Separation of water-ethanol solutions with carbon nanotubes and electric fields

Water structures in tip-charged carbon nanotubes

Carbon nanotubes (CNTs) have potential applications in separation membranes and nanofluidic devices. It is well known that the behavior of water molecules confined in CNTs is affected by surface functional groups and external electric fields, leading to structural changes. The understanding of these structural changes of water within various CNTs is crucial, particularly in the context of material separation. While there have been many investigations into the effects of individual specific functional groups, a comprehensive understanding of the effect of these functional groups and the electric fields they generate on water molecules remains elusive. In this study, we investigate the properties of water molecules in tip-charged CNTs of (8,8), (10,10), and (12,12) chiral vectors with positive charges at one tip and negative charges at the other tip. Abstraction of ionized functional groups as tip charges enables a comprehensive understanding that is independent of individual functional groups. The symmetrically arranged tip-charges spontaneously generate a strong and symmetric electric field in the CNTs. However, the strength and directionality of the electric field are non-uniform and complex. In the interiors of (8,8) and (10,10) tip-charged CNTs, helical and square structures, which have disturbances caused by the non-uniformity of the electric field, are observed. The properties of the water molecules differed significantly in the center of the CNTs and near positive and negative charges, despite the electric field symmetry. In (12,12) tip-charged CNTs with 12 charges, a local ring structure is observed in the vicinity of negative charges but not in the vicinity of positive charges. It is concluded that the water structures in tip-charged CNTs have different characteristics from those in plain CNTs under a uniform electric field.

Spontaneous sieving of water from ethanol using angstrom-sized nanopores

Ethanol is widely used as a precursor in products ranging from drugs to cosmetics. However, distillation of ethanol from aqueous solution is energy intensive and expensive. Here, we show that angstrom-sized nanopores with precisely controlled pore sizes can spontaneously remove water from ethanol-water mixtures through molecular sieving at room temperature and pressure. For small-diameter nanotubes, water-filling is observed, but ethanol is completely excluded, as evidenced by time-dependent density functional theory (TD-DFT) calculations and spectroscopy measurements. Potential of mean force calculations were performed to determine how the free energy barriers for water and ethanol-filling of the nanotubes change with increasing pore size. Water/ethanol selectivity ratio reaching as high as 6700 is observed with a (6,4) nanotube, which has a pore size of 0.204 nm. This selectivity vanishes as the pore size increases beyond 0.306 nm. These findings provide insights that may help realize energy efficient molecular sieving of ethanol and water.

Water molecules in CNT-Si3N4 membrane: Properties and the separation effect for water-alcohol solution

Water confined in carbon nanotubes (CNTs) has been intensively studied because of its unique properties and potential for various applications and is often embedded in silicon nitride (Si₃N₄) membranes. However, the understanding of the influence of Si₃N₄ on the properties of water in CNTs lacks clarity. In this study, we performed molecular dynamics simulations to investigate the effect of the Si₃N₄ membrane on water molecules inside CNTs. The internal electric field generated in the CNTs by the point charges of the Si₃N₄ membrane changes the structure and dynamical properties of water in the nanotubes, causing it to attain a disordered structure. The Si₃N₄ membrane decreases the diffusivity of water in the CNTs; this is because the Coulomb potential energy (i.e., electrostatic interaction) of water decreases owing to the presence of Si₃N₄, whereas the Lennard-Jones potential energy (i.e., van der Waals interaction) does not change significantly. Furthermore, electrostatic interactions make the water structure more stable in the CNTs. As a result, the Si₃N₄ membrane enhances the separation effect of the water-methanol mixture with CNTs in the presence of an external electric field. Furthermore, the threshold of the external electric field strength to induce water-methanol separation with CNTs is reduced owing to the presence of a silicon nitride membrane.

Electric field triggered release of gas from a quasi-one-dimensional hydrate in the carbon nanotube

We systematically investigate the effects of an axial electric field on the formation and decomposition of quasi-one-dimensional nitrogen gas hydrates within a single-walled carbon nanotube (SWNT) by using molecular dynamics (MD) simulations. We find that the nitrogen hydrate in the SWNT undergoes a series of structure phase transitions with increasing electric field. Corresponding to the structure transition, the nitrogen gas releases from the carbon nanotube in the electric field range of 1 V nm^-1 to 2 V nm^-1. However, nitrogen molecules are trapped as guest molecules, forming a molecule wire, in the ice nanotube when the electric field is less than 1 V nm^-1 or larger than 2 V nm^-1. Our simulations indicate that the nanotube is an excellent tiny gas tank that can be used to trap gas molecules and control their release triggered sensitively by electric signals. The key to this phenomenon is the change in orientations of water dipoles induced by the electric field, which leads to the structural change in the hydrogen-bonding network and the change in the diffusion coefficient of the water molecules. Our findings here may help understanding the mechanism of the electrorelease of gas from hydrates confined in the nanoscale space.

