Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Abstract

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water−energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Single-Molecule Fluorescence Investigations of Solute Transport Dynamics in Nanostructured Membrane Separation Materials

“…Single-molecule measurements on such materials will afford molecular-level information on unique solute transport phenomena that were found using ensemble-averaged methods. 85 Measurements in Complex Mixtures. Actual membrane separation processes aim to separate target species from many other species in multicomponent mixtures.…”

“…In particular, it is desired to establish simple methods that reproducibly yield materials comprising μm-long domains/pores with a diameter of ≤2 nm. Single-molecule measurements on such materials will afford molecular-level information on unique solute transport phenomena that were found using ensemble-averaged methods …”

Axial Diffusion in Liquid-Saturated Cylindrical Silica Pore Models

“…By accounting for the loading dependence of the diffusion coefficient, the agreement between different data sets could be improved . In mesoporous materials such as silica, completely saturated with fluids, the static and dynamic fluid properties may differ from their corresponding bulk values and are affected by the pore diameter and the chemical nature of the inner surface of the pore. − A structural parameter characterizing the pore topology is the tortuosity, often defined by the ratio of the actual path and the shortest path from A to B or by the ratio of the two paths squared . The latter definition can be approximated by the ratio of the bulk diffusion coefficient of a chemically inactive species and its diffusion coefficient measured under confinement. , The notion of chemical inactivity is important to avoid any superimposed effect resulting from interactions with the pore walls that may lead to rather complex diffusion behavior such as a strong reduction or even an enhanced self-diffusivity. , However, also structural properties such as constrictions, reduced connectivity, or roughness affect the diffusion coefficient ratio .…”

Probing Confinement Effects on the Infrared Spectra of Water with Deep Potential Molecular Dynamics Simulations

“…Water confined in nanopores is expected to have a hydrogen-bond (H-bond) network that deviates from that of the bulk liquid. , These deviations are particularly amplified in the narrowest nanopores, so-called single-digit nanopores (SDNs), with diameters shorter than 10 nm . An improved understanding of hydrogen bonding in SDNs is not only important to bridge knowledge gaps in the structure and dynamics of confined water but also promises to advance a wide range of technological applications, from energy storage and conversion to ion-selective membranes for water desalination. , …”

“…3 An improved understanding of hydrogen bonding in SDNs is not only important to bridge knowledge gaps in the structure and dynamics of confined water but also promises to advance a wide range of technological applications, from energy storage and conversion to ion-selective membranes for water desalination. 4,5 Among many nanoporous systems, carbon nanotubes (CNTs) represent an ideal model system for studying confinement effects. 6−9 Several attempts have been made to understand the water properties in these systems, including vibrational spectroscopy measurements that are particularly suitable for probing H-bonds.…”