Linking molecular/ion structure, solvent mesostructure, the solvophobic effect and the ability of amphiphiles to self-assemble in non-aqueous liquids

Abstract

Sixteen non-ionic molecular solvents have been found to exhibit the solvophobic effect and to support the formation of amphiphile self-assembly mesophases. The solvents were low molecular weight polar solvents which contained various combinations of amine, hydroxyl or ether moieties with relatively small proportions of hydrocarbon unit constituents. The studied amphiphiles were hexadecyltrimethylammonium bromide (CTAB), hexadecylpyridinium bromide (C₁₆PyrBr) and tetraethylene glycol monohexadecyl ether (C₁₆E₄). Lyotropic liquid crystal mesophases with lamellar, normal hexagonal and normal bicontinuous cubic, with ordered one-, two- and three-dimensional periodic structure respectively, were identified in CTAB and C₁₆PyrBr systems by using cross-polarised optical microscopy (CPOM). Mesophase diversity and thermal stability ranges correlated to the Gordon parameter (G) value, a proxy for the solvent cohesive energy density. Infrared spectroscopy confirmed that all the studied molecular solvents were associative liquids. Solvent mesostructure was studied by synchrotron small angle X-ray scattering. The small sub-set of neat solvents which were mesostructured, with polar and non-polar domain segregation, displayed the lowest G values, and amongst the lowest mesophase diversity and thermal stability ranges. It has been established that the G value is a good indicator of whether or not a molecular solvent is likely to behave as a co-surfactant, residing within the amphiphile–solvent interfacial region of self-assembled objects, thereby influencing specific mesophase structure formation. Structure–property behaviour has been explored and shows that beneficial solvent features for serving as amphiphile-self assembly media, with the potential for rich mesophase diversity, include the presence of hydroxyl > amine > ether moieties, while methyl moieties have an adverse effect larger than that of methylene moieties.