Structurally, the mechanical factor of stability lies in. Structural and mechanical factors of stability. Proteins and nucleic acids
Distinguish between thermodynamic and kinetic factors of stability,
TO thermodynamic factors include electrostatic, adsorption-solvation and entropy factors.
Electrostatic factor due to the existence of a dispersed phase of an electric double layer on the surface of the particles. The main components of the electrostatic factor are the same charge of the granules of all colloidal particles, the value of the electrokinetic potential, as well as a decrease in the interfacial surface tension due to the adsorption of electrolytes (especially in cases where the electrolytes are ionic surfactants).
The electric charge of the same name of granules leads to mutual repulsion of approaching colloidal particles. Moreover, at distances exceeding the diameter of micelles, electrostatic repulsion is mainly due 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 shift, and as a result the granules will 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 incident on 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 factor of stability was discussed in Chapter 1.
Adsorption-solvation factor associated with hydration (solvation) of both the particles of the dispersed phase and ions adsorbed on their surface or uncharged surfactant molecules. The hydration shells and adsorption layers are bonded to the particle surface by adhesion forces. Therefore, for direct contact of the aggregates, the colliding particles must have the energy required not only to overcome the electrostatic barrier, but also exceed the work of adhesion.
Entropy factor consists in the tendency of the dispersed phase to a uniform distribution of 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 are involved in intense Brownian motion.
To kinetic factors stability includes structural-mechanical and hydrodynamic factors.
Structural and mechanical factor due to the fact that the hydration (solvation) shells existing on the surface of the particles have increased viscosity and elasticity. This creates an additional repulsive force upon collision of particles - the so-called wedging pressure... The disjoining pressure is also contributed by the elasticity of the adsorption layers themselves. The theory of wedging pressure was developed by B.V.Deryagin (1935).
Hydrodynamic factor associated with 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 the particles of the dispersed phase are generally devoid of mobility.
Real-world stability dispersed systems usually provided by several factors at the same time. The highest stability is observed with the combined action of both thermodynamic and kinetic factors.
Each factor of stability corresponds to a specific method of its neutralization. For example, the effect of a structural-mechanical factor can be removed with the help of substances that thin and dissolve elastic structured layers on the surface of the particles. Solvation can be reduced or completely eliminated by lyophobization of the dispersed phase particles during the adsorption of the corresponding substances. The effect of the electrostatic factor is significantly reduced when electrolytes are introduced into the system that compress the DES. This last case is most important both for stabilization and for the destruction of dispersed systems.
Coagulation
As mentioned above, coagulation is based on the violation of the aggregate stability of the system, which leads to adhesion of particles of the dispersed phase during their collisions. Externally, coagulation of colloidal solutions manifests itself in the form of turbidity, sometimes accompanied by a color change, followed by precipitation.
In the aggregates formed during coagulation, the primary particles are connected to each other either through an interlayer of a dispersion medium, or directly. Depending on this, the aggregates can be either loose, easily peptized, or strong enough, often irreversible, which are difficult to peptize or not peptized at all. In systems with a liquid dispersion medium, especially with a high concentration of particles of the dispersed phase, the precipitation of the resulting aggregates into a precipitate is often accompanied by structure formation - the formation of a coagel or gel covering the entire volume of the system.
The first stage of sol coagulation when its stability is disturbed is latent coagulation, which consists in combining only a small number of particles. Latent coagulation is usually not detected with the naked eye and can only be noted with a special examination, for example, using an ultramicroscope. Latent coagulation is followed by explicit, when already such a significant number of particles are combined that this leads to a clearly noticeable color change, cloudiness of the sol and the loss of a loose sediment from it ( coagulum). The coagulates resulting from the loss of aggregate stability are settling (or floating) formations of various structures - dense, curdled, flaky, fibrous, crystal-like. The structure and strength of coagulates is largely determined by the degree of solvation (hydration) and the presence of adsorbed substances of various nature on the particles, including surfactants.
