Aqueous suspensions of clay particles, such as montmorillonite (MMT) platelets and sepiolite (Sep) rods, tend to form gels at concentrations around 1 vol %. For Sep rods, adsorbing sodium polyacrylate to the surface allows for an isotropic-nematic phase separation to be seen instead. Here, MMT is added to such Sep suspensions, resulting in a complex phase behavior. Across a range of clay concentrations, separation into three phases is observed: a lower, nematic phase dominated by Sep rods, a MMT-rich middle layer, which is weakly birefringent and probably a gel, and a dilute top phase. Analysis of phase volumes suggests that the middle layer may contain as much as 6 vol % MMT.

Aqueous suspensions of sepiolite clay rods in water tend to form gels on increase of concentration. Here it is shown how addition of a small amount (0.1% of the clay mass) of a common stabiliser for clay suspensions, sodium polyacrylate, can allow the observation of an isotropic-nematic liquid crystal phase transition. This transition was found to move to higher clay concentrations upon adding NaCl, with samples containing 10(-3) M salt or above only displaying a gel phase. Even samples that initially formed liquid crystals had a tendency to form gels after several weeks, possibly due to Mg(2+) ions leaching from the clay mineral.

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.

No abstract