Structurally, the mechanical stability factor lies in. Moscow State University of Printing Arts

Candidate of Chemical Sciences, Associate Professor

Topic 2. Properties dispersed systems,

their stability and coagulation

Lesson 2. Stability of dispersed systems

L e to c and I

Saratov - 2010

If the AgCl precipitate is obtained in excess of AgNO3, then the colloidal micelle will have a different structure. Potential-determining Ag+ ions will be adsorbed on the AgCl aggregate, and NO3– ions will be counterions.

For water-insoluble barium sulfate (obtained in excess of BaCl2), the structure of a colloidal particle can be represented by the formula:

BaCl2(e.g.) + NaSO4 ® BaSO4(s.f.) + 2NaCl

In an electric field, a positively charged granule will move towards a negatively charged cathode.

2. PHYSICAL THEORY OF STABILITY AND COAGULATION

Colloidal stability – the ability of a dispersed system to keep its composition unchanged (concentration of the dispersed phase and particle size distribution), as well as appearance: color, transparency, "uniformity".

The sharp difference in terms of stability between the two classes of colloids should be pointed out: freeze-dried And lyophobic . Lyophilic colloids have a high affinity for the dispersion medium, they spontaneously disperse and form thermodynamically stable colloidal solutions. In lyophobic colloids, the degree of affinity for the solvent is much lower, their dispersions are thermodynamically unstable and are characterized by high values ​​of surface tension at the interface. It is the stability and coagulation of lyophobic sols that we will study.

Colloidal stability is conditionally classified into sedimentation (kinetic) and aggregative .

Sedimentation resistance is determined by the ability of the system to counteract the settling of particles. Sedimentation, or settling of particles, leads to the destruction of the dispersed system. The dispersed system is considered sedimentation resistant , if its dispersed particles do not settle, the system does not separate into phases, i.e., it is in a stable diffusion-sedimentation equilibrium.

Sedimentation stability primarily depends on the particle size of the dispersed phase. If their size is less than 1000 nm, then the system usually has a high sedimentation stability. In the case of larger particles, the system is unstable, i.e., it separates over time, particles of the dispersed phase either float or form a precipitate.

Aggregative stability is determined by the ability of the dispersed system to counteract the adhesion of particles, that is, to keep the particle sizes of the dispersed phase unchanged. But due to the desire of systems to "get rid" of free energy (in this case, from surface energy), particles of the dispersed phase are prone to enlargement by merging or recrystallization.

Under coagulation understand the loss of aggregative stability of the disperse system, which consists in sticking and merging of particles.

If the particle sizes of the dispersed phase are constant and do not change with time, then colloidal-dispersed systems can maintain sedimentation stability indefinitely. Enlargement of particles in a dispersed system (loss of aggregative stability) leads to a violation of sedimentation stability and precipitation.

Quantitative ratios characterizing the stability of lyophobic sols in satisfactory agreement with experiment were obtained on the basis of the physical theory of stability and coagulation.

Physical Theory of Stability and Coagulation (DLFO)

In the most general view this theory was developed by Soviet scientists also in 1999, and somewhat later, independently of them, by the Dutch scientists Verwey and Overbeck. According to the first letters of the names of these scientists, the theory is called the DLVO theory.

At the heart of the theory of stability colloidal systems there must be a relationship between the forces of attraction and repulsion of particles. The DLVO theory takes into account electrostatic repulsion between particles and intermolecular attraction.

Electrostatic repulsion between like-charged particles occurs if they approach each other at a sufficiently close distance, their double electrical layers overlap and repel each other.

a) there is no repulsion b) the particles are repelled

(DES do not overlap) (DES overlap)

As a result of rather complex calculations (which we omit), we obtain expressions for the energy of electrostatic repulsion of particles. In accordance with this expression, the repulsive energy of particles increases with decreasing distance between them according to the exponential law

where Ue is the repulsion energy;

c is the potential on the particle surface;

h is the distance between particles.

The second kind of forces affecting the stability of a sol are the forces of attraction between particles. They are of the same nature as the forces acting between neutral molecules. Van der Waals explained the properties of real gases and liquids by the existence of these forces. The emergence of intermolecular forces is due to the interaction of dipoles (the Keeson effect), the polarization of one molecule of another (the Debye effect) and the dispersion forces of London, which are associated with the presence of instantaneous dipoles in neutral atoms and molecules.

