mohammad kowsari | Institute for Advanced Studies in Basic Sciences (original) (raw)
Papers by mohammad kowsari
Journal of Chemical Physics, Dec 14, 2008
Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room... more Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium ͓amim͔ + ͑alkyl= methyl, ethyl, propyl, and butyl͒ family with PF 6 − , NO 3 − , and Cl − counterions. The explicit atom transferable force field of Canongia Lopes et al. ͓J. Phys. Chem. B 108, 2038 ͑2004͔͒ is used in the simulations. In this first part, the dynamics of the ionic liquids are characterized by studying the mean-square displacement ͑MSD͒ and the velocity autocorrelation function ͑VACF͒ for the centers of mass of the ions at 400 K. Trajectory averaging was employed to evaluate the diffusion coefficients at two temperatures from the linear slope of MSD͑t͒ functions in the range of 150-300 ps and from the integration of the VACF͑t͒ functions at 400 K. Detailed comparisons are made between the diffusion results from the MSD and VACF methods. The diffusion coefficients from the integration of the VACFs are closer to experimental values than the diffusion coefficients calculated from the slope of MSDs. Both methods can show good agreement with experiment in predicting relative trends in the diffusion coefficients and determining the role of the cation and anion structures on the dynamical behavior of this family of ionic liquids. The MSD and self-diffusion of relatively heavier imidazolium cations are larger than those of the lighter anions from the Einstein results, except for the case of ͓bmim͔͓Cl͔. The cationic transference number generally decreases with temperature, in good agreement with experiments. For the same anion, the cationic transference numbers decrease with increasing length of the alkyl chain, and for the same cation, the trends in the cationic transference numbers are ͓NO 3 ͔ − Ͻ ͓Cl͔ − Ͻ ͓PF 6 ͔ −. The trends in the diffusion coefficient in the series of cations with identical anions are ͓emim͔ + Ͼ ͓pmim͔ + Ͼ ͓bmim͔ + and those for anions with identical cations are ͓NO 3 ͔ − Ͼ ͓PF 6 ͔ − Ͼ ͓Cl͔ −. The ͓dmim͔ + has a relatively low diffusion coefficient due to its symmetric structure and good packing in the liquid phase. The major factor for determining the magnitude of the self-diffusion is the geometric shape of the anion of the ionic liquid. Other important factors are the ion size and the charge delocalization in the anion.
Nashrieh Shimi va Mohandesi Shimi Iran, Aug 23, 2018
Microporous and Mesoporous Materials, Jul 1, 2018
Abstract Diffusion-selective N 2 /O 2 separation and competitive adsorption by nanoporous Li-LSX ... more Abstract Diffusion-selective N 2 /O 2 separation and competitive adsorption by nanoporous Li-LSX zeolite, as the best commercially available adsorbent, is a scientifically interesting phenomenon with important technological applications. In a recent paper ( J. Phys. Chem. C 121, 2017, 1770–1780), we reported the first molecular dynamics (MD) study of the adsorption of the binary N 2 /O 2 mixtures within this zeolite. In the present paper, the microscopic structure and intracrystalline self-diffusion coefficient of N 2 and O 2 as single-component guest species in Li-LSX zeolite are determined at different temperatures and practical loadings. The simulation results are found to be in very good complementary agreement with experimental adsorption findings as well as with the recent predictions based on the binary air mixture simulations. The influences of the internal surface dynamics of extra-framework Li + cation in site III (Li-III) on the guest static and dynamic processes within the zeolite are determined. O 2 has no significant association with Li-III, while all structural analyses prove remarkable adsorption and hence localization of N 2 around the Li-III in the supercages. The degree of localization and the residence time of N 2 on Li-III in particular increases with decreasing temperature and also with fixing of Li-III cations during the simulation. As a guide for future design of N 2 /O 2 separation by zeolite frameworks at ambient temperature, an excellent adsorption selectivity ratio can be achieved if it is practically possible to reduce the dynamics of accessible Li-III sorption sites up to extremely fixed situations. The calculated Arrhenius activation energy for N 2 diffusion process through Li-LSX zeolite is several times higher than that of O 2 . These results are due to the larger quadrupole moment of N 2 and stronger Coulombic attraction between N 2 and Li-III that causes the translational motion of N 2 to be significantly hindered. Overall, the intracrystalline self-diffusion coefficients of both N 2 and O 2 slightly decrease with loading. Current simulations also correctly approve that the Li + cations in sites I′ and II are inaccessible to interact with guest molecules.
Journal of Physical Chemistry C, Jan 18, 2017
Along with available adsorption isotherms and uptake kinetic data, microscopic knowledge of the g... more Along with available adsorption isotherms and uptake kinetic data, microscopic knowledge of the guest self-diffusion and intracrystalline movement of the simple air binary mixture of nitrogen (N 2) and oxygen (O 2) within Li−LSX zeolite is needed to optimize the design and to reach a breakthrough high-efficiency of air separation process based on the selective adsorption in this zeolite. In the current work for the first time, an all-atom molecular dynamics (MD) simulation is used to study the average single-particle dynamics, self-diffusion, and microscopic structure of the atmospheric binary gaseous mixtures of N 2 and O 2 in Li−LSX zeolite at temperatures between (260 and 700) K. The common order of magnitude of the computed guest self-diffusion coefficients at different temperatures is in the range of 10 −9-10 −8 m 2 •s −1 and corresponding activation energies obtained using the Arrhenius equation varied in the range of ∼0.6 for O 2 to 1.6−3.3 kcal•mol −1 for N 2 in simulations with mobile and with fixed extra-framework Li + on SIII sites (Li−III), respectively. Present results provide some new molecular-level insights into the link between the behaviors of the pendulum-like motion of Li−III with the guest molecules. Results show that O 2 guest molecules freely move into the supercages and channels of the zeolite without any attachment to the key sorption cationic sites and the behavior of O 2 is independent of the fixed or mobile Li−III situation during of simulations. In contrast, the oscillatory motion or immobility of the Li−III cation is found to have a surprisingly large influence on the intracrystalline N 2 self-diffusion, the local (N 2 −Li−III) structural correlation, and the mean time of attachment of N 2 to Li−III. The different observed adsorption behavior of two guest components was previously connected to the difference in their relative values of permanent quadrupole moments which causes different guest−Li−III affinities. These are well explained by a microscopic structural and dynamical analysis in current study. O 2 component diffuses faster than N 2 within the nanoporous Li−LSX zeolite, especially with a greater relative diffusivity difference for simulations with fixed Li−III at relatively low temperatures which correspond to favorable selective adsorption conditions. The computed O 2 /N 2 diffusion selectivity ratio increases with decreasing temperature.
