Suppressed terahertz dynamics of water confined in nanometer gaps

Abstract

Nanoconfined waters exhibit low static permittivity mainly due to interfacial effects that span about one nanometer. The characteristic length scale may be much longer in the terahertz (THz) regime where long-range collective dynamics occur; however, the THz dynamics have been largely unexplored because of the lack of a robust platform. Here, we use metallic loop nanogaps to sharply enhance light-matter interactions and precisely measure real and imaginary THz refractive indices of nanoconfined water at gap widths ranging from 2 to 20 nanometers, spanning mostly interfacial waters all the way to quasi-bulk waters. We find that, in addition to the well-known interfacial effect, the confinement effect also contributes substantially to the decrease in the complex refractive indices of the nanoconfined water by cutting off low-energy vibrational modes, even at gap widths as large as 10 nanometers. Our findings provide valuable insights into the collective dynamics of water molecules which is crucial to understanding water-mediated processes.

Universality in the Structure and Dynamics of Water under Lipidic Mesophase Soft Nanoconfinement

“…This red shift is attributed to the reduced degrees of freedom close to the interface compared to the center of the channel, which restricts the formation of possible hydrogen bonds. This behavior is consistent with previous experimental and theoretical results of water under geometrical restriction in other contexts. ,− Within each band one can appreciate a variation of the center frequencies for the various systems. This fine-tuning is somewhat led by the topology, with Pn 3 m phases (green symbols) generally blue-shifted with respect to Ia 3 d phases (pink symbols), pointing to a slightly increased rigidity of the network for the Pn 3 m symmetry.…”

“…In contrast, the core–shell model seems to work more generally in AOT lamellar phases, although this might not be the case for highly confined systems. When the lamellar topology is imposed by metallic matrices, the observations could be explained by assuming noninterfacial water to behave differently than in bulk . Our results show that the core–shell model can be applied to lipid-based bicontinuous cubic phases, for which the slight differences detected for the bulk-like populations with respect to pure water suggest mild soft nanoconfinement conditions, in agreement with the estimation above based on the molar ratio of water to surfactant.…”

“…The exotic behavior of nanoconfined water emerges from a complex interplay between the topological and geometrical properties of the confining space, as well as its chemical nature. Experimental and numerical works have explored multiple environments, both in natural , and artificial media. − These investigations have revealed a stark impact of the chemical identity of the confining surface on water, ,− inducing, among other features, a lower melting point ,, or a strong change in diffusivity. ,, For example, water confined within narrow carbon nanotubes exhibits an extraordinary fast diffusive behavior, which is instead significantly reduced when the nanotubes are rendered hydrophilic by carboxyl groups . Similarly, different physicochemical conditions can change the relaxation time of water under soft nanoconfinement by as many as 3 orders of magnitude .…”