P.A.Rebinder studied in detail the behavior of sols during coagulation with not completely removed protective factors and showed that in such cases coagulation structure formation is observed, leading to the appearance of gel-like systems (the structure of which will be discussed in Chapter 11).
The reverse process of coagulation is called peptization (see section 4.2.3). In ultramicroheterogeneous systems, in which the energy of Brownian motion is commensurate with the binding energy of particles in aggregates (floccules), a dynamic equilibrium can be established between coagulation and peptization. It must meet the condition
½ zE = kT ln ( V s / V To),
where z Is the coordination number of a particle in the spatial structure of the coagulum (otherwise, is the number of contacts of one particle in the resulting aggregate with other particles included in it), E Is the binding energy between particles in contact, k - Boltzmann's constant, T - absolute temperature, V h - the volume per particle in the colloidal solution, after the formation of the coagulum (if the concentration of particles is equal to n particles / m 3, then V s = 1 / n ,), V k is the effective volume per particle inside the coagulation structure (or the volume in which it oscillates relative to the equilibrium position).
In lyophobic dispersed systems after coagulation, the concentration of particles in equilibrium ash is usually negligible compared to their concentration. Therefore, according to the above equation, coagulation is usually irreversible. In lyophilic systems, the values of the binding energy between particles are small and therefore
½ zE < kT ln ( V s / V To),
that is, coagulation is either impossible or highly reversible.
The causes of coagulation can be very different. These are mechanical influences (stirring, vibration, shaking), and temperature (heating, boiling, cooling, freezing), and others, often difficult to explain and unpredictable.
But the most important in practical terms and at the same time the most well studied is coagulation under the influence of electrolytes or electrolyte coagulation.
This section discusses the phenomena and processes associated with aggregate stability dispersed systems.
First of all, we note that all dispersed systems, depending on the mechanism of the process of their formation according to P.A. Rebinder's classification, are subdivided 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 particle enlargement occur spontaneously in them, i.e. there is a decrease in surface energy due to a decrease in the specific surface area. Such systems are called aggregatively unstable.
The enlargement of particles can go in different ways. One of them called isothermal distillation , consists in the transfer of matter from small particles to large ones (Kelvin effect). As a result, small particles gradually dissolve (evaporate), and large ones grow.
The second path, the most typical and common for dispersed systems, is coagulation (from lat, coagulation, hardening), which consists in the adhesion of particles.
Coagulation in dilute systems also leads to a loss of sedimentation stability and ultimately to phase separation (separation).
The particle fusion process was named coalescence .
In concentrated systems, coagulation can manifest itself in the formation of a volumetric structure in which the dispersion medium is evenly distributed. In accordance with two different results of coagulation, the methods of observing this process are also different. The coarsening of particles leads, for example, to an increase in the turbidity of the solution, a decrease in the osmotic pressure. Structuring changes rheological properties system, its viscosity increases, the flow slows down.
A stable free-dispersed system, in which the dispersed phase is evenly distributed throughout the volume, can form as a result of condensation from a true solution. The loss of aggregate stability leads to coagulation, the first stage of which consists in bringing the particles of the dispersed phase closer together and their mutual fixation at small distances from each other. An interlayer of the medium remains between the particles.
The reverse process of formation of a stable free-dispersed system from a sediment or gel (structured dispersed system) is called peptization.
A deeper coagulation process leads to the destruction of medium layers and direct contact of 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 be converted back into a free-dispersed system only by forced dispersion. Thus, the concept of coagulation includes several processes with a decrease in the specific surface area of the system.
Fig. 33. Processes causing the loss of stability of dispersed systems.
The aggregate stability of unstabilized lyophobic dispersed systems is kinetic in nature, and it can be judged by the rate of processes caused by an excess of surface energy.
The coagulation rate determines the aggregate stability of the dispersed system, which is characterized by the process of adhesion (fusion) of particles.
Aggregate stability can also be thermodynamic in nature if the dispersed system does not have an excess of surface energy. Lyophilic systems are thermodynamically aggregatively stable, they form spontaneously and the coagulation process is generally not typical for them.
