Molecular hydrophobicity at a macroscopically hydrophilic surface.

Interfaces between water and silicates are ubiquitous and relevant for, among others, geochemistry, atmospheric chemistry, and chromatography. The molecular-level details of water organization at silica surfaces are important for a fundamental understanding of this interface. While silica is hydrophilic, weakly hydrogen-bonded OH groups have been identified at the surface of silica, characterized by a high O-H stretch vibrational frequency. Here, through a combination of experimental and theoretical surface-selective vibrational spectroscopy, we demonstrate that these OH groups originate from very weakly hydrogen-bonded water molecules at the nominally hydrophilic silica interface. The properties of these OH groups are very similar to those typically observed at hydrophobic surfaces. Molecular dynamics simulations illustrate that these weakly hydrogen-bonded water OH groups are pointing with their hydrogen atom toward local hydrophobic sites consisting of oxygen bridges of the silica. An increased density of these molecular hydrophobic sites, evident from an increase in weakly hydrogen-bonded water OH groups, correlates with an increased macroscopic contact angle.

Influence of argon and D2 tagging on the hydrogen bond network in Cs+(H2O)3; kinetic trapping below 40 K.

The influence of enthalpic and entropic effects as well as of kinetic trapping processes on the structure of Ar/D2-tagged Cs+(H2O)3 clusters is studied by temperature-dependent infrared photodissociation spectroscopy combined with harmonic vibrational spectra calculations and anharmonic free energy profiles from finite temperature metadynamics molecular dynamics simulations. Each tag favors a different hydrogen bond network of water molecules, with Ar-tagging (vs. D2-tagging) of Cs+(H2O)3 leading to the lower energy conformation. The relative population of these conformers can be tuned over a temperature range of 12 to 21 K. The formation mechanisms of these tagged clusters can be deduced from the free energy profiles. This investigation demonstrates that a variety of factors, both thermodynamic and kinetic, play a role in the structure of flexible molecular species, even at cryogenic temperatures.