Edward Maginn - Academia.edu (original) (raw)
Papers by Edward Maginn
The basic science goal in this project identifies structure/affinity relationships for selected r... more The basic science goal in this project identifies structure/affinity relationships for selected radionuclides and existing sorbents. The task will apply this knowledge to the design and synthesis of new sorbents that will exhibit increased cesium, strontium and actinide removal. The target problem focuses on the treatment of high-level nuclear wastes. The general approach can likewise be applied to non-radioactive separations. The project involves a collaboration among four organizations, with each focused on a different aspect of the problem. This document is the final report on the three years of activities conducted at the University of
The basic science goal in this project identifies structure/affinity relationships for selected r... more The basic science goal in this project identifies structure/affinity relationships for selected radionuclides and existing sorbents. The task will apply this knowledge to the design and synthesis of new sorbents that will exhibit increased cesium, strontium and actinide removal. The target problem focuses on the treatment of high-level nuclear wastes. The general approach can likewise be applied to non-radioactive separations. Research Progress During the third year of the project in addition to continuing our focus along two paths: (1) identifying the structure/affinity relationships for crystalline silicotitanate (CST) and (2) synthesis of new sorbents that include derivatives of CST, a titanosilicate analog of the mineral pharmacosiderite, sodium titanates, we are also conducting (3) insitu crystal growth and ion exchange studies at the X7B beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), Upton, NY, USA for collecting dynamic X-ray powder diffraction data. Based on the results of these studies a pathway for CST crystallization has been proposed. CST serves as the baseline or alternative technology sorbent for the separation of 137 Cs from waste solutions at several of the DOE sites. This material also exhibits a fairly high affinity for strontium, but rather low affinity for plutonium and other actinides. A sodium titanate material, monosodium titanate (MST), serves as the baseline material at the Savannah River Site for strontium and actinide separations from HLW solutions. Recent work demonstrated that pharmacosiderite and sodium nonatitanate (SNT) materials have strontium and actinide removal characteristics similar to that of MST. This year project members at Texas A&M completed two detailed structural characterizations and studied in situ crystallization process of CST. Structural studies include CST and niobium-substituted CST (Nb-CST) sorbents before and after exchange
![Research paper thumbnail of Erratum: “A method for computing the solubility limit of solids: Application to sodium chloride in water and alcohols” [J. Chem. Phys. 133, 124504 (2010)]](https://attachments.academia-assets.com/119892701/thumbnails/1.jpg)
](Journal of Chemical Physics, Jul 16, 2012
Nature Communications, 2022
Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially ... more Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES fo...
The Journal of Physical Chemistry B, 2020
Journal of The Electrochemical Society, 2021
The nitroxide radical redox organic molecule, 2-phenyl-4,4,5,5-tetrame- thylimidazoline-1-oxyl-3-... more The nitroxide radical redox organic molecule, 2-phenyl-4,4,5,5-tetrame- thylimidazoline-1-oxyl-3-oxide (PTIO), was investigated for the first time in a deep eutectic solvent (DES)-like system consisting of a 1:4 molar ratio of choline chloride and ethylene glycol (Ch1EG4) as a redox flow battery electrolyte. PTIO is a single molecule with three oxidation states, and can provide both positive and negative redox couples for a flow battery. A flow battery using the PTIO/Ch1EG4 electrolyte demonstrated nearly 50% round trip efficiency with an approximately 1 V open circuit potential. Inefficiencies were primarily due to membrane resistance which can be significantly lowered with increased temperature. While PTIO appears stable over short periods (hours), the oxidized form is not stable in the DES-like electrolyte over longer times. Molecular modeling was performed to investigate the relative stability of PTIO in DES as compared to the previously studied 4-hydroxy-TEMPO (4HT). It was fou...