Lyophobic stabilized systems are thermodynamically resistant to coagulation; they can be removed from such a state by means of actions leading to an excess of surface energy (violation of stabilization).
In accordance with the above classification, thermodynamic and kinetic factors of the aggregate stability of dispersed systems are distinguished. Since the driving force of coagulation is excess surface energy, the main factors ensuring the stability of dispersed systems (while maintaining the surface size) will be those that reduce the surface tension. These factors are referred to as thermodynamic. They reduce the likelihood 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 coagulation rate also depends on kinetic factors.
The kinetic factors that reduce the rate of coagulation are mainly associated with the hydrodynamic properties of the medium: with the slowing down of the approach of particles, the outflow and destruction of the interlayers of the medium between them.
There are the following thermodynamic and kinetic factors of stability of dispersed systems.
1.Electrostatic factor consists in a decrease in the interfacial tension due to the formation of an electric double layer on the surface of the particles, as well as in the Coulomb repulsion arising from their approach.
An electric double layer (DES) is formed by the adsorption of ionic (dissociating into ions) surfactants. The adsorption of an ionic surfactant can occur at the interface between two immiscible liquids, for example, water and benzene. The polar group of the surfactant molecule facing water dissociates, imparting a charge corresponding to the organic part of the surfactant molecules (potential-determining ions) to the surface of the benzene phase. Counterions (inorganic ions) form a double layer on the side of the aqueous phase, as they interact more strongly with it.
There are other mechanisms for the formation of an electric double layer. For example, DES is formed at the interface between water and poorly soluble silver iodide. If well-soluble silver nitrate is added to the water, then the silver ions formed as a result of dissociation can complete the AgI crystal lattice, because they are part of it (specific adsorption of silver ions). As a result, the surface of the salt is charged positively (excess of silver cations), and iodide ions will act as counterions.
Mention should also be made of the possibility of the formation of an electric double layer as a result of the transition of ions or electrons from one phase to another (surface ionization).
The DES, formed as a result of the processes of spatial charge separation described above, has a diffuse (diffuse) character, which is due to the simultaneous influence of electrostatic (Coulomb) and van der Waals interactions on its structure, as well as the thermal motion of ions and molecules.
The so-called electrokinetic phenomena (electrophoresis, electroosmosis, etc.) are caused by the presence of an electric double layer at the interface.
2. Adsorption-solvation factor consists in reducing 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 entropic repulsion of particles can be represented as the presence of constant diffusion of particles from an area with a higher concentration to an area with a lower concentration, i.e. the system constantly strives to equalize the concentration of the dispersed phase throughout the volume.
4. Structural and 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, aggregate stability is provided by several factors at the same time. Especially high stability is observed with the combined action of thermodynamic and kinetic factors.
The structural and mechanical barrier, first considered by P.A. Rebinder, is a strong stabilization factor associated with the formation of adsorption layers at the interfaces, which lyophilize the surface. The structure and mechanical properties of such layers are capable of providing 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, do not have high surface activity with respect to this interface. These substances include resins, cellulose derivatives, proteins and other so-called protective colloids, which are high molecular weight substances.
In most d.s. The processes of enlargement of particles of the d. phase occur spontaneously due to the desire to reduce the excess surface energy. The enlargement of particles can go in two ways:
1.isothermal distillation - transfer of matter from small particles to larger ones (↓ G). Driving force - the difference μ of particles of different sizes
2.coagulation - adhesion, fusion of particles of the D. phase.
Coagulation in the narrow sense is the adhesion of particles, and in the broad sense, the loss of aggregate stability. The term "coalescence" is often used to characterize particle adhesion.
Coagulation leads to sedimentation instability or increases the rate of its flow.
In concentrated solutions, coagulation can lead to the formation of bulky structures in the system. Coagulation includes several sequential stages:
Formation of floccules (aggregates of particles), separated by layers of the medium - flocculation. The reverse process is called peptization (from floccules → particles)
Destruction of interlayers, fusion of particles or the formation of rigid condensation structures.
All these processes go with ↓ G. Coagulation depends on thermodynamic and kinetic factors.
