of this process. Moreover, the larger the dimensions of the diffuse layer, the greater the forces of the disjoining pressure.
Boris Vladimirovich Deryagin (1902 - 1994)- Russian physical chemist, professor (1935), corresponding member of the USSR Academy of Sciences (1946), academician of the Russian Academy of Sciences (1992). He created the doctrine of surface forces and their influence on the disjoining pressure and the properties of thin liquid films. Prize to them. M. V. Lomonosov Academy of Sciences of the USSR (1958), State Prize of the USSR (1991). From 1936 to 1994, he headed the laboratory he created and the Department of Sorption Processes at the Institute of Physical Chemistry of the USSR Academy of Sciences. For many years he was the editor-in-chief of the journal "Colloid Chemistry". In 1962 - 1973 assumed the existence of a special kind of water - polywater. Then he refuted himself, discovering the critical influence of impurities - silicates.
If the colliding particles have sufficient kinetic energy to overcome the action of these forces, then their diffuse layers will overlap, but at this moment electrostatic repulsion forces will arise between them and the granules (because they have charges of the same sign) (Fig. 68).
Rice. 68. Scheme of interaction of colloidal particles: A– aggregatively stable system; b– overlapping of diffuse layers; V– coagulation
L 
ev Davidovich Landau (1908 - 1968), often referred to as Dau - Soviet physicist, academician of the USSR Academy of Sciences (elected in 1946). Laureate of the Nobel, Lenin and three Stalin Prizes, Hero of Socialist Labor. Member of the Academies of Sciences of Denmark, the Netherlands, the USA, France, the Physical Society of London and the Royal Society of London. The initiator of the creation and co-author of the Course of Theoretical Physics, which has gone through multiple editions and translated into many languages. A gold medal awarded since 1998 by the Department of Nuclear Physics of the Russian Academy of Sciences is named after Landau.
The larger the ζ-potential of the granules, the stronger the mutual repulsion of the particles.
In the case of overcoming these forces and approaching the granules to a distance of ≈ 10–7 cm or less (i.e., to a distance equal to or less than the size of one molecule of the dispersion medium), the so-called van der Waals forces of attraction arise between them, which have a physical nature . They cause adhesion (connection) of colloidal particles with each other.
Usually, in a stabilized hydrophobic sol, only a small fraction of the so-called active particles has a sufficient store of kinetic energy to overcome the effect of all the above forces upon impact. Therefore, such colloidal systems retain their stability for a more or less long time (depending on the degree of their stabilization). As the temperature rises, the speed and intensity of Brownian motion increase. This leads to an increase in the kinetic energy of colloidal particles. An increasing number of them are becoming active. As a result, upon impact, they more often begin to stick together with each other, and the aggregative stability of the sol will decrease.
Any other external influences exerted on the sol and leading to a decrease in the size of the diffuse layers and the value of the ζ potential will also contribute to the coagulation processes.
The least stable are colloidal systems, in which particles of the dispersed phase do not have a double electric layer and a protective shell of solvent molecules.
In this case, the electrostatic forces of repulsion and the forces of disjoining pressure do not arise between the particles, and therefore practically any collision between them will lead to mutual adhesion.
The physical theory of coagulation of the DLVO has a large mathematical apparatus and makes it possible to carry out various quantitative calculations that are in good agreement with the observed experimental facts.
The elementary act of coagulation occurs as a result of "near interaction" of particles. Precipitation is dense and irreversible, since the energy of attraction is much greater than the energy of repulsion. Here there is a direct contact between the particles, at distances corresponding to the first minimum, condensation-crystallization structures or coarse dispersions are formed. 2. If the barrier height is large and the depth of the second minimum is small, the particles cannot overcome the barrier and diverge without interaction. This is a case of "aggregatively stable system". This stability can be broken in two ways. a) An increase in the kinetic energy of the particles leads to an increase in the number of collisions. If the energy of fast particles exceeds the potential barrier, then the particles can stick together. Therefore, an increase in temperature can lead to coagulation of the system. b) The potential barrier can be reduced by adding electrolytes to the system. In this case, the DEL is compressed due to the compression of the diffuse part, as a result of which the particles approach each other at shorter distances, where the attractive forces increase. Fig.4.3 Scheme of electrolyte effect on coagulation: h2< h1 3. Если глубина второго минимума достаточно велика то, незави- симо от высоты барьера, происходит так называемое «дальнее взаимо- действие» двух частиц, отвечающее второму минимуму. Вторичный минимум на участке ВС отвечает притяжению частиц через прослойку среды. Возникает взаимодействие на дальних расстоя- ниях, осадки получаются рыхлыми и обратимыми, так как минимум не глубокий. Второму минимуму соответствует явление флокуляции или образо- вание коагуляционных структур. Интерес к этим системам в последнее время велик: фиксация час- тиц во втором минимуме при достаточной концентрации дисперсной фазы может привести к превращении. Золя в полностью структуриро- ванную систему. Реальные твердые тела, составляющие основу материальной куль- туры человечества (строительные материалы, деревянные изделия, оде- жда, бумага, полимеры) – в подавляющем большинстве являются струк- турированными дисперсными системами. Вывод: Рассмотренный классический вариант теории Дерягина-Ландау да- ет хорошее согласие с экспериментальными данными. Но может быть самым главным ее достижением является обоснование правила Шульце- Гарди, которое справедливо считается краеугольным камнем для про- верки теорий устойчивости. const g = 6 – «закон шестой степени» Дерягина, устанавливающий Z зависимость порога коагуляции от заряда иона-коагулятора. 