Critical micelle concentration. Dependence of CMC on various factors. Stalagmometric determination of CCM of surfactant solutions. Lyophilic colloidal systems. Thermodynamics of spontaneous dispersion according to Rebinder-Schukin

If the ionic strength of the solution is low, then ionic surfactants can behave like polyelectrolytes, repelling each other. With larger amounts of salt, the repulsive forces decrease and the worm-like micelles can form a network. Adding even 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 additives have little effect on nonionic surfactants. In this case, a decrease in CMC may be observed due to ion dehydration.

Alcohol additives.
Long-chain alcohols are incorporated into aggregates and form mixed micelles. In solutions containing propanol, the CMC decreases sharply with increasing alcohol concentration. With an increase in the number of methylene groups in alcohol, this decrease is more pronounced. The influence of alcohols that are more soluble in water has virtually no effect on the aggregation of surfactant solutions, but at high concentrations it can lead to an increase in CMC due to changes in the properties of the solution. The steric factor plays an important role in the formation of mixed micelles.
Additives of other organic compounds.
Water-insoluble hydrocarbons, such as benzene or heptane, entering the micellar solution are solubilized in the micelle core. At the same time, the volume of micelles increases and their sizes change. A change in the curvature of the micelle surface reduces the electrical potential on its surface, and, therefore, the electrical work of micelle formation, so the CMC decreases. Organic acids and their salts are solubilized inside the micelles near the surface, also reducing CMC2, this is especially true when adding salicylates and similar compounds due to specific interactions.

The role of hydrophilic groups in aqueous solutions of surfactants is to retain the resulting aggregates in water and regulate their size.

Hydration of counterions promotes repulsion, so less hydrated ions are more easily adsorbed onto the surface of micelles. Due to a decrease in the degree of hydration and an increase in micellar mass for cationic surfactants in the Cl - series

A comparison of the properties of ionic and nonionic surfactants having the same hydrocarbon chains shows that the micellar mass of ionic surfactants is much less than for nonionic ones.

When an indifferent electrolyte is added, the micellar mass of ionic surfactants increases and the CMC decreases, while the micellar mass of nonionic surfactants remains virtually unchanged.

The addition of nonelectrolytes to aqueous solutions of surfactants in the presence of solubilization leads to an increase in the stability of micelles, i.e. to a decrease in CMC.

Studies of aqueous solutions of colloidal surfactants have shown that micellization can only occur above a certain temperature Tk, called Kraft point ( Fig.4).

Below the temperature Tk, the solubility of the surfactant 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 surfactant concentration increases sharply with increasing temperature.

solution and through it to various types of liquid crystal systems.

For nonionic surfactants, which are liquids, there is no Krafft point. More typical for them is another temperature limit - cloud point. The turbidity is associated with an increase in the size of the micelles and the separation of the system into two phases due to the dehydration of the polar groups of the 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 one.

In this work, the conductometric method is used to determine the CMC. Conductometric determination of CMC is based on the measurement concentration dependence of electrical conductivity solutions of ionic surfactants.

At a concentration corresponding to the CMC, a kink is observed in the electrical conductivity (W) - concentration (c) graph 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

Necessary instruments and reagents.

1. Pocket conductivity meter

2. Chemical beakers with a capacity of 50 ml - 6 pcs.

3. Measuring cylinder with a capacity of 25 ml - 1 pc.

4. Solution of ionic surfactant with concentrations of 28·10 -3 mol/l, 32·10 -3 mol/l.

5. Distilled water

Electrical conductivity measurements using a conductivity meter (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 glass is 32 ml.

3. Prepare the conductometer for use: remove the protective cap, wash the working part with distilled water. Further, in order to avoid error in the result, the working part is washed with distilled water after each reading.

4. Readings are taken as follows: the working part of the device is placed in the solution (Fig. 7) , turn on the device by moving the button on the top of the device, after establishing the readings on the display, write them down, turn off and wash the working part of the device with a stream of distilled water from the rinse. The obtained data are summarized in Table 1.

Current page: 11 (book has 19 pages total) [available reading passage: 13 pages]

67. Chemical methods for producing colloidal systems. Methods for regulating particle sizes in disperse systems

There are a large number of methods for producing colloidal systems that allow fine control of particle sizes, their shape and structure. T. Svedberg proposed dividing methods for producing colloidal systems into two groups: dispersion (mechanical, thermal, electrical grinding or spraying of a macroscopic phase) and condensation (chemical or physical condensation).