A ... - Thermodynamic factors of stability:
1) electrostatic - consists in ↓ σ, due to the formation of a DES on the interface.
2) adsorption-solvation - consists in ↓ σ, due to adsorption (Gibbs equation) and adhesion (Dupre).
3) entropic - is the tendency of the system to a uniform distribution of particles. Operates in systems with Brownian motion.
B. - Kinetic factors of stability - contribute to a decrease in the rate of coagulation.
1) structural and mechanical - consists in the need to apply energy and time to destroy the film of the medium due to its certain elasticity and strength.
2) hydrodynamic - consists in a decrease in the rate of coagulation due to an increase in η and ∆ρ.
V. - Mixed factors of sustainability - consist in the emergence of a synergistic effect, i.e. the simultaneous influence of several of the above factors and their enhancement (↓ σ changes the mechanical properties of the medium film).
For each factor of resistance, if necessary, a specific method of its neutralization can be proposed.
The introduction of electrolytes reduces the electrostatic factor
The introduction of surfactants changes the mechanical strength of the interlayers
At the heart of etc. aggregate stability lies in the concept of disjoining pressure, introduced by B. Deryamin in 1935. It arises at a strong ↓ d of the film, during the interaction of approaching surface layers of particles. Surface layers begin to overlap. The wedging pressure - a total parameter that takes into account the forces of attraction (van der Waltz) and repulsive forces - have a different nature.
A decrease in d of the film leads to the disappearance of molecules of the medium with min energy in it, since the particles in it increase their excess energy due to the loss of neighbors or solvation shells. As a result, the molecules in the interlayer tend to draw other molecules into it from the volume, a kind of wedging pressure arises. Its physical meaning is the pressure that must be applied to the film in order to maintain an equilibrium thickness.
The modern theory of stability of dispersed systems is called DLFO (Deryabin-Landau-Verwey-Oberbeck). It is based on the total interaction energy of particles, defined as the algebraic sum of the energies of molecular attraction and electrostatic repulsion
The repulsive pressure is determined only by electrostatic forces. However, to date, a general theory of aggregate stability and coagulation has not yet been created.
Coagulation kinetics.
The rate of coagulation is the main factor by which the aggregate stability is judged; it can vary over a wide range.
The quantitative theory was developed in the works of M. Smolukhovsky, G. Müller, N. Fuchs. The most developed and one of the first was Smoluchowski's theory:
For monodisperse sols with spherical particles
Particle collision is the result of Brownian motion
Critical distance for interaction d = 2r
Collision of only 2 particles (single with single, single with double, double with triple).
This idea made it possible to reduce coagulation to the theory of bimolecular chemistry. reactions. As a result, the coagulation rate can be found:
; 
P - steric factor
Total number r
D - diffusion coefficient
After integration in the range from at τ = 0 to ν τ at τ:
k - it is difficult to determine, therefore Smoluchowski introduced the concept of half coagulation time - the time during which the number of particles decreases by 2 times ().
Equating these equations, we get:
, ;
The solution of the kinetic equations of coagulation can be carried out graphically.
Factors of aggregate stability colloidal systems... Types of coagulation of colloidal systems
The main method for purifying natural and waste waters from fine, emulsified, colloidal and colored impurities (groups 1 and 2) is coagulation and flocculation. The methods are based on the aggregation of dispersed phase particles with their subsequent removal from the water by mechanical settling.
The efficiency and economy of the processes of coagulation wastewater treatment is determined by the stability of the dispersed system, which depends on a number of factors: the degree of dispersion, the nature of the particle surface, the density of particles, the magnitude of the electrokinetic potential, concentration, presence in waste water other impurities, for example, electrolytes, high molecular weight compounds.
There are various methods of coagulation, the feasibility of using which depends on the factors that predetermine the aggregate stability of the systems.
Aggregate stability of colloidal systems depends on their structure.
