PubMed Health⌕ Search

Biomedical subjects

Grant D Smith

Publications and source records attributed to Grant D Smith.

At least 19 recordsLinked to original sources
PubMed2006

Molecular dynamics simulations of lithium alkyl carbonates.

A quantum chemistry-based many-body polarizable force field has been developed for two model solid-electrolyte interphase (SEI) components: dilithium ethylene dicarbonate (Li(2)EDC) and lithium methyl carbonate (LiMC). Molecular dynamics (MD) simulations of amorphous Li(2)EDC and LiMC were performed at temperatures from 393 to 600 K. Simulations reveal that Li(+) is coordinated by approximately 4.6 oxygen atoms from -COO(-) groups coming from different alkyl carbonate molecules. Charge transport in Li(2)EDC was found to be almost entirely due to Li(+). The temperature dependence of the ionic conductivity of the SEI model compounds Li(2)EDC and LiMC was found to be significantly stronger than that of liquid electrolytes (e.g., ethylene carbonate + LiTFSI), yielding extrapolated conductivities of the Li(2)EDC on order of 10(-10) S/cm at -30 degrees C.

Journal Article↗
PubMed2006

Li+ cation environment, transport, and mechanical properties of the LiTFSI doped N-methyl-N-alkylpyrrolidinium+TFSI- ionic liquids.

Molecular dynamics (MD) simulations have been performed on N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (mppy(+)TFSI(-)) and N,N-dimethyl- pyrrolidinium bis(trifluoromethanesulfonyl)imide (mmpy(+)TFSI(+)) ionic liquids (ILs) doped with 0.25 mol fraction LiTFSI salt at 303-500 K. The liquid density, ion self-diffusion coefficients, and conductivity predicted by MD simulations were found to be in good agreement with experimental data, where available. MD simulations reveal that the Li(+) environment is similar in mppy(+)TFSI(-) and mmpy(+)TFSI(+) ILs doped with LiTFSI. The Li(+) cations were found to be coordinated on average by slightly less than four oxygen atoms with each oxygen atom being contributed by a different TFSI(-) anion. Significant lithium aggregation by sharing up to three TFSI(-) anions bridging two lithiums was observed, particularly at lower temperatures where the lithium aggregates were found to be stable for tens of nanoseconds. Polarization of TFSI(-) anions is largely responsible for the formation of such lithium aggregates. Li(+) transport was found to occur primarily by exchange of TFSI(-) anions in the first coordination shell with a smaller (approximately 30%) contribution also due to Li(+) cations diffusing together with their first coordination shell. In both ILs, ion self-diffusion coefficients followed the order Li(+) < TFSI(-) < mmpy(+) or mppy(+) with all ion diffusion in mmpy(+)TFSI(-) being systematically slower than that in mppy(+)TFSI(-). Conductivity due to the Li(+) cation in LiTFSI doped mppy(+)TFSI(-) IL was found to be greater than that for a model poly(ethylene oxide)(PEO)/LiTFSI polymer electrolyte but significantly lower than that for an ethylene carbonate/LiTFSI liquid electrolyte. Finally, the time-dependent shear modulus for the LiTFSI doped ILs was found to be similar to that for a model poly(ethylene oxide)(PEO)/LiTFSI polymer electrolyte on the subnanosecond time scale.

Journal Article↗
PubMed2006

Molecular dynamics simulation study of the structure of poly(ethylene oxide) brushes on nonpolar surfaces in aqueous solution.

