Coagulation

Coagulation (from Latin coagulatio - curdling), a characteristic process of colloidal solutions (sol) consisting either in the precipitation of the dispersed phase (see Dispersed systems) or in the solidification of the entire colloidal solution into a gel. In both cases, there occurs the merging of individual smallest particles of the dispersed phase, their enlargement, or aggregation. The prerequisite for coagulation is a change in the stability of the colloidal solution by destroying or weakening those factors that ensure this stability, which for lyophobic colloids reduce to the electric charge of the particles, and for lyophilic colloids to the more or less close chemical bond of the particle surface with the solvent. Coagulation can be caused by various external influences on the colloidal solution: thermal effects (heating or cooling), the action of light, electricity, mechanical influences (shaking), etc. But the most studied is coagulation under the influence of adding various substances, especially solutions of electrolytes in the case of lyophobic sols. As has been indicated, the most important factor in the stability of these sols is the electric charge on the surface of their particles. When an electrolyte is introduced into the sol, the particles adsorb those ions of the electrolyte which carry a charge opposite in sign to the charge of the particles, and thereby neutralize it. The neutralized discharged particles, according to the theory of Haber, lack the repulsive electrostatic forces acting in the colloidal solution and therefore, being in continuous and irregular Brownian motion (see), can approach each other at such close distances that intermolecular forces of attraction or cohesion (acting only at very small distances) manifest between them, causing contact and merging of colloidal particles. According to modern electrostatic concepts, colloidal particles are surrounded by an electrical double layer (see). Its inner lining consists of a layer of ions, which form, according to some theories, the surface of the particles themselves, and according to others, are adsorbed on it. The sign and number of charges of these ions determine the charge of the particle. The outer lining of the double layer consists of ions of the opposite sign, located in the liquid surrounding the colloidal particles. They are electrostatically attracted to the particle by ions of the inner lining closely bound to it; on the other hand, thermal motion (lying at the basis of diffusion) tends to distribute them uniformly throughout the liquid. As a result of these two opposing influences, equilibrium is established between them: the ions of the outer lining are distributed throughout the liquid between the particles, but unevenly: they accumulate around the particles in a much higher concentration, gradually decreasing as the distance from the particles increases. Thus, around each particle an "ionic atmosphere" or "diffuse double layer" of decreasing outward density is formed. Since the ions entering this atmosphere have the same sign of charge around all particles, the atmospheres electrostatically repel each other and do not allow the particles to approach closely enough for intermolecular forces of attraction to begin acting. With the accumulation of electrolytes in the solution (increasing their concentration), especially in the case of multivalent ions, the thickness of the ionic atmosphere decreases, the outer lining of the double layer approaches the inner one, which is equivalent to the discharge of the particle surface. The particles can approach each other much more closely and merge under the influence of intermolecular forces: coagulation sets in. The number of charges in a unit volume and their distribution in space determine the potential of the particle, i.e., the electrical potential difference between the particle and the liquid. In view of the diffuseness of the double layer of ions, it is obvious that this potential decreases gradually. Its full value, determined thermodynamically, for colloidal particles has not yet been measured. The existing methods for measuring particle potential are applicable only to moving particles, such as those observed in cataphoresis (see). Colloidal particles carry with them the adjacent layers of liquid with ions floating in them, and the potential measured by these methods is not the full (thermodynamic) potential, but only a part of it - the potential difference between the inner part of the solution, far from the particles, and the boundary between the part carried by the particles and the part not carried by them of the ionic atmosphere. This potential is called the electrokinetic (zeta) potential. It is usually less than the full (epsilon) potential, and in some cases has the opposite sign. For most colloidal particles it ranges from 30-70 mV (0.03-0.07 volts). With an increase in the concentration of ions, especially multivalent ones, in the liquid surrounding the particles, the zeta potential decreases accordingly with the decrease in thickness (diffuseness) of the double layer. Coagulation can occur only when the zeta potential has reached a certain low value, the so-called critical potential, characteristic of each sol. In the reduction of the zeta potential