Colloid chemistry is the science of disperse systems and surface phenomena that occur at the interface, and its main theory is coagulation. The coagulation threshold depends on many factors. Colloid chemistry, in addition to it, studies adsorption, adhesion, wetting, and other phenomena in disperse systems. This article will be devoted to one of the sections, which nevertheless is closely related to the rest.
Coagulation
What is coagulation? Translated from Latin, this is coagulation, thickening, the combination of small particles in dispersed systems and their transformation into larger ones as a result of adhesion, a process that applies equally to chemistry and physics. Thus coagulation structures are formed. This theory is constructed in such a way: there is a dispersed phase where the particles are in Brownian motion (independently of each other) until the two particles come together at a distance at which their centers can be defined as the radius of the sphere of influence (denoted by d).
This distance is approximately equal to the sum of the particle radii, and direct contact is inevitable because suddenly (by an immediate jump) interaction forces appear, particles are attracted to each other and aggregated. The probability of a collision of more than two particles is negligible, and therefore either single or double with a single, or double particles with each other, triple with single ones, and so on are attracted. This is where the theory of chemical bimolecular reactions begins. This is coagulation. The coagulation threshold leads to precipitation in a colloidal solution in the form of flocs (flakes), or jelly is obtained.
Definition
What is coagulation as a process? We managed to find out, now we need to derive a definition. Coagulation is a decrease in the degree of dispersion, as well as the number of particles by sticking together. The result is sedimentation of the dispersed phase (i.e., precipitation of particles) or any changes in the dispersed system that was originally presented. You can observe in nature how spontaneous coagulation occurs. This is the aging of a colloidal solution (sol) with delamination into a dispersed medium and into a solid phase with a minimum energy. But mankind is able to artificially cause coagulation with the help of coagulants (special reagents).
Coagulation threshold - a smaller amount of electrolyte, sufficient to start the process of precipitation. Its structures are called coagulation. They are formed if the dispersed system loses sedimentation stability. A sufficient content of the dispersed phase provides the reinforcement of the full volume of the entire dispersed system. However, the entire liquid medium cannot “harden”; the colloidal dispersed phase is usually very small, several percent of the total mass.
The properties
The strength of the coagulation structure is not too high, mechanical stresses may well cause spontaneous recovery in a dispersed mobile medium. This property (thixotropy) has polymers, varnishes, paints, where coagulation structures are formed due to pigments and fillers. The most typical example is spatial grids that arise in clay dispersions when they are coagulated with electrolytes.
Sedimentation stability is a counteraction to the settling of particles that are heavy enough but do not settle under the influence of gravity. This occurs in all coarse dispersion systems, which can be seen in the examples of sediment in suspensions and cream in emulsions, where a separation of a pure dispersion medium and a layer of the dispersion phase occurs. Two patterns are characteristic of sedimentation: slow subsidence and fast. In the first case, the particles do not adhere, settling separately, and in the second, they settle together. The first case shows sedimentation stability, and the second - instability.
Sustainability problem
Everyone understands stability as the ability to maintain the original composition unchanged. This also happens in coagulation processes. The coagulation threshold violates this condition. Just then the time of constant concentration of the dispersed phase and constant distribution of particles in it ends. In colloid chemistry, one of the central problems is life or death, which disperse systems choose for themselves. These are opposite tasks, and they constantly have to be solved practically. For example, the preservation or destruction of a dispersed system.
If it is a food mass, it is necessary to maintain its stability, and if water is from any reservoirs, it is necessary to destroy stability by cleaning it. That is, to sediment all the bad impurities. Or, for example, oil - their dispersed phase consists of complex supramolecular formations that are released into an independent microphase as particles of various sizes. And here dispersed systems are the broadest field of activity.
Aging
The aging rate of a colloidal sol depends on many factors: phase separation, diffusion coefficient, particle radius, solubility, and macrophase temperature. Electrocoagulation is the acceleration of aging when the coagulating ability of an electrolyte is used. Collisions of particles far from each time cause fusion, since they are surrounded by a double electric layer, on the contrary, repelling them from each other.
Using electrolytes, this layer can be destroyed or deformed, thereby accelerating coalescence. The type of electrolyte, that is, the lyotropic rows of ions, the valency of the electrolyte affect the efficiency of this process. Hydrophobic sols can break down if little electrolytes are added. This was the object of a huge number of theoretical and experimental works.
