Water on the hydroxylated (0 0 1) surface of kaolinite: From monomer adsorption to a flat 2D wetting layer

In order to understand the role played by kaolinite in heterogeneous ice nucleation, an extensive density-functional theory study has been performed for water on its (0 0 1) basal plane. Water monomers at low coverage, water clusters, water bilayers and water multilayers have all been examined. The most important and interesting results from this study are: (i) water monomers bind strongly to kaolinite compared to many other substrates. In the preferred adsorption structure water accepts two H bonds from and donates one H bond to the substrate, revealing that kaolinite, like water, is amphoteric with the ability to accept and donate H bonds; (ii) clustering of adsorbed water molecules is not significantly favored. All water clusters (dimers, tetramers, and hexamers) examined are, at best, equally stable to water monomers; (iii) a 2D ice-like bilayer, with a stability matching that of ice Ih has been identified implying that water can wet kaolinite; (iv) multilayer ice growth is not favored, being considerably unstable compared to bulk ice, indicating that the water covered kaolinite surface is itself “hydrophobic”. Overall we see that amphoterism of the hydroxylated surface is key to many of the interesting properties of kaolinite with regard to water adsorption and ice nucleation, revealing that the behavior of water on kaolinite is more complex and interesting than previously thought to be and highlighting the need for further theoretical and experimental work.