Probing solvation electrostatics at the air–water interface

Using Born–Oppenheimer molecular dynamics simulations with a combined quantum/classical partition (QM/MM), we analyze the magnitude of the inhomogeneous local electric field stemming from the solvent reorganization around the methanol molecule, one of simplest surfactant models, adsorbed at the air–water interface. We compare the calculated electric field at the aqueous interface with the corresponding electric field in the bulk water phase. Two important findings are reported. First, we show that the electric field at the air–water interface is, on average, smaller than the corresponding electric field in the bulk phase. Secondly, we show that it is more than one order of magnitude higher than the electric field deriving from the broken symmetry at the pristine air–water interface. The broader implication of these results is that the recent discovery of reaction rate acceleration at aqueous interfaces compared to bulk phase reactivity cannot be attributed (or at least not in a general way) to the strength of the electric field, as is widely stated in the literature. Therefore, a more in-depth analysis of interface solvation electrostatics needs to be carried out in most situations.