Solubility of NaCl in water vapor at 400–700 °C

Abstract

Water significantly impacts the chemical evolution of the Earth’s crust, affecting environments from volcanic settings to hydrothermal systems. These fluids transport elements essential for geological processes, such as metal ore deposit formation. At high temperatures, as water transitions from liquid to vapor, its molecular structure changes, drastically reducing its capacity to dissolve solids and solvate ions. Here, we report experimental results of halite (NaCl_(s)) solubility in water vapor at 400–700 °C and 30–300 bar using a novel U-tube flow-through reactor system. The results show that halite solubility is low (x_NaCl,tot = 3.2 × 10⁻⁹ to 2.9 × 10⁻⁴ mol/mol) and increases with temperature and pressure, attributed to the dissolution of NaCl followed by its hydration according to the reaction:

$\text{NaC}{\text{l}}_{\text{(s)}}+n{\text{H}}_{\text{2}}{\text{O}}_{\text{(g)}}\leftrightharpoons \text{NaCl}·{\left({\text{H}}_{\text{2}}\text{O}\right)}_{n\text{(g)}}$

Thermodynamic modeling of the experimental results indicates a preference for specific hydrated species, including $\text{NaC}{\text{l}}_{\left(\text{g}\right)}$, $\text{NaCl}·{\left({\text{H}}_{\text{2}}\text{O}\right)}_{4\left(\text{g}\right)}$, $\text{NaCl}·{\left({\text{H}}_{\text{2}}\text{O}\right)}_{6\left(\text{g}\right)}$ and $\text{NaCl}·{\left({\text{H}}_{\text{2}}\text{O}\right)}_{8\left(\text{g}\right)}$. The less hydrated gaseous NaCl species become more prevalent with increasing temperature and decreasing pressure. At temperatures below 600 °C and pressures above 50 bar, halite solubility is close to stoichiometric, whereas at higher temperatures and lower pressures, the hydrolysis of NaCl_(s) to form NaOH_(lq,s) or a NaCl_xOH_(1-x)(lq,s) (x < 1) solid solution is evident, resulting in the formation of HCl_(g). The logarithm of the halite equilibrium solubility constant (logK_n) ranges from −10.18 to −4.19 for NaCl_(g), and from −13.12 to −11.94, −15.90 to −17.28, and −19.67 to –22.73 for $\text{NaCl}·{\left({\text{H}}_{\text{2}}\text{O}\right)}_{4\text{(g)}}$, $\text{NaCl}·{\left({\text{H}}_{\text{2}}\text{O}\right)}_{6\text{(g)}}$ and $\text{NaCl}·{\left({\text{H}}_{\text{2}}\text{O}\right)}_{8\text{(g)}}$, respectively. Standard thermodynamic properties derived from this data reveal a temperature-independent enthalpy ($\Delta H_{n,r}^o$), entropy ($\Delta S_{n,r}^o)$ and heat capacity ($\Delta C_{p,n,r}^o$) of reaction for each NaCl species. While the enthalpy of reaction is nearly constant, the entropy and heat capacity decrease with an increasing degree of hydration, indicating the decreasing stability of these species with increasing temperature. Our thermodynamic model results and gas species stoichiometries are in excellent agreement with previous density functional theory (DFT) calculations (Suleimenov et al., 2006). The halite solubility data obtained here compare well with prior studies and thermodynamic models for the NaCl-H₂O system.