2.B.149.  The Dehydration in Polymeric Ion Nanochannels (D-PIN) Family 

Lu et al. 2021reported experimentally resolved ion dehydration during transmembrane transport using modified in situ liquid ToF-SIMS in combination with MD simulations for mechanistic reasoning. 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. Ions generally cannot hold more than two water molecules during this sterically limited transport, but in nanopores larger than the size of the solvation shell, 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. This novel technique for studying ion solvation in situ represents an advance for the nanofluidics field and may enable advances in ion separation, biosensing, and battery applications (Lu et al. 2021).