Tetrahedral structure as a result of hydrogen bonding is a hallmark of liquid water and plays a significant role in determining its unique thermophysical properties. This particular feature of water has played a central role in understanding anomalous water properties as well as in physically interpreting and modeling the thermodynamics of hydrophobic solvation. Tetrahedrality is well-described by the relationship of any central water molecule with two of its nearest neighbors in the first coordination shell, as defined by the corresponding “three-body” angle. The distribution of three-body angles in solution shows a pronounced peak near the tetrahedral angle and thus characterizes fluctuations in structure and tetrahedrality. While order parameters and even full water models have been developed in earlier work based on specific features of the three-body angle distribution, here we present a careful examination of the distribution as a whole, relating its response to changes in temperature, density, and the presence of model solutes and interfaces. Remarkably, we find that the three-body distribution responds to such changes by varying primarily along a single degree of freedom, suggesting a remarkably simplified view of water structure. We first characterize three-body angle distributions across temperature and density space, and identify principal components of the variations with state conditions. We show that these principal components embed physical significance and can trace out boundaries in state space that enclose anomalous water properties. Moreover, we find that the ways the three-body angle distribution varies in hydration water near model colloids of different types and sizes are nearly identical to the types of variations seen in bulk water across density and temperature space. By identifying a single degree of freedom that encapsulates all such variations, remarkably, we then find that perturbations to the hydration-water distributions can well-predict the thermodynamics associated with colloid solvation.

Structural and thermodynamic information content of the three-body angle distribution of liquid water

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.