Colloid

Colloids (also known as colloidal solutions or colloidal systems) are mixtures in which microscopically dispersed insoluble particles of one substance are suspended in another substance. The size of the suspended particles in a colloid can range from 1 to 1000 nanometres (1 x 10 metres). For a mixture to be classified as a colloid, its suspended particles must not settle (in the manner that the particles of suspensions settle at the bottom of the container if left undisturbed). Colloidal solutions are known to exhibit the Tyndall Effect, which is a phenomenon in which beams of light incident on colloids are scattered due to the interactions between the light and the colloidal particles.
According to IUPAC...
Colloids usually feature substances that are evenly dispersed in another. In such mixtures, the substance that is dispersed is referred to as the dispersed phase whereas the substance through which it is dispersed is called the continuous phase.
==Preparation==
Stable colloids are also known as lyophilic sols, in these strong forces of interaction exist between the dispersed phase and the dispersion medium.
===Condensation Method===
Small solute particles are condensed to form a dispersed phase particle.
====Chemical Methods====
*Oxidation: Colloid Sulphur can be obtained by passing oxygen gas through a solution of hydrogen sulphides. Oxidising agents like HNO, HBr can be used.
 2H_2S + O_2 \rightarrow 2H_2O + 2S (sulphur sol)
* Double Decomposition: A solution of arsenic sulphide is obtained in this method. In this process hydrogen sulphide is passed through Arsenious oxide cold solution in water.
 AS_2O_3 + 3H_2S \rightarrow AS_2S_3 + 3H_2O Arsenic sulphide (sol)
* Reduction: A number of metals such as gold, silver, and platinum are obtained in a colloidal state by reacting the aqueous solution of these salts with suitable reducing agents such as formaldehyde, phenylhydrazine, hydrogen peroxide, stannous chloride etc.
 2AuCl_3 + 3SnCl_2 \rightarrow 3SnCl_4 + 2Au (gold sol)
 2AuCl_3 + 3HCHO + 3H_2O \rightarrow 2Au + 3HCOOH + 6HCl 
The gold sol prepared in the reduction of gold chloride solution has a purple colour and is called purple of Cassius.
===Dispersion Methods===
In these methods, large particles of a substance (suspension) are broken into smaller particles. The following methods are employed.
====Mechanical Dispersion====
In this method, the substance is first grounded to coarse particles. It is then mixed with dispersion medium to get a suspension. The suspension is then grinded in a colloidal mill. It consists of two metallic dyes nearly touching each other and rotating in the opposite direction at a very high speed 7000 revolution per minute. The space between the dyes of the mill is so adjusted that coarse suspension to great shearing force giving rise to particles of colloidal size. Colloidal solution of black ink, paints varnishes dyes are obtained by this method.
====Electrical Dispersion====
This method is used to prepare sols of platinum, silver copper or gold. The metal whose sol is to be prepared is made as two-electrode which immersed in a dispersion medium such as water etc.
The dispersion medium is kept cool by ice. An electric arc is placed between the electrodes. The tremendous heat generated by and give colloidal solute. Electrolytes are used for this process for stabilization and cooling.
====Peptization====
The process of converting a freshly prepared precipitate into a colloidal solution is known as peptization. In this method as the electrolyte in smaller amounts is added which is known as peptization agent or peptizing agent. Cause of peptization is the adsorption of the ions of the electrolyte by the particles of the precipitate. Important peptizing agent are sugar gem gelatin and electrolyte.
Category:Chemistry
Category:Solution