Moscow State University of Printing Arts. Stability of dispersed systems In a stable colloidal system, attractive forces prevail
Colloidal solutions are thermodynamically unstable systems in which there is a tendency to a decrease in the surface Gibbs energy due to the coarsening of particles and a decrease in the total interface. The adhesion of colloidal particles leads to an increase in their mass, as a result of which larger particles settle to the bottom (sediment) under the influence of gravity.
Known, however, numerous colloidal solutions in which the particles do not stick together for a long time. The ability of a dispersed system to maintain its state and properties over time is characterized by the stability of the dispersed system.
Distinguish sedimentation and aggregate stability.
Sedimentation resistancecharacterizes the ability of particles of a dispersed phase to be in suspension and not to settle under the influence of gravity.
Aggregate stabilitycharacterizes the ability of the particles of the dispersed phase to resist their adhesion to each other.
The sedimentation stability of colloidal systems is due to the small size of particles and their Brownian motion.
The aggregate stability of colloidal solutions with an ionic stabilizer is due to the presence of a diffuse ionic atmosphere on the surface of the particles, which prevents them from sticking together. Aggregate stability is the result of the interaction of two oppositely directed forces that simultaneously act on approaching colloidal particles: van der Waals forces of intermolecular attraction and electrostatic repulsive forces arising between like-charged particles. With a considerable length of the diffuse part of the micelle, repulsive forces prevail, arising between like-charged counterions. With a small thickness of the diffuse layer, the particles approach each other at distances at which the molecular attraction is stronger, which leads to their aggregation (sticking together).
Thus, the aggregate stability of colloidal systems depends on the conditions for the formation of a micelle - the charge of the solid phase, the thickness and charge of the diffuse layer. It is the greater, the higher the charge of the solid phase (interfacial potential), the greater the thickness of the diffuse layer, and the greater the value ζ -potential. Colloidal solutions with an ionic stabilizer are stable if the ζ-potential exceeds 50 mV, relatively stable at 30< ζ < 50 мВ и неустойчивы при ζ < 30 мВ.
Loss of aggregate stability leads to adhesion of colloidal particles with the formation of larger aggregates; this process is called coagulation ... The consequence of coagulation is the loss of sedimentation stability: enlarged particles settle under the influence of the increased gravity.
An example of coagulation of the colloidal system is the process of blood coagulation. It is facilitated by the presence of calcium cations in the blood, therefore, for long-term storage of blood intended for conservation, these ions are removed from the blood by various physicochemical methods.
Coagulation can be caused by various external influences: the addition of small amounts of electrolyte, concentration of the colloidal solution, temperature changes, the action of ultrasound, electromagnetic field and others. The greatest practical value is coagulation by electrolytes.
According to the Schulze - Hardy rule, coagulation of colloidal solutions is caused by any ions having a charge sign opposite to the charge of the granules, and their effect is the stronger, the higher the charge of the coagulant ion.
For a quantitative assessment of the coagulating ability of electrolytes, the concept was introduced coagulation threshold , i.e. the minimum electrolyte concentration, the achievement of which causes the onset of coagulation, noticeable by the cloudiness of the solution or a change in its color.
1000WITH e-mail V e-mail
WITH pore = ¾¾¾¾¾
V cr + V e-mail
where WITH pore - electrolyte coagulation threshold, mmol / l; WITH el is the initial concentration of the electrolyte solution, mol / l; V el - the added volume of electrolyte solution that caused the onset of coagulation; V cr is the initial volume of the colloidal solution.
The inverse of the coagulation threshold is called the coagulating ability of the COP:
KS = 1 / WITH since
The coagulating ability of a coagulant ion is proportional to its charge to the sixth power. For example, the coagulation of an AgCl sol obtained in an excess of Cl - ions and having negatively charged granules will be caused by positively charged ions, and when NaCl, CaCl 2 or AlCl 3 solutions are added to this sol, the coagulating effect of Na +, Ca 2+ and Al 3 cations + will be in an approximate ratio of 1 6: 2 6: 3 6 "1: 64: 729. In other words, for coagulation, you will need to add a much smaller amount of AlCl 3 solution than CaCl 2 solution and especially NaCl solution. If the AgCl sol was formed in an excess of potential-determining Ag + cations and has an inherent positive charge of the granule, then the anions will cause the coagulation of such a sol. In this case, the most effective coagulant in the series KCl - K 2 SO 4 - K 3 PO 4 will be the anion with the highest charge, since KC (Cl -): KC (SO 4 2-): KC (PO 4 3-) »1: 64 : 729.
