In Situ Characterization of Dehydration during Ion Transport in Polymeric Nanochannels

Abstract

The transport of hydrated ions across nanochannels is central to biological systems and membrane-based applications, yet little is known about their hydrated structure during transport due to the absence of in situ characterization techniques. Herein, we report experimentally resolved ion dehydration during transmembrane transport using modified in situ liquid ToF-SIMS in combination with MD simulations for a mechanistic reasoning. Notably, complete dehydration was not necessary for transport to occur across membranes with sub-nanometer pores. Partial shedding of water molecules from ion solvation shells, observed as a decrease in the average hydration number, allowed the alkali-metal ions studied here (lithium, sodium, and potassium) to permeate membranes with pores smaller than their solvated size. We find that ions generally cannot hold more than two water molecules during this sterically limited transport. In nanopores larger than the size of the solvation shell, we show that ionic mobility governs the ion hydration number distribution. Viscous effects, such as interactions with carboxyl groups inside the membrane, preferentially hinder the transport of the mono-and dihydrates. Our novel technique for studying ion solvation in situ represents a significant technological leap for the nanofluidics field and may enable important advances in ion separation, biosensing, and battery applications.

Super-assembly of freestanding graphene oxide-aramid fiber membrane with T-mode subnanochannels for sensitive ion transport

“…4h). 50,51 Fig. 4i shows the K + /Mg 2+ ion selectivity of the GO/ANF/GO composite membrane under different pH, where the ion selectivity increased with an increase in pH owning to the improved K + transport and suppressed Mg 2+ transport.…”

Salt and Water Transport in Reverse Osmosis Membranes: Beyond the Solution-Diffusion Model

“…Steric partitioning is determined by the ratio between the ion radius and pore radius (λ i ). Because ions undergo partial dehydration when partitioning through the membrane, 29,30 the Stokes radius is used for the ion size. Assuming spherical ions and cylindrical pores, the partitioning coefficient (Φ st ) can be described by 31,32 λ Φ = − (1 )…”

Enthalpic and Entropic Selectivity of Water and Small Ions in Polyamide Membranes

“…Over the past decade, a large body of research using molecular dynamic simulations has focused on exploring new mechanisms of molecular transport and selectivity in nano- and subnanopores. , Many of these simulations highlighted the role of ion dehydration (or water shell rearrangement) due to pore confinement in explaining the different transport behavior of similarly sized and charged species and precise selectivity achievable in biological channels. − However, while molecular simulations serve as an invaluable tool to gain mechanistic insights into transport in model pores, they are limited in simulating accurately the molecular transport through real polymeric membrane channels. − …”

“…22−24 Experimentally exploring transport mechanisms in membranes is also challenging due to lack of experimental techniques with adequate spatial and temporal resolution to study the kinetics of molecular transport through the membrane pore. 24 Notably, the conventional solution− diffusion model, frequently used to study transport in dense polymeric membranes such as NF and RO, describes water or solute transport through the membrane using a single parameter (i.e., the water or solute permeability coefficient), 25 which is determined experimentally by dividing the measured water or solute flux by the pressure or concentration gradient over the membrane, respectively. The permeability coefficient is often used to qualitatively explain the selectivity between species as a function of the intrinsic properties of the species (e.g., size and hydration energy) and the membrane (e.g., pore size and charge).…”