If the ionic strength of the solution is low, then ionic surfactants can behave like polyelectrolytes, repelling each other. With large amounts of salt, the repulsive forces decrease and the worm-like micelles can form a network. Adding more salt can lead to the formation of vesicles. Region(II) is the region of coexistence of various structures. The effect of similarly charged ions on solutions of ionic surfactants is small. Salt additions have little effect on nonionic surfactants. In this case, there may be a decrease in CMC due to dehydration of ions.
Alcohol additives.
Long-chain alcohols are incorporated into aggregates and form mixed micelles. In solutions containing propanol, CMC sharply decreases with increasing alcohol concentration. With an increase in the number of methylene groups in alcohol, this decrease manifests itself to a greater extent. The influence of more water-soluble alcohols has practically no effect on the aggregation of surfactant solutions, but at high concentrations it can lead to an increase in CMC due to a change in the properties of the solution. Important role the steric factor plays a role in the formation of mixed micelles.
Additives of other organic compounds.
Hydrocarbons insoluble in water, such as benzene or heptane, entering the micellar solution are solubilized in the micelle core. In this case, the volume of micelles increases and their sizes change. A change in the curvature of the micelle surface reduces the electric potential on its surface, and, hence, the electric work of micellization, so the CMC decreases. Organic acids and their salts are solubilized inside the micelles near the surface, also reducing the CMC2, this is especially evident with the addition of salicylates and similar compounds due to specific interactions.
The role of hydrophilic groups in aqueous solutions of surfactants is to keep the formed aggregates in water and control their size.
Hydration of counterions promotes repulsion, so less hydrated ions are more easily adsorbed on the surface of micelles. Due to a decrease in the degree of hydration and an increase in the micellar mass for cationic surfactants in the series Cl -
A comparison of the properties of ionic and nonionic surfactants with the same hydrocarbon chains shows that the micellar mass of ionic surfactants is much less than that of nonionic surfactants.
| |
The addition of nonelectrolytes to aqueous solutions of surfactants in the presence of solubilization leads to an increase in the stability of micelles; to a decrease in CMC.
Studies of aqueous solutions of colloidal surfactants have shown that micelle formation can occur only above a certain temperature T k, called Kraft point ( Fig.4).
Below the temperature Tk, the surfactant solubility is low, and in this temperature range there is an equilibrium between the crystals and the true surfactant solution. As a result of the formation of micelles general the concentration of surfactants increases sharply with increasing temperature.
solution and through it to various types of liquid crystal systems.
For nonionic surfactants that are liquids, there is no Kraft point. More typical for them is another temperature limit - cloud point. Turbidity is associated with an increase in the size of micelles and separation of the system into two phases due to dehydration of the polar groups of micelles with increasing temperature.
Methods for determining CMC are based on a sharp change in the physicochemical properties of surfactant solutions (surface tension s, turbidity t, electrical conductivity c, refractive index n, osmotic pressure p) upon transition from a molecular solution to a micellar solution.
In this work, the conductometric method is used to determine the CMC. The conductometric determination of CMC is based on the measurement concentration dependence of electrical conductivity solutions of ionic surfactants.
At a concentration corresponding to CMC, a break is observed on the graph of electrical conductivity (W) - concentration (c), due to the formation of spherical ionic micelles (Fig. 5). The mobility of ionic micelles is less than the mobility of ions. In addition, a significant part of the counterions is located in a dense adsorption layer, which significantly reduces the electrical conductivity of the surfactant solution.
Determination of CMC in a surfactant solution using a pocket conductometer
Required instruments and reagents.
1. Pocket conductivity meter
2. Chemical glasses with a capacity of 50 ml - 6 pcs
3. Measuring cylinder with a capacity of 25 ml - 1 pc.
4. A solution of ionic surfactant concentrations 28·10 -3 mol/l, 32·10 -3 mol/l.
5. Distilled water
Conductivity measurements using a conductometer (Fig. 7) are carried out in the following order:
1. Prepare solutions of ionic surfactants of various concentrations by dilution.
2. Pour them into beakers. The total volume of the solution in the beaker is »32 ml.
3. Prepare the conductometer for operation: remove the protective cap, wash the working part with distilled water. Further, in order to avoid an error in the result, the working part after each reading is washed with distilled water.
4. Taking readings is carried out as follows: the working part of the device is placed in a solution (Fig. 7) , turn on the device by moving the button in the upper part of the device, after setting the readings on the display, they are recorded, turned off and the working part of the device is washed with a stream of distilled water from the washer. The data obtained are summarized in Table 1.
Current page: 11 (total book has 19 pages) [accessible reading excerpt: 13 pages]
67. Chemical methods for obtaining colloidal systems. Methods for controlling particle sizes in disperse systems
There are a large number of methods for obtaining colloidal systems, which allow one to finely control the size of particles, their shape and structure. T. Svedberg proposed to divide the methods for obtaining colloidal systems into two groups: dispersive (mechanical, thermal, electrical grinding or spraying of the macroscopic phase) and condensation (chemical or physical condensation).
Receiving sols. The processes are based on condensation reactions. The process proceeds in two stages. First, nuclei of a new phase are formed, and then a weak supersaturation is created in the ash, at which no new nuclei are formed, but only their growth occurs. Examples. Obtaining sols of gold.
2KAuO 2 + 3HCHO + K 2 CO 3 \u003d 2Au + 3HCOOK + KHCO 3 + H 2 O
Aurate ions, which are potential-forming ions, are adsorbed on the formed gold microcrystals. K + ions serve as counterions
The composition of a gold sol micelle can be schematically depicted as follows:
(mnAuO 2 - (n-x)K + ) x- xK +.
It is possible to obtain yellow (d ~ 20 nm), red (d ~ 40 nm) and blue (d ~ 100 nm) gold sols.
An iron hydroxide sol can be obtained by the reaction:
When preparing sols, it is important to carefully observe the reaction conditions, in particular, strict control of pH and the presence of a number of organic compounds in the system are necessary.
To this end, the surface of the particles of the dispersed phase is inhibited by the formation of a protective layer of surfactants on it or by the formation of complex compounds on it.
Regulation of particle sizes in dispersed systems on the example of obtaining solid nanoparticles. Two identical reverse microemulsion systems are mixed, the aqueous phases of which contain substances BUT and AT, which form a sparingly soluble compound during a chemical reaction. The size of the particles of the new phase will be limited by the size of the droplets of the polar phase.
Metal nanoparticles can also be obtained by introducing a reducing agent (eg, hydrogen or hydrazine) into a microemulsion containing a metal salt, or by passing a gas (eg, CO or H 2 S) through the emulsion.