The Journal of Chemical Physics, 2020
The low melting point of room temperature ionic liquids is usually explained in terms of the pres... more The low melting point of room temperature ionic liquids is usually explained in terms of the presence of bulky, low-symmetry, and flexible ions, with the first two factors related to the lattice energy while an entropic effect is attributed to the latter. By means of molecular dynamics simulations, the melting points of 1-ethyl-3-methyl-imidazolium hexafluorophosphate and 1-decyl-3-methyl-imidazolium hexafluorophosphate were determined, and the effect of the molecular flexibility over the melting point was explicitly computed by restraining the rotation of dihedral angles in both the solid and the liquid phases. The rotational flexibility over the bond between the ring and the alkyl chain affects the relative ordering of the anions around the cations and results in substantial effects over both the enthalpy and the entropy of melting. For the other dihedral angles of the alkyl group, the contributions are predominantly entropic and an alternating behavior was found. The flexibility ...
The Journal of chemical physics, Jan 28, 2018
The miscibility of ionic liquid (IL) pairs with a common cation (1-ethyl-3-methylimidazolium [CCi... more The miscibility of ionic liquid (IL) pairs with a common cation (1-ethyl-3-methylimidazolium [CCim]) and different anions (bis(trifluoromethylsulfonyl)amide [TFSI], acetate [OAc], and chloride [Cl]) was investigated at a wide range of water concentrations at room temperature. Molecular simulations predicted that the addition of water to the [CCim][TFSI]:[CCim][OAc] and [CCim][TFSI]:[CCim][Cl] mixtures would induce a liquid-liquid phase separation and that water addition to the [CCim][OAc]:[CCim][Cl] mixture would not produce a phase separation. The effect of water on the phase behavior of the IL mixtures was verified experimentally, and the IL and water concentrations were determined in each phase. Of particular importance is the analytical methodology used to determine the species' concentration, where H NMR and a combination of F NMR, Karl Fischer titration, and ion chromatography techniques were applied.
The Journal of Physical Chemistry B, 2018
The structural and dynamical changes in the solvation shell surrounding Li + in a multi-anion env... more The structural and dynamical changes in the solvation shell surrounding Li + in a multi-anion environment are studied by Raman spectroscopy and molecular dynamics (MD) simulations. The ternary electrolyte is composed of a mixture of two ionic liquids (ILs) n-methyl-npropylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([PYR13][TFSI]), 1-ethyl-3methylimidazolium dicyanamide ([EMIM][DCA]) and a lithium bis(trifluoromethylsulfonyl) imide ([Li][TFSI]) salt (0-1 M). A 1:9 volumetric mixture of [PYR13][TFSI]:[EMIM][DCA] formed an eutectic that exhibited a lower melting point than that of either parent IL. The local structure of Li + in this eutectic is found to be heterogenous and preferentially solvated by [DCA], which is the smaller and more abundant anion. While [TFSI] is able to bridge multiple Li + at high salt concentrations and form both monodentate and bidentate conformations through its oxygen atoms, [DCA] is capable of forming only monodentate coordination with Li + through either of its end nitrogen atoms. The Raman and MD analysis suggest a wide distribution of solvation structures in the form of [Li(TFSI) m (DCA) n ]-(m+n-1) where m = 0-1 and n = 3-4. The computations showed increased ion pair lifetime for Li +-[DCA] and decreased lifetimes for Li +-[TFSI] in the ternary mixture with the increase in [Li][TFSI] concentration. These results show that the solvation and transport properties of charge carriers in ILs can be modified via the presence of multiple ions with varying degree of coordination, which provides an approach to impact the performance in electrochemical processes.
Chemical Engineering Science, 2017
The viscosity and density of the ionic liquid (IL) 1-n-hexyl-3-methylimidazolium bis(trifluoromet... more The viscosity and density of the ionic liquid (IL) 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 6 mim][N T f 2 ]), the molecular solvent tetraethylene glycol dimethyl ether (tetraglyme or G4) and their binary mixtures were measured experimentally as a function of temperature. The same systems were also studied using classical molecular dynamics (MD) simulations. The viscosities of the [C 6 mim][N T f 2 ]/G4 mixtures decrease with increasing G4 concentration, though not as much as an ideal mixing model would predict. Detailed analysis of the MD results reveals that G4 preferentially solvates cations, leading to a reduction in the interaction energy between cations and anions and a subsequent enhancement in anion mobility. A similar effect has been reported when glymes are mixed with salts containing alkali metal cations, with the resulting mixtures being called "solvate" ionic liquids. The simulations predict that the ionic conductivity will be maximized when the G4 mole fraction is around 10-20%. The ability of G4 to effectively solvate the cations stems from localized charges on the oxygens. The simulations predict that solvents having large localized positive charges would preferentially solvate anions, leading to enhanced cation mobility.
