Micellar interaction of binary mixtures of alpha olefin sulfonate and nonylphenol polyethylene glycol ethers: Length effects of ethylene oxide

The micellization behavior of binary surfactant mixtures constituted by an anionic surfactant, alpha olefin sulfonate (AOS), and a nonionic surfactant nonylphenol polyethylene glycol ether (NPE) with different numbers of ethylene oxide (EO), namely, NP10EO, NP20EO, NP30EO, and NP40EO was comprehensively investigated by surface tensiometer, conductometer, cloud point measurement, and dynamic light scattering. Theoretical treatments were carried out to explain molecular interaction in the mixed micelles based on regular solution theory of Rubingh, micellization thermodynamics of Molyneux et al. and Maeda, and molecular thermodynamic theory of Pavvada and Blankschtein. Results indicate non-ideal mixing behavior in all AOS-NPE mixtures, where nonionic surfactant EO chain length was found to play critical roles. In the absence of additional electrolytes, NPEs exhibited substantially higher activity in micelles than bulk solution; with growth of EO groups, shrinkage on the scale of synergistic interaction was evidenced. In contrary, with swamping amount of electrolytes, synergistic interactions enlarged with the rise of EO groups, and AOS activity in mixed micelles was found depending on both EO length and bulk mole fraction (alpha(A)). These findings are of great significance in mixed surfactant formulation design/optimization to maximize the synergistic efficiency of the system thus to minimize the chemical consumption and cost.