and the subsequent coagulation, the main role is played by ions carrying a charge opposite in sign to the charge of the particles: in the case of negative sols - cations, in the case of positive sols - anions. The action of ions of the same sign as on the particles is secondary and opposite in direction: they stabilize the sol. The minimum concentration of ions (of opposite sign) necessary for coagulation is determined by the Schulze-Hardy rule of valence: it is the lower, the higher the valence of the ion. For three-, two-, and univalent ions, these concentrations are related as 1:20:1,000 or 1:10:500 depending on the nature and concentration of the sol. The more concentrated the sol, the smaller these differences in the minimum concentrations of ions necessary for coagulation (in German Koagulationswert) become. As an example, a table (according to Freundlich) is given of the minimum concentrations (in millimoles per 1 liter) of various substances causing coagulation of the negative hydrosol As₂S₃ with a content of 1.857 g per 1 liter. Substance Concentration Substance Concentration Substance Concentration K-acetate 1 LiCl 58.4 NaCl 51.0 KNO₃ 50.0 KCl 49.5 K₂SO₄ 65.6 NH₄Cl 42.3 HCl 30.8 MgCl₂ 0.717 MgSO₄ 0.810 CaCl₂ 0.049 SrCl₂ 0.635 BaCl₂ 0.691 ZnCl₂ 0.685 UO₂(NO₃)₂ 0.642 AlCl₃ 0.093 Al(NO₃)₃ 0.095 Ce₂(SO₄)₃ 0.092 Guanidine-nitrate 16.4 Strychnine-nitrate 8.0 Aniline hydrochloride 2.52 Morphine hydrochloride 0.425 Neufuchsin (dye) 0.114 This table shows, besides the value of the valence of ions, the enormous influence of their adsorbability: univalent ions of alkaloids (at the end of the table) cause coagulation at lower concentrations than inorganic univalent cations, and some - even at lower concentrations than divalent ones. This is explained by the much greater adsorbability of alkaloids and dyes by colloidal particles. The adsorption of ions of inorganic electrolytes has been indisputably proven by various methods, and particles of negative sols adsorb predominantly cations, while positive sols adsorb anions. The adsorption of various ions occurs in equivalent quantities and is accompanied by the displacement from the particle surface of equivalent quantities of ions of the same sign that initially surrounded the particles, for example, in the case of acid sols - hydrogen ions. In the coagulation of lyophilic colloids, the removal of charges from particles, the decrease in zeta potential, or the decrease in diffuseness of the double layer play a much smaller role than in lyophobic colloids. Here the main factor of stability is the chemical affinity forces binding the particle surface with the solvent molecules (for example, water in the case of hydrosols). Therefore, coagulation here is caused by substances having a greater affinity for the solvent than the particles, for example, alcohol in the case of sols of agar-agar in water. Substances capable of strongly adsorbing on the particle surface and thereby creating an interlayer between the particles and the solvent, such as tannin on agar-agar, also produce coagulation. Electrolytes are also capable of causing coagulation of lyophilic sols; but the main role is played not by the discharging action of their ions, but by the dehydrating action. Therefore, the ions having the greatest affinity for water act most strongly. The coagulating ability of anions increases, for example, in the following lyotropic series found by Hofmeister: OH < CNS < J < Br < NO₃ < Cl < SO₄; for cations: organic cations < H < Cs < Rb < K < Na < NH₄ < Li. The order of ions in these series depends on the concentration of hydrogen ions (pH) of lyophilic sols; with a shift in reaction, it can change (see Hofmeister's series). When the stability of the colloidal solution is disrupted by the removal of charges (compression of the double layer) or the destruction of the intermediate shell between the particles and the dispersion medium, the obstacles to the sticking together and merging of particles during their encounters in Brownian motion disappear. If all obstacles are removed, rapid coagulation occurs; if some remain and not every collision leads to sticking together, slow coagulation is observed. The mathematical theory of these phenomena from a kinetic point of view is given by Smoluchowski (M. v. Smoluchowski).

He calculated the number of particles g_x, v₂, v₃b in the sol, consisting of 1, 2, 3, etc. simple (primary) particles at a known moment of time, and from this calculated the so-called coagulation time T, during which the total number of particles is halved: inDRv₀ where D is the diffusion constant, R is the radius of the sphere of action of molecular attractive forces, v₀ is the number of primary particles initially present in the sol. Subsequently, this theory was extended by Müller (H. Müller) to polydisperse sols, i.e., those consisting from the start of particles of different sizes, and to sols with non-spherical particles. The Smoluchowski theory was repeatedly tested on various sols both by direct counting of particles at certain intervals after the beginning of C., and by other methods (by the increase in the turbidity of the sol, the change in its color, etc.) and was found to be in good agreement with experience. Lit.-see lit. to the article Colloids. A. Rabinovich.