Monte Carlo simulations of a clay inspired model suspension: the role of rim charge

Abstract

We present a theoretical investigation of a model clay dispersion in 1-1 salt solutions by varying the particle volume fraction and ionic strength as well as the charge distribution on the clay platelets. The platelets are modeled as discs with charged sites distributed on a hexagonal lattice. The edge sites can be positively charged while the remaining sites are negative giving rise to a strong charge anisotropy. Simulations are carried out using a Monte Carlo method in the canonical ensemble. The interactions between the platelet sites are described with a screened Coulomb potential plus a short range repulsive potential. Simulations show a complex phase behavior. When the charge anisotropy is strong, i.e. all edge sites are positively charged, a fluid phase dominated by repulsion is found at low volume fraction and ionic strength. When increasing the latter an attractive liquid phase forms. At these volume fractions the platelets aggregate in an “Overlapping Coins” configuration. With increasing volume fraction the dispersion becomes unstable and the pressure goes through a van der Waals loop. A liquid crystalline phase, Smectic B, forms in the thermodynamically unstable region. On the other side of the van der Waals loop a stable gel phase is found. A phase separation between a fluid and a gel is thus predicted. The threshold value of the volume fraction at which the phase separation occurs is found to increase with the salt concentration. The gel structure is a mixture of “Overlapping Coins” and “House of Cards” configurations. When the charge anisotropy is intermediate, no phase separation occurs. Instead, a gel forms from a sol of clusters of individual particles randomly oriented that progressively grow with the volume fraction. These results are discussed in light of experimental observations on clay suspensions.