Preparation of sols. The processes are based on condensation reactions. The process occurs in two stages. First, nuclei of a new phase are formed and then a slight supersaturation is created in the ash, at which the formation of new nuclei no longer occurs, but only their growth occurs. Examples. Preparation of gold sols.

2KAuO 2 + 3HCHO + K 2 CO 3 = 2Au + 3HCOOK + KHCO 3 + H 2 O

Aurate ions, which are potential-forming ions, are adsorbed on the resulting 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.

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.

For this purpose, the surface of dispersed phase particles is inhibited due to the formation of a protective layer of surfactants on it or due to the formation of complex compounds on it.

Regulation of particle sizes in disperse systems using the example of obtaining solid nanoparticles. Two identical inverse microemulsion systems are mixed, the aqueous phases of which contain substances A And IN, forming a sparingly soluble compound during a chemical reaction. The particle sizes of the new phase will be limited by the size of the droplets of the polar phase.

Metal nanoparticles can also be produced 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 influencing 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) average concentration of reactants in the aqueous phase.

However, in all cases, the size of nanoparticles formed during the reaction processes is controlled by the size of the droplets of the original emulsion.

Microemulsion systems used to obtain organic compounds. Most of the research in this area concerns the synthesis of spherical nanoparticles. At the same time, the production of asymmetric particles (threads, disks, ellipsoids) with magnetic properties is of great scientific and practical interest.

68. Lyophilic colloidal systems. Thermodynamics of spontaneous dispersion according to Rebinder-Schukin

Lyophilic colloidal systems are ultramicrogenic systems that spontaneously form from macroscopic phases and 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 an increase in free surface energy during the destruction of the macrophase state, which may be compensated due to an increase in the entropy factor, primarily Brownian motion.

At low surface tension values, stable lyophilic systems can spontaneously arise through the decomposition of the macrophase.

Lyophilic colloidal systems include colloidal surfactants, solutions of high molecular weight compounds, and jellies. If we take into account that the critical value of surface tension strongly depends on the diameter of the lyophilic particles, then the formation of a system with large particles is possible at lower values of free interfacial energy.

When considering the dependence of the free energy of a monodisperse system on the size of all particles when changing, 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-disperse system is possible only under the condition that all particle diameters can lie precisely in the region of dispersion where the size of these particles can exceed the size of molecules.

Based on the above, the condition for the formation of a lyophilic system and the condition for its equilibrium can be represented in the form of the Rehbinder-Schukin 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 with a barely noticeable concentration of dispersed phase particles, which will significantly exceed the molecular sizes of the particles, can arise.

Criterion value R.S. can determine the equilibrium conditions of a lyophilic system and the possibility of its spontaneous emergence from the same macrophase, which decreases with increasing particle concentration.

Dispersing- This is the fine grinding of solids and liquids in any medium, resulting in powders, suspensions, and emulsions. Dispersion is used to obtain colloidal and dispersed systems in general. Dispersion of liquids is usually called atomization when it occurs in the gas phase, and emulsification when it is performed in another liquid. When solids are dispersed, 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 arise that have a dispersed nature, with certain size distributions.

Research P. I. Rebindera And E. D. Shchukina allowed us to consider the processes of stability of critical emulsions, determined the processes of formation, and provided calculations of various parameters for such systems.

69. Micelle formation in aqueous and non-aqueous media. Thermodynamics of micellization

Micelle formation– spontaneous association of molecules of surfactants (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 diphilic: a polar group and a nonpolar hydrocarbon radical.

Structure of surfactant molecules

Micelle– a mobile molecular associate that exists in equilibrium with the corresponding monomer, and monomer molecules are constantly attached to the micelle and split off from it (10–8–10–3 s). The radius of micelles is 2–4 nm, 50–100 molecules are aggregated.

Micelle formation is a process similar to a phase transition, in which a sharp transition occurs from the molecularly dispersed state of a surfactant in a solvent to the surfactant associated in micelles when the critical micelle concentration (CMC) is reached.

Micelle formation in aqueous solutions (direct micelles) is due to the equality of the forces of attraction of non-polar (hydrocarbon) parts of molecules and repulsion of polar (ionogenic) groups. Polar groups are oriented towards the aqueous phase. The process of micellization has an entropic 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 structure of water.

