XPS SPECTRA OF K-SILICATE GLASS VALENCE BANDS AND IMPLICATIONS FOR REACTIVITY

The observations: Valence bands of K-silicate glasses are composed of a lower and upper valence band (LVB and UVB) separated by an intensity minimum at ~10 eV binding energy (BE). A third band, composed of the highest occupied molecular orbitals (HOMOs), is centered at ~4.5 eV BE. These HOMOs reside solely on NBOs and are of non-bonding O 2p_y character. HOMOs of Li and Na glasses also reside solely on NBOs and are of identical O 2p_y character. The HOMOs of Li, Na and K glasses are located at ~6, ~5.5-5 and ~4.5 eV respectively whereas the HOMOs of v-SiO₂ are at ~6.5 eV, reside on BO atoms only and are of O 2p_y character. Where HOMOs determine reactivity toward a reagent (e.g., H₂O, HCl), the order of reactivity should be K>Na>Li>v-SiO₂. This is the order at which alkali ions are leached from their respective glasses by distilled water. Apparently, leaching of alkalis from glasses by water is determined by the energetic properties of the HOMOs of each glass. A mechanism for alkali leaching of glasses by distilled water is proposed where the HOMOs limit the rates of reaction.

The mechanism: We propose that H₂O molecules of distilled water become hydrogen-bonded to NBOs through Coulombic attraction of H for lone pairs of electrons on NBOs. Two such ‘bonds’ may form making the NBO tetrahedrally coordinated as is O in ice and water. Hydrogen bond formation extracts charge from the NBO-M⁺ bond of the Si-NBO-M⁺ moiety, resulting in bond rupture, with M⁺ being taken into solution. H⁺ derived from dissociation of H₂O (of bulk solution) then bonds to NBO to produce the Si-NBO-H⁺ moiety. Formation of H-bonds to NBO is proposed to be rate limiting. The reaction follows the elements of transition-state theory where the tetrahedrally coordinated NBO is the activated complex. Continued reaction produces an ‘alkali leached layer’ on the glasses and diffusion eventually controls reaction rates. The mechanism predicts leach rate order of the alkalis glasses to be K>Na>Li.