Ab Initio Molecular Dynamics Study of H 2 SO 3 Ionization in Hydrothermal Fluids and Nanodroplets

Using ab initio molecular dynamics and thermodynamic integration calculations, we have examined the speciation and stability of H 2 SO 3 from 25 to 350 °C in bulk liquid and water nanodroplets. H 2 SO 3 ionization constants have been determined by using constraint force and thermodynamic integration techniques. The temperature dependence of the first ionization (yielding HSO 3 – ) and second ionization (yielding SO 3 2– ) constants up to 350 °C is given by the following van’t Hoff equations: log K 1 = −32.5686 – 0.0297· T + 6.8894·ln( T ) and log K 2 = −86.3360 – 0.0608· T + 16.8911·ln( T ). Our calculated ionization constants are in good agreement with experimental data at 25 °C, with p K 1 = 2.08 ± 0.13 (Exp. = 1.88, [ Beyad, Y. Dalton Trans. 2014, 43 (5), 2147–2152]) and p K 2 = 8.14 ± 0.10 (Exp. = 7.12, [ Millero, F. J. J. Atmos. Chem. 1989, 8, 377–389]). With increasing temperature, values of p K 1 and p K 2 increase to 6.93 ± 0.13 and 15.66 ± 0.16 at 350 °C, respectively, highlighting the reduced acidity of H 2 SO 3 and HSO 3 – . The derived enthalpy changes (Δ H °) shift toward more exothermic values with increasing temperature from −2.67 kcal/mol at 25 °C to −33.08 kcal/mol at 350 °C, for the first ionization step, and from −1.69 kcal/mol at 25 °C to −59.90 kcal/mol at 350 °C kcal/mol for the second ionization step, the temperature-weighted entropy ( T Δ S °) shift from −5.50 kcal/mol (25 °C) to −52.85 kcal/mol (350 °C) and from −12.80 kcal/mol (25 °C) to −104.56 kcal/mol (350 °C) for the first and second ionization steps, respectively. Calculated values of Δ H ° and Δ S ° demonstrate that with increasing temperature entropic effects outweigh enthalpic contributions, giving rise to a decreased acidity of H 2 SO 3 . Vapor-phase simulations of H 2 SO 3 at 350 °C reveal a ∼30% larger ionization free energy Δ G compared to solution phase ionization, indicating an enhanced energetic penalty for deprotonation in low-density aqueous media. Simulations in H 2 SO 3 (H 2 O) 100 nanodroplets at 25 and 100 °C demonstrate surface-enhanced stabilization for H 2 SO 3, with the deprotonation free energy Δ G 2.6–4.6 kcal/mol larger at surface solvation sites versus droplet interior. Finally, these molecular dynamics results reveal that at neutral pH in bulk liquid, sulfurous acid exists as the ionized HSO 3 – form, while aqueous hydrothermal nanodroplet surfaces and interiors favor the fully associated acid H 2 SO 3 and the bisulfite ion HSO 3 –, respectively.