Surfactant-surfactant interactions in mixed monolayer and mixed micelle formation

The relationship between the interaction between the same two surfactants in a mixed monolayer at the air/aqueous solution interface (beta (sigma)) and their interaction in a mixed micelle in the aqueous phase (beta (M)) was investigated, in 0.1 M NaCl at 25 degreesC. All the interactions investigated were attractive (having negative B values) and had relatively stronger interaction in the mixed monolayer than in the mixed micelle, except for mixtures of anionic surfactants and some polyoxyethylenated (POE) nonionic surfactants, where relatively stronger: interaction occurs in the mixed micelle. It is shown that the relative strengths-of the interactions at the interface and in the micelle in the anionic-POE nonionic 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 (beta (M)) and in the mixed monolayer (beta (sigma)), with the effect on the interaction in the mixed micelle (beta (M)) greater than that in the mixed monolayer (beta (sigma)) When both the anionic and the nonionic surfactants have branched hydrophobic groups and the anionic surfactant, in addition, has branching near the hydrophilic group, interactions in both the mixed,monolayer and the mixed micelle are weak. 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 to a number sufficient to complex the Na+ of the anionic surfactant. Consequently, for synergy in surface tension reduction effectiveness to exist in anionic-POE nonionics, the number of oxyethylene units in the nonionic must be small.