§eight. Aggregative stability of dispersed systems. Factors of aggregative stability Structurally, the mechanical stability factor consists in

§eight. Aggregative stability of dispersed systems

This section discusses the phenomena and processes caused by aggregative stability dispersed systems.

First of all, we note that all disperse systems, depending on the mechanism of the process of their formation, according to the classification of P.A. Rebinder, are divided into lyophilic, which are obtained by spontaneous dispersion of one of the phases (spontaneous formation of a heterogeneous free-dispersed system), and lyophobic, resulting from dispersion and condensation (forced formation of a heterogeneous free-dispersed system).

Lyophobic systems, by definition, must have an excess of surface energy if it is not compensated by the introduction of stabilizers. Therefore, the processes of coarsening of particles take place in them spontaneously, i.e., there is a decrease in the surface energy due to a decrease in the specific surface. Such systems are called aggregatively unstable.

Particle enlargement can proceed in different ways. One of them, called isothermal distillation , consists in the transfer of matter from small particles to large ones (the Kelvin effect). As a result, small particles gradually dissolve (evaporate), while large particles grow.

The second way, the most typical and common for dispersed systems, represents coagulation (from Lat, coagulation, hardening), which consists in the adhesion of particles.

Coagulation in dilute systems also leads to loss of sedimentation stability and, ultimately, phase separation.

The process of particle fusion is called coalescence .

In concentrated systems, coagulation can manifest itself in the formation of a three-dimensional structure in which the dispersion medium is evenly distributed. In accordance with the two different results of coagulation, the methods for monitoring this process also differ. Enlargement of particles leads, for example, to an increase in the turbidity of the solution, a decrease in osmotic pressure. Structure formation changes the rheological properties of the system, its viscosity increases, and the flow slows down.

A stable free-dispersed system, in which the dispersed phase is evenly distributed throughout the volume, can be formed as a result of condensation from a true solution. The loss of aggregative stability leads to coagulation, the first stage of which is the approach of the particles of the dispersed phase and their mutual fixation at short distances from each other. A layer of medium remains between the particles.

The reverse process of formation of a stable free-dispersed system from a precipitate or gel (structured disperse system) is called peptization.

A deeper process of coagulation leads to the destruction of the interlayers of the medium and direct contact of the particles. As a result, either rigid aggregates of solid particles are formed, or they completely merge in systems with a liquid or gaseous dispersed phase (coalescence). In concentrated systems, rigid bulky solid-like structures are formed, which can only be turned back into a free-dispersed system by means of forced dispersion. Thus, the concept of coagulation includes several processes that occur with a decrease in the specific surface area of the system.

Fig.33. Processes causing loss of stability of disperse systems.

The aggregative stability of unstabilized lyophobic disperse systems is of a kinetic nature, and it can be judged by the rate of processes caused by excess surface energy.

The rate of coagulation determines the aggregative stability of a dispersed system, which is characterized by the process of adhesion (fusion) of particles.

Aggregative stability can also be of a thermodynamic nature if the dispersed system does not have an excess of surface energy. Lyophilic systems are thermodynamically aggregatively stable, they form spontaneously, and the process of coagulation is not typical for them at all.

Lyophobic stabilized systems are thermodynamically resistant to coagulation; they can be brought out of this state with the help of influences leading to an excess of surface energy (violation of stabilization).

In accordance with the above classification, thermodynamic and kinetic factors of the aggregative stability of disperse systems are distinguished. Since the driving force of coagulation is excess surface energy, the main factors that ensure the stability of disperse systems (while maintaining surface area) will be those that reduce surface tension. These factors are referred to as thermodynamic. They reduce the probability of effective collisions between particles, create potential barriers that slow down or even exclude the coagulation process. The lower the surface tension, the closer the system is to thermodynamically stable.

The rate of coagulation also depends on kinetic factors.

Kinetic factors that reduce the rate of coagulation are mainly associated with the hydrodynamic properties of the medium: with slowing down the approach of particles, leakage and destruction of the interlayers of the medium between them.

There are the following thermodynamic and kinetic stability factors for dispersed systems:

1.Electrostatic factor consists in a decrease in the interfacial tension due to the formation of a double electric layer on the surface of the particles, as well as in the Coulomb repulsion that occurs when they approach each other.

A double electric layer (DEL) is formed during the adsorption of ionogenic (dissociating into ions) surfactants. Adsorption of an ionic surfactant can occur at the interface between two immiscible liquids, such as water and benzene. The polar group of the surfactant molecule facing water dissociates, giving the surface of the benzene phase a charge corresponding to the organic part of the surfactant molecules (potential-determining ions). Counterions (inorganic ions) form a double layer on the side of the aqueous phase, since they interact with it more strongly.

