Structure and dynamic properties of water saturated CTMA-montmorillonite: molecular dynamics simulations

The interlayer structure of wet organoclays has attracted great interest due to its close relation with adsorption process but still remains ambiguous at the atomic level. In this study, the structure and dynamics of cetyltrimethylammonium (CTMA+) intercalated montmorillonite (Mt) in the water saturated condition were investigated by molecular dynamics (MD) simulations. Two Mt models with different cation exchange capacity (CEC) were selected and various amounts of CTMA+ were added to achieve different loading levels based on experiments. The simulation results show that as the surfactant loading level increases, the arrangement of CTMA+ transforms from bilayer to inclined paraffin-type with a large amount of water in the interlayer space, indicating a different configuration from dry systems. The conformations of CTMA+ are influenced by water molecules and surfactant loading level. Compared to the dry models, the percentage of gauche conformations of CTMA+ decreases in the water saturated condition. In the cases of incomplete cation exchange, the confinement from silicate surface may lead to more ordered structure of alkyl chains. In the cases of excessive surfactant loading, the percentage of gauche conformation decreases due to the steric hindrance. Moreover, the ammonium head groups of CTMA+ are found to locate close to the center of six-member ring of silicate surface and coordinated with 4–6 water molecules. Their mobility is low due to the electrostatic interactions while the alkyl chains show higher mobility. Mt with a higher CEC has a stronger confining effect on both alkylammonium and water, which then reduces the mobility of alkyl chains and water molecules within the interlayer space.