Scientists discover new behavior of membranes that could lead to unprecedented separations

Imagine a basketball game that comes down to the last shot. The chance of the ball going through the hoop may be quite small, but it would increase dramatically if the player had the opportunity to shoot it again and again.

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

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

Many commercial processes use membranes to separate solutes of different sizes, which are substances that are dissolved in water or other liquids. Nearly all commercial membranes are polydisperse, meaning their pore sizes are not consistent. For these membranes it is almost impossible to perform a sharp separation of materials, because solutes of different sizes can fit 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 are interested in looking at the properties of isoporous membranes, which are membranes in which all pores are the same size.

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

Impeded transport refers to the internal resistance of the liquid medium as the solute attempts to pass through the pore.

“The water in the pore will drag any molecule or particle that tries to get through, making it work more slowly,” Darling said.

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

“The regime we are interested in involves pores with a diameter of about 10 nanometers. With a perfect membrane and good process design, we think we can separate solutes with a size difference of only 5%. Current membranes don’t stand a chance to make that happen,” Darling said.

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

‘Until now, scientists have implicitly assumed that each solute gets only one attempt to pass through a pore, and that hindered transport would lead to rejection of many solutes smaller than the pore size, allowing them to enter the feed stream remain instead of in the pores. output stream,” Darling added.

“While it may seem obvious to some, people have never really considered a situation where the solutes might make multiple attempts to penetrate a membrane.”

To give the dissolved molecules multiple opportunities to penetrate the pore, it was necessary to circulate the nutrient solution for several weeks.

“Even with a longer period of experimentation, we still see that individual solutes try to pass through a pore a few times on average, but it makes a big difference in moving the separation curve to a sharper step-like function,” Darling said.

“Given longer time, or more likely improved process design, we think we will see a clear, sharp separation right where the pore size matched the solute size.”

The insights from isoporous membranes can apply to existing membrane materials designed to increase the number of opportunities for solutes to pass through the pores.

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