Microporous and Mesoporous Materials, Mar 1, 2017
The dynamical and structural properties of hydrogen (H 2) guest gas inside nanoporous Li-LSX zeol... more The dynamical and structural properties of hydrogen (H 2) guest gas inside nanoporous Li-LSX zeolite were studied by molecular dynamics (MD) simulation for different loadings (8, 12, 16, and 20) of H 2 per unit cell at temperatures of 200, 298, 400, and 500 K. Three equal mean-square displacement (MSD) components (in the x, y, and z-directions) for the center of mass of H 2 guest molecules show that the translational motion of H 2 in this zeolite medium is isotropic due to the high symmetry of the zeolite framework. At these conditions, H 2 guest molecules freely move without blocking each other's path into the supercages and channels of the Li-LSX zeolite. The order of calculated self-diffusion coefficient of H 2 guest molecules at different temperatures, in the range of 10-9 up to 10-7 m 2 •s-1 , and corresponding activation energy, ~ 2 kcal•mol-1 , followed using the Arrhenius equation is in good agreement with the pores size of Li-LSX zeolite (7.4-12 Å) and compatible with inter-region of wellknown Knudsen and Configurational diffusion. The H 2 self-diffusion coefficients increase with temperature, while showing no quantifiable changes with loading within this loading range. A further study with a broader guest loading range would be appropriate to fully understand the loading effect on the self-diffusion of H 2 guest molecules in the Li-LSX zeolite as the H 2 storage candidate. In addition to determining the temperature and loading effects on the H 2 guest behavior, current simulations also show that the Li-III cations are specific H 2 sorption sites and the structural correlation, dynamics, self-diffusion coefficient, and adsorption of H 2 molecules are strongly dependent on the mobility or immobility of the key extraframework Li-III cationic sites of Li-LSX zeolite. The results of the simulation help in the choice of favorable structure, best design, and operative manufacture of zeolites or other microporous materials for similar applications.
Journal of Molecular Liquids, Nov 1, 2017
insights into the thermodynamics, structure, and dynamics of ionic liquid 1-hexyl-3-methylimidazo... more insights into the thermodynamics, structure, and dynamics of ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate via molecular dynamics study,
Journal of Computational Chemistry, May 3, 2018
The reliability of a molecular dynamics (MD) simulation study mainly depends on the accuracy of t... more The reliability of a molecular dynamics (MD) simulation study mainly depends on the accuracy of the applied force field. Unlike the ability of some potential models for reasonably predicting the thermodynamic properties of acetonitrile (ACN), simulated dynamical properties such as self-diffusion are generally underestimated compared to experimental values. The present work focuses on the evaluation and refinement of several available all-atom force fields for ACN and proposes a refined flexible six-site potential model. The main modification is related to the reduction of intermolecular LJ parameters (r and E) for hydrogen atoms, especially E, significantly affecting the dynamical behavior. Besides, the adjustment of r and E for nitrile carbon and nitrogen atoms helps reach optimum results. Our refined model shows an excellent agreement with the experiment for self-diffusion coefficient and thermodynamic quantities as well as providing a qualitative description of the microscopic structure of liquid ACN. V C 2018 Wiley Periodicals, Inc.
Theoretical Chemistry Accounts, Aug 1, 2019
Hydrophobic ionic liquids (ILs) can form biphasic systems with aqueous media and are appropriate ... more Hydrophobic ionic liquids (ILs) can form biphasic systems with aqueous media and are appropriate candidates for liquidliquid extraction (LLE). In this study, a biphasic system composed of water and the hydrophobic IL, 1-hexyl-3-methylimidazolium hexafluorophosphate, [HMIM][PF 6 ] was studied using molecular dynamics (MD) simulation to understand the molecular-level distribution of the heavy metal cadmium ions, Cd 2+ , in the water-IL biphasic system in the presence of hydrochloric acid. The experimentally observed positive effect of adding chloride to the aqueous phase on the metal extraction was studied at the molecular scale. Particularly, the effect of hydrochloric acid addition on the solubility of the IL cations and anions in the water was investigated. It was found that with adding hydrochloric acid (1 M) to the water phase, the IL cation solubility in water decreased; however, the IL anion solubility almost did not change. This can affect the extraction process of the metal ions. Moreover, it was found that during the [CdCl 4 ] 2− migration to the IL phase, a gradual breaking of the hydrogen bonds occurs between the complex and the water molecules at the interface. Therefore, quantum mechanics (QM) calculations were performed to explain the interaction energies of the cadmium complex with water and the IL. The calculated interaction energy of the ternary complex of IL-[CdCl 4 ] 2−-H 2 O was found to be greater than that of the binary complexes of IL-[CdCl 4 ] 2− and H 2 O-[CdCl 4 ] 2−. The results obtained in this work give some insights into the behaviour of the IL-based extraction systems in contact with aqueous solutions containing salts or mineral acids.