The most universal component of the molecular forces of attraction is the dispersion component. Calculations carried out by Hamaker led to the following expression for the energy of molecular attraction (for parallel plates located at small distances from each other).

Rice. 2. Potential curves

The shape of the curves of the total interaction energy of particles depends on the potential on their surface, on the value of the Hamaker constant, on the particle size and shape. Therefore, depending on all these factors, there are three most characteristic species potential curves corresponding to certain states of aggregative stability (Fig. 3).

Rice. 3. Potential curves for dispersed systems

with different aggregative stability

Curve 1 corresponds to such a state of the system, in which, at any distance between particles, the energy of attraction prevails over

repulsive energy. The system is unstable, quickly coagulates.

Curve 2 indicates the presence of a sufficiently high potential barrier and a secondary minimum. In this case, floccules are easily formed, in which the particles are separated by interlayers of the medium. This occurs in the secondary minimum. This state corresponds to the reversibility of coagulation. Under certain conditions, the potential barrier can be overcome and irreversible coagulation occurs in the primary minimum.

Curve 3 corresponds to the state of the system with a high potential barrier in the absence of the second minimum. Such systems have a high aggregative stability.

3. MAIN REGULARITIES OF COAGULATION

ACTION OF ELECTROLYTES

The cause of coagulation can be the action of heat and cold, electromagnetic fields, hard radiation, mechanical effects, chemical agents.

The most common cause of coagulation is the action of the electrolyte.

Electrolytes change the structure of the DEL, reduce the zeta potential (either due to the adsorption of electrolyte ions on the particles, or due to compression of the diffuse part of the DEL), which leads to a decrease in the electrostatic repulsion between particles. According to the DLVO theory, as a result, particles can approach each other at distances at which attractive forces predominate, which can cause them to stick together and coagulate.

Coagulation threshold (denoted by Ck, g) is the minimum electrolyte concentration that causes a certain visible coagulation effect (discoloration, turbidity, sedimentation) over a certain period of time. The coagulation threshold is determined either visually, by observing changes in the disperse system when electrolyte solutions of different concentrations are introduced into it, or changes are recorded using appropriate instruments, most often by measuring the optical density or turbidity of the system.

Coagulation is subject to certain rules. Let's consider them.

coagulation rules

– Coagulation is caused by any electrolytes, if their concentration

in the system will exceed a certain minimum, called the coagulation threshold. The reason is the compression of the DES. The coagulation threshold for different electrolytes and different dispersed systems is different.

- Only that electrolyte ion has a coagulating effect, the charge of which is opposite to the charge of the colloidal particle, and its coagulating ability is expressed the stronger, the higher the valency of the counterion. This pattern is called the Schulze-Hardy rule. In accordance with this rule, the ratio of coagulation thresholds is one; two - and trivalent counterions is as follows:

For example, for arsenic sulfide sol As2S3, whose particles have a negative charge, the coagulation thresholds of various electrolytes have the following value: LiCl - 58 mmol/l; MgCl2 - 0.71 mmol/l, AlCl3 - 0.043 mmol/l.

In a series of organic ions, the coagulating effect increases with an increase in adsorption capacity, and, consequently, with charge neutralization.

- In a series of inorganic ions with the same charge, their coagulating activity increases with a decrease in their hydration (or with an increase in radius). For example, in a series of monovalent cations and anions, the coagulating activity and hydration change as follows:

Aluminum" href="/text/category/aluminij/" rel="bookmark">aluminum, silicon, iron).

3. entropy factor , like the first two, they are thermodynamic. It operates in systems in which particles or their surface forces participate in thermal motion. Its essence lies in the tendency of the dispersed phase to uniform distribution over the volume of the system, and this reduces the likelihood of particle collision and their sticking together.

Entropy repulsion can be explained based on the direct interaction of particles with surface layers, in which there are mobile counterions or long and flexible radicals of surface-active substances (surfactants) and macromolecular compounds (HMCs). Such radicals have many conformations. The approach of particles leads to a decrease in degrees of freedom or conformations, and this leads to a decrease in entropy, and, consequently, to an increase in free surface energy, which is a thermodynamically unfavorable process. Thus, this factor contributes to the repulsion of particles.

4. Structural-mechanical factor is kinetic. Its action is due to the fact that on the surface of the particles there are films with elasticity, the destruction of which requires energy and time. Typically, such a film is obtained by introducing stabilizers into the system - surfactants and Navy (colloidal protection). Surface layers acquire high strength characteristics due to the interweaving of IUD chains and long-chain surfactants, and sometimes as a result of polymerization.

