Modeling the adsorptive selectivity of carbon nanotubes for effective separation of CO2/N2 mixtures

Canonical Monte Carlo (CMC) simulations were carried out to investigate the behavior of CO₂ and N₂ mixtures upon adsorption on single walled carbon nanotubes (CNTs). In the simulation, all the particle–particle interactions between CO₂, N and C were modeled using Lennard-Jones (LJ) potential. To provide deep insight into the effect of pore width, temperature, pressure and bulk composition on the adsorption behavior of CO₂ /N₂ mixtures, five different CNTs [(6,6), (7,7), (8,8) (9,9) and (10,10) CNT] with diameters ranging from 0.807 to 1.35 nm, three temperatures (300 323 and 343 K), six pressures (0.15, 2, 4, 6, 8 and 10 MPa), and three bulk mole compositions of carbon dioxide (0.3 0.5 and 0.7) were tested. The results from all the simulation conditions investigated in this work show that CNTs preferentially adsorb carbon dioxide relative to nitrogen in a binary mixture. The results are consistent with the hypothesis that stronger interaction of one component with the nanotube surface results in a higher adsorption capacity compared to the other component. An optimized pore size of D = 8.07 nm corresponding to (6, 6) CNT, at T = 300 K and P = 0.15 MPa at a bulk mole composition of y_CO2 =0.3 was identified in which carbon nanotubes demonstrate the greatest selectivity for separation of carbon dioxide relative to nitrogen. In addition, it is worth pointing out that, under similar simulation conditions, CNTs exhibit higher selectivity compared to other carbon-based materials [carbon membrane polyimide (PI) and PI/multi-wall carbon nanotubes (MWCNTs)] for CO₂ adsorption. As a prototype, the selectivity of an equimolar mixture of CO₂ /N₂ for adsorption on (6, 6) CNTs at 300 K and 0.15 MPa reaches 9.68, which is considerably larger than that reported in carbon membrane. Therefore, it can be concluded that carbon nanotubes can act as a capable adsorbent for adsorption/desorption of CO₂ in comparison with other carbon-based materials in the literature.     

Confinement as a determinant of macromolecular structure and reactivity. II. Effects of weakly attractive interactions between confined macrosolutes and confining structures.

The effect of weak, nonspecific interaction between molecules confined within restricted elements of volume ("pores") and the boundary surfaces of the pore, upon the reactivity and physical state of the confined molecules, is explored by means of simple models. A confined molecule is represented by a rectangular parallelopiped having one of six orientations aligned with the cartesian coordinate axes, and the confining volume element is represented by a pair of parallel surfaces (planar pore), a tube of square cross section (square pore), or a cubical box (cubical pore). Weak interactions are modeled by square-well potentials having a defined range and well depth. Partition coefficients for distribution of molecules between the bulk and confined phase are calculated using an extension of the statistical-thermodynamic theory of Giddings et al. (1968). It is calculated that surface attraction with a potential of only a few kcal/mol monomer may result in large increases in the extent of self- or heteroassociation of confined molecules (as much as several orders of magnitude in favorable cases) linked to adsorption of the oligomeric species onto boundary surfaces. Calculations are also presented suggesting that surface attraction can lead to deformation of the native structure of adsorbed macromolecules. It is suggested that these findings are relevant to an understanding of the structure of eukaryotic cytoplasm.     

Gibbs Ensemble Calculations with an Equation of State: An Application to Vapor-Liquid Equilibria

Abstract

A new modification of the Gibbs ensemble Monte Carlo computer simulation method for fluid phase equilibria is described. The modification is based on a thermodynamic model for the vapor phase, and uses an equation of state to account for the weak interactions between the vapor phase molecules. Reductions in the computational time by 30–40% as compared to the original Gibbs ensemble method are obtained. The algorithm is applied to Lennard-Jones - (12,6) fluids and their mixtures and the results are in good agreement with results obtained from simulations using the full Gibbs ensemble method.     