Possessing a large specific surface, colloidal particles are able to adsorb ions from water, as a result of which the contacting phases acquire charges of the opposite sign, but equal in magnitude. As a result, an electrical double layer appears on the surface. Ions are relatively tightly bound to the dispersed solid phase are called potential defining... Οʜᴎ neutralized by excess counterions... Double layer thickness in aqueous solutions does not exceed 0.002 mm.
The degree of adsorption of ions depends on the affinity of the adsorbed ions to the surface, their ability to form nondissociated surface compounds. With the adsorption of ions of the same valence, the adsorption capacity increases with an increase in the radius of the ion and, accordingly, its polarizability, ᴛ.ᴇ. the ability to be attracted to the surface of a colloidal particle. An increase in the radius of an ion is also accompanied by a decrease in its hydration; the presence of a dense hydration shell prevents adsorption, since reduces the electrical interaction of the ion with the surface of the colloidal particle.
According to modern concepts of the structure of the electric double layer, the counter-war layer consists of two parts. One part adjoins the interface and forms an adsorption layer, the thickness of which is equal to the radius of its constituent hydrated ions. Another part of the counterions is located in a diffuse layer, the thickness of which depends on the properties and composition of the system. On the whole, the micelle is electrically neutral. The structure of a micelle - a colloidal particle - is shown in Figure 1.1.
The potential difference between the potential-determining ions and all the counterions is commonly called the thermodynamic φ-potential.
The charge on the particles prevents them from coming together, which, in particular, determines the stability of the colloidal system. In general, the stability of colloidal systems is due to the presence of a charge in the granule, diffusion layer and hydration shell.
Figure 3.1. Micelle structure: Fig. 3.2. Dual electric circuit
I - micelle core; layer in an electric field
II - adsorption layer; (I-II - granule);
III - diffusion layer;
IV - hydration shell
When a particle moves in a dispersed system or when superimposed electric field part of the counterions of the diffuse layer remains in the dispersed medium and the granule acquires a charge corresponding to the charge of the potential-determining ions. Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, the dispersion medium and the dispersed phase turn out to be oppositely charged.
The potential difference between the adsorption and diffuse layers of counterions is usually called electrokinetic ζ - potential (Fig. 1.2).
Electrokinetic potential is one of the most important parameters of the electric double layer. The magnitude ζ - potential is usually units and tens of millivolts based on the composition of the phases and the concentration of the electrolyte. The larger the value ζ– potential, the more stable the particle is.
Consider the thermodynamic and kinetic factors of stability of dispersed systems:
· Electrostatic stability factor... From the point of view of physical kinetics, the molecular attraction of particles is the main reason for the coagulation of the system (its aggregate instability). If an adsorption layer of ionic nature has formed on colloidal particles, then with a sufficient approach of like-charged particles, electrostatic repulsive forces arise. The thicker the electric double layer, the more intense the resulting repulsive force of the particles, the greater the height of the energy barrier and the less the probability of particles sticking together. Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, the stability of colloidal systems in the presence of an ionic stabilizer depends on the properties of the electric double layer.
· Solvation factor of stability. Repulsive forces are caused by the existence on the surface of approaching particles of solvation (hydration) shells or the so-called boundary phases, consisting only of molecules of the dispersion medium and having special physical properties... The micelle nucleus is insoluble in water and therefore not hydrated. Ions adsorbed on the surface of the nucleus and counterions of the electric double layer are hydrated. Due to this, an ion-hydration shell is created around the core. Its thickness depends on the distribution of the electric double layer: the more ions are in the diffuse layer, the greater is the thickness of the hydration shell.
· Entropy factor of stability. It is caused by the thermal motion of segments of surfactant molecules adsorbed on colloidal particles. When particles with adsorption layers of surfactant molecules or high-molecular substances approach each other, there is a strong decrease in the entropy of the adsorption layer, which prevents the aggregation of particles.
· Structural and mechanical factor of stability. The adsorption-solvation layers of surfactants can represent a structural-mechanical barrier that prevents the particles from coming closer together. The protective layers of counterion-stabilizers, being gel-like, have increased structural viscosity and mechanical strength.