4.7 Зависимость скорости коагуляции от концентрации электролита. Медленная и быстрая коагуляция Медленная коагуляция – это когда электролита введено в таком количестве, что небольшой барьер отталкивания сохраняется (DU), здесь не все сталкивающие частицы коагулируют. Скорость ее зависит от концентрации электролита. Быстрая коагуляция – имеет место при полном исчезновении энергетического барьера, здесь каждое столкновение частиц приводит к коагуляции. Скорость быстрой коагуляции u – не зависит от концен- трации электролита. Рис.4.4 Зависимость скорости коагуляции от концентрации электролита При небольших количествах электролита скорость коагуляции близка к нулю (участок I). Затем скорость растет при увеличении количества электролита (участок II). Коагуляция на участке II является медленной и зависит от концентрации электролита. На участке III скорость достигает максимальное значение и уже не зависит от количества прибавляемого электролита. Такая коагуляция называется быстрой и соответствует полному исчезновению потенци- ального барьера коагуляции DU . Начало участка III отвечает порогу быстрой коагуляции g б, здесь величина x -потенциала падает до нуля. Порогу быстрой коагуляции на основании теории ДЛФО можно дать строгое определение: Порог быстрой коагуляции – это количество электролита, необхо- димое для снижения энергетического барьера до нуля. 4.8 Изменение агрегативной устойчивости при помощи электролитов. Концентрационная и нейтрализационная коагуляция Одним из способов изменения агрегативной устойчивости золей является введение электролитов. Электролиты в состоянии изменить структуру ДЭС и его диффуз- ный слой, снизить или увеличить x -потенциал и электростатическое от- талкивание, т.е. способны вызвать или предотвратить коагуляцию. Воз- можны концентрационная и нейтрализационная коагуляция электроли- тами. Причина их одна и та же – снижение x -потенциала, ослабление электростатического отталкивания. Однако механизм снижения x - потенциала различный. Рис.4.5 Падение потенциала в ДЭС до (кривая 1) и после (кривая 2) введения электролита в процессе концентрационной (а) и нейтрализационной (б) коагуляции j1 и j 2 , x1 и x 2 – значения полного и электрокинетического по- тенциалов, соответственно, до и после введения электролитов; 3 и 4 – направления адсорбции ионов электролита; х – расстояние от твердой поверхности в глубь жидкости. 1. Концентрационная коагуляция наблюдается при больших заря- дах поверхности, когда j0 ³ 100 мВ, и проводится она в основном ин- дифферентными электролитами. Эти электролиты способствуют сжа- тию диффузной части ДЭС, снижению x -потенциала (x 2 < x1), но не изменяют полный потенциал j0 . Благодаря этому (сжатию ДЭС) частицы сближаются и межмоле- кулярные силы притяжения начинают превалировать, что и вызывает слияние частиц. Правило Шульце-Гарди подтвердили теоретически Б.В. Дерягин и Л.Д. Ландау, представив расклинивающее давление как суммарный эф- фект сил отталкивания и притяжения, что позволило им вывести урав- нение, связывающее порог коагуляции с зарядом иона-коагулятора. B * e (kб T) 5 Cкр = g = , (1) A2 e 6 Z 6 где B * – константа; e – диэлектрическая постоянная; kб – константа Больцмана; T – абсолютная температура; A – постоянная Ван-дер- Ваальса; e – заряд электрона; Z – заряд иона-коагулятора. Это уравнение (4) хорошо описывает зависимость порога коагуля- ции от заряда иона-коагулятора для сильно заряженных поверхностей и соответствует эмпирическому правилу Шульце-Гарди. В уравнение (1) не входит потенциал поверхности. Таким образом, правило Шульце-Гарди справедливо в случае концентрационной коагу- ляции. 2. Нейтрализационная коагуляция происходит при малых потен- циалах поверхности (j0 £ 100 м В) под действием неиндифферентных, т.е. родственных электролитов. Особенно эффективны электролиты, со- держащие ионы большого заряда и большого радиуса, то есть хорошо адсорбирующиеся. При введении таких электролитов идет частичная нейтрализация полного потенциала поверхности при адсорбции противоионов, что приводит к снижению не только полного потенциала j0 , но и j " и x - потенциала, а также к сжатию диффузной части ДЭС. Для случая нейтрализационной коагуляции при j0 £ 100 м В авторы теории ДЛФО нашли выражение для порога коагуляции: " x 4 Cкр = g = k 2 . (2) Z Из уравнения (2) следует, что для нейтрализационной коагуляции критическая концентрация зависит от x -потенциала и, следовательно, от полного потенциала поверхности j0 . Из уравнения (2) также следует: при малых j0 порог коагуляции обратно пропорционален Z 2 коагулирующего иона. Этот случай соответствует эмпирическому правилу Эйлерса- Корфа, которое оказывается справедливым для слабо заряженных по- верхностей. В реальных системах одновременно могут действовать оба меха- низма коагуляции, поэтому зависимость порога коагуляции от заряда иона-коагулятора оказывается промежуточной. 4.9 Особые явления при коагуляции. Явление неправильных рядов Коагулирующая сила ионов зависит не только от заряда и радиуса коагулирующих ионов, но и от их специфической адсорбции. Кроме того, многовалентные ионы могут вызвать перезарядку по- верхности и привести к чередованию зон устойчивого и неустойчивого состояния системы. Это явление получило название явления неправиль- ных рядов. Суть: при добавлении электролитов вначале наблюдается ус- тойчивость золя, затем – коагуляция. Далее – вновь устойчивость, и, на- конец, при избытке электролита – опять коагуляция. Это объясняется тем, что многовалентные ионы (Fe3+, Al3+, Th4+) перезаряжают частицы и переводят систему из неустойчивого в устой- чивое состояние. Введение электролита AlCl3 в золь сернистого мышь- яка, имеющего первоначально отрицательный заряд. Рис.4.6 Схема неправильных рядов На рис. 4.6 можно выделить две зоны устойчивого состояния (0-1, 2-3) и две зоны коагуляции (1-2, 3-4). Зона 0-1 – электролита добавлено недостаточно, устойчивое со- стояние. Зона 1-2 – электролита добавлено достаточно, x = xкр. Идет коагу- ляция. Далее начинается перезарядка поверхности, x -потенциал приоб- ретает противоположное значение. При достижении x >+ xcr, a steady state occurs again (section 2-3). In section 3-4, the system is coagulated again according to the scheme of concentrated coagulation. In contrast to section 1-2, where coagulation occurs with Al3+ ions, in zone 3-4, coagulation is carried out with Cl– ions, since the charge of the particles has become positive. 