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Pulling out the smallest drop.

MEMBRANES THAT FILTER BY PARTICLE SIZE CAN ALLOW emulsified liquids to slip through pores smaller than the nominal droplet size. A polymer with one layer of nano-pores and another layer of micro-pores can better separate oil from water.

After the release of petroleum into open waters, oil and water can mix in the form of an emulsion. The oil may appear to be gone, but it can linger in concentrations great enough to harm marine life and disrupt ecosystems. The Deepwater Horizon oil spill, for example, released about 5 million barrels of petroleum into the Gulf of Mexico. Years after the well was capped dolphins in the Gulf were still dying in record numbers.

The underwater use of dispersants in response to the spill may have increased the emulsion of oil and water. The denser than normal oil emulsion may also have disrupted lower levels of the food chain in the northern Gulf.

In the case of nano-emulsions, when suspended oil droplets are smaller than 1 micrometer, the combination is difficult to separate. That's why researchers at the Massachusetts Institute of Technology turned their attention to the issue and came up with a membrane designed to separate nanoemulsions of oil and water.

The research team--Kripa Varanasi, an associate professor in the department of mechanical engineering; Brian Solomon, a graduate student, and Nasim Hyder, a post-doctoral researcher--described their work in the July issue of the journal Scientific Reports. Their paper, "Separating Oil-Water Nanoemulsions Using Flux-Enhanced Hierarchical Membranes," pointed out: "A variety of techniques have been implemented in industry, including gravity separation, skimming, and dissolved air flotation. More recently, new techniques incorporating aerogels, magnetic materials, and fluorosurfactant polymers have been introduced. Though promising, these approaches are ineffective for separation of small-scale emulsions, especially for those with droplets below a micron in size."

According to the authors, membranes that filter by particle size can be effective in removing solids, but droplets in emulsified liquids can deform to slip through pores smaller than the nominal droplet size.

The team's solution combines a polymer with a layer of nano-pores and another layer of micro-pores for a structural reinforcement. The structure is treated so the surface and pores resist or attract oil or water, to suit the strategy of separation.

They said the membrane can be produced on an industrial scale.

To form the membrane, the team mixed polysulfone and polyvinylpyrrolidone and dissolved them in dimethylacetamide. The next step was to cast the PSfPVP mixture onto a glass plate and cure it in water. The polysulfone remained and the PVP macromolecules migrated to the surface.

The result was a porous matrix with a thin skin layer riddled with nano-scale pores, roughly 30 to 80 nanometers in diameter, and below that was a thicker layer with micropores, about 10 to 20 micrometers across. Because the structure was a single sheet, there was no need to bond layers together.

The membrane was treated with octadecyltrichlorosilane, which attracts oil and repels water.

The group's experiment to test the membrane used an emulsion that contained 3 percent water by weight and 97 percent n-hexadecane. The goal was to strain the oil through the membrane and leave the water behind. They added a surfactant, Span 80, to keep the emulsion stable for the length of the test.

Water droplets formed as spheres in the n-hexadecane and had a mean diameter of 1.5 pm, although there was considerable variation in size. Some droplets, detected by dynamic light scattering, were as small as 200 nm.

Little, if any, water passed through the membrane under a pressure drop of 275 kilopascals. The membrane successfully resisted even the 200 nm droplets. Dynamic light scattering returned evidence of the presence of water droplets around 10 nm across.

The authors said that a separate DLS analysis of n-hexadecane containing no water and 1 percent Span 80 showed similar results. They said that the reading may have been the result of instrumentation error or the presence of the surfactant.

The strategy of the experiment described in the paper was to separate an emulsion of water suspended in oil. The authors said the same principles could be applied to emulsions of oil in water, by treating the membrane to be hydrophilic and oleophobic.

The researchers also reported that they found a means to control the thickness of the nanoporous skin layer by mixing polyethylene glycol with the PVP as a sacrificial material.

The research was supported by Shell, through the MIT Energy Initiative.

A copy of the paper is available at edu/publications/.
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Title Annotation:TECH BUZZ
Author:Hutchinson, Harry
Publication:Mechanical Engineering-CIME
Geographic Code:1USA
Date:Dec 1, 2014
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