Phase Behaviour of Polyoxyethylene Surfactants with Water

Abstract

From a review of the major factors responsible for surfactant mesophase structure, a model phase diagram is deduced which shows phase structure as a function of surfactant volume fraction and micelle curvature. To test this model the phase behaviour of a series of pure polyoxyethylene surfactants (C, EO,) with water has been studied using optical microscopy over the temperature range 0-100 OC. The compounds studied were C, and C,,EO,,. With the other compounds optical microscopy was used to determine the number, sequence and type of mesophases. The mesophases observed were cubic-spherical-micelles (Il), hexagonal (HI), normal-cubic-bicontinuous (Vl), lamellar (La) and reversed-cubic-bicontinuous (V,). Large head groups and low temperatures favour I, and HI phases, while La and reversed phases occur for small head groups and higher temperatures. There is agreement between experiment and theory for low to medium temperatures if increasing temperature is assumed to lead to a decreased surface area per molecule at the micelle surface (a). At high temperatures and low water content the theoretical concepts were reconciled to practical behaviour only by assuming that increased interactions between EO groups occur at a critical water concentration. Two separate mechanisms are proposed for the lower consolute behaviour of surfactant +water solutions (the cloud point). One, involving van der Waals attractions between micelle cores, operates at low temperatures, while the second, involving intermicellar EO-EO attractions occurs at high temperatures. These two mechanisms can account for the 'double' cloud point phenomenon observed for surfactants with short EO groups. Polyoxyethylene surfactants are widely used as emulsifying agents and detergents. Like ionic surfactants they form micelles above a critical concentration in water (the c.m.c.) with liquid crystals frequently occurring at higher concentrations. They are unusual in having a lower consolute temperature, termed the cloud point. For many years this was attributed to the presence of giant micelles. However, it is now thought to correspond to the phase separation of a concentrated surfactant solution containing small micelles from a more dilute solution, perhaps due to the presence of significant intermicellar attra~ti0ns.l.~ Some surfactants even show a 'double cloud point ', where a 1 % aqueous solution goes cloudy then clears and clouds a second time when the temperature is increa~ed.~. 975