Water Structure Controls Carbonic Acid Formation in Adsorbed Water Films.

Reaction pathways and kinetics in highly structured HO 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 HO structure in the reaction of HO and CO to form carbonic acid (HCO) in thin HO films condensed onto silica nanoparticles from humidified supercritical CO. Rates of carbonic acid formation are correlated with spectroscopic signatures of HO 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 HO through adsorption at hydrophilic silanol groups. Within measurement uncertainty, we found no evidence that carbonic acid forms when exclusively ice-like structured HO is detected at the silica surface. Instead, formation of HCOO from HO and CO was found to be linearly correlated with liquid-like structured HO that formed on the ice-like layer.