Mixed micellization and mixed adsorbed film formation were investigated for the combination of a Gemini type cationic and a nonionic surfactants mixture: Bis-trimethyl ammonium Gemini derived from tartaric acid bromide (BAGTB) and n-Decanoyl-N-methylglucamide (MEGA-10). The surface tension of the aqueous mixed surfactant solution was measured at every 0.1 mole fraction of MEGA10 in the surfactant mixture applying a drop volume method at 30°C. From the curves of surface tension (γ) vs logarithmic concentration in molality (ln
m ), critical micellization concentration (CMC), minimum surface tension at CMC (γ
CMC), surface excess (Γ), mean surface area occupied by a molecule (
A m) and parameters related to synergism in surface activity such as pC
20 and CMC / C
20 were determined. Based on the regular solution theory, the relation of compositions of the singly dispersed phase (
X MEGA10) and the micellar phase (
Y MEGA10), and the relation of
X MEGA10 with the composition in adsorbed film phase (
Z 2) were estimated, and along with these, the interaction parameters in micelles (ω
R) and in adsorbed film (ω
A ) were calculated. Both the CMC-
X MEGA-10 and CMC-
Y MEGA-10 curves showed a negative deviation from ideal mixing and even the curve of
Z MEGA10-
m t (bulk phase concentration) produced a slightly negative ω
A . However, the synergism in surface tension reduction was found to be rather weak from examination of partial molecular area (PMA) and the minimum free energy at surface
G min(S) = (γ
CMC·
A m·
L ). As for the adsorbed film, the interaction mode between molecules, as well as two dimensional molecular packing, was observed to be separated into three regions; i. e. at
X STDS = 0.45 and at
X STDS = 0.75 different properties changed discontinuously.
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