Analysis of Electroneutrality Breakdown in Single-Digit Nanochannels
Creators: Shaw, Pragati
Publisher: name: University of Alberta Library
Year: 2024

<p>Nanotechnology has advanced in recent years to the extent that custom-tailored structures of high regularity can now be synthesized with high precision. Single-digit<br>
nanochannels (SDNs), referring to nanochannels with a diameter or conduit width of less than 10 nanometers, present unique challenges and opportunities for investigation and application. Due to the complex interplay between electrostatic, entropic, and surface-dominated phenomena in such confined regimes, new physical properties emerge. One example is the breakdown of electroneutrality, a situation in which channel surface charges are no longer exactly balanced by the ions in the channel-filling electrolyte. In past theoretical studies of charged, nanofluidic channels, electroneutrality is usually assumed to hold both globally (i.e. for the entire channel) and locally (i.e. for cross sections in the interior of the channel). However, recent work by Levy et al. [1], assuming zero-size ions, indicates that a single channel may not be electroneutral. In this work, we go beyond this initial investigation to examine the breakdown of electroneutrality in a single nanochannel using finite element-based software, COMSOL. Electroneutrality is quantified as the ratio of the ions inside the channel to the fixed charge on the channel surface. The effects of channel dimensions, surface charge density, dielectric constant of the surrounding medium, temperature, and bulk ion concentration on electroneutrality are investigated. The single nanochannel model is also investigated for finite-size ions to evaluate the significance of the steric effects within the nanochannel. Furthermore, the limiting trends depicting the convergence of the electroneutrality breakdown curves for various surface charge densities, dielectric constants, and dimensions of the surrounding medium are studied to understand the behavior of the system. The investigation is extended to explore this phenomenon in a periodic array of nanochannels. Real-world nanostructures often present such configurations, where the interactions among multiple nanochannels, also referred to as coupling of nanochannels in our work, could influence the overall system behavior. Following this, an infinite periodic array of nanochannels is studied along with the single nanochannel model to identify and examine the differences in the electroneutrality breakdown trends for various parameters.</p>

<p>Additionally, the importance of explicitly incorporating the dielectric medium is highlighted. Levy et al. [1] reported the breakdown of electroneutrality in confined nanopores embedded in a dielectric medium. A Robin boundary condition was derived which eliminates the need to include the dielectric medium explicitly when solving for the electric field within the nanopore. The issues related to the approximations made during the derivation of the boundary condition are pointed out. The errors caused by the use of this boundary condition can be significant even for nanochannels of large aspect (length to radius) ratio, a condition on which the approximations in Levy et al. [1] are based. Our contributions are aimed at critically expanding the understanding of SDNs and the roles of various physical parameters in dictating the behavior of these confined systems.</p>

<p>References:<br>
[1] A. Levy, J. P. de Souza, and M. Z. Bazant, “Breakdown of electroneutrality in nanopores,” Journal of Colloid and Interface Science, vol. 579, pp. 162–176, 2020.</p>