Theoretical analysis of the gas-phase hydration of common atmospheric pre-nucleation (HSO4-)(H2O)n and (H3O+)(H2SO4)(H2O)n cluster ions

Small atmospheric ions are always hydrated. The hydration affects their mobility, stability, and lifetime, which are critically important for the determination of nucleation rates. The gas-phase hydration of anions and cations of the atmospheric nucleation precursor H2SO4 has been studied using the Density Functional Theory (DFT). We found that the interaction between the common atmospheric ions and polar water molecules lead to the formation of the strongly hydrogen bonded hydrate complexes, whose thermodynamic stability is much higher than that of neutral atmospheric hydrates (H2SO4)(H2O)n. Both negatively and positively charged hydrates are much more stable than the aforementioned neutral form; however, the hydration of cations is much stronger than that of anions. The difference in hydration free energies is very large (∼10 kcal mol−1); however, it decreases quickly when the cluster is growing. The observed positive hydration sign preference is consistent with previous studies for pure water.