Ab initio modeling of glass corrosion: Hydroxylation and chemisorption of oxalic acid at diopside and åkermanite surfaces

Using ab initio density functional theory, we have performed a systematic study of corrosion processes at pure and at hydroxylated surfaces of the silicate minerals diopside (CaMgSi2O6)(CaMgSi2O6) and åkermanite (Ca2MgSi2O7)(Ca2MgSi2O7), serving as model systems for multicomponent glasses. The tendency of the cations to assume their ideal bulk coordination was identified as the driving force behind spontaneous surface relaxation and hydroxylation in an aqueous environment. Surface complexes formed after exposing the glass surfaces to oxalic acid form a protective hydrophobic layer on the surface and thus prevent the leaching of metal cations from the glass surface. This provides a description of the mechanism of glass corrosion inhibition at the atomic level: as the chemisorption energy of oxalic acid is larger than the physisorption energy of water, the former is the process that will actually occur.