Levich Institute Seminar Announcement, 03/13/2012
Steinman Hall, Room #312
(Chemical Engineering Conference Room)
Professor Nicolas Giovambattista
Brooklyn College of CUNY
Department of Physics
"Water Phase Behavior in Hydrophobic/Hydrophilic Nano-Scale Confinement and its effects on Water-Induced Interactions "
The behavior of water at interfaces and under nanoscale confinement is crucial in many engineering applications and biological processes. In this talk we will present results from molecular dynamics simulations of (i) water confined by nano-scale hydrophobic, hydrophilic, and "patchy" silica-based walls, and (ii) water confined by protein surfaces. We first focus on the effects of temperature and pressure on confined water phase behavior. We find that, at fixed temperature, water confined between hydrophobic plates can form vapor, liquid, or crystal (bilayer ice) phases, depending on the values of pressure (P) and separation between the plates (d). Lowering the temperature suppresses the vapor phase and stabilizes the liquid and crystal phases. Adding small hydrophilic domains to the hydrophobic plates (“patchy” surfaces) largely suppresses capillary evaporation and crystallization, highlighting the importance of chemical heterogeneity on hydrophobicity at the nanoscale. Water confined by hydrophilic surfaces is found only in the liquid phase. The effects of water phase behavior on hydrophobic interactions are addressed by calculating the potential of mean force between hydrophobic nanoscale solutes as function of temperature and pressure. We also describe the structure of water next to the walls. Simulations of water confined by heterogeneous walls decorated with hydrophobic and hydrophilic patches reveal that chemical heterogeneity reduces the effective hydrophobicity of the wall’s hydrophobic regions. Upon isobaric cooling or isothermal compression, water approaches the hydrophobic regions. Hence, the effective hydrophobicity of the walls also decreases as T decreases and/or P increases. These observations suggest that the invasion of hydrophobic cavities by water is an important mechanism underlying both pressure and cold denaturation of proteins. In the last part of this talk, we will present simulation results of water confined by nominally hydrophobic but chemically-realistic protein surfaces. Important differences in water phase behavior with respect to the ideal silica-based walls are found, with evaporation largely suppressed by the protein's chemical heterogeneity.
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