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The effect of electrolytes on the state of hydrophobic sols shows that the coagulating effect depends on the charge of ions. The coagulation rate increases significantly with an electrolyte concentration that exceeds a critical value (this is the coagulation threshold). Its formula is calculated if the concentration of the coagulator (electrolyte) is known, C, the volume of electrolyte added is V, and the total sol volume is V 30 (usually ten milligrams). The value opposing the coagulation threshold is the coagulating ability of the electrolyte, and the lower the coagulation threshold, the higher the ability of the electrolyte to coagulate.
However, not all electrolyte is involved in this process, here the main active element is precisely that ion that coincides in its charge in sign with the charge of the enemy (and the charge of an ion called up for coagulating activity is always opposite to the charge that a colloidal particle has). Such an ion is called a coagulant ion. And the greater its charge, the higher the coagulating ability, according to the Schulze-Hardy rule. The relationship between the coagulant ion and the coagulation threshold is described in the Deryagin-Landau theory. The rules of electrolyte coagulation include a rule of significance regarding the ratio of coagulation thresholds for monovalent, divalent and trivalent ions. Y 1 : Y 2 : Y 3 = 729: 11: 1. This means that a three-digit ion is able to coagulate 729 times faster than a single-bit.
Amendments
Over time and in connection with the development of colloidal chemistry as a science, some deviations have been established to the rule of significance. The coagulation threshold depends not only on the charge, the radius of the coagulant ion, the ability to hydrate and absorb, and the very nature of the ion that accompanies the coagulant also influence. The multiply charged ion gives the effect of recharging particles, that is, if the sign of the charge changes, the potential of the colloidal particle also changes.
The added ions exchange with counterions, replace them in the adsorption and diffuse layers. If a multiply charged ion is small, such as, for example, Al 3+ , Th 4+, and others, superequivalent adsorption is obtained when this ion replaces a nonequivalent amount of former ions on the particle surface in charge. And then, for example, instead of one or two K + ions, the Th 4+ ion appears. This shows a change in the potential and sign of the charge.
Physics
The colloidal mixture is stable if it is assisted by electrostatic repulsion and steric effects. That is why coagulation is carried out by the following method: electrostatic repulsion is prevented by changing the acidity or adding salts, due to which colloidal particles are able to get closer to the distance that is necessary for their adhesion.
The purpose of coagulation is the formation of flocculent clusters, which is necessary, for example, to sediment or filter water. Only if the flakes are large enough can they be removed. And without coagulation, doing this is extremely impractical, since it will take a huge amount of time. The optimal size of flocs for water treatment, for example, should be a few millimeters, otherwise impurities are almost impossible to remove.
Process
Coagulation has two stages:
1. The chemical mixes quickly with water - about one minute, so that the coagulant is properly distributed and does not destroy the resulting flocs. Usually a special mixer tank is used for mixing.
2. From half an hour to forty-five minutes , coagulation itself occurs when water, passing through several reservoirs with a decreasing mixing speed, forms a precipitate.
A separate case is when coagulation is performed by electrolytes, where two hydrophobic sols have different signs of charges. In conventional coagulation, coagulant ions are involved in recharging, and in this case, a certain ratio of the concentration of mixed sols is necessary for this reloading to occur.
Value
Mutual coagulation is very important in both natural and technological processes. For example, the formation of the soil horizon occurs due to coagulation of soil colloids by electrolytes. Salts in water are hydrolyzed, forming colloidal particles, positively charged, - 1 () 3 , which interact with colloidal particles in water, charged almost always negatively, which leads to mutual coagulation, after which coagulated particles precipitate.
Coagulation is most effective when electrolytes containing ions with an opposite charge are added to a disperse system, thus eliminating sedimentation stability. For electrolyte coagulation processes, iron or aluminum salts are used, as well as mixtures thereof. Coagulation can be triggered in a variety of ways - from mechanical stress to temperature changes. If water, for example, is boiled or frozen, a precipitate is necessarily formed. A variety of radiation, the addition of foreign substances, especially electrolytes, also affect coagulation processes. It is electrolyte coagulation that is most important, and therefore it is well studied and widely used.
Electrolyte coagulation
As already mentioned, electrolyte coagulation most clearly occurs in colloidal systems, where the stabilizer is ionic, and stability to the highest degree provides electrostatic repulsion of colloidal particles. From this we can conclude that, together with the action of the electrolyte, the electrostatic repulsion of particles decreases, and the particles are able to stick together.
Even with a not very high concentration of electrolytes, colloidal solutions begin the process of coagulation - slow or fast. But very often it is necessary to create stability protection for sols, creating adsorption layers on the surface of particles, in which the structural and mechanical properties are enhanced. Thus, electrolyte coagulation can be completely stopped or prevented by simply adding a solution of high molecular weight compounds - sodium caseinate, gelatin, egg albumin, or something similar.