Surface Condensation of CO ₂ onto Kaolinite

The fundamental adsorption of CO ₂ 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 CO ₂ (0-0.3 mmol of CO ₂ /g of KGa-2) onto the kaolinite surface up through the gaseous state (0.186 g/cm ³ ). However, in the supercritical region, the extent of CO ₂ adsorption increases dramatically, reaching a peak (0.9-1.2 mmol of CO ₂ /g of KGa-2) near 0.40 g/cm ³ , before declining rapidly. DFT studies of interactions of CO ₂ with kaolinite surface models confirm that surface adsorption is favored up to ∼0.34 g/cm ³ of CO ₂ , showing distorted T-shaped CO ₂ -CO ₂ clustering, typical of supercritical CO ₂ aggregation over the surface at higher densities. Beyond this point, the adsorption energy gain for any additional CO ₂ becomes smaller than the CO ₂ interaction energy (∼0.2 eV) in the supercritical medium, resulting in the desorption of CO ₂ from the kaolinite surface.