38 



COLLOIDS IN BIOLOGY AND MEDICINE 



Emulsifying Na oleate alone 

 agent or Na oleate in 

 or film excess of Ca 



oleate equiv 

 Emulsion 

 formed Oil in water 



Equiv. chem. Ca oleate alone 



proportion or Ca oleate in 



Na oleate and excess of Na 



Ca oleate oleate equiv. 



Critical point Water in oil 



FIG. 4a. Conversion of oil-water to 

 water-oil emulsion. 



while soaps of calcium and magnesium, being readily dispersed in oil 

 but not in water, form a film, the surface tension on the oil side of 

 which is lower than on the water, and consequently an emulsion of 

 water in oil is produced. 



CLOWES showed that emulsions of oil in water could be converted 

 into emulsions of water in oil and vice versa by varying the propor- 

 tions of certain electrolytes 



added to the system. When 



equal volumes of oil contain- 

 ing fatty acid and water con- 

 taining NaOH were shaken 

 together sodium soap was pro- 

 duced and an emulsion of oil 

 in water formed. On shaking 

 this emulsion with increasing 

 proportions of calcium chlor- 

 id, a critical point at which 

 oil and water separated into 

 two distinct layers was ob- 

 served when the CaCl 2 was 



added in sufficient amount to convert half the sodium soap into cal- 

 cium soap. Further addition of calcium chlorid led to the forma- 

 tion of a stable emulsion of water dispersed in oil. Conversely, the 

 latter emulsion could be reconverted through the critical point into 

 one of oil in water by shaking with the requisite proportions of sodium 

 soap or caustic soda. (See diagram, Fig. 4a.) 



CLOWES attributes these transformations to variations in the sur- 

 face tension relations of the water and oil phases, caused by varia- 

 tions in the proportions of the hydrophilic sodium soaps which lowers 

 the surface tension of the water phase, and the lipophilic calcium soap 

 which lowers the surface tension of the oil phase. An emulsion of oil 

 in water is produced when the surface tension is lower on the water 

 than on the oil side of the stabilizing film or membrane formed by 

 concentration of the emulsifying agent at the oil-water interface. A 

 critical point occurs when the surface tension is equal or compensa- 

 tory on both sides of the film, and an emulsion of water in oil is formed 

 when the surface tension is lower on the oil side than on the water 

 side. 



Electrolytes appear to exert a marked effect on emulsion equilib- 

 rium, those having a more reactive or more readily adsorbed anion 

 appear to promote the formation of emulsions of oil in water, while 

 those having a more reactive or readily adsorbed cation exert the 

 reverse effect, promoting the formation of emulsions of water in oil. 