· Hydrodynamic stability factor... The coagulation rate can decrease due to a change in the viscosity of the medium and the density of the dispersed phase and dispersion medium.
· Mixed factors most typical for real systems... Usually, aggregate stability is provided by several factors at the same time. Especially high stability is observed with the combination of the action of thermodynamic and kinetic factors, when, along with a decrease in interfacial tension, structural and mechanical properties of interparticle layers are manifested.
It must be borne in mind that each factor of resistance corresponds to a specific method of its neutralization. For example, the effect of the electrostatic factor is significantly reduced when electrolytes are introduced into the system, which compress the electrical double layer.
Solvation with a solvation factor should be excluded by lyophobization of the dispersed phase particles by adsorption of the corresponding substances. The effect of the structural and mechanical factor can be reduced by using substances that thin and dissolve elastic structured layers on the surface of the particles.
The destabilization of the system must be caused by various reasons, the result of many of them is the compression of the diffuse layer and, consequently, a decrease in the value of the ζ-potential. Compression of the diffuse layer also reduces the degree of ion hydration; in the isoelectric state (ζ = 0, mV), the hydration shell around the nucleus is extremely thin (10 -10 m) and does not protect micelles from sticking together during collision; as a result, particle aggregation begins.
Sedimentation stability of colloidal systems (CS) - the ability of a dispersed system to maintain a uniform distribution of particles throughout the entire volume) is due to the Brownian motion of colloidal dispersions and the diffusion of particles of the dispersed phase.
Sedimentation stability of a system depends on the action of two mutually opposite factors: the force of gravity, under the influence of which the particles settle, and diffusion, in which the particles tend to a uniform distribution over the volume. As a result, there is an equilibrium diffusion-sedimentation distribution of particles along the height, depending on their size.
Diffusion slows down with increasing particle size. With a sufficiently high degree of particle dispersion, Brownian motion, as a diffusion motion, leads to equalization of concentrations throughout the volume. How less particle, the longer it takes to establish equilibrium.
The settling speed of particles is proportional to the square of their diameter. In coarsely dispersed systems, the rate of reaching equilibrium is relatively high and equilibrium is established within a few minutes or hours. In finely dispersed solutions, it is small, and until the moment of equilibrium, years or even tens of years pass.
Coagulation types
In the modern theory of coagulation of dispersed systems developed by Deryagin, Landau, Vervey, Overbeck (DLVO theory), the degree of stability of the system is determined from the balance of molecular and electrostatic forces. There are two types of coagulation:
1) concentration, in which the loss of stability of the particles is associated with the compression of the double layer;
2) neutralization (coagulation with electrolytes), when, along with the compression of the double layer, the potential φ 1 decreases.
Concentration coagulation is characteristic of highly charged particles in highly concentrated electrolyte solutions. The higher the potential φ 1 of the DES, the stronger the counterions are attracted to the surface of the particles and by their presence screen the growth of the electric field. For this reason, at high values of φ 1, the forces of electrostatic repulsion between particles do not increase infinitely, but tend to a certain finite limit. This limit is reached at φ 1 over 250 mv. Hence it follows that the interaction of particles with a high φ 1 potential does not depend on the value of this potential, but is determined only by the concentration and charge of counterions.
As the electrolyte concentration increases, the value ζ - potential (DP) decreases, and φ 1 practically retains its value (Fig. 3.3).
Rice. 3.3. a) The relationship between the φ-potential and DP ( ζ - potential) for a highly charged particle (concentration coagulation);
b) The relationship between the φ-potential and DP for a weakly charged particle (neutralization coagulation).
To cause coagulation of the sol, it is extremely important to exceed a certain maximum concentration of ions - coagulants - the coagulation threshold.
The DLVO theory makes it possible to determine the value of the concentration coagulation threshold (γ):
Where Ck - constant weakly dependent on the ratio of the charges of the cation and anion of the electrolyte; ε- dielectric constant of the solution; A - constant characterizing the molecular attraction of particles; e - electron charge; z i - counterion valence.
