EMULSIONS 123 



emulsions. The type, permanency, consistency, and reversi- 

 bility of emulsions are determined by the interface— the stabiliza- 

 tion membrane. 



It is quite an easy task to make an emulsion. If the oil and 

 emulsifier are suitable, mere shaking in a flask is sufficient. 

 Emulsions so made usually separate, or ''break," in a short time. 

 More thorough churning is necessary to make a good stable 

 emulsion. This is accomplished either by systematic shaking 

 or preferably, by homogenizing in a homogenizer or colloid mill. 

 Commercial emulsions are made with the aid of a colloid mill 

 which forces and scatters the oil through a fine nozzle under high 

 pressure. So-called "artificial milk" can be made in the labora- 

 tory by grinding gum arable, distilled water, and butterfat 

 (melted butter) in a mortar until the oil is well emulsified; the 

 mixture is then diluted, and synthetic milk is the result. Milk 

 and cream are produced by emulsifying butterfat in a solution 

 of skimmed-milk powder. 



Emulsions are of two types — oil in water (a. Fig. 81) and 

 water in oil (6, Fig. 81). It is not always possible to say which 

 type will result when oil and water are shaken together with an 

 emulsifier; thus, one petroleum oil— kerosene — forms an oil-in- 

 water emulsion with casein as the stabilizer, while another 

 petroleum oil — Nujol — forms a water-in-oil emulsion with the 

 same stabilizer. The reason is unknown. 



There are a number of ways of ascertaining the type of an 

 emulsion; for example, the water-in-oil ones are usually much 

 thicker than the reverse type, as cold cream and milk illustrate. 

 The most reliable way to distinguish the two types is to examine 

 them under the microscope; such observations are aided by 

 staining one of the phases before making the emulsion (the oil 

 with Sudan III). The color so produced serves in another way 

 to distinguish the two types; if the red-stained oil is the outer, 

 continuous phase (the dispersion medium), the emulsion as a 

 whole appears red; if the red oil is the dispersed, or discontinu- 

 ous, phase, the emulsion is pink (Fig. 82). Another method is 

 to observe the walls of the glass container after shaking the 

 emulsion; if the glass surface is left fairly free from oil, then water 

 is the outer, continuous phase; but if the walls of the container 

 are oily, then the oil is the continuous phase. A novel method 

 is to pass an electric current through the emulsion; if 10 to 100 