Factors affecting the course of the reaction:
1) the ratio of the aqueous phase and surfactant in the system (W = / [surfactant]);
2) structure and properties of the solubilized aqueous phase;
3) dynamic behavior of microemulsions;
4) the average concentration of reactants in the aqueous phase.
However, in all cases, the size of the nanoparticles formed in the course of the reaction is controlled by the size of the droplets of the initial emulsion.
Microemulsion systems used to obtain organic compounds. Most research in this area relates to the synthesis of spherical nanoparticles. At the same time, obtaining asymmetric particles (filaments, disks, ellipsoids) with magnetic properties is of great scientific and practical interest.
68. Lyophilic colloidal systems. Thermodynamics of spontaneous dispersion according to Rehbinder-Shchukin
Lyophilic colloidal systems are called ultramicrogenic systems that spontaneously form from macroscopic phases, are thermodynamically stable both for relatively enlarged particles of the dispersed phase, and for particles when they are crushed to molecular sizes. The formation of lyophilic colloidal particles can be determined by the increase in free surface energy during the destruction of the macrophase state, which is possibly compensated due to an increase in the entropy factor, primarily the Brownian motion.
At low values of surface tension, stable lyophilic systems can spontaneously arise by decomposition of the macrophase.
Lyophilic colloidal systems include colloidal surfactants, solutions of macromolecular compounds, and jellies. If we take into account that the critical value of surface tension strongly depends on the diameter of lyophilic particles, then the formation of a system with large particles is possible at lower values of the free interfacial energy.
Considering the dependence of the free energy of a monodisperse system on the size of all particles, it is necessary to take into account the influence of dispersion on a certain value of the free specific energy of particles in the dispersed phase.
The formation of an equilibrium colloidal-dispersed system is possible only under the condition that all particle diameters can lie precisely in the area of dispersion where the size of these particles can exceed the size of molecules.
Based on the foregoing, the condition for the formation of a lyophilic system and the condition for its equilibrium can be represented as the Rebinder-Shchukin equation:
expression characteristic of the condition of spontaneous dispersion.
At sufficiently low, but initially finite values σ (change in interfacial energy), spontaneous dispersion of the macrophase can occur, thermodynamic equilibrium lyophilic disperse systems can arise with a barely noticeable concentration of particles of the dispersed phase, which will largely exceed the molecular dimensions of the particles.
Criteria value RS can determine the equilibrium conditions of the lyophilic system and the possibility of its spontaneous emergence from the same macrophase, which decreases with increasing particle concentration.
dispersion- this is a fine grinding of solid, liquid bodies in any medium, resulting in powders, suspensions, emulsions. Dispersion is used to obtain colloidal and generally dispersed systems. The dispersion of liquids is commonly referred to as atomization when it occurs in the gas phase, and emulsification when it is carried out in another liquid. During the dispersion of solids, their mechanical destruction occurs.
The condition for the spontaneous formation of a lyophilic particle of a disperse system and its equilibrium can also be obtained using kinetic processes, for example, using the theory of fluctuations.
In this case, underestimated values are obtained, since the fluctuation does not take into account some parameters (the waiting time for fluctuations of a given size).
For a real system, particles may appear that have a dispersed nature, with certain size distributions.
Research P. I. Rebinder and E. D. Schukina made it possible to consider the processes of stability of critical emulsions, determine the processes of formation, and give calculations of various parameters for such systems.
69. Micellization in aqueous and non-aqueous media. Thermodynamics of micellization
Micellization– spontaneous association of molecules of surface-active substances (surfactants) in solution.
Surfactants (surfactants)– substances whose adsorption from a liquid at the interface with another phase leads to a significant decrease in surface tension.
The structure of the surfactant molecule is amphiphilic: a polar group and a non-polar hydrocarbon radical.
The structure of surfactant molecules
Micelle is a mobile molecular associate that exists in equilibrium with the corresponding monomer, and the monomer molecules are constantly attached to the micelle and detached from it (10–8–10–3 s). The radius of micelles is 2–4 nm; 50–100 molecules are aggregated.
Micellization is a process similar to a phase transition, in which there is a sharp transition from the molecularly dispersed state of a surfactant in a solvent to a surfactant associated in micelles when the critical micelle concentration (CMC) is reached.
Micelle formation in aqueous solutions (straight micelles) is due to the equality of the forces of attraction of non-polar (hydrocarbon) parts of molecules and repulsion of polar (ionogenic) groups. The polar groups are oriented towards the aqueous phase. The micelle formation process is of an entropy nature and is associated with hydrophobic interactions of hydrocarbon chains with water: the combination of hydrocarbon chains of surfactant molecules into a micelle leads to an increase in entropy due to the destruction of the water structure.
During the formation of reverse micelles, polar groups are combined into a hydrophilic core, and hydrocarbon radicals form a hydrophobic shell. The energy gain of micellization in non-polar media is due to the advantage of replacing the “polar group–hydrocarbon” bond with a bond between polar groups when they combine into a micelle core.
Rice. 1. Schematic representation
The driving forces for the formation of micelles are intermolecular interactions:
1) hydrophobic repulsion between hydrocarbon chains and the aqueous environment;
2) repulsion of like-charged ionic groups;
3) van der Waals attraction between alkyl chains.
The appearance of micelles is possible only above a certain temperature, which is called Kraft point. Below the Krafft point, ionic surfactants, dissolving, form gels (curve 1), above, the total surfactant solubility increases (curve 2), the true (molecular) solubility does not change significantly (curve 3).
Rice. 2. Formation of micelles
70. Critical micelle concentration (CMC), basic methods for determining CMC
Critical Concentration micelle formation (CMC) is the concentration of surfactants in a solution at which stable micelles are formed in noticeable amounts in the system and a number of properties of the solution change dramatically. The appearance of micelles is fixed by changing the curve of the dependence of the properties of the solution on the concentration of the surfactant. Properties can be surface tension, electrical conductivity, EMF, density, viscosity, heat capacity, spectral properties, etc. The most common methods for determining CMC are: by measuring surface tension, electrical conductivity, light scattering, solubility of non-polar compounds (solubilization) and absorption of dyes. The CMC region for surfactants with 12–16 carbon atoms in the chain is in the concentration range 10–2–10–4 mol/l. The determining factor is the ratio of hydrophilic and hydrophobic properties of the surfactant molecule. The longer the hydrocarbon radical and the less polar the hydrophilic group, the lower the CMC value.