Monte Carlo Simulation Study of Adsorption Characteristics in Slit-Like Micropores Under Supercritical Conditions

Abstract

Adsorption characteristics of a solute diluted in supercritical fluids has been investigated by using the Monte Carlo simulation techniques. The Lennard-Jones potential function is used for describing interactions for a model system of CO₂ + benzene in slit-like micropores with infinite graphitic carbon walls. A modified μVT ensemble method with particle exchange proposed by Cracknell, Nicholson and Quirke (1993) is found to be much superior to the conventional μVT ensemble method especially for dense mixtures in a pore. Adsorption isotherms of CO₂ and benzene, in equilibrium with a dilute benzene mixture in CO₂ (mole fraction of benzene = 0.001), are computed by varying pressure, temperature, the benzene–surface interaction potential, and the slitwidth. Adsorption isotherm curve of CO₂ increases with an increase in pressure while that of benzene shows a maximum at a pressure far below the critical pressure of CO₂ and then it decreases with increasing pressure. The decrease in benzene adsorption with increasing pressure is attributable to both the enhanced solubility in supercritical CO₂ and the competitive adsorption of CO₂. The isotherm curves of each component at two temperatures, 313.2 K and 323.2 K, show to cross at a pressure near the critical pressure due to the “density effect” on the chemical potentials of a solute at supercritical fluid conditions. When the interaction between a solute and a surface increases, the adsorption isotherm increases. Narrowing the slitwidth results in the increase in the adsorption of solute since the external potential from two walls becomes deeper.     

Chain versus Ring Structures of Selenium Confined in AlPO4-5 Zeolite

A tight binding grand canonical Monte Carlo simulation of the adsorption of selenium in AlPO₄-5 zeolite is presented. We show that the structure of confined Se varies from a stretched chain to a piling of Se₅ rings, with intermediate structures combining chains and rings. It depends on the thermodynamic conditions of the adsorption: the ring structures are favored at low temperatures and high pressures; chains are favored at higher temperatures and lower pressures. These results are in qualitative agreement with recent experimental results.     

Properties of star-branched and linear chains in confined space. A Monte-Carlo study

We have studied the properties of simple models of linear and star-branched polymer chains confined in a slit formed by two parallel impenetrable walls. The polymer chains consisted of identical united atoms (homopolymers) and were restricted to a simple cubic lattice. Two macromolecular architectures of the chain: linear and regular stars with three branches of equal length, were studied. The excluded volume was the only potential introduced into the model and thus the system was athermal. Monte-Carlo simulations with the sampling algorithm based on the chains local changes of conformation were carried out for chains with different lengths as well as for different distances between the confining surfaces. We found that the properties of model chains differ for both macromolecular architectures but a universal behavior for both kinds of chains was also found. Investigation of the frequency of chain-wall contacts shows that the ends of the chains are much more mobile than the rest of the chain, especially in the vicinity of the branching point in star polymers.Figure The scheme of a star-branched (left) and a linear (right) chain located between two parallel impenetrable surfaces.     

Editorial

Abstract

Grand canonical molecular dynamics (GCMD) simulations are used to study the adsorption and desorption of Lennard-Jones nitrogen in three slit pore junction models of microporous graphite. These networks consist of two narrow pores separated by a wider (cavity) pore. We report results for cases where the narrow pore has a width of only two or three molecular diameters. Using the GCMD technique, a novel freezing transition is observed which results in pore blocking in the narrow pores of the network, which are less than 1 nm wide. This freezing results from the adsorption energy barrier at the junction between the narrow and wider pores. This type of pore blocking could account for the apparent increase in pore volume with increasing temperature that has been experimentally observed in microporous graphite systems. For networks in which the narrower pores are somewhat larger, with a width of 1.28 nm, this pore blocking effect is much reduced, and adsorbate molecules enter and fill the central cavity. In such cases, however, desorption is incomplete, some residual adsorbate remaining in the central cavity even at the lowest pressures.     

Molecular Dynamics Study of Nitrogen in Slit Micropores

Abstract

We present results of a computer simulation study of fluid nitrogen in model slit micropores. The model used for the micropore allows for the permeability of the pore wall to the confined fluid to be precisely controlled, while maintaining the atomic nature of the wall. Density and orientation profiles, wall permeabilities and diffusion coefficients have been obtained for systems with pore walls ranging from the almost impermeable to the completely permeable. Both the density and orientation profiles exhibit nonuniform behavior, while we observe anisotropy in the diffusion coefficients.     