4.10 Coagulation with a mixture of electrolytes In industrial conditions, not one electrolyte is used for coagulation, but a mixture of several electrolytes. The coagulating action of a mixture of two electrolytes is often non-additive. Sometimes an electrolyte is required in a mixture of more than one of them - this is the phenomenon of antagonism. If a mixture of electrolytes is more effective than one electrolyte, then the phenomenon of synergy appears, they need less in the mixture than each separately. In additive action, electrolytes coagulate independently of each other. To characterize a mixture of two electrolytes, it is convenient to use the dependence of the coagulation threshold g 1 on the coagulation threshold g 2 . Under additive action, the dependence g 1 – g 2 is linear. Synergism is characterized by curve 2, if the first electrolyte is taken in the amount of g 1 / 2 , then the second electrolyte is taken in the amount of g 2< g 2 / 2 . Рис.4.7 График зависимости порога коагуляции: 1 – аддитивное действие; 2 – синергетическое действие; 3 – антагонистическое действие Синергизм электролитов широко используют на практике для коа- гуляции больших количеств дисперсных систем. 4.11 Применение коагулянтов и флокулянтов в процессах очистки воды Явление коагуляции тесно связано с проблемой удаления загрязне- ний из водных сред. В основе многих методов очистки от в.д.с – загрязнений лежит яв- ление потери системой агрегативной устойчивости путем объединения частиц под внесением специально вводимых реагентов: коагулянтов и флокулянтов. Это укрупнение частиц приводит к потере седиментационной ус- тойчивости системы и образованию осадков. В настоящее время подбор реагентов для коагуляции основывается преимущественно на эмпирических исследованиях. Чаще всего коагулирование загрязнений воды производится элек- тролитами, которые содержат многозарядные ионы (Al3+, Fe3+). Ранее процесс осветления воды объясняли нейтрализацией много- валентными катионами, заряженных, как правило, отрицательно, частиц природных вод. Однако коагуляция эти ионами связана с процессами их гидролиза, в результате которого возникают полиядерные аквагидро- комплексы, обладающие более сильной коагулирующей способностью, чем ионы. Сам процесс коагуляции подобен процессу флокуляции ВМС. В процессах водоочистки постепенно расширяется применение по- лимерных флокулянтов (ВМС): длинная молекула полимера адсорбиру- ется двумя концами на двух разных частицах дисперсной фазы и соеди- няет их «мостиком». Получается рыхлый агрегат – флоккула. Здесь час- тицы не имеют непосредственного контакта между собой. Флокулянты бывают природными и синтетическими, неионоген- ными и ионогенными. В последнем случае флокуляция возможна не только по механизму мостикообразования, но и путем нейтрализации заряда частиц противоположно заряженными ионами полиэлектролита. На празднике часто эффективным оказывается совместное приме- нение коагулянтов и флокулянтов. 4.12 Кинетика коагуляции Процесс коагуляции протекает во времени. Отсюда вытекает пред- ставление о скорости коагуляции. Скорость коагуляции – это измене- ние частичной концентрации в единице объема в единицу времени. Раз- личают быструю коагуляцию, когда каждое столкновение частиц при- водит к их слипанию и медленную коагуляцию, если не все столкновения частиц являются эффективными. Термины «быстрая» и «медленная» коагуляции условны и не связаны со скоростью процесса. При опреде- ленных условиях быстрая коагуляция может протекать очень медленно и, наоборот, медленная коагуляция может идти весьма быстро. Теория кинетики быстрой коагуляции предложена С. Смолуховским. Скорость процесса уменьшения общего числа частиц (n) во времени он рассматривает как скорость реакции второго порядка, поскольку слипание частиц происходит при столкновении двух частиц, dn = k × n2 . (3) dt После интегрирования этого уравнения получим 1æ1 1 ö k= ç - ÷ (4) t è n n0 ø или n0 n= , (5) 1+ kn0t где n0 – общее число частиц в единице объема золя до коагуляции, n – число частиц к моменту времени t, k – константа скорости процесса коагуляции, которая вычисляется по уравнению (5.5). Константа k свя- зана с коэффициентом диффузии частиц D и с расстоянием d, на кото- ром действуют силы притяжения между частицами, уравнением k = 4pDd . (6) Подставив в это уравнение вместо D его значение из уравнения Эйнштейна и учитывая, что d = 2r, получим 4 RT 3 –1 k= ,м с. (7) 3h Из формулы (7) видно, что величина k не зависит от начальной концентрации золя и от размера частиц и поэтому не меняется при их слипании. Константа скорости процесса коагуляции – постоянная толь- ко для данной коллоидной системы. Если величина константы k, вычис- ленная из экспериментальных данных, не совпадает с величиной, полу- ченной из теоретической формулы (7), то это значит, что в системе про- исходит не быстрая, а медленная коагуляция. С. Смолуховский предложил формулы, позволяющие определить с к о л ь к о ч а с т и ц того или иного порядка (первичных, вторичных и т.д.) имеется в золе ко времени t. Причем для того, чтобы исключить входящие в эти формулы трудно определяемые величины D и d, он ввел в них так называемое время половинной коагуляции q (период коагуля- ции), за которое начальная концентрация первичных частиц уменьшает- ся вдвое. Тогда для первичных частиц n0 n1 = , (8) (1 + t q) 2 для вторичных частиц n0 t q n2 = (9) (1 + t q) 3 и для частиц m-го порядка n0 (t q) m-1 nm = . (10) (1 + t q) m+1 На рис. 4.8 уравнения (8-10) изображены графически. Получен- ные кривые наглядно показывают распределение числа частиц в бы- стро коагулирующем золе. В на- чальный момент, т. е. когда t = 0, все частицы – первичные: n = n1 = n0, а n2 = n3 = n4 = 0. Через некоторое время количество всех частиц равно n, число первичных n1 уменьшается, но начинают появ- ляться двойные, тройные и др. час- тицы. По мере коагуляции эти час- тицы также постепенно исчезают, уступая место частицам высших порядков – более крупным агрега- там. Поэтому кривые, выражающие Рис.4.8 Распределение числа частиц при изменение числа частиц различных быстрой коагуляции золя порядков, со временем приобрета- ют ясно выраженные максимумы. Кривые, выражающие распределение числа частиц во времени, строят также в координатах n = f (t / q) , n = f (t) или в линейной форме – в координатах 1 / n = f (t) . Согласно теории С. Смолуховского, время половинной коагуляции не зависит от времени коагуляции. Чтобы проверить применимость тео- рии, по экспериментальным данным вычисляют q для нескольких зна- чений t по формуле, полученной из (4), . (11) Если величина q не остается постоянной при различных t, то это означает, что в системе происходит не быстрая, а медленная коагуля- ция. 4.13 Примеры коагуляции. Образование почв Мы рассмотрели развитие основных идей, определяющих содержа- ние проблемы устойчивости. Так, одна из важнейших задач заключается в сохранении устойчивого состояния суспензий, эмульсий и других объектов, проходящих в процессе переработки через сложные системы производственных агрегатов. Не менее важной для народного хозяйства является и обратная задача – скорейшего разрушения дисперсных сис- тем: дымов, туманов, эмульсий, промышленных и сточных вод. Огра- ничимся здесь иллюстрацией