Water Structure Controls Carbonic Acid Formation in Adsorbed Water Films.

Reaction pathways and kinetics in highly structured H2 O adsorbed as Ångstrom to nanometer thick layers on mineral surfaces are distinct from those facilitated by bulk liquid water. We investigate the role of the interfacial H2 O structure in the reaction of H2 O and CO2 to form carbonic acid (H2 CO3 ) in thin H2 O films condensed onto silica nanoparticles from humidified supercritical CO2 . Rates of carbonic acid formation are correlated with spectroscopic signatures of H2 O structure using oxygen isotopic tracers and infrared spectroscopy. While carbonic acid virtually does not form in the supercritical phase, the silica surface catalyzes this reaction by concentrating H2 O through adsorption at hydrophilic silanol groups. Within measurement uncertainty, we found no evidence that carbonic acid forms when exclusively ice-like structured H2 O is detected at the silica surface. Instead, formation of H2 C18 O16 O2 from H2 18 O and C16 O2 was found to be linearly correlated with liquid-like structured H2 O that formed on the ice-like layer.