1. Prepare 0.2, 0.1 0.05, 0.025 and 0.0125 M solutions of three alcohols (or organic acids) one homologous series.
2. Determine the values of their surface tension using the device and the Rebinder method, write down the results and calculations in table 3.6
3. Plot on one graph the surface tension isotherms of all the surfactant solutions you used of the same homologous series.
4. From the graph, calculate the surface activities Ds/DC of all solutions for all concentrations from the initial linear plots.
5. Calculate the ratio of surface activities of the nearest neighbors of the homologous series.
6. Make a conclusion about the feasibility of the Duclos-Traube rule.
Table 3.6.
| Solutions | WITH, mol/l | P \u003d h 2 - h 1 | s, days/cm | Ds/DC |
| 0 | P o = | s o = | ||
| 0,0125 | ||||
| 0,025 | ||||
| 0,05 | ||||
| 0,1 | ||||
| 0,2 | ||||
| 0,0125 | ||||
| 0,025 | ||||
| 0,05 | ||||
| 0,1 | ||||
| 0,2 | ||||
| 0,0125 | ||||
| 0,025 | ||||
| 0,05 | ||||
| 0,1 | ||||
| 0,2 |
CONTROL QUESTIONS:
Before doing work:
1. Formulate the purpose of the work.
2. Describe the measurement procedure for determining the surface tension by the Rehbinder method.
3. Tell us the procedure for determining the surface activity of surfactant solutions and calculating the adsorption according to Gibbs.
4. Explain the procedure and calculations for checking the feasibility of the Duclos-Traube rule.
To protect your work:
1. Surface tension is ...
2. Specify the factors influencing the surface tension of liquids.
3. Is there a difference in the surface tension of soft and hard water, samples of which are at the same temperature? Justify your answer.
4. Explain the difference between the terms "absorption" and "adsorption". Give examples of adsorption and absorption.
5. Draw graphs of the dependence of adsorption on the concentration of a surfactant at temperatures T 1 and T 2, given that T 2< Т 1.
6. Draw graphs of the dependence of surface tension on the concentration of a surfactant at temperatures T 1 and T 2, given that T 2 > T 1.
7. Determine the area per molecule of aniline C 6 H 5 NH 2 at its border with air, if the limiting adsorption of aniline is G ¥ = 6.0 10 -9 kmol / m 2.
8. Give an example of a process in which the surface tension of water becomes zero.
9. From the series of compounds below, select those that increase the surface tension of water: NaOH, NH 4 OH, C 6 H 5 NH 2, CH 3 -CH 2 -CH 2 -CH 2 -COOH, CH 3 -CH 2 ONa, KCNS
10. How much do the surface activities of ethyl (CH 3 -CH 2 OH) and butyl (CH 3 -CH 2 -CH 2 -CH 2 OH) alcohols of the same concentration (at low concentrations) differ.
11. Which of the following compounds will have the highest adsorption value at the same concentration: HCOOH, CH 3 -COOH or CH 3 -CH 2 -COOH? Justify your answer.
GAS CHROMATOGRAPHY
The chromatographic method of separating a mixture of substances consists in the fact that the substances that make up the mixture move along with the non-sorbing carrier gas along the surface of the sorbent (stationary phase), and the processes of sorption and desorption of these substances continuously occur. The stationary phase is placed in the form of a nozzle in a tube called a chromatographic column, through which all the admitted substances must pass, after which they are recorded at the outlet of the column by a chromatographic detector. The movement of substances along the column occurs only together with the carrier gas flow, while in the sorbed state they do not move directionally. Therefore, the longer the average "lifetime" of the molecules of an individual substance in the adsorbed state, the lower their average velocity along the column. Figure 3.1 shows the chromatogram recorded by the detector for a mixture of four substances.
Rice. 4.1 Typical chromatogram of a mixture of four substances.
The arrow in Fig. 4.1 indicates the moment of the mixture inlet into the carrier gas flow at the column inlet. The total time for the substance to pass through the column ( retention time ) t u is the sum of the time of movement with the carrier gas t0 and the total time spent in the adsorbed state t R (corrected retention time):
t u = t o + t R 4.1
t 0 is the same for all substances, since they move along the column together with the carrier gas at the linear speed of its movement u 0 . Since the retention of substances in the adsorbed state occurs due to the interaction of the molecules of the substances being separated with the molecules of the liquid film (partition chromatography) or the surface of the solid phase (adsorption chromatography), then t R depends on the nature of the stationary phase. The components of the mixture, which differ in the energy of interaction with a given stationary phase, will have different values of t R . For example, the energy of these interactions for derivatives of hydrocarbons is determined by the length of the hydrocarbon chain and the presence of functional groups, therefore, the value of the corrected retention time t R is a qualitative characteristic of this substance under constant experimental conditions: temperature and carrier gas volumetric velocity (w ).
