Stability and coagulation of dispersed systems. Stability of colloidal solutions Attractive forces prevail in an unstable colloidal system

The main method of purification of natural and waste water from fine, emulsified, colloidal and colored impurities (groups 1 and 2) is coagulation and flocculation. The methods are based on the aggregation of particles of the dispersed phase with their subsequent removal from water by mechanical settling.

The efficiency and economy of the processes of coagulation wastewater treatment is determined by the stability of the dispersed system, which depends on a number of factors: the degree of dispersion, the nature of the surface of the particles, the density of the particles, the magnitude of the electrokinetic potential, concentration, the presence of other impurities in the wastewater, for example, electrolytes, macromolecular compounds.

Exist various ways carrying out coagulation, the expediency of which depends on the factors that determine the aggregative stability of systems.

Aggregative stability of colloidal systems depends on their structure.

Possessing a large specific surface, colloidal particles are able to adsorb ions from water, as a result of which the contacting phases acquire charges of the opposite sign, but equal in magnitude. As a result, a double electric layer appears on the surface. Ions relatively strongly associated with the dispersed solid phase are called potential-determining. They are neutralized by excess counterions. The thickness of the double layer in aqueous solutions does not exceed 0.002 mm.

The degree of adsorption of ions depends on the affinity of adsorbed ions to the surface, their ability to form non-dissociable surface compounds. When ions of the same valence are adsorbed, the adsorption capacity increases with an increase in the ion radius and, accordingly, its polarizability, i.e. the ability to be attracted to the surface of a colloidal particle. An increase in the ion radius is also accompanied by a decrease in its hydration; the presence of a dense hydration shell prevents adsorption, because reduces the electrical interaction of the ion with the surface of the colloidal particle.

According to modern ideas about the structure of the electric double layer, the counterion layer consists of two parts. One part adjoins the interfacial surface and forms an adsorption layer, the thickness of which is equal to the radius of its constituent hydrated ions. The other part of the counterions is in the diffuse layer, the thickness of which depends on the properties and composition of the system. On the whole, the micelle is electrically neutral. The structure of a micelle - a colloidal particle - is shown in Fig. 1.1.

The potential difference between the potential-determining ions and all counterions is called the thermodynamic φ-potential.

The charge on the particles prevents their approach, which, in particular, determines the stability colloid system. In general, the stability of colloidal systems is due to the presence of a charge on the granule, diffusion layer, and hydration shell.

Fig.3.1. The structure of the micelles: Fig.3.2. Double electric circuit

I – micelle core; layer in an electric field

II - adsorption layer; (I-II - granule);

III - diffusion layer;

IV - hydration shell

When a particle moves in a disperse system or when an electric field is applied, some of the counterions of the diffuse layer remain in the dispersed medium and the granule acquires a charge corresponding to the charge of the potential-determining ions. Thus, the dispersion medium and the dispersed phase turn out to be oppositely charged.

The potential difference between the adsorption and diffuse layers of counterions is called the electrokinetic ζ - potential (Fig. 1.2).

The electrokinetic potential is one of the most important parameters of the electric double layer. Value ζ - the potential is usually units and tens of millivolts, depending on the composition of the phases and the concentration of the electrolyte. The larger the value ζ– potential, the more stable the particle.

Consider the thermodynamic and kinetic stability factors disperse systems:

· Electrostatic Stability Factor. From the point of view of physical kinetics, the molecular attraction of particles is the main reason for the coagulation of the system (its aggregative instability). If an adsorption layer of an ionic nature is formed on colloidal particles, then with a sufficient approach of like-charged particles, electrostatic repulsive forces arise. The thicker the double electric layer, the more intense the resulting repulsive force of particles, the greater the height of the energy barrier, and the less likely particles stick together. Thus, the stability of colloidal systems in the presence of an ionic stabilizer depends on the properties of the electrical double layer.

· Solvation Stability Factor. Repulsive forces can be caused by the existence of solvate (hydrate) shells or so-called boundary phases on the surface of approaching particles, which consist only of molecules of the dispersion medium and have special characteristics. physical properties. The core of the micelle is insoluble in water and therefore not hydrated. The ions adsorbed on the surface of the nucleus and the counterions of the electric double layer are hydrated. Due to this, an ion-hydrate shell is created around the nucleus. Its thickness depends on the distribution of the electric double layer: the more ions are in the diffuse layer, the greater the thickness of the hydration shell.

· Entropy factor of stability. Due to the thermal motion of segments of surfactant molecules adsorbed on colloidal particles. When approaching particles with adsorption layers of surfactant molecules or macromolecular substances, there is a strong decrease in the entropy of the adsorption layer, which prevents particle aggregation.

· Structural-mechanical stability factor. Adsorption-solvation layers of surfactants can be a structural-mechanical barrier that prevents particles from approaching. Protective layers of counterions-stabilizers, being gel-like, have increased structural viscosity and mechanical strength.

