Nucleation of CO₂ Hydrate in Quasi-Free Droplets of Dilute Electrolytes

Salts are known to be thermodynamic inhibitors of gas hydrates at high concentrations by lowering the activity of water and shifting the phase boundary of gas hydrates to lower temperatures and higher pressures. However, some salts have been reported to kinetically promote the formation of gas hydrates at low concentrations. Studies on kinetic promotions of clathrate hydrate formation in dilute salt solutions are rare, and the mechanisms are poorly understood. The impact of solid walls on heterogeneous nucleation of gas hydrates is complex and depends on the nature of the solid walls, and it is difficult to decouple the impact of solid walls from that of dilute electrolytes when they are present. In particular, a solid wall often becomes charged when in contact with an aqueous phase, and the binding of the counterions to the solid wall in an aqueous phase further complicates the investigations of heterogeneous nucleation of gas hydrates. Here, we investigated the nucleation rates of CO2 hydrate in quasi-free droplets of sodium chloride (NaCl) and potassium iodide (KI) at low concentrations (<= 10 mM). The results showed that NaCl solution had no inhibition effect while KI solution had a weak promotion effect at low concentrations, and the nucleation rates were largely independent of the salt concentrations up to 10 mM. The impacts of dilute NaCl and KI solutions on the nucleation of CO2 hydrate in this study were broadly similar to the previous findings of the methane-propane mixed gas hydrates in Sowa et al.'s study.