The average linear velocity of the i-th component of the mixture along the column u i = l/t u , Where l- column length, described by the main equation:
4.2
u 0 - carrier gas velocity;
- Henry coefficient, i.e. coefficient of distribution of the i-th substance between the stationary and gas phases;
C a and C are the concentrations of the substance in these phases at equilibrium, respectively;
is called the phase ratio and is equal to the ratio of the volume V a of the stationary phase in which sorption occurs to the volume of the mobile (gas) phase in the column V = wt o., w is the volumetric velocity of the carrier gas
.
Due to the fact that Г i for different substances of the mixture differ from each other, their movement along the column occurs at different average speeds, which leads to their separation. Non-sorbing substances, as well as the carrier gas, pass the entire length of the column in time t 0 . Thus,
, 4.З
those. 
, 4.4
Where
, 4.5
Multiplying the right and left sides by w, we get
, 4.6
V R- corrected retention volume , depends only on the volume of the stationary phase in the column and the Henry coefficient. The relative retained volume of the two components 1 and 2, which is equal, does not depend on V a , but only on the nature of the substances and temperature
, 4.7
Thus, the relative retained volume is the most reproducible qualitative characteristic of a substance compared to t u , t R and V R .
A typical L/L interface is the boundary between water (W) and oil (O) - components that have little or no affinity for each other. Such a boundary is quite pronounced, although not as sharp as it is observed for the L/G interface (Fig. 1). The increase in the total contact surface by dispersion of one phase (in the form of small drops) into another is slow, while the reverse transition to the initial phases is fast, and the driving force behind the reverse process is the tendency to reduce the surface and reduce the surface energy. Amphiphilic substances (for example, fatty acids) added to the system are distributed at the L/L interface in such a way that the affinity of different parts of the molecule for different phases causes a decrease in the surface free energy and stabilizes the interface. The similarity between the types of distribution of molecules at the L/G and L/L interfaces can be seen in Figs. 4a,b; the main difference is the presence of surfactant molecules in the oil layer. The surfactant distribution shown in fig. 4b applies equally to oil-in-water (O/W) or water-in-oil (W/O) emulsions, so that both types of emulsions (or dispersions) are stabilized with suitable appropriate surfactants.
50. Adsorption of gases on the surface of solids.
51. Adsorption from solutions. ion exchange.
Adsorption isotherms of dissolved substances from a solution are similar in appearance to adsorption isotherms for gases; for dilute solutions, these isotherms are well described by the Freundlich or Langmuir equations, if the equilibrium concentration of the solute in the solution is substituted into them. However, adsorption from solutions is a much more complex phenomenon compared to gaseous one, since the adsorption of a solvent often occurs simultaneously with the adsorption of a solute.
Adsorption from aqueous solutions of electrolytes occurs, as a rule, in such a way that ions of the same type are adsorbed from the solution on the solid adsorbent. Preferential adsorption from a solution or an anion or a cation is determined by the nature of the adsorbent and ions. The mechanism of adsorption of ions from electrolyte solutions can be different; allocate exchange and specific adsorption of ions.
Ion exchange is a reversible process of equivalent exchange of ions m / y with an electrolyte solution and a solid (ion exchanger). Ion exchangers (ion exchangers) are substances capable of ion exchange upon contact with electrolyte solutions. According to the sign of the exchanged ions, cation exchangers and anion exchangers are distinguished. The cation exchanger has fixed anionic groups and cations capable of exchange with the environment. Ion exchange has some similarities with adsorption - the concentration of ions of a dissolved substance occurs on the surface of a solid.
52. Methods for obtaining and purifying disperse systems.
A dispersed system is a system in which one substance is distributed in the medium of another, and there is a phase boundary between the particles and the dispersion medium. Dispersed systems consist of a dispersed phase and a dispersion medium.
The dispersed phase is the particles distributed in the medium. Its features are dispersion and discontinuity.
Dispersion medium - the material medium in which the dispersed phase is located. Its sign is continuity.
dispersion method. It consists in mechanical crushing of solids to a given dispersion; dispersion by ultrasonic vibrations; electrical dispersion under the action of alternating and direct current. To obtain dispersed systems by the dispersion method, mechanical devices are widely used: crushers, mills, mortars, rollers, paint grinders, shakers. Liquids are atomized and sprayed using nozzles, tops, rotating discs, centrifuges. The dispersion of gases is carried out mainly by bubbling them through a liquid. In foam polymers, foam concrete, foam gypsum, gases are obtained using substances that release gas at elevated temperatures or in chemical reactions.
Despite the widespread use of dispersion methods, they cannot be used to obtain dispersed systems with a particle size of -100 nm. Such systems are obtained by condensation methods.
Condensation methods are based on the process of formation of a dispersed phase from substances that are in a molecular or ionic state. A necessary requirement for this method is the creation of a supersaturated solution from which a colloidal system must be obtained. This can be achieved under certain physical or chemical conditions.
Physical condensation methods:
1) cooling of vapors of liquids or solids during adiabatic expansion or mixing them with a large volume of air;
2) gradual removal (evaporation) of the solvent from the solution or its replacement with another solvent, in which the dispersed substance dissolves worse.
So, physical condensation refers to the condensation of water vapor on the surface of solid or liquid particles, ions or charged molecules (fog, smog) in the air.
