The structure of colloidal particles. Electrokinetic properties. Stability and coagulation of colloidal solutions. Schulze-Hardy Rule Chapter 20. Stability and coagulation of sols

The structure of colloidal particles. Electrokinetic properties. Stability and coagulation of colloidal solutions. Schulze-Hardy Rule

Chapter 20. Stability and coagulation of sols

According to the general definition, coagulation is one of the various properties exhibited by colloidal solutions. A colloid is a heterogeneous mixture of one substance of very fine particles (dispersed phase) dispersed into another substance (dispersion medium).

Substances like metals, their sulfides etc. cannot be simply mixed with the dispersion medium to form a colloidal solution. Some special methods are used to make their colloidal solutions. Such kind of sols is known as lyophobic sols. These kinds of colloidal solutions always carry some charge on them. Charge present on the colloidal sols determines their stability. If by any chance we can remove the charge present on the sol, the particles get closer to each other and they accumulate to form aggregates and precipitate under the action of gravity. This process of accumulation and settling down of particles is further known as coagulation or precipitation.

Coagulation Techniques:

The process of coagulation can be carried out in the following ways:

1. By electrophoresis: In this method, the colloidal particles are forced to move towards the oppositely charged particles and then they are discharged and collected at the bottom.

2. By mixing two oppositely charged sols: In this type of coagulation equal amounts of oppositely charged particles are mixed, they cancel out their charges and then precipitate.

3. By boiling: Whenever we boil a sol, the molecules of the dispersion medium start colliding with each other and with the surface, this, in turn, disturbs the adsorption layer. This reduces the charge on the sol due to which the particles settle down.

4. By persistent dialysis: Under the persistent dialysis parts of electrolytes are removed completely and the sol loses its stability and ultimately coagulates.The ability of the dispersed phase to maintain its original degree of dispersion of particles and their uniform distribution in the dispersion medium is called stability. N.P.Peskov introduced the concept of kinetic (sedimentation) and aggregative stability of colloidal systems.

The kinetic or sedimentation stability of colloidal systems is explained by Brownian motion, which counteracts the sedimentation of particles under the influence of gravity and diffusion.

Aggregative stability characterizes the ability of colloidal particles to maintain the degree of dispersion. This is due to the presence of electric charge on the particles. This charge prevents sticking of colloidal particles and contributes to the formation of a solvation shell in solutions around the particles.

At the loss of aggregative stability, the coarsening of particles occurs, ie sticking them together to form larger aggregates. Coagulation of sols can be caused by various factors: temperature change, irradiation with elementary particles, etc. However, the most important factor causing coagulation is the action of electrolytes.

In 1882, Schulze established that the coagulating force of an ion is greater, the higher its valence. In 1900, Hardy confirmed these results and summarized them as follows:

1. The effective ions of the electrolyte in bringing about coagulation are those which carry charge opposite to that of the colloidal particles. These ions are called coagulating ions.

2. Greater is the valency of the coagulating or the Flocculating ion, greater is its power to bring about the coagulation. When preparing for work it is necessary to study the following questions from the textbook and notes: 1. Kinetic and aggregative stability of colloidal solutions, factors of stability.2. Coagulation: slow and fast, hidden and pronounced. Signs of coagulation. Factors causing coagulation.3. Coagulation of sols with electrolytes. Coagulating ability. The rules of Schulze- Hardy. Coagulation threshold and its definition.

Task I. Determine which ions are coagulating, the effect of ion valence on coagulation and whether the data obtained correspond to the Schulze-Hardy rules

Task 2. Determine the coagulation threshold of Fe(OH)₃ hydrosol

Task 3. Compare the coagulation of lyophilic and lyophobic colloidal solutions.