Water motions and orientations in nanotubes of various dimensions. (Simulations done by Jay Mashl, University of Illinois/NCSA/Beckman Institute Computational Biology/Nanoscience Group)
System Setup Carbon nanotube (fixed) 8 sizes of nanotubes ranging 5.4-16.3 Å dia., armchair (5,5) -(12,12). Length ~40 Å. Bilayer mimetic (hcp CH2's, fixed) SPC/E water (T = 300 K) 2qH = -qO = 0.8476 e Electrostatics: PME Nose-Hoover coupling Pressure piston (Pz = 1 bar) Runs of ~2 ns each using GROMACS ( Seewww. gromacs.org) • Simulations done on NCSA IA32 and IA64 Linux superclusters
Relative Diffusion coefficients Water in Nanotube vs. bulk(=1) "Critical" slowing Dz(tube) / Dz(bulk) Nanotube diameter (Å)
Diffusion of Water: Two Dimensions 16.3 Mean-square displacement (Å2) 13.6 10.8 9.5 12.2 8.1 6.8 Time lag (ps)
Water Dipole Autocorrelation < p(t) .p(0) > / p2 Time lag (ps)
Water Velocity Autocorrelation (nm / ps)2 <v(t) . v(0) > Time lag (ps)
Snapshots of Water Configurations T = 300 K (water), fixed tube & slab (6,6) (9,9) (12,12) Single file Bulk water properties not yet achieved Critical size for order 1-D hydrogen bonding 2-D hydrogen bonding
Summary Water motions and orientations are qualitatively modified by confinement in nanotubes. Depressed transverse diffusion Modified dipole autocorrelations Collective oscillations in narrow tubes Anomalous behavior (extreme ordering and reduced mobility) is seen at critical nanotube dimensions Point for discussion and future work: Could anomolous solvent behavior at critical dimensions be modulated by an external stimulus (change in electric field for example) to serve as basis for switching behavior?