Critical micelle concentration. Dependence of CMC on various factors

Micellization, spontaneous association of surfactant molecules in solution. As a result, micelles-associates of a characteristic structure appear in the surfactant-solvent system, consisting of dozens of amphiphilic molecules with long-chain hydrophobic radicals and polar hydrophilic groups. In the so-called straight micelles, the core is formed by hydrophobic radicals, while the hydrophilic groups are oriented outward. The number of surfactant molecules forming a micelle is called the aggregation number; By analogy with the molar mass, micelles are also characterized by the so-called micellar mass. Typically, the 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).

Micellization is characteristic of various kinds Surfactants are ionic (anion- and cation-active), ampholytic and non-ionic and have a number of general patterns, however, it is also associated with the structural features of surfactant molecules (the size of the non-polar radical, the nature of the polar group), so it is more correct to talk about micellization of this class of surfactants.

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

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

The critical concentration of micelle formation is determined by the point that corresponds to the break in the curves of dependences of the properties of solutions on concentration. It is believed that at concentrations lower than CMC in surfactant solutions, only molecules are present, and the dependence of any property is determined precisely by the concentration of molecules. When micelles are formed in solutions, the property will undergo a sharp change due to an abrupt increase in the size of dissolved particles. For example, molecular solutions of ionic surfactants exhibit electrical properties, characteristic of strong electrolytes, and micellar - characteristic of weak electrolytes. This is manifested in the fact that the equivalent electrical conductivity in solutions of ionic surfactants at concentrations below 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, but due to its exceptional simplicity, is still used in laboratory practice. The determination is made by counting the drops that come off when a certain volume of liquid flows out and from the capillary opening of a special Traube stalagmometer.
• 2. Conductometric method- this is an analysis method based on studies of the electrical conductivity of the studied solutions. Direct conductometry is understood as a method by which the study of electrolyte concentrations is carried out directly. Definitions are carried out using measurements of the electrical conductivity of solutions, qualitative composition which 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 systems of solutions.

Rice. 3 Refractometer

The value of CMC is affected by:

The structure and length of the hydrocarbon chain;

The nature of the polar group;

The presence in the solution of indifferent electrolytes and non-electrolytes;

Temperature.

The influence of the first two factors is reflected by the formula

RTIn KKM = a– bp,(12.1)

where a – a constant characterizing the dissolution energy of the polar group; b– constant characterizing the dissolution energy per group – CH 2 – ; P– number of groups – CH 2 – .

From equation (12.1) it follows that the greater the dissolution energy of the hydrophobic group and the greater their number, the smaller the CMC, i.e., the easier it is to form a micelle.

On the contrary, the higher the dissolution energy of the polar group, the role of which is to retain the resulting associates in water, the greater the CMC.

The value of the CMC of ionic surfactants is much greater than that of nonionic surfactants with the same hydrophobicity of the molecules.

The introduction of electrolytes into aqueous solutions of nonionic surfactants has little effect on the CMC value and micelle size.

The introduction of electrolytes into aqueous solutions of ionic surfactants has a very significant effect, which can be estimated by the equation:

In KKM \u003d a " – b "n– k In with, (12.2)

where a" and b"– constants having the same physical meaning as a and b in equation 12.1; k– constant; with– concentration of indifferent electrolyte.

From equation 12.2 it follows that an increase in the concentration of an indifferent electrolyte (c) reduces the CMC.

The introduction of nonelectrolytes (organic solvents) into aqueous solutions of surfactants also leads to a change in the CMC. In the presence of solubilization, the stability of micelles increases, i.e. decreases KKM. If solubilization is not observed (i.e., non-electrolyte molecules do not enter the interior of the micelle), then they, as a rule, increase KKM.

EFFECT OF TEMPERATURE

The effect of temperature on the CMC of ionic surfactants and nonionic surfactants is different. An increase in temperature leads to an increase in the CMC of the ionogenic surfactant from – for the disaggregating action of thermal motion.

An increase in temperature leads to a decrease in the CMC of a nonionic surfactant due to the dehydration of oxyethylene chains (we remember that nonionic surfactants are always formed by polyoxyethylene chains and hydrocarbon "tails").

