Synergism between non-ionic and cationic surfactants in a concentration range of mixed monolayers at an air-water interface

The surface properties and adsorption behavior of mixed binary surfactants containing alkylpyridinium chloride (CnPC, n = 14 and 16) and Triton X-100 (TX100) have been studied at 298.15 K. The values of the molecular interaction parameter (beta(s)) and the mole fraction of components (X-i(s)) at the air-water interface in the pre-micellar region were calculated on the basis of Rosen's model. In the new approach, we proposed to estimate surface parameters (beta(s), X-i(s), Gamma(tot), A(tot), Gamma(TX100) and Gamma(CnPC)) in a pre-micellar region, using different fixed values of the surface tension instead of a single-value of surface tension. Then, the influence of surface tension on the surface properties was investigated. The interaction parameter (beta(s)) is negative at all compositions showing a synergistic effect between the components. The beta(s) was found to considerably decrease (less negative values) with decreasing surface tension due to more migration of TX100 to the surface layer and a reduction in electrostatic self-repulsion between head groups in the ionic surfactant. Also, the total surface area (A(tot)) at the air-water interface decreases with increasing X-1(s) (mole fraction of TX100 in mixed monolayer) or decreasing surface tension due to the dilution effect on mixing.