Promoting Effect of Common Marine Cations on Hydrate Dissociation and Structural Evolution under a Static Electric Field

Methane hydrate phase equilibrium considering dissolved methane concentrations and interfacial geometries from molecular simulations

Natural gas hydrates, mainly existing in permafrost and on the seabed, are expected to be a new energy source with great potential. The exploitation technology of natural gas hydrates is one of the main focuses of hydrate-related studies. In this study, a large-size liquid aqueous solution wrapping a methane hydrate system was established and molecular dynamics simulations were used to investigate the phase equilibrium conditions of methane hydrate at different methane concentrations and interfacial geometries. It is found that the methane concentration of a solution significantly affects the phase equilibrium of methane hydrates. Different methane concentrations at the same temperature and pressure can lead to hydrate formation or decomposition. At the same temperature and pressure, in a system reaching equilibrium, the size of spherical hydrate clusters is coupled to the solution concentration, which is proportional to the Laplace pressure at the solid-liquid interface. Lower solution concentrations reduce the phase equilibrium temperature of methane hydrates at the same pressure; as the concentration increases, the phase equilibrium temperature gradually approaches the actual phase equilibrium temperature. In addition, the interfacial geometry of hydrates affects the thermodynamic stability of hydrates. The spherical hydrate particles have the highest stability for the same volume. Through this study, we provide a stronger foundation to understand the principles driving hydrate formation/dissociation relevant to the exploitation of methane hydrates.

Facilitation of Hydrate Dissociation and Structural Evolution by Major Marine Anions under Static Electric Fields

Electric fields have been proven to be capable of significantly affecting the equilibrium state of hydrates. In this study, the thermodynamic properties and structural changes of methane hydrate (MH) in various anion solutions in an electric field at 0.7 V/nm were investigated by molecular dynamics simulations. The presence of anions significantly enhances the instability of methane hydrates under electric fields, leading to a staged dissociation process. First, the anions coexist with MH to form a temporary metastable structure under the action of an electric field. Then, the migration of anions causes the dissociation of nearby hydrates and the formation of flow channels in the hydrate layer, which leads to the complete dissociation of MH after a period. The promotive effects of F-, Br-, I-, and Cl- ions were close, while SO42- was relatively weak. The anions are still in hydration shells in the MH phase, but the structure of the hydration shells differs slightly from that in solution (the coordination numbers of I- and SO42- ions increased). The migration resistances of multiple anions to cross the surface of the hydrate layer are similar. However, inside the hydrate phase, the anions with a larger radius have a higher migration resistance. It is difficult for SO42- ions to migrate inside the hydrate phase, and they tend to form a metastable structure on the hydrate surface. Combining our previous studies, SrCl2 solution has the best hydrate promotion under an electric field environment.

Binding Study of Antibacterial Drug Ciprofloxacin with Imidazolium-Based Ionic Liquids Having Different Halide Anions: A Spectroscopic and Density Functional Theory Analysis

Herein, we have shown the interaction of an antibiotic drug ciprofloxacin (CIP) with three surface-active ionic liquids (ILs), having the same cation and different anions, namely, 1-decyl-3-methylimidazoliumtetrafluoroborate [C10mim][BF4], 1-decyl-3-methylimidazolium bromide [C10mim][Br], and 1-decyl-3-methylimidazolium chloride [C10mim][Cl]. This study has been performed by exploiting various spectroscopic techniques such as steady-state fluorescence, time-resolved fluorescence, and UV-visible spectroscopy. The fluorescence emission study of CIP with ILs was performed at various concentrations of all the three ILs. The emission spectra of CIP decreased in the presence of ILs, suggesting complex formation between CIP-IL. The effect of different concentrations of ILs on the emission spectra of CIP was exploited in terms of quenching and binding parameters. Further, fluorescence emission study was validated by the time-resolved fluorescence technique by measuring the average lifetime (τavg) of CIP in the presence of all the three ILs. The τavg value of the drug changed with the addition of ILs, which suggests complex formation between the drug and ILs. This complex formation was also confirmed by UV-visible spectroscopy results of CIP with all the three ILs. Further, for evaluating the thermodynamic parameters of the CIP-IL interactions, isothermal titration calorimetry (ITC) was performed. The ITC experiment yielded the thermodynamic parameters, ΔH (change in the enthalpy of association), ΔG (Gibbs free energy change), ΔS (entropy change), and binding constant (Ka). The binding parameters driven by ITC revealed that CIP-IL interactions are spontaneous in nature and enthalpy-driven, involving hydrophobic forces. Further, the classical density functional theory (DFT) calculations were performed, which provided deep insight for CIP-IL complex formation.