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

• Published in: Langmuir 2018-02, Vol.34 (6), p.2290-2301
• Main Authors: Jayaraman, Ashish, Mahanthappa, Mahesh K
• Format: Article
• Language: English
• Subjects: counterions; hydration; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; micelles; phase transitions; surfactants
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/acs.langmuir.7b03833

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 = K+, Cs+, or (CH3)4N+ 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) A15 and σ phases (Pm3­(−)n and P42/mnm symmetries, respectively). Previously observed in only one other minimally hydrated surfactant, the σ 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 σ-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.