Scientists discover new membrane behavior that could lead to unprecedented separations

Imagine a close basketball game that comes down to the final shot. The probability of the ball going through the hoop may be quite low, but it would increase significantly if the player had the opportunity to shoot it over and over again.

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

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

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

“Essentially, all commercial membranes, all membranes that are actually 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 pores are the same size.

Previously, scientists thought there was a limit to the sharp separations they could achieve at the nanoscale, not only due to variations in pore size, but also a phenomenon called “impeded transport”. “.

Impeded transport refers to the internal resistance of the fluid medium when the solute attempts to pass through the pore.

“Water in the pores will create drag on a molecule or particle that is trying to pass through, which will slow it down,” Darling said.

“These slower solutes appear to be rejected by the membrane. Counterintuitively, objects even half the pore size will end up being rejected about half the time.” Overcoming the rejection created by hampered transportation would enable unprecedented selectivity in size-based separations, he explained.

“The regime we are interested in involves pores approximately 10 nanometers in diameter. With a perfect membrane and proper process design, we believe we could separate solutes with a size difference as small as 5%. Current membranes have no chance of pulling that,” Darling said.

In a new study, Darling and colleagues discovered dynamics that could only be revealed by studying isoporous membranes, and which provide hope for overcoming hindered transport limitations. An article based on the study appears in the June 20 online edition of Natural water.

“Until now, scientists had implicitly assumed that each solute had only one attempt to pass through a pore, and that hindered transport would produce the rejection of many solutes smaller than the pore size, forcing them to staying in the inflow rather than the outflow,” Darling added.

“Although it may seem obvious to some, people have never really considered a situation in which solutes might make multiple attempts to cross a membrane.”

To give the solute molecules several chances to pass through the pores, the feed solution had to be circulated for several weeks.

“Even with an extended period of experimentation, we still only see individual solutes trying to pass through a pore several times on average, but it makes a big difference in moving the separation curve to a sharper step-like function. ” said darling.

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

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

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