NH_3 adsorption and competition with H_2O on a hydroxylated aluminosilicate surface

The interaction between ammonia (NH_3) and (alumino)silicates is of fundamental and applied importance, yet the specifics of NH_3 adsorption on silicate surfaces remain largely unexplored, mainly because of experimental challenges related to their electrically insulating nature. An example of this knowledge gap is evident in the context of ice nucleation on silicate dust, wherein the role of NH_3 for ice nucleation remains debated. This study explores the fundamentals of the interaction between NH_3 and microcline feldspar (KAlSi_3O_8), a common aluminosilicate with outstanding ice nucleation abilities. Atomically resolved non-contact atomic force microscopy, x-ray photoelectron spectroscopy, and density functional theory-based calculations elucidate the adsorption geometry of NH_3 on the lowest-energy surface of microcline, the (001) facet, and its interplay with surface hydroxyls and molecular water. NH_3 and H_2O are found to adsorb molecularly in the same adsorption sites, creating H-bonds with the proximate surface silanol (Si-OH) and aluminol (Al-OH) groups. Despite the closely matched adsorption energies of the two molecules, NH_3 readily yields to replacement by H_2O, challenging the notion that ice nucleation on microcline proceeds via the creation of an ordered H_2O layer atop pre-adsorbed NH_3 molecules.