Author ORCID Identifier

https://orcid.org/0000-0003-0889-8794

Defense Date

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Dusan Bratko

Abstract

The main purpose of my study was to work towards better understanding the behavior of salt solutions in nanoconfinements and its causes. To this end I have developed an in-house C++ code that can perform notoriously challenging open ensemble Monte Carlo molecular simulations, calculate relevant thermodynamic and extract structural information about each system. I use this code in my first project which deals with the intrusion/extrusion of aqueous NaCl into a nanopore open to a pressurized bulk environment. For my second project, I study the effect of explicitly accounting for intramolecular polarization and accompanying multi-body interactions on the uptake, structure, and thermodynamics of water and electrolyte in nanoconfinement.

High Pressure Simulation of Aqueous Electrolyte Uptake into a Hydrophobic Nanopore. Pressure-driven permeation of water in a poorly wettable material results in a conversion of mechanical work into surface free energy representing a new form of energy storage, or energy absorption. When water is replaced by a concentrated electrolyte solution, the storage capacity of a nanoporous medium becomes comparable to high-end supercapacitors. The addition of salt can also reduce the hysteresis of the infiltration/expulsion cycle. Our molecular simulations provide a theoretical perspective into the mechanisms involved in the process, and underlying structures and interactions in compressed nanoconfined solutions. Specifically, we consider aqueous NaCl in planar confinements of widths of 1.0 nm and 1.64 nm and pressures of up to 3 kbar. Open ensemble Monte Carlo simulations utilizing fractional exchanges of molecules for efficient additions/removal of ions have been utilized in conjunction with pressure-dependent chemical potentials to model bulk phases under pressure. Confinements open to these pressurized bulk, aqueous electrolyte phases show the intrusion can be reversed at narrow pore sizes, consistent with experiment, however, a strong hysteresis is observed at both pore sizes. The addition of salt results in significant increases in the solid/liquid interfacial tension in narrower pores and associated infiltration and expulsion pressures. These changes are consistent with strong desalination effects at the lower pore size, observed irrespective of external pressure and initial concentration.

Molecular Polarizability in Open Ensemble Simulations of Aqueous Nanoconfinements Under Electric Field. Molecular polarization in liquid water involves fast degrees of freedom that are often averaged-out in atomistic-modeling approaches. The resulting effective interactions depend on specific environment, making explicit account of molecular polarizability particularly important in solutions with pronounced anisotropic perturbations, including solid/liquid interfaces and external fields. Our work concerns polarizability effects in nanoscale confinements under electric field, open to unperturbed bulk environment. We model aqueous molecules and ions in hydrophobic pores using the gaussian-charge-on-spring BK3-AH representation. This involves nontrivial methodology developments in Expanded Ensemble Monte Carlo simulations for open systems with long-ranged multi-body interactions and necessitates further improvements for efficient modeling of polarizable ions. Structural differences between fixed-charge and polarizable models were captured in Molecular Dynamics simulations for a set of closed systems. Our open ensemble results with BK3 model in neat-aqueous systems capture the ~10% reduction of molecular dipoles within the surface layer near the hydrophobic pore walls in analogy to reported quantum mechanical calculations at water/vapor interfaces. The polarizability affects the interfacial dielectric behavior and weakens the electric-field dependence of water absorption at pragmatically relevant porosities. We observe moderate changes in thermodynamic properties and atom and charged-site spatial distributions, the Gaussian distribution of mobile charges on water and ions in the polarizable model shifts the density amplitudes and blurs the charge-layering effects associated with increased ion absorption. The use of polarizable force field indicates an enhanced response of interfacial ion distributions to applied electric field, a feature potentially important for in silico modelling of electric double layer capacitors.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

8-6-2020

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