многообразия и сложности коагуляцион- ных явлений на примерах, связанных с процессами почвообразования. Почвы образуются при разрушении горных пород в результате вы- ветривания, выщелачивания, гидролиза и т. д. Эти процессы приводят к образованию окислов: как нерастворимых, типа SiO2, Al2O3, Fe2O3 (точ- нее – их гидроокисей), так и растворимых, типа RO и R2O (где R – ме- талл). Из-за значительной гидратации нерастворимых элементов почвы и дальнему взаимодействию в процессе взаимной коагуляции образуют- ся структурированные коагуляты, близкие по свойствам к гелям, назы- ваемые коагелями. Эти коллоидно-химические процессы определяют все многообразие существующих типов почв. Например, подзолистые почвы, типичные для северных районов нашей страны, образуются в условиях малого содержания органических остатков (гуминовых веществ) и большой влажности, вымывающей окислы основного характера (RO и R2O). Остающиеся коагели характе- ризуются высоким содержанием SiO2 и малым количеством питатель- ных веществ, необходимых для растений. Наоборот, черноземные почвы средней полосы России образуются в условиях малой влажности. В этих условиях ионы Са2+ и Mg2+ не вы- мываются и, взаимодействия с гуминовыми кислотами, образуют нерас- творимые высокомолекулярные коллоидные частицы – гуматы Са2+ и Mg2+. В процессе взаимной коагуляции положительно заряженных час- тиц R2O3 с отрицательно заряженными гуматами и SiO2 возникают
To obtain a finely ground medicinal substance during its dispersion, it is recommended to add a solvent in half the amount of the mass of the crushed medicinal substance.
Explanation of the rule[edit]
The particles of the medicinal substance have cracks (Griffiths gaps) into which the liquid penetrates. The liquid exerts a disjoining pressure on the particle, which exceeds the contracting forces, which contributes to the grinding. If the substance to be ground is swelling, then it is thoroughly ground in dry form and only then the liquid is added. After grinding the medicinal substance, agitation is used to fractionate the particles. Resuspension consists in the fact that when a solid is mixed with a liquid, 10-20 times its mass in volume, small particles are in suspension, and large ones settle to the bottom. This effect is explained by different sedimentation rates of particles of different sizes (Stokes' law). The suspension of the most crushed particles is drained, and the sediment is again crushed and stirred up with a new portion of the liquid until the entire sediment passes into a fine suspension.
Application in technology[edit]
Recipe value: 200 ml of purified water is measured into a stand. 3 g of starch and 3 g of basic bismuth nitrate are crushed in a mortar with 3 ml of water (according to the Deryagin rule), then 60-90 ml of water are added, the mixture is stirred and left for several minutes. Carefully drain the fine suspension from the sediment into the vial. The wet sediment is additionally triturated with a pestle, mixed with a new portion of water, and drained. Grinding and agitation is repeated until all large particles turn into a fine suspension.
Chemist's Handbook 21
Chemistry and chemical technology
The calculated ratio is compared with the ratio of rapid coagulation thresholds, which follows from the Deryagin-Landau rule (the Schulze-Hurdy rule).
A quantitative refinement and theoretical substantiation of the Schulze-Hardy rule were given by Deryagin and Landau. To calculate the coagulation threshold, the theory gives the following formula
The Deryagin-Landau rule, derived by the authors on the basis of the concepts of the physical theory of coagulation, makes it possible to determine the value of the rapid coagulation threshold, which corresponds to the disappearance of the energy barrier on the curve of the general interaction of colloidal particles depending on the distance between them. The values of the coagulation threshold calculated according to this rule do not always coincide with the experimental values due to the fact that the coagulating effect of ions depends not only on valence, but also on specific adsorption, which is not taken into account by the above equation.
The coagulating ability of the electrolyte is characterized by the threshold of coagulation, i.e., the minimum concentration of electrolyte D in a colloidal solution, which causes its coagulation. The coagulation threshold depends on the valency of the coagulating ion. This dependence is expressed by the significance rule (Schulze-Hurdy rule). A more rigorous, theoretically substantiated quantitative relationship between the rapid coagulation threshold y and the ion valency is expressed by the Deryagin-Landau rule
This result, first theoretically obtained by Deryagin and Landau, refines the Schulze-Hardy rule.
Theoretical ideas about the causes that determine the stability of lyophobic sols were further developed in the works of B. V. Deryagin and L. D. Landau. According to Deryagin's theoretical views and experimental data, a liquid film enclosed between two solid bodies immersed in it exerts disjoining pressure on them and thereby prevents them from approaching. The action increases rapidly with thinning of the film and decreases to a large extent from the presence of electrolytes. From this point of view, the coagulation of the particles is prevented by the wedging action of the films separating them. The introduction of electrolytes into the sol leads to a change in the electrical double layer, compression of its diffuse part, and a change in the strength of the films separating particles, and thus to a violation of the stability of the sol. The harmoniously developed mathematical theory of stability and coagulation by Deryagin and Landau leads to a rigorous physical substantiation of the Schulze-Hardy valence rule and, at the same time, provides a physical basis for the empirical regularities discovered by Ostwald.
The main regularities of coagulation under the action of electrolytes. The change in the stability of sols with a change in the content of electrolytes in them was already known to the first researchers of colloidal systems (F. Selmi, T. Graham, M. Faraday, G. I. Borshchov). Later, thanks to the work of G. Schulz, W. Hardy, G. Picton, O. Linder, G. Freindlich, W. Pauli, G. Kroyt, N. P. Peskov, A. V. Dumansky and others, extensive experimental material was accumulated and made the main theoretical generalizations. A huge contribution to the development of the theory of electrolyte coagulation was made by Soviet scientists B. V. Deryagin et al., P. A. Rebinder and his school. The experimentally established regularities in coagulation with electrolytes are known as coagulation rules.
