Electric charge on small silicone oil droplets dispersed in ionic surfactant solutions

Abstract

The effect of the type and the concentration of ionic surfactants on the electric charge on small silicone oil droplets is studied by using an electrical suspension method. The adsorption process of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), is characterized by gradual neutralization of the negative charge on the oil-in-water (O/W) emulsion droplets at low surfactant concentrations. As the surfactant concentration increases, the adsorption process leads to the charge reversal. The point of charge reversal (pcr) occurs approximately at 2.5×10 −6 M CTAB. Further increasing the surfactant concentration causes an increase in the positive charge and eventually results in the maximum positive charge on the oil droplets near the critical micelle concentration (CMC) of CTAB. The limited charge reflects the saturation of the adsorption of surfactant molecules on the oil–water interface. In an anionic surfactant solution of sodium dodecyl sulphate (SDS), however, the silicone oil droplets are all negatively charged and the negative charge increases with increasing the surfactant concentration. The saturation state is also observed for the anionic surfactant adsorption. The maximum negative charge is achieved when SDS concentration approaches its CMC. In addition, two types of ionic adsorption onto the oil–water interface may occur for NaCl electrolyte solutions containing either CTAB or SDS: the surfactant adsorption and the electrolyte adsorption, although the former is much stronger than the latter. At constant surfactant concentration, their cooperation strengthens the charge at lower NaCl concentrations and leads to the strongest charge at some concentration. However, their competition at higher NaCl concentrations reduces the charge on the oil droplets.