Tricia D. Shepherd
Investigating the hydration of solutes with both hydrophobic and hydrophillic character
Jessie Eiting ('15), Tricia D. Shepherd, Valeria Molinero (Univeristy of Utah)
11th Mercury Conference on Undergraduate Computational Chemistry, Bucknell University, Lewisburg PA, July 26-28 2012
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Molecular dynamics simulations performed by LAMMPS are used to study the hydration of a diatomic system where one particle is hydrophilic and the other is hydrophobic. By varying the bond length between the two particles, it is possible to see hydration trends as it transitions from more hydrophilic (short bond lengths) to more hydrophobic (larger bond lengths). As the bond length increases, disruption of the hydration shell is observed as the molecule becomes more polar. By introducing various bond lengths of the polar molecule into an environment with a water-vacuum interface, it is possible to see the patterns in the preferred orientation and position of the molecule with respect to the interface. These findings will help research in clathrates by giving a greater understanding of the interactions between water and similar molecules in a quasi-liquid interface. With a better grasp of the interaction mechanics, the growth and properties of clathrates can be better understood.
Selected Abstracts
MD Simulations of Clathrate Growth Near the Surface of Ice
Matthew A. Koc ('13) ,Valeria Molinero (University of Utah), Tricia D. Shepherd
National Conference of Undergraduate Chemistry, Weber State University, Ogden UT, March 29-31 2012
Clathrate hydrates are crystals in which hydrogen-bonded water molecules form cages that surround small nonpolar molecules, such as methane. Clathrates have great potential in the fields of energy, carbon sequestration, industrial separations, and natural gas transportation. Molecular dynamic simulations with coarse grained models for both water and methane are used to investigate how temperature affects the growth of clathrate and ice in the same simulation cell at temperatures ranging from 250 to 270 K. The structural characteristics of the clathrate, the ice, and the interface between the two are measured at these temperatures. At low temperatures (less than 262 K), it was found that the growth rate of ice was much larger than the growth rate of clathrate; however, at higher temperatures, the growth rate of the clathrate was larger than that of the ice. A quasi-liquid layer was observed to form between the two interfaces with less tetrahedral structure than either ice or clathrate, but more tetrahedral than pure water under similar conditions. The average thickness of this layer was measured to be approximately 10 Ã… at all temperatures. This thickness is nearly twice as large as the quasi-liquid layer formed at the surface of ice without the presence of clathrate.
Heterogeneous mixing in nanoconfined solutions
Rita Okumu ('14) ,Valeria Molinero (University of Utah), Tricia D. Shepherd
National Conference of Undergraduate Chemistry, Weber State University, Ogden UT, March 29-31 2012
The description and manipulation of matter between molecular and bulk length scales is of significant interest to a variety of disciplines including biophysical, materials and nanotechnology. We are investigating the effect of solute polarity and concentration on the mixing in nanoconfined aqueous systems. Using Molecular Dynamics and a course-grained model representing water and solute particles, simulations were performed incorporating a reservoir with a fixed solute concentration, and a pore, into which the solute can diffuse. Preliminary results indicate a layering of solvent and solute particles within this system that can be tuned as a function of solute concentration and polarity. To investigate the threshold of this stratification, the concentration and solvent-solute attraction were varied and the resulting equilibrated systems analyzed. It is apparent that the larger the solute-solvent interaction a heterogeneous layering is observed independent of solute concentration. The equilibration time, however, is much longer for smaller concentrations. In addition, for larger concentrations, layering is observed for less polar solutes, but the stratification is observed more quickly the larger the solvent-solute attraction. An understanding of these aspects at a microscopic level is essential for the engineering and optimization of new materials and processes.
We gratefully acknowledge support by the National Science Foundation through award CHE-1012651 (to V.M.)