![Research paper thumbnail of Capturing the effect of [PF<sub>3</sub>(C<sub>2</sub>F<sub>5</sub>)<sub>3</sub>]<sup>−</sup><i>vs.</i> [PF<sub>6</sub>]<sup>−</sup>, flexible anion <i>vs.</i> rigid, and scaled charge <i>vs.</i> unit on the transport properties of [bmim]<sup>+</sup>-based ionic liquids: a comparative MD study](https://attachments.academia-assets.com/110535733/thumbnails/1.jpg)
](Physical Chemistry Chemical Physics, 2018
Journal of Physical Chemistry B, Oct 7, 2016
All-atom molecular dynamics (MD) simulations of 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfony... more All-atom molecular dynamics (MD) simulations of 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide ([C 6 mmim][FSI]) ionic liquid (IL) and its binary mixtures with acetonitrile (ACN) are reported for the first time. The presence of ACN as a cosolvent, similar to the effect of increasing temperature, causes an enhancement to the ion translational motion and fluidity in the IL, leading to significant improvement of ionic conductivity and self-diffusion which is well explained by a microscopic structural analysis. In neat IL and concentrated IL mixture, self-diffusion of the cation is higher than that of corresponding anion; however, further adding of ACN into the diluted mixtures with the IL molar fraction (x IL) below 0.50 results in more weakened interactions of the nearest ACN-anion neighbors rather than those of ACNcation neighbors so that the number of isolated anions is more than that of isolated cations at this condition, and the anions diffuse faster than the cations as expected based of their relative sizes. The velocity autocorrelation function (VACF) analysis indicates the inverse relation between the x IL and the mean collision time of each species. Additionally, at a fixed x IL , both the mean collision time and the velocity randomization time of ACN are shorter than those of the ions. The gradual addition of ACN changes the morphology of nano-segregated domains and tends to disrupt ionic clusters (i.e., it scatters and decomposes both the polar and non-polar domains) compared to those of pure IL, whereas both the radial and spatial distribution functions show the stabilization role of ACN on the close contact ion pair association. On the other hand, increasing of ACN causes a weakening of the structural correlations of the cation-cation and anion-anion neighbors in the solutions. ACN molecules appeared as a bridge with balanced affinities between the polar and non-polar domains, and no indication was observed for aggregation of ACN
The Journal of Chemical Physics
Atomistic-level understanding of the interfacial behavior of ionic liquids (ILs) confined in slit... more Atomistic-level understanding of the interfacial behavior of ionic liquids (ILs) confined in slit-like nanopores is of both fundamental and practical interest. Molecular dynamics (MD) is an efficient and robust approach to characterize the properties of confined systems in contrast with some limitations in direct experimental measurements at low-dimensions. In this research, MD simulations are used to study the biocompatible IL cholinium glycinate, [Cho][Gly], confined between two parallel plates of rutile or graphite, with the separation distance of 24 Å along the z-direction. As expected, both the microscopic local structure and dynamical behavior of the confined IL are very heterogeneous and depend effectively on the position of the ions to the pore walls. The ion z-density profile is used for segmentation of the inter-wall space into a central region and two outer layers. The behavior of ions in the central region is very similar to the bulk IL, while the behavior of the arrange...
![Research paper thumbnail of Fine probing the effect of replacing [PF<sub>6</sub>]<sup>−</sup> with [PF<sub>3</sub>(C<sub>2</sub>F<sub>5</sub>)<sub>3</sub>]<sup>−</sup> on the local structure and nanoscale organization of [bmim]<sup>+</sup>-based ionic liquids using MD simulation](https://attachments.academia-assets.com/110535689/thumbnails/1.jpg)
](Physical Chemistry Chemical Physics, 2019
![Research paper thumbnail of Tracing Local Nanostructure of the Aqueous Solutions of the Biocompatible [Cho][Gly] Ionic Liquid: Importance of Hydrogen Bond Attraction between Like-Charged Ions](https://attachments.academia-assets.com/106433891/thumbnails/1.jpg)
](Journal of Physical Chemistry B, Apr 10, 2020
The neat and aqueous solutions of the cholinium glycinate ionic liquid (IL), [Cho][Gly], at diffe... more The neat and aqueous solutions of the cholinium glycinate ionic liquid (IL), [Cho][Gly], at different water mole fractions, x w s, are studied by molecular dynamics simulations. The changes in the local nanostructure of systems with composition have been determined by calculation of various structural distribution functions. Hydrogen bond (H-bond) attractions determine the major relative orientations of the oppositely and like charged nearest neighbors. The cation-anion Hbonds mainly form between the hydrogen of the hydroxyl or methyl groups of the cation and the carboxylate oxygen of the anion. A preferred (antiparallel) arrangement between adjacent [Cho] + cations is due to the effective H-bond between the hydroxyl oxygen and the methyl hydrogen sites that promotes the like-charge cluster formation. Adding water decreases the occurrence probability of the [Cho] + •••[Gly]-•••[Cho] + bridge structure in the aqueous solutions due to the formation of the [Gly]-•••HOH•••[Gly]structure via H-bonding. Observed density trend versus x w is interpreted based on an interstice model and investigating the water cluster size distribution. Finally, the effect of x w on the infrared (IR) vibrational spectra were studied and blue and red shifts were observed for the stretching and bending vibrational modes of the hydroxyl group of [Cho] + , respectively. Current findings will improve the efficient engineering design and task-specific applications of aqueous solutions of bio-ILs consist of [Cho] + and amino acid anions.
