Limitations of potentiometric studies to determine the surface charge of gibbsite gamma-Al(OH)3 particles.

Surface charges of gibbsite particles were probed by potentiometric titration and subsequently analyzed to estimate intrinsic proton affinity constants of OH surface groups. A detailed spectroscopic characterization of the molecular structure of surface OH groups yielded estimates of bond lengths and bond valences of OH surface sites. Based on these results, the effects of the setting parameters of a MUSIC calculation have been shown in comparison with previous predictions yielding higher pKa values (2 < -pKa < 4) for the protonation of basal doubly coordinated OH surface groups and lower pKa values (7.9 < -pKa < 9.9) for the protonation of lateral singly coordinated OH surface groups. Comparison with experimental data is complicated by reproducible hysteresis between acid and base addition in optimal raw potentiometric titration curves at different ionic strengths. Such effects prevented the determination of a univocal intersection point to provide the global point of zero charge of gibbsite particles, even though the ionic strength dependence of the point of zero net proton charge and the different crossovers between curves indicated that the point of zero charge could be estimated between 8.1 and 9.6, in relative agreement with the lateral affinity constant calculated with the MUSIC model. Still, two main drawbacks remained to differentiate the reactivity of lateral singly and basal doubly coordinated surface groups. First, significant kinetic effects observed in acidic media indicated a dissolution process and/or protonation of basal surface groups. Second, the choice of specific surface areas, especially for a heterogeneous sample, led to several cases for the calculation of the absolute surface charge of particles. Therefore, our results demonstrated the heterogeneous reactivity of gibbsite particles and that the prediction and the experimental determination of respective surface groups are still complex even if some trends emerge.