Adsorption from Binary Liquid Solutions into Mesoporous Silica: A Capacitance Isotherm on 5CB Nematogen/Methanol Mixtures

TL;DR

This paper introduces a capacitance-based method to measure and analyze the adsorption of liquid crystal molecules from binary solutions into mesoporous silica, revealing insights into monolayer formation and pore filling transitions.

Contribution

It presents a novel capacitance measurement technique combined with a BET-inspired model to study adsorption and pore filling of liquid crystals in mesoporous silica.

Findings

Monolayer of 5CB molecules forms on pore walls.

Capillary filling transition occurs at high 5CB concentrations.

Method can be adapted to other binary liquid systems.

Abstract

We present a capacitance method to measure the adsorption of rod-like nematogens (4-cyano-4'-pentylbiphenyl, 5CB) from a binary liquid 5CB/methanol solution into a monolithic mesoporous silica membrane traversed by tubular pores with radii of 5.4 nm at room temperature. The resulting adsorption isotherm is reminiscent of classical type II isotherms of gas adsorption in mesoporous media. Its analysis by a model for adsorption from binary solutions, as inspired by the Brunauer-Emmett-Teller (BET) approach for gas adsorption on solid surfaces, indicates that the first adsorbed monolayer consists of flat-lying (homogeneously anchored) 5CB molecules at the pore walls. An underestimation of the adsorbed 5CB amount by the adsorption model compared to the measured isotherm for high 5CB concentrations hints towards a capillary filling transition in the mesopores similar to capillary condensation, i.e. film-growth at the pore walls is replaced by filling of the pore centers by the liquid crystal. The experimental method and thermodynamic analysis presented here can easily be adapted to other binary liquid solutions and thus allows a controlled filling of mesoporous materials with non-volatile molecular systems.