Water Vapor Adsorption on Goethite

Goethite (alpha-FeOOH) is an important mineral contributing to processes of atmospheric and terrestrial importance. Their interactions with water vapor are particularly relevant in these contexts. In this work, molecular details of water vapor (0.0-19.0 Torr; 0-96% relative humidity at 25 degrees C) adsorption at surfaces of synthetic goethite nanoparticles reacted with and without HCl and NaCl were resolved using vibrational spectroscopy. This technique probed interactions between surface (hydr)oxo groups and liquid water-like films. Molecular dynamics showed that structures and orientations adopted by these waters are comparable to those adopted at the interface with liquid water. Particle surfaces reacted with HCl accumulated less water than acid free surfaces due to disruptions M hydrogen bond networks by chemisorbed waters and chloride. Part, ides reacted with NaCl had lower loadings below similar to 10 Ton water vapor but greater loadings above this value than salt free surfaces Water adsorption reactions were here affected by competitive hydration of coexisting salt free surface regions, adsorbed chloride,and sodium, as well as precipitated NaCl. Collectively, the findings presented in this study add further insight into the initial mechanisms of thin water film formation at goethite surfaces subjected to variations in water vapor pressure that are relevant to natural systems.