Influence of Ca2+ substitution on the surface hydration characteristics of kaolinite: Combination of simulations and experiments

[Display omitted] •Ca2+ substitution can be enhanced the surface hydration of kaolinite.•The more Ca2+ substitution, the stronger the surface hydration of kaolinite.•Na+ enhances surface hydration by forming coordination structures.•The surface hydrophilicity of kaolinite increased after Ca2+ substitution. To investigate the changes in kaolinite surface hydration characteristics after Ca2+ substitution, water molecule adsorption onto the surface of Ca2+-substituted kaolinite was studied using density functional theory. Changes in the hydration film on the surface of kaolinite (001) and the mineral surface hydrophilicity after Ca2+ substitution were studied using molecular dynamics methods. Several Ca2+-substituted kaolinite samples were hydrothermally synthesized, and their hydration characteristics were analyzed using contact-angle and microcantilever humidity-test methods. The energy of a water molecule adsorbed on a Ca2+-substituted kaolinite surface is > 5 % lower than on pure kaolinite, and microscopic interactions between water molecules and the kaolinite surface are enhanced. Further, water molecules are more likely to be adsorbed onto the (001) surface than the (001¯) surface of kaolinite. After Ca2+ substitution, the hydration film on the kaolinite (001) surface becomes denser and the hydrophilicity of the mineral surface is enhanced. Using X-ray diffraction, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy to characterize the synthesized samples, we confirmed successful Ca2+ substitution within the kaolinite structure. Hydration-test results showed that the surface hydrophilicity of kaolinite is enhanced after Ca2+ substitution and increases with increasing Ca2+ substitution. The experimental results thus verified the simulations.