Phase and rheological behavior of aqueous mixtures of an isopropoxylated surfactant

Abstract Establishing the phase and rheological behavior of a surfactant is a critical step for assessing its potential efficacy for a range of applications, such as detergency and enhanced oil recovery (EOR). Most efforts have applied the full suite of fundamental characterization techniques to simple model surfactant systems that are often too costly to be utilized in commodity applications. Conversely, we evaluated here the behavior of aqueous mixtures of a commercial isopropoxylated anionic surfactant, and we analyzed the supramolecular structures of the surfactant aqueous mixtures using a combination of microscopy, light scattering, and x-ray scattering techniques. A phase map was obtained at 25 °C across the full range of surfactant in water concentrations, in water and in one salt concentration (9700 ppm). Specifically, the phases identified were a hexagonal liquid crystalline phase (H 1 ) and a lamellar liquid crystalline phase (L α ). The structure of the micellar solution (L 1 ) was determined by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), conductimetry, densitometry, and x-ray scattering. The data acquired from these techniques provided information about the micellar sizes and their aggregation numbers. As surfactant concentration increased, the H 1 and L α phases were observed, and the rheological behavior of these solutions was characterized. In addition, the effect of increased ionic strength on the phase behavior was quantified for both micellar solutions and liquid crystalline phases. As the ionic concentration increased from 0 to 9700 ppm (ionic strength of 150 mM), the trend of observed phases remained the same, but the structural parameters and domain sizes were changed slightly. Importantly, despite the multicomponent nature of the surfactant, the phase behavior of the aqueous surfactant mixtures and their rheological behavior followed the same trends as those observed for common single-component, single-chain, anionic surfactants. Thus, this effort points towards a potential universality of the trends of phase behavior of surfactant-water systems, even for complex commercial surfactant mixtures. Therefore, this could be of significant utility when surfactant formulations are considered for EOR and other applications.