American Chemical Society
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Aqueous Lyotropic Mesophase Behavior of Gemini Dicarboxylate Surfactants Swollen with n‑Decane

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journal contribution
posted on 2020-02-26, 22:37 authored by Carlos M. Baez-Cotto, Grayson L. Jackson, Mahesh K. Mahanthappa
We report detailed small-angle X-ray scattering (SAXS) studies of the impact of variable n-decane loadings on the lyotropic liquid crystalline (LLC) phase behaviors of homologous bis­(tetramethylammonium) gemini didecanoate surfactants TMA-7x, which derive from dimerizing decanoic acid through its α-carbon with hydrocarbyl linkers −(CH2)x– where x = 3, 4, 5, and 6. TMA-7x amphiphiles with x = 3 or 5 exhibit a strong propensity to form normal double gyroid (G) LLC network mesophases over wide surfactant hydration ranges, as compared to homologues with x = 4 or 6. On swelling aqueous TMA-7x LLC mesophases with up to 35 wt % n-decane, we demonstrate that odd-carbon linked surfactants (x = 3 or 5) form G and normal double diamond (D) phases over wide water concentration windows with T = 22−100 °C. Complementary studies of decane-swollen TMA-7x (x = 4 or 6) aqueous LLCs instead demonstrate significantly diminished network phase stability, in favor of hexagonally-packed cylinder phases and a zoo of complex quasispherical micelle packings, which include micellar C14 and C15 Laves phases (P63/mmc and Fd3­(−)­m symmetries, respectively) and high-symmetry hexagonally close packed (HCP) and body-centered cubic (BCC) arrangements. These rich phase behaviors are rationalized in terms of linker length parity-dependent surfactant conformations and the delicate free energy balance that guides the packing of these geometrically anisotropic amphiphiles by minimizing unfavorable water-hydrophobic contacts, maximizing ionic surfactant-headgroup counterion solvation with minimal local variations, and maximizing electrostatic cohesion within these supramolecular assemblies.