Effect of anionic surfactants on the stability ratio and electrophoretic mobility of colloidal hematite particles

Abstract

The effect of anionic surfactants on the charging and aggregation of hematite particles was investigated. Charging behavior was studied by the measurement and analysis of the electrophoretic mobility. The stability ratio of hematite suspension was obtained from the temporal change of turbidity, and the stability ratio was analyzed by the DLVO theory. These experiments were performed as functions of types of surfactant, surfactant dosage, and surfactant chain length. Electrophoretic mobility of bare hematite was positive at pH below 7. The mobility went through zero and became negative with increasing surfactant dosage. Required surfactant concentrations reaching the isoelectric point decreased with increasing the chain length of surfactant. These results are reasonably described by a simple theoretical model. The stability ratio showed the minimum around the isoelectric point. That is, the suspension is destabilized by charge neutralization due to the adsorption of surfactants onto the hematite. Values of the minimum stability ratio in this work were almost unity irrespective of the chain length; the fastest aggregation rate with surfactants equaled the salt-induced rate. Therefore, in the case of ionic-surfactant, charge-patch attraction and steric hindrance do not significantly work around zero mobility concentration. The DLVO theory qualitatively describes the aggregation behavior. The range of the minimum stability ratio was narrower for the suspension with fatty acids. This result indicates that the suspension with fatty acids is more stable.