Synergistic Interaction between Nonionic Octylphenol Polyoxyethylene Ethers and Effect of Hydrophilic Chain

The micellization of binary mixture composed of nonionic octylphenol polyoxyethylene ether (OP-n, n is the average number of oxyethylene unit) and its homologues, as well as the effect of hydrophilic chain were investigated. The tensiometry was adopted to determine the critical micelle concentration of binary surfactant mixture. According to the regular solution theory, some thermodynamic models were used to predict the parameters of micellization and the thermodynamic parameters. For all the three binary mixtures of nonionic/nonionic surfactants, each of which shows a nonideal mixing and can behave synergistically in aqueous solution. For both the mixture of OP-10/OP-7 and the mixture of OP-10/OP-4, the positive deviation of the mole fraction of surfactant in mixed micelle from its ideal value coexists with the negative deviation in the entire range of compositions in aqueous solution, while for the mixture of OP-7/OP-4, it only shows a positive deviation. Thermodynamic data indicate that for all the three binary surfactant mixtures, the process of micellization is entropically spontaneous, and the change of hydrophilic chain results in the difference in the stability of mixed micelle. The results can be explained by the steric effect, the hydrogen bonding and the hydration. The work helps with understanding the interaction between molecules and properties for some industrial products constituted by nonionic surfactant and its homologues.