The values of CMC depend on:
1) the positions of ionogenic groups in the hydrocarbon radical (CMC increases when they are shifted to the middle of the chain);
2) the presence of double bonds and polar groups in the molecule (the presence increases the CMC);
3) electrolyte concentration (an increase in concentration leads to a decrease in CMC);
4) organic counterions (the presence of counterions reduces the CMC);
5) organic solvents (increased CMC);
6) temperature (has a complex relationship).
Surface tension of solution σ determined by the concentration of surfactants in molecular form. Above the KKM value σ practically does not change. According to the Gibbs equation, dσ = – Гdμ, at σ = const, chemical potential ( μ ) is practically independent of the concentration at With o > KKM. Before the CMC, the surfactant solution is close in its properties to the ideal one, and above the CMC, the properties begin to differ sharply from the ideal one.
"surfactant - water" system can go into different states when changing the content of components.
CMC, in which spherical micelles are formed from monomeric surfactant molecules, the so-called. Hartley-Rebinder micelles - KKM 1 (the physicochemical properties of the surfactant solution change dramatically). The concentration at which the change in micellar properties begins is called the second CMC (CMC 2). There is a change in the structure of micelles - spherical to cylindrical through spheroidal. The transition from spheroidal to cylindrical (CMC 3), as well as from spherical to spheroidal (CMC 2), occurs in narrow concentration regions and is accompanied by an increase in the aggregation number and a decrease in the surface area of the “micelle–water” interface per surfactant molecule in a micelle. A denser packing of surfactant molecules, a higher degree of ionization of micelles, a stronger hydrophobic effect, and electrostatic repulsion lead to a decrease in the solubilizing ability of surfactants. With a further increase in the concentration of surfactants, the mobility of micelles decreases, and their adhesion occurs at the end sections, while a volumetric network is formed - a coagulation structure (gel) with characteristic mechanical properties: plasticity, strength, thixotropy. Such systems with an ordered arrangement of molecules, with optical anisotropy and mechanical properties intermediate between true liquids and solids, are called liquid crystals. With an increase in the concentration of surfactants, the gel passes into a solid phase - a crystal. Critical micelle concentration (CMC) is the concentration of surfactants in a solution at which stable micelles are formed in noticeable amounts in the system and a number of properties of the solution change dramatically.
71. Micelle formation and solubilization in direct and reverse micelles. Microemulsions
The phenomenon of the formation of a thermodynamically stable isotropic solution of a usually poorly soluble substance (solubilizate) with the addition of a surfactant (solubilizer) is called solubilization. One of the most important properties of micellar solutions is their ability to solubilize various compounds. For example, the solubility of octane in water is 0.0015%, and 2% octane is dissolved in a 10% sodium oleate solution. Solubilization increases with an increase in the length of the hydrocarbon radical of ionic surfactants, and for nonionic surfactants, with an increase in the number of oxyethylene units. Solubilization is complexly affected by the presence and nature of organic solvents, strong electrolytes, temperature, other substances, and the nature and structure of the solubilisate.
There are direct solubilization ("dispersion medium - water") and reverse ("dispersion medium - oil").
In a micelle, the solubilizate can be retained due to electrostatic and hydrophobic interaction forces, as well as others, such as hydrogen bonding.
Several methods are known for solubilization of substances in a micelle (microemulsion), depending both on the ratio of its hydrophobic and hydrophilic properties, and on possible chemical interactions between the solubilizate and the micelle. The structure of oil-water microemulsions is similar to the structure of direct micelles, so the solubilization methods will be identical. Solubilizate can:
1) be on the surface of the micelle;
2) orientate radially, i.e., the polar group is on the surface, and the non-polar group is in the micelle core;
3) be completely immersed in the core, and in the case of non-ionic surfactants, be located in the polyoxyethylene layer.
The quantitative ability to solubilize is characterized by the value relative solubilization s- the ratio of the number of moles of the solubilized substance N Sol. to the number of moles of surfactant in the micellar state N mic:
Microemulsions belong to microheterogeneous self-organizing media and are multicomponent liquid systems containing particles of colloidal size. They are formed spontaneously by mixing two liquids with limited mutual solubility (in the simplest case, water and hydrocarbon) in the presence of a micelle-forming surfactant. Sometimes, in order to form a homogeneous solution, it is necessary to add a non-micelle-forming surfactant, the so-called. co-surfactant (alcohol, amine or ether), and an electrolyte. The particle size of the dispersed phase (microdroplets) is 10–100 nm. Microemulsions are transparent due to their small droplet size.
Microemulsions differ from classical emulsions in the size of dispersed particles (5–100 nm for microemulsions and 100 nm–100 µm for emulsions), transparency, and stability. The transparency of microemulsions is due to the fact that the size of their droplets is less than the wavelength of visible light. Aqueous micelles can absorb one or more solute molecules. A microdroplet of a microemulsion has a larger surface area and a larger internal volume.
Micellization and solubilization in direct and reverse micelles. Microemulsions.
Microemulsions have a number of unique properties that micelles, monolayers, or polyelectrolytes do not have. Aqueous micelles can absorb one or more solute molecules. A microemulsion microdroplet has a larger surface area and a large internal volume of variable polarity, and can absorb significantly more molecules of the dissolved substance. In this respect, emulsions are close to microemulsions, but they have a lower surface charge, they are polydisperse, unstable, and opaque.
72. Solubilization (colloidal dissolution of organic substances in direct micelles)
The most important property of aqueous solutions of surfactants is solubilization. The solubilization process is associated with hydrophobic interactions. Solubilization is expressed in a sharp increase in solubility in water in the presence of surfactants of low-polarity organic compounds.
In aqueous micellar systems (straight micelles) substances insoluble in water, such as benzene, organic dyes, fats, are solubilized.
This is due to the fact that the micelle core exhibits the properties of a nonpolar liquid.
In organic micellar solutions (reverse micelles), in which the interior of the micelles consists of polar groups, polar water molecules are solubilized, and the amount of bound water can be significant.
The solute is called solubilisate(or substrate), and the surfactant solubilizer.
The solubilization process is dynamic: the substrate is distributed between the aqueous phase and the micelle in a ratio depending on the nature and the hydrophilic-lipophilic balance (HLB) of both substances.
Factors affecting the solubilization process:
1) surfactant concentration. The amount of solubilized substance increases in proportion to the concentration of the surfactant solution in the region of spherical micelles and additionally increases sharply when lamellar micelles are formed;
2) surfactant hydrocarbon radical length. With increasing chain length for ionic surfactants or the number of ethoxylated units for non-ionic surfactants, solubilization increases;
3) nature of organic solvents;
4) electrolytes. The addition of strong electrolytes usually greatly increases solubilization due to the reduction in CMC;
5) temperature. With increasing temperature, solubilization increases;
6) the presence of polar and non-polar substances;
7) the nature and structure of the solubilizate.
