Electric field and temperature effects on water in the narrow nonpolar pores of carbon nanotubes

Abstract

Water molecules in the narrow cylindrical pore of a (6,6) carbon nanotube form single-file chains with their dipoles collectively oriented either up or down along the tube axis. We study the interaction of such water chains with homogeneous electric fields for finite closed and infinite periodically replicated tubes. By evaluating the grand-canonical partition function term-by-term, we show that homogeneous electric fields favor the filling of previously empty nanotubes with water from the bulk phase. A two-state description of the collective water dipole orientation in the nanotube provides an excellent approximation for the dependence of the water-chain polarization and the filling equilibrium on the electric field. The energy and entropy contributions to the free energy of filling the nanotube were determined from the temperature dependence of the occupancy probabilities. We find that the energy of transfer depends sensitively on the water-tube interaction potential, and that the entropy of one-dimensionally ordered water chains is comparable to that of bulk water. We also discuss implications for proton transfer reactions in biology.

Coupling nonpolar and polar solvation free energies in implicit solvent models

“…[23][24][25] Several of these studies indicated that the magnitude of dewetting is sensitive to the nature of the solute-solvent attractive dispersion interactions. [19][20][21] A similar sensitivity was found in systems where the solutes carry charges or are exposed to an external electric field, e.g., electrostatic interactions have been shown to strongly affect the dewetting behavior of hydrophobic channels [26][27][28] and hydrophobic spherical nanosolutes. 29,30 Furthermore, two recent simulations of proteins supported the importance of solvent dewetting and its sensitivity in realistic biomolecular systems.…”

“…Importantly, this demonstrates that the present formalism captures the sensitivity of dewetting phenomena to specific solvent-solute interactions as reported in previous studies. 20,21,26,27,29,31,32 Note that the optimal shape function at ͉z͉Ӎ2 Å is closer to the solutes in VI compared to V due to the proximity of the charge to the interface. Clearly, the observed effects, particularly the transition from III to VI, cannot be described by existing solvation models which use the surface area ͑GB/SA or PB/SA͒ ͑Ref.…”

“…20,21,26,27,29,31,32 The dewetting in our model is driven by the interfacial term which favors minimizing the solute-solvent interface. A comment must be made here regarding the sensitivity of dewetting to the particular solvent-solute interactions.…”

Field-exposed water in a nanopore: liquid or vapour?

“…Carlo simulations 12,16 . An alternative setup, particularly suitable in Molecular Dynamics studies involves a simulation of both the confinement and bulk, field-free environment (reservoir) in equilibrium with each other 14,15 .…”

Coupling Hydrophobicity, Dispersion, and Electrostatics in Continuum Solvent Models

“…For example, simulations of explicit water between platelike solutes revealed that hydrophobic attraction and dewetting phenomena are strongly sensitive to the nature of solute-solvent dispersion interactions [4,5]. Similarly, simulations of hydrophobic channels [6,7] and nanosolutes [8] have shown that electrostatic potentials strongly affect the dewetting behavior and potentials of mean force (pmf). A fully atomistic simulation of the folding of the two-domain protein BphC enzyme [9] further supported coupling by showing that the region between the two domains was completely dewetted when solvent-solute van der Waals (vdW) and electrostatic interactions were turned off, but accommodated 30% of the density of bulk water with the addition of vdW attractions, and 85%-90% with the addition of electrostatics, in accord with experimental results.…”