Phase separation of non-ionic surfactant solutions. A treatment of the micellar interaction and form

Abstract

The phase separation (clouding) of aqueous solutions of some non-ionic surfactants, in particular poly(ethylene glycol) alkyl ethers like tetra- and hexa-(ethylene glycol) decyl ether, is analysed theoretically. It is found that the location of the solubility gap is incompatible with a spherical form of the micelles. The phase boundary can be reproduced theoretically if the micelles are assumed to be rod shaped and flexible at high temperatures.According to the picture presented here, the main contribution to the intermicellar interaction comes from the contact between poly(ethylene glycol)(PEG) chains belonging to different micelles. As in the case of the pure PEG–water system, there are zones with increased water structuring around the PEG chains. The existence of a lower consolute phase boundary is an effect of the “solvation” force (a modified hydrophobic interaction) that originates from the overlap of such zones. Some further consequences of this kind of force in the micellar and the liquid-crystalline phases are briefly discussed. It can lead to an increase in aggregate size with temperature.