Probing the Mechanism of Targeted Delivery of Molecular Surfactants Loaded into Nanoparticles after Their Assembly at Oil-Water Interfaces.

A targeted and controlled delivery of molecular surfactants at oil-water interfaces using the directed assembly of nanoparticles, NPs, is reported. The mechanism of NP assembly at the interface and the release of molecular surfactants is followed by laser scanning confocal microscopy and surface force spectroscopy. The assembly of positively charged polystyrene NPs at the oil-water interface was facilitated by the introduction of carboxylic acid groups in the oil phase (e.g., by adding 1 wt % stearic acid to hexadecane to produce a model oil). The presence of positively charged NPs consistently lowers the stiffness of the water-oil interface. The effect is lessened, when the NPs are present in a solution of NaCl or deionized water at pH 2, consistent with a less dense monolayer of NPs at the interface in the last two systems. In addition, the NPs reduce the interfacial adhesion (i.e., the "stickiness" of the interface or, put differently, the pull-off force experienced by the atomic force microscopy (AFM) tip during retraction). After the assembly, the NPs can release a previously loaded cargo of surfactant molecules, which then facilitate the formation of a much finer oil-water emulsion. As a proof of concept, we demonstrate the release of octadecyl amine, ODA, that has been incorporated into the NPs prior to the assembly. The release of ODA causes the NPs to detach from the interface altering the interfacial properties and leads to finer oil droplets. This approach can be exploited in applications in several fields ranging from pharmaceutical and cosmetics to hydrocarbon recovery and oil-spill remediation, where a targeted and controlled release of surfactants is wanted.

Monitoring the Early Stages of Formation of Oil-Water Emulsions Using Flow Cytometry.

Characterization of complex oil emulsions is critical yet challenging both in science and in many industrial applications. Here we demonstrate for the first time the use of flow cytometry as a fast method for characterizing complex, polydisperse oil-water emulsions. Owing to our interest in understanding how the presence of specific ions might affect the properties of oil-water emulsions including size, polydispersity, and complexity, we present a systematic study of oil emulsions in deionized water and various brines of different ionic strength. Forward scatter (FSC) and side scatter (SSC) intensities associated with detailed statistics were judiciously combined to provide a better understanding of these complex systems. We find that the type and concentration profiles of ions around the oil droplets affect significantly the properties of the emulsion. Weakly hydrated cations NH4+ and Ca2+ appear to be more effective in screening the charge of oil droplets compared to the monovalent Na+ and divalent Mg2+ ions, respectively. As a result, coalescence and formation of larger droplets are seen in the case of NH4Cl and CaCl2 compared to NaCl and MgCl2, respectively. In addition, weakly hydrated anions such as Cl- can come closer to the oil surface and, thus, decrease the effective screening that the Na+ ions provide as compared to SO42- ions, which leads to more stable emulsions in NaCl compared to Na2SO4. In addition to these specific findings, the work demonstrates the utility of the technique as a new tool for characterizing oil emulsions in a wide spectrum of fields ranging from food to oil and gas applications.

Stimuli-Responsive, Hydrolyzable Poly(Vinyl Laurate-co-vinyl Acetate) Nanoparticle Platform for In Situ Release of Surfactants.

A stimuli-responsive, sub-100 nm nanoparticle (NP) platform with a hydrolyzable ester side chain for in situ generation of surfactants is demonstrated. The NPs were synthesized via copolymerization of vinyl-laurate and vinyl-acetate [p-(VL-co-VA), 3:1 molar ratio] and stabilized with a protective poly(ethylene-glycol) shell. The NPs are ∼55 nm in diameter with a zeta potential of -54 mV. Hydrolysis kinetics in an accelerated, base-catalyzed reaction show release of about 11 and 30% of the available surfactant at 25 and 80 °C, respectively. The corresponding values in seawater are 22 and 76%. The efficiency of the released surfactant in reducing the interfacial tension, altering wettability, and stabilizing oil-water emulsion was investigated through contact angle measurements and laser confocal scanning microscopy and benchmarked to sodium laurate, a commercially available surfactant. All these measurements demonstrate both the efficacy of the NP system for surfactant delivery and the ability of the released surfactant to alter wettability and stabilize an oil-water emulsion.

Diffusiophoresis of charged colloidal particles in the limit of very high salinity.

Diffusiophoresis is the migration of a colloidal particle through a viscous fluid, caused by a gradient in concentration of some molecular solute; a long-range physical interaction between the particle and solute molecules is required. In the case of a charged particle and an ionic solute (e.g., table salt, NaCl), previous studies have predicted and experimentally verified the speed for very low salt concentrations at which the salt solution behaves ideally. The current study presents a study of diffusiophoresis at much higher salt concentrations (approaching the solubility limit). At such large salt concentrations, electrostatic interactions are almost completely screened, thus eliminating the long-range interaction required for diffusiophoresis; moreover, the high volume fraction occupied by ions makes the solution highly nonideal. Diffusiophoretic speeds were found to be measurable, albeit much smaller than for the same gradient at low salt concentrations.

Ultra-sensitive in-situ detection of near-infrared persistent luminescent tracer nanoagents in crude oil-water mixtures.

Current fluorescent nanoparticles-based tracer sensing techniques for oilfield applications suffer from insufficient sensitivity, with the tracer detection limit typically at the several hundred ppm level in untreated oil/water mixtures, which is mainly caused by the interference of the background fluorescence from the organic residues in crude oil under constant external excitation. Here we report the use of a persistent luminescence phenomenon, which enables an external excitation-free and thus background fluorescence-free measurement condition, for ultrahigh-sensitivity crude oil sensing. By using LiGa5O8:Cr(3+) near-infrared persistent luminescent nanoparticles as a tracer nanoagent, we achieved a tracer detection limit at the single-digit ppb level (down to 1 ppb concentration of nanoparticles) in high oil fraction (up to 65 wt.%) oil/water mixtures via a convenient, CCD camera-based imaging technique without any pretreatment or phase separation of the fluid samples. This detection limit is about four to five orders of magnitude lower than that obtained using conventional spectral methods. This study introduces a new type of tracer nanoagents and a new detection method for water tracer sensing in oil reservoir characterization and management.