There are other mechanisms for the formation of a double electric layer. For example, DES is formed at the interface between water and poorly soluble silver iodide. If highly soluble silver nitrate is added to water, then the silver ions formed as a result of dissociation can complete the crystal lattice of AgI, since they are part of it (specific adsorption of silver ions). As a result, the surface of the salt is positively charged (an excess of silver cations), and iodide ions will act as counterions.

We should also mention the possibility of the formation of a double electric layer as a result of the transition of ions or electrons from one phase to another (surface ionization).

DES, which is formed as a result of the processes of spatial separation of charges described above, has a diffuse (diffuse) character, which is due to the simultaneous influence on its structure of electrostatic (Coulomb) and van der Waals interactions, as well as the thermal motion of ions and molecules.

The so-called electrokinetic phenomena (electrophoresis, electroosmosis, etc.) are due to the presence of a double electric layer at the phase boundary.

2. Adsorption-solvation factor is to reduce the interfacial

tension during the introduction of surfactants (due to adsorption and solvation).

3. entropy factor, like the first two, refers to thermodynamic. It complements the first two factors and acts in systems in which particles participate in thermal motion. The entropy repulsion of particles can be represented as the presence of a constant diffusion of particles from a region with a higher concentration to a region with a lower concentration, i.e., the system constantly strives to equalize the concentration of the dispersed phase throughout the volume.

4. Structural-mechanical factor is kinetic. Its action is due to the fact that films with elasticity and mechanical strength can form on the surface of particles, the destruction of which requires energy and time.

5. hydrodynamic factor reduces the rate of coagulation due to a change in the viscosity and density of the dispersion medium in thin layers of liquid between the particles of the dispersed phase.

Usually, aggregative stability is provided by several factors simultaneously. Particularly high stability is observed under the combined action of thermodynamic and kinetic factors.

The structural-mechanical barrier, considered for the first time by P.A. Rebinder, is a strong stabilization factor associated with the formation of adsorption layers at the phase boundaries that lyophilize the surface. The structure and mechanical properties of such layers can provide a very high stability of the interlayers of the dispersion medium between the particles of the dispersed phase.

The structural-mechanical barrier arises during the adsorption of surfactant molecules, which are capable of forming a gel-like structured layer at the interface, although, possibly, they do not have a high surface activity with respect to this interface. Such substances include resins, cellulose derivatives, proteins and other so-called protective colloids, which are macromolecular substances.

§nine. Stabilization and breaking of emulsions

Let us consider the features of stabilization and destruction of dispersed systems on the example of emulsions.

Disperse systems with a liquid dispersed phase and a liquid dispersion medium are called emulsions.

Their specific feature is the possibility of forming emulsions of two types: straight, in which the dispersion medium is a more polar liquid (usually water) and reverse, in which the more polar liquid forms the dispersed phase.

Under certain conditions, there is phase reversal of emulsions when an emulsion of a given type, with the introduction of any reagents or with a change in conditions, turns into an emulsion of the opposite type.

The most important representative of emulsions is water-oil emulsion, very strongly stabilized by natural surfactants and resins. The destruction of such systems is the first and rather difficult stage of oil preparation and refining.

The aggregative stability of emulsions can be determined by many stability factors.

Their formation is possible by spontaneous dispersion under certain conditions, when the interfacial tension is so low (less than 10 2 10 1 mJ/m 2 ) that it is completely compensated by the entropy factor. This is possible at temperatures close to the so-called critical mixing temperature. In addition, colloidal surfactants and HMS solutions have the ability to reduce interfacial tension to ultra-low values, which makes it possible to obtain thermodynamically stable (spontaneously formed) emulsions even under normal conditions.

In thermodynamically stable and spontaneously formed (lyophilic) emulsions, the particles have a very high dispersion.

Most emulsions are microheterogeneous, thermodynamically unstable (lyophobic) systems. During long-term storage, agglomeration (coagulation) occurs in them, and then the droplets coalesce (coalescence).

Aggregative stability of emulsions is quantitatively characterized by the rate of their stratification. It is determined by measuring the height (volume) of the exfoliated phase at certain time intervals after obtaining the emulsion. Without an emulsifier, the stability of emulsions is usually poor. Known methods of stabilization of emulsions using surfactants, IUDs, powders. Stabilization of emulsions with surfactants is provided due to adsorption and a certain orientation of surfactant molecules, which causes a decrease in surface tension.

The orientation of surfactants in emulsions follows Rebinder's polarity equalization rule: the polar groups of the surfactant face the polar phase, and the nonpolar radicals face the nonpolar phase. Depending on the type of surfactant (ionic, nonionic), emulsion droplets acquire an appropriate charge or adsorption-solvation layers appear on their surface.

