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.