We probe the effects of solute length scale, attractions, and hydrostatic pressure on hydrophobic hydration shells using extensive molecular simulations. The hydration shell compressibility and water fluctuations both display a nonmonotonic dependence on solute size, with a minimum near molecular solutes and enhanced fluctuations for larger ones. These results and calculations on proteins suggest that the hydration shells of unfolded proteins are more compressible than of folded ones contributing to pressure denaturation. More importantly, the nonmonotonicity implies a solute curvature-dependent pressure sensitivity for interactions between hydrophobic solutes.
We report atomistically detailed molecular dynamics simulations of homogeneous nucleation of methane hydrate in bulk aqueous phase in the absence of any interface. Subcritical clusters of water and methane molecules are formed in the initial segment of the simulations, which then aggregate to give the critical hydrate nucleus. This occurs over time scales of several hundred nanoseconds, indicating that the formation and aggregation of subcritical clusters can contribute significantly to the overall rate of hydrate nucleation. The clusters have elements of sI hydrate structure, such as 5(12) and 5(12)6(2) cages as well as other uncommon 5(12)6(3) and 5(12)6(4) cages, but do not possess long-range order. Clusters are dynamic in nature and undergo continuous structural rearrangements.
Freezing in the vicinity of water-vapor interfaces is of considerable interest to a wide range of disciplines, most notably the atmospheric sciences. In this work, we use molecular dynamics and two advanced sampling techniques, forward flux sampling and umbrella sampling, to study homogeneous nucleation of ice in free-standing thin films of supercooled water. We use a coarse-grained monoatomic model of water, known as mW, and we find that in this model a vapor-liquid interface suppresses crystallization in its vicinity. This suppression occurs in the vicinity of flat interfaces where no net Laplace pressure in induced. Our free energy calculations reveal that the pre-critical crystalline nuclei that emerge near the interface are thermodynamically less stable than those that emerge in the bulk. We investigate the origin of this instability by computing the average asphericity of nuclei that form in different regions of the film, and observe that average asphericity increases closer to the interface, which is consistent with an increase in the free energy due to increased surface-to-volume ratios.
Catalytic fuel production and energy generation from biomass-derived compounds generally involve the aqueous phase, and water molecules at the catalyst interface have energetic and entropic consequences on the reaction free energies. These effects are difficult to elucidate, hindering rational catalyst design for these processes and inhibiting their widespread adoption. In this work, we combine density functional theory (DFT) and classical molecular dynamics (MD) simulations to garner molecular-level insights into H 2 O−adsorbate interactions. We obtain ensembles of liquid configurations with classical MD and compute the electronic energies of these systems with DFT. We examine CO, CH 2 OH, and C 3 H 7 O 3 intermediates, which are critical in biomass reforming and direct methanol electrooxidation, on the Pt(111) surface under various explicit and explicit/implicit water configurations. We find that liquid H 2 O molecules arrange around surface intermediates in ways that favor hydrogen bonding, with larger and more hydrophilic intermediates forming significantly more hydrogen bonds with H 2 O. For example, CO hydrogen-bonds with 1.5 ± 0.4 nearest neighbor H 2 O molecules and exhibits an interaction energy with these H 2 O molecules near 0 (−0.01 ± 0.09 eV), while CH 2 OH forms 2.2 ± 0.6 hydrogen bonds and exhibits an interaction energy of −0.43 ± 0.07 eV. C 3 H 7 O 3 forms 6.7 ± 0.9 hydrogen bonds and exhibits an interaction energy of −1.18 ± 0.21 eV. The combined MD/DFT method identifies the number of liquid H 2 O molecules that are strongly bound to surface adsorbates, and we find that these H 2 O molecules influence the energies and entropies of the aqueous systems. This information will be useful in future calculations aimed at interrogating the surface thermodynamics and kinetics of reactions involving these adsorbates.
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