Anomalous interplay of confinement, wettability, and salt concentration toward diffusion of saline water in nanochannels

Abstract

Water, characterized by its anomalous behavior, is a polar fluid whose transport properties can undergo counterintuitive modifications under confinement, especially with solvated microions, resulting in significant deviations from that of bulk pure water. Despite the observation of largely divergent transport phenomena of water induced by confinement-specific effects, the intricate interactions influencing the diffusive behavior of water remain inadequately understood. We bring forth the anomalous interplay of confinement, wettability, and salt concentration on water’s self-diffusive behavior under nanoconfinement using molecular-level investigations. In addition to the markedly different self-diffusive behavior of water compared to bulk, the combined effects of these parameters are not always intuitive, resulting in non-monotonic trends in diffusivity. Our findings reveal that dissolved ionic species induce structural perturbations that alter the hydrogen bonding network and water orientation as well as compress solvation shells in confinement-dependent ways, particularly in extreme confinements. The confined ionic inclusions lead to distinct diffusive behavior that deviates from water’s typical mobility in ion-free environments. These findings reveal the hitherto unexplored inter-coupling of various factors on water’s diffusive characteristics, not only providing valuable insights into the interfacial interactions at the nanoscale but also having far-reaching implications in the fields of ultra-filtration, point-of-care diagnostics, and energy storage devices.