Characterization of acid–base properties of two gibbsite samples in the context of literature results

Two different gibbsites, one commercial and one synthesized according to a frequently applied recipe, were studied in an interlaboratory attempt to gain insight into the origin of widely differing reports on gibbsite acid–base surface properties. In addition to a thorough characterization of the two solids, several methods relevant to the interfacial charging were applied to the two samples: potentiometric titrations to obtain the “apparent” proton related surface charge density, zeta-potential measurements characterizing the potential at the plane of shear, and Attenuated Total Reflection Infrared Spectroscopy (ATR-IR) to obtain information on the variation of counter-ion adsorption with pH (using nitrate as a probe). Values of the IEP at 9–10 and 11.2–11.3 were found for the commercial and synthesized sample, respectively. The experimental observations revealed huge differences in the charging behavior between the two samples. Such differences also appeared in the titration kinetics. A detailed literature review revealed similar disparity with no apparent systematic trend. While previously the waiting time between additions had been advocated to explain such differences among synthesized samples, our results do not support such a conclusion. Instead, we find that the amount of titrant added in each aliquot appears to have a significant influence on the titration curves. While we can relate a number of observations to others, a number of open questions and contradictions remain. We suggest various processes, which can explain the observed behavior.

Two different gibbsite samples were extensively studied. Small and big particles yield low and high surface charge densities, respectively.Figure optionsDownload high-quality image (102 K)Download as PowerPoint slideResearch highlights
► The titration results at 20 min waiting times refute the previous suggestion that increased waiting time causes higher charge on gibbsite particles.
► The difference in slope of literature charging curves are attributed to the titrant aliquot in terms of titrant added per surface area of gibbsite.
► We show in an interlaboratory comparison that titrations on one solid yield consistent results.
► Based on the interlaboratory comparison, it becomes clear that the discrepancies in interfacial properties of the two different gibbsite samples (in terms of their surface charging) studied are real.