Regulating the interfacial water structure of montmorillonite through hydrophobic modification: A strategy to promote hydrate formation during liquid CO₂ injection.

Hydrate-based carbon dioxide (CO₂) sequestration (HBCS) within marine sediments has emerged as a highly promising strategy for long-term carbon storage and climate mitigation. Nevertheless, the efficiency of this process is hindered by the inhibition of clay hydration on CO₂ hydrate formation in marine sediments. This study develops a hydrophobic modification strategy to facilitate hydrate formation by regulating the interfacial water structure at the mineral-water interface. The in-situ formation of CO₂ hydrate and the evolution of water structure were investigated using low-field nuclear magnetic resonance (LF-NMR) techniques and molecular dynamics (MD) simulations. Experimental results demonstrate that the hydrophobic modification of MMT significantly enhances hydrate growth kinetics, achieving a water-to-hydrate conversion rate of ∼41% within 30 min, which is markedly higher than that of raw MMT (∼18%). Transverse relaxation time (T₂) analysis reveals that the hydrophobic modification effectively eliminates the constraints on water molecules imposed by MMT hydration, increases the proportion of free water while reducing the bound water that is reluctant to participate in hydrate conversion. MD simulations further elucidate that the hydrophobic modification creates a "repulsive interfacial environment" through ionic substitution and the hydrophobic effect, which alleviates the confinement of water molecules on the MMT surface and favors hydrate formation. This work highlights that the regulation of interfacial water structure via hydrophobic modification is a critical mechanism for optimizing CO₂ storage in fine-grained marine sediments.