STATISTICAL-MODEL FOR THE STRUCTURE AND GELATION OF SMECTITE CLAY SUSPENSIONS

The sol-gel transition in a suspension of monodisperse, charged, diskl ike platelets (Laponite, i.e., a synthetic clay) is examined within a simplified statistical model. The initial ''primitive'' model of unifo rmly charged disks surrounded by microscopic co ions and counterions i s reduced to a model of nonintersecting disks carrying a rigid point q uadrupole, resulting from the electric double layers around the disks. The quadrupolar interactions favor edge-to-face pair configurations t hat counteract the tendency of parallel (nematic) alignment of bare di sks at high densities. The local structure and phase behavior of the q uadrupolar disk model is studied over a broad range of clay concentrat ions and quadrupole moments (which depend on the concentration of adde d salt) by extensive Monte Carlo simulations. The model suspension is found to undergo a reversible sol-gel transition above a critical quad rupolar coupling. The gel phase lacks long-range order, and is reminis cent of a ''house-of-cards'' structure in which most of the particles are edge-to-face to each other. The critical concentration and quadrup olar coupling constant are not inconsistent with recent experimental d ata on the gelation of Laponite suspensions.