Non‐linear changes in phase inversion temperature for oil and water emulsions of nonionic surfactant mixtures

Hydrophilic–lipophilic deviation (HLD) is a semiempirical framework for surfactant‐oil–water (SOW) formulation that has effectively assimilated SOW properties such as phase inversion temperature (PIT) and Winsor Type I ↔ II ↔ III ↔ IV microemulsion behavior. The HLD system's reliance on surfactant and oil parameters necessitates their experimental determination when relevant and accurate values are not available. In this study, experiments have been of the type commonly used to generate test surfactant HLD parameters based on perturbation of a well‐defined SOW reference system and assumption of linear mixing behavior for PIT versus component mole fractions. The reference SOW compositions comprised n‐octane, 0.01 M aqueous NaCl, and pure n‐decyl tetraethylene glycol ether (C10E4) reference surfactant. Test surfactants were primary alcohol ethoxylates and nonylphenol ethoxylates comprising disperse PEG (polyethylene glycol) chain lengths that are inherent to commercial production of nonionic ethoxylate surfactants. We observe non‐linear behavior of PIT versus test surfactant mole fraction, even when accounting for surfactant concentration effects, so the desired linear correlation that could otherwise be used to assign an HLD parameter has not been obtained. Since an objective of this study has been to improve the predictive utility of the HLD framework for commercial nonionic surfactants, relevant terms in the HLD equation have been assessed in terms of their potential contributions to non‐linear behavior. One current working‐explanation relates substantial non‐linear PIT contribution to oil‐like surfactant components in disperse commercial ethoxylates.