Modelling the partitioning equilibria of nonionic surfactant mixtures within the HLD framework

Commercial non-ionic surfactants are usually blends of species differing in the degree of ethoxylation, characterised by a distribution of surfactants with different affinity for the oil and the aqueous phases. This leads to an unexpected concentration dependence of the phase behaviour and the rationalization of such deviations is necessary for their exploitation in surfactant-based formulations. In this work, the phase behaviour of commercial Novel® nonionic surfactants in presence of brine and oil was studied fixing equal volumes of organic and aqueous phases and changing the amount of surfactant and brine salinity (fish cut). The conditions for the balanced state (microemulsion solubilizing equals amount of water and oil) have been determined for different alkanes. The phase behaviour of the surfactant mixture was described, without a priori information about its actual chemical composition, in terms of two pseudo-components: one hydrophilic, entirely at the interface, and the other lipophilic, able to partition itself between the interface and the oil bulk according to a partition constant P. The pseudo-components effect on the interface is quantified through surfactant characteristic parameters (SCPs), according to the Hydrophilic-Lipophilic-Difference (HLD) model. SCPs and P can be experimentally determined by fitting the dependence of the balanced state on the surfactant concentration. The model is successfully applied to the tested surfactants and can quantitively predict the behaviour of their mixtures.