Solubilization Process Steps:
1) substrate adsorption on the surface (fast stage);
2) penetration of the substrate into the micelle or orientation within the micelle (slower stage).
Method for incorporating solubilizate molecules into micelles of aqueous solutions depends on the nature of the substance. Nonpolar hydrocarbons in a micelle are located in the hydrocarbon cores of micelles.
Polar organic substances (alcohols, amines, acids) are embedded in a micelle between surfactant molecules so that their polar groups face water, and the hydrophobic parts of the molecules are oriented parallel to the surfactant hydrocarbon radicals.
In micelles of nonionic surfactants, solubilizate molecules, such as phenol, are fixed on the surface of the micelles, located between randomly bent polyoxyethylene chains.
During the solubilization of nonpolar hydrocarbons in the cores of micelles, the hydrocarbon chains move apart, as a result, the size of the micelles increases.
The phenomenon of solubilization is widely used in various processes associated with the use of surfactants. For example, in emulsion polymerization, obtaining pharmaceuticals, food products.
Solubilization- the most important factor in the detergent action of surfactants. This phenomenon plays an important role in the life of living organisms, being one of the links in the metabolic process.
73. Microemulsions, microdroplet structure, formation conditions, phase diagrams
There are two types of microemulsions (Fig. 1): the distribution of oil droplets in water (o/w) and water in oil (o/o). Microemulsions undergo structural transformations with changes in the relative concentrations of oil and water.
Rice. 1. Schematic representation of microemulsions
Microemulsions are formed only at certain ratios of components in the system. When the number of components, composition, or temperature changes, macroscopic phase transformations occur in the system, which obey the phase rule and are analyzed using state diagrams.
Usually build "pseudotriple" diagrams. Hydrocarbon (oil) is considered as one component, water or electrolyte is considered as another, and surfactant and co-surfactant as the third.
The construction of phase diagrams is carried out according to the method of sections.
Usually, the lower left corner of these diagrams corresponds to the weight fractions (percentage) of water or saline, the lower right corner - to hydrocarbons, the upper - to surfactants or a mixture of surfactants: co-surfactants with a certain ratio (usually 1:2).
In the plane of the triangle of compositions, the curve separates the region of existence of a homogeneous (in the macroscopic sense) microemulsion from the regions where the microemulsion separates (Fig. 2).
In the immediate vicinity of the curve, there are swollen micellar systems of the surfactant-water type with solubilized hydrocarbon and the surfactant-hydrocarbon type with solubilized water.
Surfactant (surfactant: co-surfactant) = 1:2
Rice. 2. Phase diagram of the microemulsion system
As the water/oil ratio increases, structural transitions occur in the system:
w/o microemulsion → water-in-oil cylinders → lamellar structure of surfactants, oils and water → w/w microemulsion.
Critical micelle concentration is the concentration of a surfactant in a solution at which stable micelles are formed. At low concentrations, surfactants form true solutions. With an increase in the surfactant concentration, CMC is achieved, that is, such a surfactant concentration at which micelles appear that are in thermodynamic equilibrium with non-associated surfactant molecules. When the solution is diluted, micelles disintegrate, and when the concentration of surfactants increases, they reappear. Above the CMC, the entire excess of the surfactant is in the form of micelles. With a very high content of surfactants in the system, liquid crystals or gels are formed.
There are two most common and frequently used methods for determining CMC: by measuring surface tension and solubilization. In the case of ionic surfactants, the conductometric method can also be used to measure KKM. Many physicochemical properties are sensitive to micelle formation, so there are many other possibilities for determining CMC.
Dependence of KKM on: 1)structure of a hydrocarbon radical in a surfactant molecule: The length of the hydrocarbon radical has a decisive effect on the process of micelle formation in aqueous solutions. The decrease in the Gibbs energy of the system as a result of micellization is the greater, the longer the hydrocarbon chain. The ability to form micelles is characteristic of surfactant molecules with a length of the y/v radical of more than 8-10 carbon atoms. 2 ) the nature of the polar group: plays a significant role in micelle formation in aqueous and non-aqueous media. 3) electrolytes: the introduction of electrolytes into aqueous solutions of nonionic surfactants has little effect on CMC and micelle size. For ionic surfactants, this effect is significant. As the electrolyte concentration increases, the micellar mass of ionic surfactants increases. The influence of electrolytes is described by the equation: ln KKM = a - bn - k ln c, where a is a constant characterizing the dissolution energy of functional groups, b is a constant characterizing the dissolution energy per one CH 2 group, n is the number of CH 2 groups, k is a constant, c is the electrolyte concentration. In the absence of an electrolyte, c = CMC. four) Introduction of non-electrolytes(organic solvents) also leads to a change in CMC. This is due to a decrease in the degree of dissociation of monomeric surfactants and micelles. If the solvent molecules do not enter the micelles, they increase the CMC. To regulate the properties of surfactants, their mixtures are used, that is, mixtures with a higher or lower micelle-forming ability.4)Temperature: an increase in temperature increases the thermal motion of molecules and contributes to a decrease in the aggregation of surfactant molecules and an increase in CMC. In the case of nonionic * surfactants, the CMC decreases with increasing temperature, while the CMC of ionic ** surfactants depends only slightly on temperature.
* Non-ionic surfactants do not dissociate into nones when dissolved; carriers of hydrophilicity in them are usually hydroxyl groups and polyglycol chains of various lengths
** Ionic surfactants dissociate in solution into ions, some of which have adsorption activity, others (counterions) are not adsorption active.
6. Foam. Properties and features of foams. Structure. Foam Stability(G/W)
They are very coarse, highly concentrated dispersions of gas in liquid. Due to the excess of the gas phase and the mutual compression of the bubbles, they have a polyhedral rather than a spherical shape. Their walls consist of very thin films of a liquid dispersion medium. As a result, the foams have a honeycomb structure. As a result of the special structure of the foam, they have some mechanical strength.
