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

Abstract

<p>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 (<i>CMC</i>) values were less than the values calculated by using the Clint’s equation. Negative β^m values calculated by using Rubingh’s equation and |ln(<i>CMC</i>₁/<i>CMC</i>₂)| < |β^m| proved synergism in the three systems of higher α_NaOL or lower <i>α</i>_NaOL. An interesting phenomenon of double inflection points was observed for pure NaOL and mixtures of high <i>α</i>_NaOL. The negative Δ<i>G</i>⁰_m 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.</p