Experimental cookery

Adsorbed film and interfacial tension theory. This theory has been developed or rather extended from earlier theories. At the present time it is probably the most universally accepted theory for the formation of emul- sions. Bancroft stated the underlying principles, basing them upon Don- nan's early work of interfacial tension ; but many others have extended the interpretations. Clayton states that with this theory "emphasis is laid upon the fact that emulsification is influenced by ( 1 ) the mass of the emulsi- fying agent present, (2) the ease with which this agent is adsorbed at the interfacial separating surface, and (3) the nature of the ions adsorbed by the resultant film." The emulsifier may be adsorbed by the water or by the oil, but it is usually adsorbed more in one liquid than in the other and thus lowers the interfacial tension of one liquid to a greater extent than that of the other. If the tension of the water is lowered more than that of the oil, the water has less tendency to form drops, flows to form a film more readily, and becomes the continuous phase. Thus the type of emulsion formed depends upon the nature of the emulsifying agent. The above is often worded some- what as follows: if the emulsifying agent is more soluble in water than in the oil the water becomes the continuous phase, or if the emulsifying agent is wetted more by the water than by the oil, the water becomes the con- tinuous phase. When the tension on each side of the film or the emulsifying agent is the same no emulsion is formed. This may occur when opposing emulsifying agents are in the mixture and the effect of each counterbalances that of the other. To form an emulsion the emulsifier must be adsorbed at the interfacial surface and form a sufficiently coherent film to stabilize the emulsion. The reversal of the emulsion depends upon the nature of the ions adsorbed by this film. Bhatnagar stresses more than previous workers the necessity for wetting of the adsorbed film. The making of permanent emulsions is important for foods, cosmetics, pharmaceutical preparations, sprays, and other products. But breaking of emulsions is important in crude oil operations for recovery of the oil. Hence emulsions have been investigated from many angles and it is not surprising that no one theory applies to all types of emulsions. The Type of Emulsion Formed The type of emulsion formed, i.e.: (1) oil-in-water or (2) water-in-oil, depends upon the nature of the emulsifying agent, the nature of the oil, and the effect of electrolytes. With one emulsifier an oil-in-water emulsion may be formed with a specific oil. Sometimes by the addition of the right substance, usually an electrolyte, the emulsion can be reversed and changed to a water-in-oil emulsion. Other emulsifiers with the same oil will form water-in-oil emulsions. Bancroft states that a hydrophilic colloid tends to make water the dis- 270 EMULSIONS persing phase, and a hydrophobic colloid tends to make water the dis- persed phase. The potassium and sodium soaps are more soluble in water than in the oil and form oil-in-water emulsions. Magnesium and calcium soaps are more soluble in oil than in water and tend to form water-in-oil emulsions. Aluminum and iron soaps are more soluble in oil than the magnesium and calcium ones and form water-in-oil emulsions.