Solvent replacement results in the formation of a sol when another liquid is added to the original solution that mixes well with the original solvent but is a poor solvent for the solute.
Chemical methods of condensation are based on the performance of various reactions, as a result of which an undissolved substance precipitates from a supersaturated solution.
Chemical condensation can be based not only on exchange, but also on redox reactions, hydrolysis, etc.
Dispersed systems can also be obtained by peptization, which consists in transferring precipitates into a colloidal “solution”, the particles of which already have colloidal sizes. There are the following types of peptization: peptization by washing the precipitate; peptization with surface-active substances; chemical peptization.
From the point of view of thermodynamics, the dispersion method is the most advantageous.
Cleaning Methods:
Dialysis is the purification of sols from impurities using semi-permeable membranes washed by a pure solvent.
Electrodialysis is dialysis accelerated by an electric field.
Ultrafiltration - cleaning by forcing the dispersion medium together with low molecular weight impurities through a semi-permeable membrane (ultrafilter).
53. Molecular-kinetic and optical properties of dispersed systems: Brownian motion, osmotic pressure, diffusion, sedimentation equilibrium, sedimentation analysis, optical properties of dispersed systems.
All molecular-kinetic properties are due to the spontaneous movement of molecules and are manifested in Brownian motion, diffusion, osmosis, and sedimentation-ionic equilibrium.
Brownian movement is called continuous, chaotic, equally probable for all directions, the movement of small particles suspended in a liquid or gases, due to the action of molecules of a dispersion medium. The theory of Brownian motion proceeds from the concept of the interaction of a random force that characterizes the impacts of molecules, a time-dependent force, and a friction force when particles of a dispersed phase move in a dispersion medium at a certain speed.
In addition to translational motion, rotational motion is also possible, which is typical for two-dimensional particles of irregular shape (threads, fibers, flakes). Brownian motion is most pronounced in highly dispersed systems, and its intensity depends on the dispersion.
Diffusion is the spontaneous spread of a substance from an area of higher concentration to an area of lower concentration. There are the following types:
1.) molecular
3) colloidal particles.
The diffusion rate in gases is the highest, and in solids it is the lowest.
Osmotic pressure is the excess pressure above the solution that is necessary to prevent the transfer of the solvent through the membrane. OD occurs when a pure solvent moves towards a solution or from a more dilute solution towards a more concentrated one, and therefore is associated with the difference in the concentration of the solute and solvent. The osmotic pressure is equal to the pressure that the dispersed phase (solute) would produce if it, in the form of a gas at the same temperature, occupied the same volume as the colloidal system (solution).
Sedimentation is the stratification of disperse systems under the action of gravity with the separation of the dispersed phase in the form of sediment. The ability of dispersed systems to sedimentation is an indicator of their sedimentation stability. Stratification processes are used when it is required to isolate one or another component from some component from some natural or artificially prepared product, which is a heterogeneous liquid system. In some cases, a valuable component is removed from the system, in others, unwanted impurities are removed. In public catering, the processes of stratification of dispersed systems are necessary when it is required to obtain transparent drinks, illuminate the broth, and free it from meat particles.
The behavior of a light beam that encounters particles of a dispersed phase on its way depends on the ratio of the light wavelength and particle size. If the particles are larger than the wavelength of light, then light is reflected from the surface of the particles at a certain angle. This phenomenon is observed in suspensions. If the particles are smaller than the wavelength of light, then the light is scattered.
organic matter with the length of the hydrocarbon radical in its molecule. According to this rule, with an increase in the length of the hydrocarbon radical by one СΗ 2 group, the surface activity of the substance increases on average by 3.2 times.Write a review on the article "Duclos-Traube rule"
Notes
K: Wikipedia: Isolated articles (type: not specified)An excerpt describing the Duclos-Traube Rule
And, going up to the bed, he took out a purse from under the clean pillows and ordered to bring wine.“Yes, and give you the money and the letter,” he added.
Rostov took the letter and, throwing money on the sofa, leaned his elbows on the table with both hands and began to read. He read a few lines and looked angrily at Berg. Meeting his gaze, Rostov covered his face with a letter.
“However, they sent you a decent amount of money,” Berg said, looking at the heavy purse pressed into the sofa. - Here we are with a salary, count, making our way. I'll tell you about myself...
“That’s what, my dear Berg,” said Rostov, “when you receive a letter from home and meet your man, whom you want to ask about everything, and I’ll be here, I’ll leave now so as not to disturb you. Listen, go away, please, somewhere, somewhere ... to hell! he shouted, and at once, grabbing him by the shoulder and looking affectionately into his face, apparently trying to soften the rudeness of his words, he added: “you know, don’t be angry; dear, my dear, I speak from the heart, as to our old acquaintance.
“Ah, pardon me, Count, I understand very well,” said Berg, getting up and speaking to himself in a throaty voice.
- You go to the owners: they called you, - Boris added.
Berg put on a clean frock coat, without a spot or a speck, fluffed up the temples in front of the mirror, as Alexander Pavlovich wore, and, convinced by Rostov's look that his frock coat had been noticed, with a pleasant smile he left the room.
