Abstract
Micelle formation in water and adsorbed film formation at air/water interface were studied by surface tension measurement for two mixed surfactant systems : combinations of sodium salts of α-sulfonatomyristic acid methyl ester (α-SMy·Me) and the propyl ester (α-SMy·Pr) with n-decanoyl-N-methyl glucamide (MEGA-10). The α-SMy·Pr/MEGA-10 and the α-SMy·Me/MEGA-10 mixed systems were found to synergistically enhance surface activity and form well-mixed micelles with the aid of strong interaction between head groups. Critical micellization concentration (CMC) as a function of mole fraction of MEGA-10 in the surfactant mixture (XMEGA-10) for both mixed systems was noted to deviate negatively from ideal mixing. Micellar phase (CMC vs YMEGA-10 relation) curves were simulated through use of the interaction parameter, ωR=-2.1 for α-SMy·Me/MEGA-10 and ωR=-2.0 for α-SMy·Pr/MEGA-10 and indicated the presence of azeotropes. Based on equations for estimating composition (Zi) and the interaction parameter (ωA) in adsorbed film equilibrated with monomers in bulk solution, phase diagrams were constructed that included the two relations of CMC vs XMEGA-10 and CMC vs ZMEGA-10. Adsorbed film of the respective mixed systems was formed through stronger interactions between anionic and nonionic surfactants compared to those of mixed micelles (-ωA>-ωR). The diagrams demonstrated that composition in micelles (Yi) differs from that in the adsorbed film (Zi). From the slope of surface tension (γ) vs logarithmic molality (ln mt) curve, just below CMC, surface excess (Γ) was determined and mean molecular area (Am) was computed as a function of XMEGA-10 or ZMEGA-10. From Am data, partial molecular area (PMA) of each component was determined as a function of ZMEGA-10; a large deviation from ideal mixing (the additivity rule) was observed for the respective mixed systems. Effects of differences in head group moiety, i.e., either methyl or propyl, were clearly evident with respect to hydrophobicity difference, but physicochemical behavior appeared essentially the same.
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