During the formation of reverse micelles, polar groups combine into a hydrophilic core, and hydrocarbon radicals form a hydrophobic shell. The energy gain of micellization in nonpolar media is due to the advantage of replacing the “polar group – hydrocarbon” bond with a bond between polar groups when they are combined into the 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 craft point. Below the Krafft point, ionic surfactants, when dissolved, form gels (curve 1), above – the total solubility of the surfactant increases (curve 2), the true (molecular) solubility does not change significantly (curve 3).

Rice. 2. Formation of micelles

70. Critical micelle concentration (CMC), main methods for determining CMC

The critical micelle concentration (CMC) is the concentration of a surfactant in a solution at which stable micelles are formed in noticeable quantities in the system and a number of properties of the solution sharply change. The appearance of micelles is detected by a change in the curve of the dependence of the solution properties on the surfactant concentration. Properties can be surface tension, electrical conductivity, emf, density, viscosity, heat capacity, spectral properties, etc. The most common methods for determining CMC: 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.

KMC values depend on:

1) the position of ionogenic groups in the hydrocarbon radical (CMC increases when they are displaced towards 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 (increasing concentration leads to a decrease in CMC);

4) organic counterions (the presence of counterions reduces the CMC);

5) organic solvents (increase in CMC);

6) temperature (has a complex dependence).

Surface tension of solution σ determined by the concentration of the surfactant 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 concentration at With o > KKM. Before the CMC, the surfactant solution is close in its properties to ideal, and above the CMC it begins to differ sharply in properties from the ideal.

System "surfactant - water" may change into different states when the content of components changes.

CMC, in which spherical micelles are formed from monomer surfactant molecules, the so-called. Hartley-Rehbinder micelles – KKM 1 (the physicochemical properties of the surfactant solution change sharply). The concentration at which the micellar properties begin to change 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 (KKM 3), as well as spherical to spheroidal (KKM 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 one surfactant molecule in the micelle. More dense 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 the surfactant. With a further increase in the concentration of the surfactant, the mobility of the micelles decreases, and their end sections adhere, and a three-dimensional network is formed - a coagulation structure (gel) with characteristic mechanical properties: plasticity, strength, thixotropy. Such systems with an ordered arrangement of molecules, possessing optical anisotropy and mechanical properties intermediate between true liquids and solids, are called liquid crystals. As the surfactant concentration increases, the gel turns into a solid phase – a crystal. The critical micelle concentration (CMC) is the concentration of a surfactant in a solution at which stable micelles are formed in noticeable quantities in the system and a number of properties of the solution change sharply.

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 (solubilizer) upon 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% solution of sodium oleate. Solubilization increases with increasing length of the hydrocarbon radical of ionic surfactants, and for nonionic surfactants, with increasing number of oxyethylene units. Solubilization is influenced in complex ways by the presence and nature of organic solvents, strong electrolytes, temperature, other substances, and the nature and structure of the solubilizate.

A distinction is made between direct solubilization (“dispersion medium – water”) and reverse solubilization (“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.

There are several known methods for solubilizing 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. The solubilizate can:

1) be on the surface of the micelle;

2) be oriented radially, i.e., the polar group is on the surface, and the nonpolar group is in the core of the micelle;

3) be completely immersed in the core, and in the case of nonionic surfactants, located in the polyoxyethylene layer.

The quantitative ability to solubilize is characterized by the value relative solubilization s– ratio of the number of moles of solubilized substance N Sol. to the number of moles of surfactant in the micellar state N mitz:

Microemulsions They 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 a hydrocarbon) in the presence of a micelle-forming surfactant. Sometimes, 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 electrolyte. The particle size of the dispersed phase (microdroplets) is 10–100 nm. Due to the small size of the droplets, microemulsions are transparent.

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 smaller than the wavelength of visible light. Aqueous micelles can absorb one or more molecules of a solute. The microemulsion microdroplet has a larger surface area and a larger internal volume.

Micelle formation 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 molecules of a solute. A microemulsion microdroplet has a larger surface area and a larger internal volume of variable polarity and can absorb significantly more molecules of the dissolved substance. Emulsions in this respect are close to microemulsions, but they have less surface charge, they are polydisperse, unstable and opaque.

72. Solubilization (colloidal dissolution of organic substances in direct micelles)

The most important property of aqueous surfactant solutions is solubilization. The solubilization process involves hydrophobic interactions. Solubilization is expressed in a sharp increase in solubility in water in the presence of surfactants of low-polar organic compounds.

