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.

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.

The value of CMC is influenced by:

Structure and length of the hydrocarbon chain;

The nature of the polar group;

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

Temperature.

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

RTIn KKM = A– bп,(12.1)

where a – a constant characterizing the energy of dissolution of a 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 lower the CMC, i.e., the easier the micelle is formed.

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

The CMC value of ionic surfactants is significantly 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 = a" – b"p– k In With, (12.2)

where a" and Kommersant"– constants that have 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 the indifferent electrolyte (c) reduces the CMC.

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

INFLUENCE 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 ionic surfactant from – for the disaggregating effect of thermal movement.

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 recording a sharp change in physical – chemical properties of surfactant solutions when changing 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 properties of the system.

On the dependence curves “surfactant solution property – surfactant concentration" a kink appears. In this case, the left side of the curves (at lower concentrations) describes the corresponding property of a surfactant solution in the molecular (ionic) state, and the right side – in colloid. The abscissa of the break point is conventionally 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

KKM DEFINITIONS

The conductometric method is based on measuring the electrical conductivity of surfactant solutions. It is clear that it can only be used for ionic surfactants. In the concentration range up to CMC, the dependences of the specific and equivalent electrical conductivity on the surfactant concentration correspond to similar dependences for solutions of medium-strength electrolytes. At a concentration corresponding to the CMC, a break in the dependence graphs is observed 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 the colloidal particle of the micelle and, therefore, significantly reduces the electrical conductivity of surfactant solutions. Therefore, with an increase in the surfactant concentration above the CMC, the increase in specific electrical conductivity is significantly weakened (Fig. 12.4), and the molar electrical conductivity decreases more sharply (Fig. 12.5)

Ln KKM Ln c Ln KKM Ln c*

Rice. 12.4 Fig. 12.5

Specific dependence, Molar dependence

conductivity electrical conductivity

from concentrations from concentration

DEFINITION OF KKM

BASED ON SURFACE MEASUREMENTS

SOLUTION TENSIONS

The surface tension of aqueous surfactant solutions decreases with increasing concentration up to the CMC. Isotherm = f(ln With) in the region of low surfactant concentrations has a curved section, where, in accordance with the Gibbs equation, the adsorption of the surfactant on the surface of the solution increases with increasing concentration. At a certain concentration with t the curvilinear 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 solution volume, and the surface tension remains virtually unchanged. The CMC is determined by the break in the isotherm when it reaches a section parallel to the In axis With(Fig. 12.6).

Surface tension measurement

Allows you to determine CMC as ionogenic,

and nonionic surfactants. Researched

Surfactants must be thoroughly cleaned from

impurities, since their presence may

cause the appearance of a minimum on

isotherm at concentrations close to

Ln c m Ln KKM Ln c KKM.

Rice. 12.6

Surface dependence

tension from nc

SPECTROPHOTOMETRIC,

OR PHOTONEPHELOMETRIC METHOD

KKM DEFINITIONS

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-aqueous solutions. When the surfactant concentration in the solution is reached, the corresponding – existing CMC, the solubility of water-insoluble dyes and hydrocarbons increases sharply. It is most convenient to use fat-soluble dyes that intensely color surfactant solutions at concentrations above the CMC. Solubilization is measured using a light scattering method or spectrophotometrically.

Factors influencing KKM

CMC depends on many factors, but is primarily determined by the structure of the hydrocarbon radical, the nature of the polar group, additions of various substances to the solution and temperature.

The length of the hydrocarbon radical R.

For aqueous solutions– in the homological series for neighboring homologues the ratio CMC ≈ 3.2 has the value of the coefficient of the Duclos-Traube rule. The higher R, the more the energy of the system decreases during micelle formation, therefore, the longer the hydrocarbon radical, the lower the CMC.

The ability to associate is manifested in surfactant molecules with R > 8-10 carbon atoms C. Branching, unsaturation, and cyclization reduce the tendency to MCO and CMC.

For organic environment at R, solubility and CMC increase.

The CMC in aqueous solutions depends most strongly on the length of the hydrocarbon radical: in the process of micellization, the decrease in the Gibbs energy of the system is 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 a surfactant molecule, the lower the concentrations where monolayer surface filling is achieved (G ) and the lower the CMC.

Studies of micellization 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 insufficient to retain the associates - they are destroyed under the influence of the thermal movement of water molecules (medium). Surfactant molecules whose hydrocarbon radical contains 8–10 or more carbon atoms acquire the ability to form micelles.

Character of the polar group.

In aqueous solutions of surfactants, hydrophilic groups hold aggregates in water and regulate their size.

for aquatic environments in organic environments

RT lnKKM = a – bn

where a is a constant characterizing the energy of dissolution of the 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 a significant role in MCO. Its influence is reflected by the coefficient a, but the influence of the nature of the polar group is less significant than the length of the radical.

