Quantifying barriers to monovalent anion transport in narrow non-polar pores

Abstract

The transport of anionic drinking water contaminants (fluoride, chloride, nitrate and nitrite) through narrow pores ranging in effective radius from 2.5 to 6.5 Å was systematically evaluated using molecular dynamics simulations to elucidate the magnitude and origin of energetic barriers encountered in nanofiltration. Free energy profiles for ion transport through the pores show that energy barriers depend on pore size and ion properties and that there are three key regimes that affect transport. The first is where the ion can fit in the pore with its full inner hydration shell, the second is where the pore size is between the bare ion and hydrated radius, and the third is where the ion size approaches that of the pore. Energy barriers in the first regime are relatively small and due to rearrangement of the inner hydration shell and/or displacement of further hydration shells. Energy barriers in the second regime are due to partial dehydration and are larger than barriers seen in the first regime. In the third regime, the pore becomes too small for bare ions to fit regardless of hydration and thus energy barriers are very high. In the second regime where partial dehydration controls transport, the trend in the slopes of the change in energy barrier with pore size corresponds to the hydration strength of the anions.

Functional Annotation of Ion Channel Structures by Molecular Simulation

“…Analysis of the pore-radius profiles for the individual umbrella sampling simulations reveals that even when an Na + ion is restrained within the hydrophobic gate, the local expansion of the pore is minimal (Figure S1C). Comparable dehydration of ions at hydrophobic gates has also been seen in simulations of model nanopores (Richards et al., 2012) and the closed state of the GLIC channel (Zhu and Hummer, 2012). …”

Factors affecting fluoride and natural organic matter (NOM) removal from natural waters in Tanzania by nanofiltration/reverse osmosis

“…5) of three key regimes for fluoride with regard to membrane pore versus ion size (Richards et al, 2012b):…”

Surface effects on ionic Coulomb blockade in nanometer-size pores

“…In our calculations, the thickness of the membrane was varied from c to 8c (8c is the typical thickness of the membranes used in studies of nano-ionic devices [15]). The radius of a cylindrical nanopore drilled through the membrane's center in most of our studies was r = 6Å(at this value of r ∼ r * the nanopore is still in the Coulomb blockade regime while dehydration effects [16] are small [11,17]) . Different radii and locations of the nanopore were also explored.…”