Surface Condensation of CO2 onto Kaolinite

The fundamental adsorption of CO2 onto poorly crystalline kaolinite (KGa-2) under conditions relevant to geologic sequestration has been investigated using a quartz crystal microbalance (QCM) and density functional theory (DFT) methods. The QCM data indicated linear adsorption of CO2 (0–0.3 mmol of CO2/g of KGa-2) onto the kaolinite surface up through the gaseous state (0.186 g/cm3). However, in the supercritical region, the extent of CO2 adsorption increases dramatically, reaching a peak (0.9–1.2 mmol of CO2/g of KGa-2) near 0.40 g/cm3, before declining rapidly. DFT studies of interactions of CO2 with kaolinite surface models confirm that surface adsorption is favored up to ∼0.34 g/cm3 of CO2, showing distorted T-shaped CO2–CO2 clustering, typical of supercritical CO2 aggregation over the surface at higher densities. Beyond this point, the adsorption energy gain for any additional CO2 becomes smaller than the CO2 interaction energy (∼0.2 eV) in the supercritical medium, resulting in the desorption of CO2 from the kaolinite surface.