- Oh, what a beast I am, however! - said Rostov, reading the letter.
- And what?
- Oh, what a pig I am, however, that I never wrote and so scared them. Oh, what a pig I am,” he repeated, suddenly blushing. - Well, send Gavrila for wine! Okay, enough! - he said…
In the letters of the relatives, there was also a letter of recommendation to Prince Bagration, which, on the advice of Anna Mikhailovna, the old countess got through her acquaintances and sent to her son, asking him to take it down for its intended purpose and use it.
- That's nonsense! I really need it, - said Rostov, throwing the letter under the table.
- Why did you leave it? Boris asked.
- What a letter of recommendation, the devil is in my letter!
- What the hell is in the letter? - Boris said, raising and reading the inscription. This letter is very important for you.
“I don’t need anything, and I’m not going to be an adjutant to anyone.
- From what? Boris asked.
- Lackey position!
PHYSICAL AND COLLOID CHEMISTRY
Abstract of lectures for students of the Faculty of Biology of the Southern Federal University (RSU)
4.1 SURFACE PHENOMENA AND ADSORPTION
4.1.2 Adsorption at the solution-vapor interface
In liquid solutions, surface tension σ is a function of the solute concentration. On fig. 4.1 shows three possible dependences of surface tension on the concentration of the solution (the so-called surface tension isotherms). Substances whose addition to a solvent reduces surface tension are called surface-active(surfactants), substances, the addition of which increases or does not change the surface tension - surface-inactive(PIAV).
Rice. 4.1 Surface isotherms Rice. 4.2 Adsorption isotherm
tension of solutions of PIAV (1, 2) and surfactant at the solution-vapor interface
Surfactant (3)
A decrease in surface tension and, consequently, surface energy occurs as a result of surfactant adsorption on the liquid-vapor interface, i.e. the fact that the concentration of surfactant in the surface layer of the solution is greater than in the depth of the solution.
The quantitative measure of adsorption at the solution-vapor interface is surface excess G (gamma), equal to the number of moles of solute in the surface layer. The quantitative relationship between the adsorption (surface excess) of a solute and the change in the surface tension of the solution with increasing solution concentration determines Gibbs adsorption isotherm:
The plot of the surfactant adsorption isotherm is shown in fig. 4.2. From equation (IV.5) it follows that the direction of the process - the concentration of a substance in the surface layer or, conversely, its presence in the volume of the liquid phase - is determined by the sign of the derivative d σ /dС. The negative value of this derivative corresponds to the accumulation of the substance in the surface layer (G > 0), the positive value corresponds to a lower concentration of the substance in the surface layer compared to its concentration in the bulk of the solution.
The value g \u003d -d σ / dС is also called the surface activity of the solute. The surface activity of surfactants at a certain concentration of C 1 is determined graphically by drawing a tangent to the surface tension isotherm at the point C = C 1 ; in this case, the surface activity is numerically equal to the tangent of the slope of the tangent to the concentration axis:
It is easy to see that with increasing concentration, the surface activity of surfactants decreases. Therefore, the surface activity of a substance is usually determined at an infinitesimal concentration of the solution; in this case, its value, denoted g o, depends only on the nature of the surfactant and solvent. Investigating the surface tension of aqueous solutions of organic substances, Traube and Duclos established the following rule of thumb for the homologous series of surfactants:
In any homologous series at low concentrations, the elongation of the carbon chain by one CH2 group increases the surface activity by a factor of 3–3.5.
For aqueous solutions of fatty acids, the dependence of surface tension on concentration is described by the empirical Shishkovsky equation :
(IV.6a)
Here b and K are empirical constants, and the value of b is the same for the entire homological series, and the value of K increases for each subsequent member of the series by 3–3.5 times.
Rice. 4.3 Limit Orientation of Surfactant Molecules in the Surface Layer
Molecules of most surfactants have a amphiphilic structure, i.e. contain both a polar group and a non-polar hydrocarbon radical. The location of such molecules in the surface layer is energetically most favorable under the condition that the molecules are oriented by the polar group to the polar phase (polar liquid), and the nonpolar group to the nonpolar phase (gas or nonpolar liquid). At a low concentration of the solution, thermal motion disrupts the orientation of surfactant molecules; with an increase in concentration, the adsorption layer is saturated and a layer of "vertically" oriented surfactant molecules is formed on the interface (Fig. 4.3). The formation of such a monomolecular layer corresponds to the minimum value of the surface tension of the surfactant solution and the maximum value of adsorption G (Fig. 4.1-4.2); with a further increase in the surfactant concentration in the solution, the surface tension and adsorption do not change.
Copyright © S. I. Levchenkov, 1996 — 2005.
