Structures of water molecules in carbon nanotubes under 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.

Formation of a two-dimensional helical square tube ice in hydrophobic nanoslit using the TIP5P water model

In extreme and nanoconfinement conditions, the tetrahedral arrangement of water molecules is challenged, resulting in a rich and new phase behavior unseen in bulk phases. The unique phase behavior of water confined in hydrophobic nanoslits has been previously observed, such as the formation of a variety of two-dimensional (2D) ices below the freezing temperature. The primary identified 2D ice phase, termed square tube ice (STI), represents a unique arrangement of water molecules in 2D ice, which can be viewed as an array of 1D ice nanotubes stacked in the direction parallel to the confinement plane. In this study, we report the molecular dynamics (MD) simulations evidence of a novel 2D ice phase, namely, helical square tube ice (H-STI). H-STI is characterized by the stacking of helical ice nanotubes in the direction parallel to the confinement plane. Its structural specificity is evident in the presence of helical square ice nanotubes, a configuration unseen in both STI and single-walled ice nanotubes. A detailed analysis of the hydrogen bonding strength showed that H-STI is a 2D ice phase diverging from the Bernal-Fowler-Pauling ice rules by forming only two strong hydrogen bonds between adjacent molecules along its helical ice chain. This arrangement of strong hydrogen bonds along ice nanotube and weak bonds between the ice nanotube shows a similarity to quasi-one-dimensional van der Waals materials. Ab initio molecular dynamics simulations (over a 30 ps) were employed to further verify H-STI's stability at 1 GPa and temperature up to 200 K.

Enhanced Ion Rejection in Carbon Nanotubes by a Lateral Electric Field

Reverse osmosis membranes hold great promise for dealing with global water scarcity. However, the trade-off between ion selectivity and water permeability is a serious obstacle to desalination. Herein, we introduce an effective strategy to enhance the desalination performance of the membrane. A series of molecular dynamics simulations manifest that an additional lateral electric field significantly promotes ion rejection in carbon nanotubes (CNTs) under the drive of longitudinal pressure. Specifically, with the increase in the electric field, the ion flux shows a deep linear decay, while the water flux decreases only slightly, resulting in a linear increase in ion rejection. The energy barriers of ions around the CNT inlet are obtained by calculating the potentials of mean force to explain enhanced ion rejection. The lateral electric field uniformly raises the energy barriers of ions by pushing them away from the CNT inlet, corresponding to the enhanced ion velocity in the field direction. Furthermore, with the increase in CNT diameter, there is a significant increase in the flux of both ions and water; however, the lateral electric field can also obviously enhance the ion rejection in wider CNTs. Consequently, the enhancement of ion rejection by lateral electric fields should be universal for different CNT diameters, which opens a new avenue for selective permeation and may have broad implications for desalination devices with large pore sizes.

Promoting Electroosmotic Water Flow through a Carbon Nanotube by Weakening the Competition between Cations and Anions in a Lateral Electric Field

Understanding the electroosmotic flow through a nanochannel is essential to the design of novel nanofluidic devices, ranging from desalination to nanometer water pumps. Nonetheless, the competition between cation and anion in electric fields inevitably leads to a limited pumping of water, and thus weakening their competition could be a new avenue for the fundamental control of water transport. In this work, through a series of molecular dynamics simulations, we find a surprising phenomenon in which under the drive of a traditional longitudinal electric field, an additional lateral electric field can significantly weaken the competitive transport of a cation and anion through a carbon nanotube, which spontaneously leads to a massive increase in electroosmotic water flux. Specifically, with the increase in the lateral electric field, the anion flux exhibits an almost linear reduction, and the cation flux is stable and can even be enhanced. As a result, the net water flux along the cation direction increases significantly. The key to this unexpected phenomenon lies in the size and mobility difference between the cation and anion. The anion is larger and has greater mobility and is thus more susceptible to the lateral electric field, which ultimately leads to the reduction of its flux. For different ion types and CNT lengths, we can observe similar electropumping phenomenon, where the friction force induced by the lateral electric field becomes nontrivial for long CNTs. Our results provide a new route to tune the competitive transport of cations and anions and should be useful for the design of novel electroosmotic pumps.

Pressure-driven water flow through a carbon nanotube controlled by a lateral electric field

Tuning the water flow through nanochannels provides a key to many physicochemical phenomena, such as energy harvesting, desalination, biosensors and so on.

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.