The Journal of Physical Chemistry B, 2001
Physical Chemistry Chemical Physics, 2012
The Journal of Chemical Physics, 1999
Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The... more Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The results are contrasted with literature dielectric spectroscopy data, in an attempt to elucidate the observed phenomena from a molecular level perspective. A bead-spring model is used and systems with chain sizes up to N=150 beads at reduced densities 0.5⩽ρ⩽0.8 are studied. The dimensions of the chains follow a universal behavior with ρ/ρ*, where ρ* is the crossover density demarcating the onset of chain overlapping. All the chains are found to follow random-walk behavior. The global motion of the chains is investigated in terms of the dielectric loss E″. As in dielectric spectroscopy experiments, the motion of the chains induces prominent dielectric relaxation at low frequencies. The shape of E″ broadens with increasing density, and a normal-mode analysis indicates that overlapping of the chains with increasing density progressively renders the distribution of relaxation times more hete...
The Journal of Chemical Physics, 1998
Molecular dynamics simulations are employed to study the static and dynamic properties of macromo... more Molecular dynamics simulations are employed to study the static and dynamic properties of macromolecules in dilute and semidilute solutions of a good solvent. The results are compared with dielectric spectroscopy experiments. Crossover concentrations, ρ*, that demarcate the dilute and semidilute regimes are identified. The shift from self-avoiding-walk to random-walk behavior is also studied. An investigation is conducted of the normal-mode dielectric relaxation of type-A polar polymers. In dilute systems, a power law molecular weight dependence of the normal mode relaxation times τ∝N2.2 is observed, in accordance with DeGennes’s scaling analysis for self-avoiding-walk chains. This result does not agree with the experimental dependence of the normal mode relaxation time on the size of the polymers. The differences between the simulations and the experimental dynamic results in the dilute regime can be ascribed to the leading assumption of the model, the neglect of hydrodynamic inter...
![Research paper thumbnail of Erratum: “A method for computing the solubility limit of solids: Application to sodium chloride in water and alcohols” [J. Chem. Phys. 133, 124504 (2010)]](https://attachments.academia-assets.com/119892732/thumbnails/1.jpg)
](The Journal of Chemical Physics, 2012
Journal of Physical Chemistry B, Mar 9, 2001
The importance of pore exit effects on the diffusion of molecules in AlPO 4-5 pores is evaluated ... more The importance of pore exit effects on the diffusion of molecules in AlPO 4-5 pores is evaluated using two molecular modeling techniques. In the first approach, a dual control volume grand canonical molecular dynamics technique is used to obtain molecular fluxes of methane out of the truncated crystal as a function of temperature and sorbate loading. The simulation results indicate the presence of a low-temperature surface barrier for diffusion, which retards the flux of methane relative to its apparent flux in the intracrystalline regions of the material. This pore exit barrier tends to diminish as temperature and loading increase. An explanation based on clustering phenomena is proposed to explain the latter. Next, a simple activated transport model is proposed to predict the relative importance of the surface barrier on the transport of sorbates in AlPO 4-5. The potential of mean force for a single sorbate molecule along the pore axis of a truncated crystal provides the required activation energy barriers for the model. The model correctly predicts the reduction in the importance of exit effects with an increase in the temperature. It is also observed that exit effects become more important as the ratio of the size of the sorbate molecule to the pore size approaches unity. In particular, exit effects are significant in micrometer-thick AlPO 4-5 crystals in the case of large molecules such as SnBr 4 and CCl 4 at room temperature.
