Water adsorption behaviour on (001) pyrophyllite surface from ab initio Density Functional Theory simulations

The present study focuses on the interaction between water and a single layer of the (001) pyrophyllite surface. Pyrophyllite [Al2Si4O10(OH)2] is an important phyllosilicate characterized by strong in-plane covalent bonds and weak van der Waals interactions holding the structural layers together. The investigation was carried out by ab initio Density Functional Theory simulations, performed both at 0 K and room conditions (300 K, 1 bar) using static and molecular dynamics approaches, respectively. The adsorption of one to three water molecules, investigated by static method, showed that the hydration is completely driven by van der Waals interactions and, as the water content increases, so does the tendency of the solvent to self-assemble. In fact, the binding energies per H2O molecule were calculated as −17.2 kJ mol−1, −11.8 kJ mol−1, and − 11.6 kJ mol−1 for the adsorption of one, two, and three water molecules, respectively. The results showed that water forms structures that resemble those typically found in the gas phase, i.e. water dimer and triangular cluster. Regarding the latter, the ab initio molecular dynamics simulations proved that this self-assembly is stable over the considered time for the run. These findings are in good agreement with the few previous data reported in the literature and could be of great use for both experimental and theoretical applications in both mineralogy and petrography and for the development of bidimensional van der Waals material from natural sources.