Electroacoustic and potentiometric studies of the hematite/water interface

The basic charging proper-ties of nearly spherical hematite particles were studied by using potentiometric titration and the electroacoustic technique . Both the pH and the ionic strength dependence of the surface charge and t he zeta -potential were studied in detail. For calculating the zeta -potent ial from mobility data a few different theories,were used and obtained diff erences are discussed. At pH values higher than 7 and at high electrolyte c oncentrations (50 mM and 100 mM NaNO3), it was difficult to fit the mobilit y data by using the full mobility spectra including both magnitude and phas e angle at several frequencies. In this regime the best fits were obtained by using a theory for aggregated complexes (porous particles). From potenti ometric titrations in 0.01, 0.1, and 1.0 M NaNO3, parameters fora 1-pK Basi c-Stem Model were determined. The model was used to examine the possibility of correlating the experimentally determined zeta -potentials to the model -calculated potentials at the Stem plane. Qualitatively, the model predicte d the correct ionic strength dependence of the zeta -potentials, and there was also a rather good quantitative agreement at high ionic strengths (50 a nd 100 mM NaNO3). However, at lower ionic strengths the model predicted val ues up to 40% higher than those found from the electroacoustic study. Surfa ce conduction behind the slip plane was discussed as a possible cause for t his discrepancy. (C) 2001 Academic Press.