Ion-Size Dependent Adsorption Crossover on the Surface of a Water Droplet.

The adsorption of ionic and neutral spherical solutes on the surface of a liquid water droplet are investigated using molecular dynamics simulations and theoretical analyses. The results reveal a crossover in the sign of the adsorption free energy as a function of ion size, with ions larger than iodide predicted to be increasingly surface active. Adsorption free energies are decomposed into competing energetic and entropic contributions arising from direct solute-water interaction energy and its fluctuations. The entropically driven surface activity of large ions is predicted to increase with ion size, while small ions are typically driven away from the interface by a more delicate balance of energetic and entropic contributions, with a nonmonotonic ion size dependence linked to the ion's hydration-shell structure and stability. The physical interpretation of the results is illuminated by comparisons with dielectric linear response and cavity formation predictions and implications to interfacial acidity and enhanced chemical reactivity are discussed.

Hiding in the Crowd: Spectral Signatures of Overcoordinated Hydrogen-Bond Environments.

Molecules with an excess number of hydrogen-bonding partners play a crucial role in fundamental chemical processes, ranging from anomalous diffusion in supercooled water to transport of aqueous proton defects and ordering of water around hydrophobic solutes. Here we show that overcoordinated hydrogen-bond environments can be identified in both the ambient and supercooled regimes of liquid water by combining experimental Raman multivariate curve resolution measurements and machine learning accelerated quantum simulations. In particular, we find that OH groups appearing in spectral regions usually associated with non-hydrogen-bonded species actually correspond to hydrogen bonds formed in overcoordinated environments. We further show that only these species exhibit a turnover in population as a function of temperature, which is robust and persists under both constant pressure and density conditions. This work thus provides a new tool to identify, interpret, and elucidate the spectral signatures of crowded hydrogen-bond networks.

Interfacial Adsorption of Neutral and Ionic Solutes in a Water Droplet.

Direct (solute-water) and indirect (water-water) contributions to adsorption at an air-water interface are identified using the Widom potential distribution theorem and quantified using molecular dynamics simulations of a liquid water droplet containing either neopentane or iodide-like solutes with charges of 0 or ±1. The results are used to quantitatively compare direct and indirect energetic and entropic contributions to adsorption, as well as to critically test surface capillary wave, linear response (LR), and mean field (MF) predictions. The negative signs of the total adsorption energies and entropies of both the anionic and cationic solutes are found to result from indirect adsorption induced changes in water-water interactions, rather than from surface capillary wave perturbations, which are found to be asymmetric with respect to solute charge. The LR and MF approximations both accurately describe the adsorption of neutral (hydrophobic) solutes, while for ionic solutes the MF approximation is entirely inappropriate and LR predictions are qualitatively (but not quantitatively) accurate.