Build graphs of the dependence of the optical density O on the concentration of the electrolyte Set (Fig. III.5). From the point of intersection of the continuation of both rectilinear sections of the curve, a perpendicular is lowered to the abscissa axis and the rapid coagulation threshold is found for each electrolyte. By dividing the obtained values of the coagulation thresholds by the smallest of them, a rule of significance is derived and compared with the Deryagin-Landau rule.
The existence of a sharp jump in properties at a certain distance from the substrate was discovered even earlier by V. V. Karasev and B. V. Deryagin when measuring the dependence of the viscosity of some organic liquids on the distance to a solid wall. All this gives the right to call such layers a special, boundary phase, since the presence of a sharp interface is the main definition of a phase. The difference with ordinary phases lies in the fact that the thickness of the boundary phase is a value quite definite for a given temperature.
The theory of Deryagin - Verwey - Overbeck establishes that Sk is inversely proportional to the sixth degree of valency of the coagulating ion. The same dependence reflects the experimentally found Schulze-Hardy rule. The obtained excellent agreement well confirms the correctness of the theory of coagulation of lyophobic sols.
Numerous objects have shown that the coagulation threshold is inversely proportional to the valency of coagulating ions in powers of 5 to 9, often to powers of 6. Lower values of the exponent (2-3) have also been observed. Thus, the Schulze - Hardy rule assumes only a high degree of dependence of the coagulation threshold on the valence (r) of counterions. Nevertheless, it is sometimes identified with the theoretically derived law 2 of Deryagin-Landau.
The influence of the valency of coagulating ions on the coagulation threshold is determined by the Schulze-Hardy rule: the greater the valence of coagulating ions, the greater their coagulating power or the lower the coagulation threshold. The theoretical substantiation of this rule was given in 1945 by B. V. Deryagin and L. D. Landau. The relationship they found between the coagulation threshold and the valence of coagulating ions is expressed in the form
If we take into account that in the case of the barrier mechanism at r
To obtain thinner and more stable aqueous suspensions of hydrophilic swelling substances (basic bismuth nitrate, zinc oxide, magnesium oxide, calcium phosphate, carbonate and glycerophosphate, coalin, sodium bicarbonate, iron glycerophosphate), it is most advisable to use the stirring method, which is a kind of dispersion method. The essence of the technique lies in the fact that the substance is dispersed first in a dry form, then - taking into account the Deryagin rule. The resulting thin pulp is diluted approximately 10 times with water (solution), triturated and the top layer of the suspension is poured into a dispensing bottle. The stirring operation is repeated until all the substance is dispersed and obtained in the form of a fine slurry.
The influence of a lubricant on friction parameters under boundary lubrication conditions is usually estimated by the adsorption value of the oil (medium) and by its chemical activity. The adsorption capacity is taken into account mainly for the case of using a chemically inactive lubricating medium. So, B. V. Deryagin proposed to evaluate the effectiveness of the oil film by the criterion of lubricity, which is the ratio of the roughness of the lubricated and non-lubricated surfaces. Another criterion of lubricity is characterized by the ratio of the difference in the work of the friction forces of unlubricated and lubricated surfaces during the time required to abrade a film of thickness /r to the thickness of this film. The lubricity criteria are mainly determined by the residence time of the oil (lubricant) molecules on the friction surface and the activity of the lubricant.
In electrolyte coagulation according to the concentration mechanism (for highly charged particles), the coagulation threshold Cc, in accordance with the Deryagin-Landau rule (justification of the empirical Schulze-Hardy rule), is inversely proportional to the charge of 2 counteriono13 to the sixth power, i.e.
The theory of the electrical double layer was developed in the works of Frumkin and Deryagin. According to their ideas, the inner layer of the ions of the electric double layer, called potential-forming ones, is closely adjacent to some of the oppositely charged ions (Fig. 50, a), called counter ions and. This part of the counterions moves along with the particle and forms a 6″ thick layer, called the adsorption layer. On fig. 50, and the boundary between such a particle and the medium is indicated by a dotted line. The remaining counterions are located in the dispersion medium, where they are distributed, as a rule, diffusely.
Recently, however, experimental data have been obtained that indicate the inapplicability in some cases of the Schulze-Hardy rule in the form of the Deryagin-Landau law. Experimentally, significant deviations from this pattern are often observed, namely, in some cases, the coagulating effect of electrolytes is proportional to the valence of counterions to a degree less than six. According to I. F. Efremov and O. G. Usyarov, this is a deviation from
The applicability of the Deryagin theory and the Schulze-Hardy rule for the coagulation of macromolecular compounds was shown by the example of rubber latexes when they interact with electrolytes of different valences (Voyutsky, Neumann, Sandomirsky).
However, even in the considered first approximation, the theory gives good agreement with experimental data (for example, the data of Schenkel and Kitchener obtained on monodisperse latexes), but perhaps its most important achievement is the substantiation of the Schulze–Hardy rule, which is rightly considered the cornerstone for testing stability theories. Consider this explanation. An analysis of the conditions for the stability of dispersed systems shows that the boundary conditions for rapid coagulation in terms of Deryagin's theory can be written as Umax = 0 and domax/ek = 0, where C/max is the maximum energy (Fig. XIII. 7). These conditions express the reduction of the barrier height to zero.
In the simplest case, u = onst. Coef. T. rest, as a rule, more coefficient. kinematic T., so that the starting force (starting torque) is greater than the resistance to uniform movement. More precisely, physical processes with dry T. are reflected in the so-called. two-term by Deryagin's law of friction ts = F / (N + PgS), where / - complements, to N the pressure caused by the forces of the intermol. interaction rubbing bodies, and S-pov-et actually. contact of rubbing bodies due to the waviness and roughness of surfaces T. contact of bodies is not complete.
In the works of 1937 and 1940. Deryagin, using the Fuchs formulas for the coagulation rate of interacting particles, derived a criterion for the aggregative stability of weakly charged colloidal particles for two limiting cases when the particle radius is much less than the thickness of ionic atmospheres, or, in other words, the characteristic Debye length, and when the particle radius is much greater than the thickness of ionic atmospheres . In the second case, the criterion generalizes and quantitatively refines the empirical rule of Eilers-Korf, which is in agreement with a number of experimental facts. At the same time, the existence of a far minimum on the curve expressing the dependence of the interaction (repulsion) force on the distance was shown.
