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We evaluate the accuracy of local-density approximations (LDAs) using explicit molecular dynamics simulations of binary electrolytes comprised of equisized ions in an implicit solvent. The Bikerman LDA, which considers ions to occupy a lattice, poorly captures excluded volume interactions between primitive model ions. Instead, LDAs based on the Carnahan-Starling (CS) hard-sphere equation of state capture simulated values of ideal and excess chemical potential profiles extremely well, as well as the relationship between surface charge density and electrostatic potential. Excellent agreement between the EDL capacitances predicted by CS-LDAs and computed in molecular simulations is found even in systems where ion correlations drive strong density and free charge oscillations within the EDL, despite the inability of LDAs to capture the oscillations in the detailed EDL profiles.

We probe shock-induced chemistry in two organic liquids by measuring broadband, midinfrared absorption in the 800–1400 cm−1 frequency range. To test this new method and understand the signatures of chemical reactions in time resolved vibrational spectra, we compared liquid benzene shocked to unreactive conditions (shocked to a pressure of 18 GPa for a duration of 300 ps) to nitromethane under reactive conditions (25 GPa). We see clear signatures of shock-induced chemistry that are distinguishable from the pressure- and temperature-induced changes in vibrational mode shapes. While shocked benzene shows primarily a broadening and shifting of the vibrational modes, the nitromethane vibrational modes vanish once the shock wave enters the liquid and simultaneously, a spectrally broad feature appears that we interpret as the infrared spectrum of the complex mixture of product and intermediate species. To further interpret these measurements, we compare them to reactive quantum molecular dynamics simulations, which gives qualitatively consistent results. This work demonstrates a promising method for time resolving shock-induced chemistry, illustrating that chemical reactions produce distinct changes in the vibrational spectra. [ABSTRACT FROM AUTHOR]

The measured spectrum that was accomplished using classical molecular dynamics (MD) simulations and ab initio electronic structure theory for the crystalline solid is analyzed. The generic ESFF force field employed in the MD studies yields less reliable predictions.

Research has been conducted on the localized-to-delocalized mixed-valence in osmium mixed-valence complexes. The use of infrared and near-infrared measurements in different solvents is described.

The time-resolved infrared spectrum of [Ru(phen)3]2+* in the fingerprint region was examined. The spectra included information about both chromophoric and ancillary ligands since the experiment is independent of the requirement of resonance enhancement. Data gathered provided evidence for the localized description of [RuIII(phen)3]3+* on the 100 ns time scale. Results also aided in the interpretation of the time-resolved resonance Raman spectrum of the ligand and gave insight into electronic distribution in the excited state.

Mixed-valance binuclear complexes reveal electronic coupling which can be extensively comprehended by adopting the uncomplicated theoretical outline derived from the Mulliken model for donor-acceptor reactions. Changes which are artificially incorporated yield information about the electrochemical potential shift. Ordinary dimeric structures reveal a resonance delocalization energy which are determined along with the extent of quantum coupling by the data obtained. The wave function coefficient is used to calculate the magnitude of wave function mixing.

Isothermal compression of poly (dimethylsiloxane), 1,4-poly(butadiene), and a model Estane® (in both pure form and a nitroplasticized composition similar to PBX-9501 binder) at pressures up to 100 kbars has been studied using atomistic molecular dynamics (MD) simulations. Comparison of predicted compression, bulk modulus, and Us-up behavior with experimental static and dynamic compression data available in the literature reveals good agreement between experiment and simulation, indicating that MD simulations utilizing simple quantum-chemistry-based potentials can be used to accurately predict the behavior of polymers at relatively high pressure. Despite their very different zero-pressure bulk moduli, the compression, modulus, and Us-up behavior (including low-pressure curvature) for the three polymers could be reasonably described by the Tait equation of state (EOS) utilizing the universal C parameter. The Tait EOS was found to provide an excellent description of simulation PVT data when the C parameter was optimized for each polymer. The Tait EOS parameters, namely, the zero-pressure bulk modulus and the C parameter, were found to correlate well with free volume for these polymers as measured in simulations by a simple probe insertion algorithm. Of the polymers studied, PDMS was found to have the most free volume at low pressure, consistent with its lower ambient pressure bulk modulus and greater increase in modulus with increasing pressure (i.e., crush-up behavior). [ABSTRACT FROM AUTHOR]

