Kosmotropes and chaotropes: modelling preferential exclusion, binding\n and aggregate stability

Kosmotropic cosolvents added to an aqueous solution promote the aggregation\nof hydrophobic solute particles, while chaotropic cosolvents act to destabilise\nsuch aggregates. We discuss the mechanism for these phenomena within an adapted\nversion of the two-state Muller-Lee-Graziano model for water, which provides a\ncomplete description of the ternary water/cosolvent/solute system for small\nsolute particles. This model contains the dominant effect of a kosmotropic\nsubstance, which is to enhance the formation of water structure. The consequent\npreferential exclusion both of cosolvent molecules from the solvation shell of\nhydrophobic particles and of these particles from the solution leads to a\nstabilisation of aggregates. By contrast, chaotropic substances disrupt the\nformation of water structure, are themselves preferentially excluded from the\nsolution, and thereby contribute to solvation of hydrophobic particles. We use\nMonte Carlo simulations to demonstrate at the molecular level the preferential\nexclusion or binding of cosolvent molecules in the solvation shell of\nhydrophobic particles, and the consequent enhancement or suppression of\naggregate formation. We illustrate the influence of structure-changing\ncosolvents on effective hydrophobic interactions by modelling qualitatively the\nkosmotropic effect of sodium chloride and the chaotropic effect of urea.\n