M.S. El-Shall

Homogeneous nucleation in supersaturated vapors of polar molecules : acetonitrile, benzonitrile, nitromethane, and nitrobenzene

The critical supersaturations (Sc) required for the homogeneous nucleation of acetonitrile, benzonitrile, nitromethane, and nitrobenzene vapors have been measured over wide temperature ranges using a thermal diffusion cloud chamber. At Tr=0.44, the experimental results are higher than the predictions of the classical nucleation theory by 20% (CH3NO2), 50% (CH3CN), 57% (C6H5NO2), and 112% (C6H5CN). This trend correlates well with the dipole moments and to a lesser extent with the polarizabilities of these substances. Corresponding states of simple fluids and scaled nucleation law show that CH3CN and CH3NO2 have similar nucleation behavior which is different from that of C6H5CN and C6H5NO2. Correlations with other less polar substances are examined. The scaled law, with Ω (the excess surface entropy per molecule) determined from the temperature dependence of the bulk surface tension, gives a better description of the experimental results than the classical theory with the exception of nitromethane. The resu...

Analysis of homogeneous nucleation data of polar molecules: Vapor association, dipole orientation, and elongated clusters

In this paper we present the results of several theoretical models examined in order to explain the observed systematic deviations of the homogeneous nucleation of highly polar substances from the predictions of the classical nucleation theory (CNT). It is found that molecular association in acetronitrile or nitromethane vapor cannot explain the observed higher supersaturations. Similarly, the use of virial coefficient data to obtain a size dependent surface energy correction factor does not reproduce the experimental results. It is shown that orientation of the surface dipoles within the spherical droplets results in higher supersaturations in agreement with the experimental findings. We also generalize the CNT for elongated clusters and show that these clusters can allow for favorable end‐to‐end and antiparallel alignments of the dipoles. It is found that for reasonable agreement with experiment, acetonitrile, and benzonitrile clusters require a larger degree of elongation compared to nitromethane and nitrobenzene clusters. These results are discussed in relation to molecular simulations of bulk liquids and clusters.

A Monte Carlo study of methanol clusters (CH3OH)N, N=5–256

The thermodynamic and structural properties of methanol clusters (CH3OH)N, N=5–15, 20, 30, 60, 128, 256 and the bulk liquid have been investigated using Monte Carlo simulation. Calculated properties as a function of size include electrostatic and dispersive contributions to the configurational energy, configurational heat capacities, fractal dimension, density profiles, order parameters characterizing dipole and bond vector orientation, and the Lindemann index. The cluster heat capacities as a function of N possess an interior maximum near N=128 and converge to the bulk value from above. Monocyclic, semiplanar structures are found to persist at liquidlike temperatures up to about N=12, followed by bi‐ and polycyclic structures for N=13–20, with the larger clusters gradually becoming more spherical. The average density of the larger clusters is fairly well represented by the bulk value. For clusters with 30 or more molecules, there is a net tendency for the molecular dipoles to lie flat on the cluster surf...

The effect of carrier gas pressure on vapor phase nucleation experiments using a thermal diffusion cloud chamber

Recent measurements of critical supersaturations for the vapor phase homogeneous nucleation of several substances using a diffusion cloud chamber technique exhibit a dependence on the pressure of the carrier gas used in the experiments. A model of droplet growth and motion in a diffusion cloud chamber, combined with the density and temperature profiles of the chamber is presented to explain the pressure dependent results. The model demonstrates that at higher carrier gas pressures the growth of the droplets is retarded and the optical scattering signal from the particles is reduced. It is concluded that the observed effect may not result from a pressure dependence of the nucleation rate, but from a pressure dependence of the droplet growth and motion.

