The UNIFAC model and the partition of alkyl and alkylphenol ethoxylate surfactants in the excess phases of middle phase microemulsions

The partition of non-ionic surfactants between oil and water is an important parameter that influences the performance of surfactant-based separations, the fate of surfactants in the environment, and the absorption by surfactants in living organisms. From a thermodynamics point of view, the partition of surfactants between oil and water can be used as a method to quantify the hydrophilic–lipophilic nature of surfactants and oil. In this work, a group contribution model, the universal functional activity coefficient (UNIFAC) framework, has been modified to fit and predict the partition of alkylphenol, and alkyl ethoxylated surfactants between aqueous and oil phases. To perform such calculations the interaction parameters of an ethoxylate functional group were optimized, including the temperature dependence for functional groups known to form hydrogen bonds (CH2CH2OH2O, OHH2O). The optimization was performed by fitting this modified UNIFAC model to experimental partition coefficient of octylphenol ethoxylated surfactants in the excess phases (water and n-heptane) of middle phase microemulsions at 25 and 55 °C and to experimental critical micelle concentration (CMC) data. The modified UNIFAC is capable of predicting the partition of alkylphenol ethoxylates and alkyl ethoxylates with varying degrees of ethoxylation, tail length and temperature and different oils. The predicted partition values correlate with experimental bioconcentration factors (BCFs) reported for alkyl ethoxylates. The model also predicts properly the critical micelle concentration (CMC) of alkyl and alkylphenol ethoxylate surfactants. The predicted partitions also helped estimate the equivalent alkane carbon numbers (EACNs) of various oils.