Main characteristics:
1) multiplicity - is expressed by the ratio of the volume of foam to the volume of the initial solution of the foaming agent ( low-fold foam (K from 3 to several tens) - the shape of the cells is close to spherical and the size of the films is small
and high-fold(K to several thousand) - a cellular film-channel structure is characteristic, in which the gas-filled cells are separated by thin films)
2) foaming ability of the solution - the amount of foam, expressed by its volume (cm 3) or column height (m), which is formed from a given constant volume of the foaming solution under certain standard foaming conditions for a constant time. ( Unsustainable foams exist only with continuous mixing of gas with a foaming p-rum in the presence. foaming agents of the 1st kind, for example. lower alcohols and org. to-t. After the gas supply is stopped, such foams quickly collapse. highly stable foam can exist for many minutes and even hours. To blowing agents of the 2nd kind, giving highly stable foams, include soaps and synthetics. Surfactant) 3) stability (stability) of the foam - its ability to maintain the total volume, dispersion and prevent the outflow of liquid (syneresis). 4) dispersion of the foam, which can be characterized by the average size of the bubbles, their size distribution or the "solution-gas" interface per unit volume of the foam.
Foams are formed when a gas is dispersed in a liquid in the presence of a stabilizer. Without a stabilizer, stable foams are not obtained. The strength and duration of the existence of the foam depends on the properties and content of the foaming agent adsorbed at the interface.
The stability of foams depends on the following main factors: 1. The nature and concentration of the foaming agent. ( foaming agents are divided into two types. 1. Foam formers of the first kind. These are compounds (lower alcohols, acids, aniline, cresols). Foams from solutions of foaming agents of the first kind quickly disintegrate as the interfilm fluid flows out. The stability of the foams increases with increasing concentration of the foaming agent, reaching a maximum value until the saturation of the adsorption layer, and then decreases to almost zero. 2 . Foaming agents of the second kind(soaps, synthetic surfactants) form colloidal systems in water, the foams of which are highly stable. The outflow of the interfilm liquid in such metastable foams stops at a certain moment, and the foam frame can be preserved for a long time in the absence of the destructive action of external factors (vibration, evaporation, dust, etc.). 2. Temperatures. The higher the temperature, the lower the stability, because the viscosity of the interbubble layers decreases and the solubility of surface-active substances (surfactants) in water increases. Foam structure: The gas bubbles in the foams are separated by the thinnest films, which together form a film frame, which serves as the basis of the foams. Such a film frame is formed if the gas volume is 80-90% of the total volume. The bubbles fit snugly together and are separated only by a thin film of foam solution. The bubbles are deformed and take the form of pentahedrons. Typically, the bubbles are arranged in the volume of the foam in such a way that three films between them are connected as shown in Fig.
Three films converge at each edge of the polyhedron, the angles between which are equal to 120 o. The joints of the films (ribs of the polyhedron) are characterized by thickenings that form a triangle in the cross section. These thickenings are called Plateau-Gibbs channels, in honor of famous scientists - the Belgian scientist J. Plateau and the American - J. Gibbs, who made a great contribution to the study of foams. Four Plateau-Gibbs channels converge at one point, forming the same angles of 109 o 28 throughout the foam
7. Characteristics of the components of dispersed systems. DISPERSIVE SYSTEM - a heterogeneous system of two or more phases, of which one (dispersion medium) is continuous, and the other (dispersed phase) is dispersed (distributed) in it in the form of separate particles (solid, liquid or gaseous). When the particle size is 10 -5 cm or less, the system is called colloidal.
DISPERSION MEDIUM - external, continuous phase of a dispersed system. The dispersion medium can be solid, liquid or gaseous.
DISPERSIVE PHASE - internal, fragmented phase of a dispersed system.
DISPERSITY - the degree of fragmentation of the dispersed phase of the system. It is characterized by the specific surface area of the particles (in m 2 /g) or their linear dimensions.
*According to the particle size of the dispersed phase, dispersed systems are conditionally divided: into coarse and fine (highly) dispersed. The latter are called colloidal systems. Dispersity is estimated by the average particle size, beats. surface or particulate composition. *According to the state of aggregation of the dispersion medium and the dispersed phase, a trace is distinguished. main types of dispersed systems:
1) Aerodisperse (gas-dispersed) systems with a gas dispersion medium: aerosols (fumes, dusts, fogs), powders, fibrous materials such as felt. 2) Systems with a liquid dispersion medium; dispersed phase m. solid (coarse suspensions and pastes, fine sols and gels), liquid (coarse emulsions, fine microemulsions and latexes) or gas (coarse gas emulsions and foams).
3) Systems with a solid dispersion medium: glassy or crystalline bodies with inclusions of small solid particles, liquid droplets or gas bubbles, eg ruby glasses, opal type minerals, various microporous materials. *Lyophilic and lyophobic disperse systems with a liquid dispersion medium differ depending on how close or different in their properties the dispersed phase and the dispersion medium are.
In lyophilic dispersed systems, intermolecular interactions on both sides of the separating phase of the surface differ slightly, therefore sp. free surface energy (for a liquid - surface tension) is extremely small (usually hundredths of mJ / m 2), the interface (surface layer) can be. is blurred and often comparable in thickness to the particle size of the dispersed phase.
Lyophilic disperse systems are thermodynamically balanced, they are always highly dispersed, form spontaneously, and, if the conditions for their formation are maintained, can exist for an arbitrarily long time. Typical lyophilic disperse systems are microemulsions, certain polymer-polymer mixtures, micellar surfactant systems, liquid crystal dispersion systems. dispersed phases. Lyophilic dispersed systems often also include minerals of the montmorillonite group that swell and spontaneously disperse in an aqueous medium, for example, bentonite clays.
In lyophobic disperse systems intermolecular interaction. in the dispersion medium and in the dispersed phase is significantly different; beats free surface energy (surface tension) is large - from several. units up to several hundreds (and thousands) mJ/m 2 ; the phase boundary is expressed quite clearly. Lyophobic disperse systems are thermodynamically nonequilibrium; large excess of surface energy causes the processes of transition in them to a more energetically favorable state. In isothermal conditions, coagulation is possible - the convergence and association of particles that retain their original shape and size into dense aggregates, as well as the enlargement of primary particles due to coalescence - the merging of drops or gas bubbles, collective recrystallization (in the case of a crystalline dispersed phase) or isothermal. distillation (mol. transfer) of the dispersed phase from small particles to large ones (in the case of dispersed systems with a liquid dispersion medium, the latter process is called recondensation). Unstabilized and, consequently, unstable lyophobic disperse systems continuously change their disperse composition in the direction of particle enlargement up to complete separation into macrophases. However, stabilized lyophobic disperse systems can maintain dispersion for a long time. time.
