Emulsion stabilisation


Emulsions are a very important class of materials. They are however intrinsically unstable. The only thing one can do is to prolong the period it takes for the emulsion to break. Emulsion stability can be improved by surfactant selection, salt content, mechanical energy added to the emulsion (sheer) and many other factors. 


Stokes law11 is a mathematical equation that helps predict how rapidly an emulsion will break. Stokes law states: 


V=d2(Dp)g/18h


 


wherein: V is Velocity of sedimentation D is diameter of particles of the dispersed phase Dp is difference between the specific gravity of the internal and external phases g is acceleration due to gravity h is viscosity of the external phase 


The most stable emulsions have a very slow ‘V’ value. To make the most stable emulsion get V to approach 0. Microemulsions have a V of close to 0 and consequently are very stable. 


Stokes law explains why: 


 Large particle size emulsions make less stable the emulsion (d in the numerator); 
Emulsions made with internal and external phases having larger the difference in specific gravity make less stable the emulsion (Dp in the numerator); 
Emulsions are more stable on the moon than on earth, and centrifugation will cause an emulsion to split more rapidly (g is in the numerator); 
Thickening the external phase will stabilise the emulsion (h in in the denominator).  
Alkyl silicones that have a melt point above room temperature, but below the temperature to which the oil phase is heated can be used to thicken the external oil phase. As that phase cools in the emulsion the oil will thicken stabilise the emulsion (see Table 5).


source from www.personalcaremagazine.com