Impact Of Oil Loading On Lyotropic Liquid Crystal Phase Behavior Of Carboxylate Surfactants

Abstract

Aqueous lyotropic liquid crystals (LLCs) are formed by solvating amphiphilic molecules with minimal amounts of water. Manipulating the extent of hydration, as well as other external conditions such as temperature, pressure, absence or presence of salt or oil, etc. drives formation of different nanoscale morphologies with important consequences for their physical properties. Particular interest has focused on bicontinuous cubic network (N) LLCs, the percolating channels of which are useful in the development of selective separations media and therapeutic delivery vehicles. However, the inability of surfactant molecules to adopt the negative Gaussian curvature that characterizes these systems hinders access to these useful phases and limits their applications. The unique twin-tail and twin-head architecture of gemini surfactants favors their lyotropic self-assembly into N phases. While reports on this surfactant class have previously demonstrated their ability to stabilize larger LLC N composition windows than their single-tail counterparts, only binary surfactant/water LLCs have been studied to date with little attention to the impact of additives on their unusual phase behaviors. In this thesis, we seek to understand the impact of the hydrophobic additive n-decane on the aqueous supramolecular self-assembly of gemini dialkanoate amphiphiles. We first describe the LLC behavior of the single-tail surfactant tetramethylammonium decanoate swelled with 40 weight percent n-decane (relative to surfactant mass). These control experiments revealed that n-decane enables access to previously unreported LLC mesophases, including a normal double diamond N LLC and a hexagonal C14 micellar LLC Laves phase. We further explored the aqueous self-assembly of gemini dialkanoate amphiphiles, in which two single-tail alkanoate amphiphiles are covalently linked through a hydrocarbon chain. By modifying linker length and counterion identity, we develop structure-self-assembly relationships to rationalize how the gemini dialkanoate architecture drives aqueous LLC microemulsion phase selection. Gemini dialkanoates with odd carbon linkers (x = 3, 5) are found to stabilize ordered network microemulsions, whereas systems with even carbon linkers (x = 4, 6) favor self-assembly into low-symmetry, complex spherical micelle packings. The observed phase behaviors may be additionally tuned by modulating counterion identity. N LLC phase windows widen as the degree of headgroup-counterion association increases. On the other hand, a highly dissociated counterion-headgroup pairs foster wide micellar LLC windows. These observations are rationalized in terms of a cooperative self-assembly that involves the unique, linker-length dependent conformational preferences of the hydrated surfactants, ion pair correlations, and the packing of the lipidic tails at constant density with minimal water-hydrophobic contacts.