The action of structural-mechanical and other factors is manifested in such a phenomenon as colloid protection

Colloidal protection is called increasing the stability of colloidal systems due to the formation of an adsorption layer on the surface of the particles when certain macromolecular substances are introduced into the sol .

Substances capable of providing colloidal protection are proteins, carbohydrates, pectins, and for systems with a non-aqueous dispersion medium, rubber. Protective substances are adsorbed on the surface of dispersed particles, which helps to reduce the surface energy of the system. This leads to an increase in its thermodynamic stability and provides colloidal stability. Such systems are so stable that they acquire the ability to form spontaneously. For example, instant coffee is a finely ground coffee powder treated with food grade surfactants.

To assess the stabilizing effect of various substances, conditional characteristics are introduced: “golden number”, “ruby number”, etc.

golden number is the minimum mass (in mg) of a stabilizing agent that is able to protect 10 ml of red gold hydrosol (prevent discoloration) from the coagulating action of 1 cm3 of 10% sodium chloride solution.

ruby number - this is the minimum mass (in mg) of a stabilizing substance that is able to protect 10 cm3 of a Congo red (Congo ruby) dye solution with a mass concentration of 0.1 kg / m3 from the coagulating effect of 1 cm3 of a 10% sodium chloride solution.

For example, the golden number of potato starch is 20. This means that 20 mg of starch, when introduced into a gold sol, prevents the coagulation of the sol when a coagulating electrolyte is added to the sol - 1 cm3 of a 10% sodium chloride solution. Without the addition of a stabilizing agent, starch, the gold sol coagulates (decomposes) instantly under such conditions.

Table 1 lists the most common numbers for some protective agents.

The protective effect is of great industrial importance. It is taken into account in the manufacture of medicines, food products, technical emulsions, catalysts, etc.

Table 1. The meanings of the most common numbers for some protective substances

protective agent	golden number	ruby number
Hemoglobin
Dextrin
Potato starch
Sodium caseinate

CONCLUSION

In today's lecture, we considered the structure of particles of the dispersed phase and the main factors affecting the stability and destruction of disperse systems. These factors must be taken into account when obtaining stable colloidal systems, such as emulsions, aerosols, suspensions, as well as in the destruction of "harmful" disperse systems formed during industrial production.

Associate Professor of the Department of Physical Education

§8. 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 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. 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 dispersed 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 dispersed systems are distinguished. Since the driving force of coagulation is excess surface energy, the main factors that ensure the stability of dispersed 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 disperse 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 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, 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.

A 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.

§9. 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 raising 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

§1. 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, viscosity 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 basic 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 international system 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 is returned upon 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.

Aggregative stability of emulsions– this is the ability to keep the size of the droplets of the dispersed phase unchanged over time, i.e., to resist coalescence. There are several factors of aggregative stability.

ELECTROSTATIC STABILITY FACTOR

EDLs are formed around the emulsion droplets and, as a result, an energy barrier arises that prevents the particles from approaching up to distances at which the forces of molecular attraction prevail over the forces of electrostatic repulsion. This stability factor is very significant for emulsions stabilized with colloidal surfactants and polyelectrolytes.

ADSORPTION-SOLVATE STABILITY FACTOR

Emulsifiers, being adsorbed on the surface of a drop, reduce the surface tension at the “droplet” boundary. – environment" and make the system more stable. But if colloidal surfactants and IUDs are used as emulsifiers, then adsorption – solvate shell, which is structured.

STRUCTURAL-MECHANICAL FACTOR

SUSTAINABILITY

On the surface of the drops, a layer of emulsifier molecules is formed, which has an increased viscosity and elasticity and prevents the drops from coalescing. This factor plays leading role if the emulsifier is HMC and non-ionic surfactants.

TYPES OF EMULSIFIERS

INORGANIC ELECTROLYTES

Inorganic electrolytes are the least effective emulsifiers. So, when adding potassium thiocyanate KNCS to a mixture of "water – oil" in a small concentration, you can get a temporary diluted emulsion of the first kind. Its relative stability can be explained by the occurrence of DZS on the water side of the interfacial surface, which is formed due to the selective adsorption of SGN . These ions create a small negative potential at the interface and the surface charge density is low. Therefore, the repulsive forces between the DEL drops are also small. This type of stabilization is too weak to obtain an emulsion of the desired concentration and with sufficient pot life.

