The density of water in carbon nanotubes

Ion Coupling, Bonding, and Transfer in Narrow Carbon Nanotubes

Narrow carbon nanotubes (nCNT) are unique mimics of biological channels with water-ion selectivity attractive for applications such as water purification and osmotic energy harvesting, yet their understanding is still incomplete. Here, an ab initio computation is employed to develop the full picture of ion transfer in nCNT including specificity and coupling between ions. The thermodynamic costs of ion transfer are computed for single ions and ion pairs and used to evaluate different local coupling scenarios including strong (pairing) and weak (free-ion) coupling as well as "electroneutrality breakdown" (EB), possible for cations only due to their chemisorption-like interaction with nCNT. The results also indicate that nCNT behaves as a highly polarizable metal-like shell, which eliminates the dielectric energy when CNT accommodates coupled cation and anion. This allows facile computation and comparison of the full transfer costs, including translation entropy, for different ions in different coupling modes to identify the dominant regime. EB transfer appears most favorable for K+, while anions strongly favor transfer as pairs, except for chloride which favors weak coupling and, at neutral pH, transfers as a trace ion coupled to both cation and OH-. The results demonstrate that, in general, observed ion permeation and conduction in nCNT, especially for anions, reflect a complex ion-specific and composition-dependent interplay between different ions.

Ion Separation Together with Water Purification via a New Type of Nanotube: A Molecular Dynamics Study

We propose a CNT-based concentric twin tube (CTT) as nanochannels for both water purification and ion separation at the nanoscale. In the model, a source reservoir dealing with the solution connects three containers via the CTT that has three subchannels for mass transfer. Before entering the three subchannels, the solution in the separating zone will form three layers (the aqua cations, water, and the aqua anions, respectively) by applying a charged capacitor with the two electrodes parallel to the flow direction of the solution. Under an electric field with moderate intensity, the three subchannels in the CTT have stable configurations for mass transfer. Since the water and the two types of aqua ions are collected by three different containers, the present model can realize both ion separation and water purification. The mass transfer in the subchannels will be sped up by an external pressure exerted on the solution in the source reservoir. The physical properties of the model, e.g., water purification speed, are analyzed with respect to the effects of the electric field, the size of CTT, and the concentration of solute, such as NaCl.

How to produce confidence intervals instead of confidence tricks: Representative sampling for molecular simulations of fluid self-diffusion under nanoscale confinement

Ergodicity (or at least the tantalizing promise of it) is a core animating principle of molecular-dynamics (MD) simulations: Put simply, sample for long enough (in time), and you will make representative visits to states of a system all throughout phase space, consistent with the desired statistical ensemble. However, one is not guaranteed a priori that the chosen window of sampling in a production run is sufficiently long to avoid problematically non-ergodic observations; one is also not guaranteed that successive measurements of an observable are statistically independent of each other. In this paper, we investigate several particularly striking and troublesome examples of statistical correlations in MD simulations of nanoconfined fluids, which have profound implications on the quantification of uncertainty for transport phenomena in these systems. In particular, we show that these correlations can lead to confidence intervals on the fluid self-diffusion coefficient that are dramatically overconfident and estimates of this transport quantity that are simply inaccurate. We propose a simple approach—based on the thermally accelerated decorrelation of fluid positions and momenta—that ameliorates these issues and improves our confidence in MD measurements of nanoconfined fluid transport properties. We demonstrate that the formation of faithful confidence intervals for measurements of self-diffusion under nanoscale confinement typically requires at least 20 statistically independent samples, and potentially more depending on the sampling technique used.

Thermodynamic Barrier for Nanoparticle Penetration into Nanotubes

It is promising yet challenging to develop efficient methods to separate nanoparticles (NPs) with nanochannel devices. Herein, in order to guide and develop the separation method, the thermodynamic mechanism of NP penetration into solvent-filled nanotubes is investigated by using classical density functional theory. The potential of mean force (PMF) is calculated to evaluate the thermodynamic energy barrier for NP penetration into nanotubes. The accuracy of the theory is validated by comparing it with parallel molecular dynamics simulation. By examining the effects of nanotube size, solvent density, and substrate wettability on the PMF, we find that a large tube, a low bulk solvent density, and a solvophilic substrate can boost the NP penetration into nanotubes. In addition, it is found that an hourglass-shaped entrance can effectively improve the NP penetration efficiency compared with a square-shaped entrance. Furthermore, the minimum separation density of NPs in solution is identified, below which the NP penetration into nanotubes requires an additional driving force. Our findings provide fundamental insights into the thermodynamic barrier for NP penetration into nanotubes, which may provide theoretical guidance for separating two components using microfluidics.