Physical Chemistry Chemical Physics, 2011
Systematic molecular dynamics simulations are used to study the structure, dynamics and transport... more Systematic molecular dynamics simulations are used to study the structure, dynamics and transport properties of the ionic liquids composed of the tetra-butylphosphonium ([TBP] + , or [P(C 4 H 9) 4 ] +) cation with six amino acid ([AA] À) anions. The structural features of these ionic liquids were characterized by calculating the partial site-site radial distribution functions, g(r), and computing the dihedral angle distribution of n-butyl side chains in the [TBP] + cations. The dynamics of the ionic liquids are described by studying the velocity autocorrelation function (VACF) and the mean-square displacement (MSD) for the centers of mass of the ions at different temperatures. The ionic diffusion coefficients and the electrical conductivities were evaluated from both the Einstein and Green-Kubo methods. The cross-correlation terms in the electric-current autocorrelation functions, which are an indication of the ion pair correlations, are investigated. The cationic transference numbers were also estimated to study the contributions of the anions and cations to the transport of charge in these ionic liquids. We determined the role of the amino acid anion structures on the dynamical behavior and the transport coefficients of this family of ionic liquids. In general, the MSD and self-diffusion coefficients of the relatively heavier non-planar [TBP] + cations are smaller than those of the lighter amino acid anions. Introducing polar functional groups (acid or amide) in the side chain of [AA] À decreases the diffusion coefficient and electrical conductivity of AAILs. The major factors for determining the magnitude of the transport coefficients are the chemical functionality and the length of the alkyl side chain of the [AA] À anion of these [TBP][AA] ionic liquids.
Silicates are multi-functional materials which are important in optical and biomedical applicatio... more Silicates are multi-functional materials which are important in optical and biomedical applications and dominant in geological environments. For this reason they are of interest to variety of scientific communities. In this work, we have used molecular dynamics simulations to study the structural, thermodynamic, and dynamical properties of alkali-metal disilicates with the formula M2Si2O5 over a wide range of temperature and pressure conditions. The model potential used in these simulations is based on that of Vessal et al. [1]. The MD calculations were performed for sodium and potassium silicates in the temperature and pressure ranges from 250 to 1000 K and 1.01325 bar to 100 kbar, respectivley. The time step for the simulations was 1 × 10-3 ps and the systems were equilibrated for a minimum of 60,000 time steps. We have obtained the partial radial distribution functions (RDFs) for all ion pairs and the total RDF over a wide temperature range for sodium and potassium disilicate glasses. The results for sodium disilicate glass at 1000 K and 100 kbar are presented in Fig. 1.
Journal of Chemical & Engineering Data, Aug 18, 2014
Journal of Chemical & Engineering Data, Jan 13, 2015
ABSTRACT Molecular dynamics simulations of four ionic liquids (ILs) based on the [Tf2N]−, bis(tri... more ABSTRACT Molecular dynamics simulations of four ionic liquids (ILs) based on the [Tf2N]−, bis(trifluoromethanesulfonyl)imide anion, and imidazolium cations with different alkyl side chains have been performed. These simulations investigate the influence of butyl side chain elimination, tail amine functional addition, and C2 methylation on the dynamics and transport properties of this family of ionic liquids at 400 K. In our earlier work (J. Chem. Eng. Data, 2014, 59, 2834-2849), a suite of thermodynamic quantities and microscopic structures of these ILs were studied by classical molecular dynamics simulations and ab initio calculations. In this work, the dynamics of the ILs are studied by calculating the mean-square displacement (MSD) and the velocity autocorrelation function (VACF) for selected atomic sites and the centers of mass of the ions. These results are used to calculate the self-diffusion and the ionic conductivity from both the Einstein and Green-Kubo formulas. The calculated ionic self-diffusion coefficients are used to estimate the cationic transference number and the Stokes-Einstein viscosity for the four ILs. In agreement with experiments, the general simulated trends in the MSD, self-diffusion, and ionic conductivity are [bmim][Tf2N] > [apmim][Tf2N] > [bmmim][Tf2N] > [mim][Tf2N]. These trends are the reverse of the trend in the viscosity of four selected ILs. As expected by applying a nonpolarizable force field, the simulation results tend to underestimate the self-diffusivity and conductivity, and overestimate the shear viscosity. The highest and the lowest degrees of ionic association are detected for [mim][Tf2N] and [bmim][Tf2N], respectively.
Microporous and Mesoporous Materials, 2017
Journal of Chemical Physics, Jan 6, 2009
A systematic molecular dynamics study is performed to determine the dynamics and transport proper... more A systematic molecular dynamics study is performed to determine the dynamics and transport properties of 12 room-temperature ionic liquids family with 1-alkyl-3-methylimidazolium cation, ͓amim͔ + ͑alkyl= methyl, ethyl, propyl, and butyl͒, with counterions, PF 6 − , NO 3 − , and Cl −. The goal of the work is to provide molecular level understanding of the transport coefficients of these liquids as guidance to experimentalists on choosing anion and cation pairs to match required properties of ionic liquid solvents. In the earlier paper ͑Part I͒, we characterized the dynamics of ionic liquids and provided a detailed comparison of the diffusion coefficients for each ion using the Einstein and Green-Kubo formulas. In this second part, other transport properties of imidazolium salts are calculated, in particular, the electrical conductivity is calculated from the Nernst-Einstein and Green-Kubo formulas. The viscosity is also determined from the Stokes-Einstein relation. The results of the calculated transport coefficients are consistent with the previous computational and experimental studies of imidazolium salts. Generally, the simulations give electrical conductivity lower than experiment while the viscosity estimate is higher than experiment. Within the same cation family, the ionic liquids with the NO 3 − counterion have the highest electrical conductivities: ͓NO 3 ͔ − Ͼ ͓PF 6 ͔ − Ͼ ͓Cl͔ −. The ͓dmim͔͓X͔ series, due to their symmetric cationic structure and good packing and the ͓bmim͔͓X͔ series due to higher inductive van der Waals interactions of ͓bmim͔ + , have the highest viscosities in these ionic liquid series. Our simulations show that the major factors determining the magnitude of the self-diffusion, electrical conductivity, and viscosity are the geometric shape, ion size, and the delocalization of the ionic charge in the anion.
