Coagulation under the influence of electrolytes

Coagulation under the influence of electrolytes.

EI 64.21 KT
Maqsudova Dilraboxon

Stability and coagulation of colloidal systems

1.Aggregative and sedimentation stability of dispersed systems.

2.Coagulation of colloidal systems and factors affecting it.

3.Fast coagulation (Smolukhovsky) theory.

4.Coagulation under the influence of electrolyte mixtures, recharging of colloids and mutual coagulation.

5.Determination of the coagulation threshold.

1. Aggregative and sedimentation stability of dispersed systems.

Lyophobic colloidal systems with a weak interaction between the dispersed phase and the dispersion medium are unstable, and their dispersion level changes over time, that is, they grow larger. The speed of the growth process is different in different colloidal systems.
A decrease in the level of dispersion in lyophobic sol can occur in two ways:

if as a result of recrystallization, the process of joining or absorption from small particles to large particles takes place;

as a result of particles of the dispersed phase sticking together and growing larger.

The process of colloidal particles becoming bigger under the influence of molecular forces is called coagulation. The growth process is very fast in some colloids, and much slower in others. In the coagulation system, particles of the dispersed phase settle to the bottom of the container (sedimentation) or separate into a liquid layer consisting of an emulsion (coacervation), depending on the relative masses of the liquid and solid. Dispersed systems in which the particles of the dispersed phase do not settle significantly under the influence of gravity are called sedimentation stable systems. According to Professor N.P. Peskov, the stability of dispersed systems is of two types: aggregative and kinetic (or sedimentation).
The aggregative stability of the dispersed system is the property of maintaining the specific level of dispersion of the dispersed system, i.e., not undergoing coagulation. There are two reasons for aggregate stability:

Colloidal particles have the same charge;

Solvent molecules surround the colloidal particle and form solvate shells around the particle. The aggregative stability of the system depends on the composition of the sol, the structure of its particles, and the state of the colloidal solution.

The sedimentation stability of the dispersed system indicates the ability of the dispersed phase particles not to separate from the dispersion medium under the influence of gravity (or centrifugal force).
Sedimentary stability depends on diffusion and Brownian motion. The speed of separation of particles from the dispersion medium also depends on the intensity of their Brownian motion and specific mass.

Due to the large size of particles in suspensions and emulsions, they cannot move on their own, that is, diffusion almost does not occur in them. Therefore, suspensions and emulsions are not stable in terms of sedimentation. Therefore, coarse dispersed systems are unstable microheterogeneous systems from the sedimentation point of view.

Due to the high level of dispersity of the colloidal solution, their micelles can move independently, that is, diffusion occurs in colloidal solutions. Therefore, colloidal solutions are stable from the point of view of sedimentation. However, the particles in the colloidal solution can become bigger under various influences and the dispersity level of the colloidal system decreases. As a result, the system loses its stability. Therefore, colloidal systems are aggregatively unstable ultramicroheterogeneous systems.

In real solutions, the particles do not join together and become larger, in these solutions the particles move by themselves. So, true solutions are aggregative and sedimentary stable homogeneous systems.
Concepts about the aggregative stability of lyophobic colloids can be divided into 5 groups:

Aggregative stability of ion-stabilized lyophobic colloids.

Stability of lyophobic colloids under the influence of structural-mechanical factors.

Stability due to solvation of particles.

Stability of the system resulting from thermodynamic properties. The reason for this is the entropy factor of the system.

Stability of spherical (spatial) factors.

Coagulation of colloidal systems and factors affecting it.
Coagulation is the phenomenon of colloid solution particles joining together and becoming larger. Under the influence of gravity, the enlarged particles begin to sink from the upper parts of the solution to the lower parts, and finally the particles are separated from the solution. Spontaneous coagulation takes a long time. Coagulation can be accelerated using various means:

adding electrolyte to the soil;

adding another colloid to the sol;

is accelerated by heating the sol.

A thorough study of the coagulation that occurs when an electrolyte is added has led to the following conclusions.
If a sufficient amount of any electrolyte is added to the colloidal solution, coagulation occurs. If coagulation can be directly seen, it is called open coagulation, if it cannot be seen, it is called hidden coagulation.

In order for open coagulation to occur, the concentration of the electrolyte must exceed the value of the coagulation concentration, that is, the solution coagulates immediately until the concentration of the electrolyte exceeds the minimum amount called the coagulation threshold.

Coagulation is caused by only one ion of the electrolyte. Positively charged colloids are coagulated under the influence of anions, and negatively charged colloids are coagulated under the influence of cations.

The coagulation threshold of this colloid first of all depends on the ionic valency of the coagulating substance.

If the valency of the coagulating ion is large, its coagulating properties will be stronger. Experience shows that if the coagulation property of a monovalent cation is 1, that of a divalent cation is approximately 70, and that of a trivalent cation is approximately 550. coagulation concentration of electrolyte (coagulation threshold) is represented by millimoles of electrolyte added to 1 l of solution.
Shultze and Gardy determined the relationship between the valency of the electrolyte ion and its coagulation power. This relationship, called the Schultze-Gardy rule, is defined as follows:
The greater the valency of the coagulating ion, the greater its coagulating power and the lower the coagulating concentration.
This rule has an approximate character, because sometimes the coagulation effect of ions of monovalent organic bases can be higher than that of 2-valent ions. The coagulation effect of Li, Na, K, Rb, Cs cations combined with the same ions (for example, NO3) changes in the following order:

Cs+ >Rb+ >NH+ >K+ >Na+ >Li+

When the cation is the same, the coagulation effect of Cl-, Br-, NO3-, J- ions on positive colloids follows the following sequence:

Cl- >, Br- >, NO-3 >, J-

These series are called lyotropic series in colloid chemistry.