The structure of poly(ethylene oxide) (PEO, M(w) = 526) brushes of various grafting density (sigma) on nonpolar graphite and hydrophobic (oily) surfaces in aqueous solution has been studied using atomistic molecular dynamics simulations. Additionally, the influence of PEO-surface interactions on the brush structure was investigated by systematically reducing the strength of the (dispersion) attraction between PEO and the surfaces. PEO chains were found to adsorb strongly to the graphite surface due primarily to the relative strength of dispersion interactions between PEO and the atomically dense graphite compared to those between water and graphite. For the oily surface, PEO-surface and water-surface dispersion interactions are much weaker, greatly reducing the energetic driving force for PEO adsorption. This reduction is mediated to some extent by a hydrophobic driving force for PEO adsorption on the oily surface. Reduction in the strength of PEO-surface attraction results in reduced adsorption of PEO for both surfaces, with the effect being much greater for the graphite surface where the strong PEO-surface dispersion interactions dominate. At high grafting density (sigma approximately 1/R(g)(2)), the PEO density profiles exhibited classical brush behavior and were largely independent of the strength of the PEO-surface interaction. With decreasing grafting density (sigma < 1/R(g)(2)), coverage of the surface by PEO requires an increasingly large fraction of PEO segments resulting in a strong dependence of the PEO density profile on the nature of the PEO-surface interaction.

Journal Article↗
PubMed2006

Structure and dynamics of N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid from molecular dynamics simulations.

Molecular dynamics (MD) simulations were performed on N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (mppy+TFSI-) from 303 to 393 K to improve understanding of the structure and ion transport of this ionic liquid. The density, ion self-diffusion coefficients, conductivity, and viscosity of mppy+TFSI- predicted from MD simulations are in good agreement with experimental measurements. The time-dependent shear modulus of the ionic liquids was calculated and compared with that for nonionic liquids. On average each mppy+ cation was found to be coordinated by four TFSI- anions. The angular distributions of N(TFSI-)-N(mppy+)-N(TFSI-) and N(mppy+)-N(TFSI-)-N(mppy+) exhibit a maximum at 80-90 degrees and a second maximum at 180 degrees . Correlation of ion motion was found to lower ionic conductivity by approximately one-third from the expected value based upon ion self-diffusion coefficients. Rotational motion of the cation and anion are anisotropic with the degree of anisotropy increasing with decreasing temperature. Electrostatic interactions are responsible for slowing down the dynamics of the ionic liquid by more than an order of magnitude and a dramatic decrease of the time-dependent shear modulus.

Journal Article↗
PubMed2006

Water-induced interactions between carbon nanoparticles.

Molecular dynamics simulations were carried out in order to study the hydration of C60 fullerenes, carbon nanotubes, and graphene sheets in aqueous solution and the nature of water-induced interactions between these carbon nanoparticles. The hydration of these nonpolar carbon nanoparticles does not exhibit classical hydrophobic character due to the high density of surface atoms (carbon) resulting in strong water-surface dispersion interactions. Water was found to wet the nanoparticle surfaces independent of nanoparticle surface curvature, with the decrease in the extent of water-water hydrogen bonding with decreasing surface curvature being offset by stronger water-surface interactions. While all carbon nanoparticles investigated are anticipated to aggregate in water due to strong direct nanoparticle-nanoparticle interactions, the water-induced interactions between nanoparticles were found to be repulsive and, in contrast to the wetting behavior, were observed to exhibit strong dependence on surface curvature. The strength of the water-induced interaction between carbon nanoparticles was found to correlate well with the number of hydration water molecules displaced upon particle aggregation, which, relative to the amount of direct nanoparticle-nanoparticle contact engendered upon aggregation, decreases with decreasing surface curvature.

Carbon↗
PubMed2006

Molecular dynamics simulations of nanoparticles in dense isotropic nematogens: the role of matrix-induced long-range repulsive interactions.

We have carried out molecular dynamics simulation studies of binary mixtures of spherical nanoparticles (NPs) in a matrix of dense isotropic rod-shaped nematogens, with the size of the nematogen length being similar to that of the NP diameter. NPs at even low concentrations were found to shift the isotropic-nematic (I-N) transition significantly to higher pressure at a given temperature, indicative of long-range perturbation of the nematogenic matrix by the NPs. The NPs were found to be dispersed in the dense isotropic nematogenic matrix over a wide range of NP concentrations due to long-range (compared with the molecular size of the nematogens) repulsion caused by NP-induced local order fluctuations and reduced local orientational correlation in the isotropic nematogenic matrix, in contrast to the phase separation predicted and observed in other studies where the particles were much larger or smaller than the nematogens. Furthermore, since the repulsion observed in the NP-nematogen mixtures is only microscopically long range (on the order of about ten molecular lengths of the nematogens), globally ordered clustering observed in mixtures of colloidal particles in nematic matrices resulting from macroscopically long-range interaction is not observed in our simulations.

