Understanding the physics of hydrophobic solvation

  • 13 October 2023
  • 1400-1500
  • Sch 1.05

Bob Evans, Bristol

Water on macroscopic hydrophobic surfaces exhibits a large contact angle; the liquid is seemingly repelled. For solute molecules or nanoparticles the term ?hydrophobic? is less clearly defined. We investigate the origins of the density depletion and accompanying enhanced density fluctuations that arise in water in the vicinity of an extended hydrophobic solute, arguing that both phenomena are remnants of the critical drying surface phase transition that occurs at liquid-vapour coexistence in the macroscopic planar limit, i.e., as the solute radius Rs ? ?. Focusing on the density profile and a sensitive spatial measure of fluctuations, the local compressibility profile (the analogue of the layer susceptibility ?n studied in models of surface magnetism) we develop a scaling theory which expresses the extent of the density depletion and enhancement in compressibility in terms of Rs, the strength of solute-water attraction, and the deviation from liquid-vapor coexistence ??.
Testing the predictions against results of i) classical density functional theory for a simple (Lennard Jones) solvent and ii) grand canonical Monte Carlo simulations of a popular water model, we find that the theory provides a firm physical basis for understanding how water behaves at a hydrophobe.

M.K. Coe, R. Evans & N. B. Wilding Phys.Rev.Lett.128, 045501 (2022) ; J.Chem.Phys.158, 034508 (2023)

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