Electrical double-layer effects on the Brownian diffusivity and aggregation rate of Laponite clay particles

Dynamic light scattering was used to study the Brownian translational diffu sion and rate of Brownian aggregation of Laponite (RD) clay particles at lo w (millimolar) electrolyte concentrations. Laponite is a manufactured clay consisting of monodisperse disk-shaped particles with a 30-nm diameter and a 1-nm thickness. The stability ratio, defined as the ratio of the coagulat ion rate for Brownian spheres with no particle interactions to the observed coagulation rate, was quite large O (10(5)), suggesting that there was a l arge potential energy barrier to Brownian aggregation. The apparent potenti al energy barrier for face-edge aggregation was rationalized on the basis o f a calculation of the electrostatic interactions between two disks with ne gative face charges and positive rim charges. The aggregation rate increase d with increasing electrolyte concentration owing to the screening of the e lectrostatic repulsion associated with the net charge on the particle. The rate decreased with increasing pH because of the decreasing positive charge on the rim. The translational diffusivity of the individual particles befo re the onset of aggregation exhibited a strong dependence on the electrolyt e concentration and was as much as 50% smaller than the diffusivity for an uncharged disk. This effect is attributed to the added drag resulting from the electroviscous effects in the deformed double layer. The electroviscous effect on the diffusion of the disk-like particles is much stronger than t hat on rods and spheres. (C) 2001 Academic Press.