Journal of Chemical Physics, Dec 14, 2008
Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room... more Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium ͓amim͔ + ͑alkyl= methyl, ethyl, propyl, and butyl͒ family with PF 6 − , NO 3 − , and Cl − counterions. The explicit atom transferable force field of Canongia Lopes et al. ͓J. Phys. Chem. B 108, 2038 ͑2004͔͒ is used in the simulations. In this first part, the dynamics of the ionic liquids are characterized by studying the mean-square displacement ͑MSD͒ and the velocity autocorrelation function ͑VACF͒ for the centers of mass of the ions at 400 K. Trajectory averaging was employed to evaluate the diffusion coefficients at two temperatures from the linear slope of MSD͑t͒ functions in the range of 150-300 ps and from the integration of the VACF͑t͒ functions at 400 K. Detailed comparisons are made between the diffusion results from the MSD and VACF methods. The diffusion coefficients from the integration of the VACFs are closer to experimental values than the diffusion coefficients calculated from the slope of MSDs. Both methods can show good agreement with experiment in predicting relative trends in the diffusion coefficients and determining the role of the cation and anion structures on the dynamical behavior of this family of ionic liquids. The MSD and self-diffusion of relatively heavier imidazolium cations are larger than those of the lighter anions from the Einstein results, except for the case of ͓bmim͔͓Cl͔. The cationic transference number generally decreases with temperature, in good agreement with experiments. For the same anion, the cationic transference numbers decrease with increasing length of the alkyl chain, and for the same cation, the trends in the cationic transference numbers are ͓NO 3 ͔ − Ͻ ͓Cl͔ − Ͻ ͓PF 6 ͔ −. The trends in the diffusion coefficient in the series of cations with identical anions are ͓emim͔ + Ͼ ͓pmim͔ + Ͼ ͓bmim͔ + and those for anions with identical cations are ͓NO 3 ͔ − Ͼ ͓PF 6 ͔ − Ͼ ͓Cl͔ −. The ͓dmim͔ + has a relatively low diffusion coefficient due to its symmetric structure and good packing in the liquid phase. The major factor for determining the magnitude of the self-diffusion is the geometric shape of the anion of the ionic liquid. Other important factors are the ion size and the charge delocalization in the anion.
Nashrieh Shimi va Mohandesi Shimi Iran, Aug 23, 2018
Microporous and Mesoporous Materials, Jul 1, 2018
Abstract Diffusion-selective N 2 /O 2 separation and competitive adsorption by nanoporous Li-LSX ... more Abstract Diffusion-selective N 2 /O 2 separation and competitive adsorption by nanoporous Li-LSX zeolite, as the best commercially available adsorbent, is a scientifically interesting phenomenon with important technological applications. In a recent paper ( J. Phys. Chem. C 121, 2017, 1770–1780), we reported the first molecular dynamics (MD) study of the adsorption of the binary N 2 /O 2 mixtures within this zeolite. In the present paper, the microscopic structure and intracrystalline self-diffusion coefficient of N 2 and O 2 as single-component guest species in Li-LSX zeolite are determined at different temperatures and practical loadings. The simulation results are found to be in very good complementary agreement with experimental adsorption findings as well as with the recent predictions based on the binary air mixture simulations. The influences of the internal surface dynamics of extra-framework Li + cation in site III (Li-III) on the guest static and dynamic processes within the zeolite are determined. O 2 has no significant association with Li-III, while all structural analyses prove remarkable adsorption and hence localization of N 2 around the Li-III in the supercages. The degree of localization and the residence time of N 2 on Li-III in particular increases with decreasing temperature and also with fixing of Li-III cations during the simulation. As a guide for future design of N 2 /O 2 separation by zeolite frameworks at ambient temperature, an excellent adsorption selectivity ratio can be achieved if it is practically possible to reduce the dynamics of accessible Li-III sorption sites up to extremely fixed situations. The calculated Arrhenius activation energy for N 2 diffusion process through Li-LSX zeolite is several times higher than that of O 2 . These results are due to the larger quadrupole moment of N 2 and stronger Coulombic attraction between N 2 and Li-III that causes the translational motion of N 2 to be significantly hindered. Overall, the intracrystalline self-diffusion coefficients of both N 2 and O 2 slightly decrease with loading. Current simulations also correctly approve that the Li + cations in sites I′ and II are inaccessible to interact with guest molecules.
