engleski

The sapphire c-plane electrolyte interface – an attempt to explain contradictory observations

We present a tentative picture for the charging of the sapphire basal plane in dilute electrolyte solutions, that allows reconciliation of the available experimental observations within a dual charging model including the MUltiSIte Complexation (MUSIC) model and autoprotolysis of interfacial water. The semi-empirical MUSIC model predicts stability constants of individual surface functional groups with respect to protonation and deprotonation based on crystal structure and bond-valence principles. On the c-plane of sapphire only doubly co-ordinated sites hydroxyl groups are expected from the ideal crystal structure, which results in quasyi zero surface potential (defined as the potential in the plane of the surface hydroxyl groups) in the near neutral pH range and rather weak charging beyond. This approach concurs strikingly well with recently published sum frequency data for the so-called ice-like band water layer located at the oxide surface. However, zeta- potential measurements at the sapphire c-plane as well as second harmonic generation data reveal an IEP of around 4 and a negative surface charges at pH values where the MUSIC model predicts the pristine point of zero charge. To explain the measured zeta- potentials and second harmonic data as well as colloid adhesion data, we need to invoke an additional charging mechanism. Enhanced autoprotolysis of interfacial water is the most probable candidate: Sum frequency data permit suggestto conclude on a net water orientation within the pH range of interest, which would allow for this unconventional mechanism at oxide-electrolyte interfaces. The similarity of the zeta- potential vs. pH plots measureds for the c-plane with those measured for hydrophobic surfaces corroborates this as do infrared spectroscopic data on thin water films on these surfaces. The proposed dual charging mode approachmodel comprises the most advanced interpretation (i.e. the MUSIC model for protonation/deprotonation at the the oxide-water interfaces and the currently advocated enhanced autoprotolysis picture for hydrophobic surfaces). It can explain the available information in a qualitative way. The importance of the dual charging mechanism is seen in its importance for charging of single crystals of alumina. The respective dominance of the protonation/deprotonation reactions of surface functional groups or the autoprotolysis will be responsible for the measured zeta- potentials and the resulting isoelectric point. There will be repercussions of one mechanism on the other and resulting in the most favourablea given interfacial water structure, which can be followed by non-linear optic techniques like sum frequency generation.

zeta-potential; isapphire; alumina; sum frequency generation; water structure; MUSIC model

nije evidentirano