Influence of salinity and electrostatic fields on CO₂ hydrate formation and the interfacial quasi-liquid layer in clay nanoconfinement: A molecular dynamics study

Abstract

Marine clay reservoirs are considered ideal substrates for hydrate-based CO₂ capture and storage owing to their large specific surface area, high porosity and favorable adsorption capacity. Understanding the evolution mechanism of quasi-liquid layers (QLLs) and hydrate formation at clay interfaces under nanoconfinement is crucial for evaluating the spatial potential of storage matrices and enhancing CO₂ sequestration efficiency. In this study, molecular dynamics (MD) simulations were conducted at 250 K and 300 bar with a time step of 2 fs to investigate the effects of salinity and electrostatic fields on CO₂ hydrate formation and QLL thickness within montmorillonite (MMT) slit pores. Simulation results indicate that CO₂ hydrate nucleation and growth predominantly occur away from MMT interfaces, with elevated salinity significantly delaying nucleation onset. Dynamic salt exclusion during hydrate formation inhibits interfacial hydrate growth toward clay surfaces. Elevated salinity increases QLL thickness from 1.2 nm to 1.6 nm near isomorphic substitution surfaces and from 1.1 nm to 1.4 nm near non-substituted MMT surfaces. Notably, under seawater salinity conditions, QLL thickening demonstrates dynamic coupling with salt exclusion effects. Electrostatic fields substantially influence nucleation stochasticity and spatial distribution through electric double-layer restructuring and ion migration. QLL thickness near isomorphic substitution surfaces shows limited sensitivity to electrostatic fields, stabilizing at about 1.15 nm. Conversely, QLLs adjacent to non-substituted surfaces exhibit significant field dependence due to surface ion redistribution, with thickness increasing to 1.55 nm at an applied field of 0.3 V/nm before reaching a plateau. These molecular-scale insights into salinity and electrostatic effects on hydrate formation and interfacial QLL evolution provide critical references for optimizing clay-based carbon storage matrices and advancing CO₂ sequestration technologies.

Keywords

CO₂ hydrate / Combustible ice / Clay nanoconfinement / Quasi-liquid layer / Salinity / Electrostatic fields / Molecular dynamics simulation / Carbon sequestration / Carbon neutrality / Carbon capture, utilization, and storage (CCUS)

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