Date of Award

Summer 2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Computational Analysis and Modeling

First Advisor

Collin Wick

Abstract

An in-depth understanding of a wide range of physical, chemical, atmospheric and biological processes can only be achieved after the structure and dynamics of interfaces and the interfacial behavior of aqueous species, such as ions, are thoroughly studied and understood. This dissertation describes computational studies conducted to gain a more comprehensive understanding of such interfaces and the behavior of ions in the bulk and interfacial regions of the (1) air/water interface, and (2) alkane/water interfaces.

At the air/water interface the effect of counterion (sodium cations) charge and the influence of ion pairing on anion (chloride) propensity for the air/water interface of water was investigated. Higher counterion charge led to greater interfacial activity of the chloride anions and also caused stronger binding between the sodium and chloride ions. Shorter sodium-chloride interatomic distance also led to greater anion interfacial propensity while dampening the interaction strength between the counterion and anion had a small effect on propensity of the anions for the interface. Another phenomenon examined at the air/water interface was the effect of the halide ion in various sodium halide electrolyte solutions on the surface tension and surface excess while including electrostatic damping in the simulation model. Divalent strontium chloride was also examined in comparison to monovalent sodium chloride. Findings suggested that the smaller halide ions were found farthest from the air/water interface—in keeping with trends from previous studies—and resulted in the largest (most negative) surface excess, which would in turn cause the greatest increase in surface tension of water. Divalent strontium chloride had a more negative surface excess when compared to sodium chloride and the inclusion of electrostatic damping in the models reduced propensity of the ions for the interface and caused overall increase in surface excess.

The alkane/water interface was investigated to determine the effect of changing the length of the alkyl chain on the water/alkane interfacial width. Two separate studies found that longer alkane chain length led to shorter alkane/water interfacial widths. A long term goal of this research is to catalog the behavior of ionic species at different interfaces. The distribution of sodium-halide ions was compared at the alkane/water and air/water interfaces. Sodium halide ions were found closer to the air/water interface than the alkane/water interface. In the future, similar studies will be carried out at the alcohol/water interface and the effects of the nature of the organic phase (alkane or alcohol with varied chain lengths, degrees of branching, and solubility in water) will be examined.

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