Investigation of the Synergism of Nonionic Surfactant Mixtures with Similar Structures and Their Relationship with Lyophobicity Using the Modified Extended Langmuir Model

This work focuses on evaluating the micellar and surface behavior of mixed systems composed of two structurally similar nonionic surfactants, BrijS10 and linear alcohol ethoxylates (7) (or LAE7), in aqueous solution at different mole fractions using surface tension measurements at 303.15 K. The critical micelle concentrations (CMC’s) were determined for pure components as well as mixed systems. The obtained surface tension data have been analyzed by Rosen and Rubingh models based on the regular solution theory (RST) and Clint’s theory for mixed binary systems. The observed negative values of interaction parameters (β m and β σ ) indicate synergistic interactions in the mixed micelle and in the mixed monolayer at all mole fractions of components. The average values of β m and β σ for this system are obtained −1.36 and −1.04, respectively. The Frumkin adsorption model was applied to evaluate the surface behavior of the pure surfactants, indicating weak interactions among the components and a higher surface concentration (Γ F ) for BrijS10 compared to LAE7. In the second part of this work, the Extended Langmuir model (EL), previously applied only to solvent mixtures, was modified and introduced as the Modified Extended Langmuir (MEL) model to quantitatively determine the lyophobicity (β) parameter of surfactants in aqueous solutions. The obtained results demonstrate that the β values of BrijS10 and LAE7 surfactants are several orders of magnitude higher than those of conventional hydrocarbon solvents in aqueous media, indicating their high lyophobic character. Moreover, the lyophobicity of the BrijS10/LAE7 mixed system was further enhanced due to the synergistic interactions between the surfactant components.