The orientational and spatial distribution of solutes in thermotropic liquids crystals

Abstract

Measurement of deuterium NMR spectra of suitably deuteriated solutes provides a powerful technique for studying solute alignment in liquid crystal. We have used this approach to investigate the orientational order of perdeuterio-n-octane and perdeuterio-benzene dissolve in the nematic and smectic A phases of 4,4′-di-n-heptylazoxybenzene (T CS A 34 °C, T S A N 54 °C, T NI 71 °C). The order parameter for each segment in n-octane is found to increase with decreasing temperature in the nematic phase. However, within the smectic A phase the four order parameters show a small but definite decrease with decreasing temperature. Thus unusual behaviour contrasts with that exhibited by benzene where the orientational order parameter increases with decreasing temperature throughout the nematic and smectic A phases. We believe that these results can be understood by considering the heterogeneous nature of the smectic layers and the non-uniform spatial distribution of the solute. The solvent molecule is composed of a rigid aromatic core which is highly ordered in a smectic layer and two flexible alkyl chains which are poorly ordered. The solute will be distributed between these two regions and its order parameter is then an average of the values in these regions. For both energetic and entropic reasons the n-octane solute prefers to be associated with the alkyl chain; a tendency which increases with decreasing temperature. Since the order parameter of the solute is smaller in the alkyl chain region that in the aromatic region the orientational order parameter for n-octane decreases with decreasing temperature even though the order of the alkyl chain increases. The behaviour of the orientational order for benzene can be understood in similar terms although now the solute prefers to be associated with the aromatic core. This region is highly ordered and the order increases with decreasing temperature; in addition the association of benzene with the aromatic core also increases with decreasing temperature. As a consequence the order parameter of benzene increases with decreasing temperature in the smectic A phase.