A well-known difficulty for the theory was that the rule of the inverse sixth degree (the Hardy-Schulze rule refined by Deryagin and Landau) is also observed when the dimensionless potential of the surface is not only small, but less than unity. This is possible, as shown by Glazman et al. , if the product of the potential and the charge of the counterion changes little when the latter changes. A quantitative explanation for this on the basis of the independence of the adsorption of counterions from the charge was given by Usyarov.
The most developed theory of the stability of ionostabilized colloidal solutions has led to a number of fundamental results. The theory of strongly charged sols, considering only concentration coagulation, made it possible to substantiate the Schulze-Hardy rule in the form of the Deryagin-Laidau law 2. At moderate potentials of colloidal particles, the coagulation thresholds change with the valence of counterions according to the law 2, where 2 a 6, which is also in accordance. with the Schulze-Hurdy rule. The theory made it possible to substantiate the various regularities of the coagulating action of electrolyte mixtures and the effect of synergism that could not be explained before. It should also be noted that, on the basis of the theory, the illegality of the widespread
Having obtained the values of the exact coagulation threshold for all electrolytes, a significance rule is derived, for which the found threshold values are divided by the smallest coagulation threshold (for AI I3). The experimental ratio of coagulation thresholds is compared with the theoretical ratio calculated according to the Deryagin-Landau rule, according to which Y a b Vai u 11 1. The results of the comparison are analyzed and the work is registered in a laboratory journal.
See pages where the term is mentioned Deryagin's rule: Synthetic polymers in printing (1961) - [ c.130 ]
Rule explanation
Application in technology
Bismuthi subnitratis ana 3.0
M.D.S. Wipe the skin of the face
Deryagin's rule- a rule developed by the chemist B.V. Deryagin regarding the technology of many dosage forms.
Aquae destillatae 200 ml
Notes
- Sinev D. N., Marchenko L. G., Sineva T. D. Reference manual on pharmaceutical technology of drugs. 2nd ed., revised. and additional - St. Petersburg: SPKhFA Publishing House, Nevsky Dialect, 2001. - 316 p.
- Nikolaev L.A. Medicine. 2nd ed., rev. and additional - Minsk: Higher School, 1988.
- Bobylev R. V., Gryadunova G. P., Ivanova L. A. et al. Technology of dosage forms. T. 2. - M .: "Medicine", 1991.
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Deryagin's rule- Deryagin's rule - a rule developed by the chemist B. V. Deryagin, concerning the technology of many dosage forms. The rule itself sounds like this: “To obtain a finely divided medicinal substance during its dispersion, it is recommended to add ... Wikipedia
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COAGULATION- (from Latin coagulatio coagulation, thickening), the association of particles of the dispersed phase into aggregates due to the adhesion (adhesion) of particles during their collisions. Collisions occur as a result of Brownian motion, as well as sedimentation, the movement of particles ... Chemical Encyclopedia
CHAPTER 20. SUSPENSIONS
Suspensions (Suspensions)- a liquid dosage form for internal, external and parenteral use, containing as a dispersed phase one or more powdered medicinal substances distributed in a liquid dispersion medium (SP XI, issue 2, p. 214). The particle size of the dispersed phase of suspensions should not exceed 50 µm. In accordance with the requirements of the US Pharmacopoeia, the British Pharmaceutical Code, it should be 10-20 microns.
Suspensions are opaque liquids with a particle size specified in private articles that do not pass through a paper filter and are visible under a conventional microscope. As microheterogeneous systems, suspensions are characterized by kinetic (sedimentation) and aggregative (condensation) instability.
Suspensions are unstable during storage, therefore:
- before use, the suspension is shaken for 1-2 minutes;
- Substances that are potent and poisonous are not released into the dosage form.
An exception is the case when the amount of the substance prescribed in the prescription does not exceed the highest single dose.
When a substance of list A is prescribed in a prescription in an amount of a higher single dose, the medicinal product is not subject to manufacture.
20.1. ADVANTAGES OF SUSPENSIONS
The advantages of suspensions over other dosage forms are:
- the convenience of the dosage form for patients, especially for children who cannot swallow tablets or capsules;
- less intense taste of suspensions than solutions. In addition, there is the possibility of correcting the taste of drugs by introducing syrups, flavorings;
— medicines in suspensions are more stable than in solution. This is especially important in the manufacture of dosage forms with antibiotics.
20.2. DISADVANTAGES OF SUSPENSIONS
The disadvantages of suspensions are:
— Physical instability: settling (sedimentation), joining and increasing particle sizes (aggregation) and joining solid and liquid phases (condensation). These physical phenomena lead to the precipitation or floating of the solid phase. The principle of dosing uniformity is violated;
- the need for the patient to intensively mix the suspension before use to restore a homogeneous state;
- unsatisfactory short shelf life - 3 days (Order of the Ministry of Health of the Russian Federation? 214).
20.3. PHYSICAL PROPERTIES OF SUSPENSIONS
The sedimentation stability of suspensions is determined by the Stokes law, according to which the sedimentation rate is directly proportional to the square of the particle diameter, the difference in particle densities and the dispersed medium, and is 18 times inversely proportional to the viscosity of the medium:
It follows from the Stokes law that the higher the degree of particle size reduction and the higher the viscosity of the medium, the higher the sedimentation stability of suspensions. In addition, the stability of suspensions depends on the degree of affinity of the medicinal substance for the dispersion medium, the presence of an electric charge of the particles. In suspensions, particles of the solid phase, in the case of good wettability by the dispersion medium, are covered with solvate shells, which prevent coalescence (combination)
particles (suspensions of substances with hydrophilic properties). Therefore, the introduction of surface-active substances (surfactants) is not required. With poor wettability, solvate shells are not formed, resulting in precipitation or floating of solid particles (suspensions of substances with pronounced hydrophobic properties).
20.4. SUSPENSION MANUFACTURING METHODS
In pharmaceutical technology, 2 methods for making suspensions are used:
- condensation (by controlled crystallization). For example, ethanolic solutions of boric, salicylic, and other acids are added to water. The precipitated crystals form a suspension;
- dispersion (by grinding crystalline substances in a dispersion medium).
20.5. AUXILIARY SUBSTANCES USED TO STABILIZE SUSPENSIONS
To increase the stability of suspensions with hydrophobic substances, use:
A. Thickeners—substances with insignificant surface activity, but ensuring the stability of the suspension by increasing the viscosity of the system.
