Rheology as a tool for identifying and characterizing optimal microemulsions formulations

Abstract

This study investigates the use of rheology to detect phase inversion in surfactant–oil–water (SOW) systems, offering a rapid method for identifying “optimal formulations.” Phase inversion through temperature or salinity variation provides a faster alternative compared with equilibrium scans. Using well-defined polyethoxylated surfactants (C₈EO₃, C₁₀EO₄, C₁₂EO₅), phase inversion was monitored through viscosity measurements at a constant shear rate, with temperature as formulation variable. Emulsion viscosity reaches a minimum at the phase inversion point, which corresponds to an ultra-low interfacial tension condition. A strong correlation between the reported fish-tail temperature (T*) and the phase inversion temperature (PIT) was observed. While identifying optimal conditions through a formulation scan in a series of test tubes is relatively quick, evaluating the surfactant system's ability to reduce interfacial tension can take several weeks due to the requirement of equilibrium. Formulation conditions at which minimal emulsion viscosity occurs are related to those where three-phase systems are obtained, with the magnitude of interfacial tension inversely proportional to this value. An empirical approach linking the emulsion destabilization zone with the interfacial tensions is proposed. By measuring this interval, it is possible to roughly predict interfacial tension for model systems.