Journal Article↗
PubMed2006

Molecular dynamics simulations studies of nanoparticles in an isotropic liquid crystal matrix: Single particle behavior and pairwise interactions.

We report results of molecular dynamics simulation studies of the behavior of spherical nanoparticles (NPs) in a dense isotropic nematogen matrix comprised of soft spherocylinders (SSCs). The SSCs exhibit a tendency for frustrated planar anchoring at the NP surface that results in a long-range (compared to the size of the NPs and SSCs) reduction in local orientational ordering and increased fluctuations in local orientational ordering compared to the pure isotropic phase of the SSCs. The potential of mean force between two nanoparticles exhibits a novel long-range repulsive tail separated from short-range molecular packing peaks by a shallow local minimum in free energy. The long-range repulsion is caused by NP-induced ordering fluctuations while the shallow minimum results from increased local ordering within the confinement region in between two NPs. The influence of the NPs on local orientational order in the nematogen matrix and the nematogen-induced interaction between NPs are found to depend strongly on the size of the NPs.

Journal Article↗
PubMed2006

Development of many-body polarizable force fields for Li-battery components: 1. Ether, alkane, and carbonate-based solvents.

Classical many-body polarizable force fields were developed for n-alkanes, perflouroalkanes, polyethers, ketones, and linear and cyclic carbonates on the basis of quantum chemistry dimer energies of model compounds and empirical thermodynamic liquid-state properties. The dependence of the electron correlation contribution to the dimer binding energy on basis-set size and level of theory was investigated as a function of molecular separation for a number of alkane, ether, and ketone dimers. Molecular dynamics (MD) simulations of the force fields accurately predicted structural, dynamic, and transport properties of liquids and unentangled polymer melts. On average, gas-phase dimer binding energies predicted with the force field were between those from MP2/aug-cc-pvDz and MP2/aug-cc-pvTz quantum chemistry calculations.

Alkanes↗
PubMed2006

Development of many-body polarizable force fields for Li-battery applications: 2. LiTFSI-doped Oligoether, polyether, and carbonate-based electrolytes.

A quantum chemistry study of Li(+) interactions with ethers, carbonates, alkanes, and a trifluoromethanesulfonylimide anion (TFSI(-)) was performed at the MP2, B3LYP, and HF levels using the aug-cc-pvDz basis set for solvents and TFSI(-) anion, and [8s4p3d/5s3p2d]-type basis set for Li. A classical many-polarizable force field was developed for the LiTFSI salt interacting with ethylene carbonate (EC), gamma-butyrolactone (GBL), dimethyl carbonate (DMC), acetone, oligoethers, n-alkanes, and perfluoroalkanes. Molecular dynamics (MD) simulations were performed for EC/LiTFSI, PC/LiTFSI, GBL/LiTFSI, DMC/LiTFSI, 1,2-dimethoxyethane/LiTFSI, pentaglyme/LiTFSI, and poly(ethylene oxide) (MW = 2380)/LiTFSI electrolytes at temperatures from 298 to 423 K and salt concentrations from 0.3 to 5 M. The ion and solvent self-diffusion coefficients, electrolyte conductivity, electrolyte density, LiTFSI apparent molar volumes, and structure of the Li(+) cation environment predicted by MD simulations were found in good agreement with experimental data.

Journal Article↗
PubMed2006

A novel mesophase formed by top-shaped molecules in the bulk and unsupported thin films: a molecular dynamics study.