- natural (gums, alginates, carrageenans, guar gum, gelatin);
- synthetic (M!, sodium carboxymethylcellulose - Carbopol?);
- inorganic (aerosil, bentonite, magnesium aluminosilicate - Veegum?).
— Surfactants that lower the interfacial tension at the phase boundary (tweens, fat sugar, pentol, T-2 emulsifier, etc.).
Table 20.1 shows the stabilizers and their concentrations used to make suspensions of hydrophobic substances.
Table 20.1. Suspension stabilizers
Amount of stabilizer (g) per 1.0 medicinal substance
with pronounced hydrophobic properties
with mildly pronounced hydrophobic properties
Note. To stabilize the suspension of sulfur for external use, it is recommended to use medical soap in the amount of 0.1-0.2 g per 1.0 g of sulfur. From a medical point of view, the addition of soap is advisable, since it loosens the pores of the skin, being a surfactant, and promotes the deep penetration of sulfur, which is used in the treatment of scabies and other skin diseases. It should be borne in mind that soap as a sulfur stabilizer is recommended to be used only under the direction of a doctor. If the recipe contains salts of divalent metals, then the amount of soap is increased to 0.3-0.4 g per 10 g of sulfur. At the same time, it is recommended to sterilize sulfur in suspensions with alcohol and glycerin.
To stabilize medicinal substances with pronounced hydrophobic properties, gelatose is used in a ratio of 1:1, and with mildly pronounced properties - 1:0.5.
Exception: sulfur slurry (see table 20.1).
20.6. TECHNOLOGY FOR OBTAINING SUSPENSIONS
The technological scheme for obtaining suspensions by the dispersion method consists of the following stages:
1. The preparatory stage includes the following technological operations:
- preparation of the workplace;
— preparation of materials, equipment;
- calculations, design of the reverse side of the PPC;
- weighing of suspended substances.
2. The grinding stage includes 2 technological operations:
- obtaining a concentrated suspension (pulp);
- obtaining a dilute suspension, including fractionation (suspension and settling).
Note. This stage is mandatory for suspensions of substances with hydrophilic properties, and is not necessary for suspensions of substances with hydrophobic properties. This is explained by the sedimentation instability of the former and the aggregative instability of the latter.
A. The operation of obtaining a concentrated suspension. To obtain a concentrated suspension, a grinding operation in a liquid medium is used. The introduction of liquid contributes to a finer grinding of particles due to the splitting action of surface tension forces (Rehbinder effect) (Fig. 20.1).
Rice. 20.1. Rebinder effect
For the first time, the wedging effect of a liquid and a decrease in the strength of solids due to this effect were studied by the Russian scientist P.A. Rehbinder in 1928. The Rehbinder effect is based on the destructive effect of the difference in the forces of the surface tension of a liquid inside a crack in a solid body (see Fig. 20.1). The effect is determined by the structure of the solid body (the presence of dislocations, cracks), the properties of the liquid (viscosity) and its amount. As a result of the action of surface tension forces, there is a multiple drop in strength, an increase in the brittleness of the solid. This facilitates and improves the mechanical grinding of various materials.
B.V. Deryagin investigated the influence of the Rebinder effect on the grinding of pharmaceutical powders. He determined the optimal ratio of the mass of a liquid to the mass of a solid, which is approximately equal to 1/2.
To obtain finely divided medicinal substances, it is recommended to first obtain a concentrated suspension by grinding the suspended substances in water, solutions of medicinal substances or other auxiliary liquid, taken in an amount of 1/2 of the mass of the crushed medicinal substance (B.V. Deryagin's rule, based on the effect Rebinder).
B. The operation of obtaining a dilute suspension, including fractionation (suspension and settling). The aim of the operation is to obtain particles smaller than 50 µm. Particles of this size form suspensions that remain homogeneous for 2–3 min, i.e. the time required for dosing and taking the dosage form by the patient.
After obtaining a concentrated suspension, water is added in an amount exceeding 10-20 times the dispersed phase. Then the suspension is intensively stirred (reception of agitation) and settled for 2-3 minutes in order to fractionate the particles. Small particles are in suspension, large particles settle to the bottom. A thin suspension is drained, the sediment is re-crushed and stirred up with a new portion of the liquid. The operation is repeated until the entire sediment passes into a fine suspension.
Bismuthi subnitratis ana 3.0 Aq. rig. 200 ml
Measure 200 ml of purified water into the stand. 3.0 g of starch and 3.0 g of basic bismuth nitrate are crushed in a mortar with 3 ml of water (B.V. Deryagin's rule), 60-90 ml of water are added, the mixture is stirred and left alone for 2-3 minutes. A thin suspension is carefully poured from the sediment into a vial. The rest in the mortar is additionally ground with a pestle, mixed with a new portion of water, drained. Grinding and agitation is repeated until all large particles turn into a fine suspension.
When preparing suspensions of hydrophobic substances with pronounced properties, it is necessary to add ethanol, as in the case of dispersion of substances that are difficult to grind.
Rp.: Solutionis Natrii bromidi 0.5% - 120 ml
Coffeini-natrii benzoatis 0.5
M.D.S. 1 tablespoon 3 times a day.
112 ml of purified water, 5 ml of caffeine-sodium benzoate solution (1:10) and 3 ml of sodium bromide solution (1:5) are measured into the stand. 1.0 g of camphor with 10 drops of 95% ethanol is ground in a mortar until dissolved, 1.0 g of gelatose and 1 ml of the prepared solution of medicinal substances are added, mixed until a thin pulp is obtained. The pulp is transferred into a dispensing vial with a solution of caffeine-sodium benzoate and sodium bromide, adding it in parts.
In the manufacture of suspensions containing medicinal substances at a concentration of 3% or more, they are prepared by weight, therefore, in the written control passport, in this case, it is necessary to indicate the tare weight and the mass of the suspension produced.
Example 3 Rp.: Zinci oxydi Talci ana 5.0
Aq. purificata 100ml
M.D.S. Wipe the skin of the face.
In a mortar, 5.0 g of zinc oxide and 5.0 g of talc are mixed first in dry form, then approximately 5 ml of purified water is added (B.V. Deryagin's rule), rubbed until a mushy mass is formed. The remaining purified water is added in parts to the thin pulp, mixed with a pestle, transferred to a vial and made out.
Suspensions are not filtered.
