(455a) Crossover From a Hydrophobic to Hydrophilic Force Law in a Coarse-Grained Water Model

Water-mediated forces, namely the so-called hydrophobic and hydrophilic interactions, govern many biological mechanisms, such as the folding of proteins, the forming of membranes, as well as the assembly of cellular machines.  Despite numerous efforts on these fundamental forces, there remains significant work for a complete understanding of them [1].  In particular, recent experiments for surfactant-coated interfaces suggest that both hydrophobic and hydrophilic interactions can be cast under a single force law [2].  Nevertheless, no definitive theory yet exists for such a surprising combined water-mediated interaction.

In this work, we address the crossover from hydrophobic to hydrophilic interactions using coarse-grained models in molecular simulations, addressing in turn the possibility of a collective force law for the water-mediated interaction.  We immerse a pair of colloids in a water bath, computing the free energy as a function of their respective separation distance [1].  To resolve the free energies, we use a novel Monte Carlo technique based on the flat-histogram and transition-matrix methods.  Importantly, we can control the colloid’s binding energy with water, going from very hydrophobic to very hydrophilic character.  We also tune the colloid’s size, going from very small to very large diameter.  In the latter case, we retrieve the empirical observation, namely, an attraction for hydrophobic surfaces and a repulsion for hydrophilic surfaces, both exhibiting an exponential decay with colloidal separation distance [2].  We find the emergence of specific scaling laws and transitions in driving-forces (entropic and enthalpic) for both hydrophobic and hydrophilic interactions.  Overall, these findings provide a unified picture of hydrophobic and hydrophilic interactions, and we suggest a simple interpretation of the corresponding water-mediated force law. 

1.         Hammer, M.U., et al., The Search for the hydrophobic force law. Faraday Discussions, 2010. 146: p. 299-308.

2.         Donaldson, S.H., et al., General hydrophobic interaction potential for surfactant/lipid bilayers from direct force measurements between light-modulated bilayers. Proceedings of the National Academy of Sciences, 2011. 108(38): p. 15699-15704.