The adsorption of carbon dioxide(CO2)on kaolinite surfaces and the roles of water on the adsorption were investigated theoretically using a density functional theory.Both of cluster and periodic models were studied.The calculation reveals that adsorbed CO2 is able to form stable complexes with the octahedral and tetrahedral surfaces of kaolinite in the presence or absence of a sodium cation and water molecule.The octahedral surface possesses a large binding affinity toward CO2 than the tetrahedral site of kaolinite partially due to the existence of the surface hydroxyl groups that are more active in the intermolecular interactions than the basal oxygen atoms of the tetrahedral sites.The adsorption energy data performs that the adsorption of CO2 on periodic model surfaces are stronger than on cluster surfaces.The addition of a sodium cation plays a significant role in this adsorption with the effect of increasing the adsorption energy weightily,while addition of one water molecule decreases the binding strength insignificantly.Estimated Gibbs free energies indicate that the adsorption of CO2 on all of the studied kaolinite surfaces is thermodynamically feasible from gas phase.Surface free energy was employed to provide predictions of the changes in surface stability as a function of temperature.