Principles for Manipulation of the Lateral Organization of Aqueous-Soluble Surface-Active Molecules at the Liquid Crystal-Aqueous Interface

We report an investigation of the lateral organization of water-soluble, surface-active molecules within monolayers formed spontaneously at interfaces between aqueous phases and immiscible, micrometer-thick films of nematic liquid crystals (LCs; 4'-pentyl-4-cyanobiphenyl and TL205, a mixture of cyclohexanefluorinated biphenyls and fluorinated terphenyls). Using both anionic (sodium dodecyl sulfate) and cationic (dodecyltiimethylammoniumbromide) surfactants, we demonstrate that the nematic order of the LCs can direct monolayers of surfactant in dynamic equilibria with bulk aqueous solutions to phase separate and assume lateral organizations at the interfaces of the LCs that are not seen in the absence of the nematic order. The lateral organization of the surfactants is readily evidenced by the patterned orientations assumed by the LCs and can be manipulated reversibly by changes in the bulk concentrations of the surfactants. Experimental observations of the effects of bulk surfactant concentration, thickness of the film of LC, nematic order, and aqueous electrolyte concentration are placed within the framework of a simple thermodynamic model. The model incorporates the dynamic equilibration of surfactant between the bulk and interface as well as the coupling between the elasticity of nematic LCs and the lateral organization of the water-soluble surfactants within the monolayers. Qualitative agreement is found between the model predictions and experimental observations, thus supporting our conclusion that LCs offer the basis of general and facile methods to direct the lateral organization of interfacial molecular assemblies.