Chemist's Handbook 21
Chemistry and chemical technology
Duclos Traube, rule
Formulate the Duclos-Traube rule and explain its physical meaning. At what structure of surface films this rule is observed What is the reversibility of this rule
The physical meaning of the Duclos-Traube rule
Colloidal surfactants exhibit high surface activity, which depends mainly on the length of the hydrocarbon radical. An increase in the length of the radical by one group. -CH2- leads to an increase in surface activity by approximately 3.2 times (Duclos-Traube rule). This rule is observed mainly for truly soluble surfactants. Since the surface activity is determined by infinite dilution of the system, it is easy to explain its dependence on the length of the hydrocarbon radical. The longer the radical, the stronger the surfactant molecule is pushed out of the aqueous solution (the solubility decreases).
The resulting expression for the ratio r (n-s) / r (u) reflects the Duclos-Traube rule.
This rule is fulfilled only for aqueous solutions of surfactants. For surfactant solutions in non-polar solvents, the surface activity decreases with an increase in the length of the hydrocarbon radical (reversal of the Duclos-Traube rule).
The whole variety of dependences of surface tension on concentration can be represented by curves of three types (Fig. 43). Surfactants are characterized by curves of type 1. Surfactants are less polar than the solvent, and have a lower surface tension than the solvent. The intensity of the interaction of solvent molecules with surfactant molecules is less than that of solvent molecules with each other. In relation to water, a polar solvent, surfactants are organic compounds consisting of a hydrocarbon radical (hydrophobic or oleophilic part) and a polar group (hydrophilic part) of carboxylic acids, their salts, alcohols, amines. Such amphiphilic structure of the molecule is a characteristic feature of surfactants. Hydrocarbon chains that do not have a permanent dipole moment are hydrophobic, interact with water molecules weaker than with each other, and are pushed to the surface. Therefore, organic substances that do not have a polar group (for example, paraffins, naphthenes) are practically insoluble in water. Polar groups such as -OH, -COOH, -NH, etc. have a high affinity for water, are well hydrated, and the presence of such a group in the molecule determines the surfactant solubility. Thus, the solubility of surfactants in water depends on the length of the hydrocarbon radical (solubility decreases with increasing length in the homologous series). For example, carboxylic acids i - C4 are infinitely soluble in water, the solubility of C5 - C12 acids decreases markedly with an increase in the number of C-atoms, and when the length of the hydrocarbon chain is more than i2, they are practically insoluble. An increase in the length of the hydrocarbon radical of a surfactant molecule by one CHa group leads to an increase in surface activity by a factor of 3.2–3.5 (this rule is called the Duclos-Traube rule).
Langmuir's ideas about adsorption also make it possible to explain the well-known Duclos-Traube rule (1878), which, like the Shishkovsky equation, was established experimentally for solutions of lower fatty acids. According to this rule, the ratio of the concentrations of two neighboring homologues, which correspond to the same A, is constant and approximately equal to 3.2. The same conclusion can be reached based on the Shishkovsky equation. For the nth and (n + 1)th homologues from (4.42) we have
Equation (39) establishes the dependence of the surface-combustion activity on the length of the direct saturated hydrocarbon radical and, in essence, contains the regularity known as the Duclos-Traube rule. Indeed, for the (n + 1)th term of the series, we can write
In accordance with equation (42), the value of the coefficient of the Duclos-Trauber rule p depends on the value of the LS increment. A decrease in this value leads to a decrease in the difference in the surface activity of homologues and vice versa.
According to Langmuir, the Duclos-Traube rule can be justified as follows. Let us assume that the thickness of the surface layer is equal to O. Then the average concentration in this layer will be Г/0. It is known from thermodynamics that the maximum work A required to compress a gas from volume Fi to volume Vit can be represented as
The ratio (VI. 37) reflects the Duclos-Traube rule. It is a constant value and for aqueous solutions at 20°C is 3.2. At temperatures other than 20 °C, the constant has other values. The surface activity is also proportional to the constant included in the Langmuir equation (or the Shishkovsky equation), since Kr = KAoo (III. 17) and the Loo-capacity of the monolayer is constant for a given homologous series. For organic media, the Duclos-Traube rule is reversed; surface activity decreases with increasing length of the surfactant hydrocarbon radical.
It is easy to see that equations (76) and (77) are similar to equation (39) expressing the Duclos-Traube rule. This indicates a relationship between the bulk and surface properties of surfactant solutions and emphasizes the commonality of adsorption and micelle formation phenomena. Indeed, in the homologous series of surfactants, the CMC value changes approximately in inverse proportion to the surface activity, so that the CMC ratio of neighboring homologues corresponds to the coefficient of the Duclos-Traube rule
It can be seen from this equation that the work of adsorption should increase by a constant value when the hydrocarbon chain is extended by the CH2 group. This means that at low concentrations, at which only the Duclos-Traube rule is observed, all CH groups in the chain occupy the same position with respect to the surface, which is possible only when the chains are parallel to the surface, i.e., lie on it. We will return to the question of the orientation of surfactant molecules in the surface layer later in this section.
That is, G is inversely proportional. Now the Duclos-Traube rule will be written as
The Duclos-Traube rule, as formulated above, is fulfilled at temperatures close to room temperature. At higher temperatures, the ratio 3.2 decreases, tending to unity, since with increasing temperature the surface activity decreases as a result of desorption of molecules and the difference between the surface activity of homologues is smoothed out.