METHODS OF DETERMINATION

CRITICAL CONCENTRATION

micelle formation

Methods for determining CMC are based on registering a sharp change in physical – chemical properties of surfactant solutions with a change in concentration. This is due to the fact that the formation of a surfactant micelle in a solution means the appearance of new phase and this leads to a sharp change in any physical – chemical property of the system.

On the dependency curves "the property of the surfactant solution – surfactant concentration” a break appears. In this case, the left side of the curves (at lower concentrations) describes the corresponding property of the surfactant solution in the molecular (ionic) state, and the right – in colloid. The abscissa of the break point is conditionally considered to correspond to the transition of surfactant molecules (ions) into micelles – i.e., the critical micelle concentration (CMC).

Let's look at some of these methods.

CONDUCTOMETRIC METHOD

DEFINITIONS OF CCM

The conductometric method is based on measuring the electrical conductivity of surfactant solutions. It is clear that it can only be used for ionogenic surfactants. In the range of concentrations up to CMC, the dependences of the specific and equivalent electrical conductivity on the surfactant concentration correspond to similar dependences for solutions of electrolytes of medium strength. At a concentration corresponding to CMC, a break is observed on the dependence graphs due to the formation of spherical micelles. The mobility of ionic micelles is less than the mobility of ions and, in addition, a significant part of the counterions is located in a dense layer of a colloidal micelle particle and, therefore, significantly reduces the electrical conductivity of surfactant solutions. Therefore, with an increase in the concentration of surfactants more than CMC, the increase in electrical conductivity is significantly weakened (Fig. 12.4), and the molar electrical conductivity decreases more sharply (Fig. 12.5)

L n KKM L n c L n KKM L n c*

Rice. 12.4 Fig. 12.5

Specific dependence, Molar dependence

conductivity electrical conductivity

from concentrations from concentration

DEFINITION OF CMC

BASED ON SURFACE MEASUREMENTS

SOLUTION TENSION

The surface tension of aqueous solutions of surfactants decreases with increasing concentration up to CMC. Isotherm = f(ln with) in the range of low surfactant concentrations has a curvilinear section, on which, in accordance with the Gibbs equation, the adsorption of surfactant on the surface of the solution increases with increasing concentration. At a certain concentration with t the curved section of the isotherm turns into a straight line with a constant value, i.e., adsorption reaches its maximum value. In this region, a saturated monomolecular layer is formed at the interface. With a further increase in the surfactant concentration (c > CMC), micelles are formed in the volume of the solution, and the surface tension practically does not change. The CMC is determined by the break of the isotherm when it reaches the section parallel to the axis In with(Fig. 12.6).

Surface tension measurement

Allows you to define CMC as ionogenic,

and nonionic surfactants. Researched

Surfactants must be thoroughly cleaned from

impurities, as their presence may

cause a minimum

isotherm at concentrations close to

Ln c m Ln KKM Ln c KKM.

Rice. 12.6

Surface dependency

tension from nc

SPECTROPHOTOMETRIC,

OR PHOTONEPHELMETRIC METHOD

DEFINITIONS OF CCM

Solubilization of dyes and hydrocarbons in surfactant micelles makes it possible to determine the CMC of ionic and nonionic surfactants, both in aqueous and non-ionic surfactants. aqueous solutions. When the concentration in the surfactant solution is reached, correspondingly – existing CMC, the solubility of water-insoluble dyes and hydrocarbons sharply increases. It is most convenient to use fat-soluble dyes that intensively color surfactant solutions at concentrations above CMC. Solubilization is measured by a method based on light scattering or spectrophotometrically.

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. 4) 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 initial solution foam concentrate ( 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 capacity 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 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 framework can persist for a long time in the absence of a destructive effect. 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. 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 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 the properties of 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, spontaneously formed, and if the conditions for their formation are maintained, they 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 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.

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 an 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. Determination of CMC by measuring surface tension

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 surfactant concentrations (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).

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 include the osmotic pressure p (see Fig. 9.4) and the 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 used up, for example, with an increase in the phase interface, some of the surfactant molecules in the solution are replenished by 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. Necessary condition micellization are 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 region of CMC, 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.