Decoding signatures of structure, bulk thermodynamics, and solvation in three-body angle distributions of rigid water models

Abstract

A tetrahedral structure resulting from hydrogen bonding is a hallmark of liquid water and plays a significant role in determining its unique thermophysical properties. This water feature has helped understand anomalous properties and physically interpret and model hydrophobic solvation thermodynamics. Tetrahedrality is well described by the geometric 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. While order parameters and even full water models have been developed using specific or average features of the three-body angle distribution, here we examine the distribution holistically, tracking its response to changes in temperature, density, and the presence of model solutes. Surprisingly, we find that the three-body distribution responds by varying primarily along a single degree of freedom, suggesting a remarkably simplified view of water structure. We 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 trace out transitions between tetrahedral and simple-fluid-like behavior. Moreover, we find that the ways three-body angles vary within the hydration shells of model colloids of different types and sizes are nearly identical to the variations seen in bulk water across density and temperature. Importantly, through the principal directions of these variations, we find that perturbations to the hydration-water distributions well predict the thermodynamics associated with colloid solvation, in particular, the relative entropy of this process that captures indirect, solvent-mediated contributions to the hydration free energy.

Generalizable Features of Interfacial Water Structure Predict the Hydrophobicity of Chemically Heterogeneous Surfaces

“…Results are presented for bulk water and for the checkered SAMs as a function of ; the separated SAMs follow approximately the same trend (ESI Figure S21). As previously suggested, 58 ( ) for water molecules with a triplet angle of 48° is shifted toward larger values of , with a maximum at = 5, for all SAMs and for bulk water. This finding indicates that ( = 48°) captures information on the likelihood of observing highly coordinated water structures.…”

“…These order parameters include information on SAM-SAM, SAM-water, and water-water hydrogen bonds, water orientations relative to the SAM, and the water triplet angle (i.e., the angle formed between an interfacial water molecule and two neighboring water molecules). [56][57][58] ESI Section S2 provides a full description for each parameter and ESI Figures S7-12 show variations in these parameters for different SAMs. Subsets of these parameters have been used previously to understand how peptide side chain chemistry affects binding, 57 surface polarity alters interfacial water orientation, 23 and SAM order affects hydrophobic interactions.…”

“…Like total , ( = 48°) exhibits different variations with respect to for Shell have suggested that a small peak in the triplet angle distribution at around 50° arises due to a fifth neighbor in the coordination shell of bulk water. 58 However, it is unclear how interfaces and surface properties affect this feature.…”

Affinity of small-molecule solutes to hydrophobic, hydrophilic, and chemically patterned interfaces in aqueous solution

“…As shown in ref. 97, 〈U sw 〉 2 reports the average interactions (potential energies) of the solute with other species in the system (i.e., water and interface) and hence the direct energetic contributions to solvation. S rel,1→2 then is the relative entropy associated with solvating the solute; it is always positive and assesses how a system's conformational fluctuations adapt to perturbations (97).…”

“…This approach is similar in spirit to inhomogeneous solvation theory (98) and associated methods that predict free energies of binding based on shifts in water structure (48,49). However, the contributing terms are fundamentally distinct (97), avoiding cancelation between energetic and entropic terms (99,100). Importantly, for relatively rigid solutes and surfaces, this relative entropy measures the penalty due to a solute's perturbation of water's fluctuating structure, and hence Eq.…”