· Hydrodynamic stability factor. The coagulation rate can decrease due to changes in the viscosity of the medium and the density of the dispersed phase and the dispersion medium.

· Mixed factors most characteristic of real systems. Usually, aggregative stability is provided by several factors simultaneously. Particularly high stability is observed under the combination of the action of thermodynamic and kinetic factors, when, along with a decrease in interfacial tension, the structural and mechanical properties of interparticle interlayers are manifested.

It must be borne in mind that each sustainability factor corresponds to a specific method of its neutralization. For example, the action of the electrostatic factor is significantly reduced when electrolytes are introduced into the system, which compress the electrical double layer.

Solvation at the solvation factor can be excluded by lyophobization of the particles of the dispersed phase by adsorption of the corresponding substances. The action of the structural-mechanical factor can be reduced with the help of substances that thin and dissolve the elastic structured layers on the surface of the particles.

The destabilization of the system can be caused by various reasons, the result of many of them is the compression of the diffuse layer and, consequently, a decrease in the value of the ζ-potential. Compression of the diffuse layer also reduces the degree of ion hydration; in the isoelectric state (ζ = 0, mV), the hydration shell around the core is extremely thin (10–10 m) and does not protect micelles from sticking together upon collision, as a result, particle aggregation begins.

Sedimentation stability of colloidal systems (CS) - the ability of a dispersed system to maintain a uniform distribution of particles throughout the volume) is due to the Brownian motion of colloidal dispersions and diffusion of particles of the dispersed phase.

The sedimentation stability of the system depends on the action of two factors that are directed mutually opposite: gravity, under which the particles settle, and diffusion, in which the particles tend to uniform distribution over the volume. As a result, an equilibrium diffusion-sedimentation distribution of particles along the height arises, depending on their size.

Diffusion slows down with increasing particle size. At a sufficiently high degree of dispersion of particles, Brownian motion, as a diffusion motion, leads to equalization of concentrations throughout the volume. The smaller the particle, the longer it takes to reach equilibrium.

The sedimentation rate of particles is proportional to the square of their diameter. In coarsely dispersed systems, the rate at which equilibrium is reached is relatively high, and equilibrium is established within a few minutes or hours. In finely dispersed solutions, it is small, and years or even tens of years pass until the moment of equilibrium.

Types of coagulation

IN modern theory coagulation of dispersed systems developed by Deryagin, Landau, Verwey, Overbeck (DLVO theory), the degree of system stability is determined from the balance of molecular and electrostatic forces. There are two types of coagulation:

1) concentration, at which the loss of stability of the particles is associated with the compression of the double layer;

2) neutralization (coagulation with electrolytes), when, along with the compression of the double layer, the potential φ 1 decreases.

Concentration coagulation is characteristic of highly charged particles in highly concentrated electrolyte solutions. The higher the potential φ 1 of the DES, the stronger the counterions are attracted to the surface of the particles and screen the growth of the electric field by their presence. Therefore, at high values ​​of φ 1, the forces of electrostatic repulsion between particles do not increase indefinitely, but tend to some finite limit. This limit is reached at φ 1 more than 250 mv. Hence it follows that the interaction of particles with a high φ 1 -potential does not depend on the value of this potential, but is determined only by the concentration and charge of counterions.

As the electrolyte concentration increases, the value ζ - potential (DP) decreases, and φ 1 practically retains its value (Fig. 3.3).

The aggregative stability/instability of the system depends on the possibility of particle contact; For sticking, the particles must approach a certain distance. In the theory of aggregative stability, known as DLVO theory(the first letters of the names of the authors of the theory: B. V. Deryagin and L. D. Landau, Russia, and E. Verwey and J. T. Overbeck, Holland), is considered combined action of attractive and repulsive forces between particles.

Historical digression

Boris Vladimirovich Deryagin is an outstanding scientist who has made an invaluable contribution to almost every branch of colloidal chemistry. Investigating the properties of clay suspensions, he found that thin layers of water between individual suspension particles have properties that are different from the properties of water in bulk, including disjoining pressure that prevents the particles from approaching. The joint consideration of the forces of attraction and repulsion explained the stability of the system. These studies, along with quantitative calculations and the identification of a stability criterion, were published by B.V. Deryagin together with Lev Davidovich Landau in several scientific articles 1935-1941; abroad about these works learned much later.

Dutch scientists E. Vervey and J.T. Overbek also did research in this area. E. Fairway in 1934 defended his dissertation on the study of the electric double layer and the stability of lyophobic colloids. Later, he published a series of articles, which considers the action of electric forces and London-van der Waals forces between colloidal particles in an electrolyte solution. And in 1948, in collaboration with Overbeck, his monograph "Theory of Stability of Lyophobic Colloids" was published.

The issue of scientific priority in relation to the creation of the theory was resolved by recognizing the merits of all four authors.

