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.

Recent work has focused attention on possible shifts in the bond angle distribution of CO(2) as a consequence of intermolecular interactions in the supercritical phase. To investigate the temperature and phase dependence of the intramolecular structure of CO(2), we performed Feynman path integral Monte Carlo calculations based on a spectroscopically derived analytical potential, first principles molecular dynamics simulations using Kohn-Sham density functional theory, and Monte Carlo simulations employing empirical interaction potentials. On the basis of various distributions used to characterize the intramolecular structure, we conclude that the aggregation state has a negligible influence on the intramolecular structure, in particular we find that in the classical limit the distributions are remarkably similar for the ideal gas, supercritical, and solid phases when considered at the same temperature. In contrast, an increase in the temperature from 325 to 673 K or inclusion of nuclear quantum effects leads to a significant broadening of the distributions. With respect to the first C-O bond vector, the second bond vector most prefers a collinear arrangement. However, due to the Jacobian factor the maximum in the bond angle distribution at 325 K is shifted to an angle of about 175.7 degrees in the classical limit or to 173.0 degrees if nuclear quantum effects are included. Nevertheless, an analysis of the temperature dependence of the constant-volume heat capacity demonstrates that carbon dioxide should be viewed as a linear molecule.

Gibbs ensemble Monte Carlo simulations were used to investigate the effect of pressure and of entrainers on the solubility of low-volatility species in CO 2. Two entrainers were examined, n-octane and methanol, as well as two solutes, hexamethylbenzene and benzoic acid. For the three pressures studied (12, 20, and 28 MPa), the simulations demonstrate that the increase in the solubility with increasing pressure is mostly due to an increase in the solute's chemical potential (as expressed by the Poynting correction) and not due to an increase in the solvent strength of supercritical CO 2. The presence of an entrainer enhances solubility, particularly when the solute and entrainer can form hydrogen bonds. The solubility of benzoic acid is enhanced by an order of magnitude upon addition of methanol entrainer, whereas the enhancements are less than 2 for the other systems.

This advanced undergraduate laboratory experiment involves the synthesis and characterization of a metal−organic framework with microporous channels that are held intact via hydrogen bonding of the coordinated water molecules. The hydrothermal synthesis of Co 3 (BTC) 2 •12H 2 O (BTC = 1,3,5-benzene tricarboxylic acid) is carried out using one of four different reactors (stainless steel, microwave, polypropylene bottle, or sealed glass ampules) and the products are evaluated and compared using thermogravimetric analysis, powder X-ray diffraction, and IR spectroscopy. Powder X-ray diffraction is also used to monitor the changes in structure of the framework during the partial or complete removal of the associated water molecules, as well as after reabsorption of water.

Monte Carlo simulations were used to investigate the phase behavior of hydrated liquid silica as a function of temperature and overall water mole fraction, x w. Simulations using the Feuston-Garofalini potential were performed in the isobaric-isothermal ensemble at p = 1 GPa for 15 temperatures (2000 < or = T < or = 9000 K) and 25 compositions (0.0 < or = x w < or = 0.4). The unusual volume minimum exhibited by tetrahedrally coordinated liquid silica is found to persist up to x w approximately 0.267, although the temperature of the volume minimum decreases with increasing water content. Structural properties of the pure and hydrated systems are compared and the addition of water to liquid silica disrupts the silica network more dramatically than temperature alone. The simulations yield very low concentrations of molecular water, e.g. only about 1.2% of the oxygen atoms are bound to exactly two hydrogen atoms at x w = 0.4 and T = 3000 K.

The structural properties of 10 nm thick lubricant films consisting of binary and ternary n-alkane mixtures (8 ≤ n ≤ 12) adsorbed on a structureless metal substrate were studied for several temperatures and compositions using Monte Carlo simulations. Configurational-bias Monte Carlo identity switch moves are essential to sample the spatial distribution in these mixtures. Longer alkanes are found to preferentially adsorb onto the substrate while shorter alkanes are enriched at the liquid-vapor interface. This preferential adsorption is evident even when the two chains differ by only one methylene unit and the longer chain is the minor component. Enhanced composition heterogeneity and orientational ordering and fewer gauche defects are characteristic features of the first layer near the substrate.

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The effect of pressure on structure and water speciation in hydrated liquid silica is examined over a range of temperatures and compositions. The Feuston-Garofalini (FG) potential is used in isobaric-isothermal Monte Carlo simulations carried out at four pressures (0.25, 1.0, 2.5, and 10 GPa) for seven temperatures (2000 < or = T < or= 9000 K) and five compositions (0.0 < or = x_w < or = 0.4). The FG potential yields a stable melt phase for p > or = 1.0 GPa and/or x_w < or = 0.1 for all temperatures. The volume minimum seen in previous simulations of pure and hydrated liquid silica using the FG potential persists up to 2.5 GPa but is no longer evident at 10 GPa. This is correlated with gradual structural changes of the liquid up to 2.5 GPa and with more significant changes at 10 GPa. Even at high overall concentrations of water (x_w = 0.4), only about 2% of oxygen atoms are present as molecular water species at the lowest temperature. This percentage decreases with increasing pressure and temperature.