Structure, dynamics, and morphology of nanostructured water confined between parallel graphene surfaces and in carbon nanotubes by applying magnetic and electric fields

Water molecules experience certain changes in their properties when they feel an external magnetic or electric field. These changes are significant in different applications, such as biological and biotechnological processes, nano-pumping, and water treatment. In this work, we have performed molecular dynamics (MD) simulations to investigate the different thermodynamics, structure, and dynamics of water molecules confined between two parallel surfaces and also confined in carbon nanotubes (CNTs). We have also applied different electric and magnetic fields in different directions to the confined molecules. In the graphene system, no polygonal shape was formed in either low or high electric fields, whereas rhombic and pentagonal shapes were formed in low and high magnetic fields. In the CNT system, applying electric fields in all three dimensions made the pentagonal shape disappear and the confined water molecules formed a ring shape when the electric field was applied in the axial direction. Applying the electric field perpendicular to the graphene surfaces increases the self-diffusion of the confined molecules, whereas applying the electric and magnetic fields along the CNT axis decreases the self-diffusion of the confined water molecules. In the graphene system, applying the electric field perpendicular to the graphene surfaces decreases the average number of hydrogen bonds (〈HB〉) whereas the magnetic field has little effect on the 〈HB〉. In the CNT system, applying E_x also leads to a smaller number of HBs. Also, applying the magnetic field along the x-direction (along the CNT direction) leads to a greater number of HBs than the other fields.

Selectivity of ion transport in narrow carbon nanotubes depends on the driving force due to drag or drive nature of their active hydration shells

Molecular dynamics simulations have revealed the important roles of hydration shells of ions transported through ultrathin carbon nanotubes (CNTs). In particular, ions driven by electric fields tend to drag their hydration shells behind them, while for ions transported by pressure, their hydration shells can actively drive them. Given the different binding strengths of hydration shells to ions of different sizes, these active roles of hydration shells affect the relative entry rates and driving speeds of ions in CNTs.

Water in Nanopores and Biological Channels: A Molecular Simulation Perspective

This Review explores the dynamic behavior of water within nanopores and biological channels in lipid bilayer membranes. We focus on molecular simulation studies, alongside selected structural and other experimental investigations. Structures of biological nanopores and channels are reviewed, emphasizing those high-resolution crystal structures, which reveal water molecules within the transmembrane pores, which can be used to aid the interpretation of simulation studies. Different levels of molecular simulations of water within nanopores are described, with a focus on molecular dynamics (MD). In particular, models of water for MD simulations are discussed in detail to provide an evaluation of their use in simulations of water in nanopores. Simulation studies of the behavior of water in idealized models of nanopores have revealed aspects of the organization and dynamics of nanoconfined water, including wetting/dewetting in narrow hydrophobic nanopores. A survey of simulation studies in a range of nonbiological nanopores is presented, including carbon nanotubes, synthetic nanopores, model peptide nanopores, track-etched nanopores in polymer membranes, and hydroxylated and functionalized nanoporous silica. These reveal a complex relationship between pore size/geometry, the nature of the pore lining, and rates of water transport. Wider nanopores with hydrophobic linings favor water flow whereas narrower hydrophobic pores may show dewetting. Simulation studies over the past decade of the behavior of water in a range of biological nanopores are described, including porins and β-barrel protein nanopores, aquaporins and related polar solute pores, and a number of different classes of ion channels. Water is shown to play a key role in proton transport in biological channels and in hydrophobic gating of ion channels. An overall picture emerges, whereby the behavior of water in a nanopore may be predicted as a function of its hydrophobicity and radius. This informs our understanding of the functions of diverse channel structures and will aid the design of novel nanopores. Thus, our current level of understanding allows for the design of a nanopore which promotes wetting over dewetting or vice versa. However, to design a novel nanopore, which enables fast, selective, and gated flow of water de novo would remain challenging, suggesting a need for further detailed simulations alongside experimental evaluation of more complex nanopore systems.

Transport of hydrated nitrate and nitrite ions through graphene nanopores in aqueous medium

Nitrate ( NO 3 - ) and nitrite ( NO 2 - ) ions are naturally occurring inorganic ions that are part of the nitrogen cycle. High doses of these ions in drinking water impose a potential risk to public health. In this work, molecular dynamics simulations are carried out to study the passage of nitrate and nitrite ions from water through graphene nanosheets (GNS) with hydrogen-functionalized narrow pores in presence of an external electric field. The passage of ions through the pores is investigated through calculations of ion flux, and the results are analyzed through calculations of various structural and thermodynamic properties such as the density of ions and water, ion-water radial distribution functions, two-dimensional density distribution functions, and the potentials of mean force of the ions. Current simulations show that the nitrite ions can pass more in numbers than the nitrate ions in a given time through GNS hydrogen-functionalized pore of different geometry. It is found that the nitrite ions can permeate faster than the nitrate ions despite the former having higher hydration energy in the bulk. This can be explained in terms of the competition between the number density of the ions along the pore axis and the free energy barrier calculated from the potential of mean force. Also, an externally applied electric field is found to be important for faster permeation of the nitrite over the nitrate ions. The current study suggests that graphene nanosheets with carefully created pores can be effective in achieving selective passage of ions from aqueous solutions.

