Direct probing of sorbent-solvent interactions for amylose tris(3,5-dimethylphenylcarbamate) using infrared spectroscopy, X-ray diffraction, solid-state NMR, and DFT modeling.

The sorbent-solvent interactions for amylose tris(3, 5-dimethylphenylcarbamate) (ADMPC) with five commonly used solvents, hexane, methanol, ethanol, 2-propanol (IPA), and acetonitrile (ACN), are studied using attenuated total reflection infrared spectroscopy (ATR-IR) of thin sorbent films, X-ray diffraction (XRD) of thin films, (13)C cross polarization/magic angle spinning (CP/MAS) and MAS solid state NMR of polymer-coated silica beads (commercially termed "Chiralpak AD"), and DFT modeling. The ADMPC-polymer-coated silica beads are used commercially for analytical and preparative scale separations of chiral enantiomers. The polymer forms helical rods with intra- and inter-rod hydrogen bonds (H-bonds). There are various nm-sized cavities formed between the polymer side-chains and rods. The changes in the H-bonding states of the C=O and NH groups of the polymer upon absorption of each of the five solvents at 25 degrees C are determined with ATR-IR. The IR wavenumbers, the H-bonding interaction energies, and the H-bonding distances of the polymer side-chains with each of the solvent molecules are predicted using the DFT/B3LYP/6-311+g(d,p) level of theory. The changes in the polymer crystallinity upon absorption of each solvent are characterized with XRD. The changes in the polymer crystallinity and the H-bonding states of C=O groups are also probed with (13)C CP/MAS solid-state NMR. The changes in the polymer side-chain mobility are detected using (13)C MAS solid-state NMR. The H-bonding states of the polymer change upon absorption of each polar solvent and usually result in an increase in the polymer crystallinity and the side-chain mobility. The polymer rods are reorganized upon solvent absorption, and the distance between the rods increases with the increase in the solvent molecular size. These results have implications for understanding the role of the solvent in modifying the structure and behavior of the polymer sorbents.

A transmission infrared cell design for temperature-controlled adsorption and reactivity studies on heterogeneous catalysts.

A design is presented for a versatile transmission infrared cell that can interface with an external vacuum manifold to undergo in situ gas treatments and receive controlled doses of various adsorbates and probe molecules, allowing characterization of heterogeneous catalyst surfaces in order to identify and quantify active sites and adsorbed surface species. Critical design characteristics include customized temperature control for operation between cryogenic and elevated temperatures (100-1000 K) and modified Cajon fittings for operation over a wide pressure range (10-2-103 Torr) that eliminates the complications introduced when using sealants or flanges to secure cell windows. The customized, hand-tightened Cajon fittings simplify operation of the cell compared to previously reported designs, because they allow for rapid cell assembly and disassembly and, in turn, replacement of catalyst samples. In order to validate the performance of the cell, transmission infrared spectroscopic experiments are reported to characterize the Brønsted and Lewis acid sites present in H-beta and H-mordenite zeolites using cryogenic adsorption of CO (<150 K).

Gas mixing system for imaging of nanomaterials under dynamic environments by environmental transmission electron microscopy.

A gas mixing manifold system that is capable of delivering a stable pressure stream of a desired composition of gases into an environmental transmission electron microscope has been developed. The system is designed to provide a stable imaging environment upon changes of either the composition of the gas mixture or upon switching from one gas to another. The design of the system is described and the response of the pressure inside the microscope, the sample temperature, and sample drift in response to flow and composition changes of the system are reported.

Temperature and gas-phase composition measurements in supersonic flows using tunable diode laser absorption spectroscopy: the effect of condensation on the boundary-layer thickness.

We used a tunable diode laser absorption spectrometer and a static-pressure probe to follow changes in temperature, vapor-phase concentration of D2O, and static pressure during condensation in a supersonic nozzle. Using the measured static-pressure ratio p/p0 and the mass fraction of the condensate g as inputs to the diabatic flow equations, we determined the area ratio (A/A*)wet and the corresponding centerline temperature of the flow during condensation. From (A/A*)wet we determined the boundary-layer displacement thickness during condensation (delta#)wet. We found that (delta#)wet first increases relative to the value of delta# in a dry expansion (delta#)Dry before becoming distinctly smaller than (delta#)Dry downstream of the condensation region. After correcting the temperature gradient across the boundary layers, the temperature determined from p/p0 and g agreed with the temperature determined by the laser-absorption measurements within our experimental error (+/-2 K), except when condensation occurred too close to the throat. The agreement between the two temperature measurements let us draw the following two conclusions. First, the differences in the temperature and mole fraction of D2O determined by the two experimental techniques, first observed in our previous study [P. Paci, Y. Zvinevich, S. Tanimura, B. E. Wyslouzil, M. Zahniser, J. Shorter, D. Nelson, and B. McManus, J. Chem. Phys. 121, 9964 (2004)], can be explained sufficiently by changes in delta# caused by the condensation of D2O, except when the phase transition occurs too close to the throat. Second, the extrapolation of the equation, which expresses the temperature dependence of the heat of vaporization of bulk D2O liquid, is a good estimate of the heat of condensation of supercooled D2O down to 210 K.

Spatially resolved gas phase composition measurements in supersonic flows using tunable diode laser absorption spectroscopy.

We used a tunable diode laser absorption spectrometer to follow the condensation of D(2)O in a supersonic Laval nozzle. We measured both the concentration of the condensible vapor and the spectroscopic temperature as a function of position and compared the results to those inferred from static pressure measurements. Upstream and in the early stages of condensation, the quantitative agreement between the different experimental techniques is good. Far downstream, the spectroscopic results predict a lower gas phase concentration, a higher condensate mass fraction, and a higher temperature than the pressure measurements. The difference between the two measurement techniques is consistent with a slight compression of the boundary layers along the nozzle walls during condensation.