If the surfactant is better soluble in water than in oil (oil is the common name for the non-polar phase in emulsions), a direct o/w emulsion is formed, if its solubility is better in oil, then an inverse o/o emulsion is obtained. (Bancroft's rule). Changing the emulsifier may cause the emulsion to reverse. So, if a solution of calcium chloride is added to an o/w emulsion stabilized with sodium soap, then the emulsifier turns into a calcium form and the emulsion reverses, i.e., the oil phase becomes a dispersion medium. This is because calcium soap is much more soluble in oil than in water.

Stabilization of inverse emulsions with surfactants is not limited to factors due to a decrease in surface tension. Surfactants, especially those with long radicals, can form films of significant viscosity (structural-mechanical factor) on the surface of emulsion droplets, as well as provide entropy repulsion. Structural-mechanical and entropy factors are especially significant if surface-active macromolecular compounds are used for stabilization. Structural-mechanical factor - the formation of an adsorption film structured and extremely solvated by a dispersion medium is of great importance for the stabilization of concentrated and highly concentrated emulsions. Thin structured interlayers between drops of a highly concentrated emulsion give the system pronounced solid-like properties.

Stabilization of emulsions is also possible with the help of highly dispersed powders. Their mechanism of action is similar to that of surfactants. Powders with a sufficiently hydrophilic surface (clay, silica, etc.) stabilize direct emulsions. Hydrophobic powders (soot, hydrophobized aerosil, etc.) are capable of stabilizing reverse emulsions. Powder particles on the surface of emulsion droplets are located in such a way that most of their surface is in the dispersion medium. To ensure the stability of the emulsion, a dense powder coating of the surface of the drop is necessary. If the degree of wetting of the particles of the stabilizer powder by the medium and the dispersed phase differs greatly, then the entire powder will be in the volume of the phase that wets it well, and it obviously will not have a stabilizing effect.

A direct emulsion stabilized with ionic emulsifiers can be destroyed by adding electrolytes with polyvalent ions. Such electrolytes cause not only compression of the electrical double layer, but also convert the emulsifier into a form that is poorly soluble in water. The emulsifier can be neutralized with another emulsifier that promotes the formation of inverse emulsions. You can add a substance more surface-active than the emulsifier, which itself does not form strong films (the so-called demulsifier). For example, alcohols (pentyl and others) displace emulsifiers, dissolve their films and promote coalescence of emulsion droplets. The emulsion can be destroyed by increasing the temperature, placing it in an electric field, settling, centrifuging, filtering through porous materials that are wetted by the dispersion medium, but not wetted by the substance of the dispersed phase, and in other ways.

CHAPTER XIV. STRUCTURAL AND MECHANICAL PROPERTIES OF DISPERSIVE SYSTEMS

§one. Basic concepts and ideal laws of rheology

The most important mechanical properties are viscosity, elasticity, plasticity, strength. Since these properties are directly related to the structure of bodies, they are usually called structural-mechanical.

Structural and mechanical properties of systems are studied by methods rheology – sciences about deformations and flow of material systems. Rheology studies the mechanical properties of systems by the manifestation of deformation under the action of external stresses. In colloid chemistry, rheological methods are used to study the structure and describe the viscous properties of dispersed systems.

Termdeformation means the relative displacement of the points of the system, at which its continuity is not violated. The deformation is divided into elastic and residual. With elastic deformation, the structure of the body is completely restored after the removal of the load (stress); residual deformation is irreversible, changes in the system remain even after the load is removed. Residual deformation, at which the body does not break, is called plastic.

Among elastic deformations, volume (tension, compression), shear and torsional deformations are distinguished. They are characterized by quantitatively relative (dimensionless) values. For example, with one-dimensional deformation, tension is expressed in terms of relative elongation:

where l 0 and l– body length before and after stretching, respectively; Δ l- absolute elongation.

Shear strain is determined by absolute shear (absolute strain) y and relative shift (fig.34) under voltage R:

(XIV.1)

where y - displacement of the upper layer (absolute deformation); X - the height over which the displacement occurs, – shear angle. .

As follows from Fig. 34, the relative shift is equal to the tangent of the shift angle , which, in turn, is approximately equal to the angle itself , if it is small and the value of this angle is expressed in radians.

Fig.34. Schematic representation of shear deformation

Liquids and gases are deformed when minimal loads are applied; under the influence of a pressure difference, they flow. The flow is one of the types of deformation, in which the amount of deformation continuously increases under the influence of a constant pressure (load). Unlike gases, liquids do not compress during flow and their density remains almost constant.

