EFFECT OF POLYOXYETHYLENE CHAIN-LENGTH ON THE INTERACTIONS BETWEEN ANIONIC AND NONIONIC SURFACTANTS .1. SODIUM DODECYLPOLYOXYETHYLENESULFATE DECYLMETHYLSULFOXIDE SYSTEMS

The surface adsorption and micelle formation of aqueous solutions of C12H25(OC2H4)jSO4NaC10H21SOCH3 systems (j=0, 1, 3) have been investigated by the surface tension method. It has been shown that there are strong molecular interactions between the two surfactant components in the surface layers and the mixed micelles. The interactions become weaker as the length of the oxyethylene (EO) chain increases. This is illustrated by the fact that there exist a maximum surface adsorption (ΓT.cmc) and a minimum c.m.c. in the C12H25SO4NaC10H21SOCH3 system (I), and a minimum c.m.c. in the C12H25OC2H4SO4NaC10H21SOCH3 system (II) (no ΓT.cmc maximum), but neither the ΓT.cmc maximum nor the c.m.c. minimum occurs in the C12H25(OC2H4)3SO4NaC10H21SOCH3 system (III). Furthermore, the absolute values of molecular interaction parameters in the micelles (βm) and in the surface layers (βs) of the three systems are in the following order: (I) ≫ (II) ≫ (III). Explanations for these findings are given. © 1990.