Phase behaviour and lyotropic liquid crystals in cationic surfactant–water systems

Abstract

The sequence of lyotropic liquid-crystalline phases occurring in a range of cationic surfactants with water has been determined using the ‘penetration’ optical microscopy technique. Of the order of 50 different surfactants were examined. The variation of the polar group structure includes the series —NH⁺₃, —NMe⁺₃, NEt⁺₃, NPr⁺₃ and NBu⁺₃. In addition, the effect of altering counter-ion and alkyl-chain length are also examined. Other surfactants studied include 2-hydroxyethyl alkylammonium salts, nitroxide ring substituents, alkyl pyridinium salts and alkyl indole surfactants where the indole substituent is part of the hydrophobic chain.

The mesophase sequences are similar to the patterns of behaviour already known for non-ionic and anionic surfactants, with hexagonal (H₁), bicontinuous cubic (V₁) and lamellar (L_α) being the most frequently observed phases (in addition to micellar solution). For long-chain surfactants, intermediate (Int) rather than V₁ phases are shown to form at temperatures just above the Krafft boundary. Qualitatively, much of the phase behaviour can be analysed in terms of alkyl chain packing constraints and intermicellar forces. For bulky alkyl headgroups mesophase formation is suppressed, but the hydroxyethyl substituent causes surprisingly little change from a trimethylammonium headgroup. The behaviour of the indole surfactants suggests that the indole group resides mainly in the micelle interior.