Journal of Physical Chemistry C, Jan 18, 2017
Along with available adsorption isotherms and uptake kinetic data, microscopic knowledge of the g... more Along with available adsorption isotherms and uptake kinetic data, microscopic knowledge of the guest self-diffusion and intracrystalline movement of the simple air binary mixture of nitrogen (N 2) and oxygen (O 2) within Li−LSX zeolite is needed to optimize the design and to reach a breakthrough high-efficiency of air separation process based on the selective adsorption in this zeolite. In the current work for the first time, an all-atom molecular dynamics (MD) simulation is used to study the average single-particle dynamics, self-diffusion, and microscopic structure of the atmospheric binary gaseous mixtures of N 2 and O 2 in Li−LSX zeolite at temperatures between (260 and 700) K. The common order of magnitude of the computed guest self-diffusion coefficients at different temperatures is in the range of 10 −9-10 −8 m 2 •s −1 and corresponding activation energies obtained using the Arrhenius equation varied in the range of ∼0.6 for O 2 to 1.6−3.3 kcal•mol −1 for N 2 in simulations with mobile and with fixed extra-framework Li + on SIII sites (Li−III), respectively. Present results provide some new molecular-level insights into the link between the behaviors of the pendulum-like motion of Li−III with the guest molecules. Results show that O 2 guest molecules freely move into the supercages and channels of the zeolite without any attachment to the key sorption cationic sites and the behavior of O 2 is independent of the fixed or mobile Li−III situation during of simulations. In contrast, the oscillatory motion or immobility of the Li−III cation is found to have a surprisingly large influence on the intracrystalline N 2 self-diffusion, the local (N 2 −Li−III) structural correlation, and the mean time of attachment of N 2 to Li−III. The different observed adsorption behavior of two guest components was previously connected to the difference in their relative values of permanent quadrupole moments which causes different guest−Li−III affinities. These are well explained by a microscopic structural and dynamical analysis in current study. O 2 component diffuses faster than N 2 within the nanoporous Li−LSX zeolite, especially with a greater relative diffusivity difference for simulations with fixed Li−III at relatively low temperatures which correspond to favorable selective adsorption conditions. The computed O 2 /N 2 diffusion selectivity ratio increases with decreasing temperature.
Microporous and Mesoporous Materials, Mar 1, 2017
The dynamical and structural properties of hydrogen (H 2) guest gas inside nanoporous Li-LSX zeol... more The dynamical and structural properties of hydrogen (H 2) guest gas inside nanoporous Li-LSX zeolite were studied by molecular dynamics (MD) simulation for different loadings (8, 12, 16, and 20) of H 2 per unit cell at temperatures of 200, 298, 400, and 500 K. Three equal mean-square displacement (MSD) components (in the x, y, and z-directions) for the center of mass of H 2 guest molecules show that the translational motion of H 2 in this zeolite medium is isotropic due to the high symmetry of the zeolite framework. At these conditions, H 2 guest molecules freely move without blocking each other's path into the supercages and channels of the Li-LSX zeolite. The order of calculated self-diffusion coefficient of H 2 guest molecules at different temperatures, in the range of 10-9 up to 10-7 m 2 •s-1 , and corresponding activation energy, ~ 2 kcal•mol-1 , followed using the Arrhenius equation is in good agreement with the pores size of Li-LSX zeolite (7.4-12 Å) and compatible with inter-region of wellknown Knudsen and Configurational diffusion. The H 2 self-diffusion coefficients increase with temperature, while showing no quantifiable changes with loading within this loading range. A further study with a broader guest loading range would be appropriate to fully understand the loading effect on the self-diffusion of H 2 guest molecules in the Li-LSX zeolite as the H 2 storage candidate. In addition to determining the temperature and loading effects on the H 2 guest behavior, current simulations also show that the Li-III cations are specific H 2 sorption sites and the structural correlation, dynamics, self-diffusion coefficient, and adsorption of H 2 molecules are strongly dependent on the mobility or immobility of the key extraframework Li-III cationic sites of Li-LSX zeolite. The results of the simulation help in the choice of favorable structure, best design, and operative manufacture of zeolites or other microporous materials for similar applications.
Journal of Molecular Liquids, Nov 1, 2017
insights into the thermodynamics, structure, and dynamics of ionic liquid 1-hexyl-3-methylimidazo... more insights into the thermodynamics, structure, and dynamics of ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate via molecular dynamics study,
Journal of Computational Chemistry, May 3, 2018
The reliability of a molecular dynamics (MD) simulation study mainly depends on the accuracy of t... more The reliability of a molecular dynamics (MD) simulation study mainly depends on the accuracy of the applied force field. Unlike the ability of some potential models for reasonably predicting the thermodynamic properties of acetonitrile (ACN), simulated dynamical properties such as self-diffusion are generally underestimated compared to experimental values. The present work focuses on the evaluation and refinement of several available all-atom force fields for ACN and proposes a refined flexible six-site potential model. The main modification is related to the reduction of intermolecular LJ parameters (r and E) for hydrogen atoms, especially E, significantly affecting the dynamical behavior. Besides, the adjustment of r and E for nitrile carbon and nitrogen atoms helps reach optimum results. Our refined model shows an excellent agreement with the experiment for self-diffusion coefficient and thermodynamic quantities as well as providing a qualitative description of the microscopic structure of liquid ACN. V C 2018 Wiley Periodicals, Inc.
Theoretical Chemistry Accounts, Aug 1, 2019
Hydrophobic ionic liquids (ILs) can form biphasic systems with aqueous media and are appropriate ... more Hydrophobic ionic liquids (ILs) can form biphasic systems with aqueous media and are appropriate candidates for liquidliquid extraction (LLE). In this study, a biphasic system composed of water and the hydrophobic IL, 1-hexyl-3-methylimidazolium hexafluorophosphate, [HMIM][PF 6 ] was studied using molecular dynamics (MD) simulation to understand the molecular-level distribution of the heavy metal cadmium ions, Cd 2+ , in the water-IL biphasic system in the presence of hydrochloric acid. The experimentally observed positive effect of adding chloride to the aqueous phase on the metal extraction was studied at the molecular scale. Particularly, the effect of hydrochloric acid addition on the solubility of the IL cations and anions in the water was investigated. It was found that with adding hydrochloric acid (1 M) to the water phase, the IL cation solubility in water decreased; however, the IL anion solubility almost did not change. This can affect the extraction process of the metal ions. Moreover, it was found that during the [CdCl 4 ] 2− migration to the IL phase, a gradual breaking of the hydrogen bonds occurs between the complex and the water molecules at the interface. Therefore, quantum mechanics (QM) calculations were performed to explain the interaction energies of the cadmium complex with water and the IL. The calculated interaction energy of the ternary complex of IL-[CdCl 4 ] 2−-H 2 O was found to be greater than that of the binary complexes of IL-[CdCl 4 ] 2− and H 2 O-[CdCl 4 ] 2−. The results obtained in this work give some insights into the behaviour of the IL-based extraction systems in contact with aqueous solutions containing salts or mineral acids.
