THEME Disperse systems. Colloidal solutions: the preparation, purification and properties.

Plan Disperse systems. Classification. Colloidal stat. Types of colloidal solution. Preparation of colloidal solutions. Method of purification colloidal solution. Structure of colloidal parts. Stability of disperse systems Coagulation. The coagulation threshold. Sedimentation.

Disperse systems are called systems, which consist of two phases, one of which is scattered or dispersed in other. The disperse phase - phase which is scattered (dispersed) in medium. The disperse medium - phase in which dispersion done.

By stat of dispersed phase and dispersed medium Classification: By stat of dispersed phase and dispersed medium

By size of dispersed phase

Fog

Colloidal solution Colloidal solutions have dispersed phase particle, which size between 10-9 to 10-7m or 1 nm to 100 nm.

Classification of colloidal solution Lyophilic colloids - solvent-attracting Lyophobic colloids - solvent-repelling

Preparation of colloidal solutions 1. Dispersion or disintegration methods: Mechanical disintegration Electro-disintegration (Bredig s method). 2. Peptization 3. Condensation or aggregation methods: chemical reaction: - double decomposition; - reduction; - oxidation; - hydrolysis. exchange of solvent; condensing vapours of a substance into the solvent; excessive cooling.

Dispersion These methods involve the breaking of the bigger particles to colloidal size.

Mechanical disintegration The mechanical disintegration is carried out in a machine called «colloid mill». It consists of two steel discs with a little gap in between and capable of rotating in the opposite directions at high speed. А suspension of the substance in water introduced into the mill. The size of suspension particles is reduced to that of colloidal size

Electro-disintegration (Bredig’s method) This method is employed for obtaining colloidal solutions of metals like gold, silver, platinum etc. An electric arc is set up between two metallic electrodes suspended in а trough of water. The intense heat of the arc converts the metal into vapours which are condensed immediately in the ice cold water bath resulting in the formation of particles of colloidal size

Peptization - is a process of passing of a precipitate into colloidal particles on adding suitable electrolyte. The electrolyte added is called peptizing agent.

Condensation or aggregation methods These methods involve the joining together of а large number of smaller particles to form particles of colloidal size.

Chemical reactions By double decomposition: As2O3 + 3H2S = As2S3 + 3H2O By reduction: 2 AuCl3 + 3SnCl2=2 Аu + 3SnCl4 By oxidation: Br2 + H2S = S + 2 HBr By hydrolysis: FeCl3 + 3 H2O = Fe(OH)3 + 3 HCl

By exchange of solvent Substances like sulphur and phosphorous are fairly soluble in alcohol but less soluble in water. If their alcoholic solutions are poured in water, colloidal solutions of sulphur and phosphorus are obtained

By condensing vapours of а substance into the solvent Colloidal solutions of sulphur and mercury in water are prepared by passing their vapours cold water containing а little stabilizing agent like ammonium nitrate.

By excessive cooling Colloidal solution of ice in an organic solvent like ether is obtained by freezing a mixture of the solvent and water

Purification colloidal solutions When a colloidal solution is prepared, quite often it contains certain impurities of electrolytes which are crystalloidal in nature and tend to destabilize the solution. Hence their removal is very essential.

Method of purification colloidal solution Dialysis: electrodialysis; compensatory dialysis; vividialisis; hemodialysis; Artificial kidney. Ultrafiltration Ultracentrifugation

Dialysis The process of separating the particles of colloids from those of crystalloids by diffusion of the mixture through semipermeable membrane (а parchment or an animal membrane) is known as dialysis. The above process can be quickened if an electric field is applied around the membrane (the process is then called Electro- dialysis).

Electro-dialyser

The most important application of dialysis is in the purification of blood in the artificial kidney

artificial kidney

Structure of colloidal parts {[nuclear] + adsorptive layer} diffuse laeyr

Structure of a micelle Fe (OH)3. The iron hydroxyde sol a formed, if iron chlorate is hydrolyzed: FeCl3 + H2O = Fe(OH)3 + HCl Reaction products interact: Fe (OH)3 + HC1 = FeOCI + 2H2O Iron oxychloride can dissocied: FeOCI = FeO+ + Cl- The potential-determining ions will be only FeO+ as the ion Cl-: {[Fe(OH)3]m n FeO+ (n-x)Cl-}x+ xCl-

Properties Heterogeneous character Stability Filterability Visibility 1. Physical Properties Heterogeneous character Stability Filterability Visibility 2. Colligative properties - osmotic pressure 3. Mechanical properties – Brownian movement 4. Optical properties – Tyndall affect 5. Electrical properties

Kinetic stability А major source of kinetic stability of colloids is the existence of an electric charge on the surfaces of the particles. On the account of this charge, ions of opposite charge tend to cluster nearby, and an ionic atmosphere is formed.

On placing а colloidal solution under the influence of an electric field, the particles of the dispersion medium move towards oppositely charged electrode, provided the colloidal particles is called electro-osmosis.

The movement of colloidal particles under the influence of an electric field is called electrophoresis or cataphoresis. а) Before electrophoresis (b) After electrophoresis

Flocculation (coagulation) Aggregation of the particles arising from the stabilizing effect of this secondary minimum is called flocculation.

Hardy-Schulze Law Greater is the valency of the oppositely charged ion of the electrolyte being added, the faster is the coagulation.

Sedimentation In а gravitational field, heavy particles settle towards the foot of а column of solution by the process called sedimentation.