posted on 2020-02-26, 22:37authored byCarlos
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.