Transport behavior of pressure-driven electrolyte solution through a surface-charged nanochannel

The transport behavior of a pressure-driven electrolyte solution through a surface-charged nanochannel is investigated using molecular dynamics (MD) simulations. Similar to pure water, the relationship between the applied pressure (P) and the average transport velocity ([Formula: see text]) of the electrolyte solution is roughly linear, which matches with the theoretical solution very well. The friction coefficient λ is used to describe the transport behavior (a higher λ leads to a lower [Formula: see text]), which scales with the slope of the P- [Formula: see text] relationship and increases with the increases of both the charge density of the channel wall σ and the electrolyte concentration n. The physical mechanism is found as follows: the solid-liquid interaction energy between the channel wall and the liquid inside the channel decreases with both σ and n (being more negative), which makes it more difficult for the liquid boundary layer to slide against the channel wall, leading to a higher λ. In addition, the increase of σ also causes a significant decrease of the liquid-liquid interaction energy but the opposite effect is found with the increase of n. However, λ increases with the increase of both σ and n, suggesting that the relationship between λ and the liquid-liquid interaction energy is more complicated for an electrolyte solution, different from the corresponding result of pure water.

A Molecular Dynamics Study on Rotational Nanofluid and Its Application to Desalination

In this work, we systematically study a rotational nanofluidic device for reverse osmosis (RO) desalination by using large scale molecular dynamics modeling and simulation. Moreover, we have compared Molecular Dynamics simulation with fluid mechanics modeling. We have found that the pressure generated by the centrifugal motion of nanofluids can counterbalance the osmosis pressure developed from the concentration gradient, and hence provide a driving force to filtrate fresh water from salt water. Molecular Dynamics modeling of two different types of designs are performed and compared. Results indicate that this novel nanofluidic device is not only able to alleviate the fouling problem significantly, but it is also capable of maintaining high membrane permeability and energy efficiency. The angular velocity of the nanofluids within the device is investigated, and the critical angular velocity needed for the fluids to overcome the osmotic pressure is derived. Meanwhile, a maximal angular velocity value is also identified to avoid Taylor-Couette instability. The MD simulation results agree well with continuum modeling results obtained from fluid hydrodynamics theory, which provides a theoretical foundation for scaling up the proposed rotational osmosis device. Successful fabrication of such rotational RO membrane centrifuge may potentially revolutionize the membrane desalination technology by providing a fundamental solution to the water resource problem.

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

In the past few decades great effort has been devoted to the study of water confined in hydrophobic geometries at the nanoscale (tubes and slit pores) due to the multiple technological applications of such systems, ranging from drug delivery to water desalination devices. To our knowledge, neither numerical/theoretical nor experimental approaches have so far reached a consensual understanding of structural and transport properties of water under these conditions. In this work, we present molecular dynamics simulations of TIP4P/2005 water under different nanoconfinements (slit pores or nanotubes, with two degrees of hydrophobicity) within a wide temperature range. It has been found that water is more structured near the less hydrophobic walls, independently of the confining geometries. Meanwhile, we observe an enhanced diffusion coefficient of water in both hydrophobic nanotubes. Finally, we propose a confined Stokes-Einstein relation to obtain the viscosity from diffusivity, whose result strongly differs from the Green-Kubo expression that has been used in previous works. While viscosity computed with the Green-Kubo formula (applied for anisotropic and confined systems) strongly differs from that of the bulk, viscosity computed with the confined Stokes-Einstein relation is not so much affected by the confinement, independently of its geometry. We discuss the shortcomings of both approaches, which could explain this discrepancy.

Combined effects of molecular geometry and nanoconfinement on liquid flows through carbon nanotubes

Molecular dynamics simulations are carried out to investigate the geometry effects of diatomic molecules on liquid flows in carbon nanotubes (CNTs). Oxygen molecules are considered as the fluid inside armchair (n,n) (n=6-20) CNTs. The simulated fluid temperature and bulk pressure for the liquid state are T=133 K and ρ_{b}=1346kg/m^{3}, respectively. In the agglomerated molecular cluster, nanoconfinement-induced structural changes are observed. As the CNT diameter decreases, it is confirmed that the flow rate significantly increases with irregular trends (discontinuity points in the profiles). From the discussion of the structure of the agglomerated fluid molecules, it is found that those trends are not simply caused by the structural changes. The main factor to induce the irregularity is confirmed to be the interlayer molecular movement affected by the combination of the molecular geometry and the arrangement of the multilayered structure.

Effects of CNT size on the desalination performance of an outer-wall CNT slit membrane

We investigate the effect of varying carbon nanotube (CNT) size on the desalination performance through slit confinements formed by horizontally aligned CNTs stacked on top of one another.

Negative effect of nanoconfinement on water transport across nanotube membranes

Nanoconfinement environments are commonly considered advantageous for ultrafast water flow across nanotube membranes. This study illustrates that nanoconfinement has a negative effect on water transport across nanotube membranes based on molecular dynamics simulations. Although water viscosity and the friction coefficient evidently decrease because of nanoconfinement, water molecular flux and flow velocity across carbon nanotubes decrease sharply with the pore size of nanotubes. The enhancement of water flow across nanotubes induced by the decreased friction coefficient and water viscosity is markedly less prominent than the negative effect induced by the increased flow barrier as the nanotube size decreases. The decrease in water flow velocity with the pore size of nanotubes indicates that nanoconfinement is not essential for the ultrafast flow phenomenon. In addition, the relationship between flow velocity and water viscosity at different temperatures is investigated at different temperatures. The results indicate that flow velocity is inversely proportional to viscosity for nanotubes with a pore diameter above 1 nm, thereby indicating that viscosity is still an effective parameter for describing the effect of temperature on the fluid transport at the nanoscale.

