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The surface adsorption and micelle formation of aqueous solutions of C12H25(OC2H4)jSO4NaC10H21SOCH3 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 C12H25SO4NaC10H21SOCH3 system (I), and a minimum c.m.c. in the C12H25OC2H4SO4NaC10H21SOCH3 system (II) (no ΓT.cmc maximum), but neither the ΓT.cmc maximum nor the c.m.c. minimum occurs in the C12H25(OC2H4)3SO4NaC10H21SOCH3 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.

The surface adsorption and micelle formation of the aqueous solutions of alkyl (C8 and C10) methylsulfoxide [A]-ionic surfactant (C7F15COONa [I], C10H21SO4Na [II] or C10H21N(CH3)3Br [III]) mixtures were investigated by the surface tension method. It was found that there is significant synergism in the surface activity of the A-I and II systems, but not in that of the A-III system. This is shown by the fact that we can obtain the γCMC and CMC minima in the A-I system and the CMC minimum in the A-II system, but neither of these in the A-III system; furthermore, the negative values of βσ and βm are much larger in the A-I and II systems than in the A-III system.