`=>` We know that solutions are homogeneous systems.

`=>` We know that sand in water when stirred gives a suspension, which slowly settles down with time.

`=>` Between the two extremes of suspensions and solutions we come across a large group of systems called colloidal dispersions or simply colloids.

`text(Definition :)` A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium.

`=>` The essential difference between a solution and a colloid is that of particle size.

`=>` In a solution, the constituent particles are ions or small molecules.

`=>` In a colloid, the dispersed phase may consist of particles of a single macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules.

`=>` Colloidal particles are larger than simple molecules but small enough to remain suspended.

`=>` Their range of diameters is between `1` and `1000` nm (`10^(–9)` to `10^(–6) m`).

`=>` Colloidal particles have an enormous surface area per unit mass as a result of their small size.

`=>` Consider a cube with 1 cm side. It has a total surface area of `6 cm^2`. If it were divided equally into 1012 cubes, the cubes would be the size of large colloidal particles and have a total surface area of `60,000 cm^2` or `6 m^2`.

Colloids are classified on the basis of the following criteria :

(i) Physical state of dispersed phase and dispersion medium

(ii) Nature of interaction between dispersed phase and dispersion medium

(iii) Type of particles of the dispersed phase.

`=>` Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases, eight types of colloidal systems are possible.

`=>` A gas mixed with another gas forms a homogeneous mixture and hence is not a colloidal system.

`=>` The examples of the various types of colloids along with their typical names are listed in Table.

`=>` Many familiar commercial products and natural objects are colloids.

● `text(Examples :)` (i) Whipped cream is a foam, which is a gas dispersed in a liquid.

(ii) Firefighting foams, used at emergency airplane landings are also colloidal systems.

(iii) Most biological fluids are aqueous sols (solids dispersed in water). Within a typical cell, proteins and nucleic acids are colloidal-sized particles dispersed in an aqueous solution of ions and small molecules.

`=>` Out of the various types of colloids given in Table, the most common are sols (solids in liquids), gels (liquids in solids) and emulsions (liquids in liquids).

`=>` Further, it may be mentioned that if the dispersion medium is water, the sol is called aquasol or hydrosol and if the dispersion medium is alcohol, it is called alcosol and so on.

`=>` Depending upon the nature of interaction between the dispersed phase and the dispersion medium, colloidal sols are divided into two categories

(i) Lyophilic (solvent attracting)

(ii) Lyophobic (solvent repelling).

● If water is the dispersion medium, the terms used are hydrophilic and hydrophobic.

(i) `text(Lyophilic Colloids :)` The word ‘lyophilic’ means liquid-loving.

● Colloidal sols directly formed by mixing substances like gum, gelatine, starch, rubber, etc., with a suitable liquid (the dispersion medium) are called lyophilic sols.

● An important characteristic of these sols is that if the dispersion medium is separated from the dispersed phase (say by evaporation), the sol can be reconstituted by simply remixing with the dispersion medium. That is why these sols are also called reversible sols.

● Furthermore, these sols are quite stable and cannot be easily coagulated as.

(ii) `text(Lyophobic Colloids :)` The word ‘lyophobic’ means liquid-hating.

● Substances like metals, their sulphides, etc., when simply mixed with the dispersion medium do not form the colloidal sol.

● Their colloidal sols can be prepared only by special methods. Such sols are called lyophobic sols.

● These sols are readily precipitated (or coagulated) on the addition of small amounts of electrolytes, by heating or by shaking and hence, are not stable.

● Further, once precipitated, they do not give back the colloidal sol by simple addition of the dispersion medium. Hence, these sols are also called irreversible sols.

● Lyophobic sols need stabilising agents for their preservation.

Depending upon the type of the particles of the dispersed phase, colloids are classified as :

(i) `text(Multimolecular Colloids :)` On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size in the colloidal range (diameter < 1 nm). The species thus formed are called multimolecular colloids.

● Example : A gold sol may contain particles of various sizes having many atoms. Sulphur sol consists of particles containing a thousand or more of `S_8` sulphur molecules.

(ii) `text(Macromolecular Colloids :)` Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects.

● Examples of naturally occurring macromolecules are starch, cellulose, proteins and enzymes; and those of man-made macromolecules are polythene, nylon, polystyrene, synthetic rubber, etc.

(iii) `text(Associated Colloids (Micelles) :)` There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids.

● The formation of micelles takes place only above a particular temperature called Kraft temperature (`T_k`) and above a particular concentration called critical micelle concentration (CMC).

● On dilution, these colloids revert back to individual ions.

● Surface active agents such as soaps and synthetic detergents belong to this class.

● For soaps, the CMC is `10^(–4)` to `10^(–3) mol L^(–1)`. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as `100` molecules or more.

Let's take the example of soap solutions.

`text(Soap :)` It is sodium or potassium salt of a higher fatty acid and may be represented as `RCOO^(-)Na^+` (e.g., sodium stearate `CH_3(CH_2)_(16)COO^(–) Na^+`, which is a major component of many bar soaps).

`=>` When dissolved in water, it dissociates into `RCOO^–` and `Na^+` ions. The `RCOO^-` ions, however, consist of two parts — a long hydrocarbon chain `R` (also called non-polar ‘tail’) which is hydrophobic (water repelling), and a polar group `COO^–` (also called polar-ionic ‘head’), which is hydrophilic (water loving).

`=>` The `RCOO^–` ions are, therefore, present on the surface with their `COO^–` groups in water and the hydrocarbon chains `R` staying away from it and remain at the surface.

`=>` 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 chains pointing towards the centre of the sphere with `COO^–` part remaining outward on the surface of the sphere.

`=>` An aggregate thus formed is known as ‘ionic micelle’. These micelles may contain as many as `100` such ions.

`=>` Similarly, in case of detergents, e.g., sodium laurylsulphate, `CH_3(CH_2)_(11)SO_4^( –)Na^+`, the polar group is` – SO_4^(–)` along with the long hydrocarbon chain. Hence, the mechanism of micelle formation here also is same as that of soaps.

`=>` A micelle consists of a hydrophobic hydrocarbon – like central core.

`=>` The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles (Fig. 5.7).

`=>` 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 helps in emulsification and washing away of oils and fats.

`=>` The negatively charged sheath around the globules prevents them from coming together and forming aggregates.