Physical Chemistry Chemical Physics, 2018
Journal of Physical Chemistry B, Oct 7, 2016
All-atom molecular dynamics (MD) simulations of 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfony... more All-atom molecular dynamics (MD) simulations of 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide ([C 6 mmim][FSI]) ionic liquid (IL) and its binary mixtures with acetonitrile (ACN) are reported for the first time. The presence of ACN as a cosolvent, similar to the effect of increasing temperature, causes an enhancement to the ion translational motion and fluidity in the IL, leading to significant improvement of ionic conductivity and self-diffusion which is well explained by a microscopic structural analysis. In neat IL and concentrated IL mixture, self-diffusion of the cation is higher than that of corresponding anion; however, further adding of ACN into the diluted mixtures with the IL molar fraction (x IL) below 0.50 results in more weakened interactions of the nearest ACN-anion neighbors rather than those of ACNcation neighbors so that the number of isolated anions is more than that of isolated cations at this condition, and the anions diffuse faster than the cations as expected based of their relative sizes. The velocity autocorrelation function (VACF) analysis indicates the inverse relation between the x IL and the mean collision time of each species. Additionally, at a fixed x IL , both the mean collision time and the velocity randomization time of ACN are shorter than those of the ions. The gradual addition of ACN changes the morphology of nano-segregated domains and tends to disrupt ionic clusters (i.e., it scatters and decomposes both the polar and non-polar domains) compared to those of pure IL, whereas both the radial and spatial distribution functions show the stabilization role of ACN on the close contact ion pair association. On the other hand, increasing of ACN causes a weakening of the structural correlations of the cation-cation and anion-anion neighbors in the solutions. ACN molecules appeared as a bridge with balanced affinities between the polar and non-polar domains, and no indication was observed for aggregation of ACN
The Journal of Chemical Physics
Atomistic-level understanding of the interfacial behavior of ionic liquids (ILs) confined in slit... more Atomistic-level understanding of the interfacial behavior of ionic liquids (ILs) confined in slit-like nanopores is of both fundamental and practical interest. Molecular dynamics (MD) is an efficient and robust approach to characterize the properties of confined systems in contrast with some limitations in direct experimental measurements at low-dimensions. In this research, MD simulations are used to study the biocompatible IL cholinium glycinate, [Cho][Gly], confined between two parallel plates of rutile or graphite, with the separation distance of 24 Å along the z-direction. As expected, both the microscopic local structure and dynamical behavior of the confined IL are very heterogeneous and depend effectively on the position of the ions to the pore walls. The ion z-density profile is used for segmentation of the inter-wall space into a central region and two outer layers. The behavior of ions in the central region is very similar to the bulk IL, while the behavior of the arrange...
Physical Chemistry Chemical Physics, 2019
Journal of Physical Chemistry B, Apr 10, 2020
The neat and aqueous solutions of the cholinium glycinate ionic liquid (IL), [Cho][Gly], at diffe... more The neat and aqueous solutions of the cholinium glycinate ionic liquid (IL), [Cho][Gly], at different water mole fractions, x w s, are studied by molecular dynamics simulations. The changes in the local nanostructure of systems with composition have been determined by calculation of various structural distribution functions. Hydrogen bond (H-bond) attractions determine the major relative orientations of the oppositely and like charged nearest neighbors. The cation-anion Hbonds mainly form between the hydrogen of the hydroxyl or methyl groups of the cation and the carboxylate oxygen of the anion. A preferred (antiparallel) arrangement between adjacent [Cho] + cations is due to the effective H-bond between the hydroxyl oxygen and the methyl hydrogen sites that promotes the like-charge cluster formation. Adding water decreases the occurrence probability of the [Cho] + •••[Gly]-•••[Cho] + bridge structure in the aqueous solutions due to the formation of the [Gly]-•••HOH•••[Gly]structure via H-bonding. Observed density trend versus x w is interpreted based on an interstice model and investigating the water cluster size distribution. Finally, the effect of x w on the infrared (IR) vibrational spectra were studied and blue and red shifts were observed for the stretching and bending vibrational modes of the hydroxyl group of [Cho] + , respectively. Current findings will improve the efficient engineering design and task-specific applications of aqueous solutions of bio-ILs consist of [Cho] + and amino acid anions.
