Effect of electrolyte and temperature on interfacial tensions of alkylbenzene sulfonate solutions

The important role the hydrophilic-lipophilic ability of the surfactant played in interfacial tension (IFT) behavior has been recognized early. The thermodynamic model based on the surfactant affinity difference (SAD) concept has been developed and successfully employed to microemulsions. However, according to the theoretical models developed by Blankschtein et al., the molecular size of the surfactant is another important factor on IFT, which can be reflected by the cross-sectional area of the adsorbed surfactant molecules, a(s). In this paper, based on the concept of a(s), the influences of temperature and electrolyte on IFT were expounded. The experimental results show that ultralow IFTs can be obtained only when the surfactant has appropriate hydrophilic-lipophilic ability and a(s). The addition of electrolyte can enhance the hydrophobic ability of the surfactant, which improves the n(min) value. However, the IFTmin values pass through a minimum with increasing NaCl concentration, resulting from the responsible mechanisms of enhancement of adsorption amounts and reducing the size of the hydrophilic part during the low and high NaCl concentration range, respectively. An increase in the temperature appears to cause an increase in the surfactant solubility and a consequent decrease in the n(min) value. Moreover, the IFTmin increases with increasing temperature until a plateau because the molecular motion speeds up, which increases the solvent molecules in the adsorbed layer.