Tuning the Dielectric Response of Water in Nanoconfinement through Surface Wettability

Freezing of Water Solvation Dynamics in Nanoconfinement by Reverse Micelles at Room Temperature

Much attention has recently been paid to anomalously low dielectric constants of nanoconfined water between two slabs at room temperature (Fumagalli et al. Science, 2018, 360, 1339). These low values imply that the dipole rotation of the interfacial water on the slab is completely suppressed. Such freezing has so far been observed for water confined between solids. In contrast, it remains unclear whether this holds for water in soft confinement, which is omnipresent naturally and artificially. Here, we address this question using encapsulated reverse micelles with a dye molecule, allowing us to study water sandwiched between the surfactant and dye molecules in solution. Moreover, we examine the solvation related to the dielectric property of water, which is reorientational motion in the hydration layer of the dye molecule, by persistent hole-burning spectroscopy. We first show that the dye molecule is surrounded by water without contact with the surfactant and that the dye molecule has two or three hydration layers on average. We next demonstrate that the solvation dynamics is frozen below the water droplet size of ∼4 nm, whereas they become liquid-like when the RM size is further increased. The average gap distance (∼1.5 nm) for freezing the solvation agrees with the gap distance with no rotational water motions between slabs. Our findings may have biological relevance, providing a new aspect for understanding biological function in cells.

Noncontact In Situ Multidiagnostic NMR/Dielectric Spectroscopy

Introduction of a dielectric material in a nuclear magnetic resonance (NMR) probe head modifies the frequency response of the probe circuit, a phenomenon revealed by detuning of the probe. For NMR spectroscopy, this detuning is corrected for by tuning and matching the probe head prior to the NMR measurement. The magnitude of the probe detuning, "the dielectric shift", provides direct access to the dielectric properties of the sample, enabling NMR spectrometers to simultaneously perform both dielectric and NMR spectroscopy. By measuring sample dielectric permittivity as a function of frequency, dielectric permittivity spectroscopy can be performed using the new methodology. As a proof of concept, this was evaluated on methanol, ethanol, 1-propanol, 1-pentanol, and 1-octanol using a commercial cross-polarization magic angle spinning (CPMAS) NMR probe head. The results accurately match the literature data collected by standard dielectric spectroscopy techniques. Subsequently, the method was also applied to investigate the solvent-surface interactions of water confined in the micropores of an MFI-type, hydrophilic zeolite with a Si/Al ratio of 11.5. In the micropores, water adsorbs to Bro̷nsted acid sites and defect sites, resulting in a drastically decreased dielectric permittivity of the nanoconfined water. Theoretical background for the new methodology is provided using an effective electric circuit model of a CPMAS probe head with a solenoid coil, describing the detuning resulting from the insertion of dielectric samples in the probe head.

Origin of dielectric polarization suppression in confined water from first principles

It has long been known that the dielectric constant of confined water should be different from that in bulk. Recent experiments have shown that it is vanishingly small, however the origin of the phenomenon remains unclear. Here we used ab initio molecular dynamics simulations (AIMD) and AIMD-trained machine-learning potentials to understand water's structure and electronic properties underpinning this effect. For the graphene and hexagonal boron-nitride substrates considered, we find that it originates in the spontaneous anti-parallel alignment of the water dipoles in the first two water layers near the solid interface. The interfacial layers exhibit net ferroelectric ordering, resulting in an overall anti-ferroelectric arrangement of confined water. Together with constrained hydrogen-bonding orientations, this leads to much reduced out-of-plane polarization. Furthermore, we directly contrast AIMD and simple classical force-field simulations, revealing important differences. This work offers insight into a property of water that is critical in modulating surface forces, the electric-double-layer formation and molecular solvation, and shows a way to compute it.

Dielectric exclusion, an éminence grise

Dielectric exclusion has long been well-established as the key mechanism in membrane desalination, critical for delivering the required levels of salt rejection, also playing important role in electro-membrane processes, nanofluidics, and biomimetics. Unfortunately, its elusive nature and many features, such as dependence on the pore size, membrane hydration, and ion size and charge, make it deceivingly similar to the other ion exclusions mechanisms, steric and Donnan, which has led to much controversy and misconceptions. Starting from the Born model and the concept of self-energy, the present paper reviews and highlights the physical basis of dielectric exclusion, its main features and the ways it may be looked at. It discusses what makes the dielectric exclusion both similar and distinctly different from the other mechanism and its synergy and intimate connection with other phenomena, such as Donnan exclusion, permeability-selectivity upper-bound, and selectivity of charged membranes towards uncharged solutes. The paper also addresses subjects that still cause much controversy at present, such as appropriate measures of ionic radii and the subtle distinction between the dielectric exclusion and primary ion hydration. It also points to gaps that need to be bridged towards more complete theory. The points addressed here are important for understanding, modeling and development of various next-generation separation technologies including water purification, resource recovery and reuse, and green energy generation and storage.

Microscopic Simulations of Electrochemical Double-Layer Capacitors

Electrochemical double-layer capacitors (EDLCs) are devices allowing the storage or production of electricity. They function through the adsorption of ions from an electrolyte on high-surface-area electrodes and are characterized by short charging/discharging times and long cycle-life compared to batteries. Microscopic simulations are now widely used to characterize the structural, dynamical, and adsorption properties of these devices, complementing electrochemical experiments and in situ spectroscopic analyses. In this review, we discuss the main families of simulation methods that have been developed and their application to the main family of EDLCs, which include nanoporous carbon electrodes. We focus on the adsorption of organic ions for electricity storage applications as well as aqueous systems in the context of blue energy harvesting and desalination. We finally provide perspectives for further improvement of the predictive power of simulations, in particular for future devices with complex electrode compositions.