Seminar

Molecular solvation plays a critical role in the transport and reactivity of volcanic and hydrothermal vapors. For instance, interactions between volcanic sulphur dioxide (SO₂) and water vapor (cold and hot) give rise to a wide range of gaseous acids, e.g. H₂SO₃ and H₂SO₄ with unusual acidities and solvation behavior. These materials are therefore important in our understanding of hydrothermal geochemistry, but equally relevant to solvation processes taking place in atmospheric droplets and at liquid-gas interfaces.

This work focuses in part on the speciation of SO₂ (and related sulfur species) in nano-scale water droplets with varying numbers of H₂O molecules. I will initially present results from electronic structure calculations for the smallest complex, i.e. SO₂(H₂O), its product acid H₂SO_3, a precursor compound critical in the formation of atmospheric sulfuric acid H₂SO₄. Preliminary results indicate that the SO₂(H₂O) dimer complex is stable up to ~200 K, unstable at 298 K and at higher temperatures, and would likely dissociate back to SO₂ and H₂O in low-density water vapor. This presentation will also include a summary of results obtained from first-principles molecular dynamic simulations (Car-Parrinello Molecular Dynamics) of SO₂(H₂O)_n nanodroplets (with up to 40 waters), by considering both droplet surface and interior solvation sites. The goal of this work is to probe the propensity of SO₂ for droplet surfaces or interiors (vs bulk fluid), and to evaluate the temperature-dependent stability of SO₂ in a nanodroplet and that of the droplet itself. These CPMD simulations are currently also being expanded to include scenarios in which neutral and ionic solutes are deposited onto mixed H₂O-H₂S nanodroplet surfaces, interiors, and bulk solution environments.

Additional information: Mr. HUI Chi Hang Wallace, wallh817@connect.hku.hk