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Molecular and collective reorientations in interfacial water are by-and-large decelerated near surfaces subjected to outgoing electric fields (pointing from surface to liquid, i.e., when the surface carries positive charge). In incoming fields at negatively charged surfaces, these rates show a nonmonotonic dependence on field strength where fastest reorientations are observed when the field alignment barely offsets the polarizing effects due to interfacial hydrogen bonding. This extremum coincides with a peak of local static permittivity. We use molecular dynamics simulations to explore the impact of background static field on high frequency AC permittivity in hydration water under an electric field mimicking the conditions inside a capacitor where one of the confinement walls is subject to an outgoing field and the other one to an incoming field. At strong static fields, the absorption peak undergoes a monotonic blue shift upon increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under an incoming field (the negative capacitor plate) features a red shift coinciding with maximal static-permittivity and reorientation-rate. The shift is mostly determined by the variation of the inverse static dielectric constant as proposed for mono-exponentially decaying polarization correlations. Conversely, hydration water at the opposite (positively charged) surface features a monotonic blue shift consistent with conventional saturation. The sensitivity of absorption peaks on the field suggests that surface charge densities could be deduced from sub-THz dielectric spectroscopy experiments in porous materials when interfaces accommodate a major fraction of water contained in the system. 

Kinetics of molecular and collective reorientations at solid/water interfaces are known to depend on local electric field. Deceleration is observed near surfaces subjected to an outgoing static field (pointing from surface to liquid) as is the case when the solid carries positive charge. In an incoming field, both the reorientation rates and local permittivity in hydration layer show a nonmonotonic dependence on field strength with fastest reorientations and highest permittivity observed when the field alignment barely offsets the orienting bias at the wall. (Mulpuri and Bratko, J. Chem. Phys. 158,134716, 2023). Here, we use Molecular Dynamics simulations to explore the impact of background field (or, equivalently, surface charge density) on high frequency (GHz to THz) AC permittivity in hydration water inside a nanosized aqueous film under perpendicular DC field. Our model system mimics conditions inside a capacitor where one of the confinement walls is subject to outgoing and the other one to incoming field. In very strong static fields, the frequency corresponding to the maximal imaginary part of AC permittivity, features a blue shift with increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under ingoing field (adjacent to the negative capacitor plate) features a red shift, which is especially pronounced at the field strength corresponding to the maxima of static-permittivity and reorientation-rate. The shift reflects the variation of the inverse static dielectric constant in normal direction, (Gekle and Netz, J. Chem. Phys. 137, 104704, 2012) with marginal effect of librational motions on the local AC permittivity. Hydration water at the opposite surface (closer to the positive capacitor plate), on the other hand, features a monotonic blue shift consistent with conventional saturation. The sensitivity of imaginary peaks on the field suggests surface charge densities could be deduced from THz dielectric spectroscopy experiments in a porous material where hydration layers comprise a major fraction of water contained in the system.