Adsorption of Binary Mixtures in a Zeolite Micropore

Abstract

We investigate the selective adsorption of xenon, argon, and methane in zeolite NaA by applying the grand canonical ensemble Monte Carlo simulation technique to an adsorbed binary mixture and to two reference systems: i) an adsorbed single component system and ii) a bulk mixture. We define and calculate selectivities and excess densities due to i) mixing and ii) adsorption in terms of differences between the binary adsorbed system and these reference systems. We observe that xenon selectively adsorbs in both xenon-argon and xenon-methane mixtures at low chemical potential (low pressure) due to its greater energetic interaction with the zeolite. However, a reversal in selectivity occurs at higher chemical potential in both of these mixtures. This is due in large part to the greater efficiency in which the smaller component “packs” in the pore as compared to the bulk. We show that the crossover in selectivity occurs at a lower chemical potential for a mixture where one component can occupy regions of the porespace inaccessible to the other. We suggest that this crossover in selectivity may be a general feature of microporous adsorption.     

Phase Equilibria of Quadrupolar Fluids by Simulation in the Gibbs Ensemble

Abstract

Vapour-liquid phase diagrams for pure fluids and mixtures of molecules with Lennard-Jones plus quadrupole-quadrupole interaction potentials were determined by Monte Carlo simulation in the Gibbs ensemble [1]. This is the first reported application of the method to molecular fluids. We have demonstrated that the Gibbs method works reliably for strongly interacting molecular fluids at liquid densities. Pure fluid calculations were performed for reduced quadrupole strengths, Q* = Q/(εσ⁵)^1/2 equal to 1 and √2, typical of molecules like C₂H₂ and C₂H₄. It was found that the critical temperature of the quadrupolar fluid increased rapidly with increasing quadrupolar strength, in good agreement with previous computer simulation and theoretical results. A single mixture with components characterized by identical Lennard-Jones parameters and Q*₁ = + 1, Q*₂ = - 1 was studied at three temperatures. A negative azeotrope was observed at the lowest temperature studied, as seen experimentally in the CO₂/C₂H₂ mixture. The perturbation theory calculations are in good agreement with the simulation results for all properties except coexisting liquid densities. The results illustrate some of the strengths and limitations of perturbation theories based on the Padé approximant for the free energy of polar fluids.     

Monte Carlo Simulation of Binary Gas Adsorption in Zeolite Cavities

Abstract

Grand canonical Monte Carlo simulations have been performed for binary adsorption of Lennard-Jones molecules with point multipole moments in zeolite cavities of type X. Fluid-solid electrostatic interactions were taken into account. Phase diagrams and total coverage were calculated for three binaries and compared with experimental measurements. MC simulations gave good agreement with experiment for two mixtures (C₂H₄-CO₂ and CO₂-CH₄) but there were discrepancies between simulation and experiment for the system i-C₄H₁₀-C₂H₄. The dependence of excess Gibbs free energy on the composition and pressure was studied. Negative deviations from ideality are due to energetic heterogeneity and size effects. Unlike liquid-vapor equilibrium, deviations from the Lorentz-Berthelot mixing rules for the adsorbates have little effect upon the phase behavior. Density distributions show that the components compete for the high energy sites inside the cavity; depending on its relative strength of adsorption, one component may be excluded from such positions (CH₄ in CO₂-CH₄), or the two species may share sites inside the cavity (C₂H₄-CO₂).     

N-octane diffusivity enhancement via carbon dioxide in silica slit-shaped nanopores – a molecular dynamics simulation

Equilibrium molecular dynamics simulations were conducted to study the competitive adsorption and diffusion of mixtures containing n-octane and carbon dioxide confined in slit-shaped silica pores of width 1.9 nm. Atomic density profiles substantiate strong interactions between CO₂ molecules and the protonated pore walls. Non-monotonic change in n-octane self-diffusion coefficients as a function of CO₂ loading was observed. CO₂ preferential adsorption to the pore surface is likely to attenuate the surface adsorption of n-octane, lower the activation energy for n-octane diffusivity, and consequently enhance n-octane mobility at low CO₂ loading. This observation was confirmed by conducting test simulations for pure n-octane confined in narrower pores. At high CO₂ loading, n-octane diffusivity is hindered by molecular crowding. Thus, n-octane diffusivity displays a maximum. In contrast, within the concentration range considered here, the self-diffusion coefficient predicted for CO₂ exhibits a monotonic increase with loading, which is attributed to a combination of effects including the saturation of the adsorption capacity of the silica surface. Test simulations suggest that the results are strongly dependent on the pore morphology, and in particular on the presence of edges that can preferentially adsorb CO₂ molecules and therefore affect the distribution of these molecules equally on the pore surface, which appears to be required to provide the effective enhancement of n-octane diffusivity.     

