Highly selective adsorption of methanol in carbon nanotubes immersed in methanol-water solution

Partition Constant of Binary Mixtures for the Equilibrium between a Bulk and a Confined Phase

It is well-known that the thermodynamic, kinetic and structural properties of fluids, and in particular of water and its solutions, can be drastically affected in nanospaces. A possible consequence of nanoscale confinement of a solution is the partial segregation of its components. Thereby, confinement in nanoporous materials (NPM) has been proposed as a means for the separation of mixtures. In fact, separation science can take great advantage of NPM due to the tunability of their properties as a function of nanostructure, morphology, pore size, and surface chemistry. Alcohol-water mixtures are in this context among the most relevant systems. However, a quantitative thermodynamic description allowing for the prediction of the segregation capabilities as a function of the material-solution characteristics is missing. In the present study we attempt to fill this vacancy, by contributing a thermodynamic treatment for the calculation of the partition coefficient in confinement. Combining the multilayer adsorption model for binary mixtures with the Young equation, we conclude that the liquid-vapor surface tension and the contact angle of the pure substances can be used to predict the separation ability of a particular material for a given mixture to a semiquantitative extent. Moreover, we develop a Kelvin-type equation that relates the partition coefficient to the radius of the pore, the contact angle, and the liquid-vapor surface tensions of the constituents. To assess the validity of our thermodynamic formulation, coarse grained molecular dynamics simulations were performed on models of alcohol-water mixtures confined in cylindrical pores. To this end, a coarse-grained amphiphilic molecule was parametrized to be used in conjunction with the mW potential for water. This amphiphilic model reproduces some of the properties of methanol such as enthalpy of vaporization and liquid-vapor surface tension, and the minimum of the excess enthalpy for the aqueous solution. The partition coefficient turns out to be highly dependent on the molar fraction, on the interaction between the components and the confining matrix, and on the radius of the pore. A remarkable agreement between the theory and the simulations is found for pores of radius larger than 15 Å.

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.

Designing Solute-Tailored Selectivity in Membranes: Perspectives for Water Reuse and Resource Recovery

Treatment of nontraditional source waters (e.g., produced water, municipal and industrial wastewaters, agricultural runoff) offers exciting opportunities to expand water and energy resources via water reuse and resource recovery. While conventional polymer membranes perform water/ion separations well, they do not provide solute-specific separation, a key component for these treatment opportunities. Herein, we discuss the selectivity limitations plaguing all conventional membranes, which include poor removal of small, neutral solutes and insufficient discrimination between ions of the same valence. Moreover, we present synthetic approaches for solute-tailored selectivity including the incorporation of single-digit nanopores and solute-selective ligands into membranes. Recent progress in these areas highlights the need for fundamental studies to rationally design membranes with selective moieties achieving desired separations.

Electric Field Induced Dewetting of Hydrophobic Nanocavities at Ambient Temperature

The understanding of water dewetting in nanoporous materials is of great importance in various fields of science and technology. Herein, we report molecular dynamics simulation results of dewetting of water droplet in hydrophobic nanocavities between graphene walls under the influence of electric field. At ambient temperature, the rate of dewetting induced by electric field is significantly large. Whereas, it is a very low rate of dewetting induced by high temperature (423 K) due to the strong interaction of the hydrogen-bonding networks of water droplets in nanocavities. In addition, the electric filed induced formation of a water column has been found in a vacuum chamber. When the electric field is turned off, the water column will transform into a water droplet. Importantly, the results demonstrate that the rate of electric field-induced dewetting increases with growth of the electric field. Overall, our results suggest that electric field may have a great potential application for nanomaterial dewetting.

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.

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.

Molecular dynamics simulations of nano-confined methanol and methanol-water mixtures between infinite graphite plates: Structure and dynamics

Molecular dynamics simulations are used to investigate microscopic structures and dynamics of methanol and methanol-water binary mixture films confined between hydrophobic infinite parallel graphite plate slits with widths, H, in the range of 7-20 Å at 300 K. The initial geometric densities of the liquids were chosen to be the same as bulk methanol at the same temperature. For the two narrowest slit widths, two smaller initial densities were also considered. For the nano-confined system with H = 7 Å and high pressure, a solid-like hexagonal arrangement of methanol molecules arranged perpendicular to the plates is observed which reflects the closest packing of the molecules and partially mirrors the structure of the underlying graphite structure. At lower pressures and for larger slit widths, in the contact layer, the methanol molecules prefer having the C-O bond oriented parallel to the walls. Layered structures of methanol parallel to the wall were observed, with contact layers and additional numbers of central layers depending on the particular slit width. For methanol-water mixtures, simulations of solutions with different composition were performed between infinite graphite slits with H = 10 and 20 Å at 300 K. For the nanoslit with H = 10 Å, in the solution mixtures, three layers of molecules form, but for all mole fractions of methanol, methanol molecules are excluded from the central fluid layer. In the nanopore with H = 20 Å, more than three fluid layers are formed and methanol concentrations are enhanced near the confining plates walls compared to the average solution stoichiometry. The self-diffusion coefficients of methanol and water molecules in the solution show strong dependence on the solution concentration. The solution mole fractions with minimal diffusivity are the same in confined and non-confined bulk methanol-water mixtures.

Dynamic behavior and selective adsorption of a methanol/water mixture inside a cyclic peptide nanotube

Present molecular dynamics simulations indicate that the methanol component in a methanol/water mixture is more likely to be trapped in a cyclic peptide nanotube (CPNT), while water molecules tend to be present at the channel mouths as transient guests. Channel water resides mainly between methanol and the CPNT wall, resulting in a distinct decrease in the H-bond number per channel methanol. Six designed CPNTs with different channel diameters and outer surface characteristics all possess distinct selectivity to methanol over water. Of these, the amphipathic 8 × (AQ)₄-CPNT exhibits the best performance. Results in this study provide basic information for the application of a CPNT to enrich methanol from a methanol/water mixture. Graphical Abstract Typical overview of water and methanol molecular distribution in cyclic peptide nanotubes.

