  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.
