The significance of the solid-to-liquid ratio in the electrokinetic studies of the effect of ionic surfactants on mineral oxides

Abstract

The effect of SDS on the electrokinetic behavior of TiO(2) and Al(2)O(3) was studied by electrophoresis at various solid-to-liquid ratios. Additionally, the effect of CTAB on electrokinetic curves of Al(2)O(3) single crystal and of Al(2)O(3) particles was studied by streaming potential. At a sufficiently low solid-to-liquid ratio, the electrokinetic potential was negative and almost pH-independent in the presence of SDS and positive and pH-independent in the presence of CTAB. Further decrease in the solid-to-liquid ratio had a limited effect on the course of the electrokinetic curves. At a sufficiently high solid-to-liquid ratio, the electrokinetic potential was not affected by the presence of the surfactant. At moderate solid-to-liquid ratios, the electrokinetic potential in the presence of SDS was negative and almost pH-independent at very high and at very low pH, and less negative or even positive electrokinetic potential (more positive at higher solid-to-liquid ratios) was observed at moderate pH with a peak 1 to 2 pH units below the pristine IEP. The inspection of the results (obtained at single solid-to-liquid ratio) from the literature confirmed the above trends, also for oxides other than TiO(2) or Al(2)O(3). The range of solid-to-liquid ratios, which can be covered by electrophoresis is limited by insufficient signal, and by insufficient transparency at low and at high solid-to-liquid ratios, respectively. The available range of solid-to-liquid ratios can be extended by using the electroacoustic method. Apparently, the significance of the solid-to-liquid ratio in the electrokinetic studies of oxide-ionic surfactant systems is underrated. To our best knowledge, this is the first systematic study of such an effect ever published, and in many publications, the solid-to-liquid ratio was not reported and probably not even controlled.