ST A n I LISA TIOX 105 



cause nmtiial repulsion, and (c) nieelianieally keep the dioplets 

 separate. The presence oi' an ennilsoid seems to confer stalnhty 

 on an oil-water emulsion. 



Various theories have been put forward to explain why the 

 presence of a hydrophilie colloid permits of the formation of a 

 permanent enudsion and why some stabilising colloids ])r()duce 

 a dispersion of oil-in-water while others favour the water-in- 

 oil type. 



(1) Quincke, Hilly er, Donnan and Potts are of opinion that the 

 stability is due mainly to a lowering of interfaeial tension by a 

 thin layer of the colloid or semi-colloid deposited on the surface of 

 the droplets of the disperse phase. This interphase reduces the 

 surface tension on the film-water interface, confers a charge on the 

 droplets by adsorption, and, by having the remainder of the 

 colloid as an outer phase, provides a medium sufficiently viscous 

 to keep the droplets in suspension. That is, an emulsion is 

 triphasic. 



(2) According to Bancroft, a deposition of the stabiliser in 

 accordance with the Gibbs-Thomson principle {q.v.) provides the 

 necessary electric charge and confers protection. He adduces as 

 proof the fact that if another surface is brought into competition 

 with the oil- water surface for the stabilising agent, some proportion 

 of the stabiliser will be adsorbed to this new surface. For example, 

 if the emulsion is allowed to stand the air-fluid interface will 

 capture some of the stabiliser, i.e. the cream will carry off some of 

 the colloid, and this, we know, it does. 



(3) Fischer considers that an emulsion may be triphasic but need 

 not necessarily be so. His idea is that a diphasic system is all that 

 is required for stability, e.g. oil and a lyophilic colloid in water. 

 A film of the solvated colloid is adsorbed to the surface of the 

 disperse phase. 



(4) R. E. Wilson is of opinion that the film on the dispersed 

 fluid is really a plastic solid, i.e. butter-like. 



All these theories have one fact in common, viz. the nature of 

 the emulsoid determines the type of emulsion produced. If the 

 colloid is one which is " wetted " by water (hydrosol or hydrogel), 

 and is adsorbed by oil, it (or its solution) will form a film round the 

 oil droplets and give an emulsion of oil in water. On the other 

 hand, if the colloid is dispersed through oil and is adsorbed by 

 water, it will emulsify water in oil. 



The oil cannot be dispersed throughout a hydrated colloid 

 until a certain lower limit of water content has been exceeded, 

 nor can it be divided permanently into a hydrated colloid after 

 an upper limit has been passed. 



