The effect of water-water hydrogen bonding on the hydrophobic hydration of macroparticles and its temperature dependence

Abstract

A theoretical model for the effect of water hydrogen bonding on the thermodynamics of hydrophobic hydration is proposed as a combination of the classical density functional theory with the recently developed probabilistic approach to water hydrogen bonding in the vicinity of a hydrophobic surface. The former allows one to determine the distribution of water molecules in the vicinity of a macroscopic hydrophobic particle and calculate the thermodynamic quantities of hydrophobic hydration as well as their temperature dependence, whereas the latter allows one to implement the effect of the hydrogen bonding ability of water molecules on their interaction with the hydrophobic surface into the DFT formalism. This effect arises because the number and energy of hydrogen bonds that a water molecule forms near a hydrophobic surface differ from their bulk values. Such an alteration gives rise to a hydrogen bond contribution to the external potential field whereto a water molecule is subjected in that vicinity. This contribution is shown to play a dominant role in the interaction of a water molecule with the surface. Our approach predicts that the free energy of hydration of a planar hydrophobic surface in a model liquid water decreases with increasing temperature in the range from 293 K to 333 K. This result is indirectly supported by the counter-intuitive experimental observation that under some conditions the hydration of a molecular hydrophobe is entropically favorable as well as by the molecular dynamics simulations predicting positive hydration entropy for sufficiently large (nanoscale) hydrophobes.