(50h) Phase and Rheological Behavior of Aqueous Mixtures of an Isopropoxylated Surfactant

Establishing the phase and rheological behavior of a surfactant with water or other liquids 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 a full suite of fundamental characterization techniques to systems of simple model surfactant that are often too costly to be utilized in large-scale applications. For this reason, we have evaluated aqueous mixtures of a commercial isopropoxylated extended anionic surfactant, for which few results have been reported in the literature. This surfactant has a hydrophobic group that has a hydrocarbon part and an isopropoxylated part, and one sodium sulfate polar group, and is a viscous and isotropic liquid at room temperature. We have analyzed and report here the supramolecular structures of several 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 concentrations, in water and in one brine (with 9,700 ppm of total concentration of several salts, and an ionic strength of 150 mM; see Chung, J.; Boudouris, W.B.; and Franses, E.I Colloids and Surfaces A: Physicochemical and Engineering Aspects 2018, 537, 163-172). Above the critical micelle concentration (CMC), which is very low for water, ca 12 ppm, and even lower in the brine, the average size and aggregation number of the micelles were estimated from dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), conductimetry, and x-ray scattering data. Densitometry data were used also to estimate the partial densities of the micelles in solution. As the surfactant concentration increased beyond the solubility, which was quite high, in the range of 20-25 wt %, (Chung et al. 2018), first a hexagonal liquid crystalline phase (H₁) and then a lamellar liquid crystalline phase (L_α) were observed both with water and with brine. At even higher concentrations, a surfactant-rich liquid phase was observed. The rheological behavior of the micellar solutions and the liquid crystalline phases were characterized by dynamic amplitude and dynamic frequency sweep tests. The micellar solutions were found to behave as Newtonian fluids. The liquid crystalline phases displayed a range of non-Newtonian and viscoelastic behavior. The effect of the higher ionic strength on the phase behavior was quantified for the micellar solutions and the liquid crystalline phases. With this brine as with water, the trend of the observed phases as the surfactant concentration increased remained the same, while the structural parameters and the domain sizes in the liquid crystals changed only slightly. Importantly, despite the complex 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-non-extended-chain, anionic surfactants. This effort points towards a potential universality of the phase behavior trends of systems of extended or non-extended surfactants with water or brine. Therefore, this could be of significant utility when surfactant formulations are considered for EOR and other applications.