8. Changing the aggregative stability of dispersed systems with the help of electrolytes (Rule of Schulze - Hardy).
As a measure of the aggregative stability of disperse systems, one can consider the rate of its coagulation. The system is more stable the slower the coagulation process. Coagulation is the process of particles sticking together, the formation of larger aggregates with subsequent phase separation - the destruction of the disperse system. Coagulation occurs under the influence of various factors: aging of the colloidal system, changes in temperature (heating or freezing), pressure, mechanical influences, the action of electrolytes (the most important factor). The generalized Schulze-Hardy rule (or rule of significance) says: Of the two electrolyte ions, the one whose sign is opposite to the sign of the charge of the colloidal particle has a coagulating effect, and this effect is the stronger, the higher the valency of the coagulating ion.
Electrolytes can cause coagulation, but they have a noticeable effect when a certain concentration is reached. The minimum electrolyte concentration that causes coagulation is called the coagulation threshold, it is usually denoted by the letter γ and is expressed in mmol / l. The threshold of coagulation is determined by the beginning of the cloudiness of the solution, by the change in its color, or by the beginning of the release of the substance of the dispersed phase into the precipitate.
When an electrolyte is introduced into the sol, the thickness of the electric double layer and the value of the electrokinetic ζ potential change. Coagulation occurs not at the isoelectric point (ζ = 0), but when a certain rather small value of the zeta potential (ζcr, critical potential) is reached.
If │ζ│>│ζcr│, then the sol is relatively stable, at │ζ│<│ζкр│ золь быстро коагулирует. Различают два вида коагуляции коллоидных растворов электролитами − concentration and neutralization.
Concentration coagulation is associated with an increase in the concentration of an electrolyte that does not enter into chemical interaction with the components of a colloidal solution. Such electrolytes are called indifferent; they do not have ions capable of completing the micelle core and reacting with potential-determining ions. As the concentration of the indifferent electrolyte increases, the diffuse layer of micelle counterions shrinks, passing into the adsorption layer. As a result, the electrokinetic potential decreases, and it can become equal to zero. This state of the colloidal system is called isoelectric. With a decrease in the electrokinetic potential, the aggregation stability of the colloidal solution decreases, and at a critical value of the zeta potential, coagulation begins. The thermodynamic potential does not change in this case.
During neutralization coagulation, the ions of the added electrolyte neutralize the potential-determining ions, the thermodynamic potential decreases and, accordingly, the zeta potential also decreases.
When electrolytes containing multiply charged ions with a charge opposite to the charge of the particle are introduced into colloidal systems in portions, the sol first remains stable, then coagulation occurs in a certain concentration range, then the sol becomes stable again, and finally, at a high electrolyte content, coagulation occurs again, finally . A similar phenomenon can also be caused by bulk organic ions of dyes and alkaloids.
Let us consider in more detail the distribution of surfactant molecules in solution (see Fig. 21.1). Some of the surfactant molecules are adsorbed at the liquid-gas (water-air) interface. All the regularities that were previously considered for the adsorption of surfactants at the interface between a liquid and a gaseous medium (See Chapters 4 and 5) are also valid for colloidal surfactants. Between surfactant molecules in the adsorption layer 1 and molecules in solution 2 there is a dynamic balance. Some surfactant molecules in solution are able to form micelles 3 ; there is also an equilibrium between the surfactant molecules in solution and the molecules that make up micelles. This is the equilibrium in Fig. 21.1 is shown by arrows.
The process of formation of micelles from molecules of dissolved surfactants can be represented as follows:
mm? (M) m (21.5)
where M-- molecular weight of the surfactant molecule; m is the number of surfactant molecules in a micelle.
The state of surfactants in solution depends on their concentration. At low concentrations (10-4 --10-2 M) true solutions are formed, and ionic surfactants exhibit the properties of electrolytes. When the critical micelle concentration (CMC) is reached, micelles are formed that are in thermodynamic equilibrium with surfactant molecules in solution. At a surfactant concentration above the CMC, the excess surfactant passes into micelles. With a significant content of surfactants, liquid crystals (see paragraph 21.4) and gels can form.
In the area close to the CMC, spherical micelles are formed (Fig. 21.3). With an increase in the concentration of surfactants, lamellar (Fig. 21.1) and cylindrical micelles appear.
Micelles consist of a liquid hydrocarbon core 4 (Fig. 21.1), covered with a layer of polar ionogenic groups 5 . The liquid state of hydrocarbon chains is structurally ordered and thus differs from the bulk liquid (aqueous) phase.
The layer of polar groups of surfactant molecules protrudes above the surface of the nucleus by 0.2-0.5 nm, forming a potential-forming layer (see paragraph 7.2). A double electric layer appears, which determines the electrophoretic mobility of micelles.
The hydrophilic polar shell of micelles sharply reduces the interfacial surface tension y at the micelle-liquid (water) boundary. In this case, condition (10.25) is observed, which means the spontaneous formation of micelles, the lyophilicity of the micellar (colloidal) solution, and its thermodynamic stability.
The most important surface property in surfactant solutions is the surface tension y (see Fig. 2.3), and the bulk properties should include osmotic pressure p (see Fig. 9.4) and molar electrical conductivity?l, which characterizes the ability of a solution containing ions to conduct electricity.
On fig. 21.2 shows changes in the surface tension of the ZhG (curve 2 ), osmotic pressure p (curve 3 ) and molar electrical conductivity l (curve 4 ) depending on the concentration of the sodium dodecyl sulfate solution, which dissociates according to equation (21.3). The area in which the decrease in the surface tension of solutions of colloidal surfactants stops and is called the critical concentration of micellization. (KKM).
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Osmotic pressure p (curve 3 ) first, in accordance with formula (9.11), as the surfactant concentration increases, it increases. In the CMC region, this growth stops, which is associated with the formation of micelles, the size of which significantly exceeds the size of the molecules of dissolved surfactants. The cessation of the growth of osmotic pressure due to an increase in particle size follows directly from formula (9.13), according to which the osmotic pressure is inversely proportional to the cube of the particle radius r 3 . The binding of surfactant molecules into micelles reduces their concentration in solution as electrolytes. This circumstance explains the decrease in the molar electrical conductivity in the CMC region (curve 4 ).
Mathematically, CMC can be defined as an inflection point on the curves "property of colloidal surfactant solutions - concentration" (see Fig. 21.2), when the second derivative of this property becomes equal to zero, i.e. d 2 N/dc 2 = 0. Micellization should be considered as a process analogous to the phase transition from a true surfactant solution to an associated state in micelles; in this case, micellization occurs spontaneously.
The concentration of surfactants in the micellar form significantly, by several orders of magnitude, exceeds the concentration of surfactants in solution. Micelles make it possible to obtain solutions of colloidal surfactants with a high content of the dissolved substance compared to true solutions. In addition, micelles are a kind of storage of surfactants. The equilibrium between the different states of surfactants in solution (see Fig. 21.1) is mobile, and as the surfactant is consumed, for example, with an increase in the phase interface, some of the surfactant molecules in the solution are replenished due to micelles.
