Proton Diffusion through Bilayer Pores

Abstract

The transport of protons through channels in complex environments is important in biology and materials science. In this work, we use multistate empirical valence bond simulations to study proton transport within a well-defined bilayer pore in a lamellar L phase lyotropic liquid crystal (LLC). The LLC is formed from the self-assembly of dicarboxylate gemini surfactants in water, and a bilayer-spanning pore of radius of approximately 3-5 Å results from the uneven partitioning of surfactants between the two leaflets of the lamella. Local proton diffusion within the pore is significantly faster than diffusion at the bilayer surface, which is due to the greater hydrophobicity of the surfactant/water interface within the pore. Proton diffusion proceeds by surface transport along exposed hydrophobic pockets at the surfactant/water interface and depends on the continuity of hydronium-water hydrogen bond networks. At the bilayer surface, there is a reduced fraction of the "Zundel" intermediates that are central to the Grotthuss transport mechanism, whereas the fraction of these species within the bilayer pore is similar to that in bulk water. Our results demonstrate that the chemical nature of the confining interface, in addition to confinement length scale, is an important determiner of local proton transport in nanoconfined aqueous environments.

Proton Transport in [BMIM+][BF4–]/Water Mixtures Near the Percolation Threshold

“…The motif that hydronium ions nucleate their own water clusters/transport networks is similar to that observed in studies of other systems: for example, Peng et al . have shown that an excess proton can create its own water wires in confined environments such as carbon nanotubes and other hydrophobic cavities. ,, We note that previous work on proton transport in surfactant-based lyotropic liquid crystals emphasized the importance of interactions of the hydronium oxygen atom with hydrophobic moieties on the surfactants. , We have computed correlation functions between the oxygen atom of the hydronium ion and aliphatic chains of the BMIM + cation, which are shown in Figure S14; while there is a tendency for coordination with the hydrophobic BMIM + tail, concentration effects are subtle and appear relatively minor.…”

“…Additionally, a different choice of anion may favor the Zundel configuration instead of the Eigen configuration, with benefits for proton transport rates. The many previous examples of enhanced proton transport in low-water-content systems ,,, suggest much room for further optimization of ionic liquid based proton conducting solutions.…”

“…For starting configurations for the MS-EVB simulations, one BMIM + cation was removed from each system and replaced with a hydronium ion. MS-EVB simulations were conducted with an in-house code, following the algorithm described by Voth and co-workers. ,− The particle mesh Ewald (PME) method was used for electrostatics, van der Waals interactions were truncated at 1.4 nm cutoff distance, and a Langevin thermostat was used with a 0.1 ps –1 friction coefficient. Comparison to benchmark NVE ensemble simulations verified that this thermostat choice does not significantly alter predictions of proton transport dynamics.…”

“…27,30,116 We note that previous work on proton transport in surfactant-based lyotropic liquid crystals emphasized the importance of interactions of the hydronium oxygen atom with hydrophobic moieties on the surfactants. 53,54 We have computed correlation functions between the oxygen atom of the hydronium ion and aliphatic chains of the BMIM + cation, which are shown in Figure S14; while there is a tendency for coordination with the hydrophobic BMIM + tail, concentration effects are subtle and appear relatively minor.…”

“…Theoretical and computational studies can significantly aid interpretation of environment effects on proton transport. Ab initio and reactive molecular dynamics (MD) simulations have provided a multitude of insights into the transport mechanisms of excess protons in diverse systems ranging from bulk water to interfaces to biological and synthetic channels. ,− The multistate empirical valence bond (MS-EVB) model developed by Voth and co-workers has been particularly successful at enhancing theoretical understanding of proton transport. ,,− The MS-EVB methodology allows for explicit and efficient simulation of the bond breaking and forming process that accompanies proton structural diffusion, allowing for an assessment of solvent and environmental effects on proton transport mechanisms. , MS-EVB simulations have been applied to evaluate proton solvation and transport motifs in ionic and/or confined systems that may possess characteristics similar to IL/water mixtures, such as Nafion, , carbon nanotubes, , biological/synthetic channels, ,,− and salt solutions. , For example, while an order of magnitude faster diffusion is observed for proton transport in model hydrophobic pores and carbon nanotubes of sufficiently low channel radius, , MS-EVB simulations have shown that more complex cavities have varied effects on the Grotthuss mechanism; while narrower channels still favor the Zundel cation, substituents located inside pores such as those found in cell membranes can serve to aid or hinder proton transport depending on the local solvated environment of the excess proton. ,,− Additionally, MS-EVB simulations of protons in chloride salt solutions , indicate decreased proton transport rates with increasing ion concentration, partly due to a disruption in the hydrogen-bonded water structure needed for Grotthuss transport and also due to increased electrostatic interactions.…”