Extremely Narrow Nanopores on the Rise
Understanding mass transport in nanopores is critical in a wide variety of emerging energy and environmental technologies, such as water desalination and supercapacitors. Not all nanopores are created equal. For starters, their diameters vary between 1 and 100 nanometers (nm). The smallest of these nanopores, called single digit nanopores (SDNs), have diameters of less than 10 nm, and have only recently been accessible experimentally for precision transport measurements.
Broadly, SDNs underpin a large array of material systems and technological applications. For example, adsorbent technology for chemical and air separations employs activated carbon with pores as small as 0.6 nm in size, and zeolites, with a pore diameter around 0.6 nm, have been used for a wide variety of catalysis, adsorption, and pollution abatement applications. Energy storage is another area, where carbon porous materials with diameters less than 10 nm have been utilized as electrode materials in supercapacitors to store energy using either ion adsorption or fast surface redox reactions.
However, detailed studies of SDN transport at the single pore level have only become possible recently with the advent of isolated pore systems combined with sophisticated experimental measurements. More interestingly, recent studies of transport in such systems reveal many counterintuitive behaviors that challenge existing theories in the areas of nanofluidics and fluid confinement.
In a recent review article featured on the cover of the The Journal of Physical Chemistry, a team of scientists and colleagues from eight institutions, led by the Massachusetts Institute of Technology (MIT) in the Center for Enhanced Nanofluidic Transport (CENT), have reviewed recent SDN experiments and identified critical gaps in understanding nanoscale behavior in the areas of hydrodynamics, molecular sieving, fluidic structure, and thermodynamics.
For example, recent studies have found that the narrowest nanopores demonstrate the highest mass transport rates. Other notable knowledge gaps include fluid phase boundaries in SDNs that are distorted relative to their bulk fluid counterparts, and nonlinear, correlative effects in ion transport through SDNs that are not observed in larger diameter nanopores.
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. For instance, a better understanding of transport at the nanoscale can lead to innovative technologies such as new membranes for water purification, new gas-permeable materials, and energy storage devices. SDNs can be tailored to sieve ions eﬃciently from seawater and serve as membranes for seawater desalination, diﬀerentiate between polar and nonpolar ﬂuids, enhance proton transport in fuel cell applications, and generate electricity from osmotic power harvesting.
The manuscript can be seen as a summary of the main activities of the Center for Enhanced Nanofluidic Transport. As an EFRC, CENT will apply precision model systems, transformative experimental tools, and predictive multiscale theories to address emerging and compelling gaps in our knowledge of fluid flow and molecular transport in single digit nanopores and establish the scientific foundation for developing transformative molecular separation technologies impacting the Water-Energy Nexus.
Faucher S, N Aluru, MZ Bazant, D Blankschtein, AH Brozena, J Cumings, J Pedro de Souza, M Elimelech, R Epsztein, JT Fourkas, AG Rajan, HJ Kulik, A Levy, A Majumdar, C Martin, M McEldrew, RP Misra, A Noy, TA Pham, M Reed, E Schwegler, Z Siwy, Y Wang, and M Strano. 2019. “Critical knowledge gaps in mass transport through single-digit nanopores: a review and perspective.” J. Phys. Chem. C 123(35):21309-21326. DOI: 10.1021/acs.jpcc.9b02178
This work and all authors were supported as part of the Center for Enhanced Nanofluidic Transport, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0019112. S.F. acknowledges support by the National Science Foundation Graduate Research Fellowship under Grant No. 1122374.