Water's hydrogen bonds in the hydrophobic effect: a simple model

Area of Science:

• Physical Chemistry
• Chemical Physics
• Computational Chemistry

Background:

• The hydrophobic effect is crucial in biological systems and materials science.
• Understanding the role of water's hydrogen bonding network is key to explaining this effect.
• Existing models often struggle to quantitatively capture temperature-dependent hydrophobic phenomena.

Purpose of the Study:

• To develop a simple analytical model for the hydrophobic effect that explicitly includes water's hydrogen bonding.
• To investigate how hydrogen bond orientational restrictions influence solvation thermodynamics.
• To explain the temperature dependence of the hydrophobic effect and associated entropic behavior.

Main Methods:

• A mean-field partition function approach was used to model water molecules in the first solvation shell.
• The model incorporates orientational constraints imposed by hydrogen bonding.
• Key quantities were derived from bulk water simulations, and a 2D model illustrated the principles.

Main Results:

• The model successfully predicts the heat capacity of hydrophobic solvation.
• It reproduces solvation energies and entropies across different temperatures with a single fitting parameter.
• The model accounts for the dependence of the hydrophobic effect on solute size.

Conclusions:

• Water's hydrogen bonding propensity is a primary driver of the hydrophobic effect's temperature dependence.
• The model provides a theoretical basis for the experimentally observed positive entropy of dissolving nonpolar solutes in hot water.
• This work offers a simplified yet powerful framework for understanding hydrophobic interactions.