Hexagonal closest-packed spheres liquid crystalline phases stabilised by strongly hydrated counterions

Abstract

The sequence and structure of lyotropic liquid crystals formed in C₁₂–C₁₆ alkyltrimethylammonium surfactants with hydrolysable and multivalent phosphate (PO₄³⁻, HPO₄²⁻ and H₂PO₄⁻), oxalate (HC₂O₄⁻ and C₂O₄²⁻), and carbonate (HCO₃⁻/CO₃²⁻) counterions were determined using a concentration gradient method coupled with polarising optical microscopy and small angle X-ray scattering. In addition to the discrete cubic (I₁, space group Pm3n) and hexagonal (H₁, p6m) phases, almost all of these surfactants also formed the (previously) rare hexagonally closest-packed spheres (HCPS, P6₃/mmc) phase at compositions between the Pm3n cubic and L₁ micellar phases. This structure has not been previously observed in cationic surfactants, but is readily achieved by using strongly hydrated counterions to stabilise spherical micelles at high concentrations.