In aqueous micellar systems (straight micelles) Substances that are insoluble in water, such as benzene, organic dyes, and 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 micelles consists of polar groups, polar water molecules are solubilized, and the amount of bound water can be significant.

The substance being dissolved is called solubilized(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 hydrophilic-lipophilic balance (HLB) of both substances.

Factors influencing the solubilization process:

1) surfactant concentration. The amount of solubilized substance increases in proportion to the concentration of the surfactant solution in the area of spherical micelles and additionally increases sharply with the formation of lamellar micelles;

2) length of the surfactant hydrocarbon radical. With increasing chain length for ionic surfactants or the number of ethoxylated units for nonionic surfactants, solubilization increases;

3) the nature of organic solvents;

4) electrolytes. The addition of strong electrolytes usually greatly increases solubilization due to a decrease in CMC;

5) temperature. As temperature increases, solubilization increases;

6) the presence of polar and non-polar substances;

7) nature and structure of the solubilizate.

Stages of the solubilization process:

1) adsorption of the substrate 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. Non-polar hydrocarbons in a micelle are located in the hydrocarbon cores of the 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 hydrocarbon radicals of the surfactant.

In micelles of nonionic surfactants, solubilizate molecules, such as phenol, are fixed on the surface of the micelle, located between randomly bent polyoxyethylene chains.

When nonpolar hydrocarbons are solubilized in the micelle cores, the hydrocarbon chains move apart, resulting in an increase in the size of the micelles.

The phenomenon of solubilization is widely used in various processes involving the use of surfactants. For example, in emulsion polymerization, production of pharmaceuticals, food products.

Solubilization– the most important factor in the cleaning action of surfactants. This phenomenon plays a large role in the life of living organisms, being one of the links in the metabolic process.

73. Microemulsions, structure of microdroplets, conditions of formation, phase diagrams

There are two types of microemulsions (Fig. 1): distribution of oil droplets in water (o/w) and water in oil (w/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 in the system, macroscopic phase transformations occur, which obey the phase rule and are analyzed using phase diagrams.

Typically, “pseudo-ternary” diagrams are constructed. One component is a hydrocarbon (oil), another is water or an electrolyte, and the third is a surfactant and co-surfactant.

Phase diagrams are constructed using the section method.

Typically, the lower left corner of these diagrams corresponds to the weight fractions (percentages) of water or saline solution, the lower right corner to a hydrocarbon, the upper corner to a surfactant or a mixture of surfactants: co-surfactants with a certain ratio (usually 1:2).

In the plane of the composition triangle, the curve separates the region of existence of a homogeneous (in the macroscopic sense) microemulsion from the regions where the microemulsion stratifies (Fig. 2).

Directly near the curve there are swollen micellar systems of the “surfactant – water” type with solubilized hydrocarbon and “surfactant – hydrocarbon” 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 → cylinders of water in oil → lamellar structure of surfactant, oil and water → o/w microemulsion.

Micelle formation, spontaneous association of surfactant molecules in solution. As a result, associate micelles of a characteristic structure appear in the surfactant-solvent system, consisting of dozens of amphiphilic molecules having long-chain hydrophobic radicals and polar hydrophilic groups. In so-called straight micelles, the core is formed by hydrophobic radicals, and the hydrophilic groups are oriented outward. The number of surfactant molecules forming a micelle is called the aggregation number; By analogy with molar mass, micelles are also characterized by the so-called micellar mass. Typically, aggregation numbers are 50-100, micellar masses are 10 3 -10 5. The micelles formed during micelle formation are polydisperse and are characterized by size distribution (or aggregation numbers).

Micelle formation is characteristic of various types of surfactants - ionic (anion- and cation-active), ampholytic and nonionic and has a number of general principles, however, it is also associated with the structural features of the surfactant molecules (size of the non-polar radical, nature of the polar group), so it is more correct to talk about micellization of this surfactant class.

Micelle formation occurs in a temperature range specific to each surfactant, the most important characteristics of which are the Kraft point and the cloud point. The Kraft point is the lower temperature limit for micellization of ionic surfactants, usually it is 283-293 K; at temperatures below the Krafft point, the solubility of the surfactant is insufficient for the formation of micelles. Cloud point is the upper temperature limit of micellization of nonionic surfactants, its usual values are 323-333 K; at higher temperatures, the surfactant-solvent system loses stability and separates into two macrophases. Micelles of ionic surfactants at high temperatures (388-503 K) disintegrate into smaller associates-dimers and trimers (so-called demicellization).

