Hydration-Controlled Coupling of Water Polarization and Proton Transport in Nafion Under External Electric Fields

We use molecular dynamics simulations to study hydrated Nafion at hydration levels λ=3,9,15 under external electric fields. At zero field, the static dielectric constant of the aqueous subsystem, obtained from water dipole fluctuations, grows with hydration, from ϵ≈2.9 at λ=3 to ϵ≈17.2 at λ=15; because the single-molecule dipole is essentially unchanged, this enhancement is cooperative in origin, reflecting the dipolar correlations that build up as the aqueous domains percolate. Under an applied field the fluctuation formula no longer defines an equilibrium dielectric constant, so we characterize the response through the polarization itself: the collective water dipole orients along the field already at low fields, while its magnitude 〈Mz〉 saturates gradually, on a field scale that grows with hydration. This polarization build-up is accompanied by a reorganization of the first H3O+–SO3(H) coordination shell and by a crossover from diffusive to drift-dominated proton transport. These signatures do not evolve independently but shift together to higher fields as hydration increases, showing that the water polarization and the proton transport are coupled to a common, hydration-controlled field scale: electrostatic screening by the added water simultaneously sets the equilibrium dielectric response and the field required for directed hydronium transport in perfluorinated membranes.