NaCl Dissociation Dynamics at the Air-Water Interface

The rate for NaCl dissociation was compared between the air-water interface and water bulk using the transition path sampling formulism to begin understanding how an interface affects reaction dynamics. The results showed that the dissociation dynamics at the interface was much slower than in the bulk. Free energy of dissociation calculations were carried out utilizing umbrella sampling for NaCl dissociation at the interface, and a higher barrier to dissociation was observed, which is consistent with the slower interfacial dissociation dynamics. Using the dissociation free energy profile at the interface, transition-state theory was used to predict a rate constant that was lower than the one calculated using transition path sampling. This led to the conclusion that it is difficult to capture the complex nature of interfacial reactions with mean free energy profiles. Analysis of transition states harvested from the simulations found that the NaCl vector was significantly less aligned to the normal of the air-water interface at the transition state than on average and that the Cl- ion had a high probability to be found at the Gibbs dividing surface at the transition state.