Computer simulation studies of the structure and dynamics of ions and non-polar solutes in water

The mobility of simple ions such as alkali-metal and halide ions at room te mperature shows two anomalies. Firstly, there are maxima in mobilities as a function of ion size for both positive and negative ions and, secondly, th e maximum for negative ions occurs at a larger ionic radius than the maximu m for positive ions. Theoretical treatments of this problem are reviewed an d it is concluded that a molecular treatment of the system is needed to und erstand the results. Computer simulation using the simple point charge mode l (SPC/E) for water reproduced the observations and is used to discuss the application of theories. In particular, the nature of the first solvation s hell is correlated with ion mobility. Simulation reveals a further anomaly, namely that if the charge is removed from a large ion, then it moves more slowly. This is interpreted as the result of formation of a solvent cage ar ound the hydrophobic solute. The changes in local structure resulting from changes in charge and size also affect the solvation thermodynamics. Simula tions show that the solvation entropy has a double maximum when viewed as a function of charge. The local minimum near zero charge is interpreted as b eing due to hydrophobic order, and the maxima as the result of structure br eaking. This double maximum in the entropy of solvation is a signature of t he hydrophobic cage effect. Comparisons are made between ion mobilities in liquid water at ambient and supercritical conditions.