Physical Chemistry Chemical Physics, 2011
Systematic molecular dynamics simulations are used to study the structure, dynamics and transport... more Systematic molecular dynamics simulations are used to study the structure, dynamics and transport properties of the ionic liquids composed of the tetra-butylphosphonium ([TBP] + , or [P(C 4 H 9) 4 ] +) cation with six amino acid ([AA] À) anions. The structural features of these ionic liquids were characterized by calculating the partial site-site radial distribution functions, g(r), and computing the dihedral angle distribution of n-butyl side chains in the [TBP] + cations. The dynamics of the ionic liquids are described by studying the velocity autocorrelation function (VACF) and the mean-square displacement (MSD) for the centers of mass of the ions at different temperatures. The ionic diffusion coefficients and the electrical conductivities were evaluated from both the Einstein and Green-Kubo methods. The cross-correlation terms in the electric-current autocorrelation functions, which are an indication of the ion pair correlations, are investigated. The cationic transference numbers were also estimated to study the contributions of the anions and cations to the transport of charge in these ionic liquids. We determined the role of the amino acid anion structures on the dynamical behavior and the transport coefficients of this family of ionic liquids. In general, the MSD and self-diffusion coefficients of the relatively heavier non-planar [TBP] + cations are smaller than those of the lighter amino acid anions. Introducing polar functional groups (acid or amide) in the side chain of [AA] À decreases the diffusion coefficient and electrical conductivity of AAILs. The major factors for determining the magnitude of the transport coefficients are the chemical functionality and the length of the alkyl side chain of the [AA] À anion of these [TBP][AA] ionic liquids.
Silicates are multi-functional materials which are important in optical and biomedical applicatio... more Silicates are multi-functional materials which are important in optical and biomedical applications and dominant in geological environments. For this reason they are of interest to variety of scientific communities. In this work, we have used molecular dynamics simulations to study the structural, thermodynamic, and dynamical properties of alkali-metal disilicates with the formula M2Si2O5 over a wide range of temperature and pressure conditions. The model potential used in these simulations is based on that of Vessal et al. [1]. The MD calculations were performed for sodium and potassium silicates in the temperature and pressure ranges from 250 to 1000 K and 1.01325 bar to 100 kbar, respectivley. The time step for the simulations was 1 × 10-3 ps and the systems were equilibrated for a minimum of 60,000 time steps. We have obtained the partial radial distribution functions (RDFs) for all ion pairs and the total RDF over a wide temperature range for sodium and potassium disilicate glasses. The results for sodium disilicate glass at 1000 K and 100 kbar are presented in Fig. 1.
Journal of Chemical & Engineering Data, Aug 18, 2014
Journal of Chemical & Engineering Data, Jan 13, 2015
ABSTRACT Molecular dynamics simulations of four ionic liquids (ILs) based on the [Tf2N]−, bis(tri... more ABSTRACT Molecular dynamics simulations of four ionic liquids (ILs) based on the [Tf2N]−, bis(trifluoromethanesulfonyl)imide anion, and imidazolium cations with different alkyl side chains have been performed. These simulations investigate the influence of butyl side chain elimination, tail amine functional addition, and C2 methylation on the dynamics and transport properties of this family of ionic liquids at 400 K. In our earlier work (J. Chem. Eng. Data, 2014, 59, 2834-2849), a suite of thermodynamic quantities and microscopic structures of these ILs were studied by classical molecular dynamics simulations and ab initio calculations. In this work, the dynamics of the ILs are studied by calculating the mean-square displacement (MSD) and the velocity autocorrelation function (VACF) for selected atomic sites and the centers of mass of the ions. These results are used to calculate the self-diffusion and the ionic conductivity from both the Einstein and Green-Kubo formulas. The calculated ionic self-diffusion coefficients are used to estimate the cationic transference number and the Stokes-Einstein viscosity for the four ILs. In agreement with experiments, the general simulated trends in the MSD, self-diffusion, and ionic conductivity are [bmim][Tf2N] > [apmim][Tf2N] > [bmmim][Tf2N] > [mim][Tf2N]. These trends are the reverse of the trend in the viscosity of four selected ILs. As expected by applying a nonpolarizable force field, the simulation results tend to underestimate the self-diffusivity and conductivity, and overestimate the shear viscosity. The highest and the lowest degrees of ionic association are detected for [mim][Tf2N] and [bmim][Tf2N], respectively.
Microporous and Mesoporous Materials, 2017
Journal of Chemical Physics, Jan 6, 2009
A systematic molecular dynamics study is performed to determine the dynamics and transport proper... more A systematic molecular dynamics study is performed to determine the dynamics and transport properties of 12 room-temperature ionic liquids family with 1-alkyl-3-methylimidazolium cation, ͓amim͔ + ͑alkyl= methyl, ethyl, propyl, and butyl͒, with counterions, PF 6 − , NO 3 − , and Cl −. The goal of the work is to provide molecular level understanding of the transport coefficients of these liquids as guidance to experimentalists on choosing anion and cation pairs to match required properties of ionic liquid solvents. In the earlier paper ͑Part I͒, we characterized the dynamics of ionic liquids and provided a detailed comparison of the diffusion coefficients for each ion using the Einstein and Green-Kubo formulas. In this second part, other transport properties of imidazolium salts are calculated, in particular, the electrical conductivity is calculated from the Nernst-Einstein and Green-Kubo formulas. The viscosity is also determined from the Stokes-Einstein relation. The results of the calculated transport coefficients are consistent with the previous computational and experimental studies of imidazolium salts. Generally, the simulations give electrical conductivity lower than experiment while the viscosity estimate is higher than experiment. Within the same cation family, the ionic liquids with the NO 3 − counterion have the highest electrical conductivities: ͓NO 3 ͔ − Ͼ ͓PF 6 ͔ − Ͼ ͓Cl͔ −. The ͓dmim͔͓X͔ series, due to their symmetric cationic structure and good packing and the ͓bmim͔͓X͔ series due to higher inductive van der Waals interactions of ͓bmim͔ + , have the highest viscosities in these ionic liquid series. Our simulations show that the major factors determining the magnitude of the self-diffusion, electrical conductivity, and viscosity are the geometric shape, ion size, and the delocalization of the ionic charge in the anion.