The effect of the electrolyte on the coagulation of colloidal solutions should be taken into account when introducing electrolytes into living organisms. For example, a physiological solution of NaCl (0.9%) cannot be replaced with an isotonic solution of MgSO 4, since the doubly charged ions Mg 2+ and SO 4 2- have a significantly greater coagulating effect than the singly charged ions Na + and Cl -. When electrolytes are injected into muscle tissue they should be introduced gradually so as not to cause local excess of the coagulation threshold, which will lead to coagulation of biosubstrates.
The course of the coagulation process can be judged by the value of the ζ-potential (Fig. 24). Coagulation becomes possible with a decrease in the thickness of the diffuse layer of the micelle, which is accompanied by a decrease in the electrokinetic potential. A decrease in the value of the ζ-potential to 25-30 mV indicates the onset of coagulation, although external signs(haze or discoloration) may not be observed due to the low speed of this process (the so-called "hidden" coagulation). A further decrease in the ζ-potential is accompanied by an increase in the rate of coagulation and turbidity of the solution ( "explicit" coagulation), and at ζ = 0 the coagulation rate is maximum. The state of colloidal particles, in which the electrokinetic potential is equal to 0, is called isoelectric state. In this state, the charge of the granules is equal to 0, therefore, in an electric field, they do not acquire directional motion.
Coagulation
latent explicit
v slow fast
 |
ζ> 30 mV ζ< 30 мВ ζ = 0
Fig. 24. Dependence of the coagulation rate on the concentration of the electrolyte-coagulant
Coagulation can also be induced by acting mixtures of electrolytes ... Moreover, there are three possible options interactions between electrolyte-coagulants:
1) additive action - summation of the coagulating action of ions; for example, a mixture of KCl and NaNO 3 salts, which do not interact with each other, exhibits an additive effect in relation to colloids with granules charged both positively and negatively (in the first case, anions cause coagulation, in the second - salt cations);
2) antagonism - weakening of the coagulating effect of one electrolyte in the presence of another; for example, the addition of Na 2 SO 4 weakens the coagulating effect of Ba 2+ cations due to the fact that the reaction Ba 2+ + SO 4 2- ® BaSO 4 proceeds in the solution, leading to a decrease in the concentration of these cations;
3) synergy- strengthening of the coagulating action of one electrolyte in the presence of another; for example, the coagulating effect of FeCl 3 and KSCN on colloids with positively charged granules (coagulants - singly charged anions) increases sharply when they are present together, because as a result of the reaction Fe 3+ + 6SCN - ® 3-, a triply charged complex anion is formed, exhibiting a very high coagulating ability.
When mixing two colloidal solutions containing particles with opposite charges of the granules occurs mutual coagulation - sticking together of oppositely charged granules into large aggregates. In this case, coagulation occurs the more completely, the more completely the charges of the granules are neutralized.
The sediment freshly obtained during coagulation can be returned to the colloidal state. The reverse process of coagulation - the transformation of the sediment into a stable colloidal solution, is called peptization ... Peptization is facilitated by washing the precipitate with a pure solvent, which flushes out coagulant ions from the system, and adding a peptizing electrolyte containing ions capable of being adsorbed on the surface of the precipitate particles to restore ionic atmospheres around them and transform them into a colloidal state. Peptization is enhanced with stirring and heating.
The peptization process underlies the treatment of many diseases: resorption of atherosclerotic plaques on the walls of blood vessels, kidney and liver stones. However, old blood clots and hardened stones are practically not peptized.
The stability of colloidal solutions can be increased by adding to them some high molecular weight compounds (HMC). This phenomenon is called colloidal protection. The protective effect of IUDs is explained by the fact that they are adsorbed on the surface of colloidal particles. In this case, the hydrophobic parts of their structures (hydrocarbon radicals) are facing the particles of the dispersed phase, and the hydrophilic fragments (polar groups) are facing outward, towards water. An additional shell of IUD macromolecules and their own hydration shells is formed around the micelle, which prevents the colloidal particles from approaching.
In relation to aqueous colloidal solutions, water-soluble proteins, polysaccharides, pectins have a protective effect. Proteins prevent the precipitation of poorly soluble cholesterol and calcium salts on the walls of blood vessels, the formation of stones in the urinary and biliary tract. In pharmacy, the protective properties of the IUD are used to increase the stability of drugs in a colloidal state.
To ensure colloidal protection, it is necessary to create a sufficiently high concentration of IUDs, which ensures the formation of a monomolecular protective shell around the micelle. The introduction of a small amount of IUDs can lead to the opposite effect: macromolecules interact simultaneously with several colloidal particles, binding them to form loose flakes. The aggregation of particles of the dispersed phase in lyophobic colloidal solutions under the influence of small amounts of IUD is called flocculation.
The flocculation phenomenon is based on the method of purification of natural and drinking water... A highly water-soluble synthetic polymer, polyacrylamide, is used as a flocculant.
