An Experimental System for Imaging the Reversible Adsorption of Amphiphiles at Aqueous−Liquid Crystal Interfaces

Abstract

We report an experimental system that permits optical imaging of the reversible adsorption of amphiphiles at stable, planar interfaces formed between aqueous phases and immiscible thermotropic liquid crystals. Copper grids (hole sizes of 19−292 μm and thicknesses of 18−20 μm) supported on glass surfaces treated using octadecyltrichlorosilane were impregnated with nematic 4-cyano-4‘-pentylbiphenyl (5CB). Films of 5CB confined within the grids were stable (did not dewet the grid) to immersion into either water, aqueous solutions of sodium dodecyl sulfate (SDS), or aqueous solutions of SDS containing NaCl. Whereas the anchoring of 5CB on the copper grid dominated the appearance of the 5CB when using grids with hole sizes of 19 μm, reversible changes in the orientation of the liquid crystal (observed using polarized light) caused by adsorption of SDS at the liquid crystal−aqueous interface were readily observed when using grids with hole sizes larger than 19 μm. With increasing concentrations of SDS in the aqueous phase, a series of reproducible and distinct surface-driven distortions were observed within the 5CB confined to the grid. We also observed the effects of added NaCl on the distortions induced within the liquid crystal to be consistent with increased adsorption of SDS caused by screening of the electrostatic interactions between the SDS adsorbed at the interface. This result suggests that the orientation of the 5CB is influenced by the areal density of SDS molecules at the liquid crystal−aqueous interface (probably through steric interactions). Because the aqueous phase contacting the liquid crystal can be exchanged (thus permitting the addition and removal of reactants), this experimental system is a simple and broadly useful one for investigations in which liquid crystals are used to amplify interfacial phenomena at fluid interfaces into optical images.