Expanded Salinity Window of Middle-Phase Microemulsions and Reduced Surfactant Adsorption by Hydrotrope.

Using surfactant blends to mobilize residual oil offers a promising technique for enhanced oil recovery (EOR) and surfactant-enhanced aquifer remediation (SEAR). A major financial setback for broader application of this method is the loss of surfactants, as they get absorbed onto reservoir mineral surfaces. This loss becomes even more costly in oil fields with high-salinity formation water. Our research delved into the use of hydrotropes to minimize the surfactant absorption. The impacts of surfactant adsorption with hydrotrope additives were quantified and compared to three representative porous media. Initial tests studied the ideal salinity range influenced by hydrotropes with the observations of Winsor Type III microemulsions with selected surfactants, and four specific hydrocarbons were confirmed through interfacial tension measurements. When tested on three types of porous media, the presence of hydrotropes reduced the adsorption rates: up to 65% on Indiana limestone, 21% on Ottawa sand, and 53% on activated carbon. Notably, our study revealed urea's role in reducing surfactant retention in porous media. This discovery can help modify the salinity range of middle-phase microemulsions, which is crucial for EOR by easing salinity constraints of target reservoirs. The large middle-phase microemulsion window is also very advantageous for other potential applications. Moreover, urea proves to be more effective than typical sacrificial agents for reservoirs, as it binds the surfactant to the liquid rather than acting as a mere sacrificial component. Our research underscores the potential of improving surfactant flooding results by integrating hydrotropes, offering substantial cost savings in surfactant consumption and enhancing the overall efficiency of EOR and SEAR projects.

Oil-Induced Viscoelasticity in Micellar Solutions of Alkoxy Sulfate.

Rheological properties of the solution of an extended surfactant, sodium alkoxy sulfate (C8-(PO)4-(EO)1-SO4Na), are investigated as a function of the presence of various paraffinic oils over a range of salt conditions in the Winsor III microemulsion region at oil fractions where the microemulsion is "oil-starved". The addition of as small as 3 vol % alkane to 2 wt % surfactant solutions at salt concentrations where the oil-water interfacial tension is minimized induces a sudden shift in the rheological behavior. The solution viscosity increases by 5 orders of magnitude, with solid-like behaviors (G' > G″) being observed in the entire frequency region investigated (0.01-100 rad/s). Commonly, in the cases where wormlike micelles are present in the solution, alkanes are believed to be solubilized in the core of micelles, leading to a radial growth of the cylindrical part of the wormlike micelle, resulting in a drop of end-cap energy (EC) and micelle length and a reduction in viscosity. In this study, however, the addition of oil causes the formation of wormlike micelles. The viscosity of solubilized-oil samples does, however, decrease with an increase in incorporated oil volume. We hypothesize that this "abnormal oleo-responsive" viscoelastic behavior is related to a spacer of intermediate hydrophilicity, that is, polypropylene oxide (PO) segment of the alkoxy sulfate, being inserted between the hydrophobic tail and hydrophilic head (the ethoxylated sulfate segment) of the extended surfactant. The addition of a small amount of oil likely extends the PO moiety and increases the tail length of the surfactant in the aggregates as well as reducing the headgroup size, driving the formation of wormlike micelles from a solution that initially had a viscosity consistent with the absence of such structures.

Counterion binding on coacervation of dioctyl sulfosuccinate in aqueous sodium chloride.

A simple coacervate-forming system consisting of sodium dioctyl sulfosuccinate (DOSS) in aqueous NaCl solution was investigated by turbidity measurement, electromotive force measurement (EMF), dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM) to reveal the role of counterion binding in the microstructural changes behind the evolution of the coacervate phase. Coacervation phase boundaries of DOSS against different NaCl concentrations were obtained; the pseudo-coacervation constant, Ksp,co = [DOSS-][Na+], was determined to be 1.35 ± 0.15 × 10-4 M2 at 25 °C. Sodium ion activity from EMF measurements confirmed a drastic rise in counterion binding to DOSS aggregates near the coacervate phase boundary. For DOSS/NaCl solution concentrations near the coacervate phase boundary, the turbidity changed, starting from a clear, isotropic solution far from the phase boundary, transitioning to a turbid solution near the phase boundary, and exhibiting a distinct growth of the hydrodynamic diameter of DOSS aggregates as detected by DLS. Cryo-TEM evidenced the presence of vesicles at concentrations close to the coacervate phase boundary; both unilamellar and multilamellar vesicles were observed. Increased counterion binding on the aggregates led to fusion of the larger vesicles and eventually to formation of a coacervate phase; the DOSS aggregates in the clear supernatant phase were predominately small vesicles of approximately 100 nm diameter. This study suggests that the mechanism for coacervate formation in DOSS solutions is an increase in counterion binding coincident with formation of multilamellar vesicles near the phase boundary, followed by flocculation of the multilamellar vesicles beyond the phase boundary to form the coacervate phase.