However, this explanation contradicts the fact that the values of Goo measured on the same objects correspond to the standing, rather than lying, position of the molecules, due to which they are almost independent of n. Duclos-Traube is satisfied, the adsorbed molecules lie on the surface, and as their density increases, they gradually rise. But it is obvious that such an interpretation is incompatible with the strict application of the Langmuir isotherm, in which Goo is assumed to be a constant value independent of the degree of filling of the adsorption layer.
The extent to which the Duclos-Traube rule is observed for the homologous series of fatty acids can be seen from the data in Table. V, 4. The Duclos-Traube rule is observed not only for fatty acids, but also for other homologous series - alcohols, amines, etc.
Another formulation of the Duclos-Traube rule is that when fatty acid chain length increases exponentially, surface activity increases exponentially. A similar relationship must be observed when the molecule is elongated and for the value jA, since the surface activity of substances at sufficiently low concentrations is proportional to the specific capillary constant.
It should also be noted that the Duclos-Traube rule is observed only for aqueous solutions of surfactants. For solutions of the same substances in non-polar solvents, the Duclos-Traube rule is inverted, since with increasing
In the first approximation, it can also be assumed that the better the medium dissolves the adsorbent, the worse the adsorption in this medium. This provision is one of the reasons for the reversal of the Duclos-Traube rule. So, when the adsorption of a fatty acid occurs on a hydrophilic adsorbent (for example, silica gel) from a hydrocarbon medium (for example, from benzene), adsorption does not increase with an increase in the molecular weight of the acid, as follows from the Duclos-Traube rule, but decreases, since higher fatty acids are more soluble in a non-polar medium.
It is clear that such a reversal of the Duclos-Traube rule cannot be observed on non-porous adsorbents with smooth surfaces.
Duclos-Traube rule
The Duclos-Traube rule for soluble surfactants is fulfilled in a wide range of concentrations, starting from dilute solutions and ending with the maximum saturation of surface layers. In this case, the Traube coefficient can be expressed as the ratio of the concentrations corresponding to the saturation of the surface layer
The Duclos-Traube rule is of great theoretical and practical importance. It indicates the right direction in the synthesis of highly active surfactants with long chains.
How the Duclos-Traube rule is formulated How it can be written How do the surface tension isotherms of two neighboring homologues with the number of carbon atoms n and n- look like -
The connection between the constants included in the Shishkovsky equation and the structure of surfactant molecules can be established by referring to the pattern established by Duclos and Traube. Duclos found that the ability of surfactants to reduce the surface tension of water in the homologous series increases with increasing number of carbon atoms. Traube supplemented Duclos' observations. The relationship between the surface activity and the number of carbon atoms found by these researchers was called the Duclos-Traube rule. With an increase in the number of carbon atoms in the homologous series in an arithmetic progression, the surface activity increases exponentially, and an increase in the hydrocarbon part of the molecule by one CH3 group corresponds to an increase in surface activity by about 3-3.5 times (average 3.2 times).
The Duclos-Traube rule is most accurate at low solute concentrations. That's why
An important conclusion follows from the Duclos-Traube rule: the area per molecule at maximum saturation of the adsorption layer remains constant within one homologous series.
Aliphatic reversible competitive inhibitors. As can be seen from fig. 37, the affinity site of the active center is not very specific with respect to the structure of the aliphatic chain in the inhibitor molecule (alkanols). Regardless of whether the aliphatic chain is normal or branched, the efficiency of the reversible binding of the KOH alkanol to the active center is determined by the gross hydrophobicity of the K group. Namely, the value of log i, which characterizes the strength of the complex, increases linearly (with a slope close to unity) with the degree of distribution 1 R of these compounds between water and standard organic phase (n-octanol). The observed increment of free energy of CHa-group transfer from water to the active center medium is approximately -700 cal/mol (2.9 kJ/mol) (for the lower members of the homologous series). This value is close to the value of the free energy increment, which follows from the Duclos-Traube rule known in colloidal chemistry and is characteristic of the free energy of the transition of a liquid CH-group from water to a non-aqueous (hydrophobic) medium. All this makes it possible to consider the hydrophobic region of the active center of chymotrypsin as a drop of an organic solvent located in the surface layer of the protein globule. This droplet either adsorbs the hydrophobic inhibitor from the water onto the interface, or, being somewhat deepened, completely extracts it. From the point of view of the microscopic structure of the hydrophobic region, it would be more correct to consider it as a fragment of a micelle, however, such detailing seems unnecessary, since it is known that the free energy of the transition of n-alkanes from water to the microscopic medium of a dodecyl sulfate micelle differs little from the free energy of the release of the same compounds from water into a macroscopic liquid non-polar phase..
