Maxwell Displacement Current Across Monolayers with Dielectric Anisotropy due to Biaxiality on a Water Surface

Abstract

Maxwell displacement currents (MDCs) across mesogenic liquid crystal monolayers of 4-cyano-4'-n-alkyl-biphenyl (nCB) with positive anisotropy, a mixture of 4-alkoxyphenyl 4-alkyl-cyclohexanecarboxylates of k-17-N-73-I (DON103) with negative dielectric anisotropy, and LVI-035L without dielectric anisotropy on a water surface were measured by compression of the monolayer film in the range of immeasurably low surface pressures by means of a MDC-measuring technique. It is shown that the MDCs generated in the Langm\\ddot uir films in the range just after the phase transition from the planar surface alignment isotropic phase to the polar orientational alignment phase mainly depend on the orientational change of the long-axis-orienting dipole moment of organic molecules. A novel theoretical explanation was introduced for study of this dependence. That is, a biaxial molecule model expressible with two order parameters S∥ and S⊥ was developed for the first time, and applied to the analysis of the aforementioned experimental results. Here S∥ and S⊥ are the orientational order parameter in the direction parallel and perpendicular to the molecular long-axis, respectively. It is found that the monolayer films of LVI-035L without dielectric anisotropy exhibit no MDC generation as the Langmüir films are compressed, and the extent of MDC generation in monolayers with positive dielectric anisotropy is much larger than that in monolayers with negative dielectric anisotropy. We concluded that the developed mathematical expressions based on the biaxial molecule model are useful for the analysis of the MDC generation across biaxial monolayers.