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