The catalytic properties of zeolites, which are primarily determined by the framework topology an... more The catalytic properties of zeolites, which are primarily determined by the framework topology and active centers (e.g. Ti4+, Sn4+ or Al3+), remain challenging to be controlled in zeolite synthesis. Here, we combined first-principal and classical molecular simulations to investigate how Al siting, and OSDA orientation impacts the energy of a zeolite supercell. 36 T-site CHA zeolite with TMAda+ (OSDA) was chosen as the model system. By applying a Boltzmann factor to each Al configuration and as a function of TMAda+ orientation, we came to the conclusion Al pairs prefer to locate in 8-MRs compared to 6-MR, 4-MR and D6R, which is consistent with our previous experimental finding. We also found that the potential energy was governed by the distance between the anionic AlO4 tetrahedra and the cationic quaternary ammonium 1 ar X iv :2 11 0. 12 52 3v 3 [ co nd -m at .m tr lsc i] 3 0 O ct 2 02 1 groups (the Al-N distance), which is the key factor in determining the Al distribution. These re...
The catalytic properties of zeolites, which are primarily determined by the framework topology an... more The catalytic properties of zeolites, which are primarily determined by the framework topology and active centers (e.g. Ti4+, Sn4+ or Al3+), remain challenging to be controlled in zeolite synthesis. Here, we combined first-principal and classical molecular simulations to investigate how Al siting, and OSDA orientation impacts the energy of a zeolite supercell. 36 T-site CHA zeolite with TMAda+ (OSDA) was chosen as the model system. By applying a Boltzmann factor to each Al configuration and as a function of TMAda+ orientation, we came to the conclusion Al pairs prefer to locate in 8-MRs compared to 6-MR, 4-MR and D6R, which is consistent with our previous experimental finding. We also found that the potential energy was governed by the distance between the anionic AlO4 tetrahedra and the cationic quaternary ammonium groups (the Al-N distance), which is the key factor in determining the Al distribution. These results highlight opportunities of using classical molecular simulation com...
The basic science goal in this project identifies structure/affinity relationships for selected r... more The basic science goal in this project identifies structure/affinity relationships for selected radionuclides and existing sorbents. The task will apply this knowledge to the design and synthesis of new sorbents that will exhibit increased cesium, strontium and actinide removal. The target problem focuses on the treatment of
The basic science goal in this project identifies structure/affinity relationships for selected r... more The basic science goal in this project identifies structure/affinity relationships for selected radionuclides and existing sorbents. The task will apply this knowledge to the design and synthesis of new sorbents that will exhibit increased cesium, strontium and actinide removal. The target problem focuses on the treatment of high-level nuclear wastes. The general approach can likewise be applied to non-radioactive separations. The project involves a collaboration among four organizations, with each focused on a different aspect of the problem. This document is the final report on the three years of activities conducted at the University of
The basic science goal in this project identifies structure/affinity relationships for selected r... more The basic science goal in this project identifies structure/affinity relationships for selected radionuclides and existing sorbents. The task will apply this knowledge to the design and synthesis of new sorbents that will exhibit increased cesium, strontium and actinide removal. The target problem focuses on the treatment of high-level nuclear wastes. The general approach can likewise be applied to non-radioactive separations. Research Progress During the third year of the project in addition to continuing our focus along two paths: (1) identifying the structure/affinity relationships for crystalline silicotitanate (CST) and (2) synthesis of new sorbents that include derivatives of CST, a titanosilicate analog of the mineral pharmacosiderite, sodium titanates, we are also conducting (3) insitu crystal growth and ion exchange studies at the X7B beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), Upton, NY, USA for collecting dynamic X-ray powder diffraction data. Based on the results of these studies a pathway for CST crystallization has been proposed. CST serves as the baseline or alternative technology sorbent for the separation of 137 Cs from waste solutions at several of the DOE sites. This material also exhibits a fairly high affinity for strontium, but rather low affinity for plutonium and other actinides. A sodium titanate material, monosodium titanate (MST), serves as the baseline material at the Savannah River Site for strontium and actinide separations from HLW solutions. Recent work demonstrated that pharmacosiderite and sodium nonatitanate (SNT) materials have strontium and actinide removal characteristics similar to that of MST. This year project members at Texas A&M completed two detailed structural characterizations and studied in situ crystallization process of CST. Structural studies include CST and niobium-substituted CST (Nb-CST) sorbents before and after exchange
Journal of Chemical Physics, Jul 16, 2012
Nature Communications, 2022
Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially ... more Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES fo...