Adsorption from the organic phase. In this case, only the polar group passes into the neighboring (aqueous) phase. Consequently, the work of adsorption is determined only by the difference in the energy of the intermolecular interaction of polar groups in the organic phase and water, i.e., by the change in their energy state during the transition from an organic liquid to water. Since the hydrocarbon radicals remain in the organic phase, PAAUdaO and the work of adsorption from the organic phase is V0. In this case, the work of adsorption should not depend on the length of the hydrocarbon radical, and the Duclos-Traube rule should not be observed. Indeed, as experimental data show, all normal alcohols and acids are approximately equally adsorbed from paraffinic hydrocarbons at the boundary with water. This is well illustrated in Fig. 4 . Greatness-
Consequently, the surface activity of the compound is the greater, the stronger the polar asymmetry of the molecule is expressed. The influence of the non-polar part of the surfactant molecule on the surface activity is most pronounced in the homologous series (Fig. 20.1). G. Duclos discovered this regularity, which was then more precisely formulated by P. Traube in the form of a rule called the Duclos-Traube rule
The value of p is called the Traube coefficient. The theoretical explanation of the Duclos-Traube rule was given later by I. Langmuir. He calculated the energy gain for two neighboring homologues during the transition of their hydrocarbon chains from water to air and found that the difference corresponding to the energy of the transition of one CH3 group is constant in the homologous series and is close to 3 kJ / mol. The gain in energy is due to the fact that when a nonpolar circuit is forced out of an aqueous medium into air, the dipoles of water combine and the Gibbs energy of the system decreases. At the same time, the Gibbs energy and the surfactant chain, which has passed into the medium, to which it has a high polarity affinity, decrease.
Effect of surfactant chain length. In the homologous series, with increasing surfactant molecular weight, the CMC value decreases approximately in inverse proportion to the surface activity (CMCl 1/0m). For neighboring homologues, the CMC ratio has the value of the Duclos-Traube rule coefficient (CMC) / (CMC) +1 P = 3.2.
Langmuir showed that the Duclos-Traube rule can be used to calculate the energy of group transfer - Hj - from the volume of the solution to the gas phase. Indeed, considering b as a constant of adsorption equilibrium [on p. 61 It was shown that for the equivalent value of K, K = kJ is valid, in accordance with the equation of the standard reaction isotherm, we have
See pages where the term is mentioned Duclos Traube, rule: Colloidal Chemistry 1982 (1982) — [ c.54 ]
surface activity. Surface-active and surface-inactive substances. Duclos-Traube rule.
surface activity, the ability of a substance during adsorption at the interface to lower the surface tension (interfacial tension). Adsorption G in-va and the decrease in surface tension s caused by it is associated with the concentration With in-va in the phase from which the substance is adsorbed to the interfacial surface, the Gibbs equation (1876): 
Where R- gas constant, T-abs. temperature (see Adsorption). Derivative
serves as a measure of the ability of a substance to lower surface tension at a given interfacial boundary and is also called. surface activity. Denoted G (in honor of J. Gibbs), measured in J m / mol (gibbs).
Surfactants (surfactants), substances whose adsorption from a liquid at the interface with another phase (liquid, solid or gaseous) leads to a mean. lowering surface tension (see Surface activity). In the most general and practical case, adsorbed surfactant molecules (ions) have an amphiphilic structure, i.e., they consist of a polar group and a nonpolar hydrocarbon radical (amphiphilic molecules). Surface activity in relation to a non-polar phase (gas, hydrocarbon liquid, non-polar surface of a solid) is possessed by a hydrocarbon radical, which is pushed out of the polar medium. In an aqueous solution of surfactants, an adsorption monomolecular layer with hydrocarbon radicals oriented towards air is formed at the boundary with air. As it becomes saturated, the molecules (ions) of the surfactant, condensing in the surface layer, are located perpendicular to the surface (normal orientation).
The concentration of surfactants in the adsorption layer is several orders of magnitude higher than in the bulk of the liquid, therefore, even with a negligible content in water (0.01-0.1% by weight), surfactants can reduce the surface tension of water at the border with air from 72.8 to 10 -3 to 25 10 -3 J/m 2 , i.e. almost to the surface tension of hydrocarbon liquids. A similar phenomenon takes place at the interface between an aqueous solution of a surfactant and a hydrocarbon liquid, which creates prerequisites for the formation of emulsions.
Depending on the state of surfactants in solution, truly soluble (molecularly dispersed) and colloidal surfactants are conditionally distinguished. The conditionality of such a division is that the same surfactant can belong to both groups, depending on the conditions and chem. the nature (polarity) of the solvent. Both groups of surfactants are adsorbed at phase boundaries, i.e., they exhibit surface activity in solutions, while only colloidal surfactants exhibit bulk properties associated with the formation of a colloidal (micellar) phase. These groups of surfactants differ in the value of a dimensionless quantity, which is called. hydrophilic-lipophilic balance (HLB) and is determined by the ratio:
Duclos-Traube rule- dependence connecting the surface activity of an aqueous solution of organic matter with the length of the hydrocarbon radical in its molecule. According to this rule, with an increase in the length of the hydrocarbon radical by one СН 2 group, the surface activity of a substance increases on average by a factor of 3.2. Surface activity depends on the structure of surfactant molecules; the latter usually consist of a polar part (groups with a large dipole moment) and a non-polar part (aliphatic or aromatic radicals). Within the boundaries of the homologous series of organic substances, the concentration required to lower the surface tension of an aqueous solution to a certain level decreases by a factor of 3-3.5 with an increase in the carbon radical by one -СΗ 2 -group.
