Interfacial tension and the behavior of microemulsions and macroemulsions of water and carbon dioxide with a branched hydrocarbon nonionic surfactant

Surface tensions at elevated pressure depend strongly on bulk phase saturation

HYPOTHESIS

Understanding interfacial phenomena at elevated pressure is crucial to the design of a variety of processes, modeling important systems, and interpreting interfacial thermodynamics. While many previous studies have offered insight into these areas, current techniques have inherent drawbacks that limit equilibrium measurements.

EXPERIMENTS

In this work, we adapt the ambient microtensiometer of Alvarez and co-workers into a high pressure microtensiometer (HPMT) capable of experimentally quantifying a wide range of interfacial phenomena at elevated pressures. Particularly, the HPMT uses a microscale spherical interface pinned to the tip of a capillary to directly measure surface tension via the Laplace equation. The stream of microscale bubbles used to pressurize the system ensures quick saturation of the bulk phases prior to conducting measurements. The HPMT is validated by measuring the surface tension of air-water as a function of pressure. We then measure the surface tension of CO₂ vapor and water as a function of pressure, finding lower equilibrium surface tension values than originally reported in the literature.

FINDINGS

This work both introduces further development of a useful experimental technique for probing interfacial phenomena at elevated pressures and demonstrates the importance of establishing bulk equilibrium to measure surface tension. The true equilibrium state of the CO₂-water surface has a lower tension than previously reported. We hypothesize that this discrepancy is likely due to the long diffusion timescales required to ensure saturation of the bulk fluids using traditional tensiometry. Thus we argue that previously reported elevated pressure measurements were performed at non-equilibrium conditions, putting to rest a long standing discrepancy in the literature. Our measurements establish an equilibrium pressure isotherm for the pure CO₂-water surface that will be essential in analyzing surfactant transport at elevated pressures.

Synthesis of PDMS-b-POEGMA Diblock Copolymers and Their Application for the Thermoresponsive Stabilization of Water-Supercritical Carbon Dioxide Emulsions

In this study, PDMS₁₃-b-POEGMA_x diblock copolymers consisting of a CO₂-philic poly(dimethylsiloxane) (PDMS) block connected to a thermosensitive hydrophilic poly(oligoethylene glycol methacrylate) (POEGMA) block were synthesized by reversible addition-fragmentation chain-transfer (RAFT) radical polymerization. Their ability to decrease the water-supercritical CO₂ (scCO₂) interfacial tension (γ) and to stabilize water-scCO₂ emulsions was investigated using an original homemade device developed in the laboratory. This device is able to control the pressure from 1 to 250 bar and the temperature from 40 to 80 °C. It was implemented with 2 visualization windows, a drop tensiometer and a remote optical head for dynamic light scattering (DLS) measurements. These experiments revealed that PDMS-b-POEGMA decreased γ down to 1-2 mN/m and was the most efficient at high pressure (250 bar) and low temperature (40 °C) where PDMS and POEGMA blocks exhibited the highest affinity for their respective phase. The diblock copolymers were shown to stabilize water-scCO₂ emulsions. Moreover, the thermosensitive behavior of the POEGMA block in water (with a lower critical solubility temperature around 65 °C) resulted in the formation of temperature-responsive emulsions that could reversibly switch at 100 bar from stable at 40 °C to unstable at 80 °C. These results were rationalized based on the solubility of each individual block of the copolymers in water and scCO₂ as a function of temperature and pressure.

Interfacial Phenomena of Natural Dispersants for Crude Oil Spills

A natural surfactant was studied to simulate the dispersion process of crude oil in water. The interfacial phenomena of this natural dispersant was compared with a commercially available chemical dispersant, COREXIT EC9500A. This functional surfactant was extracted from the mucilage of the Opuntia ficus-indica cactus species. The evaluation to determine the efficacy to disperse crude oil of the cactus-based mucilage extract (nongelling extract, NE) was based on characterizing surface and interfacial tension, dispersion efficiency, mixing effects, salinity effects, stability, and droplets size distributions. We found that surface tension values follow a linear relationship with respect to the natural logarithm of the concentrations of NE. The application of NE in the water phase led to decreasing oil/water interfacial tensions. Surface tension tests were also used to quantify the effect of oil-in-water (O/W) emulsion ratios once either natural or commercialized dispersants were added. A key finding of our work is that the surface tension between typical 6% and 3% v/v O/W emulsions was significantly reduced with the addition of discrete amounts of NE. This result indicated that the dynamic balance between O/W and water-in-oil (W/O) emulsions was thermodynamically more stable toward O/W emulsion states with NE. We also found that O/W emulsions with higher dispersion effectiveness were formed for both 10 and 35 practical salinity units, as the dispersant to oil ratios increased, with a significant correlation to the mixing energy. We observed that the O/W emulsions with natural dispersants had a significantly smaller weighted average diameter compared to those with COREXIT EC9500A. Such a phenomenon can be explained by understanding intermolecular interactions due to the structure and type of dispersant. In conclusion, cactus-based mucilage extracts could be used as environmentally benign dispersants and, therefore, reduce negative social perceptions of the application of dispersants to clean up spilled oil.

Synergistic formation and stabilization of oil-in-water emulsions by a weakly interacting mixture of zwitterionic surfactant and silica nanoparticles

Oil-in-water emulsions were formed and stabilized at low amphiphile concentrations by combining hydrophilic nanoparticles (NPs) (i.e., bare colloidal silica) with a weakly interacting zwitterionic surfactant, caprylamidopropyl betaine, to generate a high hydrophilic-lipophilic balance. The weak interaction of the NPs with surfactant was quantified with contact angle measurements. Emulsions were characterized by static light scattering to determine the droplet size distributions, optical photography to quantify phase separation due to creaming, and both optical and electron microscopy to determine emulsion microstructure. The NPs and surfactant acted synergistically to produce finer emulsions with a greater stability to coalescence relative to the behavior with either NPs or surfactant alone. As a consequence of the weak adsorption of the highly hydrophilic surfactant on the anionic NPs along with the high critical micelle concentration, an unusually large surfactant concentration was available to adsorb at the oil-water interface and lower the interfacial tension. The synergy for emulsion formation and stabilization for the two amphiphiles was even greater in the case of a high-salinity synthetic seawater aqueous phase. Here, higher NP adsorption at the oil-water interface was caused by electrostatic screening of interactions between (1) NPs and the anionic oil-water interface and (2) between the NPs. This greater adsorption as well as partial flocculation of the NPs provided a more efficient barrier to droplet coalescence.