CMC is the most important and distinctive property of colloidal surfactants. CMC corresponds to the surfactant concentration at which micelles appear in the solution, which are in thermodynamic equilibrium with surfactant molecules (ions). In the area of CMC, the surface and bulk properties of solutions change dramatically.
CMC is expressed in moles per liter or as a percentage of a dissolved substance. For calcium stearate at 323K, the CMC is 5.10-4 mol/l, and for sucrose esters (0.51.0) 10-5 mol/l.
The CMC values are not high, a small amount of surfactants is enough to reveal the bulk properties of their solutions. We emphasize once again that not all surfactants are able to form micelles. A necessary condition for micellization is the presence of a polar group in the surfactant molecule (see Fig. 5.2) and a sufficiently large length of the hydrocarbon radical.
Micelles are also formed in non-aqueous solutions of surfactants. The orientation of surfactant molecules in nonpolar solvents is opposite to their orientation in water, i.e. hydrophobic radical facing the hydrocarbon liquid.
CMC manifests itself in a certain range of surfactant concentration (see Fig. 21.2). With an increase in the surfactant concentration, two processes can occur: an increase in the number of spherical micelles and a change in their shape. Spherical micelles lose their regular shape and can turn into lamellar ones.
Thus, in the CMC region, the most significant change occurs in the bulk and surface properties of solutions of colloidal surfactants, and inflections appear on the curves characterizing these properties (see Fig. 21.2).
The bulk properties of colloidal surfactants are manifested in such processes as solubilization, formation of foams, emulsions and suspensions. The most interesting and specific of these properties is solubilization.
Solubilization called the dissolution in solutions of colloidal surfactants of those substances that are usually insoluble in a given liquid. For example, as a result of solubilization in aqueous solutions of surfactants, hydrocarbon liquids, in particular gasoline and kerosene, as well as fats that do not dissolve in water, dissolve.
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Solubilization is associated with the penetration of substances into micelles, which are called solubilisates. The mechanism of solubilization for the different nature of the solubilisates can be explained using Fig. 21.3. During solubilization, non-polar substances (benzene, hexane, gasoline, etc.) are introduced into the micelle. If the solubilizate contains polar and non-polar groups, then it is located in the micelle with the hydrocarbon end inward, and the polar group is turned outward. With regard to solubilisates containing several polar groups, adsorption on the outer layer of the surface of micelles is most probable.
Solubilization begins when the surfactant concentration reaches the CMC. At a surfactant concentration above the CMC, the number of micelles increases and solubilization proceeds more intensively. The solubilizing ability of colloidal surfactants increases within the given homologous series as the number of hydrocarbon radicals increases. Ionic surfactants have a greater solubilizing ability compared to non-ionic surfactants.
Especially significant is the solubilizing ability of biologically active colloidal surfactants - sodium chelate and deoxychelate. Solubilization and emulsification (see paragraph 15.4) are the primary processes of fat digestion; As a result of solubilization, fats are dissolved in water, and then absorbed by the body.
Thus, the bulk properties of solutions of colloidal surfactants are due to the formation of micelles.
Factors affecting the CMC
CMC depends on many factors, but primarily determined by the structure of the hydrocarbon radical, the nature of the polar group, additives to the solution of various substances and temperature.
The length of the hydrocarbon radical R.
For aqueous solutions– in the homologous series for neighboring homologues, the CMC ratio ≈ 3.2 has the value of the coefficient of the Duclos-Traube rule. The larger R, the more the energy of the system decreases during micelle formation; therefore, the longer the hydrocarbon radical, the smaller the CMC.
The ability to associate is manifested in surfactant molecules at R > 8-10 carbon atoms C. Branching, unsaturation, cyclization reduce the tendency to MCO and CMC.
For organic environment at R, the solubility and CMC increase.
The CMC in aqueous solutions most strongly depends on the length of the hydrocarbon radical: in the process of micellization, the decrease in the Gibbs energy of the system is the greater, the longer the hydrocarbon chain of the surfactant, i.e., the longer the radical, the smaller the CMC. Those. The longer the hydrocarbon radical of the surfactant molecule, the lower the concentrations of the monolayer filling of the surface (Г ) and the lower the CMC.
Micellization studies have shown that the formation of associates of surfactant molecules also occurs in the case of hydrocarbon radicals consisting of 4–7 carbon atoms. However, in such compounds, the difference between the hydrophilic and hydrophobic parts is not sufficiently pronounced (high HLB value). In this regard, the aggregation energy is not sufficient to retain the associates - they are destroyed under the action of the thermal motion of water (medium) molecules. The ability to form micelles is acquired by surfactant molecules, the hydrocarbon radical of which contains 8–10 or more carbon atoms.
The nature of the polar group.
In aqueous solutions of surfactants, hydrophilic groups hold aggregates in water and regulate their size.
for aquatic environment in organic environment
RT lnKKM = a – bn
where a is a constant characterizing the dissolution energy of a functional group (polar parts)
c is a constant characterizing the energy of dissolution per one group –CH 2 .
The nature of the polar group plays an essential role in the MCO. Its influence reflects the coefficient a, however, the influence of the nature of the polar group is less significant than the length of the radical.
At equal R, that substance has a large CMC, in which its polar group dissociates better (the presence of ionogenic groups, the solubility of surfactants), therefore, at an equal radical, CMC IPAV > CMC NIPAV.
The presence of ionic groups increases the solubility of surfactants in water, so less energy is gained for the transition of ionic molecules into a micelle than for nonionic molecules. Therefore, the CMC for ionic surfactants is usually higher than for nonionic surfactants, with the same hydrophobicity of the molecule (the number of carbon atoms in the chains).
Influence of additives of electrolytes and polar organic substances.
The introduction of electrolytes into solutions of IPAV and NIPAV causes an unequal effect:
1) in solutions of IPAV Sal-ta ↓ CMC.
The main role is played by the concentration and charge of counterions. Ions charged with the same name as the surfactant ion in the MC have little effect on the CMC.
The lightening of the MCO is explained by the compression of the diffuse layer of counterions, the suppression of the dissociation of surfactant molecules, and the partial dehydration of surfactant ions.
Decreasing the charge of micelles weakens the electrostatic repulsion and facilitates the attachment of new molecules to the micelle.
The addition of electrolyte has little effect on the MCO NIPAV.
