Exploring the thermodynamic signature of water interactions by molecular dynamic simulations

Jill Erin Tomlinson-Phillips, Purdue University

Abstract

This thesis describes the application of computer simulations and fundamental theoretical strategies to probe water’s interactions with various solutes, from small, simple polar and non-polar solutes, like the alkanes and alcohols, to the macroscopic solutes, like oil drops. There is a specific focus on exploring and expanding our knowledge of hydrophobic interactions and the dewetting phenomenon. First, partial molar volume simulations, performed over the entire ambient fluid temperature range, are used to investigate the volume changes associated with the hydration of idealized hydrophobic (hard sphere) particles of varying size. This provides insight into relevant thermodynamic quantities, such as the pressure derivative of the particle chemical potential. The results are used to develop a revised Cavity Equation of State (C-EOS) to better predict the partial molar volume of cavities from 0.2 to at least 1 nm in size. Second, molecular dynamic (MD) simulations of solutes of varying size and polarity in water are implemented to determine and quantify several thermodynamic features of hydration. Simulations of solute-water interaction energies and hydration free energies are used to infer energy and entropy changes in response to solute attractive forces. Evaluation of the range of applicability and eventual break down of first and second order perturbation theory is discussed. Of particular interest are simulations of nanometer-sized solutes, as the behavior of water is expected to be quite different on that scale. Quantifying these interaction and free energies provides a first look at these distinctive thermodynamic features. Finally, electric field strength calculations are performed on aqueous solutions of small molecules for comparison with experimental Raman spectra to elucidate spectral features that are obtained experimentally using Multivariate Curve Resolution (MCR). Specifically, simulations have focused on confirming the assignment of a relatively sharp peak, found at ∼3660 cm-1 , to a dangling OH bond. Furthermore, simulations confirm that the strength of the hydrogen bonding around small organic solutes is shown to be slightly weaker than those of pure water. Finally, vibrational frequency simulations are investigated over the ambient fluid temperature range to evaluate effect of increasing temperature on the dangling OH and strength of H-bonds in the hydration shell around n-alkanes.

Degree

Ph.D.

Advisors

Ben-Amotz, Purdue University.

Subject Area

Physical chemistry

COinS