Effect of Organic Matter on CO₂ Hydrate Phase Equilibrium in Phyllosilicate Suspensions

In this study, we examined various CO₂ hydrate phase equilibria under diverse, heterogeneous conditions, to provide basic knowledge for successful ocean CO₂ sequestration in offshore marine sediments. We investigated the effect of geochemical factors on CO₂ hydrate phase equilibrium. The three-phase (liquid–hydrate–vapor) equilibrium of CO₂ hydrate in the presence of (i) organic matter (glycine, glucose, and urea), (ii) phyllosilicates [illite, kaolinite, and Na-montmorillonite (Na-MMT)], and (iii) mixtures of them was measured in the ranges of 274.5–277.0 K and 14–22 bar. Organic matter inhibited the phase equilibrium of CO₂ hydrate by association with water molecules. The inhibition effect decreased in the order: urea < glycine < glucose. Illite and kaolinite (unexpandable clays) barely affected the CO₂ hydrate phase equilibrium, while Na-MMT (expandable clay) affected the phase equilibrium because of its interlayer cations. The CO₂ hydrate equilibrium conditions, in the illite and kaolinite suspensions with organic matter, were very similar to those in the aqueous organic matter solutions. However, the equilibrium condition in the Na-MMT suspension with organic matter changed because of reduction of its inhibition effect by intercalated organic matter associated with cations in the Na-MMT interlayer

CO₂ Hydrate Nucleation Kinetics Enhanced by an Organo-Mineral Complex Formed at the Montmorillonite–Water Interface

In this study, we investigated experimentally and computationally the effect of organo-mineral complexes on the nucleation kinetics of CO₂ hydrate. These complexes formed via adsorption of zwitter-ionic glycine (Gly-zw) onto the surface of sodium montmorillonite (Na-MMT). The electrostatic attraction between the −NH₃⁺ group of Gly-zw, and the negatively charged Na-MMT surface, provides the thermodynamic driving force for the organo-mineral complexation. We suggest that the complexation of Gly-zw on the Na-MMT surface accelerates CO₂ hydrate nucleation kinetics by increasing the mineral–water interfacial area (thus increasing the number of effective hydrate-nucleation sites), and also by suppressing the thermal fluctuation of solvated Na⁺ (a well-known hydrate formation inhibitor) in the vicinity of the mineral surface by coordinating with the −COO^– groups of Gly-zw. We further confirmed that the local density of hydrate-forming molecules (i.e., reactants of CO₂ and water) at the mineral surface (regardless of the presence of Gly-zw) becomes greater than that of bulk phase. This is expected to promote the hydrate nucleation kinetics at the surface. Our study sheds new light on CO₂ hydrate nucleation kinetics in heterogeneous marine environments, and could provide knowledge fundamental to successful CO₂ sequestration under seabed sediments