Critical Knowledge Gaps in Mass Transport through Single-Digit Nanopores: A Review and Perspective

Abstract

Not all nanopores are created equal. By definition, nanopores have characteristic diameters or conduit widths between ∼1 and 100 nm. However, the narrowest of such pores, perhaps best called Single Digit Nanopores (SDNs) and defined as those with regular diameters less than 10 nm, have only recently been accessible experimentally for precision transport measurements. This Review summarizes recent experiments on pores in this size range that yield surprising results, pointing toward extraordinary transport efficiencies and selectivities for SDN systems. These studies have identified critical gaps in our understanding of nanoscale hydrodynamics, molecular sieving, fluidic structure, and thermodynamics. These knowledge gaps are, in turn, an opportunity to discover and understand fundamentally new mechanisms of molecular and ionic transport at the nanometer scale that may inspire a host of new technologies, from novel membranes for separations and water purification to new gas-permeable materials and energy storage devices. Here we highlight seven critical knowledge gaps in the study of SDNs and identify the need for new approaches to address these topics.

The relative insignificance of advanced materials in enhancing the energy efficiency of desalination technologies

“…22,23 Ultra-fast water transport through novel material channels results from flow enhancement under slip flow conditions. 24 Slip flow occurs when the interactions between water molecules and the channel surface result in a nonzero velocity, leading to a breakdown of the commonly assumed no-slip boundary condition. This case often arises when considering atomically smooth, hydrophobic surfaces, such as graphitic planes.…”

“…where P W is the intrinsic water permeability of the channel, Z is the viscosity of water, l s is the slip length, and h is the channel size. Though theoretical simulations have failed to successfully predict the results of experimental measurements, 24 the potential for ultra-high permeabilities has spearheaded research efforts in the development of novel materials for next-generation desalination membranes. Despite being exciting in concept, the practical implications of ultra-permeable membranes toward increased energy efficiency in RO desalination are minimal.…”

Molecular Mean-Field Theory of Ionic Solutions: A Poisson-Nernst-Planck-Bikerman Model

“…Water and ions give life. Their electrostatic and kinetic interactions play essential roles in biological and chemical systems such as DNA, proteins, ion channels, cell membranes, physiology, nanopores, supercapacitors, lithium dendrite growth, porous media, corrosion, geothermal brines, environmental applications, and the oceanic system [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ]. Poisson, Boltzmann, Nernst, and Planck laid the foundations of classical electrostatic and kinetic theories of ions in 1813–1890 [ 35 , 36 , 37 , 38 , 39 ].…”

“…Water molecules are treated as a dielectric medium (constant) without volumes either. However, advanced technologies in ion channel experiments [ 56 , 57 ] and material science [ 33 , 34 , 58 ] have raised many challenges for classical continuum theories to describe molecular mechanisms of ions and water (or solvents) with specific size effects in these systems at nano or atomic scale [ 9 , 12 , 14 , 16 , 17 , 18 , 19 , 30 , 33 ].…”

Revisiting Sampson's theory for hydrodynamic transport in ultrathin nanopores

“…Advances in experimentation [1][2][3][4][5] and computational studies [6][7][8] of fluids in nanoconfinement have led to the observation of unique properties of fluids. For example, water in carbon-based nanopores has been shown to have giant transport rates [5,[8][9][10][11], fast rotational motion [12,13], and rotation-translation coupling [14].…”