Sol-gel and isotropic/nematic transitions in aqueous suspensions of natural nontronite clay. Influence of particle anisotropy. 2. Gel structure and mechanical properties.

After size-selection, the phase behavior of aqueous suspensions of nontronite clay was analyzed by osmotic pressure measurements, rheological experiments, and small-angle X-ray scattering. All the measurements confirm that for ionic strength < or =10(-3) M/L, the system is purely repulsive. By combining results from osmotic pressure measurements and X-ray scattering, it appears that the pressure of the system can be well-described using a simple Poisson-Boltzmann treatment based on the interaction between charged infinite parallel planes. In terms of rheological properties, even if the status of the sol/gel transition remains partially unclear as the number density of particles at the sol-gel transition exhibits a -2 power dependence with average particle size, the yield stress and elasticity of the gels can be easily renormalized for all particle sizes on the basis of the volume of the particles. Furthermore, rheological modeling of the flow curves shows that for all the particles, an approach based on excluded volume effects captures most features of nontronite suspensions. Still, the high shear flow properties of the suspensions that reveal a strong orientation of particles in the flow are affected by electrostatic interactions. This study then shows that the rich phase behavior of clay minerals, notably the fact that some clay minerals display an isotropic/nematic transition while others exhibit a sol-gel transition, requires a full understanding of all the interactions in the system that can only be achieved by working on well-characterized size-selected samples.