PEOPLE

RESEARCH

December 2019

Reversible double water in oil in water (W/O/W) emulsions were developed to contain subsurface hydrocarbon spills during their remediation using surfactant flushing. Double emulsions were prepared by emulsifying CaCl2 solutions in canola oil, and subsequently by emulsifying the W/O emulsions in aqueous sodium alginate solutions. The formation of double emulsions was confirmed with confocal and optical microscopy. The double emulsions reversed and gelled when mixed with the surfactants sodium dodecyl sulfate (SDS) and cocamidopropyl betaine (CPB). Gels can act as ‘emulsion locks’ to prevent spreading of the hydrocarbon plume from the areas treated with surfactant flushing, as shown in sand column tests. Shear rheology was used to quantify the viscoelastic moduli increase (gelation) upon mixing the double emulsion with SDS and CPB. SDS was more effective than CPB in gelling the double emulsions. CPB and SDS could adsorb at the interface between water and model hydrocarbons (toluene and motor oil), lowering the interfacial tension and rigidifying the interface (as shown with a Langmuir trough). Bottle tests and optical microscopy showed that SDS and CPB produced W/O and O/W emulsions, with either toluene or motor oil and water. The emulsification of motor oil and toluene in water with SDS and CPB facilitated their flow through sand columns and their recovery. Toluene recovery from sand columns was quantitated using Gas-Chromatography Mass-Spectroscopy (GC-MS). The data show that SDS and CPB can be used both for surfactant flushing and to trigger the gelation of ‘emulsion locks’. Ethanol also gelled the emulsions at 100 mL/L. (K. Lamont, A. Marangoni, and E. Pensini. "‘Emulsion locks’ for the containment of hydrocarbons during surfactant flushing." Journal of Environmental Sciences (2019).)

November 2019

Switchable double emulsions (water in oil in water, W/O/W) are proposed for the in situ immobilization of subsurface organic contaminants such as toluene, hexane or benzene. Primary W/O emulsions were prepared by emulsifying 250 mL of 0.36 M CaCl2 aqueous solutions in 1 L of canola oil (with 12.5 g/L of ethylcellulose, EC, and 2.5 g/L of calcium stearate). In the primary W/O emulsion the water droplets in oil were ≈8 μm, as observed using an optical and a confocal microscope. EC and calcium stearate adsorbed at the oil water interface (as demonstrated by interfacial tension measurements), forming films which stabilized the W/O emulsions (as verified with bottle tests). Experiments conducted using a Langmuir trough suggest that EC and calcium stearate films did not desorb from the oil-water interface upon compression. Crumpling tests and optical microscopy observations indicate that EC and calcium stearate films were skin-like, and buckled when deformed. To obtain double W/O/W emulsions the primary emulsions were emulsified in a 0.75 wt% solution of sodium alginate, with 2 mL/L of Tween 20 and 10 g/L of NaCl. The formation of W/O/W emulsions was verified through optical microscopy and confocal microscopy observations. In the absence of the contaminants the double emulsions were stable, as observed by resting them on the bench over three days and agitating them with a multi-action wrist shaker for 30 min. Also, they had low shear elastic (G′ = 2.67 ± 0.58 Pa) and viscous (G″ = 1.69 ± 0.24 Pa) moduli, which should facilitate their transport through geological media (e.g. soil) to polluted areas. Upon mixing with toluene, hexane or benzene at concentrations ranging from 5% to 17%, the double emulsions were destabilized. Emulsion destabilization caused the release of CaCl2, which crosslinked sodium alginate and formed gels in which the contaminants were incorporated. The gelation rate and the magnitude of the viscoelastic moduli depended on the contaminant type and concentration, and on the mixing time. Gelation occurred fastest with the highest toluene concentrations tested (9% to 17%), but the highest elastic moduli were measured with 9% toluene concentrations for the longest mixing times tested (90 s). Gelation occurred slowest with hexane, likely due to the poor solubility of EC in hexane. Because of their ability to gel exclusively in contaminant proximity, the double emulsions studied offer a potential strategy to control the migration of plumes of contaminants such as toluene, hexane or benzene. (Lamont, Kristine, Erica Pensini, and Alejandro Marangoni. "Gelation on Demand Using Switchable Double Emulsions: A Potential Strategy for the In Situ Immobilization of Organic Contaminants." Journal of Colloid and Interface Science (2019))

August 2019

Natural sorbents for the removal of a free toluene phase from water were made using lecithin, food-grade oils (canola, sunflower, safflower and corn oil) and water, or lecithin, hydroxystearic acid (HSA) and canola oil, or lecithin, HSA and soy wax. Lecithin (5 g), food-grade oils (5 mL), and water (5 mL) formed emulsions, in which clusters of water droplets were dispersed in oil, as probed using confocal microscopy and cryo-scanning electron microscopy (cryo-SEM). These emulsions were gel-like, with shear elastic moduli (G’) greater than the shear viscous moduli (G”). Emulsion gels obtained with lecithin, canola oil and water could absorb up to 47% (volume based) of toluene freely floating on deionised water in 20 h. G’ increased from 1621 ± 203 Pa to 6372 ± 168 Pa upon mixing with up to 20% of toluene (volume based), and decreased to 2130 ± 376 Pa and to 846 ± 60 Pa with 33% and 47% toluene (volume based), respectively. However, the gels remained cohesive enough to be recovered from water even with 47% toluene, facilitating it removal. The gels lost instead cohesiveness with 67% of toluene. Similar trends were observed with all other food-grade oils used and in the presence of CaCl2 salt. With 35 g/L NaCl and 33% toluene gels less cohesive than with DI or CaCl2 salt. Optical microscopy showed that lecithin formed thick, heterogeneous films at the oil-water interface in the absence of toluene. Toluene addition to lecithin in canola decreased the interfacial tension (as probed with the pendant drop method), rendered the lecithin interfacial films homogeneous and improved the miscibility between the oil and the water phase, as observed through confocal microscopy. Toluene addition also affected the lamellar swelling of lecithin bilayers, which were studied using Wide and Small Angle X-ray Scattering (WAXS and SAXS). Addition of HSA or HSA and soy wax to the emulsion gels increased their shear viscoelastic moduli before and after mixing with toluene. Gels comprised of HSA (0.5 g), lecithin (4.5 g), canola oil (2.5 mL) and water (5 mL) or of HSA (0.5 g), lecithin (3 g), soy wax (1.5 g) and water (5 mL) could absorb approximately 10 mL of toluene (i.e. 67% of the lecithin gel volume) in 20 h. Addition of HSA also increased the cohesiveness of gels with toluene and 35 g/L NaCl. (Safieh, Peter, Erica Pensini, Alejandro Marangoni, Kristine Lamont, Saeed Mirzaee Ghazani, Nukhalu Callaghan-Patrachar, Michaela Strüder-Kypke, Fernanda Peyronel, Jay Chen, and Braulio Macias Rodriguez. "Natural emulsion gels and lecithin-based sorbents: A potential treatment method for organic spills on surface waters." Colloids and Surfaces A: Physicochemical and Engineering Aspects 574 (2019): 245-259)