We have used molecular dynamics simulations to investigate the ordering of top-shaped molecules in bulk phases and in unsupported thin films. Each rigid anisotropic molecule was composed of 11 Lennard-Jones interaction centers (beads). In an attempt to enhance the nematic stability in preference to smectic, the three central beads were assigned a larger Lennard-Jones diameter than the tail beads, giving the molecule a shape resembling a top. The molecular model was found to exhibit an unusual bulk mesophase with long-range orientational order and with molecular center-of-mass positions arranged in parallel interdigitated layers, with layer spacing smaller than half the length of the long axis of a molecule. However, despite the toplike molecular shape, no nematic phase was observed in the pressure range studied. Unsupported films of the isotropic liquid were cooled in order to locate a triple point between the novel mesophase, vapor, and isotropic liquid. At temperatures slightly above the triple point, enhanced surface ordering of molecules was found to occur in the unsupported film. At temperatures slightly below the triple point, the preferred molecular alignment in the unsupported film was parallel to the interface, in violation of arguments that have been proposed based on the relative enthalpies of various cleavage planes for close-packed structures.

Journal Article↗
PubMed2006

LiTFSI structure and transport in ethylene carbonate from molecular dynamics simulations.

Molecular dynamics (MD) simulations using a many-body polarizable force field were performed on ethylene carbonate (EC) doped with lithium bistrifluoromethanesulfonamide (LiTFSI) salt as a function of temperature and salt concentration. At 313 K Li+ was coordinated by 2.7-3.2 EC carbonyl oxygen atoms and 0.67-1.05 TFSI- oxygen atoms at EC:Li = 10 and 20 salt concentrations. In completely dissociated electrolytes, however, Li+ was solvated by approximately 3.8 carbonyl oxygen atoms from EC on average. The probability of ions to participate ion aggregates decreased exponentially with an increase in the size of the aggregate. Ion and solvent self-diffusion coefficients and conductivity predicted by MD simulations were in good agreement with experiments. Approximately half of the charge was transported by charged ion aggregates with the other half carried by free (uncomplexed by counterion) ions. Investigation of the Li+ transport mechanism revealed that contribution from the Li+ diffusion together with its coordination shell to the total Li+ transport is similar to the contribution arising from Li+ exchanging solvent molecules in its first coordination shell with solvents from the outer shells.

Journal Article↗
PubMed2006

Monte Carlo simulation study of the kinetics of brush formation by irreversible adsorption of telechelic polymers onto a solid substrate.

The irreversible adsorption of telechelic polymer chains from solution and melts onto solid substrates has been studied using the bond fluctuation Monte Carlo model. Complex brush formation kinetics dominated by diffusion of chains to the substrate at short times (diffusion-limited regime or DLR) and by penetration of chains through the maturing brush at longer times (penetration-limited regime or PLR) were observed. During the entire adsorption process, the rate of chain adsorption decreases monotonically with time. In the DLR, characterized by a maximum in the concentration of singly bound chains and a rapidly increasing fraction of doubly bound chains (loops), this decrease is due primarily to the depletion of free chains near the substrate and the formation of concentration gradients of free (nonadsorbed) chains in the bulk solution. The DLR and PLR are separated by an intermediate regime during which the brush becomes dominated by doubly bound chains and both penetration of the maturing brush and diffusion of chains to the brush surface play a role in determining the kinetics of brush growth. The PLR is characterized by steep gradients of free chains within the growing brush and the disappearance of concentration gradients for free chains in the bulk solution. In the PLR, the concentration of singly bound chains is low and decreases slowly while surface coverage and the fraction of doubly bound chains increase slowly. The rates of adsorption of new chains and the formation of loops in the PLR slow dramatically with increasing surface coverage and increasing chain length and less dramatically with decreasing bulk concentration.

Journal Article↗
PubMed2006

Thermal relaxation mechanism and role of chemical functionalization in fullerene solutions.

Using molecular-dynamics simulations we investigate thermal relaxation of C60 and C84 molecules suspended in octane liquid. Pristine fullerenes exhibit relatively slow relaxation due to weak thermal coupling with the liquid. A comparison of the interfacial transport characteristics obtained from relaxation simulations with those obtained from equilibrium simulations and fluctuation-dissipation theorem analysis demonstrates that the relaxation process involves two main steps: (i) energy flow from high- to low-frequency modes within the fullerene, and (ii) energy flow from low-frequency fullerene modes to the liquid. Functionalization of fullerenes with alkene chains leads to significant reduction of the thermal relaxation time. The relaxation time of functionalized fullerenes becomes independent from the functionalizing chain length beyond approximately 10 carbon segments; this can be understood in terms of thermal conductivity along the chain and heat transfer between the chain and the solvent.