Adsorption of bacterial capsular exopolymers to hydrophilic and hydrophobic surfaces: A 2D‐FTIR analysis

Analysis of the adsorption of capsular exopolymers (EPS) from Pseudomonas sp. NCIMB 2021 to hydrophilic and hydrophobic gold surfaces was examined, in situ, using Fourier transform infrared spectroscopy. The molecular sequence of events occurring upon EPS adsorption to hydrophilic and hydrophobic surfaces has been elucidated using dynamic 2D‐FTIR correlation spectroscopy. This method of analysis enables the enhancement of the resolution of overlapping spectral features and the elucidation of time‐dependent changes. The data reveal the existence of surface dependent adsorption mechanisms. At both surfaces, the aromatic tyrosyl side chains of the protein moiety displace water. This is followed by an adsorption step dominated by carboxylate groups. However, at the hydrophobic surface, the two steps are interrupted by the ingress of water back to the surface. Furthermore, the amount of neutral exopolymer present was greater at the hydrophilic surface than the hydrophobic surface.     

Modeling of adsorption in nanopores

Adsorption in nonporous materials has been studied using Grand Canonical Monte Carlo simulations. We discuss three types of materials: (a) a model of cylindrical pores with smooth walls, representing MCM-41 like materials, (b) a model of cylindrical pores with regular structured walls (model of carbon nanotubes) and (c) a material with crystalline wall structure (zeolites). Typical problems related to the stability of adsorbed layers have been analyzed. We have shown that the mechanism of adsorption is strongly dependent on the structure of the pore walls. In the case of amorphous walls it may lead to metastable configurations. In nanotubes, the ordered corrugation structure of walls determines the low temperature structure of the adsorbed system. In 3D ordered porous system, such as zeolites, the mechanism of adsorption is mostly determined by characteristic sites of adsorption.Figure Adsorbed atoms and energy fluctuations at the pressure of the first layer formation of krypton atoms: (a) instantaneous numbers of adsorbed atoms (per nm² of the pore wall) as a function of the time of simulation (Monte Carlo steps) observed in a relatively long run, (b) the bimodal distribution of the energy fluctuations is a consequence of the behavior of the systems as shown in (a).     

Fluorine-free mixed amphiphilic polymers based on PDMS and PEG side chains for fouling release applications

Fluorine-free mixed amphiphilic block copolymers with mixtures of short side groups of polydimethyl siloxane (PDMS) and polyethylene glycol (PEG) were synthesized and studied for their ability to influence the surface properties and control the adhesion of marine organisms to coated surfaces. The settlement (attachment) and strength of adhesion of two different marine algae, the green seaweed Ulva and the diatom Navicula, were evaluated against the surfaces. It is known that hydrophobic coatings based on polydimethyl siloxane elastomers (PDMSe) are prone to protein adsorption and accumulation of strongly adherent diatom slimes, in contrast to PEG-based hydrophilic surfaces that inhibit protein adsorption and moderate only weak adhesion of diatoms. By incorporating both PDMS and PEG side chains into the polymers, the effect of incorporating both polar and non-polar groups on fouling-release could be studied. The dry surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The ability of these mixed amphiphilic polymers to reconstruct in water was examined using underwater bubble contact angle and dynamic water contact angle experiments. To understand more about surface reconstruction behavior, protein adsorption experiments were carried out with fluorescein isothiocyanate-labeled bovine serum albumin (BSA-FITC) on both dry and pre-soaked surfaces.     