COLLOID SURFACE-ACTIVE

SUBSTANCES

Recall that colloidal surface – active substances – amphiphilic molecules containing in their hydrocarbon radical at least 8 – 10 carbon atoms. The relationship between the hydrophilic properties of the polar group and the lipophilic (“lipos” – fat) properties of a non-polar group (hydrocarbon radical) is determined by hydrophilically– lipophilic balance– HLB number, Stabilization of emulsions by ionic colloidal surfactants is associated with adsorption and a certain orientation of surfactant molecules on the surface of droplets. In accordance with Rehbinder's polarity equalization rule surfactant polar groups face the polar phase, and non-polar radicals – to the non-polar phase. In order for a surfactant to protect a drop from merging with another, it must create a protective shell outside drops. Therefore, it should dissolve better (but not completely! 14) in the liquid that is the dispersion medium than in the liquid that makes up the drop. The solubility of surfactants is characterized

number of GLBs. The larger it is, the more the balance is shifted towards the hydro – filial properties, the better this substance is soluble in water.

Surfactants with an HLB number of 8 to 13 are better soluble in water than in oil, they form type I emulsions. Surfactants with an HLB number of 3 to 6 form emulsions of type II.

The most effective emulsifiers for the preparation of type I emulsions are sodium salts of fatty acids (soaps) with 8 carbon atoms. – 10 and above, as well as alkyl sulfates, alkyl sulfonates, etc. In the series of fatty acids, the best emulsifiers are lauric (C 11 H 20 COOH) and myristidine (C 13 H 27 COOH) acids, which, according to the Traube rule, give the greatest decrease in surface tension compared to the previous members of the homologous series.

Ionic surfactants form a double electric layer. It is essential that to prevent direct contact and coalescence of droplets, there is no need

14 If the surfactant is completely dissolved in one of the liquids, it will not be at the interface, but will go into the volume of this liquid.

in the formation of a continuous protective layer, it is sufficient if this layer occupies 40 – 60% of the drop surface.

Hydrocarbon radicals of surfactants in emulsions of the first kind go deep into the droplets, and for good vertical orientation they must consist of at least 8 – 10 carbon atoms.

The vertical orientation of nonionic surfactants on the interface leads to the formation of a layer of polar groups, which are centers of hydration – a protective hydrate layer is created.

Stabilization of inverse emulsions (W/O) with surfactants is not limited to factors that cause a decrease in surface tension. Surfactants, especially those with long radicals, can form films of considerable viscosity on the surface of water droplets (implemented structurally). – mechanical stability factor), as well as provide entropy repulsion due to the participation of radicals in thermal motion.

In cooking, natural products containing surfactants are usually used as emulsifiers: ground pepper, mustard, egg yolks, etc. In the food industry, synthetic surfactants are more often used for these purposes: oleates, propyl alcohol, fatty acid monoglycerides, sugar glycerides.

HIGH MOLECULAR SUBSTANCES

Even greater emulsion stability can be achieved using HMCs: proteins, rubber, resin, rubber, starch and other polysaccharides (eg dextrin, methyl cellulose), as well as synthetic polymers (eg polyvinyl alcohol). Unlike soaps, long valuable molecules of these substances with a uniform distribution of polar groups are located horizontally in the plane of the section "drop – environment”, where they can easily intertwine with the formation of two-dimensional structures. Adsorption of macromolecular compounds is usually slow and almost irreversible. Some proteins, when adsorbed, become insoluble in water. If such layers are compressed, they are destroyed with the formation of microscopic deposits, which remain on the interfacial surface in the form of a strong elastic shell. It is clear that a drop, being in such a "capsule", is infinitely stable against coalescence, but the quantitative patterns of this phenomenon are unknown. A high molecular weight emulsifier that forms an elastic gel can be considered effective: it swells in the continuous phase, and attempts to compress this gel are hindered by large osmotic forces (swelling pressure).

Thus, when used as emulsifiers, HMC is primarily implemented structurally – mechanical stability factor – a structured strong film is formed on the surface of the drop. In the case of highly concentrated emulsions, in which drops are in the form of polyhedrons, and the medium is in the form of thin layers between them, these layers are at the same time structured protective shells, they give the whole system pronounced solid properties.

