Study Describes Mechanics of Dispersant Adsorption at Oil-Water Interface

Oil in the wake of a boat. (Photo credit:  NOAA Gulf Spill Restoration)

(Click to enlarge) Oil in the wake of a boat. (Photo credit: NOAA Gulf Spill Restoration)

Scientists are trying to develop more stable, safer dispersants.  

Carnegie Mellon researchers examining the chemical interaction of Tween 80 (a primary component in typical dispersants) at the point where oil and water meet have characterized dispersants behavior in variable test environments. They observed reorganization, irreversible adsorption, and high values of surface elasticity. The researchers discuss these and other results in terms of their impacts on oil spill response measures.  They published their findings in the January 2013 issue of Langmuir: Interfacial Tension Dynamics, Interfacial Mechanics, and Response to Rapid Dilution of Bulk Surfactant of a Model Oil−Water-Dispersant System.

The usefulness of dispersants is their ability to help oil stay broken up in small particles so that bacteria have a longer time to eat it, despite the reduction of dispersant concentration as it mixes with the ocean and its use under varying water conditions. Understanding the “more complex adsorption processes where application conditions include step changes in concentration” is critical knowledge that affects decisions about dispersant application. To do this, scientists used Tween 80 as a dispersant proxy, along with squalane and artificial sea water, to develop a model to better track and measure the mixing of oil and water and the stability of that mixture.

The image on the right visually depicts the oil-solution interface. The plot on the left shows pre-rinse interfacial tension versus post-rinse interfacial tension for four different solutions in simulated sea water (SSW) and deionized water (DI). (Image provided by Lynn Walker)

(Click to enlarge) The image on the right visually depicts the oil-solution interface. The plot on the left shows pre-rinse interfacial tension versus post-rinse interfacial tension for four different solutions in simulated sea water (SSW) and deionized water (DI). (Image provided by Lynn Walker)

Researchers observed and characterized several behaviors. They quantified “the interfacial tension of Tween 80 at an oil-water interface, which has not been reported previously,” to include the finding that “the initial adsorption (of Tween 80) is irreversible,” and “is not strongly associated with the oil phase.”  They showed “a clear increase in transport rates to the interface under higher convective fluxes, demonstrating the role of diffusion in the transport process,” and “the lack of impact of salt concentration.” Tests on this component showed that it “has two adsorption states that are dependent on how much surfactant is present on the surface at the point of adsorption,” and indicates “reorganization at the interface that allows the surfactant to adsorb in different states.” Their findings on surface elasticity and viscosity demonstrated “the rigidity of the interface changes with rinsing,” and the researchers suggest that their comparisons of pre-rinsed and rinsed interfaces “should provide otherwise unattainable insight into understanding potential impacts on aqueous film drainage between oil droplets after a solution is diluted.”

The study authors are Matthew D. Reichert and Lynn M. Walker (Langmuir 2013, 29 (6), 1857-1867).

This research is made possible by a grant from BP/The Gulf of Mexico Research Initiative (GoMRI) to the Consortium for the Molecular Engineering of Dispersant Systems (C-MEDS). The GoMRI is a 10-year, $500 million independent research program established by an agreement between BP and the Gulf of Mexico Alliance to study the effects of the Deepwater Horizon incident and the potential associated impact of this and similar incidents on the environment and public health.

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