The Importance of Dehydration in Determining Ion Transport in Narrow Pores

Abstract

The transport of hydrated ions through narrow pores is important for a number of processes such as the desalination and filtration of water and the conductance of ions through biological channels. Here, molecular dynamics simulations are used to systematically examine the transport of anionic drinking water contaminants (fluoride, chloride, nitrate, and nitrite) through pores ranging in effective radius from 2.8 to 6.5 Å to elucidate the role of hydration in excluding these species during nanofiltration. Bulk hydration properties (hydrated size and coordination number) are determined for comparison with the situations inside the pores. Free energy profiles for ion transport through the pores show energy barriers depend on pore size, ion type, and membrane surface charge and that the selectivity sequence can change depending on the pore size. Ion coordination numbers along the trajectory showed that partial dehydration of the transported ion is the main contribution to the energy barriers. Ion transport is greatly hindered when the effective pore radius is smaller than the hydrated radius, as the ion has to lose some associated water molecules to enter the pore. Small energy barriers are still observed when pore sizes are larger than the hydrated radius due to re-orientation of the hydration shell or the loss of more distant water. These results demonstrate the importance of ion dehydration in transport through narrow pores, which increases the current level of mechanistic understanding of membrane-based desalination and transport in biological channels.

Mechanosensitive Channel Activation by Diffusio-Osmotic Force

“…Inside the hydrophobic constriction, on the other hand, perfect slip is assumed, consistent with the plug-like flow found in hydrophobic nanotubes [26]. Note that assuming no slip inside the hydrophobic constriction instead leads pmf µ (z) in narrow channels exhibits a peak, reaching a maximum of µ 0 ∼ 10-30 k B T in the center of the channel, which decreases with increasing channel radius [18][19][20][21]36]. At a radius of R = 1 nm, µ 0 is still several k B T 's in short nanopores [36].…”

“…In weakly polar channels, water can fill constrictions down to the size of a single water molecule [14], but even strongly hydrophobic channels are intermittently filled with water [14][15][16]. Ions, on the other hand, are subject to a strongly repulsive potential of mean force (pmf) up to channel radii much larger than the ionic radius, caused by their hydration shells [17][18][19], steric and van der Waals interactions and self energy [20]. Using molecular dynamics simulations, the energy barrier for ion permeation through mscs has been estimated to be 17-34 k B T [21], explaining the lack of electric conductivity of mscs in the closed state despite its relatively wide permeation pathway.…”

Molecular Dynamics Simulations of Anion Exclusion in Clay Interlayer Nanopores

“…The answer to this question has important implications for the migration of salts and groundwater through clay-rich rocks (Fritz, 1986;Leroy et al, 2006), the design of engineered nanofluidic systems (Schoch et al, 2008;Bocquet and Charlaix, 2010), the performance of desalination membranes (Richards et al, 2012), and the transport of ions through channels in cell membranes (Nikaido and Rosenberg, 1981;Nikaido, 2003). It also is of importance to highlevel radioactive waste (HLRW) management, where the performance of geologic repositories is determined, in many cases, by the migration of anionic, long-lived fission products ( 129 I À , 99 TcO 4 À , 36 Cl À ) through nanoporous clay barriers (Tournassat et al, 2007;Altmann, 2008;Claret et al, 2010;Glaus et al, 2011;Altmann et al, 2012).…”

Moving Fe 2+ from ferritin ion channels to catalytic OH centers depends on conserved protein cage carboxylates

“…iron traffic | oxidoreductase enzyme activity | antioxidant | ferrihydrite | BioIron B iological and synthetic ion channels transport metal ions through barriers, including those in ferritin protein cages (1)(2)(3)(4)(5)(6)(7)(8). How ions reach specific destinations, after exiting ion channels, remains unknown.…”