Surface Properties and Micellar Molecular Interaction in Binary Systems of a Biosurfactant Sodium Deoxycholate (NaDC) with Conventional Surfactants

Surface tension for the binary mixtures of a biosurfactant sodium deoxycholate (NaDC) with three different conventional surfactants (p-(1,1,3,3-tetramethylbutyl)phenol polyoxyethylene, polyoxethylene-10-oleyl ether, and sodium oleate) were measured in buffer solutions at different temperatures. Surface properties were determined as well as the micellar characteristics like composition, micellar molecular interaction parameter (??), activity coefficients (f ₁, f ₂), and the free energy of micellization were analyzed using some theoretical models, including Clint, Rubingh, Motomura and Maeda models. The results reveal that all the three investigated systems show non-ideality in the micelle formation by the deviation in the experimental critical micelle concentration (CMC), f ₁, f ₂, and negative ?? values from the ideal values. The proportion of NaDC in the mixed micelle (X ₁) calculated by all models increases with increasing the molar fraction of NaDC in solution (?? _NaDC), but the X ₁ values are very small even at the high ?? _NaDC. Moreover, by using the Maeda theory, the results suggest the chain-chain interaction among surfactants does not seem to be strong. It is noted that two break points were observed in the surface tension plots of pure NaOL and NaDC/NaOL mixture solutions. Interestingly, by introducing moderate NaDC or decreasing the temperature, the first break point disappeared for the two systems.