Modern physical theory electrolyte coagulation is based on general principles statistical physics, theory of molecular forces and theory of solutions. Its authors are: B.V. Deryagin, L. D. Landau (1937-1941), E. Vervey, J. Overbeck (after the first letters of the DLFO).
The essence of the theory: between any particles as they approach each other, a wedging pressure of the separating liquid layer arises as a result of the action of the forces of attraction and repulsion. The wedging pressure is a total parameter that takes into account the action of both attractive and repulsive forces.
The state of the system depends on the balance of the energy of attraction (U pr) and the energy of repulsion (U ex). U ott prevails - a stable system. U pr prevails - violation of aggregate stability - coagulation.
The change in the interaction energy between two particles as they approach is depicted graphically (Fig. 5.3).
The total energy of a system of two particles (curve 3) is obtained by adding Utm and Upr:
U = U OT + U Pr =
where: B is a factor that depends on the values of the electrical potentials of the DES, the properties of the medium, and the temperature;
e is the base of the natural logarithm;
c is the reciprocal of the thickness of the diffuse layer;
h is the distance between particles;
A is the constant of molecular forces of attraction.
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Figure 5.3. Potential interaction curves
colloidal particles:
1 - change in repulsive energy with distance;
2 - change in the energy of attraction;
3 - the resulting curve.
Consider the resulting curve 3 in Figure 5.3. It has characteristic areas:
In the region of small distances, there is a deep primary minimum (potential well) - U pr prevails significantly. The primary minimum corresponds to direct adhesion of particles (I).
In the region of large distances, there is a secondary shallow minimum (the second potential well corresponds to attraction through the medium layer). Diagram II.
In the region of average distances on the curve there is a maximum and, if it is located above the abscissa axis, then an energy barrier of repulsive forces appears (DU b).
The resulting curve 3 can have different kind depending on the stability of the dispersed system (Figure 5.4.).
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Rice. 5.4. Potential curves for certain
stability states of a dispersed system:
1 - in the system, at any distance between particles, the energy of attraction prevails over the energy of repulsion. In such a system, rapid coagulation is observed with the formation of aggregates.
2 - a sufficiently high potential barrier and the presence of a secondary minimum. The particles interact, but do not have direct contact and are separated by medium layers.
3 - a system with high aggregate stability (high potential barrier and the absence of a secondary minimum, or at its depth, less thermal energy kТ).
Depending on the height of the energy barrier and the depth of the potential wells, different variants of the behavior of particles when approaching are possible (Fig. 5.5), the particles have kinetic energy- kT.
Figure 5.5. Interaction schemes of colloidal particles
| State c: Low barrier height and shallow secondary minimum: DU b @DU i £ kT particles enter into short-range interaction, i.e. in direct contact - coagulation occurs | State a: Characterized by the fact that diffuse layers are overlapped and interlayers of the medium between the particles (gels) are preserved. The energy barrier is quite high, the secondary minimum is not deep: DU I ³kT The interacting particles cannot disperse (keep the forces of attraction) and cannot come close (the forces of repulsion prevent them). The addition of electrolyte most often leads to coagulation (h decreases). | State b: High energy barrier DU b ³kT and absence or shallow secondary minimum DU i £ kT: Particles cannot overcome the barrier and diverge without interaction. Such a system is aggregatively stable. |
A dispersed system is aggregatively stable with a high energy barrier of repulsive forces.
Under resistance dispersed systems understand the invariability of their properties and composition over time, including the dispersion of the phase, interparticle interaction. Here we consider the issues of stability of systems with respect to the enlargement or aggregation of particles of the dispersed phase, to their precipitation. The elimination of aggregate stability is necessary in the processes of precipitation during phase separation, during purification Wastewater and industrial emissions.
According to the classification of P.A. Rebinder disperse systems are divided into lyophilic, resulting from spontaneous dispersion of one of the phases, and lyophobic, resulting from forced dispersion and condensation with supersaturation. Lyophobic systems have an excess of surface energy, and processes of particle enlargement can occur spontaneously in them, i.e. a decrease in surface energy can occur due to a decrease in the specific surface area. Such systems are called aggregatively unstable.
The aggregation of particles can consist in the transfer of matter from small particles to large ones, since the chemical potential of the latter is less / isothermal distillation /. Large particles grow, and small particles gradually dissolve / evaporate /. Aggregation of particles can also occur by sticking / merging / particles - the most typical way for dispersed systems / coagulation /.
Distinguish between thermodynamic and kinetic factors of aggregate stability of dispersed systems. The driving force behind coagulation is excess surface energy. The main factors affecting the stability of systems are factors that reduce the surface tension while maintaining the surface size. These factors are referred to as thermodynamic. They reduce the likelihood of effective particle collisions, create potential barriers that slow down or even exclude the coagulation process. The lower the surface tension, the greater the thermodynamic stability of the system.