The determination of CMC can be carried out by studying almost any property of solutions depending on changes in their concentration. Most often in research practice, the dependences of solution turbidity, surface tension, electrical conductivity, light refractive index and viscosity on the total concentration of solutions are used.

The critical concentration of micellization is determined by the point that corresponds to the break in the curves of the properties of solutions depending on concentration. It is believed that at concentrations lower than the CMC in surfactant solutions, only molecules are present and the dependence of any property is determined precisely by the concentration of the molecules. When micelles form in solutions, the property will undergo a sharp change due to a sudden increase in the size of the dissolved particles. For example, molecular solutions of ionic surfactants exhibit electrical properties characteristic of strong electrolytes, and micellar solutions characteristic of weak electrolytes. This is manifested in the fact that the equivalent electrical conductivity in solutions of ionic surfactants at concentrations below the CMC, depending on the square root of the solution concentration, turns out to be linear, which is typical for strong electrolytes, and after CMC, its dependence turns out to be typical for weak electrolytes.

Rice. 2

1. Stalagmometric method, or the method of counting drops, although inaccurate, is still used in laboratory practice due to its exceptional simplicity. The determination is made by counting the drops that come off when a certain volume of liquid flows out of the capillary opening of a special Traube stalagmometer device.
2. Conductometric method is an analysis method based on studies of the electrical conductivity of the solutions under study. Direct conductometry is understood as a method by which studies of electrolyte concentrations are carried out directly. Determinations are made using measurements of the electrical conductivity of solutions whose qualitative composition is known.
3. Refractometric method of analysis(refractometry) is based on the dependence of the refractive index of light on the composition of the system. This dependence is established by determining the refractive index for a number of standard mixtures of solutions. The refractometry method is used for the quantitative analysis of binary, ternary and various complex solution systems.

Rice. 3 Refractometer

The critical micelle concentration is the concentration of surfactant in solution at which stable micelles are formed. At low concentrations, surfactants form true solutions. As the surfactant concentration increases, the CMC is achieved, that is, the surfactant concentration at which micelles appear that are in thermodynamic equilibrium with unassociated surfactant molecules. When the solution is diluted, the micelles disintegrate, and when the surfactant concentration increases, they reappear. Above the CMC, all excess surfactants are in the form of micelles. With a very high surfactant content in the system, liquid crystals or gels are formed.

There are two most common and frequently used methods for determining CMC: surface tension and solubilization measurements. 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 the hydrocarbon radical in the surfactant molecule: The length of the hydrocarbon radical has a decisive effect on the process of micellization in aqueous solutions. The decrease in the Gibbs energy of the system as a result of micellization is greater, the longer the hydrocarbon chain. The ability to form micelles is characteristic of surfactant molecules with a radical length of more than 8-10 carbon atoms. 2 ) character of the polar group: plays a significant role in micellization in aqueous and non-aqueous media. 3) electrolytes: the introduction of electrolytes into aqueous solutions of nonionic surfactants has little effect on the CMC and micelle size. For ionic surfactants, this effect is significant. With increasing electrolyte concentration, the micellar mass of ionic surfactants increases. The effect of electrolytes is described by the equation: ln KKM = a - bn - k ln c, Where a is a constant characterizing the energy of dissolution of functional groups, b is a constant characterizing the energy of dissolution 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 electrolyte c = KMC. 4) Introduction of non-electrolytes(organic solvents) also leads to a change in CMC. This occurs due to a decrease in the degree of dissociation of monomeric surfactants and micelles. If solvent molecules do not enter the micelle, they increase the CMC. To regulate the properties of surfactants, mixtures of them are used, that is, mixtures with higher or lower micelle-forming ability.

4)temperature: An increase in temperature increases the thermal movement of molecules and helps reduce the aggregation of surfactant molecules and increase the CMC. In the case of nonionic * surfactants, the CMC decreases with increasing temperature; the CMC of ionic** surfactants depends weakly on temperature.

* Nonionic surfactants do not dissociate into nones when dissolved; the 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 resistance (G/F)

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 spherical shape. Their walls consist of very thin films of a liquid dispersion medium. As a result, the foams have a honeycomb-like structure. As a result of the special structure of the foam, they have some mechanical strength.

