John L. Daschbach

Adsorption, desorption, and clustering of H2O on Pt(111)

The adsorption, desorption, and clustering behavior of H2O on Pt111 has been investigated by specular He scattering. The data show that water adsorbed on a clean Pt111 surface undergoes a structural transition from a random distribution to clustered islands near 60 K. The initial helium scattering cross sections as a function of temperature are found to be insensitive to the incident H2O flux over a range of 0.005 monolayers (ML)/s-0.55 ML/s indicating that the clustering process is more complex than simple surface diffusion. The coarsening process of an initially random distribution of water deposited at 25 K is found to occur over a broad temperature range, 60<T<140 K, during thermal annealing. The desorption kinetics for submonolayer water are determined to be zero order for surface coverages greater than 0.05 ML and temperatures between 145 and 172 K. The zero-order desorption kinetics are consistent with a two-dimensional two-phase coexistence between a high-density H2O condensed phase (islands) and a low-density two-dimensional gaslike phase on the Pt surface.

HCl Adsorption and Ionization on Amorphous and Crystalline H2O Films below 50 K

Molecular beams were used to grow amorphous and crystalline H(2)O films and to dose HCl upon their surface. The adsorption state of HCl on the ice films was probed with infrared spectroscopy. A Zundel continuum is clearly observed for exposures up to the saturation HCl coverage on ice upon which features centered near 2530, 2120, 1760, and 1220 cm(-1) are superimposed. The band centered near 2530 cm(-1) is observed only when the HCl adlayer is in direct contact with amorphous solid water or crystalline ice films at temperatures as low as 20 K. The spectral signature of solid HCl (amorphous or crystalline) was identified only after saturation of the adsorption sites in the first layer or when HCl was deposited onto a rare gas spacer layer between the HCl and ice film. These observations strongly support conclusions from recent electron spectroscopy work that reported ionic dissociation of the first layer HCl adsorbed onto the ice surface is spontaneous.

A new angle into time-resolved photoacoustic spectroscopy: A layered prism cell increases experimental flexibility

A new pulsed photoacoustic calorimetry cell that uses transmission of light through a pair of dovetail prisms is discussed. The layered prism cell (LPC) combines the enhanced time-resolution capabilities of the “layered” front-face irradiation geometry with the zero-background and broadband flexibility of the classical cuvette geometry. This work provides a phenomenological description of photoinduced pressure changes to yield an analytical expression to calculate the magnitude of the photoinduced acoustic pressure wave in a series of solvents. The mechanical to electrical conversion efficiency for an ultrasonic transducer coupled to the LPC is presented to provide a comparison of the experimentally observed photoinduced acoustic signal amplitudes to the empirically calculated acoustic signal amplitudes. An analysis of the background signals due to absorption and electrostriction of the media provides insight into the issues of sensitivity and limitations of pulsed photoacoustic experiments. The LPC provi...

Molecular Beam Studies of Nanoscale Films of Amorphous Solid Water

What is Amorphous Solid Water? Amorphous solid water (ASW) is a solid phase of water that is metastable with respect to the crystalline phase [1,2]. It is metastable because it is “trapped” in a configuration that has a higher free energy than the equilibrium crystalline configuration [3]. Amorphous solids, also known as glasses, are often described as structurally arrested or “frozen” liquids. Amorphous solids are most often formed when a liquid is cooled fast enough that crystallization does not occur prior to the system reaching a temperature where the structural relaxation timescale is long compared to the laboratory timescale, i.e. 100 s. The temperature where this occurs is called the glass transition temperature, T g .

Free energies of CO2/H2 capture by p -tert-butylcalix [4 ]arene : A molecular dynamics study

The interactions of CO2∕H2 with p-tert-butylcalix[4]arene (TBC4) were studied using potential of mean force (PMF) and free energy perturbation approaches. To the best of our knowledge, the present work is one of the first to employ the constrained mean force approach to evaluate solute selectivity by the TBC4 molecule. The computed PMFs for the interaction of CO2∕H2 with a single TBC4 molecule establish that the interaction of CO2 with the open end of the cage structure is attractive while the interaction with H2 is repulsive. Free energy perturbation calculations were performed for the same two guest molecules with a pair of facing TBC4 molecules used as a representative model as found in the TBC4 molecular solid. At low temperature, both CO2∕H2 have favorable interactions with the TBC4 pair, with the CO2 interaction being considerably greater. These results are in agreement with recent experimental data showing considerable CO2 uptake by TBC4 at moderate pressures.

