By means of surface tension measurement (Wilhelmy method), micellization and adsorbed film formation were investigated for three combinations of mixed surfactant systems: a typical anionic surfactant, sodium dodecylsulfate (SDS) was combined with three nonionic surfactants used as membrane-protein solubilizers,
n-alkyl (octyl, nonyl and decyl)-
N-methylglucamides (MEGA-8, MEGA-9 and MEGA-10, respectively) in water at 30°C. The data of surface tension (
γ) versus logarithmic total molality of both single and mixed surfactant systems (
mt) plots as a function of mole fraction of surfactant 2 (2 corresponds to MEGA-n’s),
X2, enabled us to determine critical micellization concentration (CMC), minimum surface tension at CMC (
γCMC), surface excess (
Γt), mean surface area occupied by a molecule (
Am) and parameters related to synergism in surface activity such as pC
20 and CMC/C
20. Based on the regular solution theory, the relation of compositions of the singly dispersed phase (
X2) and the composition of the adsorbed film phase (
Z2) were estimated, and then the interaction parameters in micelles (
ωR) and in adsorbed film (
ωA) were also calculated. Both the CMC-
X2 and CMC-
Y2 curves were found for all combinations to show a negative deviation from ideal mixing, and even the curve of
mt (bulk phase concentration) versus
Z2 produced a prominent negative
ωA. A marked synergism in surface tension reduction was observed for these MEGA-n’s mixtures with SDS; the extent of the synergism increased in the order of MEGA-8, -9 and -10. A positive synergism was observed, even from the partial molecular area (PMA) and the minimum free energy at surface
G(S)min= (
γCMC·
Am·
L) where,
L is the Avogdro’s number.
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