The rule was formulated by I. Traube (German) Russian. in 1891 as a result of his experiments carried out on solutions of many substances (carboxylic acids, esters, alcohols, ketones) in water. The previous studies of E. Duclos, although they were close in spirit to the works of Traube, did not offer any clear dependence of concentration, therefore, in foreign literature, the rule bears only the name of Traube. . The thermodynamic interpretation of the Traube rule was given in 1917 by I. Langmuir.
Duclos-Traube rule
Large English-Russian and Russian-English dictionary. 2001 .
Duclos-Traube rule- Duclos Traube's rule: with an increase in the length of the carbon chain of substances of one homologous series, adsorption on a non-polar adsorbent from a polar solvent increases by about 3 times with an increase in the hydrocarbon chain by one methylene group CH2 ... ... Chemical terms
Duclos' rule- Traube dependence linking the surface activity of an aqueous solution of an organic substance with the length of the hydrocarbon radical in its molecule. According to this rule, with an increase in the length of the hydrocarbon radical by one group ... ... Wikipedia
General chemistry: textbook. A. V. Zholnin; ed. V. A. Popkova, A. V. Zholnina. . 2012 .
See what the "Duclos-Traube rule" is in other dictionaries:
SURFACE PRESSURE- (flat pressure, two-dimensional pressure), the force acting per unit length of the interface (barrier) of a clean liquid surface and the surface of the same liquid covered with adsorption. layer of surfactant. P. d. directed to the side ... ... Physical Encyclopedia
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Traube-Duclos rule;
As already noted, surface-active molecules capable of being adsorbed on the solution–gas interface must be amphiphilic; have polar and non-polar parts.
Duclos and then Traube, studying the surface tension of aqueous solutions of the homologous series of saturated fatty acids, found that the surface activity (−) of these substances at the solution–air interface is the greater, the longer the length of the hydrocarbon radical, and on average it increases by 3–3 .5 times for each group -CH 2 -. This important pattern is called Traube-Duclos rules.
Traube's rule — Ducloglasite:
in the homologous series of normal fatty monobasic acids, their surface activity (-) with respect to water increases sharply by 3-3.5 times for each group -CH 2 - at an equal molar concentration.
Another formulation of Traube's rule — Duclos: “When the length of a fatty acid chain increases exponentially, surface activity increases exponentially.” Traube's rule — Duclos is well illustrated in Figure 18.1.
As can be seen from the figure, the higher the substance in the homologous series, the more it lowers the surface tension of water at a given concentration.
The reason for the dependence established by Traube's rule — Duclos, lies in the fact that with an increase in the length of the radical, the solubility of the fatty acid decreases and the tendency of its molecules to move from the volume to the surface layer increases. It has been established that Traube's rule — Duclos is observed not only for fatty acids, but also for other homologous series - alcohols, amines, etc.
Rice. 18.1 Traube's rule — Duclos:
1- acetic acid, 2- propionic acid, 3- butyric acid, 4- valeric acid.
1) only at low concentrations, when the value - - is maximum;
2) for temperatures close to room temperature. At higher temperatures, the factor 3–3.5 decreases and tends to unity. An increase in temperature promotes the desorption of molecules and therefore their surface activity decreases (the difference between the surface activity of homologues is smoothed out);
3) only for aqueous solutions. surfactant.
The American physical chemist Langmuir found that the Traube rule is valid only for small concentrations of surfactants in a solution with a free arrangement of adsorbed molecules on the surface (Fig. 18.6).
Rice. 18.6 Location of adsorbed molecules at the interface:
a – at low concentrations; b - at medium concentrations;
c - in a saturated layer at the maximum possible adsorption
DUCLAU-TRAUBE RULE
It follows from the Gibbs equation that the value of the derivative is a characteristic of the behavior of a substance during adsorption, but its value changes with a change in concentration (see Fig. 3.2). To give this quantity the form of a characteristic constant, its limiting value is taken (at c 0). P. A. Rebinder (1924) called this value the surface activity g:
[g] = J – m 3 / m 2 -mol \u003d J – m / mol or N-m 2 / mol.
The more the surface tension decreases with increasing concentration of the adsorbed substance, the greater the surface activity of this substance, and the greater its Gibbs adsorption.
Surface activity can be defined graphically as the negative value of the tangent of the slope of the tangent drawn to the curve =f(c) at the point of its intersection with the y-axis.
Thus, for surfactants: g > 0; 0. For TIDs: g 0, Г i
This also explains the inactivity of sucrose, the molecule of which, along with a non-polar hydrocarbon skeleton, has many polar groups, therefore, the molecule has a balance of the polar and non-polar parts.
2. In the homologous series, there are clear patterns in the change in surface activity (g): it increases as the length of the hydrocarbon radical increases.
