Scott L. Wallen

Rubidium ion hydration in ambient and supercritical water

X‐ray absorption fine structure (XAFS) measurements and analyses are presented for Rb+ in supercritical water solutions. The structure of the first hydration shell at ambient conditions is compared to that in the supercritical region at a temperature of 424 °C and pressures from 382 to 633 bar. For all reported studies, RbBr at a concentration of 0.5 molal was used. XAFS results show that there is a well‐defined hydration shell around the cation even at 424 °C but at these high temperatures the extent of hydration of the Rb cation is reduced by about 40%. A slight contraction of this first shell distance by about 0.10 A is also observed under supercritical conditions. The reduction in the number of water‐ion bonds is analogous to the reduction in the amount of water–water hydrogen bonding that has been observed by others under supercritical conditions. The reduction in waters‐of‐hydration under supercritical conditions may also be in part due to formation of contact‐ion pairs.

The ion pairing and hydration structure of Ni2+ in supercritical water at 425 °C determined by x-ray absorption fine structure and molecular dynamics studies

The ion pairing structure of Ni(Br)2 solutions (0.2 and 0.4 molal) under supercritical conditions was determined using x-ray absorption fine structure (XAFS) spectroscopy. These first measurements of the average bulk structure show that approximately one Br− counterion is associated with each Ni2+. The Ni2+-to-Br− distance of 2.40 A is very accurately determined and the strength of this interaction, as indicated by the Debye–Waller factor (σ2=0.009 A2), shows that the bromine anion is very tightly bound to the nickel cation under these supercritical conditions. In addition to the onset of ion pairing interactions, there is also a dramatic transition in the hydration structure. Results show a loss of about 50% of the waters in the first shell upon going from ambient to a hydrothermal condition of 425 °C and 690 bar. Finally, we use molecular dynamics simulations with refined intermolecular potentials to directly calculate XAFS spectra that are shown to quantitatively reproduce the experimental results for ...

Sugar Acetates as CO2-philes: Molecular Interactions and Structure Aspects from Absorption Measurement Using Quartz Crystal Microbalance

Sugar acetates, recognized as attractive CO(2)-philic compounds, have potential uses as pharmaceutical excipients, controlled release agents, and surfactants for microemulsion systems in CO(2)-based processes. This study focuses on the quantitative examination of absorption of high pressure CO(2) into these sugar derivatives using quartz crystal microbalance (QCM) as a detector. In addition to the absorption measurement, the QCM is initially found to be able to detect the CO(2)-induced deliquescence of sugar acetates, and the CO(2) pressure at which the deliquescence happens depends on several influencing factors such as the temperature and thickness of the film. The CO(2) absorption in alpha-D-glucose pentaacetate (Ac-alpha-GLU) is revealed to be of an order of magnitude larger in comparison with its anomer Ac-beta-GLU, whereas alpha-D-galactose pentaacetate (Ac-alpha-GAL) absorbs CO(2) less than Ac-alpha-GLU due to the steric-hindrance between the acetyl groups on the anomeric and C4 carbons, implying the significant importance of the molecular structure and configuration of sugar acetates on the absorption. The effects of molecular size and acetyl number of sugar acetates on the CO(2) absorption are evaluated and the results indicate that the conformation and packing of crystalline sugar acetate as well as the accessibility of the acetyls are also vital for the absorption of CO(2). It is additionally found that a CO(2)-induced change in the structure from a crystalline system to an amorphous system results in an order of magnitude increase in CO(2) absorption. Further investigation illustrates the interaction strength between sugar acetates and CO(2) by calculating the thermodynamic parameters such as Henrys law constant, enthalpy and entropy of dissolution from the determined CO(2) absorption. Experiments and calculations demonstrate that sugar acetates exhibit high CO(2) absorption, as at least comparable to ionic liquids. Since the ionic liquids have potential uses in the separation of acidic gases, it is evident from this study that sugar acetates could be used as possible materials for CO(2) separation.

High‐pressure, capillary x‐ray absorption fine structure cell for studies of liquid and supercritical fluid solutions

A method is described to acquire x‐ray absorption fine structure (XAFS) spectra of high‐pressure liquid and supercritical fluid solutions. The technique employs a short length of fused‐silica capillary tubing that has an inner diameter of 250 μm and an outer diameter of 360 μm. A hairpin bend is formed near the center of the capillary and the bend is then placed end‐on directly in the focused x‐ray beam. Fluorescence spectra were acquired in a 90° geometry using a 13 element Ge detector. Demonstration XAFS spectra are reported for a Mn organometallic complex dissolved in subcritical and supercritical CO2. Although the maximum pressure of these studies was 160 bar, with slight modification, the method will be applicable to studies requiring pressures as high as 4 kbar.

High-pressure NMR study of metal complexes in supercritical fluids

Abstract Results of high-pressure NMR investigations of subcritical and supercritical-fluid solutions of metal complexes in ethylene and C02 are presented. To our knowledge, this is the first report of high-pressure NMR studies of metal chelates in supercritical fluids. Examples presented include 19 F NMR studies of two β-diketone metal chelates dissolved in subcritical CO 2 -methanol mixtures and a 1 H NMR study of the (1,5-cyclooctadiene) dimethylplatinum (II) (Pt-COD) complex in supercritical ethylene. High-pressure NMR chemical shifts as a function of density for supercritical ethylene are also presented. The results show the utility of the NMR technique to study a variety of metal complexes in subcritical and supercritical fluids.