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.

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.

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

Bioethanol has been used as an alternative energy source for transportation vehicles to reduce the use of fossil fuels. The separation of water-ethanol solutions from fermentation processes is still an important issue in the production of anhydrous ethanol. Using molecular dynamics simulations, we investigate the effect of axial electric fields on the separation of water-ethanol solutions with carbon nanotubes (CNTs). In the absence of an electric field, CNT-ethanol van der Waals interactions allow ethanol to fill the CNTs in preference to water, i.e., a separation effect for ethanol. However, as the CNT diameter increases, this ethanol separation effect significantly decreases owing to a decrease in the strength of the van der Waals interactions. In contrast, under an electric field, the energy of the electrostatic interactions within the water molecule structure induces water molecules to fill the CNTs in preference to ethanol, i.e., a separation effect for water. More importantly, the electrostatic interactions are dependent on the water molecule structure in the CNT instead of the CNT diameter. As a result, the separation effect observed under an electric field does not diminish over a wide CNT diameter range. Moreover, CNTs and electric fields can be used to separate methanol-ethanol solutions too. Under an electric field, methanol preferentially fills CNTs over ethanol in a wide CNT diameter range.

Effect of an external electric field on capillary filling of water in hydrophilic silica nanochannels

Development of functional nanofluidic devices requires understanding the fundamentals of capillary driven flow in nanochannels.

Voltage Gating of a Biomimetic Nanopore: Electrowetting of a Hydrophobic Barrier

It is desirable that nanopores that are components of biosensors are gated, i.e., capable of controllable switching between closed (impermeable) and open (permeable) states. A central hydrophobic barrier within a nanopore may act as a voltage-dependent gate via electrowetting, i.e., changes in nanopore surface wettability by application of an electric field. We use "computational electrophysiology" simulations to demonstrate and characterize electrowetting of a biomimetic nanopore containing a hydrophobic gate. We show that a hydrophobic gate in a model β-barrel nanopore can be functionally opened by electrowetting at voltages that do not electroporate lipid bilayers. During the process of electrowetting, voltage-induced alignment of water dipoles occurs within the hydrophobic gate region of the nanopore, with water entry preceding permeation of ions through the opened nanopore. When the ionic imbalance that generates a transbilayer potential is dissipated, water is expelled from the hydrophobic gate and the nanopore recloses. The open nanopore formed by electrowetting of a "featureless" β-barrel is anionic selective due to the transmembrane dipole potential resulting from binding of Na⁺ ions to the headgroup regions of the surrounding lipid bilayer. Thus, hydrophobic barriers can provide voltage-dependent gates in designed biomimetic nanopores. This extends our understanding of hydrophobic gating in synthetic and biological nanopores, providing a framework for the design of functional nanopores with tailored gating functionality.

Deformation of water nano-droplets on graphene under the influence of constant and alternative electric fields

Influence of constant and oscillating electric fields on the dynamics of a water nano-droplet on graphene.

Molecular dynamics simulations of electric field induced water flow inside a carbon nanotorus: a molecular cyclotron

A nano-flow is induced by applying gigahertz rotating electric fields (EFs) of different strengths and frequencies on a carbon nanotorus filled with water molecules, using molecular dynamics simulations.

Electric fields can control the transport of water in carbon nanotubes

The properties of water confined inside nanotubes are of considerable scientific and technological interest. We use molecular dynamics to investigate the structure and average orientation of water flowing within a carbon nanotube. We find that water exhibits biaxial paranematic liquid crystal ordering both within the nanotube and close to its ends. This preferred molecular ordering is enhanced when an axial electric field is applied, affecting the water flow rate through the nanotube. A spatially patterned electric field can minimize nanotube entrance effects and significantly increase the flow rate.

Water–methanol separation with carbon nanotubes and electric fields

Methanol is used in various applications, such as fuel for transportation vehicles, fuel cells, and in chemical industrial processes. Conventionally, separation of methanol from aqueous solution is by distillation. However, this method consumes a large amount of energy; hence development of a new method is needed. In this work, molecular dynamics simulations are performed to investigate the effect of an electric field on water–methanol separation by carbon nanotubes (CNTs) with diameters of 0.81 to 4.07 nm. Without an electric field, methanol molecules fill the CNTs in preference to water molecules. The preference of methanol to occupy the CNTs over water results in a separation effect. This separation effect is strong for small CNT diameters and significantly decreases with increasing diameter. In contrast, under an electric field, water molecules strongly prefer to occupy the CNTs over methanol molecules, resulting in a separation effect for water. More interestingly, the separation effect for water does not decrease with increasing CNT diameter. Formation of water structures in CNTs induced by an electric field has an important role in the separation of water from methanol.