Gold(I)-bisulfite complexation in hydrothermal nanodroplets: A molecular dynamics study

Abstract

Water nanodroplets present a unique environment for gold hydrothermal transport, with fluid properties in aqueous nanodroplets distinct from bulk liquid and vapor phases. By performing classical and ab initio molecular dynamics simulations, we have probed the stability of water nanodroplets (H₂O)_n (n = 100, 1000) at 25 °C and 100 °C. The solvation and complexation of gold(I)-bisulfite AuHSO₃ in nanodroplet environments were also examined, with a particular focus on surface and interior solvation. Classical TIP4P/2005 molecular dynamics simulations reveal extreme densities in the interior of (H₂O)₁₀₀ and (H₂O)₁₀₀₀ nanodroplets compared to droplet surface regions. At 25 °C, the interior region of (H₂O)₁₀₀ exhibits fluctuating densities at 1.016–1.079 g/cm³, with two maxima at 1.079 g/cm³ and 1.074 g/cm³, corresponding to pressures of ∼ 2.23 kbar and 2.07 kbar, respectively; Reduced densities are predicted for the larger (H₂O)₁₀₀₀ systems, these being 1.013 g/cm³ (25 °C, 370 bar) and 0.968 g/cm³ (100 °C, 220 bar). The outer regions, on the other hand, featured densities intermediate between saturated liquid and vapor conditions, as part of a transition from liquid to vapor-like densities at the edge of the droplet. Born-Oppenheimer molecular dynamics simulations at 100 °C show that the gold(I)-bisulfite complex H₂O-AuHSO₃ maintains a near linear solvation structure (θ_O-Au-S = 172°-174°) in bulk aqueous fluids and at surface and interior sites of (H₂O)₁₀₀ nanodroplets. Distance constrained simulations reveal that, upon extension of the gold(I)-bisulfite Au-S contact (equilibrium r_Au-S = 2.3 Å), HSO₃⁻ is displaced by a water molecule, forming a two-water solvation shell around Au⁺. Thermodynamic integration gives gold(I)-bisulfite dissociation energies (ΔG) of 17.65 ± 0.37 kcal/mol (bulk), 20.22 ± 0.38 kcal/mol (nanodroplet surface), and 18.31 ± 0.31 kcal/mol (nanodroplet interior). Our ab initio molecular dynamics results demonstrate that water nanodroplets are stable at hydrothermal conditions and would play an important role in the speciation and transport of gold in volcanic and hydrothermal vapors.