THE EFFECT OF SMECTITE ON THE CORROSION OF IRON METAL

The combination of zero-valent iron (ZVI) and a clay-type amendment is often observed to have a synergistic effect on the rate of reduction reactions. In the present study, electrochemical techniques were used to determine the mechanism of interaction between the iron (Fe) and smectite clay minerals. Iron electrodes coated with an evaporated smectite suspension (clay-modified iron electrodes, CMIEs) were prepared using five different smectites: SAz-1, SWa-1, STx-1, SWy-1, and SHCa-1. All the smectites were exchanged with Na+ and one sample of SWy-1 was also exchanged with Mg2+. Potentiodynamic polarization scans and cyclic voltammograms were taken using the CMIEs and uncoated but passivated Fe electrodes. These electrochemical experiments, along with measurements of the amount of Fe2+ and Fe3+ sorbed in the smectite coating, suggested that the smectite removed the passive layer of the underlying Fe electrode during the evaporation process. Cyclic voltammograms taken after the CMIEs were biased at the active-passive transition potential for varying amounts of time suggested that the smectite limited growth of a passive layer, preventing passivation. These results are attributed to the Brønsted acidity of the smectite as well as to its ability to sorb Fe cations. Oxides that did form on the surface of the Fe in the presence of the smectite when it was biased anodically were reduced at a different electrochemical potential from those that form on the surface of an uncoated Fe electrode under otherwise similar conditions; this difference suggested that the smectite reacted with the Fe2+ formed from the oxidation of the underlying Fe. No significant correlation could be found between the ability of the smectite to remove the Fe passive film and the smectite type. The results have implications for the mixing of sediments and Fe particles in permeable reactive barriers, underground storage of radioactive waste in steel canisters, and the use of smectite supports in preventing aggregation of nano-sized zero-valent iron.

Grand Canonical Monte Carlo studies of CO2 and CH4 adsorption in p-tert-butylcalix[4]arene

Grand Canonical Monte Carlo simulations were performed for single component isotherms of CO(2) and CH(4) in the p-tert-butylcalix[4]arene structure. Comparison with literature data for adsorption used the Peng-Robinson equation of state to map simulated fugacities to experimentally determined pressures. CO(2) binding in the high-pressure structure of TBC4 (TBC4-H) occurs in two distinct waves. The cage sites in TBC4 completely fill up, followed by the filling of interstitial sites, resulting in the sum of two Langmuir isotherms being the best way to describe the total absorption isotherms. Our simulation results capture the essential experimental feature that the cage sites are the major contributor to the absorption isotherms, and the contribution of interstitial sites are significantly less. We found that CH(4) does not exhibit the same two-site binding characteristic and has a smaller temperature dependence, which arises from a smaller negative entropy change upon absorption compared with the case for CO(2). Our calculations give higher binding than observed experimentally for the cage site but lower binding for the interstitial site. We also demonstrate that by rescaling the interaction between CO(2) and the lattice, the results can reproduce the experimental data well at low loadings. The rescaled potentials are within the range found in other studies. This makes the discrepancy between experiment and simulation at high loadings greater, which is unexpected for this system. It is postulated that the simulation points to structural changes or defects being partially responsible for the relatively higher absorption found experimentally.

Computational Studies of Load-Dependent Guest Dynamics and Free Energies of Inclusion for CO2 in Low-Density p-tert-Butylcalix(4)arene at Loadings up to 2:1

The structure, dynamics, and free energies of absorption of CO(2) by a low-density structure (P4/n) of calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO(2):TBC4 have been studied by using molecular dynamics simulations with two sources of initial TBC4 structures (TBC4-T and TBC4-U). The CO(2)/TBC4 complex structure is very sensitive to the initial lattice spacing of TBC4. From the computed radial distribution functions of CO(2) molecules, a CO(2) dimer is observed for TBC4-T and a cage-interstitial CO(2) structure is suggested for TBC4-U. The dynamics of the CO(2) molecules show little initial TBC4 structural dependency. The free energy of inclusion for a single CO(2) in this TBC4 structure for various loadings is -4.0 kcal/mol at 300 K and -1.8 kcal/mol at 450 K, showing that CO(2) inclusion is favored. The fully loaded 1:1 CO(2):TBC4 system is slightly less favorable at -3.9 and -1.2 kcal/mol at 300 and 450 K, respectively. The first CO(2) added beyond 1:1 loading shows a significant drop in absorption energy to -1.9 and +1.9 kcal/mol at 300 and 450 K. These data are consistent with experimental results showing that low-density structures of TBC4 are able to absorb CO(2) at loadings greater than 1:1 but retention is lower than for 1:1 loaded systems indicating the free energy of inclusion for addition of the CO(2) above 1:1 is less favorable.