Main characteristics:

1) multiplicity - expressed as the ratio of the volume of foam to the volume of the original foam concentrate solution ( 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(up to several thousand) - characterized by a cellular film-channel structure, in which gas-filled cells are separated by thin films)

2) foaming ability of a solution - the amount of foam, expressed by its volume (cm 3) or column height (m), which is formed from a given constant volume of a foaming solution subject to certain standard foaming conditions over a constant period of time. ( Low-resistant foams exist only with continuous mixing of gas with a foaming solution in the presence. foaming agents of the 1st kind, for example. lower alcohols and org. kt. After the gas supply is stopped, such foams quickly collapse. Highly stable foams can exist for many years. minutes and even hours. Type 2 foaming agents that produce highly stable foams include soaps and synthetics. Surfactant) 3) stability (stability) of foam - its ability to maintain total volume, dispersion and prevent liquid leakage (syneresis). 4) foam dispersion, which can be characterized by the average size of bubbles, their size distribution or the “solution-gas” interface per unit volume of foam.

Foams are formed when gas is dispersed in a liquid in the presence of a stabilizer. Without a stabilizer, stable foams cannot be obtained. The strength and lifespan 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. Foaming agents of the first kind. These are compounds (lower alcohols, acids, aniline, cresols). Foams from solutions of foaming agents of the first type quickly disintegrate as the interfilm liquid flows out. The stability of foams increases with increasing foaming agent concentration, reaching a maximum value until the adsorption layer is saturated, and then decreases to almost zero. 2 . Foaming agents of the second type(soaps, synthetic surfactants) form colloidal systems in water, the foams of which are highly stable. The flow of 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 surfactants in water increases. Foam structure: Gas bubbles in foams are separated by thin films, which together form a film frame, which serves as the basis of the foam. Such a film frame is formed if the gas volume is 80-90% of the total volume. The bubbles fit tightly together and are separated only by a thin film of foam solution. The bubbles are deformed and take the shape of pentahedrons. Usually the bubbles are located in the foam volume 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°. The junctions of the films (polyhedron edges) are characterized by thickenings that form a triangle in cross section. These thickenings are called Plateau-Gibbs channels, in honor of famous scientists - the Belgian scientist J. Plateau and the American scientist J. Gibbs, who made a great contribution to the study of foams. Four Plateau-Gibbs channels converge at one point, forming identical angles of 109 about 28 throughout the foam

7. Characteristics of components of disperse systems. DISPERSED 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 individual 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 the dispersed system. The dispersion medium can be solid, liquid or gas.

DISPERSED PHASE - internal, crushed phase of the dispersed system.

DISPERSITY - the degree of fragmentation of the dispersed phase of the system. It is characterized by the size of the specific surface of particles (in m 2 /g) or their linear dimensions.

*According to the particle size of the dispersed phase, dispersed systems are conventionally divided: into coarse and finely dispersed. The latter are called colloidal systems. Dispersity is assessed by the average particle size, sp. surface or dispersed composition. *Based on the state of aggregation of the dispersion medium and the dispersed phase, the following is distinguished. basic types of disperse systems:

1) Aerodispersed (gas-dispersed) systems with a gas dispersion medium: aerosols (smoke, dust, mists), powders, fibrous materials such as felt. 2) Systems with liquid dispersion medium; dispersed phase m.b. solid (coarse suspensions and pastes, highly dispersed sols and gels), liquid (coarsely dispersed emulsions, highly dispersed microemulsions and latexes) or gas (coarsely dispersed 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, for example, 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 the dispersed phase and the dispersion medium are in their properties.

In lyophilic in dispersed systems, intermolecular interactions on both sides of the separating phase surface differ slightly, therefore the beat. free surface energy (for a liquid - surface tension) is extremely low (usually hundredths of mJ/m2), the interphase boundary (surface layer) may be blurred and often comparable in thickness to the particle size of the dispersed phase.

Lyophilic disperse systems are thermodynamically equilibrium, they are always highly dispersed, form spontaneously and, if the conditions for their formation are maintained, can exist for an indefinitely long time. Typical lyophilic disperse systems are microemulsions, certain polymer-polymer mixtures, micellar surfactant systems, dispersed systems with liquid crystals. dispersed phases. Lyophilic disperse systems also often include minerals of the montmorillonite group that swell and spontaneously disperse in an aqueous environment, for example, bentonite clays.

