Colloids

A colloidal is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles into another substance called   dispersion medium. Colloidal particles are larger than simple molecules but smaller enough to remains suspended their range of diameter is between 1and 1000nm (10^-9 to 10^-6m)

Classification of collides: –  

Classification of collides on the basis of

• Physical state of dispersed phase and dispersion medium
• Nature of interaction between dispersed phase and dispersion medium.
• Types of particles of dispersed phase

I.    Classification based on physical state of dispersed phase and dispersion medium:-

1. classification based on nature of interaction between dispersed phase and dispersion medium :-in  colloidal state  soles  are  divided  into two categories  :- Lyophillic (solvent  attractive) ,Lyophobic (solvent repelling)
2. a)Lyophillic colloids: – 

The word ‘Lyophillic’ means liquid loving. Colloidal sols directly formed by mixing substances like gum, gelatin, starch, rubber etc. With a suitable liquid (dispersion medium) are called Lyophillic solution. These solutions are also called reversible solutions.

1. b)Lyophobic Colloids: –

These words ‘Lyophobic’ means liquid hating substance like metals sulphides etc. When simply mixed with the dispersion medium do not form the colloidal solution. Their colloidal sols can be prepared by only special methods; such sols are called Lyophobic sols. These sols are also called irreversible sols.

III.        Classification based on type of particle of dispersed phase:- In this  colloids are classified into multi-molecular, macro-molecular and associated colloids.

1. a)Multi molecular Colloids:- 

On dissolution a large no of substance aggregate together to form  species having size in the colloidal range (diameter <1nm) the species thus formed are called  Multi molecular colloids

1. b)Macromolecular Colloids:-

Macro molecules in suitable solvents form solution in which the size of macro molecules may be in colloidal range. Such systems are called macromolecule colloids

1. c)Associated Colloids (Micelles):- 

These are some substances which at low concentration behave as a normal strong electrolyte, but at higher concentration exhibit colloidal behaviors due to the formation of aggregates, the aggregates particle thus formed are called micelles or associated colloids .the formation of micelles takes place only above a particular temperature called Kraft temperature, and above a particular concentration called critical concentration (CMC).

Mechanism of Micelles formation:- 

soap is the sodium or potassium salt of higher fatty acid and may be represented as RCOO^– Na (e.g. sodium stearate , (CH₃(CH₂)16COO^–Na⁺])  when dissolve into  RCOO^– and  Na⁺ ions ,the RCOO^– ions ,consist of two parts – a long hydrocarbon chain (also called  non – polar tail) which is hydrophobic (water repelling ) and a polar group  COO^–   (polar head) which is hydrophilic (water loving ).

But at  critical micelle concentration the  anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chain pointing towards the center of the sphere with COO^–   part remaining outward on the surface of the sphere . An aggregate thus formed is known as Ionic micelle.

Cleansing action of soaps :- the soap molecule in such a way that hydrophobic part of the stearate ions is in oil droplet and hydrophobic part projects out of the grease droplet like the bristles (hairs)

Since the polar groups can interact   with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface thus soap help in emulsification and washing away of oils and fats.