Traube's rule, its interpretation and illustrative examples. Duclos-Traube rule. Generalized equation of Dubinin's theory of volume filling of micropores, special cases of this equation
This rule works in solutions of surfactant homologous series and is formulated as follows:
With an increase in the length of the hydrocarbon radical by one CH group 2 , surface activity increases in the homologous series by 3 - 3.5 times.
Let's illustrate this rule graphically:
Fig.2.21. Isotherms of surface tension (a) and adsorption (b) of surfactant solutions of the same homologous series (1,2,3 - number of -CH groups 2 - in hydrocarbon radical)
Note that the value G ∞ for one homologous series remains permanent. This is explained by the fact that the capacitance of the monolayer in this case depends only on the area occupied by the surfactant molecule in this layer. In the series of carboxylic acids, alcohols, this area is determined by the size of the polar group, which is the same for the entire series of surfactants.
This rule is observed for truly soluble surfactants. Because surface activity is determined for infinitely dilute systems, 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, because the incorporation of a radical into water increases ΔG, and the process of molecules coming to the surface is energetically very favorable.
Shishkovsky equation ( * )
For the case of adsorption of surfactant molecules at the phase boundary, both proposed adsorption equations on a homogeneous surface can be used. Let's compare them to each other:
=
(2.56)
Separate the variables and integrate these equations:
,
(2.57)
, (2.58)
Since in surfactant solutions, due to their high surface activity, the values of absolute adsorption A almost equal to excess adsorption G, so the resulting equation can be written in the form:. (2.59)
The resulting equation is called Shishkovsky's equations. Initially, it was derived empirically by him to describe the dependence of surface tension on surfactant concentration:
The equation (2.60) includes the coefficients B and A, the physical meaning of which is visible from the above derived equation (2.59).
The relationship between surface tension and adsorption can be traced in the equation Frumkin (*) :
, (2.61)
from which it follows that for the same adsorption, all homologues reduce the surface tension by the same value ∆σ.
The difference in surface activity in the homologous series of surfactants is due to their different adsorption properties, i.e. the same value of G is achieved for short-chain surfactants at significantly higher C than for long-chain surfactants. But if the concentrations of the homologues are such that their adsorptions are the same, then they lower σ by the same amount.
Experimental determination of the geometric dimensions of a surfactant molecule
Let us show that, knowing the value of the capacitance of the monolayer, we can calculate S o- the area occupied by the polar group and δ - the length of the hydrocarbon radical of the surfactant molecule. The calculated data can be compared with independently determined other methods.
,
Area occupied by the polar group 
(2.62)
Volume occupied by one molecule V 1 = δ S o (2.63)
The molar mass of a monolayer can be determined by the formula:
M=ρ δS o N a , (2.64)
where ρ is the surfactant density, N a is Avogadro's number (*) . And since
S o *N a \u003d 1 / G ∞, then the length of the hydrocarbon radical can be determined based on the equation:
. (2.65)
Numerous experimental checks of the resulting equation showed good agreement between the values of δ calculated from the above equation and measured by other methods.
Features of the structure of the surface layer of the phase.
Intermediate phase containing one or more molecular layers
Peculiarities:
– Inside the volume of a pure substance, all forces of intermolecular interaction are balanced
– The resultant of all forces acting on surface molecules is directed inside the liquid
– Surface phenomena are negligible if the ratio between body mass and surface is in favor of body mass
– Surface phenomena acquire significance when the substance is in a fragmented state or in the form of the thinnest layer (film)
1 cm 3 arrow 10 -7, S = 6,000 m 2
1 mm of blood arrow 4 - 5 million erythrocytes; 1l arrow> 30 mlr cells, S = 1000 m 2
S alveoli = 800 -1000 m 2; S liver capillaries = 600 m 2
Gibbs surface energy
σ– surface tension
Gibbs energy reduction:
By reducing the surface area (coarse particles)
By reducing the surface tension (sorption)
403)surface tension
Work done to create a unit of surface
Units J / m 2
Force acting per unit length of a line bounding the surface of a liquid and directed in the direction of decreasing this surface
Units N/m2
Dependence of surface tension on the nature of substances, temperature and pressure.
The surface tension of liquids decreases with increasing temperature and becomes zero near the critical temperature. With increasing pressure, the surface tension at the liquid-gas interface decreases, because the concentration of molecules in the gas phase increases and the force decreases. Dissolved substances can increase, decrease and practically influence the practical tension of liquids. The surface tension at the liquid-liquid interface depends on the nature of the adjacent phases. It is the greater, the smaller the force of molecular interaction between dissimilar molecules.
Methods for measuring the surface tension of a liquid.
The method of tearing off the ring from the surface of the liquid
Method for counting the number of drops of a certain volume of the test liquid flowing from the capillary (stalagmometric)
Method for determining the pressure required to detach an air bubble from a capillary immersed in a liquid (Rehbinder method)
Method for measuring the height of rise of a liquid in a capillary, the walls of which are well wetted by it
The distribution of the dissolved substance between the surface layer and the volume of the phase.
theoretically, it is possible to imagine three cases of the distribution of the dissolved substance between the surface layer and the volume of the phase: 1) the concentration of the dissolved substance in the surface layer is greater than in the volume of the phase. 2) the concentration of the dissolved substance in the surface layer is less, than in the volume of the phases. 3) the concentration of the dissolved substance in the top layer is the same as in the volume of the phases.
