The Donnan electric potential arises from an imbalance of charges at the interface of a charged membrane and a liquid but detecting it directly has not been possible. Until now.

It is named after the chemist Frederick Donnan, who first probed the phenomenon in the early 20th century using a solution of Congo red. In a paper published in 1911, Donnan described experiments in which a membrane separated two charged solutions and only allowed some ions to pass through. As the two solutions reach equilibrium, he found, they may also scatter charges unevenly across the membrane — and therefore produced an electric potential. The Donnan potential plays a role in any system that brings together a material with fixed ions — like a charged polymer or the membrane of a cell — and an electrolyte solution. The charges in the solution are free to move, and some can pass into the membrane. 

The Donnan potential plays a critical role in transporting ions through a cellular membrane which ties it to biological functions ranging from muscle contractions to neural signaling. Ion exchange membranes are also important in energy storage strategies and water purification technologies.

Credit: Marilyn Sargent/Berkeley Lab

Chemist Pinar Aydogan-Gokturk, Ph.D., postdoctoral scholar at Lawrence Berkeley National Lab, said the new measurement will also improve previous thermodynamic models of the Donnan equilibrium. Those models have long relied on uncertain assumptions and indirect measurements. “Using our method, we are hoping to be able to answer questions about fluid dynamics in non-ideal conditions at the membrane interfaces,” she said.

To make the measurement, Aydogan-Gokturk, Crumlin, and their collaborators at the University of Texas at Austin’s Center for Materials for Water and Energy Systems used a technique called “tender” ambient pressure x-ray photoelectron spectroscopy, or tender-APXPS.

That x-ray photoelectron spectroscopy can reveal the chemical composition, and lesser known (but just as important) local potentials of the surface of a material. When x-rays are focused on the materials’ surface, they trigger the release of electrons, and the energy levels of those electrons give away the constituent atoms. In 1981, Swedish physicist Kai Siegbahn won the Nobel Prize in Physics for work on using XPS. Surface spectroscopy tools like XPS typically require vacuum environments to work, but pioneering work at Berkeley Lab led to the use of XPS at ambient pressure. About 10 years ago, ALS scientists pushed the technology further, combining ambient pressure XPS with higher-energy x-rays. That advance allowed them to probe solid-liquid interfaces.

Crumlin, Aydogan-Gokturk, and their team collected time intensive spectroscopic data sets to probe the Donnan potential. They immersed a charged membrane in a salt solution, fired x-rays at the interface, and studied the electrons that emerged. To help validate the experiments, Berkeley Lab Staff Scientist Jin Qian compared the measured Donnan potential values to simulated thermodynamic models. A tool that’s usually used to probe chemical composition may not seem like an obvious instrument for studying membranes, but Crumlin predicted that using tender-APXPS in membrane science will continue to reveal new insights about interfacial phenomena.