The Journal of Physical Chemistry B, 2020
Journal of The Electrochemical Society, 2021
The nitroxide radical redox organic molecule, 2-phenyl-4,4,5,5-tetrame- thylimidazoline-1-oxyl-3-... more The nitroxide radical redox organic molecule, 2-phenyl-4,4,5,5-tetrame- thylimidazoline-1-oxyl-3-oxide (PTIO), was investigated for the first time in a deep eutectic solvent (DES)-like system consisting of a 1:4 molar ratio of choline chloride and ethylene glycol (Ch1EG4) as a redox flow battery electrolyte. PTIO is a single molecule with three oxidation states, and can provide both positive and negative redox couples for a flow battery. A flow battery using the PTIO/Ch1EG4 electrolyte demonstrated nearly 50% round trip efficiency with an approximately 1 V open circuit potential. Inefficiencies were primarily due to membrane resistance which can be significantly lowered with increased temperature. While PTIO appears stable over short periods (hours), the oxidized form is not stable in the DES-like electrolyte over longer times. Molecular modeling was performed to investigate the relative stability of PTIO in DES as compared to the previously studied 4-hydroxy-TEMPO (4HT). It was fou...
The Journal of Chemical Physics, 2020
The low melting point of room temperature ionic liquids is usually explained in terms of the pres... more The low melting point of room temperature ionic liquids is usually explained in terms of the presence of bulky, low-symmetry, and flexible ions, with the first two factors related to the lattice energy while an entropic effect is attributed to the latter. By means of molecular dynamics simulations, the melting points of 1-ethyl-3-methyl-imidazolium hexafluorophosphate and 1-decyl-3-methyl-imidazolium hexafluorophosphate were determined, and the effect of the molecular flexibility over the melting point was explicitly computed by restraining the rotation of dihedral angles in both the solid and the liquid phases. The rotational flexibility over the bond between the ring and the alkyl chain affects the relative ordering of the anions around the cations and results in substantial effects over both the enthalpy and the entropy of melting. For the other dihedral angles of the alkyl group, the contributions are predominantly entropic and an alternating behavior was found. The flexibility ...
The Journal of chemical physics, Jan 28, 2018
The miscibility of ionic liquid (IL) pairs with a common cation (1-ethyl-3-methylimidazolium [CCi... more The miscibility of ionic liquid (IL) pairs with a common cation (1-ethyl-3-methylimidazolium [CCim]) and different anions (bis(trifluoromethylsulfonyl)amide [TFSI], acetate [OAc], and chloride [Cl]) was investigated at a wide range of water concentrations at room temperature. Molecular simulations predicted that the addition of water to the [CCim][TFSI]:[CCim][OAc] and [CCim][TFSI]:[CCim][Cl] mixtures would induce a liquid-liquid phase separation and that water addition to the [CCim][OAc]:[CCim][Cl] mixture would not produce a phase separation. The effect of water on the phase behavior of the IL mixtures was verified experimentally, and the IL and water concentrations were determined in each phase. Of particular importance is the analytical methodology used to determine the species' concentration, where H NMR and a combination of F NMR, Karl Fischer titration, and ion chromatography techniques were applied.
