Interactions between two different surfactants in mixed monolayers at various interfaces and in mixed micelles

The relationship between the interaction between the same two surfactants in a mixed monolayer at the air/aqueous solution interface (β σ) and their interaction in a mixed micelle in the aqueous phase (βM) was investigated. It is shown that the relative strengths of the interactions at the interface and in the micelle in the anionic-polyoxyethylene (POE) nonionic surfactant mixtures are determined by the branching and bulkiness of the hydrophobic and hydrophilic groups of the surfactants in the mixture. Interaction in the mixed micelle is reduced by branching in the hydrophobic group in either surfactant of the mixture. Branching close to the hydrophilic group of the anionic surfactant in the mixture reduces the interactions both in the mixed micelle (βM) and in the mixed monolayer. Interaction in the anionic-POE nonionic mixed micelle is sharply enhanced by an increase of the oxyethylene units in the POE group of the nonionic surfactant of the mixture.; Regular solution theory has been used to explain the values of surfactant molecular interaction (β) parameters observed in mixed monolayers and mixed micelles. Reduction in electrostatic self-repulsion interaction energy of the ionic surfactant, due to the dilution effect upon mixing, is suggested as a major contributor to the negative β values observed for mixed monolayers and mixed micelles. Steric effects appear when surfactant molecular structure varies in the size of the head groups and in the branching of the hydrophobic groups. Bulkiness due to branching in the hydrophobic group of the ionic surfactant decreases electrostatic self-repulsion before mixing with a nonionic surfactant having a linear hydrophobic group. Branching in the hydrophobic group of a POE nonionic surfactant results in slightly more negative βσ values upon mixing with an anionic surfactant with a linear hydrophobic group, than with the corresponding linear hydrophobic chain nonionic surfactant.; Molecular interactions at the solid/aqueous solution interface were also investigated. It is suggested that the observed spreading enhancement from mixed solutions on hydrophobic polyethylene film resulted from the stronger attractive interactions at the polyethylene/aqueous solution interface, as indicated by the negative $bsSL$ values, than that at the air/aqueous solution interface, as indicated by the small negative $bsLA$ values. Lower dynamic contact angles of the mixture than either of the components on hydrophobic polyethylene film, caused by adsorption enhancement and faster adsorption at the solid/aqueous solution interface after mixing, imply spreading enhancement for the mixture.