Synergistic inhibition effects of hydrophilic monomeric substances on CH4 hydrate as revealed by experimental and computational approaches

Abstract In this study, urea (U), acetamide (A), and glycine (G), which are easily decomposable, were suggested as kinetic hydrate inhibitors (KHIs) and potential synergists for CH4 hydrate. The inhibition performance of these substances was evaluated by measuring the onset temperature of CH4 hydrate via both a stirred high-pressure autoclave and a non-stirred high-pressure micro-differential scanning calorimeter. In addition, two quantum mechanics approaches of COnductor-like Screening MOdel for Real Solvents (COSMO-RS) and quantum theory of atoms in molecules (QTAIM), were used to examine the hydrophilicity of the inhibitor candidates and to elucidate the synergism of the inhibitor mixtures. The σ-profile and σ-potential analyses using COSMO-RS indicated that the three monomeric substances have a significant potential as gas hydrate inhibitors by forming hydrogen bonds with host water molecules. The experimental results demonstrated that glycine was the best KHI among the three candidates, and either of two combinations (U + A, U + G, and A + G) had synergistic inhibition effects on CH4 hydrate. The A + G mixture showed the lowest onset temperature, i.e., the best synergism. QTAIM analysis showed that when two different inhibitor molecules were bound to the bi-cage hydrate structure, the absolute value of hydrogen bond energy between inhibitors and water molecules was larger compared to the attachment of two identical inhibitor molecules. The overall combined experimental and computational analyses not only suggest the possibility of hydrophilic monomeric substances as potent KHIs but also offer a new way to reveal the mechanism of hydrate inhibition.