Journal Article↗
PubMed2005

A molecular-dynamics simulation study of solvent-induced repulsion between C60 fullerenes in water.

Molecular-dynamics simulations of a single C(60) fullerene and pairs of C(60) fullerenes in aqueous solution have been performed for the purpose of obtaining improved understanding of the nature of solvent-induced interactions between C(60) fullerenes in water. Our simulations reveal repulsive solvent-induced interactions between two C(60) fullerenes in aqueous solution in contrast to the associative effects observed for conventional nonpolar solutes. A decomposition of the solvent-induced potential of mean force between fullerenes into entropy and energy (enthalpy) contributions reveals that the water-induced repulsion between fullerenes is energetic in origin, contrasting strongly to entropy-driven association observed for conventional nonpolar solutes. The dominance of energy in the solvent-induced interactions between C(60) fullerenes arises primarily from the high atomic density of the C(60) molecule, resulting in strong C(60)-water van der Waals attraction that is reduced upon association of the fullerenes. The water-induced repulsion is found to decrease with increasing temperature due largely to an increasing contribution from a relatively weak entropy-driven association.

Journal Article↗
PubMed2005

Equilibrium sampling of self-associating polymer solutions: a parallel selective tempering approach.

We present a novel simulation algorithm based on tempering a fraction of relaxation-limiting interactions to accelerate the process of obtaining uncorrelated equilibrium configurations of self-associating polymer solutions. This approach consists of tempering (turning off) the attractive interactions for a fraction of self-associating groups determined by a biasing field h. A number of independent configurations (replicas) with overlapping Hamiltonian distributions in the expanded (NVTh) ensemble with constant NVT but different biasing fields, forming a chain of Hamiltonians, were simulated in parallel with occasional attempts to exchange the replicas associated with adjacent fields. Each field had an associated distribution of tempered interactions, average fraction of tempered interactions, and structural decorrelation time. Tempering parameters (number of replicas, fields, and exchange frequencies) were chosen to obtain the highest efficiency in sampling equilibrium configurations of a self-association polymer solution based on short serial simulation runs and a statistical model. Depending on the strength of the relaxation-limiting interactions, system size, and thermodynamic conditions, the algorithm can be orders of magnitude more efficient than conventional canonical simulation and is superior to conventional temperature parallel tempering.

Journal Article↗
PubMed2005

Molecular dynamics simulation study of the role of evenly spaced poly(ethylene oxide) tethers on the aggregation of C60 fullerenes in water.

The aggregation behavior of C60 fullerenes and C60 fullerenes with six symmetrically tethered poly(ethylene oxide) oligomers [(PEO)-6-C60] in aqueous solutions has been studied using implicit solvent molecular dynamics simulations. Our simulations reveal that while the attraction between two (PEO)-6-C60 fullerenes in aqueous solution is stronger and longer range than that between two bare C60 fullerenes, the (PEO)-6-C60 fullerenes do not phase-separate in water but rather aggregate in chain-like clusters at concentrations where unmodified fullerenes completely phase-separate.

Letter↗
PubMed2005

Molecular dynamics simulation study of the alpha- and beta-relaxation processes in a realistic model polymer.

Molecular dynamics simulations of a melt of freely rotating chains of 1,4-polybutadiene (FRC-PBD) have been performed over a wide range of temperature. Removal of the dihedral barriers in FRC-PBD allows for complete resolution of the Johari-Goldstein beta process from the primary alpha process in the simulation time window. We find that relaxation in the beta regime occurs as the result of large-angle excursions of all backbone dihedrals. While during the beta process each dihedral visits all available states, the complete relaxation of torsional autocorrelation function (alpha process) occurs only when the polymer matrix shows significant motion and each dihedral populates all states with equilibrium probability.

Journal Article↗