NH$_3$ adsorption and competition with H$_2$O on a hydroxylated aluminosilicate surface

TL;DR

This study investigates how ammonia (NH₃) interacts with aluminosilicate surfaces, especially microcline feldspar, revealing that water can displace NH₃ despite similar adsorption energies, impacting understanding of ice nucleation processes.

Contribution

It provides detailed atomic-level insights into NH₃ adsorption on microcline and demonstrates water's ability to replace NH₃, challenging previous assumptions about ice nucleation mechanisms.

Findings

NH₃ and H₂O adsorb on the same sites with similar energies

H₂O can displace NH₃ on the surface

Implications for ice nucleation theories

Abstract

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$_3$O$_8$), 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$_2$O 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$_2$O, challenging the notion that ice nucleation on microcline proceeds via the creation of an ordered H$_2$O layer atop pre-adsorbed NH$_3$ molecules.