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.

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.

Investigation of Transport Properties of Water-Methanol Solution through a CNT with Oscillating Electric Field

Molecular dynamics simulations were used to investigate the transport properties of water-methanol solution getting through a carbon nanotube (CNT) with an oscillating electric field. Eight alternating electric fields with different oscillation periods were used in this work. Under the oscillating electric field, water molecules have the advantage of occupying a CNT over methanol molecules. Meanwhile, the space occupancy of water-methanol solution in the CNT increases as the oscillating period increases. More importantly, we found that the oscillating period of electric field affects the van der Waals interaction of the solution inside the CNT and the shell of the CNT, which results in the change in the number of hydrogen bonds in the water-methanol solution confined in the CNT. And the change in the hydrogen-bond network leads to the change in transport properties of water-methanol solution.

Flue gas adsorption by single-wall carbon nanotubes: A Monte Carlo study

Adsorption of flue gases by single-wall carbon nanotubes (SWCNT) has been studied by means of Monte Carlo simulations. The flue gas is modeled as a ternary mixture of N2, CO2, and O2, emulating realistic compositions of the emissions from power plants. The adsorbed flue gas is in equilibrium with a bulk gas characterized by temperature T, pressure p, and mixture composition. We have considered different SWCNTs with different chiralities and diameters in a range between 7 and 20 Å. Our results show that the CO2 adsorption properties depend mainly on the bulk flue gas thermodynamic conditions and the SWCNT diameter. Narrow SWCNTs with diameter around 7 Å show high CO2 adsorption capacity and selectivity, but they decrease abruptly as the SWCNT diameter is increased. For wide SWCNT, CO2 adsorption capacity and selectivity, much smaller in value than for the narrow case, decrease mildly with the SWCNT diameter. In the intermediate range of SWCNT diameters, the CO2 adsorption properties may show a peculiar behavior, which depend strongly on the bulk flue gas conditions. Thus, for high bulk CO2 concentrations and low temperatures, the CO2 adsorption capacity remains high in a wide range of SWCNT diameters, although the corresponding selectivity is moderate. We correlate these findings with the microscopic structure of the adsorbed gas inside the SWCNTs.

From dimers to collective dipoles: Structure and dynamics of methanol/ethanol partition by narrow carbon nanotubes

Alcohol partitioning by narrow single-walled carbon nanotubes (SWCNTs) holds the promise for the development of novel nanodevices for diverse applications. Consequently, in this work, the partition of small alcohols by narrow tubes was kinetically and structurally quantified via molecular dynamics simulations. Alcohol partitioning is a fast process in the order of 10 ns for diluted solutions but the axial-diffusivity within SWCNT is greatly diminished being two to three orders of magnitude lower with respect to bulk conditions. Structurally, alcohols form a single-file conformation under confinement and more interestingly, they exhibit a pore-width dependent transition from dipole dimers to a single collective dipole, for both methanol and ethanol. Energetic analyses demonstrate that this transition is the result of a detailed balance between dispersion and electrostatics interactions, with the latter being more pronounced for collective dipoles. This transition fully modifies the reorientational dynamics of the loaded particles, generating stable collective dipoles that could find usage in signal-amplification devices. Overall, the results herein have shown distinct physico-chemical features of confined alcohols and are a further step towards the understanding and development of novel nanofluidics within SWCNTs.

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.

Interactions of carbon nanotubes with the nitromethane–water mixture governing selective adsorption of energetic molecules from aqueous solution

Ordered structures of nitromethane are formed in the tubular cavity through selective adsorption by carbon nanotubes from aqueous solution.

Porous carbon nanotubes: molecular absorption, transport, and separation

We use classical molecular dynamics simulations to study nanofluidic properties of porous carbon nanotubes. We show that saturated water vapor condenses on the porous nanotubes, can be absorbed by them and transported in their interior. When these nanotubes are charged and placed in ionic solutions, they can selectively absorb ions in their interior and transport them. Porous carbon nanotubes can also be used as selective molecular sieves, as illustrated on a room temperature separation of benzene and ethanol.

Spontaneous formation of one-dimensional hydrogen gas hydrate in carbon nanotubes

We present molecular dynamics simulation evidence of spontaneous formation of quasi-one-dimensional (Q1D) hydrogen gas hydrates within single-walled carbon nanotubes (SW-CNTs) of nanometer-sized diameter (1-1.3 nm) near ambient temperature. Contrary to conventional 3D gas hydrates in which the guest molecules are typically contained in individual and isolated cages in the host lattice, the guest H2 molecules in the Q1D gas hydrates are contained within a 1D nanochannel in which the H2 molecules form a molecule wire. In particular, we show that in the (15,0) zigzag SW-CNT, the hexagonal H2 hydrate tends to form, with one H2 molecule per hexagonal prism, while in the (16,0) zigzag SW-CNT, the heptagonal H2 hydrate tends to form, with one H2 molecule per heptagonal prism. In contrast, in the (17,0) zigzag SW-CNT, the octagonal H2 hydrate can form, with either one H2 or two H2 molecules per pentagonal prism (single or double occupancy). Interestingly, in the hexagonal or heptagonal ice nanotube, the H2 wire is solid-like as the axial diffusion constant is very low (<5 × 10(-10) cm(2)/s), whereas in the octagonal ice nanotube, the H2 wire is liquid-like as its axial diffusion constant is comparable to 10(-5) cm(2)/s.