At equal R, the substance has a larger CMC, in which its polar group dissociates better (the presence of ionogenic groups, surfactant solubility), 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).

Effect of additives of electrolytes and polar organic substances.

The introduction of electrolytes into IPAS and NIPAV solutions causes different effects:

1) in solutions of IPAS Sel-ta ↓ KKM.

The main role is played by the concentration and charge of counterions. Ions charged with the same charge as the surfactant ion in the MC have little effect on the CMC.

The facilitation of MCO is explained by compression of the diffuse layer of counterions, suppression of the dissociation of surfactant molecules and partial dehydration of surfactant ions.

Decreasing the charge of the micelles weakens the electrostatic repulsion and makes it easier for new molecules to attach 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 affects the CMC in different ways:

low molecular weight compounds (alcohols, acetone) KKM (if there is no solubilization)

long-chain compounds ↓ CMC (micelle stability increases).

3). Effect of temperature T.

There is a different nature of the influence of T on IPAV and NIPAV.

An increase in T for IPAS solutions enhances thermal movement and prevents the aggregation of molecules, but intense movement reduces the hydration of polar groups and promotes their association.

Many surfactants with high R do not form micellar solutions due to poor solubility. However, with a change in T, the solubility of the surfactant may increase and MCO is detected.

T, with cat. IPAS solubility increases due to the formation of MC, called the Krafft point (usually 283-293 K).

T. Kraft does not coincide with T PL TV. Surfactant, but lies below, because in the 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 Krafft point

To obtain a surfactant with a low Craft point value:

a) introduce additional CH 3 - or side substituents;

b) introduce an unsaturated relationship “=”;

c) polar segment (oxyethylene) between the ionic group and the chain.

Above the K raft point, the IPAS MCs disintegrate into smaller associates—demicellization occurs.

(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.

Crafting point- 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- 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.)

2) T in NIPAV solutions ↓ CCM due to dehydration of oxyethylene chains.

In NIPAV solutions, a cloud point is observed - the upper temperature limit of the NIPAV MCO (323-333 K); at higher temperatures, the system loses stability and separates into two phases.

Thermodynamics and micelle formation mechanism (MCM)

(The true solubility of a surfactant is due to an increase in entropy S during dissolution and, to a lesser extent, interaction with water molecules.

IPAS are characterized by dissociation in water, and their dissolution rate is significant.

NIPAS interact weakly with H 2 O, their solubility is lower at the same R. More often ∆H>0, therefore solubility at T.

Low solubility of surfactants manifests itself in “+” surface activity, and with C - in a significant association of surfactant molecules, which turns 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):

∆Н f.p. >0 ∆S f.p. >0 ∆Н sol. >

∆Н solvate.

∆ G= ∆Н dissolve . - T∆S sol.

For IPAV :

∆Н solvate. large in size, ∆Н sol. 0 and ∆G dist.

For NIPAV ∆Н sol. ≥0, therefore, at T, solubility is due to the entropy component.

The MCO process is characterized by ∆Н MCO. ∆ G MCO = ∆Н MCO . - T∆S MCO.

Methods for determining CMC

Based on recording 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 solutions; 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 of them 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 the surfactant solution; study of optical properties - refractive index of solutions, light scattering; study of absorption spectra and NMR spectra, etc. The CMC is well determined when plotting the dependence of surfactant solubility on the value of 1/T (reverse temperature). Simple and reliable methods of potentiometric titration and ultrasound absorption, etc.

The second group of methods for measuring CMC is based on adding additional substances to solutions and their solubilization (colloidal dissolution) in surfactant micelles, which can be recorded using spectral methods, fluorescence, ESR, etc. Below is a brief description of some methods for determining CMC from the first group.

Rice. 7.2. Determination of CMC by the conductometric method (left).

Fig. 7.3. Determination of CMC by surface tension measurement method

The conductometric method for determining CMC is used for ionic surfactants. If there were 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 coordinates  = f() would lie along a straight line (Fig. 7.2) . This is done at low concentrations of surfactants (10 -3 mol/l), starting from the CMC, ionic micelles are formed, surrounded by a diffuse layer of counterions, the course of the dependence  = f() is disrupted and a kink is observed on the line.

Another method for determining CMC is based on measuring the surface tension of aqueous solutions of surfactants, which decreases with increasing concentration up to CMC, and then remains almost 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 coordinates  = f(lnC) (Fig. 7.3).

Isotherms σ=f(C) differ from 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 CMC point 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). From this dependence, it is possible to determine the CMC of surfactants in aqueous and non-aqueous media.

In the CMC region, the true (molecular) solution transforms into a colloidal solution, and the light scattering of the system sharply increases (everyone could observe the scattering of light on dust particles suspended in the air). To determine the CMC by the light scattering method, the optical density of the system D is measured depending on the surfactant concentration (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 light scattering method (right).

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.

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 precipitate.

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.

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 may 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, may 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.