Forces of attraction - these are the forces of intermolecular interaction (London-van der Waals forces). The forces of attraction that arise between individual atoms manifest themselves at very small distances of the order of atomic dimensions. When particles interact, due to the additivity of dispersion forces, attraction between particles manifests itself at much greater distances. The attraction energy is inversely proportional to the square of the distance between the particles:

repulsive forces between particles are electrostatic in nature. The electrostatic repulsive energy that occurs when the diffuse layers overlap decreases exponentially with increasing distance:

In the above formulas for the energies of attraction and repulsion A* - Gamaxra constant; X - distance between particles; e is the permittivity of the dispersion medium; e ° \u003d 8.85 K) 12 F / m - electrical constant; (p^ is the potential of the diffuse layer; A. is the thickness of the diffuse layer of the electrical double layer (DEL).

For more details on the structure of the DEL, including the adsorption and diffuse layers, see paragraph 4.3.

Attractive energies are assigned a minus sign, repulsive energies are assigned a plus sign. The energies of attraction and repulsion are considered in the DLVO theory as components of the disjoining pressure between particles. The action of the energies of attraction and repulsion depending on the distance between the particles is shown in Fig. 4.2.

Rice. 4.2.

On the resulting curve of the total energy in Fig. 4.2 can be divided into three areas.

Plot a. At small distances between colloidal particles (up to 100 nm), attractive forces predominate, an energy well or a near energy minimum appears. If the particles come close to such a distance, coagulation will occur under the influence of attractive forces. Coagulation in such cases is irreversible.

Plot b. At medium distances, the electrostatic repulsive forces are greater than the forces of intermolecular attraction, an energy maximum arises - a potential barrier that prevents particles from sticking together; the barrier height depends on the surface charge and the thickness of the diffuse layer.

If the potential barrier is high, the particles are not able to overcome it, then coagulation does not occur. The possibilities of overcoming the barrier are determined by its decrease (decrease in the surface charge and repulsion forces between particles, for example, when exposed to an electrolyte) or by an increase in the particle energy (heating).

The effect of electrolytes on the structure of the electrical double layer is discussed in subparagraph 4.3.3.

Further, under the influence of attractive forces, the particles approach each other, and coagulation occurs. If the particles cannot overcome the barrier, then coagulation does not occur and the system can maintain aggregative stability for a long time.

Plot in At relatively large distances (about 1000 nm), attractive forces also prevail, forming the so-called far minimum. The depth of the far minimum is individual for each system. At an insignificant far minimum, the approach of particles is prevented by a potential barrier.

If the far minimum is deep enough, then the particles, when approaching, cannot leave the potential well and remain in an equilibrium state at an appropriate distance from each other, retaining their individuality.

The presence of a high potential barrier prevents closer approach of the particles, and a layer of liquid remains between them. The system as a whole retains dispersion, representing a loose sediment - a coagulant, or a flocculant. This state corresponds to the reversibility of coagulation; it is possible to transfer the system to the sol state (peptization).

« Peptization is one of the methods for obtaining dispersed systems, see paragraph 2.4.

At a high concentration of the dispersed phase, a structured system - a gel - can form.

Features of structured systems are discussed in more detail in Section 9.4.

Summary

Aggregative stability of the system (resistance to coagulation) largely determined by the presence electric charge on a surface.

• Vetvey E. J., Overbeek J. Th. G. Theory of the stability of lyophobic colloids. N.Y.: Elsevier, 1948.

Magnetic fluid, which includes highly dispersed magnetic materials (iron, cobalt, magnetite, ferrites, etc.) with a particle size of 50-200 E as a dispersed phase, liquid hydrocarbons, silicone and mineral oils, water, organofluorine as a dispersion medium compounds, etc., can be classified as colloidal solutions or sols.

The stability of colloidal systems is the central problem of colloidal chemistry, and its solution is of great practical importance in geology, agriculture, biology, and technology. Using the basic concepts of modern stability theory, let us briefly consider the conditions for the stability of magnetic fluids.

It is necessary to distinguish between aggregative stability, that is, the resistance of particles to aggregation, and sedimentation stability - resistance to the effects of gravitational magnetic and electric fields, centrifugal forces, etc.

Sedimentation consists in the free settling of particles of the dispersed phase under the action of gravity, as a result of which the concentration of dispersed particles in the volume of the dispersion medium changes depending on the height of the layer, the system is stratified and a highly concentrated precipitate is formed. Free sedimentation of particles is prevented, on the one hand, by the viscous resistance force of the dispersion medium (Stokes force), and, on the other hand, by the diffusion movement of particles, however, in this case, the particle size must be small enough to ensure their Brownian thermal motion. The condition for sedimentation stability is the smallness of the settling rate compared to the rate of Brownian motion. In particular, for magnetic fluids based on kerosene, water, and mineral oil, using magnetite as a ferrophase, respectively, the following values ​​were obtained maximum dimensions particles: d = 8 10 -6 m, d = 7 10 -6 m and d = 20 10 -6 m.

