Isotropic/nematic and sol/gel transitions in aqueous suspensions of size selected nontronite NAu1

The phase behaviour of aqueous suspensions of NAu1 nontronite was studied on size-selected particles by combining osmotic pressure measurements, visual observations under polarized light, rheological experiments and Small Angle X-ray Scattering (SAXS). NAu1 suspensions display a liquid crystalline behaviour as they exhibit a Isotropic/Nematic (I/N) transition that occurs before the sol/gel transition for ionic strengths below 10-3 M/L. This I/N transition shifts towards lower volume fractions for increasing particle anisotropy and its position in the phase diagram agrees well with the theoretical predictions for platelets. SAXS measurements reveal the presence of characteristic interparticular distances in the isotropic, nematic and gel phases. In the gel phase a local lamellar order is observed which shows that the "house of cards" model is not appropriate for describing the gel structure in swelling clay materials at low ionic strength. Furthermore, 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 repulsion between charged infinite parallel planes. In terms of rheological properties, even if the thermodynamical status of the sol/gel transition remains partially unclear, 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 modelling of the flow curves shows that for all the particles an approach based on excluded volume effects captures most features of nontronite suspensions.