Classification of dissolved substances according to their effect on the surface tension of a liquid (water).
classification. 1) dissolved in lower tension p-la. Alcohols, to-you. 2) the dissolved content slightly increases the sodium content. Inorg to-you, bases, salts. Sucrose.
Gibbs equation for characterizing the adsorption of dissolved substances. Equation analysis.
Г=-(C/RT)*(∆σ/∆C). G-value of adsorption on the surface of the solution. ∆σ/∆C-pov activity in-va. Analysis: ∆σ/∆C=0, Г=0. This is NVD. ∆σ/∆C>0, Г<0-поверхностно инактивные в-ва. ∆σ/∆C<0, Г>0-surfactant.
Molecular structure and properties of surfactants.
sv-va: Limited solubility
Have lower surface tension than liquids
Dramatically change the surface properties of the liquid
Structure: Amphiphilic - different parts of the molecule are characterized by a different relationship to the solvent
Hydrophobic properties: hydrocarbon radical
Hydrophilic properties: OH, NH 2 , SO 3 H
Classification of surfactants, examples.
Molecular or non-ionic - alcohols, bile, proteins
Ionic anionic - soaps, sulfonic acids and their salts, carboxylic acids
Ionic cationic - organic nitrogen-containing bases and their salts
Influence of the nature of surfactants on their surface activity. Duclos-Traube rule.
Chain elongation by a radical - CH 2 - increases the ability of fatty acids to adsorb by 3.2 times
Applicable only for dilute solutions and for temperatures close to room temperature, because desorption increases with increasing temperature
Read:
|
As already noted, the molecules of surface-active substances (surfactants) capable of being adsorbed at the solution-gas interface must be amphiphilic, i.e., have polar and non-polar parts.
The polar part of surfactant molecules can be groups with a sufficiently large dipole moment: -СООН, - ОН, -NH 2, - SH, -CN, -NO 2 .-СNS,
CHO, -SO 3 N.
The non-polar part of the surfactant molecule is usually aliphatic or aromatic radicals. The length of the hydrocarbon radical strongly affects the surface activity of the molecule.
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 greater, the longer the hydrocarbon radical is. Moreover, when the hydrocarbon radical is extended by one - CH 2 - group, the surface activity increases by 3-3.5 times (3.2 times on average). This position became known as Duclos-Traube rule .
Another wording of it boils down to the following: as the fatty acid chain grows exponentially, surface activity increases exponentially.
What is the reason (physical meaning) of such a dependence, established first by Duclos, and then, in a more general form, by Traube? It lies in the fact that with an increase in the chain length, the solubility of the fatty acid decreases and, thereby, the tendency of its molecules to move from the volume to the surface layer increases. For example, butyric acid is miscible with water in all respects, valeric acid gives only a 4% solution, all other fatty acids, with a higher molecular weight, are even less soluble in water.
The Duclos-Traube rule, as it was later found, is observed not only for fatty acids, but also for other surfactants that form homologous series, alcohols, amines, etc. Its theoretical (thermodynamic) justification was given by Langmuir.
When a surfactant is introduced into water, practically non-hydrating hydrocarbon chains push the water molecules apart, incorporating into its structure. To accomplish this, work must be done against molecular forces, since the interaction between water molecules is much greater than between water molecules and surfactant molecules. The reverse process - the release of surfactant molecules to the interfacial surface with the orientation of hydrocarbon chains in the non-polar phase of the gas - occurs spontaneously with a decrease in the Gibbs energy of the system and the "gain" of the work of adsorption. The longer the hydrocarbon radical, the greater the number of water molecules it separates and the greater the tendency of surfactant molecules to come to the surface, i.e. the greater their adsorption and the work of adsorption. The work of adsorption when the chain is extended by one link - CH 2 - increases by the same value, which leads to an increase in the adsorption equilibrium constant (adsorption coefficient K) by the same number of times (3.2 times at 20 ° C) . This, in turn, leads to an increase in surface activity by ~3.2 times.
It should be noted that with this formulation, the Duclos-Traube rule is observed only for aqueous solutions and for temperatures close to room temperature.
For solutions of the same surfactants in nonpolar solvents, the Duclos-Traube rule is reversed: with an increase in the length of the hydrocarbon radical, the solubility of surfactants increases and they tend to pass from the surface layer into the solution.
With more high temperatures the average factor3,2 decreases, tending to unity in the limit: with increasing temperature, the surface activity decreases as a result of molecular desorption, and the difference between the surface activity of members of the homologous series is smoothed out.
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 for separating a mixture of substances is that the substances that make up the mixture move together with a 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 sorbed 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 qualitative characteristic given substance under constant experimental conditions: temperature and carrier gas volumetric velocity (w ).
Medium line speed movement 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 various substances mixtures 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 .
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. This amphiphilic structure of the molecule is hallmark surfactant. 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 has an important theoretical and practical value. 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 site 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 increment value observed in this case free energy the transfer of the CHa group 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 phase 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 Р = 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 the non-polar phase (gas, hydrocarbon liquid, non-polar surface solid body) has a hydrocarbon radical that is pushed out of the polar environment. 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 concentration dependence, therefore, in foreign literature the rule bears only the name 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.