Equation (1.1.) Shows that the coagulation threshold does not depend on φ 1, and is inversely proportional to the sixth degree of valence of counterions. For one-, two-, three- and tetravalent ions, the ratio of the coagulation thresholds will be
Neutralizing coagulation is characteristic of weakly charged particles. The loss of aggregate stability is due to the adsorption of counterions and a decrease in the potential of the diffuse layer φ 1.
At low electrolyte concentrations, when the thickness of the diffuse layer is large, the values of φ 1 and ζ - the potentials are close (Fig. 3.3.). For this reason, the value ζ - potential during neutralization coagulation rather reliably characterizes the degree of sol stability.
According to Deryagin's theory, the critical value of the potential () is related to the conditions of neutralization coagulation by the relation
where C n - constant; Aχ is the reciprocal of the thickness of the diffuse layer.
3) Coagulation should be caused by the addition of electrolytes to the system and under the influence of physicochemical factors (stirring the system, heating, freezing followed by thawing, exposure to magnetic or electric fields, ultracentrifugation, ultrasonic exposure, etc.).
Factors of aggregate stability of colloidal systems. Types of coagulation of colloidal systems - concept and types. Classification and features of the category "Factors of aggregate stability of colloidal systems. Types of coagulation of colloidal systems" 2017, 2018.
thermodynamic kinetic
(↓). (↓ coagulation rate due to the hydrodynamic properties of the medium)
a) electrostatic factor - ↓ due to a) hydrodynamic factor
DES formation
b) adsorption-solvation factor - ↓ b) structural-mechanical
due to adsorption and surface solvation factor
c) entropy factor
Thermodynamic factors:
Electrostatic factor promotes the creation of electrostatic repulsive forces that increase with an increase in the surface potential of particles, and especially ζ- potential.
Adsorption-solvation factor due to a decrease in the surface of particles as a result of solvation. In this case, the surface of the particles is lyphilic 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 molecules on their surface with which their environment interacts. These are surfactants, IUDs, and in the case of emulsions, fine powders wetted by the medium.
The adsorption of such substances is accompanied by the solvation and orientation of molecules in accordance with the polarity of the contacting phases (Rebinder's rule). The adsorption of surfactants 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 Brownian motion, particles of the dispersed phase are uniformly distributed over the volume of the system. As a result, the chaos of the system increases (its chaos is less if the particles are in the form of 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 in the course of some process S> 0, then according to the equation
G = H - TS,
this process proceeds with a decrease in the Gibbs energy G<0.
Kinetic factors:
Structural and mechanical factor sustainability occurs during the adsorption of surfactants and IUDs on the surface of particles, which lead to the formation of adsorption layers with enhanced structural and mechanical properties. Such substances include: long-chain surfactants, most IUDs, for example, gelatin, casein, proteins, soaps, resins. Concentrating on the surface of particles, they can form a gel-like film. These adsorption layers act as a barrier to the approach of particles and their aggregation.
A simultaneous decrease in surface tension in this case leads to the fact that this factor becomes universal for the stabilization of all dispersed systems.
The hydrodynamic stability factor is manifested in highly viscous and dense dispersion media, in which the speed of motion of particles of the dispersed phase is low and their kinetic energy is insufficient to overcome even a small potential barrier of repulsion.
In real colloidal systems, several thermodynamic and kinetic stability factors usually act at once. For example, the stability of polystyrene latex micelles (see Chapter 5) is provided by ionic, structural-mechanical, and adsorption-solvation factors of stability.
It should be noted that each factor of stability has its own specific method of neutralizing it. For example, the effect of the ionic factor is significantly reduced with the introduction of electrolytes. The action of the structural and mechanical factor can be prevented with the help of substances - the so-called. demulsifiers(these are usually short-chain surfactants), thinning elastic structured layers on the surface of particles, as well as by mechanical, thermal and other methods. As a result, there is a loss of aggregate stability of systems and coagulation.
Mechanisms of action of stabilizers
Stabilizers create a potential or structural-mechanical barrier on the way of particle adhesion, and at a 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 dispersed systems.
