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Solvation science is an integral part of many fields across physics, chemistry, and biology. Liquids, interfaces, and the ions that populate them are responsible for many poorly understood natural phenomena such as ion specific effects. Establishing a single-ion solvation free energy thermodynamic scale is a necessary component to unraveling ion-specific effects. This task is made difficult by the experimental immeasurability of quantities such as the interfacial potential between two media, which sets the scale. Computer simulations provide a necessary bridge between experimental and theoretical results. However, computer models are limited by the accuracy-efficiency dilemma, and results are misinterpreted when the underlying physics is overlooked. Classical molecular dynamic techniques, while efficient, lack transferability. Quantum-based ab initio techniques are accurate and transferable, but their inefficiency limits the accessible simulation size and time. This thesis seeks to determine the physical origin of the interfacial potential at the liquid-vapor interface using classical models. Additionally, I assess the ability of Neural Network Potential (NNP) simulation methods to produce electrostatic properties of bulk liquids and interfaces. Complicating factors are minimized through a simple water model (SPC/E) free of experimental parametrization and a finite droplet simulation free of Ewald effects. Multipolar decomposition of the potential in the region of zero charge density provides a direct method for determining the potential felt by ions near interfaces. Non-aqueous solvents are studied through an OPLS-AA based model of the organic liquid ethylene carbonate (EC) using the same approach to compare aqueous and non-aqueous solvents. Neural network potentials may be a step towards the higher-level need for predictive models, but they require further testing. Using an existing NNP framework, I train models for dynamics, as well as multipole electrostatics. Combining the two in both a bulk and interfacial system allows for the calculation of interfacial electrostatic properties.My results for water elucidate the reason for widely varying net potentials calculated for various models with similar dipole but differing quadrupole moments. Near-cancelling dipole contributions between the droplet interface and the cavity interface of a solvated ion leaves the quadrupole as the dominant contribution to the net potential. Molecular density profiles and potential profiles show that a length scale of 5 {\AA} from cavity boundary is needed for a convergent potential. A theoretical argument for the radial dependence of each contribution is made, which supports my results. EC also has this radial dependence but has a different length scale of convergence. Differences in the molecular size, orientation, hydrogen-bonding capabilities, and multipole moments results in solvent-specific net potential contributions. This is evidenced by the results for charged cavities and an orientational analysis of EC.NNPs are shown to provide excellent agreement with ab initio electrostatic properties. This is encouraging evidence for the use of NNPs in the calculation of thermodynamic properties and in force field development. |
