(335f) Tetrahedrality At Hydrophobic Interfaces in a Coarse-Grained Water Model

The Lum-Chandler-Weeks theory highlights that hydrophobicity behaves differently for nonpolar spheres of different sizes: entropy dominates the hydration of small hydrophobes, while enthalpy dominates the hydration of large ones [1].  Similarly, Sarupria and Garde have found corresponding signatures for solvent density at hydrophobic interfaces, with low fluctuations near small solutes and high fluctuations near large ones [2].  One outstanding question is how this crossover with length scale can be interpreted in terms of the tendency of water to form a tetrahedral network.  While tetrahedrality arguments have been frequently used to describe hydrophobicity, an exact picture of water’s structure near nonpolar molecules remains unclear. 

In this work, we address water’s tetrahedral structure near hydrophobic colloids using coarse-grained models in molecular simulations.  The water model is an isotropic (“core-softened”) potential that is developed through a novel procedure which minimizes the information lost upon coarse-graining [3].  For the hydrophobes, we assume they are composed of Lennard-Jones points, and we use the Hamaker approximation to attain an isotropic potential for colloids whose size can vary.  While the system might appear overly simplified,  such a description has been shown to adequately capture several signatures of hydrophobic association [4].  Here, we show that the models, despite being isotropic in nature, manifest tetrahedrality through a careful balancing of driving-forces at different length scales.  To enumerate the effects of tetrahedrality on hydrophobicity, we modify a well-studied tetrahedral order parameter so that it is suitable for interfaces, and we also propose several novel related metrics.  We measure the disturbance of the hydrophobe on interfacial tetrahedrality, and we find that a select few of these order parameters show signatures of the corresponding Lum-Chandler-Weeks crossover.  Finally, we present simple arguments for this crossover, emphasizing the manner in which this description manifests tetrahedrality, [1,2].

1.         Lum, K., D. Chandler, and J.D. Weeks, Hydrophobicity at Small and Large Length Scales. The Journal of Physical Chemistry B, 1999. 103(22): p. 4570-4577.

2.         Sarupria, S. and S. Garde, Quantifying Water Density Fluctuations and Compressibility of Hydration Shells of Hydrophobic Solutes and Proteins. Physical Review Letters, 2009. 103(3): p. 037803.

3.         Chaimovich, A. and M.S. Shell, Anomalous waterlike behavior in spherically-symmetric water models optimized with the relative entropy. Physical Chemistry Chemical Physics, 2009. 11(12): p. 1901-1915.

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