Surface Chemistry of Class 12

An emulsion is a colloidal system consisting of two immiscible liquid phases, one of which is dispersed as globules in another. The familiar examples of emulsion are milk (liquid fat) liver oils etc. The emulsion can be classified into two classes.

(i) Water in oil type and (ii) Oil in water type

When a little oil is shaken with excess of water, an emulsion of oil in water (O−W) type is formed. On the other hand if a little water is shaken with an excess of oil, an emulsion of water in oil (W−O) type is formed. For example milk belongs to oil in water type emulsion while butter is an example of water in oil type emulsion.

An emulsion formed by mere shaking of an oil with excess of water or vice versa is not stable and soon separates into two distinct layers when allow to stand. To stabilize an emulsion a third substance like gelatin, soap, gum arabic etc (called emulsifying agents or emulsifier) has to be added. The emulsifying agent forms a film around every drop of the dispersed phase thereby preventing these drops from coalescence and consequent separation into two layers. In milk and cod liver oil emulsifiers are albumin + casein and egg yolk + casein + gums respectively. The emulsifiers decrease the surface tension of the two liquids thereby the tendency of the tiny droplets to coalesce and form aggregates is reduced.

Soaps and detergents are the most commonly employed emulsifiers. Other emulsifying agents used are gums, proteins, agar−agar etc., The nature of emulsifying agent determines the type of the emulsion formed. According to Bancroft rule the phase in which the emulsifier (or stabilizer) is more soluble becomes the outer phase. For example, alkali metal soaps gelatin etc which are insoluble in hydrocarbons but soluble in water give rise to oil in water type emulsion. On the other hand heavy metal soaps (eg., Ca, Al oleate/palmitate etc) which are more soluble hydrocarbons then water yield water in oil type emulsion. In case the stabilizer is insoluble in both phases (i.e oil as well as water) the phase which wets the emulsifier better becomes the external phase e.g., clay glass CaCO3 and pyrites which are more easily wetted by water give rise to oil in water emulsions (also called quietus emulsion) while lamp black which is wetted more easily by oil than water yields water in oil emulsions (i.e., oily emulsions). Emulsion has wide applications.
Such as

(i) Emulsion are of great importance in everyday life: Examples include butter margarine hair cream, many cosmetic creams, ointments and certain paints, varnishes and liquid polishes.

(ii) Mayonnaise, is an oil in water emulsion of an edible oil in a dilute solution of an edible organic acid. The emulsifying agent is the protein in egg yolk. Molecules of this protein form a skin around each microdroplet of the oil which keeps the microdroplets from coalescing. During the making of mayonnaise, the mixture must be whipped very vigorously as it is made. This makes the oil droplets very small, and ensures that each gets its coating of the emulsifying agent.

(iii) The emulsifying properties of soaps and detergents have a direct bearing on washing of clothes, crockery, etc. They own their cleansing capability to the structures of their molecules which allows them to assume the role of emulsifiers between dirt, oil, grease etc. and water. The dispersed soap particles adsorb (or emulsify) fine dirt particles and form protective envelopes around them, thereby the dirt particles are held in suspension and hence washed away. Similarly oil (or grease) drops are adsorbed by dispersed soap particle forming protective films around them. These oil soap particles are held in suspension and hence washed away.

(iv) Emulsification helps in digestion of fats in the intestines. A small amount of fat reacts with alkaline solution (always present in intestines) forming an alkali soap (e.g., RCOONa), which emulsifies the remaining fat thereby the digestive enzymes are helped (by emulsification) in carrying out their metabolic functions.

(v) Many drugs are available in the form of emulsions. Many cosmetics (like lotions, face creams, ointments), pharmaceuticals and other such preparation are emulsions either of oil in water or water in oil type.

(vi) In froth flotation process for the concentration of ores, finely divided ore particles in a water tank are treated with oil (like eucalyptus oil). On agitating by blowing compressed air oil disperses in the form of colloidal particles, which possess a stronger adsorptive attraction for the metallic sulphide particles then for the gangue particles. So oil particles attach to the metallic sulphide (or ore) particles and move up in the form of the froth (or foam) leaving behind the gangue particles in water tank.

(vii) Sometimes emulsions do not serve useful purpose, e.g., petroleum obtained from oil well is always found emulsified with water. To recover the oil, oil−water emulsion has to be broken. The process of breaking an emulsion into two separate layers is called demulsification. Emulsions can be broken to recover the constituent liquids in separate layers by freezing or boiling or filtration or centrifugation or by electrostatic precipitation. Thus centrifugation method is used to separate cream from milk. Chemical methods which destroy the emulsifiers are also employed.

Characteristics of emulsions

(i) All emulsions show usual properties of colloids

(ii) They usually carry negative charge

(iii) Their stability is determined by the charge on the dispersed phase and the film of emulsifying agent.

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