Vapor phase homogeneous nucleation of acetonitrile: the effect of dipole—dipole interaction

Abstract The critical supersaturation ( S c ) required for the homogeneous nucleation of CH 3 CN vapor has been measured over the temperature range 240–300 K by use of a diffusion cloud chamber. The experimental results are in serious disagreement with the predictions of the classical nucleation theory. The discrepancy has been explained in terms of dipole—dipole interaction within the curved surface of the embryonic droplets. This correction seems to be necessary to explain the homogeneous nucleation of highly polar liquids (μ

Evidence for changes in the electronic and photoluminescence properties of surface-oxidized silicon nanocrystals induced by shrinking the size of the silicon core

Abstract Web-like aggregates of Si nanocrystals produced by laser vaporization–controlled condensation technique are allowed to oxidize slowly in air and the photoluminescence (PL) is measured. A significant shift in the PL red band from 1.83 to 1.94 eV is observed. The bonding structure is established by correlating the PL data with the photon-yield electronic structure measurements using soft-X-ray fluorescence (SXF) and photon-yield near-edge X-ray absorption fine structure (NEXAFS) techniques. The results indicate that as the nanoparticles oxidize, the radius of the crystalline core decreases, which gives rise to a larger bandgap and consequently to the observed blue shift in the PL band.

Interaction of Zn+ with isobutylene in the gas phase and in clusters. Metal ion induced cationic polymerization

Abstract A variety of closed shell hydrocarbon ions have been observed following charge transfer from Zn+ to isobutylene (C4H8) in a high-pressure mass spectrometric (HPMS) source. The time profiles of these ions and their strong pressure dependence suggest that consecutive and sequential ion-molecule reactions are responsible for their formation. The interaction of Zn+ with isobutylene clusters (C4H8)n generated by a supersonic beam expansion results in efficient generation of the C4H9(C4H8)n+ series. These observations suggest has phase and intracluster polymerization behavior induced by Zn+. Clusters provide a feasible and valuable approach for understanding the mechanism of the early stages of cationic polymerization.

Termolecular proton transfer reactions assisted by ionic hydrogen bond formation: Reactions of aromatic cations with polar molecules

We present a new method that applies resonant‐two‐photon ionization to generate reactant ions selectively in the source of a high‐pressure mass spectrometer (R2PI‐HPMS) for kinetic and equilibrium studies. Applications to reactions that would be obscured otherwise in a complex system are illustrated in mixtures of benzene with polar solvent molecules (S). We observe a novel type of proton transfer reactions from C6H6+• to two S molecules where S=CH3CN, CH3OH, C2H5OH and CH3COOC2H5, and from C6H5CH3+• to two S molecules where S=CH3OH and C2H5OH to form protonated solvent S2H+ dimers. The reactions are driven by the strong hydrogen bonds in the S2H+ dimers and therefore require the formation of the hydrogen bond concertedly with proton transfer, to make the process energetically feasible. The adducts (C6H6+•)S are observed with blocked solvent molecules where the subsequent switching reaction to yield S2H+ is slow, but not with alcohol reactants that can form hydrogen‐bonded chains that facilitate fast subs...

Morphology, photoluminescence and electronic structure in oxidized silicon nanoclusters

Abstract The dependence of quantum size effects on bonding structure in oxidized silicon nanoclusters is established by correlating photoluminescence data with photon-yield electronic structure measurements at the advanced light source. The nanoclusters were synthesized using a laser ablation technique that utilizes a convective He environment to control the size of the particles. After removal from the growth chamber, our ex situ photoluminescence (PL) results indicate that, as the nanoclusters oxidize, the main PL peak moves from 1.83 to 1.94 eV in energy. The central focus of the present work is to establish the origin of the main PL peak, and to determine why its energy shifts as the nanoclusters are allowed to oxidize slowly in air. The changes in the morphology and bonding structure of the clusters was established using soft-X-ray fluorescence spectroscopy (SXF) and photon-yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, which probe the element-specific density of occupied (SXF) and unoccupied (NEXAFS) electronic structure. Our conclusion is that the as-synthesized nanoclusters consist of a pure, crystalline Si core within a nearly pure SiO2 shell, with little or no sub-oxides present. As the nanoclusters oxidize, the radius of the crystalline core decreases in size, which gives rise to the change in the position of the PL signal.

Modeling nucleation and droplet growth for ion-induced nucleation experiments in diffusion cloud chambers

Abstract A model of ion-induced nucleation, droplet growth and motion processes in a diffusion cloud chamber is presented. The model predicts the arrival time of the droplets with an accuracy greater than 95%. The model also explains the increase in the time corresponding to the maximum of the nucleation pulse with increasing carrier gas pressure. Using this model it is possible to correlate the arrival time of the droplets with the height (temperature and supersaturation) where the nucleation events take place.