From Molecular Dynamics to the Conductivity of Sulfuric Acid: Ultrafast Optical Kerr Effect Experiments and Ab Initio...

Extensive use of sulfuric acid in technological applications calls for knowledge of its molecular scale properties. Here, we report a study of aqueous sulfuric acid solutions across a broad concentration range using optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments and ab initio molecular dynamics (AIMD) simulations. The OHD-OKE experiments measured the time derivative of the polarizability-polarizability correlation function (PPCF). By comparison of distinct components of the OKE signal to the excess proton identity correlation functions calculated from AIMD simulations, it was found that the experimental t₃ components quantitatively agreed with the proton hopping time from one water to another, which suggested that the origin of t₃ was associated with proton hopping. The proton hopping distances within the t₃ time scale were used to determine the proton hopping diffusion constants at several concentrations. Using information from the literature and the Nernst-Einstein conductivity equation, it was shown that the vehicular mechanism was insufficient to describe the conductivity. The experimental concentration-dependent conductivities were reproduced by adding the AIMD proton hopping contribution to the conductivity, and proton hopping was shown to be the dominant component of the conductivity. Finally, the experimental concentration-dependent function, C₃/t₃, where C₃ is the amplitude coefficient of the third PPCF component, closely tracked the concentration dependence of the hopping component of the conductivity.