Entropy deepens loading chemical potentials of small alcohols by narrow carbon nanotubes

Small alcohol confinement within narrow carbon nanotubes has been extensively and systematically studied via rigorous free-energy calculations.

Atomistic insights into the separation mechanism of multilayer graphene membranes for water desalination

Graphene-based membranes have been extensively explored owing to their excellent separation properties. In this paper, multiple factors regarding desalination performance were investigated by molecular dynamics (MD) simulations. These factors include the interlayer spacing distance (H), the gap width (dG), offset (O), and the number of gaps and layers in a multilayer graphene membrane (MGM). It is found that salt rejection is influenced significantly by the interlayer spacing distance owing to the largest free energy between ions and graphene sheets as well as the relatively larger size of the hydration layer around the ions. The optimal desalting parameter (dG = 1 nm, H = 0.8 nm) was selected; MGM systems based on the optimized parameter exhibited excellent salt rejection for NaCl, MgCl2 and CaCl2 solutions. These results can provide some ideas for the future design of graphene-based membranes.

Separation of water-alcohol mixtures using carbon nanotubes under an electric field

Carbon nanotubes (CNTs) are a promising candidate for separation membranes because of their ability to transport substances at very high flow rates. However, there is a tradeoff between achieving a high selectivity using small pore sizes and the reduction of water flux. Here, using molecular dynamics simulations, we report that CNTs can effectively separate water-methanol mixtures under an electric field. Without an electric field and under piston pressure, both water and methanol flow through a CNT, resulting in no separation effect. In contrast, under an electric field and high piston pressure, CNTs allow selective water permeation while rejecting the permeation of methanol molecules. This separation effect is caused by the ordered structures of water molecules in the CNT. A high filtering effect is observed under the conditions of high methanol concentration in the solution or even with large-diameter CNTs up to 3.39 nm. As long as the ordered structure of water in the CNTs can be maintained, the strong filtering effect can be maintained.

Mechano-optical effects in multiwall carbon nanotubes ethanol based nanofluids

A highly sensitive technique for analyzing surface tension and dynamic viscosity of nanofluids was reported. Multiwall carbon nanotubes suspended in ethanol were evaluated. The assistance of a Fabry-Perot interferometer integrated by a small sample volume fluid allowed us to explore the stability and mechanical properties exhibited by the nanostructures. The surface tension and dynamic viscosity of the colloid was examined by using interferometric optical signals reflected from a remnant drop pending at the end of an optical fiber. Nanosecond pulses provided by a Nd:YAG laser source with 9.5 MW/mm² at 532 nm wavelength were used to induce mechano-optical effects in the liquid drop. The mechanical parameters were approximated, taking into account single optical pulses interacting with an inelastic mass-spring-damper system.

Electric field mediated separation of water–ethanol mixtures in carbon-nanotubes integrated in nanoporous graphene membranes

The tunable separation of water–ethanol mixtures inside CNTs by varying the electric field orientation angle θ.

Rotating-Electric-Field-Induced Carbon-Nanotube-Based Nanomotor in Water: A Molecular Dynamics Study

Using molecular dynamics simulations, it is shown that a carbon nanotube (CNT) suspended in water and subjected to a rotating electric field of proper magnitude and angular speed can be rotated with the aid of water dipole orientations. Based on this principle, a rotational nanomotor structure is designed and the system is simulated in water. Use of the fast responsiveness of electric-field-induced CNT orientation in water is employed and its operation at ultrahigh-speed (over 10¹¹ r.p.m.) is shown. To explain the basic mechanism, the behavior of the rotational actuation, originated from the water dipole orientation, is also analyzed . The proposed nanomotor is capable of rotating an attached load (such as CNT) at a precise angle as well as nanogear-based complex structures. The findings suggest a potential way of using the electric-field-induced CNT rotation in polarizable fluids as a novel tool to operate nanodevices and systems.

Multiscale molecular simulations on interfacial adsorption and permeation of nanoporous graphynes

Unique interfacial adsorption and permeation on nanoporous graphynes have been revealed by a multiscale simulation strategy.