Kinetic factors are mainly associated with the hydrodynamic properties of the medium: slowing down the approach of particles, destruction of the interlayers of the medium between the particles. In general, the following factors of stability of dispersed systems are distinguished:
1. Hydrodynamic - due to changes in the viscosity of the medium and the density of the phase and dispersion medium, the rate of coagulation decreases;
2. Structural - mechanical factor due to the presence on the surface of the particles of an elastic, mechanically strong film, the destruction of which requires energy and time;
3. Electrostatic - due to the formation of a double electrostatic layer / DES / on the surface of the particles, the interfacial tension decreases. The appearance of an electric potential at the interface is possible due to surface electrolytic dissociation or adsorption of electrolytes;
4. The entropy factor manifests itself in systems in which particles or their surface layers are involved in thermal motion. Its essence lies in the tendency of the dispersed phase to be uniformly distributed over the volume of the system;
5. Adsorption-solvation - manifests itself in a decrease in interfacial tension due to adsorption and solvation during the interaction of particles with a dispersion medium.
V real systems aggregate stability is determined simultaneously by a combination of thermodynamic and kinetic factors.
According to modern concepts, the stability of systems (lyophobic colloids) is determined by the balance of forces of molecular attraction and electrostatic repulsion between particles. A universal property of dispersed systems is the presence of an electric double layer (DEL) at the interface.
The surface charge of particles is formed as a result of one of the processes:
- dissociation of surface groups of particles;
- adsorption of potential-determining ions, i.e. ions included in the composition of the crystal lattice or similar to them;
- adsorption of ionic surfactants;
- isomorphic substitution, for example, the charge of particles of most clays is formed due to the replacement of tetravalent silicon ions by Al +3 or Ca +2, with a deficit of positive charge on the particle.
In the first three cases, the surface charge can be controlled, within certain limits, the amount of charge and sign can be adjusted by changing the concentration of ions in the system. For example, as a result of the dissociation of surface silanol groups, silica particles can acquire a charge:
The surface charge density is equal to the number of elementary charges per unit surface. The surface charge of a particle in a dispersed system is compensated by the sum of charges localized in the diffuse and dense (directly adjacent part of the counterion monolayer) parts of the DES.
The phenomenon of the appearance of a potential difference during the deposition of a dispersed phase is called the potential of sedimentation / sedimentation /. With a relative displacement of the phases, regardless of the reasons causing the displacement, a DES rupture occurs in terms of the slip density. The slip plane usually passes through the diffuse layer of the DES, and some of its ions remain in the dispersion medium. As a result, the dispersion medium and its dispersed phase turn out to be oppositely charged. The potential arising on the slip plane when a part of the diffuse layer is detached is called the electrokinetic potential, or z / zeta / potential. The zeta potential, reflecting the properties of DES, characterizes the nature of the phases and interphase interaction. The magnitude of the electrokinetic potential depends on the speed of movement of the phases, the viscosity of the medium, the nature of the phases, and other factors. A decrease in temperature, the introduction of electrolytes into the system that specifically interact with the surface, an increase in the charge of electrolyte ions leads to a decrease in the zeta potential.
The magnitude of the zeta potential depends on the nature of the surface of the contacting phases. On the surfaces of polyelectrolytes containing ionic groups, as well as on the surface of many inorganic oxides, the zeta potential can reach high values - 100 mV or more. If counterions are adsorbed on the surface, then the electrokinetic potential decreases. The pH value of the medium has a significant effect, since the H + and OH - ions have a high adsorption capacity. The sign and value of the zeta potential are widely used to characterize the electrical properties of surfaces when considering the aggregate stability of dispersed systems.
As a first approximation, it is generally accepted that the stability of dispersed systems is determined by the value of the electrokinetic z (zeta) potential. When electrolytes or surfactants are added to the systems, there is a change in the structure of DES, a change in the value of z - potential with a constant value of the surface potential. This change (decrease) is most significant with an increase in the counterion charge at the same electrolyte concentration (Figure 2.1).
Highly charged counterions / Al +3, Fe +3 /, complex organic ions due to the action of van der Waals forces can be adsorbed superequivalently, i.e. in quantities exceeding the number of charges on the surface, accumulating in the layer. As a result, it is possible to change both the magnitude and sign of the electrokinetic potential. Such phenomena are often encountered when polyelectrolytes and coagulants are introduced into dispersed systems.
In disperse systems, when equally charged particles approach each other, they are repulsed, which is not purely Coulomb, since the surface charge is fully compensated by the counterion charge. Repulsive forces appear when diffuse ionic atmospheres overlap. At the same time, van der Waals attraction acts between particles, consisting of orientational, induction and dispersion forces. V certain conditions these forces are commensurate with the repulsive forces. The total interaction energy of dispersed particles is the sum of the energies of attraction and repulsion. The value of the total energy of particles from the distance between them is schematically shown in Fig. 2.2.
