Surface properties, micellar molecular interaction, and physical properties for binary systems of sodium oleate with three anionic surfactants

The mixed micellization of the anionic surfactant sodium oleate (NaOL) and three anionic surfactants (NaDC, AOT, SDS) of different structures in 0.01 mol/L NaCl solution was investigated by surface tension measurement at different temperatures. All the investigated mixtures exhibited nonideal behaviors as the experimental critical micelle concentration (CMC) values were less than the values calculated by using the Clint’s equation. Negative βm values calculated by using Rubingh’s equation and |ln(CMC1/CMC2)| < |βm| proved synergism in the three systems of higher αNaOL or lower αNaOL. An interesting phenomenon of double inflection points was observed for pure NaOL and mixtures of high αNaOL. The negative ΔG0m values for the mixed systems imply that the micelle formations are spontaneous process. The NaOL/NaDC system was also studied in buffer solution (pH 8, 12) at 30°C which also exhibited nonideal behavior. Synergism existed at pH 12 but not at pH 8 for the NaOL/NaDC system. It is noteworthy that there were no double inflection points for NaOL/NaDC system in buffer solution (pH 8, 12). Krafft point and calcium stability data show that these three anionic surfactants can improve the water solubility and hard-water resistance of soap.