Looking deep into the smallest pores

Membranes of vertically aligned carbon nanotubes (VaCNT) can be used to purify or purify water at high flow rates and low pressures. Recently, researchers and collaborators at the Karlsruhe Institute of Technology (KIT) conducted steroid hormone absorption experiments to study the interaction of forces in small pores.

They found that VaCNT of specific pore geometry and pore surface structure are suitable for use as highly selective membranes. Study them Published in Nature Communications.

Clean drinking water is of utmost importance to all people around the world. Membranes are used to effectively remove micropollutants, e.g Steroid hormones which are harmful to health and environment. A very promising membrane material is made of vertically aligned carbon nanotubes (VaCNT).

“This material is amazing – with small pores of 1.7 to 3.3 nanometer diameters, an almost perfectly cylindrical shape, and small torsion,” says Prof. Andrea Ares Schäfer, head of KIT’s Institute for Advanced Membrane Technology (IAMT). “Nanotubes should have a very absorbent effect, but only friction is very low.” Currently, the pores are too large for effective retention, but smaller pores are not yet technically feasible.

Interaction of forces

In experiments with steroid micropollutants, IAMT researchers studied why VaCNT membranes are perfect water filters. They used membranes produced by the Lawrence Livermore National Laboratory (LLNL) in Livermore (California). Findings: Low adsorption, i.e. surface deposition, of VaCNT is essential for highly selective membranes targeting specific substances.

Studies show that adsorption in membrane nanopores depends not only on the adsorption surface and finite mass transfer, but also on the interplay of hydrodynamic forces, friction, and attractive and repulsive forces at the liquid-wall interface. Highly water-permeable nanopores exhibit low interaction due to small friction and high flow rates.

“When molecules are not held together by their size, the interaction with the material will often determine what happens. The molecules bounce off the membrane like a climber climbing a wall. Times are much easier when there are many good climbing holds.” Schäfer explains. Such studies conducted by IAMT specifically help in designing pore geometry and pore surface structure.

Ten years to turn an idea into an experiment

The membrane was developed by Dr. Francesco Fornasero and his team at LLNL. Experiments with micropollutants were conducted and evaluated using state-of-the-art analytical equipment at IAMT. “It took about 10 years to turn this idea into a successful experiment that was met with widespread interest from the membrane technology community,” Schaeffer says.

The production of such nearly perfect membranes is extremely difficult. Over areas as large as a few square centimeters, the probability of defects is very high. And defects will affect the results. In recent years, LLNL has been able to produce membranes over large areas. In parallel, IAMT researchers built very small filtration systems for experiments to retain trace pollutants at two square centimeters.

“Downscaling is extremely difficult. Managing it together is a huge achievement,” Schaeffer says. “Now, we look forward to developing membranes with even smaller pores.”

This study was the first to focus on the interplay of hydrodynamic forces, friction, and the forces of attraction and repulsion. It provides basic results regarding water processing. These can benefit ultra- and nanofiltration processes controlled by nanopores.