The aggregative stability of colloidal systems is determined by the balance of repulsive and attractive forces between particles. Attractive forces are London forces, and repulsive forces are electrostatic or steric repulsion forces.

This is due to the fact that, due to their small size, colloid particles are single-domain and have their own magnetic moment. The interaction between magnetic particles leads to their sticking together into aggregates, which ultimately leads to the sedimentation of magnetic particles. In addition, when particles approach each other, London forces arise, which also lead to particles sticking together. To prevent coagulation of the particles, their surface is covered with a layer of long, chain structure, surfactant molecules. The shell of PAB molecules prevents the particles from approaching each other, since when it is compressed, repulsive forces arise. And, finally, electrostatic forces act between the particles, which arise due to the interaction of double electrical layers surrounding the particles. The resistance to particle aggregation and coagulation determines the aggregative stability of colloidal systems and depends on the balance of forces acting between ferromagnetic particles - forces of attraction (van der Waals forces, dipole-dipole interaction and magnetic forces) and repulsive forces (forces of electrical and steric nature). The nature and intensity of the forces mentioned above have been discussed in detail in a number of papers.

Electrostatic repulsion is due to the existence of double electrical layers consisting of ions on the surface of dispersed particles in a liquid medium.

Since the liquids we are considering are colloidal systems, the laws of colloidal chemistry will be valid for them. An important feature and the main difference between magnetic fluids (MF) and conventional colloidal systems is their magnetic properties. And therefore, in addition to the main forces of interaction between particles (the forces of London attraction, the forces of electrostatic and steric repulsion), it is also necessary to take into account the forces of magnetic interaction. The balance of these forces or the predominance of repulsive forces will ensure the stability of the colloidal system. Stability is one of the most important characteristics of magnetic fluids and to a large extent determines the possibility of their successful application. Stability is understood as the ability of particles of magnetic fluids not to aggregate and retain, but for a certain time constant, their physical, chemical and magnetic properties. Moreover, this time, as for any colloidal system, will depend primarily on the size of the particles of the dispersed phase, chemical composition and physical characteristics of the colloid, external conditions(e.g. temperatures, values magnetic field etc.) and can vary from a few seconds to several years.

Magnetic particles in a colloid, due to their small size, are single-domain and super-non-magnetic, that is, they are completely magnetized in one direction and their magnetic interaction can be approximately described as the interaction of point dipoles.

Between particles covered with a layer of long chain molecules, when they come into contact, a repulsive force arises, called steric. Steric repulsion occurs due to an increase in the local concentration of long polymer molecules (surfactants) in the zone of intersection of adsorption layers (osmotic effect).

In order for the adsorption layer on magnetic particles not to be destroyed, it is necessary that the steric repulsion forces exceed the dipole-dipole interaction forces.

However, the sufficient strength of the adsorption layer does not yet mean the absence of coagulation, since two particles separated by the adsorption layer 2e can be held together by magnetic attraction forces. Such an agglomerate can be destroyed by the thermal motion of the particles. Since with increasing thickness of the solvation layer the distance between the particles increases, the energy of the dipole-dipole interaction decreases and, therefore, the influence increases. thermal motion particles for their aggregation.

The thickness of the solvate shell, which prevents the aggregation of particles, taking into account their thermal energy and dipole-dipole interaction, depends on temperature, particle size, and their magnetic characteristics. Specifically for magnetic magnetite particles at room temperature:

e is the length of the surfactant molecules.

If oleic acid (d = 20?) is used as a surfactant for magnetite particles, then the condition d cr<<д говорит о том, что в этом случае от коагуляции будут защищены частицы, диаметр которых существенно меньше 190Е. С другой стороны, очень малые частицы (10-20Е) теряют свои магнитные свойства вследствие малости энергии обменного взаимодействия по сравнению с тепловой энергией. Поэтому наиболее приемлемым, с точки зрения агрегативной устойчивости, является размер частиц магнетита 40-160Е, а применение поверхностно-активных веществ с большей, чем у олеиновой кислоты, длиной молекул, обеспечит стабилизацию более крупных частиц магнетита.

Thus, the stability of an MF is determined by the balance of all possible factors of interaction (intermolecular, magnetic, structural-mechanical, and for polar media - electrostatic) between the particles of the dispersed phase. If the forces of attraction are dominated by repulsive forces, the system is in a stable state. Otherwise, the system tends to destroy the colloidal structure.

