Simulating vapor-liquid nucleation of water: A combined histogram-reweighting and aggregation-volume-bias Monte Carlo investigation for fixed-charge and polarizable models

The method of histogram-reweighting was integrated with a recently developed approach using aggregation-volume-bias Monte Carlo and self-adaptive umbrella sampling to develop the AVUS-HR algorithm that allows for exceedingly efficient calculations of nucleation properties over a wide range of thermodynamic conditions. Simulations were carried out for water using both fixed-charge and polarizable force fields belonging to the TIP4P family (namely, TIP4P, TIP4P-FQ, TIP4P-pol2, and TIP4P-pol3) to investigate the nucleation of water over a temperature range from 200 to 300 K and the concentration of water clusters in the atmosphere at elevations up to 15 km. It was found that the nucleation free energy barriers and atmospheric concentrations are extremely sensitive to the force field, albeit all of the models investigated in this study support the following general conclusions: (i) ice nucleation is not present under the thermodynamic conditions and cluster-size range investigated here, i.e., the critical nuclei possess liquidlike structures, and (ii) the atmospheric concentrations of water clusters under homogeneous conditions are very low with the mole fraction of hexamers being about 10(-10), a number probably too low to influence the solar radiation balance. Compared to the experimental data, the TIP4P-pol3 model yields the most accurate nucleation results, consistent with its excellent performance for the second virial coefficient and the minimum cluster energies.     

Partial molar volume and solvation structure of naphthalene in supercritical carbon dioxide: a Monte Carlo simulation study

Monte Carlo simulations were used to investigate the solvation of naphthalene in supercritical carbon dioxide at a temperature of 308.38 K just above the solvent's critical temperature and at pressures of 74.6, 79.7, 87.8, and 310.2 bar covering a range from just below to far above the solvent's critical pressure and at a slightly elevated temperature of 318.15 K and a pressure of 93.0 bar. The Monte Carlo simulations were carried out in the isobaric-isothermal ensemble and employed the transferable potentials for phase equilibria (TraPPE) force field. Systems containing 2000 carbon dioxide molecules and from 0 to 4 solute molecules were used for all five state points, and additional simulations with 16 000 solvent molecules were carried out at the lower temperature and p = 79.7 bar. In agreement with experiment, the simulations yield large, negative partial molar volumes of naphthalene near the critical pressure at 79.7 bar, with values of -4340 +/- 750 and -3400 +/- 620 cm(3) mol(-1) for the 2000 and 16 000 molecule systems, respectively. Structural analysis through radial distribution functions and the corresponding number integrals yields good agreement with neutron diffraction data and shows evidence for a long-range density enhancement around solutes but lacking any specific solute-solvent clustering. Solvatochromic shifts estimated from the local solvent structure correlate well with the experimental data over the entire pressure range.     

Free energies and phase equilibria of chain molecules

In this Article, a review is presented of recent developments in Monte Carlo simulations of chain molecules. The Rosenbluth chain insertion technique is used to calculate the free energy of the chain molecules. Furthermore, this insertion method is used to generate biased Monte Carlo moves. It is shown that this bias can be removed by adjusting the acceptance rules such that configurations are generated with their correct Boltzmann weight. This configurational-bias Monte Carlo method can be combined with the Gibbs-ensemble technique which results in an efficient method to simulate phase equilibria of chain molecules.     

Transferable potentials for phase equilibria. 8. United-atom description for thiols, sulfides, disulfides, and thiophene

An extension of the transferable potentials for phase equilibria united-atom (TraPPE-UA) force field to thiol, sulfide, and disulfide functionalities and thiophene is presented. In the TraPPE-UA force field, nonbonded interactions are governed by a Lennard-Jones plus fixed point charge functional form. Partial charges are determined through a CHELPG analysis of electrostatic potential energy surfaces derived from ab initio calculations at the HF/6-31g+(d,p) level. The Lennard-Jones well depth and size parameters for four new interaction sites, S (thiols), S(sulfides), S(disulfides), and S(thiophene), were determined by fitting simulation data to pure-component vapor-equilibrium data for methanethiol, dimethyl sulfide, dimethyl disulfide, and thiophene, respectively. Configurational-bias Monte Carlo simulations in the grand canonical ensemble combined with histogram-reweighting methods were used to calculate the vapor-liquid coexistence curves for methanethiol, ethanethiol, 2-methyl-1-propanethiol, 2-methyl-2-propanethiol, 2-butanethiol, pentanethiol, octanethiol, dimethyl sulfide, diethyl sulfide, ethylmethyl sulfide, dimethyl disulfide, diethyl disulfide, and thiophene. Excellent agreement with experiment is achieved, with unsigned errors of less than 1% for saturated liquid densities and less than 3% for critical temperatures. The normal boiling points were predicted to within 1% of experiment in most cases, although for certain molecules (pentanethiol) deviations as large as 5% were found. Additional calculations were performed to determine the pressure-composition behavior of ethanethiol+n-butane at 373.15 K and the temperature-composition behavior of 1-propanethiol+n-hexane at 1.01 bar. In each case, a good reproduction of experimental vapor-liquid equilibrium separation factors is achieved; both of the coexistence curves are somewhat shifted because of overprediction of the pure-component vapor pressures.     

