Equilibrium Surface Adsorption Behavior in Complex Anionic/Nonionic Surfactant Mixtures

Abstract

Neutron reflectivity (NR) and small angle neutron scattering (SANS) have been used to investigate the equilibrium\nsurface adsorption behavior and the solution microstructure of mixtures of the anionic surfactant sodium 6-dodecyl\nbenzene-4 sulfonate (SDBS) with the nonionic surfactants monododecyl octaethylene glycol (C₁₂EO₈) and monododecyl\ntriiscosaethylene glycol (C₁₂EO₂₃). In the SDBS/C₁₂EO₈ and SDBS/C₁₂EO₂₃ solutions, small globular mixed micelles\nare formed. However, the addition of Ca²⁺ ions to SDBS/C₁₂EO₈ results in a transition to a vesicle phase or a mixed\nvesicle/micellar phase for SDBS rich compositions. In contrast, this transition hardly exists for the SDBS/C₁₂EO₂₃\nmixture, and occurs only in a narrow composition region which is rich in SDBS. The adsorption of the SDBS/C₁₂EO₈\nmixture at the air−solution interface is in the form of a mixed monolayer, with a composition variation that is not\nconsistent with ideal mixing. In water and in the presence of NaCl, the nonideality can be broadly accounted for by\nregular solution theory (RST). At solution compositions rich in SDBS, the addition of Ca²⁺ ions results in the formation\nof multilayer structures at the interface. The composition range over which multilayer formation exists depends upon\nthe Ca²⁺ concentration added. In comparison, the addition of a simple monovalent electrolyte, NaCl, at the same ionic\nstrength does not have the same impact upon the adsorption, and the surface structure remains as a monolayer.\nCorrespondingly, in solution, the mixed surfactant aggregates remain as relatively small globular micelles. In the\npresence of Ca²⁺ counterions, the variation in surface composition with solution composition is not well described\nby RST over the entire composition range. Furthermore, the mixing behavior is not strongly correlated with variations\nin the solution microstructure, as observed in other related systems.