3. Mixing stage includes the introduction of other medicinal substances in the form of solutions. A feature of this stage is the need to check the compatibility of both medicinal substances and their effect on the sedimentation stability of suspensions. Strong electrolytes and polar substances drastically deteriorate the stability of suspensions.
If the suspension contains inorganic salts, then it is better to prepare a concentrated suspension by rubbing the substance with purified water, then adding a stabilizer, and then salt solutions in ascending order of concentration.
4. Stage of design and packaging. Suspensions are packed similarly to liquid dosage forms in a container that ensures the preservation of the quality of the drug during the expiration date. The most convenient is the packaging of suspensions in syringes equipped with adapters and dispensers (Fig. 20.2).
When registering, it is necessary to have additional warning labels on the label: “Shake before use”, “Freezing is unacceptable”, “Shelf life 3 days”.
5. Evaluation of the quality of suspensions. The quality of the prepared suspensions is evaluated in the same way as for other liquid dosage forms, i.e. check the document
Rice. 20.2. Syringes and nozzles for dispensing suspensions
tion (recipe, passport), design, packaging, color, smell, absence of mechanical impurities, deviations in volume or mass. Specific quality indicators for suspensions are the resuspendability and uniformity of the particles of the dispersed phase.
resuspendability. In the presence of sediment, the suspensions are restored to a uniform distribution of particles throughout the volume with shaking for 20-40 s after 24 hours of storage and 40-60 s after 24-72 hours of storage.
Homogeneity of the particles of the dispersed phase. There should be no heterogeneous large particles of the dispersed phase.
Note. Particle size determination is carried out by microscopy. The particle size of the dispersed phase should not exceed the sizes specified in private articles on suspensions of individual medicinal substances (FS, VFS).
20.7. EXAMPLES OF SUSPENSION RECIPES (ORDER OF THE MOH OF THE USSR? 223 OF 12.08.1991)
1. Suspension of iodoform and cynic oxide in glycerin Rp.: Iodoformii 9.0
Zinci oxydi 10.0 Glycerini ad 25.0 M.D.S. External.
Action and indications: antiseptic.
2. Suspension of sulfur with chloramphenicol and salicylic acid alcohol
Rp.: Laevomycetini Ac. salicylici ana 1.5 Sulfuris praecip. 2.5sp. aethylici 70% - 50 ml M.D.S. Wipe the skin.
Action and indications: antibacterial and antiseptic for skin diseases.
3. Suspension of zinc oxide, talc and starch Rp.: Zinci oxydi
Aq. pur. 100 ml M.D.S. External.
Action and indications: antiseptic, astringent.
4. Suspension "Novocindol" Rp.: Zinci oxydi
sp. aethylici 96% - 21.4 ml
Aq. rsh \ ad 100.0 M.D.S. Lubricate the skin.
Action and indications: antiseptic, astringent and local anesthetic.
5. Suspension of zinc oxide, talc, starch and anesthesin alcohol-glycerine
Anaesthesini ana 12.0
sp. aethylici 70% - 20.0 ml Aq. pur. ad 100.0
M.D.S. Apply to skin.
Action and indications: antiseptic, astringent, local anesthetic.
6. Suspension of zinc oxide, starch, talc, anestezin and boric acid, water-glyceric
Rp.: Zinci ohidi Amyli
Talciana 30.0 Anaesthesini 5.0
Sol. Ac. borici 2% - 200.0
1. What is the definition of suspensions as a dosage form? What are her
features as a heterogeneous system?
2. What are the types of suspension stability as a heterogeneous system?
3. What factors affect the stability of suspensions?
4. How to prepare a suspension of hydrophilic substances?
5. How to explain the application of the rule of prof. B.V. Deryagin and the method of resuspension in the manufacture of suspensions?
6. What is the role of stabilizers and their mechanism of action?
7. How to justify the choice of a stabilizer for suspensions of hydrophobic substances?
8. How to prepare suspensions from substances with mild hydrophobic properties?
9. How to prepare suspensions from substances with pronounced hydro-
10. What are the features of the preparation of sulfur suspension?
11. What are the main indicators for assessing the quality of a suspension?
12. What changes can suspensions undergo during storage?
1. Before use, the suspension is shaken for:
2. Toxic substances in suspensions:
2. They are released if the amount of the poisonous substance prescribed in the prescription does not exceed the highest single dose.
3. The sedimentation rate is directly proportional to:
1. The square of the particle diameter.
2. Densities of particles and a dispersed medium.
3. Viscosity of the medium.
4. The advantages of suspensions over other dosage forms are:
1. Physical stability (sedimentation).
2. The convenience of the dosage form for patients (children) who cannot swallow tablets or capsules.
3. Short shelf life - 3 days.
5. It follows from the Stokes law: the higher the degree of particle size reduction, the sedimentation stability of suspensions:
6. It follows from the Stokes law: the greater the viscosity of the medium, the sedimentation stability of suspensions:
7. To stabilize medicinal substances with pronounced hydrophobic properties, gelatose is used in the ratio:
8. To stabilize medicinal substances with mildly pronounced hydrophobic properties, gelatose is used in the ratio:
9. Fractionation (suspension and settling) is mandatory for suspensions of substances that have:
1. Hydrophilic properties.
2. Hydrophobic properties.
10. To obtain finely divided medicinal substances, it is recommended to first obtain a concentrated suspension by grinding the suspended substances in water, solutions of medicinal substances or other auxiliary liquid in the amount of:
1. 1/1 of the mass of the crushed medicinal substance.
2. 1/2 of the mass of the crushed medicinal substance.
3. 2/1 of the mass of the crushed medicinal substance.
11. In the manufacture of suspensions containing medicinal substances at a concentration of 3%, they are prepared:
13. If the suspension contains inorganic salts, then it is better to prepare a concentrated suspension by rubbing the substance with:
1. Salt solution.
2. Purified water.
14. For making a recipe:
Rp.: Solutionis Natrii bromidi 0.5% 120 ml Camphorae 1.0 Coffeini-natrii benzoatis 0.5
15. Total Recipe Volume:
Rp.: Solutionis Natrii bromidi 0.5% 120 ml Camphorae 1.0 Coffeini-natrii benzoatis 0.5:
3. The recipe is made by weight.
16. Rp.: Zinci oxydi; Talciana 5.0 Aquae purificata 100 ml
