An anomalous maximum in the ionic strength dependent electrophoretic mobility curves has been observed in previous reports from particles dispersed in colloids. This maximum has been considered anomalous because it is contradictory with the Gouy-Chapman model. The existence of such a maximum has been attributed to specific ionic adsorption, a hairy layer at the surface, or the effect of the anomalous change of surface conductivity in different studies. It was also pointed out that the O'Brien-White approach based on the Gouy-Chapman model could be used to understand this maximum in electrophoretic mobility curves and lead to understandable zeta potential curves. This implied that the observed maximum was actually not "anomalous". In this work we report our simulation of ionic strength dependent zeta potential curves based on the O'Brien-White approach and experimental studies of the ionic strength dependent electrophoretic mobility of the hexadecane droplets in the hexadecane-water emulsions at different pH or in the presence of sodium dodecyl sulphate at varied concentrations. In some cases, the simulation shows that the calculation with the O'Brien-White approach does change the trend in the concerned ionic strength dependent curves. However, the simulation in some other cases also leads to similar trends in the ionic strength dependent electrophoretic mobility curves and zeta potential curves. In the experiments, both the existence and non-existence of such a maximum were observed and demonstrated to be system dependent. The corresponding molecular structure of the oil-water interface was then discussed with the analyses of the zeta potential curves and second harmonic generation signals recorded at the hexadecane-water interface.
It is important to investigate the influence of surfactants on structures and physical/chemical properties of oil/water interfaces. This work reports a second harmonic generation study of the adsorption of malachite green (MG) on the surfaces of oil droplets in a hexadecane/water emulsion in the presence of surfactants including sodium dodecyl sulfate, polyoxyethylene-sorbitan monooleate (Tween80), and cetyltrimethyl ammonium bromide. It is revealed that surfactants with micromolar concentrations notably influence the adsorption of MG at the oil/water interface. Both competition adsorption and charge-charge interactions played very important roles in affecting the adsorption free energy and the surface density of MG at the oil/water interface. The sensitive detection of the changing oil/water interface with the adsorption of surfactants at such low concentrations provides more information for understanding the behavior of these surfactants at the oil/water interface.
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