A Monte Carlo study of interlayer and surface structures of tetraphenylphosphonium-modified Na-montmorillonite

Na-montmorillonite modified with tetraphenylphosphonium (TPP+) chloride has been shown to possess a high sorption capacity for iodide and a high thermal stability. We studied TPP+ arrangements in the interlayer space and on the external surface of Na-montmorillonite using Monte Carlo simulations in order to contribute to an explanation of such unresolved experimental issues as interlayer structure and the origin of excess positive surface charge of this clay. The layer spacing simulated for a monomolecular arrangement of interlayer TPP+ ions, which is characterized by a segregation of organic and inorganic species to the opposite montmorillonite surfaces, was found to show the best agreement with the experiment. The simulations revealed further that the maximum surface coverage of adsorbed TPP+ ions equals the negative charge density of the external montmorillonite surface. This TPP+ adsorption does not lead to a desorption of Na+ ions associated with tetrahedral substitutions in the montmorillonite layer, which explains the experimental observation of the excess positive charge of TPP+-modified montmorillonite.