Dynamics of the NO ₃ ^– ···NO ⁺ Ion Pair in Thin Water Films on Solid Surfaces of Tunable Hydrophobicity and Implications for NO ₂ Hydrolysis

Understanding the role of the medium on chemical reactions occurring in thin water films covering solid surfaces is an important issue in many areas of chemistry, particularly in atmospheric and environmental sciences. The hydrolysis of nitrogen dioxide NO₂ into nitric and nitrous acids has long been considered a prototypical example of this type of chemistry. Various experiments have shown that this process, which is not favorable in gas-phase, occurs easily in supported water films. In general, the reacting species has been assumed to be the (NO₂)₂ dimer, which is known to spontaneously form the contact ion pair NO₃^-···NO⁺ in the presence of water. Using advanced molecular dynamics simulations based on combined quantum-classical force-fields we have studied the properties of this dimer in water films on hydrophobic surfaces. We show that solvent effects on several reactivity indices of the contact ion pair in this medium are larger compared to solvent effects in bulk water. Analysis of this unexpected result reveals that there is a significant shift in the electrostatic potential generated by the reorganization of water molecules around the ion pair in the films compared to the bulk solution. This finding bears similarities to previous research on solvent effects at the air-water interface of microdroplets, which are well-known to enhance chemical reactivity. The results presented here will be useful for designing new experiments and calculations to further explore the potential catalytic effect of thin water films supported on surfaces.