The modern physical theory of aggregative stability and coagulation of dispersed systems was proposed in 1945 by B.V. Deryagin and L.D. Landau.

According to this theory, the forces of attraction and repulsion between the particles of the dispersed phase affect each other.

In this theory, it is shown and based on the distance from which the effect of repulsion (Vander-Waal's molecular forces) is higher than the force of Brownian motion. At the same time, it has been shown that there are different forces between the particles, called "pressure pressure".

Theory of rapid coagulation.
The process of coagulation, like chemical reactions, takes place in a certain time, therefore it has a kinetic character.

The rate of coagulation depends on the Brownian motion of the particles of the colloidal system, their interaction (in other words, the size of the radius of the sphere of mutual attraction of particles and the diffusion coefficient D) and the initial concentration of particles in the system.

From the theoretical point of view, the simplest process of coagulation can be imagined as follows: if two particles collide with each other once and form a larger particle, such coagulation is called rapid coagulation, and its speed depends on the intensity of the Brownian motion of colloidal particles. depends, but if it depends on the concentration of added coagulating electrolyte, such coagulation is called slow coagulation. The theory of rapid coagulation was created in 1946 by M. Smolukhovsky.

According to Smolukhovsky's theory, due to the mutual repulsion between colloidal particles, these particles cannot unite with each other, but when they are very close to each other, these particles attract each other. if no electrolyte is added, the colloidal solution is stable because the colloidal particles are far from each other.

After the electrolyte is added to the colloidal solution, the particles come closer to each other and begin to attract each other; as a result, the colloid coagulates slowly. if the electrolyte is added again, the coagulation will accelerate and the particles will start to unite with each other.

To find the rapid coagulation rate constant - K, we use the following formula K =2DL
where D is the diffusion constant, L is the distance between the particles affected by gravitational forces.

Smolukhovsky's theory was tested in the experiment and its correctness was confirmed.

It has been proved that Smolukhovsky's theory of fast coagulation can be applied to "slow coagulation", but here it is necessary to take into account the property of effective coagulation. In that case, the slow coagulation rate constant is found from the following formula.

K =2 Dl

where  is the effective collision coefficient.

Coagulation under the influence of electrolyte mixtures, recharging of colloids and mutual coagulation.
Coagulation also occurs under the influence of several different electrolyte mixtures and can be of three types.

The coagulation ability of one electrolyte is added to that of the other electrolyte. This property is called "additivity" of the electrolyte effect.

One electrolyte enhances the effect of another electrolyte, this phenomenon is called "synergism" or "sensitization".

The coagulant effect of one electrolyte decreases when the second electrolyte is added, that is, the phenomenon of antagonism occurs.

Antagonism and synergism phenomena often occur when cells are coagulated, and additivity is less common.

As a result of studying the coagulation of dispersed systems under the influence of electrolytes, the so-called "recharging" phenomenon of particles (colloids) was revealed. At the same time, it was shown that there is an alternation of coagulation with the absence of coagulation depending on the concentration of the electrolyte added to the sol, that is, there are processes known as coagulation zones, in other words, false lines.

It is known that dispersed systems do not undergo coagulation under the influence of electrolytes, but when another colloid with an opposite charge is added to the colloid solution, a coagulation process occurs, that is, it undergoes "mutual coagulation". Coagulation of colloids with colloids depends on their charge and concentration.

For example, the mutual coagulation between positive and negative salts of AgJ (when they are taken in equivalent amounts) can be shown by the following scheme:

[nAgJ]xJ-+[nAgJ]xAg+(2n+x)AgJ

If an excess of positively charged sol is added, the sol remains positively charged and does not coagulate:

[mAgJ] xAg+ [nAgJ] yJ-(m+n+y)AgJ,(x-y)Ag+

When colloidal solutions are heated, sometimes they coagulate quickly, sometimes heating has little effect. In general, coagulation is accelerated when colloids are boiled. The reason for this is that when the sol is boiled, its charge decreases, and the balance between particles and ions in the solution is disturbed. When the sol is heated, colloidal particles poorly adsorb ions, as a result of which their charge decreases, and such particles meet each other and the sol coagulates.

Determination of the coagulation threshold.
In colloidal chemistry, changes in sol turbidity, sedimentation, and sol color change are signs of coagulation, and the "coagulation threshold" is determined as a result of observing these signs. The minimum amount of electrolyte needed to coagulate the sol is called the "coagulation threshold" of the sol. To determine it, equal amounts of electrolyte solutions of different concentrations are successively added to the sol placed in test tubes.

First, water is poured into test tubes, and its volume is increased as it passes from one test tube to another test tube. Then electrolyte solution is added so that the volume of the liquids in the test tubes is the same. After that, two adjacent test tubes are taken, one of which is turbid, and the other is not turbid. Let us assume that the initial concentration of the electrolyte is S, and its coagulation volume is V. In it, the number of millimoles of electrolyte of the same volume is equal to Coagulation threshold is usually calculated for 1 liter of solution. If W ml of sol was taken for the experiment, the coagulation threshold of the sol is calculated by the following formula:

The following formula is usually used in the experiment:

Here, N- the concentration of the electrolyte (expressed in normals), Vel- the volume of the electrolyte solution, W- the volume of the solution expressed in liters, - the coagulation "threshold" of the electrolyte.

B.V. Deryagin found the following connection between the coagulation threshold of the sol and the valence (Z) of the coagulating ion:

where A is the general constant,  is the dielectric constant, and T is the absolute temperature.