Wetting Effects and Molecular Adsorption at Hydrated Kaolinite Clay Mineral Surfaces

In this study, classical molecular dynamics simulations have been used to understand the key interactions and surface structure of a set of organic molecules at the hydrated surfaces of the 1:1 clay mineral kaolinite. Decane, decanoic acid, and decanamine have been modeled at both the hydroxylated and silicate surfaces of kaolinite. Additionally, the effect of pH is observed via looking at the protonated decanamine and decanoate anion forms. The key results show that relative affinity of the organic molecules to the kaolinite surface may be readily switched between the hydroxylated and the silicate surfaces according to the pH and the nature of the organic head functional group. Decane molecules readily form droplets atop the silicate surface and do not adsorb to the hydroxyl surface, as do protonated decanoic acids. In stark contrast,, decanoate anions do not adsorb to the silicate surface, yet adsorb to the hydroxyl surface through an anion exchange mechanism. Decanamine readily adsorbs to both silicate and hydroxyl surfaces, though the hydroxyl amine interactions are mediated through water bridges. Once charged, the decanamine remains adsorbed to both surfaces, however, both interactions are ionically mediated, rather than through van der Waals and hydrogen bonds. Furthermore, protonated decanamine is observed to adsorb to the hydroxyl surface via anion bridges, a phenomenon that is typically associated with positively charged layered double hydroxides rather:than negatively charged clay minerals.