Wrapping Up Hydrophobic Hydration: Locality Matters

Abstract

Water, being the universal solvent, acts as a competing agent in fundamental processes, such as folding, aggregation or biomolecular recognition. A molecular understanding of hydrophobic hydration is of central importance to understanding the subtle free energy differences, which dictate function. Ab initio and classical molecular dynamics simulations yield two distinct hydration water populations in the hydration shell of solvated tert-butanol noted as "HB-wrap" and "HB-hydration2bulk". The experimentally observed hydration water spectrum can be dissected into two modes, centered at 164 and 195 cm^-1. By comparison to the simulations, these two bands are attributed to the "HB-wrap" and "HB-hydration2bulk" populations, respectively. We derive a quantitative correlation between the population in each of these two local water coordination motifs and the temperature dependence of the solvation entropy. The crossover from entropy to enthalpy dominated solvation at elevated temperatures, as predicted by theory and observed experimentally, can be rationalized in terms of the distinct temperature stability and thermodynamic signatures of "HB-wrap" and "HB-hydration2bulk".

Figure 1

THz fingerprints of the hydration water around tert-butanol. (A) Comparison of experimental (top) and theoretical (bottom) difference spectra, obtained by subtracting the bulk water spectrum from the spectrum of a 0.5 M aqueous solution of tert-butanol, for two temperatures (310 and 290 K), over the range of 100–600 cm^–1. (B) In the top panel, each concentration- and temperature-dependent difference spectrum can be dissected into two hydration water components. Here, we show these in orange and blue with their respective amplitudes for a 0.5 M aqueous solution of tert-butanol at 290 K. In the inset, the concentration-dependent amplitudes of each of the two hydration water components are shown for concentrations of 0.5 M and 5.0 M at 290 K. The bottom panel shows theoretical difference spectra calculated for each of the two interfacial H-bond populations, HB-wrap (orange) and HB-hydration2bulk (blue), at 0.5 M and 290 K. For comparison, the difference spectrum calculated for the HB population forming the 2D HB network at the air–water interface (AW-2DN) is also reported (red). (C) Snapshot from DFT-MD simulations highlighting in orange the HB-wrap population, forming the 2D polygonal HB network wrapped around the alcohol.

Figure 2

Solvation environment of tert-butanol. (A) Illustration of the comparison between HB connectivity patterns at the air–water surface and in the inner hydration shell of tert-butanol in a bulk solution. (B) Corresponding numerical values in addition to occurrences of free OH bonds. (C) Spatial distribution of the tetrahedral order parameter for water molecules in the solvation environment of tert-butanol at 293 K (Δq = q – q_bulk). Shown are 0.5 ų voxels for which the local average tetrahedral order differs significantly from that of the bulk. Orange (blue) colors indicate a decrease (an increase) in q for the local coordination environment of a water molecule with respect to the bulk.

Figure 3

Temperature-dependent changes in hydration water structure affect solvation entropy. (A) THz-active contributions to the solvation entropy, with T_ref = 400 K (see the Supporting Information and refs (16) and (29)), of tert-butanol in the investigated temperature range as deduced from MD simulations (black circles), changes in the THz spectra (red triangles), and conventional calorimetry (blue squares). (B) Effective number of hydration waters contributing to each THz band (ν₁₆₄ and ν₁₉₅) obtained from the experimental spectra. (C) Illustration of the tert-butanol molecule and its first hydration shell at 273, 293, and 313 K. Voxels indicate regions with a >30% increase in the water number density relative to the bulk liquid (the 3D analogue of the first peak in a radial distribution function), and the color illustrates local variations in the absolute entropy per water molecule relative to bulk water at the corresponding temperature [ΔS(r) = S(r) – S_bulk].