Molecular beam studies of HCl dissolution and dissociation in cold salty water

Abstract

Gas-liquid scattering experiments are used to explore collisions and reactions of HCl and DCl with 12 mol% LiBr solutions of H₂O and D₂O at 208–218 K. These ∼6 M aqueous salt solutions have vapor pressures just below 0.01 Torr, requiring special consideration of the effects of gas-vapor collisions. We find that impinging HCl molecules readily equilibrate on the surface of the solution even at incident energies of 90 kJ mol⁻¹. Approximately 90% of the thermalized HCl molecules dissolve and dissociate for long times in the cold salty solution, while the remaining 10% desorb from the surface intact. There is no evidence for rapid, interfacial conversion of HCl into DCl, in striking contrast to previous observations of distinct submicrosecond DCl→HCl exchange in collisions of DCl with salty glycerol at 292 K. These results indicate that cold salty water efficiently captures impinging HCl molecules and suppresses interfacial proton exchange, most likely because of the long interaction times of the HCl molecules in contact with the cold surface and because of facile transport of H⁺ and Cl⁻ from the interfacial region into the bulk solution.