This talk will be focused on polymer porous materials that are negatively charged and cation selective in aqueous solutions of monovalent salts, but switch their selectivity towards anions when exposed to multivalent cations or high-dipole moment, aprotic organic solvents. The ions and solvent tunable selectivity will be explained via correlations between ions and between solvent molecules and ions that lead to unusual structure of solid/liquid interfaces. Single nanopores in polymer films prepared by irradiation with energetic heavy ions and etching provide a model system of nanoconfinement containing a high density of carboxyl groups. Using a series of nanopores with well-defined diameters we have identified conditions when the addition of trivalent cations of tris(ethylenediamine)chromium(III) leads not only to reversible switch of the effective surface charge from negative to positive but also modulation of ionic transport. Due to ~1 nm diameter of the trivalent chromium ions, the effective positive charge induced by the ions is a function of the pore opening diameter and the salt concentration. Consequently, the transmembrane current can be enhanced in nanopores with opening above 10 nm, and diminished and nearly complete blocked in nanopores whose opening is below 5 nm. In the second part of the talk, we will show that transport properties of polymer nanopores can also be modified by a solvent. The same polymer nanopores with carboxyl groups when exposed to salt solutions (LiClO<sub>4</sub>) in propylene carbonate or acetone remain negatively charged in low salt concentrations and become positively charged when the salt concentration exceeds 1 mM. The effective surface charge cannot be attributed to the charges on the pore walls because both solvents are aprotic. We postulate that the effective surface charge stems from spatial organization of solvent molecules that determine local distributions of cations and anions. These two examples show that properties of solid/liquid interfaces as well as ion selectivity of nanoporous materials can be drastically modulated not only by the ionic composition of the solution but even the choice of solvent.

Selectivity of nanoporous materials tuned by ions and solvent

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