Paper title:
Water transport through nanotubes with varying interaction strength between tube wall and water
Journal:
J. Phys. Chem. Lett. 2, 2978?2983 (2011)
Author/s:
Matthew Melillo[1], Fangqiang Zhu[2], Mark A. Snyder[1], and Jeetain Mittal*[1]
[1] Lehigh University, Department of Chemical Engineering, Bethlehem, Pennsylvania 18015, United States
[2] Laboratory of Chemical Physics, NIDDK, National Institutes of Health, Bethesda, Maryland, United States
Summary:
For this study, it is assumed that the interactions of atoms are solely dictated by their Lennard-Jones (LJ) potential parameters, well-depth ε and the characteristic distance σ. From this, the authors look at the effect of changing the interaction strength, εNT-OW, between the atoms of a nanotube (NT) and the oxygen atoms of water. Concretely, this means that they are investigating the differences between nanotubes made up of different materials (i.e., varying ε). The paper aimed to study the sensitivity of water flow to changes in the physical properties of the NT materials and to obtain the range of εNT-OW values that will provide the optimum water occupancy and flux.
MD simulations were performed using NAMD. Periodic boundary conditions were applied in x, y, and z directions. Langevin thermostat and Langevin piston Nose-Hoover method were used to maintain temperature and pressure at 300 K and 1 atm, respectively. Particle Mesh Ewald (PME) method was used for electrostatic interactions.
Simulations were conducted for a wide range of wettability: 0.02 kcal mol (highly hydrophobic NTs) to 0.20 kcal/mol (highly hydrophilic NTs). Standard LJ potential form was used for the interaction parameters. Water was modeled using TIP3P, wherein interaction between the H atom of water and NT atoms is zero. Twelve hexagonally packed NTs of types (6,6), (7,7), (8,8), (9,9), and (12,12) with length 1.34 nm were used for the simulations. A 5.6-nm (6,6) NT was also considered to study the possible effects of NT length on the observed behavior.
From the MD simulations, the authors were able to observe that there is a narrow transition region wherein water occupancy within the nanotube and water flux through the tube increase dramatically with increasing εNT-OW. It was observed that this transition region narrows as the diameter of nanotube increases; for instance, for tube diameter 1.6 nm, water transport changes from no flow to high water flux between εNT-OW values of 0.05 and 0.055 kcal/mol. Specifically, in the narrow region from 0.05 to 0.075 kcal/mol, the characteristics of a nanotube can change abruptly from being very hydrophobic with no water flux and low water occupancy to being very hydrophilic with high water flux and high occupancy. This transition region also coincides with water contact angles close to 900, signifying that there might be a relationship between nanotube wettability and water transport through it.
It was also observed that if the εNT-OW value is higher than 0.075 kcal/mol, water flux decreased because water molecules tend to stay inside the nanotube more than pass through the other end of the tube (occupancy and residence time are higher). In addition, when nanotube diameter is increased, sharper transition regions, more abrupt changes in water flux, and more rapid changes in occupancy were observed. This is expected because there is greater interaction between water molecules and the nanotube wall.
It was also found that water flux is proportional to the average water occupancy divided by the average residence time within the nanotube. The proportionality constant calculated for this relationship was 0.36, which is independent of LJ parameters and nanotube diameter and length.
Contribution and application:
This study opens the possibility of using nanotubes made from materials other than the common ones like carbon and boron nitride. Other types of nanotubes may be developed for allowing water to pass through them. Then, for further research, these nanotubes could be made desalination devices by applying charges on one end of the nanotube to separate ions and allow only water to pass through with high flux.
By: Hannah Ebro
hannah@gist.ac.kr