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 environment.
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. four . 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 a non-polar phase (gas, hydrocarbon liquid, non-polar surface of a solid body) 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 .
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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.
Adsorption - the phenomenon of spontaneous accumulation of one substance on the surface of another. The substance that is adsorbed is called adsorbent; substance on whose surface adsorption occurs adsorbent.
Adsorption on the surface of liquids
Particles of substances dissolved in liquids can be adsorbed on the surface of liquids. Adsorption accompanies the dissolution process, influencing the distribution of solute particles between the surface layers of the solvent and its internal volume.
Adsorption on the liquid surface can be calculated using the Gibbs equation:
G - the value of specific adsorption, mol / m 2;
C - molar concentration, mol / m 3;
dσ - change in surface tension corresponding to a change in concentration ΔС;
surface activity.
If with an increase in the concentration of a substance, the surface tension decreases Δ σ< 0, то его адсорбция Г считается положительной (Г >0). This means that the concentration of the substance in the surface layer is greater than in the volume of the solution.
If with an increase in the concentration of a substance, the surface tension at the phase boundary increases Δ σ > 0, then adsorption is considered negative Г< 0, это означает, что концентрация вещества в объеме раствора больше, чем в поверхностном слое.
Surfactant adsorption
Surfactants are diphilic substances in nature, they have polar (hydrophilic) and non-polar (hydrophobic) parts.
For example, soap: C 17 H 35 COONa
non-polar symbol polar symbol
parts parts
Surfactants are positively adsorbing substances, these include: fats, fatty acids, ketones, alcohols, cholesterol, soaps and other organic compounds. When such substances are dissolved in water, positive adsorption occurs, accompanied by the accumulation of the substance in the surface layer. The process of release of the molecules of these substances to the surface is very beneficial, because. leads to a decrease in surface tension at the interface. Scheme of surfactant adsorption:
The ability of a substance to lower surface tension at the interface is called surface activity.
Duclos-Traube rule
The value of the surface activity of surfactants - members of the same homologous series of organic compounds depends on the length of the hydrocarbon radical: elongation of the surfactant by one group -CH 2 - increases the surface activity of the substance by 3-3.5 times.
Consider the Duclos-Traube rule using the example of four representatives of the homologous series of alcohols.
Surface Tension Isotherm:
PIAV adsorption
In relation to polar water, such substances are electrolytes: inorganic acids, salts, alkalis. The dissolution of these substances increases the surface tension, so the surfactants will be pushed out of the surface layer into the adsorbent. Such adsorption is called negative. For example: the dissolution of KS1 in water is accompanied by the dissociation of the salt, followed by hydration of the resulting ions.
Scheme of PIAV adsorption.
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
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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.
At higher 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.
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.
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 3-3.5 times 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.
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