Figure 2.1. Dependence of the value of z - potential on the concentration of counterions. The curves show the counterion charge
The stability of dispersed systems and coagulation directly reflect the interaction of particles of the dispersed phase with each other or with any macrosurfaces. The theory of stability is based on the relationship between the forces of attraction and repulsion of particles. The theory of stability, first proposed by B.V. Deryagin and L.D. Landau, which takes into account the electrostatic component of the disjoining pressure (repulsion) and its molecular component (attraction).
In a simplified version, the total interaction energy between two particles per unit area is equal to
E = E pr + E from. (2.1)
Figure 2.2. The dependence of the interaction energy of particles (E total) on the distance between them ( L), E total = E attraction + E repulsive
Each of these components can be expressed as a function of the distance between particles
dЕ pr = Р pr dh, (2.2)
dE from = P from dh, (2.3)
where P pr is the gravitational pressure, i.e. the molecular component of the proppant pressure; P from is the repulsive pressure, in this case the electrostatic component of the wedging pressure.
The attraction pressure is usually caused by the tendency of the system to decrease the surface energy; its nature is associated with van der Waals forces. The repulsive pressure is due only to electrostatic forces, therefore
dР from = d, (2.4)
where is the volumetric charge density in the EMF, is the electric potential of the double layer.
If the particles are located at distances at which no interaction occurs, then the DELs do not overlap, and the potentials in them are practically equal to zero. As the particles approach each other, the DELs overlap; as a result, the potentials significantly increase up to 2 and the repulsive forces increase.
In the region of low potentials, the electrostatic component of pressure strongly depends on the value of the potential; with an increase in the potential, this dependence becomes less noticeable. The repulsive energy of particles increases exponentially with decreasing distance h between them.
According to the simplified equation 2.5, the energy of attraction of particles is inversely proportional to the square of the distance between them.
P pr = -, (2.5)
where n is the number of atoms per unit volume of the particle; K is a constant depending on the nature of the interacting phases;
The energy of attraction between particles decreases much more slowly with distance than the energy of attraction between molecules (atoms). Hence, it follows that particles of dispersed systems interact at farther distances than molecules.
The stability of dispersed systems or the rate of coagulation depends on the sign and value of the total potential energy of particle interaction. Positive repulsive energy E from with increasing h decreases exponentially, and negative E pr is inversely proportional to the square of h. As a result, at small distances (at h®0, E from ®const, E pr ®) and at large distances between particles, the energy of attraction prevails, and at medium distances, the energy of electrostatic repulsion.
The primary minimum I (Fig. 2.2) corresponds to the direct adhesion of particles, and the secondary minimum II - to their attraction through the medium layer. The maximum corresponding to the average distances characterizes the potential barrier that prevents particles from sticking together. The forces of interaction can spread over distances of up to hundreds of nm, and the maximum energy value can exceed 10 -2 J / m 2. An increase in the potential barrier is facilitated by an increase in the potential on the particle surface in the region of its small values. Already at 20 mV, a potential barrier arises that provides the aggregate stability of dispersed systems.
In various industries, there are dispersed systems containing dissimilar particles that differ in their chemical nature, sign and magnitude of the surface charge, and size. The aggregation of such particles (coagulation) is called heterocoagulation. This is the most common case of particle interaction during dyeing, flotation, formation bottom sediments, sewage sludge. The term mutual coagulation means more special case- aggregation of oppositely charged particles.
The process of mutual coagulation is widely used in practice to destroy the aggregate stability of dispersed systems, for example, in wastewater treatment. Thus, the treatment of wastewater under certain conditions with aluminum or iron salts causes rapid coagulation of suspended negatively charged substances interacting with positively charged particles of aluminum and iron hydroxides formed during the hydrolysis of salts.
Lyophilic colloids are characterized by intense interaction of dispersed particles with the medium and thermodynamic stability of the system. The decisive role in the stabilization of lyophilic colloids belongs to the solvation layers formed on the surface of the dispersed phase as a result of polymolecular adsorption of solvent molecules. The ability of the solvation shell to prevent adhesion of particles is explained by the presence of shear resistance, which prevents the squeezing of the molecules of the medium from the gap between the particles, as well as the absence of noticeable surface tension at the boundary of the solvation layer and the free phase. The stabilization of dispersed systems is facilitated by the introduction of surfactants into the system. Nonionic surfactants, adsorbed on hydrophobic dispersed particles, convert them into hydrophilic ones and increase the stability of sols.
