8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.158Wed 711:10-14:00A Derjaguin- hypernetted chain equation (D-HNC) view ofthe stability and yield stress of clay materialsBelky Sulbarán, 1 Werner Zambrano, 2 and Wilmer Olivares-Rivas 31 Instituto Tecnológico de Ejido (IUTE), Dept. de Investigación, Area de CienciasAplicadas, Instituto Tecnológico de Ejido (IUTE) 5201, Ejido, Venezuela2 Universidad de Los Andes, Merida, Venezuela3 Universidad de Los Andes, Mérida, VenezuelaClay dispersions at high volume fractions exhibit a yield stress, namely, a stress above which thematerial flows like a viscous fluid. The description of this property is rather important in a varietyof industrial systems and processes and particularly in the formulation of drilling fluids, ceramicproducts and paints. Determination of the yield stress as a true material property is known to bedifficult, because it not only depends on the sample preparation and the measurement technique,but also on the model and the theory used to evaluate rheological data. When modeling clays, thecommon assumptions are that the platelet thickness d is infinite, the ion diameter a is zero, and thatthe surface potential is a constant. However, we have shown before that the ion-ion correlationsdue to the finite sizes, a and d, give rise to local electro-neutrality violation and repulsive/attractiveoscillations of the disjoining pressure, as a function of the separation between the like-like chargedplates. In this work we present a study of swelling pressure and the yield stress of a face-facemodel for planar clays, such as montmorillonite, with finite thickness d, at fixed surface chargedensity, and in the presence of 1-1 or 2-2 RPM electrolytes also with finite ionic size. Here, theclassical misleading view of the problem, the so called DLVO approach, is extended by evaluatingthe Derjaguin approximation, using a van der Waals potential for finite d, in conjunction with thenon-linear modified Gouy-Chapman equation (D-MGC) and the numerical HNC integral equation(D-HNC). So, the stability and yield stress of concentrated clay materials is revisited as a functionof the ratio a/d and volume fraction.158