Diffusivity and Solubility of Organic Solutes in Supported Liquid Crystal Membranes

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Abstract
The electro-optical properties of thermotropic liquid crystalline (LC) materials have been the subject of significant research effort due to their well-established applications in display technology; however, relatively little work has been done concerning the transport of dissolved solutes in LC phases, limiting their potential use in applications including chemical separation and sensing. Supported liquid crystal membranes were synthesized by impregnating porous cellulose nitrate (CN) membranes with 4-cyano-4′-octylbiphenyl (8CB) from chloroform solutions under vacuum. The resulting membranes were stable under aqueous conditions. Measurements of the 8CB-CN membrane transport properties were performed at 38 °C (nematic) and 44 °C (isotropic) for 11 aromatic solutes, including positional isomers, in aqueous solution. Solute diffusivity and solubility was calculated using a time-lag technique based on the study of the transient (unsteady state) and steady-state permeation regimes. The solubility and diffusivity of aqueous aromatic solutes in 8CB LCs depended significantly on intra- and intermolecular hydrogen bonding, and more specifically the number of hydrogen-bonding sites on a solute that are available for interactions with the aqueous and LC phases. In the nematic phase of 8CB, shape specific affinity was observed for para isomers and other rodlike solutes. A decreased activation energy for diffusion was observed at the isotropic to nematic phase transition for o-hydroxybenzoic acid, as expected based on the increased order in the nematic phase. Permeation selectivities for the separation of positional isomers by the 8CB-CN membranes indicated high selectivity only for the hydroxybenzoic acid isomers, due to the propensity of o-hydroxybenzoic acid to form intramolecular hydrogen bonds.