Ion concentration assumptions break down in nanofluidic devices

Nanofluidics can examine electrolytes in a well-controlled fashion within tiny, manufactured channels. But characterizations of such devices often assume the ion concentration in the channel is the same as that in the larger reservoirs of solution they connect.

Using a one-dimensional channel geometry, Robin et al. show that this ideal assumption breaks down in channels narrower than a few nanometers across.

In a typical nanofluidic system, a channel connects two reservoirs of salt solution. Submerged in each reservoir is an electrode, which creates an electric current after imposing a voltage drop. The current is carried through the channel by ions, so the current-voltage characteristics provide insight into ion transport properties under nanoscale confinement.

“In a macroscopic reservoir of electrolyte solution (say, a glass of salted water), the ions behave essentially as particles of an ideal gas: they have almost no interactions with each other. This ideal gas picture is often assumed to also apply to an electrolyte solution in nanoscale confinement. Then, the ion concentration inside the channel should be identical to the one in the reservoir,” said author Nikita Kavokine.

However, being close to the channel wall is electrostatically unfavorable for an ion. They can get around this by creating a tightly bound pair, but that has a cost in terms of entropy.

“Our exact solution of the ion filling problem gives access to all the statistical properties of the confined electrolyte, without any prior assumptions,” said Kavokine.

The team plans to extend the solution to channels with charged walls. They believe this problem is an exciting example of the emerging interface between fluid dynamics and condensed matter physics.

Source: “Ion filling of a one-dimensional nanofluidic channel in the interaction confinement regime,” by Paul Robin, Adrien Delahais, Lydéric Bocquet, and Nikita Kavokine, Journal of Chemical Physics (2023). The article can be accessed at https://doi.org/10.1063/5.0142110 .

This paper is part of the 2023 JCP Emerging Investigators Special Collection, learn more here .