Carbon dioxide binding in supercooled water nanofilms on nanominerals†
Moist CO2-bearing air flowing in Earth's terrestrial environments and now warming cryosphere can be captured by thin nanometric water films supported by mineral nanoparticles. Molecular-level mechanisms driving the fate of CO2 by these water films at 25, −10 and −50 °C were resolved by vibration spectroscopy of mineral nanoparticles of well-known crystal habits and surface structures. This work shows that mineral-supported water films cooled below the freezing point of water do not freeze to ice and instead host hydrated carbonate species of comparable properties to those formed at 25 °C. CO2 uptake by water films is driven by nucleophilic attack of surface hydroxo groups. Conversion to carbonate species is, in turn, stabilised by hydrogen bonding with neighbouring hydroxo groups and water molecules. The lower CO2 uptake under extremely cold conditions (−50 °C) is, as such, explained by the reduced mobility of water needed to hydrate carbonate ions. Carbonate species initially entrapped by cooling of warmer films to −50 °C are nonetheless resilient to outgassing, even under vacuum. This implies that CO2 initially entrapped by cooling of warm CO2-bearing water films can have prolonged lifetimes under extremely cold conditions. Our findings shine new light on how nanominerals and the nanofilms they host alter the fate of CO2 in cold terrestrial environments. This chemistry is even strongly relevant to nanominerals in Earth's atmosphere and on the planet Mars.