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 interface were investigated by surface tension measurement of a mixed surfactant system: the combination of sodium salt of α-sulfonatomyristic acid methyl ester (α-SMy·Me) with decanoyl- N -methylglucamide (MEGA-10). α-SMy·Me and MEGA-10 can form well-mixed micelles with the aid of a strong interaction between headgroups, and accordingly the critical micelle concentration (cmc) as a function of mole fraction of MEGA-10 in the surfactant mixture ( X MEGA10 ) deviates negatively from ideal mixing. The micellar phase curve (cmc− Y MEGA10 relation) was simulated by using the interaction parameter ω R = −2.1; the curve indicated the existence of an azeotrope formed by a 3:2 mixture (at X MEGA10 = Y MEGA10 = 0.4). Further, we derived equations related to the composition in the adsorbed film ( Z i ) equilibrated with monomers in bulk solution and to the interaction parameter ( W A ), and then constructed a phase diagram including two relations of cmc vs X MEGA10 and cmc 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 composition in micelles ( Y i ) differs from that in the adsorbed film ( Z i ). The surface tension (γ) vs logarithmic molality (ln m ) curve at every 0.1 increment in X MEGA10 showed synergistically enhanced surface activity. From the slope of the γ vs ln m curve just below cmc, the surface excess ( Γ ) was determined and then the mean molecular area ( A m ) was calculated as a function of X MEGA10 . By analysis of A m data, the partial molecular area (PMA) of each component was determined as a function of X MEGA10; this also showed a large deviation from ideal mixing (the additivity rule).