Thus, the behavior of the MF can be predicted by summing the repulsive energy (electrostatic for polar media and due to surfactants) with the energy of magnetic and intermolecular attraction. A positive addition result indicates the predominance of repulsive forces, from which we can conclude that the system is stable. A negative result suggests that the system is kinetically unstable. Based on all of the above, it can be concluded that the most optimal variant of the MF colloidal solution is the following system: magnetic particles 50–200 E in size, coated with a surfactant layer and distributed in a liquid medium free of low molecular weight electrolytes. It is in this case that the forces of electrostatic repulsion are minimal, the forces of intermolecular and magnetic attraction are minimal, and the structural-mechanical factor stabilizes the system in the most effective way, and the MF as a whole is, therefore, the most stable colloidal system in time, space, gravitational and electromagnetic fields.

ion, while in-and molecular systems is determined

3. HETEROGENEITY OF COLLOID SYSTEMS AS THE MAIN DIFFERENCE FROM MOLECULAR SOLUTIONS

We have already said that aggregative instability is a specific feature of colloidal systems. This property of colloidal systems is of great practical importance. It would not be an exaggeration to say that the main task of the technologist of the production process in which colloidal systems take place is either to maintain the aggregative stability of the system, or, conversely, to provide known coagulation conditions.

Aggregative instability is the central problem of colloidal chemistry, and already at the beginning of the course it is necessary to consider, at least in the most general form, what causes the aggregative instability of colloidal systems and why many colloidal systems, despite their fundamental aggregative instability, exist for a very long time. The reasons for the instability of colloidal systems can be explained from two points of view - thermodynamic and kinetic.

According to thermodynamics, the aggregative instability of colloidal systems is due to a sufficiently large and always positive free surface energy concentrated on the interfacial surface of the system. Since surface energy represents free energy and since all systems with excess free energy are unstable, this determines the ability of colloidal systems to coagulate. During coagulation, the particles stick together, while the interfacial surface at least partially disappears and, thus, the free energy of the system decreases. However, Smolukhovsky, and recently G. A. Martynov, drew attention to the fact that in order to reduce the free energy of the system, direct contact of particles is not necessary. The free energy can also decrease when the particles do not come into direct contact, but approach only a certain distance, allowing them to interact through the layer separating their media.

Indeed, let

where F is the free surface energy of the entire system; st, % - interfacial surface; f is the specific free surface energy.

The value of f is the sum of the interfacial surface energy fa, determined by the state of the monolayer at the phase boundary, and the free energy fv near the surface, i.e., f = fa + fv. The volume-surface contribution fv is due to a change in the state of the liquid layers near the interface. Despite the fact that in general fa ^ fv, the stability of the system "in most cases is associated precisely with a change in fv, since the phase boundary usually does not disappear during the formation of aggregates from solid particles. Therefore, during coagulation, the value of /a remains practically constant, and fv changes , and the degree of change depends on the decrease in the distance between the particles.Of course, all this does not apply to emulsions, where coalescence takes place, that is, the merging of particles with the complete elimination of the interfacial surface that initially separates the particles.

Since colloidal systems with a large specific surface area and high free energy are fundamentally non-equilibrium systems, the well-known phase rule is inapplicable to them. Such systems, obviously, will always tend to an equilibrium state corresponding to the separation of the system into two continuous phases with a minimum interfacial surface, although this equilibrium may never actually occur. The thermodynamic interpretation of the reasons for the stability or instability of colloidal systems is extremely simple. However, like any thermodynamic interpretation, this explanation is formal, i.e., it does not reveal the essence of the property of aggregative instability. In addition, thermodynamics does not establish a relationship between the free energy of a system and how long the system can remain in a non-equilibrium state. Therefore, in this case, it is more complete to explain the aggregative instability or stability of colloidal systems from the standpoint of physical kinetics.

According to kinetic concepts, the instability or stability of a colloidal or microheterogeneous system is determined by the ratio of forces acting between its individual particles. These forces include forces of two kinds: forces of cohesion, or attraction forces, tending to bring the particles together and form an aggregate from them, and repulsive forces that prevent coagulation.

Cohesive forces are usually of the same nature as intermolecular (van der Waals) forces. It is essential that the forces acting between the particles increase very rapidly as the particles approach each other.

Repulsive forces can be electrical forces resulting from selective adsorption by the interfacial surface of one of the electrolyte ions present in the system. Since the particles of the dispersed phase are identical in nature and always adsorb a certain ion, they all acquire an electric charge of the same sign and experience mutual repulsion, which prevents them from approaching such distances where very significant attraction forces can already act. Another reason that prevents colloidal particles from approaching to distances at which cohesion forces begin to prevail may be the formation of a solvate shell of medium molecules on the particle surface. Such a shell arises as a result of adsorption by the dispersed phase of either molecules of the medium, or molecules or ions of the third component (stabilizer) of the system. In addition to these two factors, there are other factors that ensure the aggregate stability of colloidal systems. All sustainability factors are discussed in detail in Chap. IX.

Thus, the relative stability of a colloidal system is determined by whether the repulsive forces are strong enough to prevent particles from approaching close distances. It is clear that such an explanation does not contradict the fundamental instability of the vast majority of colloidal systems, since with the close proximity of particle surfaces, the cohesive forces, as a rule, are greater than the repulsive forces, and it is usually energetically more favorable for two separate particles to form an aggregate. In what follows, we will see that there are many ways to reduce the repulsive forces, and in particular, one of these methods is the introduction of electrolytes into the system.

