Enhanced Dehydration by External Pressure Driving Forces Decreases Ion Trans-subnanochannel Selectivity

The external driving force as an operating condition significantly determines membrane separation performance, but it is not clear how the selectivity of ions and intrinsic transport mechanisms are affected accordingly. Herein, the selective ratio of three kinds of representative cations (alkali metal ions, bivalent cations, and polyatomic cations) and their underlying Eyring's enthalpy and entropy of activation for transporting through regular confined channels under pressure, concentration gradient, and electric field were quantified. Compared with the diffusion-only process, the increase in the enthalpic barriers under external pressure was attributed to the increase in the degree of ion dehydration and deformation of the ion's own structure, especially for polyatomic ions (NH₄⁺ and TMA⁺) with lower hydration energies. Moreover, the splitting of the integrated ion fluxes under pressure by the DSPM-DE model further demonstrated the enhancement of ion dehydration by forced convection, which reduced the ion transport variability under steric sieving effects while increasing the ion fluxes. The ion selectivity was greatest for electromigration, but 60-fold higher voltage (about 20 V) was required for reaching equal ion fluxes as pressure-driven transport. The thermodynamic analysis indicated that compared with the ion migration alone under an electric field, osmotic pressure in concentration diffusion and cotransport of ions and water under pressure increased the transmembrane energy barriers. This study informs the choice of membrane separation modes in different application scenarios, which could help balance selectivity and energy consumption associated with driving forces.