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Open Access Publications from the University of California

Fluctuations in Water and their Relation to the Hydrophobic Effect

  • Author(s): Varilly, Patrick Stephen
  • Advisor(s): Chandler, David
  • Liphardt, Jan T
  • et al.

The hydrophobic effect, or the tendency for oil and water not to mix, is a fundamental force that strongly influences the shape, behavior and assembly of solutes in solution. Hydrophobicity emerges from the collective behavior of large numbers of solvent molecules, so its accurate treatment is challenging. A decade ago, Lum, Chandler and Weeks (LCW) addressed this challenge indirectly by modeling how solvent density fluctuations couple to external solutes and constraints, and then inferring hydrophobic behavior from the resulting mean solvent density. LCW theory is successful because it distinguishes between, and separately models, small-length-scale and large-length-scale density fluctuations. In this thesis, we develop methods for probing the statistics of large-length-scale density fluctuations in computer simulations of water. We use these tools to study solvation phenomena in model systems and in proteins, in bulk water and near surfaces, and we rationalize these phenomena in terms of LCW ideas. Building on these ideas and on past efforts by others, we construct a tractable, efficient and accurate theory of solvation on a coarse-grained lattice. The final theory allows us to model the solvation behavior of uncharged, static solutes of arbitrary shape, and we outline the steps necessary to model charged, dynamic solutes in the future. A unifying thread in our solvation studies is the importance of fluctuations of liquid-vapor interfaces. At the end of this thesis, we describe how these fluctuations may play a role in water evaporation.

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