The aggregate stability / instability of the system depends on the possibility of particle contact; for adhesion, the particles must approach a certain distance. In the theory of aggregate stability, known as DLFO theory(the first letters of the names of the authors of the theory: B.V.Deryagin and L.D. Landau, Russia, and E. Vervey and J.T. Overbeck, Holland) combined action of attractive and repulsive forces between the particles.
Historical excursion
Boris Vladimirovich Deryagin is an outstanding scientist who made an invaluable contribution to almost every section colloidal chemistry... Investigating the properties of clay suspensions, he found that thin layers of water between individual particles of the suspension have properties that are different from the properties of water in the volume, including wedging pressure, which prevents the particles from approaching. The joint consideration of the forces of attraction and repulsion explained the stability of the system. These studies, along with quantitative calculations and the identification of the stability criterion, were published by B.V.Deryagin together with Lev Davidovich Landau in several scientific articles 1935-1941; abroad they learned about these works much later.
Dutch scientists E. Vervey and J.T. Overbek has also done research in this area. E. Vervey in 1934 defended his thesis on the study of the electric double layer and the stability of lyophobic colloids. Later, he published a series of articles, which examines the action of electric forces and the forces of London - Van der Waals between colloidal particles in an electrolyte solution. And in 1948, in co-authorship with Overbeck, his monograph "The Theory of Stability of Lyophobic Colloids" was published.
The issue of scientific priority regarding the creation of the theory was resolved by the recognition of the merits of all four authors.
The forces of gravity - these are the forces of intermolecular interaction (the forces of London - Van der Waals). The forces of attraction that arise between individual atoms are manifested at very small distances of the order of atomic dimensions. In the interaction of particles, due to the additivity of dispersion forces, the attraction between the particles manifests itself at much greater distances. The energy of attraction is inversely proportional to the square of the distance between particles:
Repulsive forces between the particles are of an electrostatic nature. The repulsive electrostatic energy arising from the overlap of diffuse layers decreases exponentially with increasing distance:
In the above formulas for the energies of attraction and repulsion A* - Gamaxra constant; NS - distance between particles; e is the dielectric constant of the dispersion medium; e ° = 8.85 K) 12 F / m - electrical constant; (p ^ is the potential of the diffuse layer; A. is the thickness of the diffuse layer of the electric double layer (DES).
For more details on the structure of a DES, including adsorption and diffuse layers, see paragraph 4.3.
The energy of attraction is assigned the minus sign, the repulsive energy - the plus sign. The energies of attraction and repulsion are considered in the DLVO theory as components of the disjoining pressure between particles. The action of the energies of attraction and repulsion depending on the distance between the particles is shown in Fig. 4.2.
Rice. 4.2.
The resulting total energy curve in Fig. 4.2, three areas can be distinguished.
Plot a. At small distances between colloidal particles (up to 100 nm), attractive forces prevail, an energy well or a near energy minimum appears. If the particles come close to such a distance, coagulation will occur under the influence of the forces of attraction. Coagulation in such cases is irreversible.
Plot b. At medium distances, the electrostatic repulsive forces are greater than the forces of intermolecular attraction, an energy maximum arises - a potential barrier that prevents particles from sticking together; the height of the barrier depends on the surface charge and the thickness of the diffuse layer.
If the potential barrier is high, the particles are not able to overcome it, then coagulation does not occur. The ability to overcome the barrier is determined by its decrease (a decrease in the surface charge and repulsive forces between particles, for example, when exposed to an electrolyte) or an increase in the energy of the particles (heating).
The effect of electrolytes on the structure of the electric double layer is discussed in Subsection 4.3.3.
Further, under the influence of the forces of attraction, the particles approach each other, and coagulation occurs. If the particles cannot overcome the barrier, then coagulation does not occur and the system can maintain aggregate stability for a long time.
Plot in. At relatively large distances (about 1000 nm), attractive forces also prevail, forming on the resulting curve the so-called far minimum. The depth of the far minimum is individual for each system. At an insignificant far minimum, the potential barrier prevents the particles from approaching.
If the far minimum is deep enough, then the particles cannot leave the potential well upon approaching and remain in an equilibrium state at an appropriate distance from each other, preserving their individuality.
The presence of a high potential barrier prevents the particles from coming closer together, and a liquid interlayer remains between them. The system as a whole retains its dispersion, representing a loose sediment - a coagulant, or flocculant. This state corresponds to the reversibility of coagulation; it is possible to transfer the system to the state of a sol (peptization).
« Peptization is one of the methods for obtaining dispersed systems, see paragraph 2.4.
At a high concentration of the dispersed phase, a structured system - a gel - can form.
The specifics of structured systems are discussed in more detail in Section 9.4.
Summary
Aggregate stability of the system (coagulation resistance) is largely determined by the presence of an electric charge on the surface.
