Classification of Colloids

Since the colloidal solutions consist of dispersed phase and dispersion medium, the components of these phases have certain specific characteristics of their own. In view of these observations, the colloidal solutions can be classified into the various types as under;

(I).       CLASSIFICATION BASED ON PHYSICAL STATE OF DISPERSED PHASE AND DISPERSION MEDIUM

                        Based on the physical state of dispersed phase and dispersion medium, there are eight types of colloidal solutions. Either these phases may be solid, liquid or gas. Since gases gives homogeneous mixture in all proportions, hence colloidal solution can not be obtained in which dispersed phase and dispersion medium are gases.

S.No.	Dispersed Phase	Dispersion Medium	Name of Colloidal Solution	Examples
1.	Gas	Liquid	Foam	Soap lather, whipped cream, soda water, froth on beer.
2.	Gas	Solid	Solid foam	Cake, Bread, Pumice stone lava.
3.	Liquid	Gas	Aerosol	Mist, fog, Clouds.
4.	Liquid	Liquid	Emulsion	Milk, cream, butter, oil in water
5.	Liquid	Solid	Gel.	Curd, Cheese, Jellies, boot polish.
6.	Solid	Gas	Solid Aerosol	Smoke, dust.
7.	Solid	Liquid	Sols	Paints, gold sol, inks.
8.	Solid	Solid	Solid Sols	Alloys, coloured glass, gem stones.

(II).     CLASSIFICATION BASED ON ELECTRICAL CHARGE ON DISPERSION PHASE

                        Based on the nature of charge on the dispersed phase, there are two types of colloidal solution;

            (a).       Positive Colloids

                                     When dispersed phase in a colloidal solution carries a positive charge, it is known as Positive Colloid.

                        Examples :- Metal hydroxides like Fe(OH)3, Al(OH)3, TiO₂, Methylene blue sols etc.

            (b).       Negative colloids

                                     When dispersed phase in a colloidal solution carries a negative charge, it is known as Negative Colloid.

                        Examples: AS2S3 Sol, Cu or Ag Sol.

(III). CLASSIFICATION BASED ON APPEARANCE OF THE COLLOIDS

                        On this basis, the colloidal solutions can be divided into two types;

            (a).       Sols

                                    When a colloidal solution appears as fluid it is referred to as Sol. The sols are generally named after dispersion medium. For Instance, when the dispersion medium is water, the sol is known as Hydrosol or Aquosol. When the dispersion medium is alcohol and benzene, the sols are called Alcosol and Benzosol respectively.

            (b).       Gels

                                    When a colloidal has solid like appearance it is called Gel. The rigidity of gels varies from substance to substance. Some examples of gels are: Jelly, butter, cheese, curd.

(IV).    CLASSIFICATION BASED ON MOLECULAR SIZE OF THE DISPERSED PHASE

                        On the basis of molecular size the colloids can be divided into three types:

            (a).       Multimolecular Colloids       

                                    In these colloids, the Individual particles of the dispersed phase consists of aggregates of atoms or small molecules having diameter less than 10–7cm. For instance a gold sol consists of particles of various size containing several atoms. Likewise, a sulphur sol contains particles each having eight sulphur atoms (S8). In these colloids, the particles are held by weak vanderwaal’s forces.

            (b).       Macro Molecular Colloids:

                                    In these colloids, the particles of dispersed phase are sufficiently large in size enough to be of colloidal dimensions. These are called Natural Polymers. Some typical examples are starch, cellulose and proteins. Some artificial or synthetic polymers are polythene, polystyrene, nylon and plastics.

            (c).       Associated Colloids

                                    These colloids behave as normal electrolytes at low concentrations but behave as colloids at higher concentration. These associated colloids are also referred as Micelles. Sodium Stearate behaves as electrolyte in dilute solution but as colloid at higher concentration. Some other examples of associated colloids are; Soaps, higher alkyl sulphonates, polythene oxide etc.

(V)      CLASSIFICATION BASED ON THE INTERACTION OR AFFINITY OF TWO PHASES

                        The colloidal solutions can also be classified in terms of the attraction between the dispersed phase and dispersion medium as:

            (a).       Lyophilic Colloids

                                    Colloidal solutions in which the dispersed phase has a great affinity or love for the dispersion medium are termed as Lyophilic Colloids. Such substances have a tendency to pass directly into colloidal solution when brought in contact with dispersion medium. The lyophilic colloids are also called Intrinsic Colloids.

                                    If the dispersion medium is water, they are called Hydrophilic or Emulsoids. The lyophilic colloids are generally self-stablised, reversible in nature and are heavily hydrated. The typical examples of lyophilic colloids are starch, geletin, rubber, protein etc.

            (b).       Lyophobic Colloids

                                    Colloidal solutions in which the dispersed phase has no affinity or hates the dispersion medium are called Lyophobic Colloids. These are also referred as Extrinsic Colloids. Such substances have no tendency to pass into colloidal solution when brought in contact with dispersion medium. The lyophobic colloids are relatively unstable and readily tend to change into a suspension. They are irreversible by nature and are stablised by adding small amount of electrolyte. They are poorly hydrated.

                                    If the dispersion medium is water, the lyophobic colloids are called Hydrophobic or Suspensoids. The important typical instances of lyophobic colloids are sols of metals like Au, Ag, sols of metal hydroxides like Al(OH)3, Fe(OH)3 etc and sols of metal sulphides like AS2S3.