Effect of Thiouronium-Based Ionic Liquids on the Formation and Growth of CO₂ (sI) and THF (sII) Hydrates

In this manuscript, two thiouronium-based ionic liquids (ILs), namely 2-ethylthiouronium bromide [C₂th][Br] and 2-(hydroxyethyl)thiouronium bromide [C₂OHth][Br], were tested at different concentrations (1 and 10 wt%) for their ability to affect CO₂ (sI) and tetrahydrofuran (THF) (sII) hydrate formation and growth. Two different methods were selected to perform a thermodynamic and kinetic screening of the CO₂ hydrates using a rocking cell apparatus: (i) an isochoric pressure search method to map the hydrate phase behavior and (ii) a constant ramping method to obtain the hydrate formation and dissociation onset temperatures. A THF hydrate crystal growth method was also used to determine the effectiveness of the ILs in altering the growth of type sII hydrates at atmospheric pressure. Hydrate-liquid-vapor equilibrium measurements revealed that both ILs act as thermodynamic inhibitors at 10 wt% and suppress the CO₂ hydrate equilibria ~1.2 °C. The constant ramping methodology provides interesting results and reveals that [C₂OHth][Br] suppresses the nucleation onset temperature and delays the decomposition onset temperatures of CO₂ hydrates at 1 wt%, whereas suppression by [C₂th][Br] was not statistically significant. Normalized pressure plots indicate that the presence of the ILs slowed down the growth as well as the decomposition rates of CO₂ hydrates due to the lower quantity of hydrate formed in the presence of 1 wt% ILs. The ILs were also found to be effective in inhibiting the growth of type sII THF hydrates without affecting their morphology. Therefore, the studied thiouronium ILs can be used as potential dual-function hydrate inhibitors. This work also emphasizes the importance of the methods and conditions used to screen an additive for altering hydrate formation and growth.