Phase behaviour of polyoxyethylene surfactants with water. Mesophase structures and partial miscibility (cloud points)

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_nEO_m) with water has been studied using optical microscopy over the temperature range 0–100 °C. The compounds studied were C₈EO₃, C₈EO₄, C₈EO₈, C₈EO₁₂, C₁₀EO₃, C₁₂EO₃-C₁₂EO₆, C₁₂EO₈, C₁₂EO₁₂, C₁₄EO₃, C₁₄EO₆, C₁₆EO₃, C₁₆EO₄, C₁₆EO₆, C₁₆EO₈, C₁₆EO₁₂, C₉PhEO₈ and C₁₂(2-C₁₀)EO₁₀. Phase diagrams were determined for C₈EO₄, C₁₂EO₃-C₁₂EO₆, C₁₂EO₈, C₁₆EO₄, C₁₆EO₈ and C₁₆EO₁₂. With the other compounds optical microscopy was used to determine the number, sequence and type of mesophases.

The mesophases observed were cubic–spherical-micelles (I₁), hexagonal (H₁), normal-cubic–bicontinuous (V₁), lamellar (L_α) and reversed-cubic–bicontinuous (V₂). Large head groups and low temperatures favour I₁ and H₁ phases, while L_α 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 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.