Chemistry Reference
In-Depth Information
where is the sum of all surface species. The ionization of the silica/aqueous
interface through protonation and deprotonation reactions means that the silica surface
acquires a surface charge and this, in turn, generates a surface potential As a
result, the proton concentration immediately adjacent to the solid surface differs from
the bulk according to Boltzmann's distribution law:
The concentration of SiOH, using Eqs. (4.10)-(4.12), (4.20), and (4.21), can then be
expressed as
The dissolution rate is determined by reaction (4.13), which is the rate-determining
step, and can be expressed as
where [SiF] and [SiOH] are the surface concentrations of adsorbed fluoride and
hydroxyl sites; according to Eq. (4.12).
It is seen from Eq. (4.22) that at low pH, [SiOH] increases with increasing pH,
whereas at high pH, it decreases with increasing pH. Since [SiF] is proportional to
[SiOH], this means that there is a maximum [SiF] as a function of pH, which explains
the observed etch rate maximum shown in Fig. 4.39. By considering the equilibria in
solutions, Osseo-Asare also explained the occurrence of the etch rate
maximum as a function of in these solutions. For a fixed concentration of HF,
increasing the concentration of dissolved has two major effects: (1) It increases
the total concentration of dissolved fluoride which in turn increases the surface
