Phase transitions of small molecules and liquid crystals in restricted geometries

Abstract

Restricted geometry effects on phase transitions of small molecules (cyclohexane, cyclohexanone, and ethyl alcohol) and liquid crystals (p-azoxyanisole (PAA) and 4-heptyloxybenzoic acid (HOBA)) were investigated using Raman spectroscopy and the differential scanning calorimetry (DSC) technique. The host materials were porous sol-gel glass and Vycor glass with average pore diameters varying from 2.9 nm to 22.0 nm. Selected porous samples had their internal surfaces modified to evaluate the effects due to surface-molecule interactions. The phase transition temperatures of molecular systems inside small pores were found to be depressed, and the depressions were pore size dependent. This phenomenon was clearly observed for the cubic to monoclinic solid-solid phase transition of cyclohexane, the cubic to orthorhombic solid-solid phase transition of cyclohexanone, the solid to nematic phase transition of PAA, and the solid to smectic phase transition of HOBA. The experimental results showed that the temperature depressions were linearly dependent on the reciprocal of the pore diameter, which was predicted by the capillary condensation theory. For the liquid-solid transition of cyclohexane, the rotational motion of cyclohexane molecules was hindered inside small pores. The experimental data were in good agreement with Brodka and Zerda's computer simulation results. The amorphous structures of these molecular systems were observed inside the pores. The surface interactions affect the amount of the amorphous structure which increases with the strength of the molecule-pore surface interactions. In small pores the amorphous structure is more important, because a large portion of molecules are the surface monolayer molecules which are completely disorganized and resemble the liquid phase. Surface modification had a profound effect on the phase transitions of cyclohexane, particularly on the cubic to monoclinic transformation. For other molecules, cyclohexanone, PAA, and HOBA, the surface modification had less influence mainly because of stronger intermolecular couplings. For molecular systems with strong intramolecular couplings, the boundary condition change due to the replacement of -OH groups by -Si(CH$\sb3)\sb3$ is not sufficient to modify the phase behavior, and it is the restricted space that limits the size of a crystal and dominates the changes of phase transitions.