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Nonlocal dielectric response of water Badali, Matthew

Abstract

Water, possessing a permanent dipolar moment, generates a peculiar potential of mean force between solvated ions, the investigation of which is the goal of this thesis. Other authors have observed overscreening between two charges in water, leading to attractive regimes in otherwise repulsive potentials, and vice versa. In this thesis, atomistic molecular dynamics simulations of Extended Simple Point Charge (SPC/E) water are used to generate a nonlocal dielectric function that matches with other computational and experimental results from literature. Nonlocality means the consideration of the orientation of neighbouring water molecules when finding the displacement field caused by an electric field. Specifically, instead of the displacement field at a given point depending only on the electric field at that point, with nonlocality the displacement field involves an integral of the electric field over all space. To compare to this nonlocal dielectric function derived from simulations of only water, simulations of water with ions are performed. Simulations of two ions in water demonstrate the existence of nonlocality, also in agreement with current scientific understanding. Further, simulations of three ions show that the effective mean force in more complicated systems is not simply a summation of pair-wise forces but instead must account for multi-body effects.

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