Pressure–volume–temperature (PVT) equation-of-state (EOS) information for polymers and polymeric composites is valuable for predicting their response to extreme conditions. An obstacle in determining equations of state for polymeric materials is the lack of a simple, static experimental method for acquiring PVT data for solid networks and liquids at pressures greater than several kilobars. Here, we report a novel approach in determining static EOS for polymers using high-pressure diamond-anvil cells coupled with optical microscopy and image analysis. Results are presented for a cross-linked poly(dimethylsiloxane) polymer, SylgardR 184. Static isothermal results were fitted to empirical and semiempirical equations of state, including the Tait, Birch–Murnaghan, and Vinet forms. Static PV data were also converted to pseudoshock velocity–pseudoparticle velocity (Us–up) for comparison to dynamic Hugoniot data. A linear Rankine–Hugoniot fit Us=sTup+cT gives cT=1.572 km/s and sT=1.703. sT is related to the pressure derivative of the bulk modulus B0′ by sT=(B0′+1)/4 and B0′=5.8. A comparison of the static and shock data is given, along with an estimate of the Grüneisen parameter, and a discussion of the free volume content in the polymer network, and limitations of this novel method. [ABSTRACT FROM AUTHOR]

We use the recently developed reactive force field ReaxFF with molecular dynamics to study thermal induced chemistry in RDX [cyclic-[CH2N(NO2)]3] at various temperatures and densities. We find that the time evolution of the potential energy can be described reasonably well with a single exponential function from which we obtain an overall characteristic time of decomposition that increases with decreasing density and shows an Arrhenius temperature dependence. These characteristic timescales are in reasonable quantitative agreement with experimental measurements in a similar energetic material, HMX [cyclic-[CH2N(NO2)]4]. Our simulations show that the equilibrium population of CO and CO2 (as well as their time evolution) depend strongly of density: at low density almost all carbon atoms form CO molecules; as the density increases larger aggregates of carbon appear leading to a C deficient gas phase and the appearance of CO2 molecules. The equilibrium populations of N2 and H2O are more insensitive with respect to density and form in the early stages of the decomposition process with similar timescales. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ReaxFF reactive force field is expanded to describe carbon-silicon systems, in order to investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives. It is found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH(sub 3) radicals and methane molecules.

We report molecular dynamics simulations designed to investigate the effective size of colloidal particles suspended in a fluid in the vicinity of a rigid wall where all interactions are defined by smooth atomic potential functions. These simulations are used to assess how the behavior of this system at the atomistic length scale compares to continuum mechanics models. In order to determine the effective size of the particles, we calculate the solvent forces on spherical particles of different radii as a function of different positions near and overlapping with the atomistically defined wall and compare them to continuum models. This procedure also then determines the effective position of the wall. Our analysis is based solely on forces that the particles sense, ensuring self-consistency of the method. The simulations were carried out using bothWeeks-Chandler- Andersen andmodified Lennard-Jones (LJ) potentials to identify the different contributions of simple repulsion and van derWaals attractive forces. Upon correction for behavior arising the discreteness of the atomic system, the underlying continuum physics analysis appeared to be correct down to much less than the particle radius. For both particle types, the effective radius was found to be ~ 0.75σ, where σ defines the length scale of the force interaction (the LJ diameter). The effective "hydrodynamic" radii determined by this means are distinct from commonly assumed values of 0.5σ and 1.0σ, but agreewith a value developed from the atomistic analysis of the viscosity of such systems. [ABSTRACT FROM AUTHOR]

Research was conducted to examine vibrational mapping at the mixed-valence, localized-to-delocalized transition. The complexes, cis,cis-((bpy)2(Cl)OsII(BL)OsII(Cl)(bpy)2)2+ or pyrazine were prepared and charaterized previously. Results provide clear evidence that the v(bpy) modes experience and averaged electronic environment and that OsIIOsIII electron transfer is rapid on the vibrational time scale.

Self-consistent field theory is used to study the self-assembly of a triblock copolymer melt. Two different external factors (temperature and solvent) are shown to affect the self-assembly. Either one or two-step self-assembly can be found as a function of temperature in the case of a neat triblock melt, or as a function of increasing solvent content (for non-selective solvents) in the case of a triblock-solvent mixture. For selective solvents, it is shown that increasing the solvent content leads to more complicated self-assembly mechanisms, including a reversed transition where order is found to increase instead of decreasing as expected, and re-entrant behavior where order is found to increase at first, and then decrease to a previous state of disorder. [ABSTRACT FROM AUTHOR]