Counterion-Dependent Access to Low-Symmetry Lyotropic Sphere Packings of Ionic Surfactant Micelles

The water-driven self-assembly of homologous dianionic surfactants into lyotropic liquid crystals (LLCs) is investigated, with a focus on understanding how surfactant headgroup and counterion identities guide supramolecular spherical mesophase selection. Using temperature dependent small angle X-ray scattering (SAXS), we demonstrate that 2 alkylmalonate surfactants (C(n)Mal-M-2) with n = 8 (octyl) or 10 (decyl) and M = Cs+, or (CH3)(4)N+ form both simple and complex micelle packings. Observed spherical morphologies include body-centered cubic (BCC), hexagonally closest packed (HCP), and tetrahedrally closest-packed Frank-Kasper (FK) AlS and a phases (Pm3(-)n and P4(2)/mnm symmetries, respectively). Previously observed in only one other minimally hydrated surfactant, the a phase is a rare LLC morphology comprising a low symmetry unit cell containing 30 sub 2 nm quasispherical micelles, each of which belongs to one of five symmetry equivalent classes with discrete aggregation numbers. Temperature versus water concentration phase maps for C(n)Mal M-2 LLCs reveal that sigma-phase formation depends sensitively on the size and polarizability of the surfactant counterion and the length of the surfactant alkyl tail. These observations are rationalized in terms of a delicate interplay between global packing symmetry and local particle symmetry, and the extent to which counterion headgroup correlations enforce the latter structures in these LLC phases.