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Vaithee


Graduate Student in Physics


am studying water in confined systems using computer simulations. My first project is a constant pressure and temperature molecular dynamics study of water between two plates acted upon by an external electric field. This system is immersed in a larger "bath" with which it is in chemical and thermal equilibrium.
These simulations show a very different behavior of water compared to constant volume simulations. Our results imply that for fluids with large molecular dipole moments like water, current ideas of electrostriction need revision.


The second project is a Monte Carlo study of single-file water chains inside a carbon nanotube. We have studied the structure and energetics of the confined water at different electric fields and temperatures.
By evaluating the grand-canonical partition function we show that external electric fields favor the filling of previously empty nanotubes from the bulk phase. A simple two-state statistical mechanics model provides an excellent approximation for the electric field dependence of the polarization of the water chain and its effect on the filling equilibrium. We also find that the filled state of the tube is favored by increasing temperature near ambient conditions. This implies, counter-intuitively, that the entropy of the confined water is greater than that of bulk water.


Electric field dependence of the free energy of filling for the long modified tube





Electric field dependence of the free energy of filling for the long modified tube

molecule
Electric field dependence of the total dipole moment of the water chain.

Comparison of simulations with the two-state model.

We have also conducted a Monte Carlo study of water occupancy in hydrophobic cavities similar to those found in proteins.