Water adsorption on a model silicate surface: wollastonite (100)

TL;DR

This study combines advanced microscopy and DFT calculations to elucidate the atomic-scale structures of water overlayers on wollastonite (100), revealing how water-surface and water-water interactions influence adsorption patterns.

Contribution

It provides new atomic-scale insights into water adsorption mechanisms on calcium silicate surfaces using combined experimental and theoretical approaches.

Findings

Water forms distinct structures depending on coverage and interactions.

At low coverage, water follows the surface lattice due to water-surface interactions.

At higher coverage, water clusters emerge as water-water interactions dominate.

Abstract

Water adsorption on silicate surfaces is a critical yet poorly understood process relevant to, e.g., mineral weathering and cement hydration. This study investigates the structure of water overlayers on a model calcium silicate, the lowest-energy (100) surface of wollastonite (CaSiO3). It combines atomically resolved non-contact atomic force microscopy (nc-AFM), acquired with qPlus sensors and functionalized tips in ultrahigh vacuum (UHV), with density functional theory (DFT) calculations employing the metaGGA r2SCAN+rVV10 functional. Adding incremental doses of water to the sample at cryogenic temperatures produces distinct structures governed by the competition between water-surface and water-water interactions. With two water molecules per surface unit cell, water-surface interactions dominate: In line with previous theoretical predictions, adsorbates follow the surface lattice. As the coverage increases, intermolecular hydrogen bonding competes with bonding to the surface, leading to the emergence of complex, coexisting patterns. While their small energy differences prevent an unambiguous identification of the most stable structure by DFT, the experimentally observed symmetries help constrain plausible structural models. Above a critical density of four water molecules per unit cell, water-water interactions prevail, and water clusters are formed. The results provide an atomic-scale framework for understanding water interactions with calcium silicate surfaces.