Phase Equilibrium Data and Phase Behavior in Formation Systems of Semiclathrate Hydrates Formed with Partly Asymmetric Quaternary Ammonium Salts under Methane, Carbon Dioxide, and Nitrogen Gas Pressure

Semiclathrate hydrates are water-based crystalline host–guest materials that consist of water and large ionic substances. To expand their unique functions, investigation into asymmetric quaternary ammonium salts, i.e., n-butyl chains of tetra-n-butylammonium (TBA or N4444 where N and 4 denote nitrogen and n-butyl group, respectively) cation is partly substituted by other groups, is necessary. In this study, we report the three-phase (gas–hydrate–liquid) equilibrium data in the systems of (N3444Cl or N4445Cl) + (CH4, CO2, or N2) + H2O up to 10 MPa of the pressure, where 3 and 5 denote n-propyl and n-pentyl, respectively. A total of 99 equilibrium data sets were obtained with aqueous compositions with 0.1, 0.2, and 0.3 salt mass fractions. Both N3444Cl and N4445Cl promoted formation conditions of CH4, CO2, and N2 gas hydrates. N3444Cl partly acted as an inhibitor for pure CH4 and CO2 hydrates under some conditions and also showed polymorphic phase behavior. A comparison with the literature data for other ionic guests was made. The present findings help in understanding the phase behavior of the semiclathrate hydrates, which can be applied for further development of applications based on these materials.