Diffusivity and Solubility of Organic Solutes in Supported Liquid Crystal Membranes

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 degrees C (nematic) and 44 degrees 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.

Enantioselective cholesteric liquid crystalline membranes characterized using nonchiral HPLC with circular dichroism detection

“…The cholesteric to isotropic transition, in which the light became extinct under crossed-polarization due to the lack of birefringence, was observed at 37 • C. Below the transition temperature, focal conic structures were observed, which indicate the presence of a cholesteric phase in the presence of untreated substrates (see supporting information). It has been reported that transition temperatures for LCs in confined geometries decrease slightly or remain unchanged [7,19]. The presence of a liquid crystal to isotropic phase transition at 37 • C was confirmed using polarized optical microscopy for the SLCM produced by embedding a 5CB-COC mixture embedded in a CN support (see supporting information).…”

“…Chiral liquid crystalline (LC) phases were produced by doping achiral nematic LC materials (4-cyano-4 -pentylbiphenyl -5CB) with cholesterol based chiral dopants (cholesteryl oleyl carbonate -COC). Methods for the production of these membranes were previously developed by our laboratory for the preparation of achiral SLCMs [7]. This is the first use of cholesteric liquid crystals as the selective phase in enantiomer separation membranes, although a similar selective adsorption mechanism of enantiomers has been reported previously for chromatographic separations using cholesteric LC phases [8,9].…”

“…The presence of a liquid crystal to isotropic phase transition at 37 • C was confirmed using polarized optical microscopy for the SLCM produced by embedding a 5CB-COC mixture embedded in a CN support (see supporting information). We have previously reported that other LCs embedded in cellulose nitrate (CN) supports also exhibited only small variations from the bulk transition temperature [7].…”

Thermoregulated gas transport through electrospun nanofiber membranes

“…Disks (3.7 cm in diameter) were cut from the three-layered membranes and sandwiched between two pieces of aluminum tape (Han and Martin, 2009) having circular openings to expose an effective membrane area of 0.95 cm 2 . Silicone glue (RTV Silicone Gasket Maker, Ultra Copper, Permatex®) was used to seal the three-layered membrane where it contacts the aluminum tape, so that gas transport takes place only through the three-layered membrane itself.…”

“…The slower heating rate was used to ensure equilibration of the fibers at each temperature up to the transition temperature. According to Fick's law (Han and Martin, 2009;Treybal, 1980), , the feed side outlet was closed so that the feed gas flows through the membrane under an applied pressure. Gas flow rate on the permeate side was measured using a flow meter (ProFLOW 6000, Restek) with an accuracy of 0.1 ml/min.…”