Lyotropic phases reinforced by hydrogen bonding

Amphiphilic guanidinium alkylbenzenesulfonates (GCnBS; n = number of carbons in the alkyl chain) exhibited lyotropic behavior in aqueous and organic solvents. The GCnBS compounds formed gel-like phases in certain cyclic organic solvents (e.g. p-xylene, cyclohexane) through the formation of swollen interdigitated lamellar phases reinforced by hydrogen bonding between the guanidinium ions and sulfonate moieties. This behavior was not observed for the homologous sodium alkylbenzenesulfonates, indicating that hydrogen bonding, mediated by the guanidinium (G) ion, was required for gel formation. Infrared spectroscopy unambiguously demonstrated the existence of the quasi-hexagonal hydrogen-bonded sheet typically adopted by G ions and the sulfonate groups in layered, solvent-free crystalline phases of the compounds, supporting lamellar structures in the gels. Small-angle X-ray scattering analysis of these gels revealed GCnBS lamellar phases with interlayer spacings (d) that increased with increasing temperature, consistent with increased absorption of solvent by the nonpolar regions of the gelator. At the lower gelator concentrations, the increase in d-spacing achieved at the higher temperatures exceeded the sum of the alkylbenzene chain lengths, suggesting either long-range interactions between the GS sheets or undulation stabilized lamellae, which have been reported in aqueous lamellar gels. The GCnBS compounds also formed lyotropic phases in water, but the phase behavior was more complex than that of the organogels. The rheology suggested gel-like behavior associated with entangled worm-like micelles at these higher concentrations. These lyotropic phases were reminiscent of crystalline layered and tubular architectures exhibited by various guanidinium organomonosulfonate compounds. These lyotropic phases expand the liquid crystal behavior observed for GS compounds beyond recently observed thermotropic smectic phases, adding to the portfolio of phase behavior exhibited by these materials.