Getting the inside track on ion-selective membranes

In-depth analysis of polyethylene terephthalate (PET) membrane fabrication has revealed how to unlock ion-selective transport in these materials¹. Precise control over ion movement could prove useful in applications including water desalination, ultrafiltration and even biological research where track-etched PET membranes are already used to control cellular environments.

PET membranes with smooth, cylindrical pores are routinely fabricated using a process called track etching, in which raw PET films are bombarded with high-energy ions. This approach, which can produce pore sizes down to tens of nanometers, has found wide use in filtration. Increasingly though, researchers are looking to create ion-selective membranes by tweaking chemical structures inside the track structures.

According to Pavel Apel from Dubna State University in Moscow, one issue with track-etched membranes is that many details of their formation processes are unknown, particularly when it comes to track surfaces. Conventional preparation techniques use chemical attacks to remove remnant material from the membrane pores after the initial bombardment — an approach that can alter the entire PET polymer.

As an alternative, Apel and his co-workers targeted only the track zones through ‘softer’ treatments involving the exposure of ion-irradiated PET films to ultraviolet radiation for several hours. In the journal Membranes and Membrane Technologies, the team reported that using infrared spectroscopy showed that the extended light treatment led to the formation of water-soluble products in the tracks, which could be readily washed out. These measurements also showed that carboxyl groups were the dominant chemical species remaining on the track surfaces.

Intriguingly, the researchers discovered that their PET membranes exhibited distinct selectivity for singly charged cations after water extraction. However, this preference began to shift when the average separation between tracks approached 20 nanometres. Electrical transport measurements revealed that at these pore densities, ion transport routes may deviate from the nanometre-scale cylindrical channels.

“The change in selectivity and permeability is a sign that the channels interact with each other, and, therefore, their diameter is much wider than one nanometre,” says Apel. “It seems that the ions pass through a kind of three-dimensional structure resembling the gel-like structure of ion-exchange membranes.”

The team also uncovered a pronounced pH-dependence of the liquid extraction process that could be used to shed light on the formation of charged ion-conductive gels in the tracks.

“Quantitative characterization of alterations in the polymer structure, such as concentration of carboxyl groups or water uptake, helps us better understand the mechanisms of ion permeability,” explains Apel. “There are no doubts that these characteristics relate to membrane performance.”