Determination of Acid-Base Equilibrium Constants on Aqueous Mesoporous Silica Surfaces

Abstract

We used the potentiometric titration technique to study the deprotonation reactions at aqueous mesoporous silica surfaces. The concentration of surface proton binding sites was obtained by the Gran plot method using the acid-base titration data. The relevant equilibrium constants of the surfaces in terms of the constant capacitance model (CCM) were determined based on the experimental results using the FITEQL 4.0 program. The results indicate that the surface deprotonation behavior of the mesoporous silica suspensions is significantly different from that of amorphous silica. This behavior can be described by two surface reactions with surface bidentate and monodentate deprotonation constants of pKa1=6.78±0.15 and pKa2=10.25±0.22, respectively. Using these deprotonation constants, we established a surface speciation diagram for mesoporous silica in aqueous suspensions as a function of pH and discussed the effect of surface capacitance on surface speciation.