- Vetvey E. J., Overbeek J. Th. G. Theory of the stability of lyophobic colloids. N. Y .: Elsevier, 1948.
So, the occurrence of electrokinetic phenomena is due to the diffuse structure of the electric double layer. The difference in the charges of the phases leads to the movement of counterions together with the liquid phase (electroosmosis), and in the case of a dispersed system, to the movement of particles of the dispersed phase (electrophoresis). In this case, the effective electric force (equal to the product of the charge and the potential gradient) will be the greater, the more charges of the diffuse layer are in the mobile liquid. Thus, the greater the mobile charge of the diffuse layer and the proportional electrokinetic potential, the more developed electrokinetic phenomena should be. Hence, it follows that the electrokinetic potential can serve as a measure of the intensity of electrokinetic phenomena and, at the same time, a measure of the degree of diffusion of the diffusion part of the electric double layer. Therefore, it can be used when considering the properties of a system associated with the existence of a diffuse layer, in particular, the stability of hydrophobic sols.
19 Stability of dispersed systems
According to the proposal of N.P. Peskov (1920), the stability of dispersed systems is divided into two types: resistance to precipitation of a dispersed phase (sedimentation stability) and resistance to aggregation of its particles - aggregate stability... With respect to aggregation, dispersed (heterogeneous) systems can be thermodynamically and kinetically stable. Thermodynamically stable dispersed systems are formed as a result of spontaneous dispersion of one of the phases. According to PA Rebinder's classification, thermodynamically stable systems (formed by spontaneous dispersion) are called lyophilic. Thermodynamically unstable disperse systems are called lyophobic systems; they have different kinetic resistance to particle aggregation. Kinetically stable dispersed systems cannot be obtained by spontaneous dispersion; they are stable for a certain time, sometimes very long.
B.D. Summ proposes to distinguish 4 types of instability of colloidal systems:
1) Thermodynamic (aggregate) instability manifests itself in a gradual increase in the size of dispersed particles or the formation of aggregates from coalesced particles.
The evolution of an aggregatively unstable disperse system is quantitatively characterized by the dependence of the particle size and their size distribution on time, as well as by the time dependence of the particle concentration.
Two different processes of reducing the surface energy of a dispersed system are possible:
Coarsening of dispersed particles, leading to an increase in their size (ζ = const). This process is called coalescence (fusion). It is typical for systems with liquid or gaseous particles.
Decrease in specific surface energy (ζ = const). The enlargement of particles can go in two ways. One of them called isothermal distillation, consists in the transfer of matter from small particles to large ones, since the chemical potential of the latter is less (the Kelvin effect). As a result, small particles gradually dissolve (evaporate), and large ones grow. The second way, the most typical and common for dispersed systems, is coagulation, which consists in sticking together (merging) of particles of the dispersed phase. In a general sense, coagulation is understood as the loss of aggregate stability of a dispersed system. The coagulation process also includes the adhesive interaction of dispersed phase particles with macrosurfaces. It consists in the formation of aggregates of many dispersed particles separated by thin layers of a dispersion medium.
A stable free-dispersed system, in which the dispersed phase is uniformly distributed throughout the entire volume, can be formed as a result of condensation from 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 mutual fixation at short distances from each other. Interlayers of the medium remain between the particles. As a result, either floccules are formed (flocculation is the formation of aggregates of several particles separated by medium layers), or coagulation structures, characterized by the mobility of particles relative to each other under the action of relatively small loads (contact points are separated by medium layers). 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 the medium layers and direct contact of the particles. As a result, either rigid aggregates of solid particles are formed, or they completely merge in systems with a liquid or gaseous dispersed phase (coalescence). In concentrated systems, rigid bulk condensation structures are formed solids, which can again be converted into a free-dispersed system only by dispersion (forced).
2) Sedimentation instability. It is caused by the difference in the densities of the substances of the dispersed phase (ρ d) and the dispersion medium (ρ o). This difference leads to gradual settling (sedimentation) of larger particles (if ρ d> ρ o) or their floating
(if ρ d< ρ o ).
The size of dispersed particles affects the aggregate and sedimentation stability in the opposite way. The higher the degree of dispersion (the smaller the particle size), the more pronounced their aggregative instability, but their resistance to sedimentation increases.
3) Phase instability. This refers to a change in the structure of particles while maintaining their size. For example, in the synthesis of colloidal solutions of metals, oxides, and hydroxides, dispersed particles are usually amorphous, and with time, an energetically favorable crystallization process can occur inside the particles.
4) Surface instability. Its reasons are different. For example, surfactants with a large molecular weight (proteins) slowly diffuse from the volume of the dispersion medium onto the surface of the particles and form an adsorption layer over time. Another possible mechanism is the dissolution of the substance of dispersed particles in a dispersion medium. It determines several processes:
The change chemical composition solution near the surface of particles and changes in the structure of the DES;
A change in the microrelief of a solid surface and, as a consequence, a change in the contact angles of wetting.