4. PRESSURE PRESSURE*

* This section of the chapter was written by B. V. Deryagii.

With thinning of a layer of liquid separating the surfaces of two solid bodies or, in general, of any two phases adsorbing ions, two kinds of interaction forces arise between the surfaces of these phases. First, the forces that depend on the attraction between the molecules of both bodies, between the molecules of the liquid and between the molecules of the liquid and each body (or phase).

If both bodies are the same, then these forces lead to the attraction of the bodies, which tends to thin the liquid layer. Secondly, as a result of the action of forces of an electrical nature, repulsion always occurs between identical bodies, causing a thickening of the liquid layer. Therefore, so that the thickness of the interlayer does not change and the system as a whole retains t

Lecture 5 Stability and coagulation of colloidal systems

The concept of the stability of disperse systems.

Types of DS stability.

Coagulation.

The effect of electrolytes on coagulation.

Joint action of electrolytes during coagulation.

DLVO stability theory.

coagulation rate.

Sol aging. Colloidal protection.

Questions of the stability of disperse systems occupy a central place in colloid chemistry, since these systems are mainly thermodynamically unstable.

Under the stability of the system is understood the constancy in time of its state and basic properties: the dispersion of the uniform distribution of particles of the dispersed phase in the volume of the dispersion medium and the nature of the interaction between the particles.

The particles of a dispersed system, on the one hand, experience the effect of gravity; on the other hand, they are subject to diffusion, which tends to equalize the concentration at all points in the system. When an equilibrium occurs between these two forces, the particles of the dispersed phase are located in a certain way relative to the surface of the Earth.

At the suggestion of N.P. Peskov (1920), the stability of dispersed systems is divided into two types:

- kinetic(sedimentation) stability - the property of dispersed particles to be held in suspension without settling (resistance of particles to gravity).

(stability conditions - high dispersion of particles, participation of particles of the dispersed phase in Brownian motion);

- aggregative stability - the ability of particles of a dispersed phase to resist sticking (aggregation) and thereby maintain a certain degree of dispersion of this phase as a whole.

Dispersed systems are divided into two classes according to their stability:

Thermodynamically stable (lyophilic colloids);

Thermodynamically unstable (lyophobic systems).

The former spontaneously disperse and exist without a stabilizer. These include surfactant solutions, IUD solutions.

The Gibbs free energy of a thermodynamically stable system decreases (DG<0).

Sols, suspensions, emulsions (DG>0) belong to thermodynamically unstable systems.

Recently, there are also condensation resistance: the system forms unstable aggregates (flocculi) or loose sediments - the particles lose their individual mobility, but remain as such for a long time.

Coagulation

Lyophobic colloids are thermodynamically unstable systems that exist due to stabilization due to the appearance of protective ionic or molecular layers. Consequently, a change in the state of these layers can lead to a loss of stability and then to the separation of a dispersed phase.

Coagulation- the process of adhesion (fusion) of colloidal particles with the formation of larger aggregates with subsequent loss of kinetic stability.

In a general sense, coagulation is understood as the loss of aggregative stability of a disperse system.

The latent stage of coagulation is very fast - the particle size increases, but the precipitate does not fall out - discoloration, turbidity.

Explicit stage - precipitation, separation of two phases in solution. The precipitate is called coagulate.

The end result of coagulation can be two results: phase separation and the formation of a volumetric structure in which the dispersion medium is evenly distributed (concentration of the system). In accordance with two different results of coagulation, methods of their study are also distinguished (for the first result - optical, for example, for the second - rheological).

The main processes that can occur in dispersed systems are shown in Figs. 5.1.

It can be seen from the diagram that the concept of coagulation includes several processes (flocculation, coalescence, aggregation, structure formation) that occur with a decrease in the specific surface area of ​​the system.

Rice. 5.1. Processes occurring in disperse

systems.

Coagulation can be caused by various factors:

The introduction of electrolytes;

Heating or freezing of the dispersed system;

mechanical impact;

High frequency vibrations;

Ultracentrifugation and other factors.

The most important and studied is the action of electrolytes.

The effect of electrolytes on coagulation

A number of empirical patterns of the effects of electrolytes have been established, which are known as coagulation rules:

1. All electrolytes can cause coagulation, but they have a noticeable effect when a certain concentration is reached.

Coagulation threshold- the minimum concentration of electrolyte that causes coagulation (g, mol / l; sometimes C c).

The coagulation threshold is determined by turbidity, discoloration, or by the beginning of the separation of the dispersed phase into the precipitate.

2. Schulze-Hardy rule (significance rule, empirical):

The electrolyte ion that has a charge opposite to the charge of the potential-determining ions of the micelles (granules) has a coagulating effect, and the higher the charge, the stronger the coagulating effect.

where K is the coagulating ability (let's take it as a unit).

