We all are familiar with solutions. They play an important role in our life. A large number of substances such as milk, butter, cheese, creams, coloured gems, boot polish, rubber, ink etc also play an important role in our daily life. They are also solutions of some kind. They are colloidal solutions.

3. partly biology- biological materials are colloids- the mechanisms of living systems are related to colloid- and interfacial chemistry

1. partly physical chemistry- it is not the chemical composition which is important- the state is independent of the composition

2. partly physics- the physical properties are of great importance- basic law of physics can be applied

Thomas Graham (1861) studied the ability of dissolved substances to diffuse into water across permeable membrane. He observed that crystalline substances such as sugar, urea and sodium chloride passed through the membrane, while others like glue, gelatin and gum Arabic did not. The former he called crystalloids and the latter colloids (Greek, kolla= glue; eidos=like).

Graham thought that the difference in the behavior of crystalloids and colloids was due to the particle size. Later it was realized that any substance, regardless of its nature could be converted into a colloid by subdividing it into particles of colloid size.

Word colloid refers to the state of matter with certain degree of physical division. The colloidal state is usually associated with intermediate spatial scales between the scale of atoms(nm scale) and the size of small macroscopic objects like observable by a microscope (mm scale).

The nature of all types of solutions depend upon the size of the solute particles. When the size of the solute particles lies between 1 to 100 nm, it behaves as colloid or colloidal solution, if the size of the solute particles is greater than 100nm, it exits as a suspension and if the size of the solute particle is less than 1nm, it exits as a true solution. Therefore the colloidal solution is an intermediate state between a true solution and a suspension.

solutions colloids suspensions

< 1 nm > 100 nm

Property True solution Colloidal solution Suspension

Size less than 1 nm between 1nm and 100 nm Greater than 100nm

Filtrability Pass through ordinary filter paper and also through animal membrane

Pass through ordinary filter paper but not through animal membrane

Do not pass through filter paper or animal membrane

Setling Particles do not settle down on standing

Particles do not settle down on their own but can be made to settle down by centrifugation

Particles settle down on their own under gravity

Visibility Particles are invisible to the naked eye as well as under a microscope

Particles are invisible to the naked eye but their scattering effect can be viewed with the help of a microscope

Particles are visible to the naked eye

Separation The solute particles and solvent cannot be separated by ordinary filteration or by ultrafilteration

The solute and solvent cannot be separated by ordinary filteration but can be separated by ultrafilteration

The solute and solvent can be separated by ordinary filteration

Diffusion Diffuse quickly Diffuse slowly Do not diffuse

Heterogeneous in nature- consists of 2 phases.

Dispersed phase: consists of particles of colloidal size of any nature (e.g. solid, liquid or gasDispersion medium: continuous phase of a different composition in which colloid particles are dispersed

Two phases namely dispersed phase and dispersion medium can be solid, liquid or a gas. Thus different types of colloidal solutions are possible depending on the physical states of the two phases.

Examples of some colloidal systems

Out of the various types of colloidal solutions most common are-Sols (solid in liquid type), Gels (liquid in solid type) and Emulsions (liquid in liquid type)

if the dispersion medium is water then the ‘sol’ is called a Hydrosol and if the dispersion medium is alcohol then the ‘sol’ is called a Alcosol and so on.

1. Lyophilic colloids2. Lyophobic colloids3. Association colloids

association colloids (surfactants)

chemical structure of a single surfactant molecule: sodium dodecyl sulfate

lyophobic sols have to be prepared by special method.These methods fall into two categories:a. Dispersion methods in which larger macro sized particles are broken down to colloidal size.b. aggregation methods in which colloidal particles are build up by aggregating single ions or molecules.

Dispersion Methods:In these methods, the coarser particles are broken down into smaller particles of colloidal dimension through suitable means including the following:1. Colloid mill: The solid along with the liquid dispersion medium is fed into a colloid mill. The mill consists of two steel plates nearly touching each other and rotating in opposite directions with high speed. The solid particles are ground down to colloidal size and are dispersed in the liquid to give the sol.Collodal graphite (a lubricant) and printing inks are made by this method.2. Electric dispersion: Certain metals may be dispersed by passage of an electric arc between electrodes made up of the metal and immersed in the dispersion medium.3. Ultrasonic radiation: the passage of ultrasonic waves through a dispersion media produces high frequencies oscillation (20,000 to 200,000 cycles per second) resulting in size reduction of coarse particles to colloidal size.4. Peptization: The process of converting a precipitate into a colloidal sol by shaking it with the dispersion medium, in the presence of a small amount of electrolyte, is called peptization. The electrolyte used is called the peptizing agent. This method is used to convert a freshly prepared precipitate into a colloidal sol.In the process of peptization, the precipitate adsorbs one of the ion of the electrolyte onto its surface. The ion adsorbed on the surface is generally common with those of the precipitate. Adsorption of ion results in the development of positive or negative charge on precipitates and which ultimately break up into colloidal size particles. For e.g., a precipitate of silver iodide already formed can be dispersed by the addition of potassium iodide. Here potassium iodide is the peptizing agent.

Condensation Method:These involve the rapid production of supersaturated solutions of the colloidal material under Conditions in which it is deposited in the dispersion medium as colloidal particles and not as a precipitate. The supersaturation is often obtained by means of a chemical reaction that results in the formation of the colloidal material.e.g: Colloidal silver iodide may be obtained by reacting together dilute solutions of silver nitrate and potassium iodide.A change of solvent may also cause the production of colloidal particles by condensation methods. In this method, a true solution in mixed with an excess of another solvent in which the solute is insoluble but the solvent is miscible. For e.g., a solution of sulphur in alcohol mixed with excess of water results in a colloidal sol of sulphur.