Transferable potentials for phase equilibria. 7. Primary, secondary, and tertiary amines, nitroalkanes and nitrobenzene, nitriles, amides, pyridine, and pyrimidine

The transferable potentials for phase equilibria (TraPPE) force fields are extended to amine, nitro, nitrile, and amide functionalities and to pyridine and pyrimidine. In many cases, the same parameters for a functional group are used for both united-atom and explicit-hydrogen representations of alkyl tails. Following the TraPPE philosophy, the nonbonded interaction parameters were fitted to the vapor-liquid coexistence curves for selected one-component systems. Coupled-decoupled configurational-bias Monte Carlo simulations in the Gibbs ensemble were applied to neat (methyl-, dimethyl-, trimethyl-, ethyl-, diethyl-, or triethyl-)amine, nitromethane, nitroethane, nitrobenzene, acetonitrile, propionitrile, acetamide, propanamide, butanamide, pyridine, and pyrimidine. Excellent agreement with experimental results was found, with the mean unsigned errors being less than 1% for both the critical temperature and the normal boiling temperature. Similarly, the liquid densities at low reduced temperatures are reproduced to within 1%, and the deviation for the critical densities is about 4%. Additional simulations were performed for the binary mixtures of methylamine + n-hexane, diethyl ether + acetonitrile, 1-propanol + acetonitrile, and nitroethane + ethanol. With the exception of the methylamine/n-hexane mixture for which the separation factor is substantially overestimated, agreement with experiment for the other three mixtures is very satisfactory.     

Correlative imaging for polymer science

The characterization of polymeric materials is key towards the understanding of structure–activity relations and therefore for the rational design of novel and improved materials for a myriad of applications. Many microscopy techniques are currently used, with electron microscopy, fluorescence microscopy, and atomic force microscopy being the most relevant. In this perspective paper, we discuss the use of correlative imaging, that is, the combination of multiple imaging methodologies on the same sample, in the field of polymeric materials. This innovative approach is emerging as a powerful tool to unveil the structure and functional properties of biological and synthetic structures. Here we discuss the possibilities of correlative imaging and highlight their potential to answer open questions in polymer science.     

Hollow and Yolk‐Shell Iron Oxide Nanostructures on Few‐Layer Graphene in Li‐Ion Batteries

We report a simple and template‐free strategy for the synthesis of hollow and yolk‐shell iron oxide (FeO_x) nanostructures sandwiched between few‐layer graphene (FLG) sheets. The morphology and microstructure of this material are characterized in detail by X‐ray diffraction, X‐ray absorption near‐edge structure, X‐ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Its properties are evaluated as negative electrode material for Li‐ion batteries and compared with those of solid FeO_x/FLG and two commercial iron oxides. In all cases, the content of carbon in the electrode has a great influence on the performance. The use of pristine FLG improves the capacity retention and further enhancement is achieved with the hollow structure. For a low carbon loading of 18 wt. %, the presence of metallic iron in the hollow and yolk‐shell FeO_x/FLG composite significantly enhances the capacity retention, albeit with a relatively lower initial reversible capacity, retaining above 97 % after 120 cycles at 1000 mA g^?1 in the voltage range of 0.1–3.0 V.     

Importance of the number of acid molecules and the strength of the base for double-ion formation in (H2SO4)m x base x (H2O)6 clusters

Sulfuric acid and water clusters are important for new particle formation in the atmosphere. Recent experimental studies demonstrate that critical clusters in diverse atmospheric environments contain two acid molecules and may also include additional N-containing molecules (i.e., a base). We use first-principles molecular dynamics simulations to show that the presence of two sulfuric acid molecules in (H2SO4)m x base x (H2O)6 clusters is always sufficient to form a double ion, whereas a single acid molecule, even in the presence of a base, is not.     

Excited state hydrogen bond dynamics: coumarin 102 in acetonitrile-water binary mixtures

The time dependent change in the intermolecular response of solvent molecules following photoexcitation of Coumarin 102 (C102) has been measured in acetonitrile-water binary mixtures. Experiments were performed on mixtures of composition x(CH3CN) = 0.25, 0.50, 0.75, and 1.00. At low water concentrations (x(H2O) < or = 0.25) the solvent response is consistent with previous measurements probing dipolar solvation. With increasing water concentration (x(H2O) > or = 0.50) an additional response is found subsequent to dipolar solvation, exhibited as a rapid gain in the solvent's polarizability on a approximately 250 fs time scale. Monte Carlo simulations of the C102:binary mixture system were performed to quantify the number of hydrogen-bonding interactions between C102 and water. These simulations indicate that the probability of the C102 solute being hydrogen bound with two water molecules, both as donors at the carbonyl site, increases in a correlated fashion with the amplitude of the additional response in the measurements. We conclude that excitation of C102 simultaneously weakens and strengthens hydrogen bonding in complexes with two inequivalently bound waters.