Voltage (R ), which causes deformation of the body, is determined by the ratio of the force to the area on which it acts. The acting force can be decomposed into two components: normal, directed perpendicular to the surface of the body, and tangential (tangential), directed tangentially to this surface. Accordingly, two types of stresses are distinguished: normal and tangential, which correspond to two main types of deformation: tension (or compression) and shear. Other types of deformation can be represented using various combinations of these basic types of deformations. The SI unit of voltage is the pascal ( Pa).

Any material system has all the rheological properties . The main ones, as already mentioned, are elasticity, plasticity, toughness and strength. All these properties are manifested in shear deformation, which is therefore considered the most important in rheological studies.

Thus, the nature and magnitude of the deformation depend on the properties of the material of the body, its shape and the method of application of external forces.

In rheology, the mechanical properties of materials are represented in the form of rheological models, which are based on three basic ideal laws that relate stress to deformation. They correspond to three elementary models (elements) of idealized materials that meet the main rheological characteristics (elasticity, plasticity, viscosity): Hooke's ideally elastic body, Newton's ideally viscous body (Newtonian fluid) and Saint-Venant-Coulomb's ideally plastic body.

Hooke's ideally elastic body represent in the form of a spiral spring (Fig. 35). In accordance with Hooke's law deformation in an elastic body is proportional to the shear stress R:

or
(XIV.2)

where G- proportionality factor, or shear modulus.

Shear modulus G is a characteristic of the material (its structure), quantitatively reflecting its elastic properties (stiffness). From equation (XIV.2) it follows that the unit of shear modulus is pascal (SI), i.e. the same as for voltage, since the value γ dimensionless. The shear modulus can be determined from the cotangent of the slope of the straight line characterizing the dependence of the deformation γ from shear stress R(see Fig. 35, b). The modulus of elasticity for molecular crystals is ~ 10 9 Pa, for covalent crystals and metals - 10 11 Pa and more. After the load is removed, Hooke's ideally elastic body instantly returns to its original state (shape).

Fig.35. Hooke's ideal elastic body model (a) and the dependence of the deformation of this body on the shear stress (b)

Newton's ideally viscous body depicted as a piston with holes placed in a cylinder with liquid (Fig. 36). An ideally viscous fluid flows in accordance with Newton's law . According to this law, the shear stress in a laminar fluid flow is proportional to the gradient of the absolute shear rate (absolute deformation) dU/ dx:

(XIV.3),

where η – proportionality factor, called dynamic viscosity (dynamic viscosity is also sometimes denoted by the letter symbol  ).

With plane-parallel (laminar) motion of two layers of fluid, one layer shifts relative to the other. If the rate of absolute shear of fluid layers is denoted by U= dy/ dt and take into account that the coordinate X and time t are independent variables, then by changing the order of differentiation, taking into account (XIV.1), we can obtain the following relation:

(XIV.4)

where
is the relative shear strain rate.

Thus, Newton's law can also be stated as follows: shear stress is proportional to the relative strain rate:

(XIV.5)

The rheological properties of ideal fluids are uniquely characterized by viscosity. Its definition is given by equations (XIV.3) and (XIV.5). dependency graph P – is a straight line emerging from the origin, the cotangent of the angle of inclination of this straight line to the x-axis determines the viscosity of the liquid. The reciprocal of viscosity is called fluidity. If viscosity characterizes the resistance of a fluid to movement, then fluidity characterizes its mobility.

Fig.36. Model of an ideally viscous Newton's fluid (a) and dependence of the strain rate of this fluid on shear stress (b)

The units of viscosity follow from Equation (XIV.5). Since in the international system of units, stress is measured in pascals, and the relative strain rate in with -1 , then the unit of viscosity will be pascal second ( Pass). In the CGS system, the poise is taken as the unit of viscosity ( P) (1 Pass = 10 P). The viscosity of water at 20.5°C is 0.001 Pass or 0.01 P, i.e. 1 centipoise ( sp). Viscosity of gases is about 50 times less, for highly viscous liquids the viscosity values can be thousands and millions of times higher, and for solids it can be 10 15 -10 20 Pass and more. The dimension of fluidity is the inverse of the dimension of viscosity, therefore, the units of viscosity are the inverse of the units of fluidity. For example, in the CGS system, fluidity is measured in poise to the minus one power ( P -1 ).

The model of the ideally plastic body of Saint-Venant - Coulomb is a solid body located on a plane, during the movement of which the friction is constant and does not depend on the normal (perpendicular to the surface) force (Fig. 37). This model is based on the law of external (dry) friction, according to which there is no deformation if the shear stress is less than a certain value R* , called the yield strength, i.e. at

PP*

If the stress reaches the yield point, then the developed deformation of an ideally plastic body has no limit, and the flow occurs at any speed, i.e., at

P= P* >0 >0

This dependence is shown in Fig. 37, b. It follows from it that a stress exceeding P*. Value P* reflects the strength of the body structure. Given that R = P* the structure of an ideal plastic body is destroyed, after which the resistance to stress is completely absent.

