NaCl increases the dielectric constant of nanoconfined water in phospholipid multilamellar vesicle by enhancing intermolecular orientation correlation rather than rotational freedom of individual molecules

Highlights: • The first demonstration that ion increases the dielectric constant of nanoconfined water. • The molecular mechanism is different in two water regions, membrane surface-bound water and water beyond it. • NaCl enhances the rotational freedom of surface-bound water. • Beyond surface-bound water, NaCl enhances the intermolecular orientation correlation rather than the rotational freedom of individual molecules. We investigate how the NaCl affects the dielectric constant of nanoconfined water in phospholipid multilamellar vesicles using dielectric relaxation spectroscopy. An anomalous increase in the dielectric constant is observed near the physiological NaCl concentration ($0<c<0.3M$), which has been consistently proposed in molecular dynamic simulations but is first confirmed by experiment. NaCl accelerates the reorientation motion of surface-bound water, which implies an increase of the rotational degree of freedom. However, for water beyond the hydration layer, retarded collective reorientation motion and increased dielectric strength are observed, suggesting NaCl enhances intermolecular orientation correlation rather than rotational freedom.