Isotropic-nematic phase transition of polydisperse clay rods.

Rod-like colloidal particles are known to display an isotropic-nematic phase transition on increase of concentration, as predicted already by Onsager. Both natural clay particles and synthetic rods tend to be polydisperse, however, and the question arises how to allow for this in comparing experimental observations with theory. Experimental data for a wide range of samples (both from the literature and the new results) have been collated, with aspect ratios ranging from 14 to 35. As a characteristic, the concentration is taken where half of the sample volume is nematic. Experimental data agree well with predictions for monodisperse finite aspect ratio rods. However, compared to these predictions, the width of the transition (taken as the ratio of isotropic and nematic limiting concentrations) is noticeably broadened. Still, in most cases, the transition can be characterised by a linear increase of the nematic phase volume with sample concentration. The transition width is in broad agreement with theoretical predictions for infinitely thin rods.