Analysis of the causes and forms of instability of dispersed systems leads to the following fundamental conclusion: nonequilibrium causes the evolution of dispersed systems.
Thus, the characteristics of dispersed systems can change significantly over time.
The main problem of the theory of stability of dispersed systems is to determine the specific reasons and the mechanism for the combination of individual dispersed particles into larger aggregates and to elucidate the factors that prevent their aggregation.
The theory of stability of hydrophobic sols was developed in detail by B. Deryagin and L. Landau and independently by E. Vervey and T. Overbeck (DLVO theory). According to this theory, two forces act on dispersed particles - the repulsive force (f e) due to the electrostatic and thermodynamic components (wedging pressure) and the attractive force (f d |) (Van der Waals forces). Depending on the ratio of these forces, two variants of the behavior of the colloidal solution are possible:
1) If the attractive force prevails (| f d |> | f e |), then dispersed particles approach each other, contact arises between them, and they combine into a larger aggregate (colloidal
"dimer"). Thus, in this case, an elementary act of the coagulation process can take place.
2) If electrostatic repulsion prevails (| f d |<|f e |), то частицы могут не вступать в непосредственное соприкосновение, и коагуляция золя не происходит.
Thus, the electrostatic (Coulomb) repulsion of dispersed particles is taken as the main factor of the thermodynamic stability of a dispersed system in the DLVO theory.
Additional concepts are introduced to calculate the coagulation conditions:
1) The particles have a prismatic shape and are separated by a plane-parallel gap of width h (see Fig. 11).
2) The particles only move in a direction perpendicular to the gap. Brownian motion is excluded.
To calculate the conditions, not the forces of attraction are compared, but the corresponding interaction energies (U d, U e).
A 12 * |
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12 h 2 |
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where A 1 *, 2 is a complex Hamaker constant; the "-" sign indicates mutual attraction.
The energy of electrostatic interaction (U e) is created due to the overlapping of diffuse layers of counterions in a thin film of electrolyte solution in the gap between the particles.
U e, which depends on the film thickness, creates additional pressure in the film - wedging pressure (Π)... Π is the thermodynamic parameter of a thin liquid film in the space between particles:
dW f, (19.2) dh
where W f is the work required to increase the surface area of the thin film per unit area at a constant temperature.
W f 2 W f, (19.3)
where W f is the additional energy of the film, which must be spent to bring the surface layers ABB'A 'and CDD'C' closer together.
Figure 11 - Occurrence of wedging pressure in a flat thin foam film with overlapping surface layers (h< 2δ)
Physically, the value of W f can be considered as an energy definition of the surface tension of a thin film.
The physical meaning of the quantity Π is the excess pressure in a thin film in comparison with the hydrostatic pressure in a large volume of liquid.
(h) p f p o, (19.4)
where p f is the pressure in the thin film.
Positive wedging pressure prevents film thinning! The appearance of Π is associated with surface forces of a different nature
(electrical, magnetic, molecular). For colloidal chemistry, the former and the latter are especially important.
With a liquid film thickness of 1 μm, Π can |
reach 400 Pa, and 0.04 μm - |
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1.88 ∙ 104 Pa. |
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64 Co RT |
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æh) |
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where 1 / æ = δ is the thickness of the ionic atmosphere.
You don't have to memorize formulas! The main thing is to understand that U e and U d have different signs and depend differently on the thickness of the separating film h:
Figure 12 - Change in energy (U) of a thin electrolyte film depending on its thickness (h)
As can be seen from the figure, U e changes according to the exponential law (proportional to e - æh), U d - according to the power law (proportional to 1 / h 2). Therefore, at small distances, attraction will prevail (at h → 0 U d → ∞). At large distances, attraction also predominates, since the power function decreases more slowly than the exponential. At medium distances, a local (far) maximum is possible. It corresponds to an energy (potential) barrier that prevents particles from coming together and coagulating.
Analysis of the equation and graph allows us to distinguish three cases of the behavior of a dispersed system depending on the ratio of the height of the energy barrier U M, the depth of the potential well U N at large distances, and at small distances the energy of thermal vibrations k B T.
Figure 13 - Change in energy (U) of a thin electrolyte film depending on distance
Curve 1 in Fig. 13 corresponds to such a state of a dispersed system when, at any distance between particles, the energy of attraction prevails over the energy of repulsion. The thermal motion of the particles does not change this ratio either. In this state of the dispersed system, rapid coagulation is observed with the formation of aggregates; in systems with liquid and gaseous dispersed phases, coalescence occurs. Curve 2 indicates the presence of a sufficiently high potential barrier and secondary