Comparison of ideal elements (rheological models) shows that the energy expended on the deformation of Hooke's elastic body returns during unloading (after the termination of the stress), and when the viscous and plastic bodies are deformed, the energy is converted into heat. In accordance with this, Hooke's body belongs to conservative systems, and the other two belong to dissipative (energy-losing) systems.

This section discusses the phenomena and processes caused by aggregative stability dispersed systems.

Lyophobic systems, by definition, must have an excess of surface energy if it is not compensated by the introduction of stabilizers. Therefore, processes of particle enlargement spontaneously take place in them, i.e. there is a decrease in surface energy due to a decrease in specific surface area. Such systems are called aggregatively unstable.

Particle enlargement can proceed in different ways. One of them, called isothermal distillation , consists in the transfer of matter from small particles to large ones (the Kelvin effect). As a result, small particles gradually dissolve (evaporate), while large particles grow.

The second way, the most characteristic and common for disperse systems, is coagulation (from Lat, coagulation, hardening), which consists in the adhesion of particles.

Coagulation in dilute systems also leads to loss of sedimentation stability and, ultimately, phase separation.

The process of particle fusion is called coalescence .

In concentrated systems, coagulation can manifest itself in the formation of a three-dimensional structure in which the dispersion medium is evenly distributed. In accordance with the two different results of coagulation, the methods for monitoring this process also differ. The enlargement of the particles leads, for example, to an increase in the turbidity of the solution, a decrease in the osmotic pressure. Structure formation changes the rheological properties of the system, its viscosity increases, and the flow slows down.

The reverse process of formation of a stable free-dispersed system from a precipitate or gel (structured disperse system) is called peptization.

A deeper process of coagulation leads to the destruction of the interlayers of the medium and direct contact of the particles. As a result, either rigid aggregates of solid particles are formed, or they completely merge in systems with a liquid or gaseous dispersed phase (coalescence). In concentrated systems, rigid bulk solid structures are formed, which can only be converted back to a free-dispersed system by forced dispersion. Thus, the concept of coagulation includes several processes that occur with a decrease in the specific surface area of the system.

Fig.33. Processes causing loss of stability of disperse systems.

The aggregative stability of unstabilized lyophobic disperse systems is of a kinetic nature, and it can be judged by the rate of processes caused by excess surface energy.

The rate of coagulation determines the aggregative stability of a dispersed system, which is characterized by the process of adhesion (fusion) of particles.

In accordance with the above classification, thermodynamic and kinetic factors of the aggregative stability of disperse systems are distinguished. Since the driving force of coagulation is excess surface energy, the main factors that ensure the stability of disperse systems (while maintaining the size of the surface) will be those that reduce surface tension. These factors are referred to as thermodynamic. They reduce the probability of effective collisions between particles, create potential barriers that slow down or even exclude the coagulation process. The lower the surface tension, the closer the system is to thermodynamically stable.

The rate of coagulation also depends on kinetic factors.

There are the following thermodynamic and kinetic stability factors for dispersed systems.

There are other mechanisms for the formation of a double electric layer. For example, DES is formed at the interface between water and poorly soluble silver iodide. If a highly soluble silver nitrate is added to water, then the silver ions formed as a result of dissociation can complete the crystal lattice of AgI, because they are included in its composition (specific adsorption of silver ions). As a result, the surface of the salt is positively charged (an excess of silver cations), and iodide ions will act as counterions.

We should also mention the possibility of the formation of a double electric layer as a result of the transition of ions or electrons from one phase to another (surface ionization).

The so-called electrokinetic phenomena (electrophoresis, electroosmosis, etc.) are due to the presence of a double electric layer at the phase boundary.

2. Adsorption-solvation factor is to reduce the interfacial

tension during the introduction of surfactants (due to adsorption and solvation).

3. entropy factor, like the first two, refers to thermodynamic. It complements the first two factors and acts in systems in which particles participate in thermal motion. The entropy repulsion of particles can be represented as the presence of a constant diffusion of particles from a region with a higher concentration to a region with a lower concentration, i.e. the system constantly tends to equalize the concentration of the dispersed phase throughout the volume.

4. Structural-mechanical factor is kinetic. Its action is due to the fact that films with elasticity and mechanical strength can form on the surface of the particles, the destruction of which requires energy and time.

5. hydrodynamic factor reduces the rate of coagulation due to a change in the viscosity and density of the dispersion medium in thin layers of liquid between the particles of the dispersed phase.

