In recent years, there has been growing interest in gas hydrates as technological applications, such as for energy (methane and hydrogen) storage and transportation, separation (gas and desalination), and carbon capture. However, there are several challenges that deter large-scale applications and commercialization of these hydrate-based technologies. One of the main challenges is the long induction time and slow growth of hydrate particles, which can increase the overall operating costs of these technologies. It has been reported that the addition of additives (known as hydrate promoters) can help improve the nucleation and growth rate of hydrates. In general, there are two types of hydrate promoters: thermodynamic hydrate promoters and kinetic hydrate promoters. Thermodynamic hydrate promoters shift the hydrate equilibrium curve to milder conditions (i.e., lower pressures and higher temperatures), while kinetic hydrate promoters reduce the induction time for hydrate formation and increase the growth rate. In this review, we provide a comprehensive review of the two types of hydrate promoters (thermodynamic and kinetic) and their effects on hydrate phase equilibria, induction time, and growth rate.

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Additives like Tetrahydrofuran (THF) and Sodium Dodecyl Sulfate (SDS) improve CO2 hydrates thermal stability and growth rate when used separately. It has been hypothesised that combining them could improve the kinetics of growth and the thermodynamic stability of CO2 hydrates. We exploit atomistic molecular dynamics simulation to investigate the combined impact of THF and SDS under different temperatures and concentrations. The simulation insights are verified experimentally using pendant drop tensiometry conducted at ambient pressures and high-pressure differential scanning calorimetry. Our simulations revealed that the combination of both additives is synergistic at low temperatures but antagonistic at temperatures above 274.1 K due to the aggregation SDS molecules induced by THF molecules. These aggregates effectively remove THF and CO2 from the hydrate-liquid interface, thereby reducing the driving force for hydrates growth. Experiments revealed that the critical micelle concentration of SDS in water decreases by 20% upon the addition of THF. Further experiments showed that only small amounts of SDS with THF is sufficient to increase the CO2 storage efficiency by over 40% compared to results obtained without promoters. These results provide microscopic insights into the mechanisms of THF and SDS promoters on CO2 hydrates, which allow for determining the optimal condition for hydrate growth.