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.

Curvature-dependent adsorption of water inside and outside armchair carbon nanotubes

The curvature dependence of the physisorption properties of a water molecule inside and outside an armchair carbon nanotube (CNT) is investigated by an incremental density-fitting local coupled cluster treatment with single and double excitations and perturbative triples (DF-LCCSD(T)) study. Our results show that a water molecule outside and inside (n, n) CNTs (n = 4, 5, 6, 7, 8, 10) is stabilized by electron correlation. The adsorption energy of water inside CNTs decreases quickly with the decrease of curvature (increase of radius) and the configuration with the oxygen pointing toward the CNT wall is the most stable one. However, when the water molecule is adsorbed outside the CNT, the adsorption energy varies only slightly with the curvature and the configuration with hydrogens pointing toward the CNT wall is the most stable one. We also use the DF-LCCSD(T) results to parameterize Lennard-Jones (LJ) force fields for the interaction of water both with the inner and outer sides of CNTs and with graphene representing the zero curvature limit. It is not possible to reproduce all DF-LCCSD(T) results for water inside and outside CNTs of different curvature by a single set of LJ parameters, but two sets have to be used instead. Each of the two resulting sets can reproduce three out of four minima of the effective potential curves reasonably well. These LJ models are then used to calculate the water adsorption energies of larger CNTs, approaching the graphene limit, thus bridging the gap between CNTs of increasing radius and flat graphene sheets.

Calculation of Raman parameters of real-size zigzag (n, 0) single-walled carbon nanotubes using finite-size models

Structural and selected Raman features of real-size single-walled carbon nanotubes (SWCNTs) were studied using finite-size pristine SWCNT models at the DFT level.

Nanostructured water and carbon dioxide inside collapsing carbon nanotubes at high pressure

We present simulations of the collapse under hydrostatic pressure of carbon nanotubes containing either water or carbon dioxide.

Confinement effects on liquid-flow characteristics in carbon nanotubes

Liquid flow dynamics through the armchair (6,6)-(160,160) carbon nanotubes (CNTs) is elucidated by molecular dynamics simulations. The liquid is modeled by nonpolar argon atoms to understand the fundamental flow physics. The velocity profiles and slip lengths are discussed considering the radial distributions of the fluid density by the presently proposed finite difference-based velocity fitting method. It is found that as the CNT diameter D increases, the slip length and the flow rate enhancement show three-step transitional profiles in the region of D≤2.3 nm. The slip length and the flow rate stepwise increase at the first transition while they drop at the second and third transitions. The first transition corresponds to the structural change from the single-file chain to single-ring structures of the molecule cluster. The second and third transitions take place when the ring structure starts to develop another inner layer.

Structure, dynamics and thermodynamics of single-file water under confinement: effects of polarizability of water molecules

Various structural, dynamic and thermodynamic properties of water molecules confined in single-wall carbon nanotubes are investigated using both polarizable and non-polarizable water models.

Molecular dynamics simulations of proton-ordered water confined in low-diameter carbon nanotubes

Metastable ferroelectric, ferrielectric, and antiferroelectric structures of water confined in carbon nanotubes and their abrupt or continuous transitions are identified.

Solitons transport water through narrow carbon nanotubes

Transformative technologies for desalination and chemical separations call for understanding molecular transport through man-made and biological nanochannels. Using numerical simulation of single-file flow of water through carbon nanotubes, we find that flow is due to fast-moving density variations (solitons) that are additive so flow rate is proportional to number of solitons. Simulation results match predictions from a theoretical model for soliton propagation. From 1-300 K flow rates increase as temperature decreases. Our results build a fundamentally new understanding of nanochannel flows and suggest new principles for the design of nanoscale devices.

Enhanced carbon dioxide adsorption through carbon nanoscrolls

Over the last few years, significant efforts have been devoted to exploring the capabilities of carbon based structures for gas separation and filtration. In the present study the layering behavior of carbon dioxide transported through carbon nanoscrolls is examined through molecular dynamics simulations. The layering arrangements are investigated for carbon nanoscrolls with intralayer distances spanning from 4.2 to 8.3 Å at temperature of 300 K and pressures ranging from 5 to 20 bars. Characteristic layering structures are developed around the internal and external surfaces of the nanoscroll for all the examined cases. It is shown that the number of layers, their relative strength, and the starting point of bifurcation phenomena vary as a function of the nanoscrolls' intralayer distance, scroll's core radius, CO2 density, and gas structure interactions. It is also shown that the number of carbon dioxide molecules adsorbed per scroll's carbon particles is a function of the scroll's surface-to-volume ratio and is maximized under certain structural configurations.