Effect of Nonionic Head Group Size on the Formation of Worm-Like Micelles in Mixed Nonionic/Cationic Surfactant Aqueous Systems

Abstract

The phase behavior, microstructure and rheological properties of mixtures of polyoxyethylene type nonionic surfactants (C₁₂EO_n, n = 0–4) and alkyltrimethylammonium bromide cationic surfactants in water were investigated. Upon addition of C₁₂EO₃ to the aqueous cationic system at high surfactant concentration, both the interfacial curvature of aggregates and the effective cross-sectional area per surfactant molecule, a_S, decrease, resulting in a hexagonal-lamellar liquid crystal phase transition. This effect becomes more pronounced as the alkyl chain-length of the cationic surfactant increases. At low surfactant concentration, addition of nonionic amphiphile to micellar solution above a particular concentration, C^*, induces a rapid unidimensional micellar growth, and particularly in CTAB systems viscoelastic micellar solutions of entangled wormlike micelles are formed. With successive addition of the nonionic amphiphile ultimately a micellar to lamellar phase transformation occurs. Rheological measurements within the W_m-phase show that with decreasing EO chain-length, C^* also decreases and viscosity increases more swiftly with increasing concentration of nonionic amphiphile, suggesting a rapid micellar growth. This is attributed to the decrease in a_S, as predicted by a simple geometrical model.