STABILISATION 85 



under what conditions each of these types is formed. One 

 might at first imagine that the governing factor would be the 

 relative amounts of oil in water, much water and little oil producing 

 the oil-in-water type and excess of oil over water producing the 

 water-in-oil emulsion. This is not so. The nature of the emulsoid- 

 colloid determines the type of the emulsion absolutely. The relative 

 amounts of oil and water have nothing to do with it. To under- 

 stand the significance of this, one must examine the function of 

 the colloid. 



Some means must be adopted once the oil has been dispersed 

 to (a) decrease the interfacial tension between the droplets and 

 the dispersion medium so that the dispersed particles will not 

 coalesce, (b) confer on the droplets an electrical charge so as to 

 cause mutual repulsion, and (c) mechanically keep the droplets 

 separate. The presence of an emulsoid seems to confer stability 

 on an oil-water emulsion. Two theories to account for this are 

 of sufficient importance to warrant attention. 



1. Most physical chemists prefer the theory which postulates 

 a third phase, namely, a thin layer of colloid or semi-colloid 

 separating the disperse from the continuous phase. This inter- 

 facial film 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. 



2. Recently Fischer and Hooker have promulgated their theory 

 of the action of the protective colloid. They consider that an 

 emulsion may be triphasic but need not necessarily be so. Their 

 idea is that a diphasic system is all that is required for stability, 

 e.g. oil and an emulsoid in water. 



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. Most 

 emulsions are of the first type, oil-in-water. 



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. 



Emulsions are broken through the institution of conditions 

 that are the reverse of those that make for their stabilisation. 