According to the Schultz-Hurdy rule, the coagulation thresholds for counterions with charges 1, 2, and 3 are related as 1:1/20:1/500, i.e. the higher the charge, the less electrolyte is required to cause coagulation.

For example, we coagulate the sol of arsenic sulfide (As 2 S 3): or Fe (OH) 2

The Schulze-Hurdy rule is approximate and describes the action of ions of inorganic compounds only.

3. In a series of organic ions, the coagulating effect increases with an increase in adsorption capacity.

4. In a series of inorganic ions of the same charge, their coagulating activity increases with a decrease in hydration.

Lyotropic series or Hofmeister series is the order in which ions are arranged according to their ability to hydrate (bind water).

The word "lyotropic" means "tending to liquid" (a more appropriate term for the case of aqueous media is hydrotropic).

5. Very often, the beginning of coagulation corresponds to a decrease in the zeta potential to a critical value (about 0.03 V).

6. In the sediments obtained during coagulation with electrolytes, there are always ions that cause it.

Joint action of electrolytes

during coagulation

Electrolyte mixtures rarely act independently when coagulating sols. The observed phenomena can be reduced to the following three: additivity, antagonism and synergy electrolytes. These phenomena when using mixtures of electrolytes are shown in Fig.5.2.

Dependence 1 - characterizes the additive effect of electrolytes. The coagulating effect in the mixture is determined by the rule of simple addition:

KCl + KNO 3 ; NaCl+KCl

Curve 2 - electrolyte antagonism - the content of each electrolyte in the mixture exceeds its own threshold concentration

Al(NO 3) 3 + K 2 SO 4; Ti(NO 3) 4 +Na 2 SO 4

Curve 3 demonstrates the synergism of the action of electrolytes. The action of each of the electrolytes is enhanced - for coagulation they require less in the mixture than each separately.

LiCl+CaCl 2 act on hydrosol H 2 S

Rice. 5.2. The combined action of electrolytes in

coagulation.

Theory of stability of hydrophobic disperse systems DLVO

The modern physical theory of coagulation with electrolytes is based on the general principles of statistical physics, the theory of molecular forces and the theory of solutions. Its authors are: B.V. Deryagin, L.D. Landau (1937-1941), E. Fairway, J. Overbeck (in the first letters of the DLFO).

The essence of the theory: between any particles when they approach, a disjoining pressure of a separating liquid layer arises as a result of the action of forces of attraction and repulsion. The disjoining pressure is a total parameter that takes into account the action of both attractive and repulsive forces.

The state of the system depends on the balance of the energy of attraction (U pr) and the energy of repulsion (U ot). U otm prevails - a stable system. U pr prevails - violation of aggregative stability - coagulation.

The change in the interaction energy between two particles as they approach is depicted graphically (Fig. 5.3).

The total energy of a system of two particles (curve 3) is obtained by adding U ot and U pr:

U \u003d U ott + U pr \u003d

where: V is a multiplier that depends on the values ​​of the electrical potentials of the DES, the properties of the medium, temperature;

e is the base of the natural logarithm;

c is the reciprocal of the diffuse layer thickness;

h is the distance between particles;

A is the constant of molecular forces of attraction.

Fig.5.3. Potential interaction curves

colloidal particles:

1 – change of repulsion energy with distance;

2 - change in the energy of attraction;

3 - resulting curve.

Consider the resulting curve 3 in Figure 5.3. It has characteristic areas:

In the region of small distances, there is a deep primary minimum (potential well) - U predominates significantly. The primary minimum corresponds to the direct adhesion of particles (I).

In the area of ​​large distances - a secondary shallow minimum (the second potential well, corresponds to attraction through the medium layer). On diagram II.

In the region of average distances, there is a maximum on the curve, and if it is located above the abscissa axis, then an energy barrier of repulsive forces (DU b) appears.

The resulting curve 3 may have a different form depending on the stability of the disperse system (Fig. 5.4.).

Rice. 5.4. Potential curves for certain

states of stability of a dispersed system:

1 - in the system at any distance between the particles, the energy of attraction prevails over the energy of repulsion. In such a system, rapid coagulation with the formation of aggregates is observed.

2 - a sufficiently high potential barrier and the presence of a secondary minimum. The particles interact, but do not have direct contact and are separated by interlayers of the medium.

3 - a system with high aggregate stability (high potential barrier and the absence of a secondary minimum or at its depth, less than the thermal energy kT).

Depending on the height of the energy barrier and the depth of potential wells, various behaviors of particles are possible when approaching (Fig. 5.5), particles have a kinetic energy - kT.

