Tricia D. Shepherd
Thermodynamic driving forces for aqueous self-assembly
Marcus Parry ('14), Rita Okumu ('14) ,Valeria Molinero (University of Utah) , Tricia D. Shepherd
12th Mercury Conference on Undergraduate Computational Chemistry, Bucknell University, Lewisburg PA, July 25-27 2013
Recent molecular dynamics simulations have revealed the spontaneous formation of liquid crystals from randomly mixed binary solutions of simple solutes in water modeled using a coarse-grained potential. The liquid crystals sustain their shape through anisotropic pressure simulations, and are stable in a wide range of water-solute interaction potentials. In this work, we use molecular dynamics simulations to investigate the driving force behind the liquid crystal formation. The extent of the hydrophobic/hydrophilic interaction between the solute pair in mW water is examined at temperatures from 260 K to 340 K. The potential of mean force (PMF) of the solute pair is calculated, along with the thermodynamic signatures relating to the solute pair, including the free energy of association, enthalpy of association, and entropy of association. These calculations reveal the driving force behind the association of the solute pair, providing insight into the more complex formation of liquid crystals. Further understanding of the theoretical formation of liquid crystals may reveal candidate solutes for experimental reproduction, potentially useful in nanofluidic applications and in the experimental control of the assembly of macromolecules.
Formation of Stripe Liquid Crystal Phases in Water Mixtures
​Rita Okumu ('14), Valeria Molinero (University of Utah), Tricia D. Shepherd
239th ACS National Meeting, New Orleans LA, April 6-9, 2013
Liquid crystals have widespread applications in nano-optical and nano-electrical technology. We report simulations of water and a mildly hydrophilic solute that produce stripe liquid crystal phases. Using classical Molecular Dynamics simulations with the mW model of water, we determined the range of stability of this liquid crystal phase and quantified the ratio of water to solute in the stripes as a function of the strength of the water-solute attraction. In addition we investigated the affect of temperature and excess solute and water on these structures. These preliminary results contribute towards an understanding of the mechanism behind the formation of this novel phase of water.
Hydration of a coarse-grained methyl-ion solute
Marcus Parry ('14) and Tricia D. Shepherd
239th ACS National Meeting, New Orleans LA, April 6-9, 2013
​
Studying toxic methyl mercury in aqueous environments is essential to understanding mechanisms of food chain contamination. While there is much research investigating solute-solvent interactions of aqueous metal ions or nonpolar molecules in water, we are interested in the solution properties of a simple methyl-ion model developed to mimic the behavior of aqueous methyl mercury. Coarse-grained parameters for several hydrophilic ion-like particles without charge are presented representing metal ions Zn2+, Hg2+, Na+, and K+. Despite the use of only short-ranged interactions, the coarse-grained ion models accurately reproduce the solvation structure of each ion in reference to both experimental observation and atomistic molecular dynamics simulations. Molecular dynamics simulations of these hydrophilic ion particles bonded to a single hydrophobic methane-like particle in water surrounded by a vacuum were performed. Methyl-zinc and methyl-mercury models were found to exhibit more hydrophobic character than smaller charge methyl-potassium and methyl-sodium for a range of bond lengths.
Selected Abstracts
My students use a variety of computational resources including our local server blackburn - 4X AMD Opteron 6274, 2.2GHz, 16C
funded by the Myriad Excellence in Learning Leadership Award (2011)
This project was funded though the Building Research, Innovation, and Novel Experimentation (BRINE) program supported through a grant from the Keck Foundation.
This project was funded though support from the Westminster College Gore Math & Science Endowment