Experimental and Theoretical Studies of Cationic Gemini Surfactant and Anionic Sodium Lauryl Ether Sulfate

Abstract: The mixed micellization of dimeric 1,2-bis(dodecyldimethyl ammonium)ethane dibromide (CGSI) and sodium lauryl ether sulfate (SLES) has been investigated. The surface tension was measured as a function of the concentration of the surfactants in aqueous solutions at 25 and $35^{\circ}C$ using a surface tensiometry and electrical conductometry apparatus. From these measurements, the critical micelle concentration (CMC), the thermodynamic parameters ($X_{i}$, $\gamma_{i}$, $a_{i}^{M}$, $\DeltaH_{mic}$), and the interfacial parameters ($\gamma_{CMC}$, $pC_{20}$, $\Gamma_{max}$, $-d_{\gamma}/dlogC$, $A_{min}$, $CMC/C_{20}$) were calculated and analyzed by means of equations derived from the nonideal mixed micelle model. Structural effects on the adsorption and micellization and the effectiveness of surface tension reduction are discussed in terms of these parameters. The kinds and degrees of interactions between the different surfactant molecules are explained on the basis of a regular solution model. The interaction parameter, $\beta$, was calculated to be -4.38 for the SLES/CGSI system at $25^{\circ}C$ and -4.72 at $35^{\circ}C$. These negative values indicate a reduction in the free energy of micellization over that predicted by the ideal solution theory, and reflect the synergistic behavior of the components (SLES and CGSI) in the mixed micelle. The behavior of this mixed surfactant system at interfaces and in dilute solution (viscosity, solubilization, and foaming power) is also reviewed.