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

摘要

The phase behavior,microstructure and rheological properties of mixtures of polyoxyethylene type nonionic surfactants (C_(12)EO_n,n=0-4) and alkyltrimethylammonium bromide cationic surfactants in water were investigated. Upon addition of C_(12)EO_3 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 mi-cellar growth. This is attributed to the decrease in a_s,as predicted by a simple geometrical model.