Water transport through functionalized nanotubes with tunable hydrophobicity

Abstract

Molecular dynamics simulations are used to study the occupancy and flow of water through nanotubes comprised of hydrophobic and hydrophilic atoms, which are arranged on a honeycomb lattice to mimic functionalized carbon nanotubes (CNTs). We consider single-file motion of TIP3P water through narrow channels of (6,6) CNTs with varying fractions (f) of hydrophilic atoms. Various arrangements of hydrophilic atoms are used to create heterogeneous nanotubes with separate hydrophobic/hydrophilic domains along the tube as well as random mixtures of the two types of atoms. The water occupancy inside the nanotube channel is found to vary nonlinearly as a function of f, and a small fraction of hydrophilic atoms (f ≈ 0.4) are sufficient to induce spontaneous and continuous filling of the nanotube. Interestingly, the average number of water molecules inside the channel and water flux through the nanotube are less sensitive to the specific arrangement of hydrophilic atoms than to the fraction, f. Two different regimes are observed for the water flux dependence on f - an approximately linear increase in flux as a function of f for f < 0.4, and almost no change in flux for higher f values, similar to the change in water occupancy. We are able to define an effective interaction strength between nanotube atoms and water's oxygen, based on a linear combination of interaction strengths between hydrophobic and hydrophilic nanotube atoms and water, that can quantitatively capture the observed behavior.

Breakdown of 1D water wires inside charged carbon nanotubes

“…The average number of confined water molecules increase exponentially with the charge percentage as seen in figure 2(b). This increase in average number of confined water molecules is in correspondence with earlier computational study 32 and suggests that the electrostatic interactions between the CNT and water play a dominant role compared to the vDW interactions 13 during the filling process.…”

“…There are several theoretical models 12,13 which describe the permeation of water through nanotube. Simulations have provided invaluable insights about the structure, dynamics and thermodynamics 14,15 of water confined inside pristine nanotubes.…”

Molecular dynamics study of nanoconfined TIP4P/2005 water: how confinement and temperature affect diffusion and viscosity

“…With this method, we estimated the contact angle for the hydrophobic case to be around 96.5 • which indicates a slight hydrophobicity of the surface, similarly to the one computed by Hummer et al [21], Moskowitz et al, [71] and Kohler et al [48] considering the same interaction strength (ε = 0.2703 kJ/mol). In the superhydrophobic system, where ε sh co is 0.047 kJ/mol, we obtained a contact angle of ≈ 132 • , much larger than 90 • .…”

“…The value of ε CO established the amount of hydrophobicity of the carbon wall. In this work we have used a value of ε CO widely used for water in graphite confinement [21,48,71,72] correponding to a contact angle of around 96.5 • . For comparison, we have also used a superhydrophobic water-carbon interaction value previouly , filled with water molecules (red for oxygens and white for hydrogens).…”

2D nanoporous membrane for cation removal from water: Effects of ionic valence, membrane hydrophobicity, and pore size

“…While graphene is a purely hydrophobic material, MoS 2 sheets have both hydrophobic (S) and hydrophilic (Mo) sites. Recent studies have shown that the water dynamics and structure inside hydrophobic or hydrophilic pores can be quite distinct regarding the pore size [28][29][30] and even near hydrophobic or hydrophilic protein sites. 31 Three cations are considered: the standard monovalent sodium (Na + ), the divalent zinc (Zn 2+ ), and the trivalent iron (Fe 3+ ).…”