Fig.5.5. Schemes of interaction of colloidal particles

State V: Low barrier height and shallow secondary minimum: DU b @DU i £kT particles enter into short-range interaction, i.e. directly in contact - comes coagulation

State A: It is characterized by the fact that the diffuse layers overlap and the interlayers of the medium between the particles (gels) are preserved. energy barrier quite high The secondary minimum is shallow: The interacting particles cannot disperse (they hold the forces of attraction) and cannot come close (the forces of repulsion hinder). The addition of an electrolyte most often leads to coagulation (h decreases).

State b: High energy barrier DU b ³kT and absence or shallow secondary minimum DU i £kT: The particles cannot overcome the barrier and diverge without interaction. Such a system is aggregatively stable.

The disperse system is aggregatively stable at a high energy barrier of repulsive forces.

Coagulation rate

The course of coagulation, depending on the concentration of the coagulating electrolyte, can be divided into two stages: slow and fast.

Fig.5.6. Dependence of coagulation rate on

electrolyte concentration

In area slow coagulation rate is highly dependent on concentration (segment AB). At point B, the speed becomes constant and does not depend on the concentration of the electrolyte - here the value of the z - potential is zero - the beginning fast coagulation. The concentration of electrolyte, starting from which the coagulation rate remains constant, is called rapid coagulation threshold.

Theories of coagulation kinetics were developed by Smoluchowski (1916).

Coagulation is considered as a second-order reaction, in the elementary act of which two particles participate: .

Smoluchowski's equation for calculating the number of particles sticking together m-pieces in time t:

;

Initial number of particles;

Half coagulation time ().

With rapid coagulation, all colliding particles react (DU b =0).

Smoluchowski's equation for the rate constant of fast coagulation:

where h is the viscosity of the medium.

With slow coagulation, not all collisions lead to sticking. Smoluchowski's equation for slow coagulation:

;

where P is a steric factor that takes into account favorable spatial arrangements of particles in a collision and their physical dimensions. With fast coagulation, all collisions are effective and P=1, with slow P<1.

DE – potential barrier, with fast coagulation DE=0, with slow coagulation DE¹0.

h is the viscosity.

The coagulation threshold can be calculated from the ratio theoretically found by Deryagin and Landau and called law of the 6th degree:

the energy barrier between colloidal particles disappears when the critical concentration (g) is reached, which is inversely proportional to the sixth degree of charge of the coagulating ion:

;

C is a constant depending on the number of charges of the cation and anion;

e is the permittivity of the solution;

A is the van der Waals attraction constant;

e is the electron charge;

k is the Boltzmann constant;

z is the charge of the coagulating ion.

In accordance with this equation, the values ​​of g for elements with counterion charges 1, 2, and 3 are related as 1:1/2 6:1/3 6 =1:1/64:1/729.

The equation well substantiates the empirical Schulze-Hardy rule.

In cases where the role of the adsorption-solvation factor of stability is large, the approximation of the DLVO theory is manifested, since it does not take into account the role of specific adsorption and the affinity of the ion for the solvent.

The connection between the effectiveness of collisions and the potential barrier during coagulation was shown by Fuchs N.A.

If DE is much greater than kT, then the coagulation rate may approach zero and the system will be aggregatively unstable.

In the theory developed by Fuchs, the concept of the coagulation deceleration coefficient W is used, which shows how many times the rate constant of slow coagulation is less than the rate constant of fast coagulation. Taking into account the expressions for K b and K m, we get:

The coefficient W is called the stability factor or the stability coefficient.

Sol aging

Lyophobic colloids have a weak interaction between the dispersed phase and the dispersion medium and are characterized by a tendency to decrease in dispersion with time.

The excess of free surface energy received by particles during their formation is (according to the second law of thermodynamics) the main reason for the transition to a more stable state, which is determined by particle enlargement.

The spontaneous process of particle enlargement (decrease in the degree of dispersion) in lyophobic sols is called aging or autocoagulation.

The rate of aging is much slower than coagulation under the influence of electrolytes.

The protective effect of molecular

absorbent layers

Some systems have a very high stability, they even acquire the ability to form spontaneously - colloidal solubility.

In most sols, on the interface between two phases, there are adsorption layers formed by surfactant molecules. Adsorption layers prevent particles from sticking together, but they do not cover the entire surface, but approximately 40 ... 60% of it.

Maximum stability is achieved when a complete adsorption layer is formed.

Increasing the stability of disperse systems under the influence of surfactants is called colloidal protection or colloid stabilization.

The following are used as stabilizers: high-molecular surfactants, gelatin, albumin, casein, starch, pectin, rubbers, hemoglobin, etc.

For a quantitative assessment of the stabilizing effect of a particular colloid, R. Zsigmondy proposed the so-called golden number.

The golden number is the minimum mass (in mg) of a stabilizing agent that is able to protect 10 ml of red gold sol (prevent red-blue discoloration) from the coagulating effect of 1 ml of 10% NaCl solution.

The smaller the golden number, the greater the protective effect of the colloid.

The protective effect was also determined with respect to silver sols - silver number, congo ruby ​​- ruby ​​number, sulfur - sulfur number, etc.