Systematic study of orientational wetting and anchoring at a liquid-crystal-surfactant interface

The molecular anchoring and orientational wetting properties of a liquid crystal close to the nematic-isotropic transition temperature confined to the 0.2 {μ}m cylindrical channels of alumina membranes are investigated for various surface preparations. The cavity walls of the confining pores are chemically modified with an aliphatic acid (CₙH₂ₙ₊₁-COOH) to establish surface anchoring. Radical changes in deuterium nuclear magnetic resonance (²H-NMR) line shapes in the nematic phase reveal the existence of a discontinuous homeotropic-to-planar anchoring transition that is induced by either changing the length of the surfactant (vary carbon number n), the density of the surfactant on the surface (vary concentration), or by varying temperature. The transition to planar anchoring drives the planar-polar nematic director field to a stable uniform axial structure. Above the nematic-isotropic transition temperature, the thickness of the surfactant monolayer is found to strongly influence the degree of the surface-induced orientational ordering. The corresponding order parameter of the liquid-crystal molecules at the surfactant interface increases as n increases, until a maximum ordering surface ($n=17$) is reached; thereafter, the surface order parameter decreases as n increases. An orientational wetting transition from partial to quasicomplete is observed as the length of the aliphatic acid increases. The effect is manifested in the change of the pretransitional temperature dependence of the adsorption parameter from weak to strong but still nondivergent. Further increase in n results in a reentrant phenomenon back to the partial wetting regime. Similar coupling mechanisms and wetting behaviors exhibited by the long chain aliphatic acids and the more rigid benzoic acid surfactants indicate minimal interdigitation of the liquid-crystal molecules into the surfactant aligning layer.