Micellization and adsorbed film formation of a binary mixed system of anionic/nonionic surfactants

Micelle formation in water and adsorbed film formation at the air/water int erface were investigated by surface tension measurement of a mixed surfacta nt system: the combination of sodium salt of alpha-sulfonatomyristic acid m ethyl ester (alpha-SMy.Me) with decanoyl-N-methylglucamide (MEGA-10). alpha -SMy.Me and MEGA-10 can form well-mixed micelles with the aid of a strong i nteraction between headgroups, and accordingly the critical micelle concent ration (cmc) as a function of mole fraction of MEGA-10 in the surfactant mi xture (X-MEGA10) deviates negatively from ideal mixing. The micellar phase curve (cmc-Y-MEGA10 relation) was simulated by using the interaction parame ter omega(R) = -2.1; the curve indicated the existence of an azeotrope form ed by a 3:2 mixture (at X-MEGA10 = Y-MEGA10 = 0.4) Further, we derived equa tions related to the composition in the adsorbed film (Z(i)) equilibrated w ith monomers in bulk solution and to the interaction parameter (W-A), and t hen constructed a phase diagram including two relations of cmc vs X-MEGA10 and cme vs Z(MEGA10). From the diagram an azeotrope was found to be formed by the 1:1 mixture (at X-MEGA10 = Z(MEGA10) = 0.5), suggesting that the com position in micelles (Y-i) differs from that in the adsorbed film (Z(i)). T he surface tension (gamma) vs logarithmic molality (ln m) curve at every 0. 1 increment in X-MEGA10 showed synergistically enhanced surface activity. F rom the slope of the gamma vs ln m curve just below cmc, the surface excess (Gamma) was determined and then the mean molecular area (A(m)) was calcula ted as a function of X-MEGA10 BY analysis of A(m) data, the partial molecul ar area (PMA) of each component was determined as a function of X-MEGA10; t his also showed a large deviation from ideal mixing (the additivity rule).