EMULSIONS 133 



oil-in-water emulsion (Fig. 88). Colloidal clay (kaolin) and 

 lampblack make good emulsifiers. Many colloidal suspensions 

 and precipitates (ferric hydroxide and basic copper sulphate) 

 stabilize emulsions well. 



Certain Arab tribes, and in general primitive people the world 

 over, use fine earth for washing. This method of cleansing and 

 our more civiHzed use of soap involve identical methods, viz., 

 emulsification. Both fine earth particles and soap emulsify 

 the film of oil on our hands and thus permit washing the oil away. 

 An oil-in-water emulsion leaves a surface wet with water and 

 therefore relatively clean. 



Breaking of Emulsions. — When Oc/fer li quiol 



an emulsion is being reversed with 



an electrolyte, there is a theoretical ^ 



point at which no emulsion exists. 



In practice, before the last vestige 



of one type has disappeared, a small 



quantity of the other type is already Fig. 88.— Solid particles at the 



formed; but while there is usually surface of an emulsion globule func- 

 ' . tioning as a stabilization membrane. 



no sharply marked breaking point, 



there is always a region in which the emulsification is very 

 poor. Emulsions may be broken by adding just enough of the 

 proper electrolyte to de-emulsify. Breaking an emulsion by 

 gradually bringing it to the reversal point with electrolytes is 

 not what the commercial chemist usually has in mind when he 

 speaks of breaking. He is thinking of emulsions that were good 

 when first formed but later break or separate. Mechanical 

 agitation such as jarring may cause an otherwise very stable 

 emulsion to break. Thomas cites a number of interesting cases 

 of the breaking of emulsions by agitation. Vibrations from 

 shipment by truck or railway may incite breakdown. This is 

 known in commercial mayonnaise handling, where trucks equipped 

 with special springs to reduce shock are sometimes used for the 

 transportation of mayonnaise. The reverse situation exists in the 

 finely emulsified water in petroleum distillates. The emulsion 

 is so stable that getting the water out presents a very difficult 

 commercial problem. On the other hand, an oil-field emulsion, 

 so stable as to resist separation in a centrifuge, separated into 

 water and oil during transit in an express train. Once an emul- 

 sion breaks, it is usually re-emulsified with difficulty. 



