Evidence of the existence of three types of species at the quartz-aqueous solution interface at pH 0-10: XPS surface group quantification and surface complexation modeling

The surface composition of a quartz surface reacted with various aqueous solutions of pH 0-10 was qualitatively and quantitatively evaluated using X-ray photoelectron spectroscopy (XPS). The positions and intensities of the recorded Si 2p and O 1s lines change depending on solution conditions. The O 1s line, where the position varies more significantly, was analyzed in detail showing three components corresponding to three surface species: >SiOH2+, >SiOH0, and >SiO- (where > represents the bulk quartz). The changes in the Si 2p spectra support these findings. The atomic ratio between the surface oxygen and silicon atoms was found to be 1.8. These data allow proposing a two-step deprotonation model of the quartz surface where two surface oxygen atoms are bonded to one silicon surface atom and where the most deprotonated surface sites show the SiO- configuration. Physisorbed water in the amount of around 10% of the monolayer was also found on all samples under spectroscopic investigation. The density of the >SiO- group increases significantly with an increase of pH, whereas the surface concentration of the >SiOH2+ group is the highest at pH 0. The maximum of the neutral > SiOH0 group is observed at pH 6. These results indicate immediately the validity of a 2-pK model of protonation of quartz/electrolyte interface versus a 1-pK model. The 2-pK surface capacitance model of the electric double layer having pK(1) = - 1.0 and pK(2) = 4.0 was derived from the XPS data. The new surface stability constants allow a quantitative description of quartz surface charge and dissolution kinetics in neutral to alkaline solutions and explicitly account for an increase of quartz dissolution rate at pH < 2 due to significant increase of the concentration of the >SiOH2+ surface species. These results provide, for the first time, direct atomic level spectroscopic evidence of the validity of the chemical surface speciation approach, notably the existence of the charged >SiOH2+ and >SiO- species at the quartz surface and their relative densities in acidic and alkaline solutions.