title: Structure of liquid crystalline aerosol-OT and its alkylammonium salts.
abstract: Aerosol-OT is a widely used anionic surfactant, and its lyotropic properties have been studied extensively. However, neat AOT is itself liquid crystalline. We carried out an X-ray study of neat AOT sodium salt, as well as of AOT-n-decyl-, n-dodecyl-, n-tetradecyl-, and n-hexadecylammonium salts. We confirm an earlier report that pure AOT forms a hexagonal columnar phase but propose a different packing model. This involves a relatively highly ordered structure with each column cross-section containing three tessellated molecules in the plane normal to the column axis. The structure is trigonal locally but hexagonal over the long-range. This mode of assembly is supported by electron density reconstruction and molecular modeling. At subambient temperatures, the AOT-alkylammonium complexes Cn-AOT, with n=10-16, also display a hexagonal columnar phase, but this is more disordered, and each column cross-section contains only two ion pairs. Unusually, molar enthalpy and entropy of the columnar-isotropic transition in Cn-AOT salts decrease with increasing n. This is attributed to a disproportionally high conformational disorder of the radial chains in the columnar phase, which is required for efficient space filling.
authors.affiliation: Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, UK. g.ungar@shef.ac.uk
authors.forename: Goran
authors.initials: G
authors.lastname: Ungar
authors.forename: Vlasta
authors.initials: V
authors.lastname: Tomasić
authors.forename: Fangning
authors.initials: F
authors.lastname: Xie
authors.forename: Xiang-bing
authors.initials: XB
authors.lastname: Zeng