Preparation of monovalent cation perm-selective membranes by controlling surface hydration energy barrier

Cationic separating the with different valence states is greatly desirable but technically challenging. Achieving such a precise separation using membranes requires accurately distinguishing tiny differences of cations on physico-chemical properties. Here, inspired by the ion dehydration effect, a series of cation exchange membrane with adjustable surface hydrophobicity based on quaternized polypyrrole were prepared. We demonstrate that improving hydrophobicity of modified layer substantially hinder cations with higher hydration energy to enter the modified layer due to their stronger binding ability with water molecules. When applied in electro-driven cation separation, the optimal membrane exhibited an outstanding perm-selectivity between monovalent and divalent cations (P Na Mg  = 12.95, P Li Mg  = 1.28) than commercial monovalent cation perm-selective membrane (P Na Mg  = 2.46, P Li Mg  = 1.07). Overall, our results indicate that the hydrophobicity of the modified layer governs the cations dehydration behavior through the polymeric pores due to ion-pore wall interactions, providing the scientific basis for the construction of membranes with high monovalent cation perm-selectivity.