| Abstract
| - The relationship between the interaction between the same two surfactants in a mixed monolayerat the air/aqueous solution interface (βσ) and their interaction in a mixed micelle in the aqueousphase (βM) was investigated, in 0.1 M NaCl at 25 °C. All the interactions investigated were attractive(having negative β values) and had relatively stronger interaction in the mixed monolayer than in themixed micelle, except for mixtures of anionic surfactants and some polyoxyethylenated (POE) nonionicsurfactants, where relatively stronger interaction occurs in the mixed micelle. It is shown that therelative strengths of the interactions at the interface and in the micelle in the anionic−POE nonionicmixtures are determined by the branching and bulkiness of the hydrophobic and hydrophilic groups ofthe 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 theanionic surfactant in the mixture reduces the interactions both in the mixed micelle (βM) and in themixed monolayer (βσ), with the effect on the interaction in the mixed micelle (βM) greater than that in themixed monolayer (βσ). When both the anionic and the nonionic surfactants have branched hydrophobicgroups and the anionic surfactant, in addition, has branching near the hydrophilic group, interactionsin both the mixed monolayer and the mixed micelle are weak. Interaction in the anionic−POE nonionicmixed 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.
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