Thermodynamic data of adsorption reveal the entry of CH4 and CO2 in smectite clay interlayer

The ability of smectite clays to incorporate gases in their interlayers is shown to be conditioned by interlayer spacing, depending, in turn, on phyllosilicate layer composition and effective size of the charge-balancing cations. As illustrated by earlier in situ X-ray diffraction and spectroscopic characterization of gas/clay interface, most smectites with small-size charge-balancing cations, such as Na+ or Ca2+, accommodate CO2 and CH4 in their interlayers only in a partially-hydrated state resulting in the opening of the basal spacing, above a certain critical value. In the present study CH4 and CO2 adsorption isotherms where measured for Na- and Mg-exchanged montmorillonite up to 9 MPa using manometric technique. The processess of dehydration of these clays was thorougly charactrized by thermogrvimetric analysis and powder X-ray diffraction. A dramatic decrease in specific surface area, methane and carbon dioxide adsorption capacities for fully-dehydrated samples in comparison to partially-dehydrated ones, is assigned to the shrinkage of interlayer spacing resulting in its inaccessibility for the entry of CH4 and CO2 molecules. This observation is a direct evidence of CH4 and CO2 adsorption capacity variation depending on the opening of smectite clay interlayer spacing.