Tuning the Hydrophobicity of Layer-Structure Silicates To Promote Adsorption of Nonaqueous Fluids: Effects of F- for OH- Substitution on CO2 Partitioning into Smectite Interlayers

The intercalation of nonaqueous fluids in the nanopores of organic and inorganic materials is of significant interest, particularly in the energy science community. Recently, X-ray powder diffraction and computational modeling results have shown that structural F- for OH- substitution in layered silicates makes them more hydrophobic. Here, we use grand canonical molecular dynamics calculations to investigate how increasing the F-/(F- + OH-) ratio of a prototypical layered silicate (the smectite Na-hectorite) impacts the intercalation behavior of CO2 and H2O at elevated temperature and pressure. At the conditions of this study (T = 323 K, P = 90 bar, water-saturated CO2), increasing F- for OH- substitution causes decreasing total CO2 + H2O intercalation, increasing CO2/(CO2 + H2O) ratios in the interlayer galleries, and an increasing energy barrier to CO2 and H2O intercalation. CO2 intercalation is the greatest at monolayer basal spacings, and the results support the idea that with Na+ as the exchangeable cation, the interlayers must be propped open by some H2O molecules to allow CO2 to enter the interlayer galleries. The computed immersion energies suggest that the bilayer or a more expanded structure is the stable state under these conditions, in agreement with experimental results, and that the basal spacings of the minimum energy 2L structures increase with increasing F- for OH- substitution. These results are consistent with a wide range of experimental data for smectites at ambient conditions and elevated pressures and temperatures and suggest that F- for OH- substitution in conjunction with reduced structural charge and exchange with large, low charge cations may increase the ability of smectite minerals to incorporate hydrophobic species such as CH4, CO2, H-2, and other organic compounds.