Usually, aggregative stability is provided by several factors simultaneously. Particularly high stability is observed under the combined action of thermodynamic and kinetic factors.

thermodynamic kinetic

(↓) .(↓coagulation rates due to the hydrodynamic properties of the medium)

a) electrostatic factor - ↓ due to a) hydrodynamic factor

formation of DES

b) adsorption-solvation factor - ↓ b) structural-mechanical

due to adsorption and solvation of the surface factor

c) entropy factor

Thermodynamic factors:

Electrostatic factor contributes to the creation of electrostatic repulsive forces, which increase with an increase in the surface potential of the particles, and especially the ζ-potential.

Adsorption-solvation factor due to a decrease in the surface of the particles as a result of solvation. In this case, the surface of the particles is liphilic in nature or due to the adsorption of non-electrolyte stabilizers. Such systems can be aggregatively stable even in the absence of a potential on the particle surface.

It is possible to lyophilize lyophobic systems by adsorbing on their surface molecules with which their medium interacts. These are surfactants, HMS, and in the case of emulsions, finely dispersed powders wetted by the medium.

The adsorption of such substances is accompanied by solvation and orientation of molecules in accordance with the polarity of the contacting phases (Rehbinder's rule). Surfactant adsorption leads to a decrease in the surface Gibbs energy and, thereby, to an increase in the thermodynamic stability of the system

entropy factor plays a special role in systems with small particles, since due to the Brownian motion, the particles of the dispersed phase are uniformly distributed over the volume of the system. As a result, the randomness of the system increases (its randomness is less if the particles are in the form of a sediment at the bottom of the vessel), as a result, its entropy also increases. This leads to an increase in the thermodynamic stability of the system, achieved by reducing the total Gibbs energy. Indeed, if during any process S > 0, then according to the equation

G = H - TS,

such a process occurs with a decrease in the Gibbs energy G

Kinetic factors:

Structural-mechanical stability factor occurs during the adsorption of surfactants and HMS on the surface of particles, which lead to the formation of adsorption layers with enhanced structural and mechanical properties. These substances include: long-chain surfactants, most IUDs, such as gelatin, casein, proteins, soaps, resins. Concentrating on the surface of the particles, they can form a gel-like film. These adsorption layers are, as it were, a barrier to the approach of particles and their aggregation.

The simultaneous decrease in surface tension in this case leads to the fact that this factor becomes universal for the stabilization of all disperse systems.

The hydrodynamic stability factor manifests itself in highly viscous and dense dispersion media, in which the velocity of particles of the dispersed phase is low and their kinetic energy is not enough to overcome even a small potential repulsion barrier.

In real colloidal systems, several thermodynamic and kinetic stability factors usually act simultaneously. For example, the stability of polystyrene latex micelles (see Chapter 5) is provided by ionic, structural-mechanical, and adsorption-solvation stability factors.

It should be noted that each sustainability factor has its own specific method of neutralizing it. For example, the effect of the ionic factor is significantly reduced by the introduction of electrolytes. The action of the structural-mechanical factor can be prevented with the help of substances - the so-called. demulsifiers(these are usually short-chain surfactants) that thin the elastic structured layers on the surface of the particles, as well as by mechanical, thermal and other methods. As a result, there is a loss of aggregative stability of systems and coagulation.

Mechanisms of action of stabilizers

Stabilizers create a potential or structural-mechanical barrier on the way of particles sticking together, and with its sufficient height, a thermodynamically unstable system can exist for a long time for purely kinetic reasons, being in a metastable state.

Let us consider in more detail the electrostatic factor of stability or the ionic factor of stabilization of disperse systems.

6.3. Ion factor of stabilization of dispersed systems

Theory of stability of lyophobic sols DLVO

Adsorption, electrostatic and a number of other theories of stability and coagulation could not explain a number of facts observed for dispersed systems. Their most important provisions have become an integral part of the modern theory of stability, which is in good agreement with the behavior of typically lyophobic sols.

The formation of DEL leads, on the one hand, to a decrease in interfacial tension, which increases the thermodynamic stability of systems, and, on the other hand, creates a potential barrier of electrostatic repulsion on the way of particle aggregation, causing the so-called. ionic (electrostatic) stability factor.

