Swelling Phenomena of the Nonswelling Clay Induced by CO₂ and Water Cooperative Adsorption in Janus-Surface Micropores

With the development of microscopy and sensor techniques, it becomes evident that nonswelling clays show swelling behavior under CO₂-water mixture environments at high pressures and temperatures. The examples include Illite, muscovite, and kaolinite-rich rock samples. Here, we investigated the underlying mechanisms of kaolinite swelling induced by CO₂ and water using molecular simulations and low-pressure gas adsorption experiments. The results suggest the cooperative adsorption behavior of CO₂ and water on contact with kaolinite micropores, which have distinct wettabilities on the two adjoining interlayer surfaces. Even if clay-bound water exists, CO₂ can enter the micropores to induce swelling. The measured micropore volume, simulated equilibrium stable interlayer distance with pure water, and that with CO₂-water mixture were used in the swelling estimation, which shows good agreement with our experiments. The CO₂ and water molecule distributions inside the interlayer micropores verify the importance of the wettabilities of the kaolinite surfaces in this cooperative adsorption behavior. The result extends the traditional understanding of the swelling mechanism, i.e., cation hydration and subsequent osmotic processes. In addition to earlier observations of kaolinite swelling behavior with potassium acetate, our study indicates the significance of the subtle balance of the noncovalent interactions between CO₂, water, and the kaolinite Janus surfaces.