Quantitative Evaluation of Long-Range and Cooperative Ion Effect on Water in Polyamide Network

Despite long-standing research efforts to elucidate the specific ion effect on the structure and dynamics of water, the spatial extent affected by ions and the cooperativity of ions and counterions are still controversial. Here, we demonstrate an undoubtable evidence of long-range and cooperative ion effect on water molecules in a polyamide network by using a precision ion sensor of photonic crystal hydrogel membrane. The ion effect was quantitatively evaluated by means of the osmotic work per unit cell volume change of photonic crystal, W_unit, required for the ion-inhibited dehydration, which means a suppressed migration of water molecules by the extensive effect of ions beyond their immediate hydration shells. It was found that  W_unit required for 14 vol % contraction of the membrane sensor in LiCl aqueous solutions was 7.7 times larger than that in Sr(NO₃)₂ solutions. The combination of structure-making Ca²⁺ and Sr²⁺ with nitrate anions lowered the ion effect than the chloride salts of borderline Na⁺ and Ba²⁺. Furthermore, the nitrate salt of Sr²⁺ exhibited a lower ion effect than the chloride salts of structure-breaking K⁺ and Cs⁺. These results have revealed that the ion effect acts to water extensively, which is modulated by cooperative interactions of ions and counterions.