In lyophobic dispersed systems intermolecular interaction. in a dispersion medium and in a dispersed phase are significantly different; beat free surface energy (surface tension) is high - from several. units to several hundreds (and thousands) mJ/m2; the phase boundary is expressed quite clearly. Lyophobic disperse systems are thermodynamically nonequilibrium; large excess of free surface energy determines the occurrence of transition processes 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 droplets 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, therefore, unstable lyophobic disperse systems continuously change their disperse composition towards particle enlargement until complete separation into macrophases. However, stabilized lyophobic disperse systems can remain dispersive for long periods of time. time.

8. Changing the aggregative stability of dispersed systems using electrolytes (Schulze-Hardy rule).

As a measure of the aggregative stability of dispersed systems, one can consider the rate of its coagulation. The slower the coagulation process, the more stable the system is. Coagulation is the process of particle adhesion, the formation of larger aggregates, followed by phase separation—the destruction of the dispersed system. Coagulation occurs under the influence of various factors: aging of the colloid system, changes in temperature (heating or freezing), pressure, mechanical stress, the action of electrolytes (the most important factor). The generalized Schulze-Hardy rule (or significance rule) states: 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 stronger, the higher the valence of the coagulating ion.

Electrolytes can cause coagulation, but they have a noticeable effect when they reach a certain concentration. The minimum electrolyte concentration that causes coagulation is called the coagulation threshold; it is usually denoted by the letter γ and expressed in mmol/l. The coagulation threshold is determined by the beginning of turbidity of the solution, by a change in its color, or by the beginning of the release of a dispersed phase substance into a sediment.

When an electrolyte is introduced into the sol, the thickness of the electrical double layer and the value of the electrokinetic ζ-potential change. Coagulation does not occur at the isoelectric point (ζ = 0), but when a certain rather small value of 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 interact chemically with the components of the 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 counterions in the micelle contracts, turning into an adsorption layer. As a result, the electrokinetic potential decreases and can become equal to zero. This state of the colloidal system is called isoelectric. With a decrease in the electrokinetic potential, the aggregative 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 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 at first remains stable, then coagulation occurs in a certain concentration range, then the sol again becomes stable 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.

Aqueous solutions of many surfactants have special properties that distinguish them both from true solutions of low molecular weight substances and from colloidal systems. One of the distinctive features of surfactant solutions is the possibility of their existence both in the form of molecular-true solutions and in the form of micellar - colloidal ones.

CMC is the concentration at which, when a surfactant is added to a solution, the concentration at the phase boundary remains constant, but at the same time occurs self-organization surfactant molecules in a bulk solution (micelle formation or aggregation). As a result of such aggregation, the so-called micelle formation is formed. A distinctive sign of micelle formation is the turbidity of the surfactant solution. Aqueous solutions of surfactants, during micellization, also acquire a bluish tint (gelatinous tint) due to light refraction micelles.

The transition from the molecular state to the micellar state occurs, as a rule, in a fairly narrow concentration range, limited by the so-called boundary concentrations. The presence of such boundary concentrations was first discovered by the Swedish scientist Ekval. He found that at limiting concentrations, many properties of solutions change dramatically. These boundary concentrations lie below and above the average CMC; Only at concentrations below the minimum limit concentration are surfactant solutions similar to true solutions of low molecular weight substances.

Methods for determining CMC:

Fig. 1 - surface tension (s) of sodium dodecyl sulfate solutions at 25 o C

Fig. 2 - equivalent electrical conductivity (l) of solutions of decyltrimethylammonium bromide at 40 o C

Fig. 3 - specific electrical conductivity (k) of sodium decyl sulfate solutions at 40 o C

Fig. 4 - viscosity (h/s) of sodium dodecyl sulfate solutions at 30 o C

The study of any property of surfactant solutions depending on its concentration makes it possible to determine average concentration, at which the system makes a transition to the colloidal state. To date, more than a hundred different methods for determining the critical concentration of micelle formation have been described; Some of them, in addition to QCM, also allow one to obtain rich information about the structure of solutions, the size and shape of micelles, their hydration, etc. We will focus only on those methods for determining CMC that are used most often.

