Scientists discover new membrane behavior that could lead to unprecedented separations

Imagine a tight basketball game that comes down to the last shot. The probability of the ball going through the goal may be quite low, but it would increase dramatically if the player was given the opportunity to shoot at it over and over again.

A similar idea is at play in the scientific field of membrane separations, a key process central to industries that include everything from biotechnology to petrochemicals to water treatment to food and beverage.

“Separation lies at the heart of so many products we use in our daily lives,” said Seth Darling, director of the Center for Advanced Materials for Energy Water Systems (AMEWS) at the US Department of Energy’s (DOE) Argonne National Laboratory. “Membranes are the key to achieving efficient separations.”

Many commercial processes use membranes to separate different sizes of solutes, which are substances that are dissolved in water or other fluids. Almost all commercial membranes are polydisperse, meaning their pore sizes are not consistent. With these membranes, it is almost impossible to make a sharp separation of materials because different sizes of solutes can pass through different pores.

“Basically all commercial membranes, all membranes that are really used for anything, have a wide range of pore sizes — small pores, medium pores and large pores,” Darling said.

Darling and his colleagues at Argonne and the Pritzker School of Molecular Engineering at the University of Chicago were interested in the properties of isoporous membranes, which are membranes in which all the pores are the same size.

Previously, researchers believed that the sharpness of the separations they could achieve at the nanoscale was limited, not only because of changes in pore size, but also because of a phenomenon called “impeded transport.”

Impeded transport refers to the internal resistance of a fluid medium as a solute attempts to pass through the pores.

“The water in the pores causes a drag on the molecule or particle that’s trying to get in and causes it to slow down,” Darling said.

“These slower solutes seem to be rejected by the membrane. Counterintuitively, objects that are even half the pore size will end up being rejected about half of the time.” Overcoming the rejection caused by perturbed transport would enable unprecedented selectivity in size-based separations, he explained.

“The regime we’re interested in involves pores around 10 nanometers in diameter. With the perfect membrane and the right process design, we believe we could separate solutes with a size difference of just 5%. Current membranes have no chance of pulling this off,” Darling said.

In the new study, Darling and his colleagues have revealed dynamics that could only be revealed by studying isoporous membranes, and which give hope for overcoming limited transport constraints. A paper based on the study will appear in the June 20 online edition Natural water.

“Until now, researchers have implicitly assumed that each solute gets only one attempt to pass through the pore, and that hindered transport will result in the rejection of many solutes that are smaller than the pore size, causing them to remain in the inflow rather than the outflow.” Darling added.

“While this may seem obvious to some, people have never really considered the situation where solutes might try to cross a membrane multiple times.”

This required cycling the feed solution for several weeks to give the solute molecules more chances to penetrate the pores.

“Even after longer experiments, we still see that individual solutes try to penetrate the pore only a few times on average, but it makes a big difference in the shift of the separation curve towards a sharper step function.” Darling said.

“Given a longer time, or more likely an improved process design, we believe we will see a clear and sharp separation just where the pore size matches the solute size.”

The insights gained from isoporous membranes could be applicable to existing membrane materials designed to increase the number of opportunities for solutes to pass through the pores.

“If these basic studies can be successfully transferred to industrial membrane separations, it could have a huge impact in many sectors of our economy,” he said.