THE EFFECTS OF OXYETHYLENE AND METHYLENE GROUPS ON THE INTERACTION BETWEEN ANIONIC AND CATIONIC SURFACTANTS

The surface tension of aqueous solutions of the 1 : 1 mixed anionic-cationic surfactant systems C8H17(OC2H4)nSO4Na-C8H17(OC2H4)3NC5H5Cl (C8EnS-C8E3Py) C8H17NC5H5Br-C8E3S (C8Py-C8E3S) CmH2m+1NC5H5Br-C8E3S (CmPy-C8E3S), where n=0, 1, 3 and m=8, 10, 12, 14, have been measured. From the surface tension-log concentration (gamma-log C) curves, the critical micelle concentration (CMC), the surface tension at the CMC, gamma(CMC), and the saturated surface adsorption of various systems were determined, and the molecular interaction parameter beta(m) and the mole fractions (x1m, x2m) of the two components in the mixed micelles were calculated. The results have shown that (1) the interaction between C8EnS and C8Py (or C8E3Py) becomes weaker as n increases, and (2) the interaction between C8E3S and C(m)Py becomes stronger as m increases. It is noted that the interaction between C8S and C8E3Py is significantly greater than that between C8E3S and C8Py. An explanation based on the electropositivity of the oxyethylene (EO) group is presented. The smaller interaction between components of the polyoxyethylenated anionic-cationic surfactants may be ascribed to the hydrophilicity and the steric hindrance effect of the EO group.