The Journal of Physical Chemistry B, 2018
The structural and dynamical changes in the solvation shell surrounding Li + in a multi-anion env... more The structural and dynamical changes in the solvation shell surrounding Li + in a multi-anion environment are studied by Raman spectroscopy and molecular dynamics (MD) simulations. The ternary electrolyte is composed of a mixture of two ionic liquids (ILs) n-methyl-npropylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([PYR13][TFSI]), 1-ethyl-3methylimidazolium dicyanamide ([EMIM][DCA]) and a lithium bis(trifluoromethylsulfonyl) imide ([Li][TFSI]) salt (0-1 M). A 1:9 volumetric mixture of [PYR13][TFSI]:[EMIM][DCA] formed an eutectic that exhibited a lower melting point than that of either parent IL. The local structure of Li + in this eutectic is found to be heterogenous and preferentially solvated by [DCA], which is the smaller and more abundant anion. While [TFSI] is able to bridge multiple Li + at high salt concentrations and form both monodentate and bidentate conformations through its oxygen atoms, [DCA] is capable of forming only monodentate coordination with Li + through either of its end nitrogen atoms. The Raman and MD analysis suggest a wide distribution of solvation structures in the form of [Li(TFSI) m (DCA) n ]-(m+n-1) where m = 0-1 and n = 3-4. The computations showed increased ion pair lifetime for Li +-[DCA] and decreased lifetimes for Li +-[TFSI] in the ternary mixture with the increase in [Li][TFSI] concentration. These results show that the solvation and transport properties of charge carriers in ILs can be modified via the presence of multiple ions with varying degree of coordination, which provides an approach to impact the performance in electrochemical processes.
Chemical Engineering Science, 2017
The viscosity and density of the ionic liquid (IL) 1-n-hexyl-3-methylimidazolium bis(trifluoromet... more The viscosity and density of the ionic liquid (IL) 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 6 mim][N T f 2 ]), the molecular solvent tetraethylene glycol dimethyl ether (tetraglyme or G4) and their binary mixtures were measured experimentally as a function of temperature. The same systems were also studied using classical molecular dynamics (MD) simulations. The viscosities of the [C 6 mim][N T f 2 ]/G4 mixtures decrease with increasing G4 concentration, though not as much as an ideal mixing model would predict. Detailed analysis of the MD results reveals that G4 preferentially solvates cations, leading to a reduction in the interaction energy between cations and anions and a subsequent enhancement in anion mobility. A similar effect has been reported when glymes are mixed with salts containing alkali metal cations, with the resulting mixtures being called "solvate" ionic liquids. The simulations predict that the ionic conductivity will be maximized when the G4 mole fraction is around 10-20%. The ability of G4 to effectively solvate the cations stems from localized charges on the oxygens. The simulations predict that solvents having large localized positive charges would preferentially solvate anions, leading to enhanced cation mobility.
The Journal of Physical Chemistry B, 2001
Physical Chemistry Chemical Physics, 2012
The Journal of Chemical Physics, 1999
Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The... more Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The results are contrasted with literature dielectric spectroscopy data, in an attempt to elucidate the observed phenomena from a molecular level perspective. A bead-spring model is used and systems with chain sizes up to N=150 beads at reduced densities 0.5⩽ρ⩽0.8 are studied. The dimensions of the chains follow a universal behavior with ρ/ρ*, where ρ* is the crossover density demarcating the onset of chain overlapping. All the chains are found to follow random-walk behavior. The global motion of the chains is investigated in terms of the dielectric loss E″. As in dielectric spectroscopy experiments, the motion of the chains induces prominent dielectric relaxation at low frequencies. The shape of E″ broadens with increasing density, and a normal-mode analysis indicates that overlapping of the chains with increasing density progressively renders the distribution of relaxation times more hete...
The Journal of Chemical Physics, 1998
Molecular dynamics simulations are employed to study the static and dynamic properties of macromo... more Molecular dynamics simulations are employed to study the static and dynamic properties of macromolecules in dilute and semidilute solutions of a good solvent. The results are compared with dielectric spectroscopy experiments. Crossover concentrations, ρ*, that demarcate the dilute and semidilute regimes are identified. The shift from self-avoiding-walk to random-walk behavior is also studied. An investigation is conducted of the normal-mode dielectric relaxation of type-A polar polymers. In dilute systems, a power law molecular weight dependence of the normal mode relaxation times τ∝N2.2 is observed, in accordance with DeGennes’s scaling analysis for self-avoiding-walk chains. This result does not agree with the experimental dependence of the normal mode relaxation time on the size of the polymers. The differences between the simulations and the experimental dynamic results in the dilute regime can be ascribed to the leading assumption of the model, the neglect of hydrodynamic inter...