Consider the nature of this barrier. According to the theory of stability of hydrophobic colloids Deryagin (*) , Landau (*) , Verweya (*) , Overbeck (*) (DLVO theory), between particles having a DEL, attractive and repulsive forces act. The repulsive forces are caused by the disjoining pressure: when the particles approach, the diffuse parts of the DEL overlap, and the concentration of counterions between the particles becomes higher than inside the phase. There is a flow of the dispersion medium into the space between the particles, which tends to separate them. This flow creates disjoining pressure. According to the DLVO theory, the particle repulsion energy is expressed by the equation:

The modern physical theory of stability was developed by Russian scientists Deryagin and Landau (1937) and received universal recognition. Somewhat later (1941), a theoretical development that led to the same results was carried out by the Dutch scientists Verwey and Overbeck. In accordance with the first letters of the authors, the theory of stability is known as the theory DLFO(DLVO).

Interfacial surface tension of dispersed systems is not the only reason for aggregative stability. When similarly charged sol particles approach each other, their diffuse layers overlap. This interaction takes place in a thin layer of the dispersion medium that separates the particles.

Stability of lyophobic sols determined by the special properties of these liquid layers. The thinning of this layer ends either with its rupture at a certain small thickness, or with the achievement of a certain equilibrium thickness, which does not decrease further. In the first case, the particles stick together, in the second they do not.

The thinning of the thin layer occurs by flowing out of its liquid. When the liquid layer becomes thin (100 - 200 nm), the properties of the liquid in it begin to differ greatly from the properties of the liquid in the volume. Appears in the layer extra pressure , which Deryagin called "disjoining pressure" π.

The disjoining pressure is the excess pressure that must be applied to the surfaces bounding a thin film so that its thickness remains constant or can be reversibly changed in a thermodynamically equilibrium process.

positive disjoining pressure occurs when:

"+" P in layer 0. This prevents liquid from flowing out of it, i.e. the approach of particles;

"disjoining pressure", i.e. spreads, wedges:

Negative disjoining pressure π

"-" when the pressure in the layer increases, which contributes to the convergence of particles

Let us consider the cases of approaching the particles of the dispersed phase at different distances:

No disjoining pressure, h > 2δ

(diffuse layer thickness)

R o R o "+" - R

In a thin layer

"-" - liquid will flow out of the gap, and

P P particles approach

Fig.6.1. Formation of disjoining pressure in thin layers

Before overlapping diffuse layers, the energy E of free dispersed systems was unchanged, and P in the gap = P o (pressure inside the free liquid).

After overlapping, the free energy changes, and in the liquid layer, a R.d. directed towards the contacting bodies appears.

The concept of disjoining pressure is one of the fundamental disperse systems in physical chemistry. Disjoining pressure always occurs when a thin layer of liquid forms between the particles of the dispersed phase (solid, liquid or gaseous). In a layer of water 1 µm thick enclosed between two surfaces of mica, the disjoining pressure is 430 Pa. With a water layer thickness of 0.04 µm, the disjoining pressure is significantly higher and amounts to 1.8810 4 Pa.

Optical and, above all, interoferometric methods are usually used to study the film structure and measure its thickness.

The intensity I of the reflected light due to interference depends in a complex way on the ratio of the film thickness to the length of the incident light wave.

1/4 3/4 5/4 7/4 h/λ

Rice. 6.2. Dependence of I of the reflected monochromatic light on the relative thickness of the film.

For thick films: h=(k+½)λ/2n.

k is the order of interference

n is the refractive index.

In white light, thin films are colored in different colors. Thin films with h≤ λ/10 appear gray in reflected light, while thinner films appear black.

For gray and black films, the measurement of the intensity I makes it possible to determine their h, and the dependence I=f(t) determines the thinning kinetics.

The repulsive forces in thin films are electrostatic in nature: a colloidal system consisting of water, proteins ... uses the achievements of organic, inorganic and analytical chemistry, processes and devices of chemical and...

Proteins and nucleic acids

Study Guide >> Chemistry

TECHNOLOGIES OF HIGH MOLECULAR COMPOUNDS BIOLOGICAL CHEMISTRY ABSTRACT LECTURES for students of specialties 49 ... under certain conditions, protein solutions form colloidal systems - gels or jellies ... water surrounding a thick layer colloidal protein particles...

Environmental aspects of teaching the topic P-elements in the classroom chemistry and ecology

Coursework >> Pedagogy

holding lectures on the topic, the teacher, together with the students, draws up reference abstracts. ... reactions with homework. Colloidal Particles Lab Experience 21...in chemistry. Moscow: Enlightenment, 1981, 192 p. Rudzitis G. E. Chemistry: Inorganic chemistry. Organ. Chemistry: Tutorial...

Fundamentals of Ecology (10)

Abstract >> Ecology

The need for preparation and publication abstract lectures, which may be ... or brown, enriched colloidally- dispersed minerals. The underlying horizon... settling tanks, centrifugation, filtering. Khim, physical chem and biol cleaning. flotation...