To determine the CMC by changes in the surface tension of surfactant solutions, they are often used methods of maximum pressure in a gas bubble, With thalagmometer, tearing off a ring or balancing a plate, measuring the volume or shape of a hanging or lying drop, weighing drops, etc. The determination of CMC by these methods is based on the cessation of changes in the surface tension of the solution at the maximum saturation of the adsorption layer at the interface “water - air”, “hydrocarbon - water”, “solution - solid phase”. Along with determining the CMC, these methods make it possible to find the value of the limiting adsorption, the minimum area per molecule in the adsorption layer. Based on experimental values of surface activity at the solution-air interface and the maximum areas per molecule in a saturated adsorption layer, the length of the polyoxyethylene chain of nonionic surfactants and the size of the hydrocarbon radical can also be determined. The determination of CMC at various temperatures is often used to calculate the thermodynamic functions of micellization.

Research shows that the most accurate results are obtained by measuring the surface tension of surfactant solutions plate balancing method. The results found are reproduced quite well stalagmometric method. Less accurate, but fairly correct data is obtained when using ring tearing method. The results of purely dynamic methods are poorly reproduced.

When determining KKM viscometric method experimental data are usually expressed as the dependence of reduced viscosity on the concentration of surfactant solutions. The viscometric method also makes it possible to determine the presence of boundary concentrations of micellization and the hydration of micelles by intrinsic viscosity. This method is especially convenient for nonionic surfactants due to the fact that they do not have an electroviscous effect.

Definition of cash register by light scattering based on the fact that when micelles form in surfactant solutions, the scattering of light by particles sharply increases and the turbidity of the system increases. The CMC is determined by a sharp change in the turbidity of the solution. When measuring the optical density or light scattering of surfactant solutions, an abnormal change in turbidity is often observed, especially if the surfactant contains some impurities. Light scattering data is used to determine micellar mass, micelle aggregation numbers, and micelle shape.

Definition of cash register by diffusion carried out by measuring diffusion coefficients, which are related both to the size of micelles in solutions and to their shape and hydration. Typically, the CMC value is determined by the intersection of two linear sections of the dependence of the diffusion coefficient on the dilution of solutions. Determination of the diffusion coefficient allows one to calculate the hydration of micelles or their size. By combining the measurements of the diffusion coefficient and the sedimentation coefficient in an ultracentrifuge, the micellar mass can be determined. If the hydration of micelles is measured by an independent method, then the shape of the micelles can be determined from the diffusion coefficient. Observation of diffusion is usually carried out when an additional component is introduced into surfactant solutions - a micelle label; therefore, the method can give distorted results when determining the CMC if a shift in the micellar equilibrium occurs. Recently, the diffusion coefficient has been measured using radioactive labels on surfactant molecules. This method does not shift the micellar equilibrium and gives the most accurate results.

Definition of cash register refractometric method based on the change in the refractive index of surfactant solutions during micellization. This method is convenient in that it does not require the introduction of additional components or the use of a strong external field, which can shift the “micelle-molecule” equilibrium, and evaluates the properties of the system almost under static conditions. It requires, however, careful thermostatting and accurate determination of the concentration of solutions, as well as the need to take into account the time of the experiment in connection with the change in the refractive index of the glass due to the adsorption of surfactants. The method gives good results for nonionic surfactants with a low degree of ethoxylation.

The basis of the definition of KKM ultraacoustic method lies in the change in the nature of the passage of ultrasound through the solution during the formation of micelles. When studying ionic surfactants, this method is convenient even for very dilute solutions. Solutions of nonionic substances are more difficult to characterize by this method, especially if the solute has a low degree of ethoxylation. Using the ultraacoustic method, it is possible to determine the hydration of surfactant molecules both in micelles and in dilute solutions.

Widespread conductometric method limited only to solutions of ionic substances. In addition to the CMC, it allows you to determine the degree of dissociation of surfactant molecules in micelles, which is necessary to know to correct the micellar masses found by light scattering, as well as to introduce a correction for the electroviscous effect when calculating hydration and association numbers using methods related to transport phenomena.

Sometimes methods like this are used like nuclear magnetic resonance or electron paramagnetic resonance, which make it possible, in addition to QCM, to measure the “lifetime” of molecules in micelles, as well as ultraviolet and infrared spectroscopy, which make it possible to identify the location of solubilizate molecules in micelles.

Polarographic studies, as well as measurements of the pH of solutions, are often associated with the need to introduce a third component into the system, which naturally distorts the results of determining the CMC. Methods for dye solubilization, solubilization titration and paper chromatography, unfortunately, are not accurate enough to measure CMC, but they do allow one to judge the structural changes of micelles in relatively concentrated solutions.