Aqueous CO2 Solutions at Silica Surfaces and within Nanopore Environments. Insights from Isobaric-Isothermal Molecular Dynamics

We present a detailed molecular-based characterization via isobaric isothermal-molecular dynamics simulation of the microstructure and dynamics of water-rich aqueous CO2 solutions at silica surfaces and under extreme confinement between finite silica plates at state conditions relevant to geologic capture and sequestration of carbon dioxide. The study comprises three types of slit-pore plates to represent two extreme cases of surface polarity and a mismatched pair of plates to interrogate the fluid behavior at and confined between heterogeneous surfaces. We found layer formation of H2O and CO2 whose strength depends on the nature of the plate surface, i.e., stronger H2O layering at hydrophilic than at hydrophobic plates with simultaneous weaker water-mediated CO2/hydrophilic-surface interactions. We observed the opposite behavior with the hydrophobic plates in which the weaker water layering results from the CO2-mediated H2O/hydrophobic-surface interactions. Moreover, we illustrate how the interplay between these types of interactions and extreme fluid confinement, i.e., strong overlapping of interfacial structures, can induce a drying out of the pore environment whose immediate consequence is a significant CO, concentration enhancement relative to that of the bulk environment. Finally, we assessed the effect of the nature of the plate surfaces on the translational diffusion coefficient of water. We found that this property changes monotonically at purely interfacial regions but nonmonotonically under confinement.