Theory chemistry. organic and inorganic chemistry and teaching methods

Synopsis >> Chemistry

... chemistry. Har-ka struct. e-comrade with-we. In l-s-me lecture... pinning at the end lectures. Didactic conditions lectures: highly focused lectures, heightened awareness, ... water, in hot water forms colloidal solution. Starch macromolecules are built...

There are thermodynamic and kinetic stability factors,

To thermodynamic factors include electrostatic, adsorption-solvation and entropy factors.

electrostatic factor due to the existence of a double electric layer on the surface of particles of the dispersed phase. The main components of the electrostatic factor are the similar charge of the granules of all colloidal particles, the value of the electrokinetic potential, as well as a decrease in interfacial surface tension due to the adsorption of electrolytes (especially in cases where ionogenic surfactants are electrolytes).

The same electric charge of the granules leads to mutual repulsion of approaching colloidal particles. Moreover, at distances exceeding the diameter of micelles, electrostatic repulsion is due mainly to the charge of counterions in the diffuse layer. If fast moving particles collide with each other, then the counterions of the diffuse layer, being relatively weakly bound to the particles, can move, and as a result, the granules come into contact. In this case, the main role in the repulsive forces is played by the electrokinetic potential. Namely, if its value exceeds 70 - 80 mV, then particles colliding with each other as a result of Brownian motion will not be able to overcome the electrostatic barrier and, having collided, will disperse and aggregation will not occur. The role of surface tension as a thermodynamic stability factor was discussed in Chapter 1.

Adsorption-solvation factor associated with hydration (solvation) of both the particles of the dispersed phase and adsorbed on their surface ions or uncharged surfactant molecules. Hydration shells and adsorption layers are bound to the particle surface by adhesion forces. Therefore, for the direct contact of the aggregates, the colliding particles must have the energy necessary not only to overcome the electrostatic barrier, but also to exceed the work of adhesion.

entropy factor consists in the tendency of the dispersed phase to a uniform distribution of the particles of the dispersed phase over the volume of the system as a result of diffusion. This factor manifests itself mainly in ultramicroheterogeneous systems, the particles of which participate in intense Brownian motion.

To kinetic factors stability include structural-mechanical and hydrodynamic factors.

Structural-mechanical factor due to the fact that the hydrated (solvate) shells existing on the surface of the particles have increased viscosity and elasticity. This creates an additional repulsive force when particles collide - the so-called disjoining pressure. The elasticity of the adsorption layers themselves also contributes to the disjoining pressure. The doctrine of disjoining pressure was developed by BV Deryagin (1935).

hydrodynamic factor is related to the viscosity of the dispersion medium. It reduces the rate of destruction of the system by slowing down the movement of particles in a medium with high viscosity. This factor is least pronounced in systems with a gaseous medium, and its greatest manifestation is observed in systems with a solid medium, where particles of the dispersed phase are generally devoid of mobility.

Under real conditions, the stability of dispersed systems is usually ensured by several factors simultaneously. The highest stability is observed under the combined action of both thermodynamic and kinetic factors.

Each stability factor corresponds to a specific method of its neutralization. For example, the action of the structural-mechanical factor can be removed with the help of substances that thin and dissolve the elastic structured layers on the surface of the particles. Solvation can be reduced or completely eliminated by lyophobization of particles of the dispersed phase during the adsorption of the corresponding substances. The action of the electrostatic factor is significantly reduced when electrolytes are introduced into the system, which compress the DEL. This last case is the most important both in the stabilization and in the destruction of dispersed systems.

thermodynamic kinetic

(↓ ).(↓coagulation rates due to the hydrodynamic properties of the medium)

a) electrostatic factor - ↓ due to a) hydrodynamic factor

formation of DES

b) adsorption-solvation factor - ↓ b) structural-mechanical

due to adsorption and solvation of the surface factor

c) entropy factor

Thermodynamic factors:

Electrostatic factor contributes to the creation of electrostatic repulsive forces, which increase with an increase in the surface potential of the particles, and especially ζ- potential.

G = H - TS,

such a process occurs with a decrease in the Gibbs energy G<0.

Kinetic factors:

Structural-mechanical factor sustainability occurs during the adsorption of surfactants and HMS on the surface of particles, which lead to the formation of adsorption layers with enhanced structural and mechanical properties. These substances include: long-chain surfactants, most IUDs, such as gelatin, casein, proteins, soaps, resins. Concentrating on the surface of the particles, they can form a gel-like film. These adsorption layers are like a barrier to the approach of particles and their aggregation.

The simultaneous decrease in surface tension in this case leads to the fact that this factor becomes universal for the stabilization of all disperse systems.

Mechanisms of action of stabilizers

Let us consider in more detail the electrostatic factor of stability or the ionic factor of stabilization of dispersed systems.