The sol-gel transition in a suspension of monodisperse, charged, disklike platelets ~Laponite, i.e., a synthetic\ud clay! is examined within a simplified statistical model. The initial ‘‘primitive’’ model of uniformly charged\ud disks surrounded by microscopic co ions and counterions is reduced to a model of nonintersecting disks\ud carrying a rigid point quadrupole, resulting from the electric double layers around the disks. The quadrupolar\ud interactions favor edge-to-face pair configurations that counteract the tendency of parallel ~nematic! alignment\ud of bare disks at high densities. The local structure and phase behavior of the quadrupolar disk model is studied\ud over a broad range of clay concentrations and quadrupole moments ~which depend on the concentration of\ud added salt! by extensive Monte Carlo simulations. The model suspension is found to undergo a reversible\ud sol-gel transition above a critical quadrupolar coupling. The gel phase lacks long-range order, and is reminiscent\ud of a ‘‘house-of-cards’’ structure in which most of the particles are edge-to-face to each other. The critical\ud concentration and quadrupolar coupling constant are not inconsistent with recent experimental data on the\ud gelation of Laponite suspensions
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