The Sisyphus Factor
Researchers validate model of ion action at the interface of oil and water
Cesium ions (red) initially in oil (green) tend be sucked into the neighboring water (blue). PNNL for the first time measured that long-range aquatic attraction by balancing it against the attraction to a solid metal, represented by the gray bar at the far left. Enlarged View
Results: Everyone knows that oil and water don't mix. But what happens when you add charged metal atoms or ions, such as cesium, to the brew? Do the ions latch onto the water or the oil? How strong is that attraction?
A team of scientists from Pennsylvania State University, Peking University, and Pacific Northwest National Laboratory conducted an experiment that, for the first time, confirms the accuracy of a simple, longstanding model of how ions behave at the interface of oil and water. The new information may give scientists more confidence in applying the “Born potential” to some of the complicated challenges in predicting ion transport near and through chemical and biological systems.
Why it matters: “We actually measured a very useful theoretical concept,” said Dr. James Cowin, project leader and a PNNL research scientist. “From this study, we now have a better understanding of how water can influence ion motion. This can help us understand such things as how ions move from water into a cell membrane, or how to extract Hanford’s radioactive cesium from water using oil or other organic solvents.”
The study also will aid scientists in selecting which molecular models to use for their research. Some situations require extremely detailed, and therefore costly, simulations. Now researchers know they can obtain satisfactory results in certain applications from the less elaborate, less expensive, Born model.
Methods: Like most metals, cesium dissolves in water as charged atoms called ions. These ions strongly prefer to be in water, rather than oil. Thus, it takes a lot of energy to drag a cesium ion initially in water to a neighboring oil. It is much like having to push a heavy rock (the ion), from a deep valley (the water), uphill into the oil. Because of the long-range nature of the ion-water interactions, the ions still “want” the water, even when moved a long way away from the interface. This means the hill has a long “slippery slope” that tries to suck the ions back toward the water, from deep within the oil.
Many chemical reactions of ions occur only where the oil and water meet. The rates and outcomes of these reactions depend critically upon the height of that hill, and the shape and reach of that slippery slope, that describe the ion interactions. Theory, guided by experiments, is key to predicting these interactions. Simple theories, such as the Born potential, were created as early as 1920. Recent, very expensive and elaborate molecular dynamic theories can now estimate the shape of the hill, as well as how far it reaches into the oil.
“Experiments to measure that hill, particularly its shape, are needed so that researchers can decide which theory to use,” Cowin said. “These experiments were essentially non-existent until this study.”
The team employed molecular beam epitaxy and other resources in the Department of Energy’s EMSL, a national scientific user facility at PNNL, to measure the slope of the hill under different circumstances. Knowing the slope of the hill at many distances from a film of water allowed researchers to reconstruct the shape of the hill, rather like a roofer specifies a roof shape by calculating a series of slopes and distances.
“The trick was to warp the hill by a known amount, until we bent it flat,” Cowin said. To do this, the team created an oil-water-oil sandwich with a metal slab on one side, roughly 100 oil molecule diameters away from one of the oil-water interfaces. Forces in the metal created a new hill with a precisely predictable slope going toward the metal, in the opposite direction from the hill’s slippery slope toward the water layer. (See illustration.)
“It turns out ions would like to be in the metal even more than they want to be in the water layer,” said PNNL scientist Dr. Greg Schenter.
Researchers studied the ion motion as the layered system was slowly warmed. Near 90 degrees above absolute zero, the syrupy oil (3-methylpentane) is just thin enough to permit the ions to move.
The motion of the ions is very sensitive to the warping of the hill. At a critical warping, the hill becomes flat at the position where the ions have been placed. Now the ions find themselves at the very top of the new bent hill.
From how the ions moved, and by what fraction they went toward the water versus toward the metal, the team calculated the precise rise of the slope of the hill at that distance from the water. Repeating the experiment with the ions placed initially further and further away from the water, gave the slope of the hill at each distance. “And from that, for the first time, we reconstructed the actual shape of the hill,” Schenter said.
What’s next: Experiments such as the one above are conducted in a vacuum at very low temperatures. Cowin and his team are now developing an instrument to permit examination of reactions on the surface of water in a vacuum at higher temperatures. They especially want to study the photochemistry of the “polar dawn” in which sunlight releases pollutants from chemicals that have collected on Arctic and Antarctic snows during the winter.
Acknowledgments: The research was funded by a DOE Office of Basic Energy Sciences chemical sciences grant and the DOE Office of Biological and Environmental Research (OBER). The work was performed at DOE’s EMSL, a national scientific user facility at PNNL. The interdisciplinary research team consisted of Richard C. Bell at Pennsylvania State University, Kai Wu at Peking University, and Martin J. Iedema, Gregory K. Schenter, and James P. Cowin at PNNL.
Reference: Bell RC, K Wu, MJ Iedema, GK Schenter, and JP Cowin. 2009. "The Oil-Water Interface: Mapping the Solvation Potential." Journal of the American Chemical Society 131(3):1037-1042.