ABSTRACT
Air-liquid interfacial processing of volatile organic compound photooxidation has been suggested as an important source of secondary organic aerosols. However, owing to the lack of techniques for studying the air-liquid interface, the detailed interfacial mechanism remains speculative. To obviate this, we enabled in situ synchrotron-based vacuum ultraviolet single photon ionization mass spectrometry using the system for analysis at the liquid-vacuum interface microreactor to study glyoxal photooxidation at the air-liquid interface. Determination of reaction intermediates and new oxidation products, including polymers and oligomers, by mass spectral analysis and appearance energy measurements has been reported for the first time. Furthermore, an expanded reaction mechanism of photooxidation and free radical induced reactions as a source of aqueous secondary organic aerosol formation is proposed. Single photon ionization can provide new insights into interfacial chemistry.
ABSTRACT
Bilge water from ships is regarded as a major pollutant in the marine environment. Bilge water exists in a stable oil-in-water (O/W) emulsion form. However, little is known about the O/W liquid-liquid (l-l) interface. Traditional bulk characterization approaches are not capable of capturing the chemical changes at the O/W l-l interface. Although surfactants are deemed essential in droplet formation, their roles in bilge water stabilization have not been fully revealed. We have utilized novel in situ chemical imaging tools including in situ scanning electron microscopy (SEM) and in situ time-of-flight secondary ion mass spectrometry (ToF-SIMS) to study the evolving O/W interface using a NAVY bilge model for the first time. The droplet size distribution (DSD) does not change significantly without the addition of X-100 surfactants under static or rocking conditions. Both the oil components and the water clusters are shown to evolve over time at the O/W droplet interface by in situ liquid SIMS imaging. Of particular interest to droplet stabilization, the contribution of surfactants to the aged bilge droplets becomes more significant as the droplet size increases. The higher mass surfactant component does not appear on the droplet surface immediately while many lower mass surfactants are solvated inside the droplet. We have provided the first three-dimensional images of the evolving O/W interface and demonstrated that in situ surface chemical mapping is powerful enough to reveal the complex and dynamic l-l interface in the liquid state. Our observational insights suggest that surfactants are important in mediating droplet growth and facilitating effective separation of bilge water emulsion.
ABSTRACT
Bilgewater is a regulated shipboard produced waste stream that often contains oil-in-water emulsion. Fundamental knowledge of emulsion surface changes is required for improved wastewater treatment; however, limited information is currently available. We have reported the first surface characterization of synthetic bilgewater emulsions using time-of-flight secondary ion mass spectrometry (ToF-SIMS) coupled with optical microscopy. A Navy standard bilgewater solution consisting of a hydrocarbon and detergent mixture is used as the synthetic bilgewater emulsion model. Both fresh and aged emulsion samples are analyzed to determine their droplet size distributions (DSDs) and surface chemical composition. Our results show that fresh emulsions are largely mono-modal with hydrocarbon fragments as the main surface composition. Aged emulsions are also mono-modal with slightly larger size. Both SIMS spectral comparison and Principal Component Analysis (PCA) show that some surfactant components appear on the fresh emulsion surface while larger molecular weight components appear at the aged bilge droplet surface. Our results indicate that the oil-water interface evolves after emulsion droplet formation. More importantly, surface evolution not only changes the bilgewater DSD, but also alters the surface chemical composition and reactivity.
