Molecular dynamics analysis of interfacial structures and sum frequency generation spectra of aqueous hydrogen halide solutions

Molecular dynamics simulation for gas/liquid interfaces of aqueous hydrochloric (HCl) and hydroiodic (HI) acid solutions is performed to calculate and analyze their sum frequency generation (SFG) spectra. The present MD simulation supports the strong preference of hydronium ions at the topmost surface layer and a consequent formation of ionic double layers by the hydronium and halide ions near the interface. Accordingly, the orientational order of surface water in the double layers is reversed in the acid solutions from that in the salt (NaCl or NaI). The calculated SFG spectra of the O-H stretching region reproduce the experimental spectra of ssp and sps polarizations well. In the ssp spectra, the strong enhancement in the hydrogen-bonding region for the acid solutions is elucidated by two mechanisms, ordered orientation of water in the double layer and symmetric OH stretching of the surface hydronium ions. In the sps spectra, reversed orientation of surface water is evidenced in the spectral line shapes, which are quite different from those of the salt solutions.