Molecular simulation of water removal from simple gases with zeolite NaA

Water vapor removal from some simple gases using zeolite NaA was studied by molecular simulation. The equilibrium adsorption properties of H(2)O, CO, H(2), CH(4) and their mixtures in dehydrated zeolite NaA were computed by grand canonical Monte Carlo simulations. The simulations employed Lennard-Jones + Coulomb type effective pair potential models, which are suitable for the reproduction of thermodynamic properties of pure substances. Based on the comparison of the simulation results with experimental data for single-component adsorption at different temperatures and pressures, a modified interaction potential model for the zeolite is proposed. In the adsorption simulations with mixtures presented here, zeolite exhibits extremely high selectivity of water to the investigated weakly polar/non-polar gases demonstrating the excellent dehydration ability of zeolite NaA in engineering applications.     

Molecular Dynamics Simulation of Platinum Particles Between Graphite Walls

Abstract

We report preliminary molecular dynamics simulations results for platinum atoms confined between two parallel graphite surfaces. The system shows phase transition characteristics corresponding to a second order transition. Significant structural changes are also observed in the range of temperature studied. We have also investigated the effects of two dfferent Pt-wall interaction potentials: the 9-3 form suggested by Crowell and the 10-4 form originally proposed by Steele. The results show that the two systems have rather different structural characteristics but similar thermodynamic behavior.     

Adsorption Behavior of Multicomponent Protein Mixtures Containing α1−Proteinase Inhibitor with the Anion Exchanger, 2-(Diethylamino) ethyl-Spherodex

The equilibrium binding behavior of α₁-proteinase inhibitor (α₁-PI) in the presence of human serum albumin (HSA) has been determined in packed bed systems with the anion exchanger, 2-(diethylamino) ethyl (DEAE) -Spherodex. Experimental data derived for the individual proteins were compared with the corresponding data obtained from batch adsorption studies as well as studies in which mixtures of these two proteins were loaded at different concentration ratios onto columns of the same anion exchange adsorbent. The results confirm that α₁-PI has a greater affinity for the anion exchanger, although competitive adsorption was observed as the inlet concentration of HSA was increased. Under these conditions, decreased binding capacities and lower dynamic adsorption rates were observed for α₁-PI with the DEAE-Spherodex anion exchange adsorbent. The results are discussed in terms of the influence which various contaminants that occur in multicomponent mixtures of proteins from human plasma can have on the equilibrium binding characteristics of a target protein with weak or strong ion exchange adsorbents under conditions approaching concentration overload in preparative chromatographic systems. These investigations have also addressed, as the first part of an iterative approach for the simulation of the adsorption behavior of multicomponent mixtures of human plasma proteins with ion exchange and affinity chromatographic adsorbents, the ability of noncompetitive and competitive Langmuirean models to simulate the adsorption of α₁-PI in the presence of different concentrations of HSA to DEAE-Spherodex.     

Computer simulation of protein self-association during small-zone gel filtration. Estimation of equilibrium constants.

A simulation is developed that qualitatively describes the small-zone-gel-filtration behaviour of a reversibly associating protein. The results reflect the dependence of the apparent molecular weight of a reversibly associating protein on the equilibrium constant (KD) and initial concentration of the protein as well as the column length. The behaviour of a protein on an individual column is characterized and thus a means is provided for estimation of KD. The procedure is extended to describe the behaviour of a mixture of two proteins capable of heterologous as well as homologous association. This computer simulation has been applied in association studies of immunoglobulin light chains [Stevens, Westholm, Solomon & Schiffer (1980) Proc. Natl. Acad. Sci. 77, 1144--1148]. The KD value determined for the Bence--Jones protein Au (10(5) M-1) is close to the value (6.6 X 10(4) M-1) determined by other methods [Maeda, Steffen & Engel (1978) Biophys. Chem. 9, 57-64].     

Fluctuation Simulations and the Calculation of Mechanical Partial Molar Properties

Abstract

A method for the direct calculation of partial molar volumes, energies, and enthalpies in multicomponent mixtures in which all species have finite concentrations is presented. The approach, which is based on fluctuation theory, allows the simultaneous determination of the properties of all components in the mixture. The advantages and limitations of the method are illustrated through the (N, U, V) molecular dynamics calculation of the mechanical partial molar properties of two binary Lennard-Jones mixtures.