Many HMS contain ionogenic groups and decompose in solutions to form polyions. group – COOH, for example, contain alginates, soluble starch, a group – OSO 2 – agar. Polyelectrolytes can simultaneously – must contain both acidic and basic groups. Their prominent representatives are proteins containing groups – UNSD and – NH2. In these cases, in addition to the above-mentioned structural – an electrostatic factor is added to the mechanical stability factor.

In the food industry, whey proteins, soy protein isolate, sodium caseinate, blood plasma proteins, bovine serum albumin, waste products from the processing of food raw materials (blood from slaughterhouses, cheese whey, potato starch) are widely used, from which proteins are used as emulsifiers.

Gelatin is often used in culinary practice. – polydisperse protein, which is a mixture of polymer homologues of various molecular weights from 12,000 to 70,000 a.u. eat.

FINE GRINDED INSOLUTION POWDERS

This type of stabilizer is typical only for emulsions. It has long been known that certain fine powders effectively stabilize emulsions against coalescence. The chemical nature of these particles is less important than their surface properties. Basic requirements for powders:

The particle size must be very small compared to the size

Particles must have a certain wetting angle in the "oil – water – solid." The action of the powder is mainly to prevent thinning of the liquid layer between the drops. Smooth spherical powder particles are unsuitable; good results are obtained with lamellar particle-shaped powders such as bentonite clay.

Solid powdery substances (gypsum, graphite, etc.) are able to accumulate at the interface between drops and the medium, due to selective wettability solid bodies. For example, gypsum particles in an O/W emulsion, due to their hydrophilicity, enter almost completely into water and only partially into an oil drop, as a result of which they surround the oil drop in a continuous layer and prevent it from sticking to other drops. However, the selective wetting need not be complete, since in this case the stabilizer particles would be entirely in the aqueous phase and the oil droplets would be unprotected.

With incomplete selective wetting of hydrophilic particles (graphite, ZnS, CuS, etc.), they can be stabilizers of W/O emulsions. Thus, the mechanism of action of powders is similar to that of surfactants.

DETERMINING THE TYPE OF EMULSION

In the process of obtaining an emulsion, especially by dispersion methods, drops of both one and the other liquid are inevitably formed. However, in time, drops of one liquid persist and gradually accumulate, drops of the other almost instantly coalesce. If oil droplets accumulate, a direct emulsion (O/W) is formed, if water – an inverse emulsion (W/O) is formed. The type of emulsion formed depends on a number of factors, but is largely determined by the nature of the emulsifier. Following the Bancroft rule, we can say that the liquid that dissolves the emulsifier better or wets it better (if it is a powder) is a dispersion medium. Thus, knowing the nature of the emulsifier, one can predict the type of emulsion formed. However, such an estimate is very approximate, especially if the emulsion is multicomponent.

There are several experimental methods determining the type of emulsions.

DILUTION METHOD

A drop of emulsion is introduced into a test tube with water, which, when gently shaken, is evenly distributed in the volume of water if it is an M / W type emulsion. If the emulsion is inverted (W/O), then the drop is not dispersed. This test gives the best results in the case of dilute emulsions.

METHOD OF WETTING HYDROPHOBIC

SURFACES

When a drop of emulsion is applied to a paraffin plate, the drop spreads if the dispersion medium is oil (W/O emulsion).

CONTINUOUS PHASE DEFINITION

A drop of the emulsion is placed on a microscope slide next to several crystals of the dye dissolved in water. The plate is tilted so that the drop and the dye are in contact. If it turns out that the continuum (water) is colored, then it is an M/W type emulsion. Otherwise, the experiment is repeated with a fat-soluble dye, proving that the emulsion – type V/M. Water-soluble dyes are, for example, methyl orange and brilliant blue, and oil-soluble – sudan III and magenta. This test can be carried out by pouring a certain amount of emulsion into a test tube and adding a few crystals of a water-soluble dye. Uniform coloring of the liquid will indicate that it is an O/W type emulsion. Tronner and Bassus (1960) developed this method. On filter paper mugs moistened with 20% – m solution of cobalt chloride and then dried, they placed a drop of the emulsion. The O/W type emulsion causes a rapid development of a pink color, with the W/O emulsion no color change was observed. If there is a mixture of O/W and W/O emulsions – slowly appears weakly – pink coloration.

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).

eponymous electric charge 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 the rapidly 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.

IN 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 disperse systems.