2) The addition of organic substances to aqueous solutions of surfactants has a different effect on CMC:
low molecular weight compounds (alcohols, acetone) CMC (if there is no solubilization)
long-chain compounds ↓ CMC (micelle stability increases).
3). Influence of temperature T.
There is a different nature of the influence of T on IPAV and NIPAV.
An increase in T for IPAV solutions enhances thermal motion and prevents the aggregation of molecules, but intense motion reduces the hydration of polar groups and promotes their association.
Many surfactants with large R do not form micellar solutions due to poor solubility. However, with a change in T, the surfactant solubility can increase and MCO is detected.
T, with a cat. the solubility of surfactant increases due to the formation of MC, is called the Kraft point (usually 283-293 K).
T. Kraft does not coincide with T PL TV. surfactant, but lies below, because in a swollen gel, the surfactant is hydrated and this facilitates melting.
C, mol/l surfactant + solution
R 
ast-mot MC+rr
Rice. 7.2. Phase diagram of a colloidal surfactant solution near the Kraft point
To obtain a surfactant with a low crafting point value:
a) introduce additional CH 3 - or side substituents;
b) introduce an unlimiting relationship "=";
c) a polar segment (oxyethylene) between the ionic group and the chain.
Above the K point of the raft, the MCs of surfactants disintegrate into smaller associates—demicellization occurs.
(Micelle formation occurs in a specific temperature range for each surfactant, the most important characteristics of which are the Kraft point and cloud point.
Craft Point- the lower temperature limit of micellization of ionic surfactants, usually it is 283 - 293K; at temperatures below the Kraft point, the surfactant solubility is insufficient for the formation of micelles.
cloud point- the upper temperature limit of micellization of nonionic surfactants, its usual values are 323 - 333 K; at higher temperatures, the surfactant-solvent system loses its stability and separates into two macrophases.)
2) Т in NIPAV solutions ↓ CMC due to dehydration of oxyethylene chains.
In NIPAV solutions, a cloud point is observed - the upper temperature limit of NIPAV MCO (323-333 K), at higher temperatures, the system loses stability and separates into two phases.
Thermodynamics and mechanism of micelle formation (MCO)
(The true solubility of surfactants is due to an increase in the entropy S during dissolution and, to a lesser extent, interaction with water molecules.
For surfactants, dissociation in water is characteristic, S of their dissolution is significant.
NIPS interact weakly with H 2 O, their solubility is less at the same R. More often ∆Н> 0, therefore, solubility at T.
The low solubility of surfactants is manifested in the "+" surface activity, and with C - in a significant association of surfactant molecules, passing into MCO.)
Let us consider the mechanism of surfactant dissolution. It consists of 2 stages: phase transition and interaction with solvent molecules - solvation (water and hydration):
∆N f.p. >0 ∆S f.p. >0 ∆N sol. >
∆H solvate.
∆ G= ∆N solution . - T∆S sol.
For IPAV :
∆H solvate. large in size, ∆Н sol. 0 and ∆G sol.
For NIPAV ∆H sol. ≥0, so at T the solubility is due to the entropy component.
The MCO process is characterized by ∆H MCO. ∆ G ICO = ∆N ICO . - T∆S ICO.
Methods for determining CMC
They are based on the registration of a sharp change in the physicochemical properties of surfactant solutions depending on their concentration (turbidity τ, surface tension σ, equivalent electrical conductivity λ, osmotic pressure π, refractive index n).
Usually there is a break in these curves, because one branch of the curve corresponds to the molecular state of the solutions, while the second part corresponds to the colloidal state.
The CMC values for a given surfactant-solvent system may differ when they are determined by one or another experimental method or when using one or another method of mathematical processing of experimental data.
All experimental methods for determining CMC (more than 70 are known) are divided into two groups. One group includes methods that do not require the introduction of additional substances into the surfactant-solvent system. This is the construction of surface tension isotherms = f(C) or = f(lnC); measurement of electrical conductivity ( and ) of a surfactant solution; study of optical properties - the refractive index of solutions, light scattering; study of absorption spectra and NMR spectra, etc. CMC is well determined when plotting the dependence of surfactant solubility on 1/T (inverse temperature). Simple and reliable methods of potentiometric titration and absorption of ultrasound, etc.
The second group of methods for measuring CMC is based on the addition of additional substances to solutions and their solubilization (colloidal dissolution) in surfactant micelles, which can be recorded using spectral methods, fluorescence, EPR, etc. Below is a brief description of some methods for determining CMC from the first group.
Rice. 7.2. Determination of CMC by conductometric method (left).
Fig.7.3 Definition KKM method surface tension measurements
The conductometric method for determining CMC is used for ionogenic surfactants. If there was no micellization in aqueous solutions of ionic surfactants, for example, sodium or potassium oleate, then, in accordance with the Kohlrausch equation (), the experimental points of the dependence of the equivalent electrical conductivity on the concentration C in the coordinates = f () would lie along a straight line (Fig. 7.2) . This is done at low concentrations of surfactants (10 -3 mol/l), starting from CMC, ionic micelles are formed, surrounded by a diffuse layer of counterions, the course of the dependence = f() is broken and a break is observed on the line.
Another method for determining CMC is based on measuring the surface tension of aqueous surfactant solutions, which decreases with increasing concentration up to CMC, and then remains practically constant. This method is applicable to both ionic and nonionic surfactants. To determine the CMC, experimental data on the dependence of on C are usually presented in the coordinates = f (lnC) (Fig. 7.3).
The isotherms σ=f(C) differ from the isotherms of true surfactant solutions by a sharper ↓σ with C and the presence of a break in the region of low concentrations (about 10 -3 - 10 -6 mol/l), above which σ remains constant. This point of the CMC is revealed more sharply on the isotherm σ=f ln(C) in accordance with
Dσ= Σ Γ i dμ i , for a given component μ i = μ i o + RT ln a i dμ i = μ i o + RT dln a i
= - Γ i = - Γ i RT
The graph of the dependence of the refractive index n on the concentration of the surfactant solution is a broken line of two segments intersecting at the CMC point (Fig. 7.4). This dependence can be used to determine the CMC of surfactants in aqueous and non-aqueous media.
In the CMC region, a true (molecular) solution passes into a colloidal solution, and the light scattering of the system sharply increases (everyone could observe the scattering of light by dust particles suspended in air). To determine the CMC by light scattering, the optical density of the system D is measured depending on the concentration of the surfactant (Fig. 7.5), the CMC is found from the graph D = f (C).
Rice. 7.4. Determination of CMC by measuring the refractive index n.
Rice. 7.5. Determination of CMC by the light scattering method (right).