The Journal of Chemical Physics, 2012
Journal of Physical Chemistry B, Mar 9, 2001
The importance of pore exit effects on the diffusion of molecules in AlPO 4-5 pores is evaluated ... more The importance of pore exit effects on the diffusion of molecules in AlPO 4-5 pores is evaluated using two molecular modeling techniques. In the first approach, a dual control volume grand canonical molecular dynamics technique is used to obtain molecular fluxes of methane out of the truncated crystal as a function of temperature and sorbate loading. The simulation results indicate the presence of a low-temperature surface barrier for diffusion, which retards the flux of methane relative to its apparent flux in the intracrystalline regions of the material. This pore exit barrier tends to diminish as temperature and loading increase. An explanation based on clustering phenomena is proposed to explain the latter. Next, a simple activated transport model is proposed to predict the relative importance of the surface barrier on the transport of sorbates in AlPO 4-5. The potential of mean force for a single sorbate molecule along the pore axis of a truncated crystal provides the required activation energy barriers for the model. The model correctly predicts the reduction in the importance of exit effects with an increase in the temperature. It is also observed that exit effects become more important as the ratio of the size of the sorbate molecule to the pore size approaches unity. In particular, exit effects are significant in micrometer-thick AlPO 4-5 crystals in the case of large molecules such as SnBr 4 and CCl 4 at room temperature.
The catalytic properties of zeolites, which are primarily determined by the framework topology an... more The catalytic properties of zeolites, which are primarily determined by the framework topology and active centers (e.g. Ti4+, Sn4+ or Al3+), remain challenging to be controlled in zeolite synthesis. Here, we combined first-principal and classical molecular simulations to investigate how Al siting, and OSDA orientation impacts the energy of a zeolite supercell. 36 T-site CHA zeolite with TMAda+ (OSDA) was chosen as the model system. By applying a Boltzmann factor to each Al configuration and as a function of TMAda+ orientation, we came to the conclusion Al pairs prefer to locate in 8-MRs compared to 6-MR, 4-MR and D6R, which is consistent with our previous experimental finding. We also found that the potential energy was governed by the distance between the anionic AlO4 tetrahedra and the cationic quaternary ammonium 1 ar X iv :2 11 0. 12 52 3v 3 [ co nd -m at .m tr lsc i] 3 0 O ct 2 02 1 groups (the Al-N distance), which is the key factor in determining the Al distribution. These re...
The catalytic properties of zeolites, which are primarily determined by the framework topology an... more The catalytic properties of zeolites, which are primarily determined by the framework topology and active centers (e.g. Ti4+, Sn4+ or Al3+), remain challenging to be controlled in zeolite synthesis. Here, we combined first-principal and classical molecular simulations to investigate how Al siting, and OSDA orientation impacts the energy of a zeolite supercell. 36 T-site CHA zeolite with TMAda+ (OSDA) was chosen as the model system. By applying a Boltzmann factor to each Al configuration and as a function of TMAda+ orientation, we came to the conclusion Al pairs prefer to locate in 8-MRs compared to 6-MR, 4-MR and D6R, which is consistent with our previous experimental finding. We also found that the potential energy was governed by the distance between the anionic AlO4 tetrahedra and the cationic quaternary ammonium groups (the Al-N distance), which is the key factor in determining the Al distribution. These results highlight opportunities of using classical molecular simulation com...
The basic science goal in this project identifies structure/affinity relationships for selected r... more The basic science goal in this project identifies structure/affinity relationships for selected radionuclides and existing sorbents. The task will apply this knowledge to the design and synthesis of new sorbents that will exhibit increased cesium, strontium and actinide removal. The target problem focuses on the treatment of