Biomedical subjects

Wei Gan

Publications and source records attributed to Wei Gan.

6 recordsLinked to original sources

PubMed2006

An empirical approach to the bond additivity model in quantitative interpretation of sum frequency generation vibrational spectra.

Knowledge of the ratios between different polarizability betai'j'k' tensor elements of a chemical group in a molecule is crucial for quantitative interpretation and polarization analysis of its sum frequency generation vibrational spectroscopy (SFG-VS) spectrum at interface. The bond additivity model (BAM) or the hyperpolarizability derivative model along with experimentally obtained Raman depolarization ratios has been widely used to obtain such tensor ratios for the CH3, CH2, and CH groups. Successfully, such treatment can quantitatively reproduce the intensity polarization dependence in SFG-VS spectra for the symmetric (SS) and asymmetric (AS) stretching modes of CH3 and CH2 groups, respectively. However, the relative intensities between the SS and AS modes usually do not agree with each other within this model even for some of the simplest molecular systems, such as the air/methanol interface. This fact certainly has cast uncertainties on the effectiveness and conclusions based on the BAM. One of such examples is that the AS mode of CH3 group has never been observed in SFG-VS spectra from the air/methanol interface, while this AS mode is usually very strong for SFG-VS spectra from the air/ethanol interface, other short chain alcohol, as well as long chain surfactants. In order to answer these questions, an empirical approach from known Raman and IR spectra is used to make corrections to the BAM. With the corrected ratios between the betai'j'k' tensor elements of the SS and AS modes, all features in the SFG-VS spectra of the air/methanol and air/ethanol interfaces can be quantitatively interpreted. This empirical approach not only provides new understandings of the effectiveness and limitations of the bond additivity model but also provides a practical way for its application in SFG-VS studies of molecular interfaces.

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PubMed2006

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface.

Here we report a detailed study on spectroscopy, structure, and orientational distribution, as well as orientational motion, of water molecules at the air/water interface, investigated with sum frequency generation vibrational spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analyses of the SFG data in different polarizations with four sets of experimental configurations can shed new light on our present understanding of the air/water interface. Firstly, we concluded that the orientational motion of the interfacial water molecules can only be in a limited angular range, instead of rapidly varying over a broad angular range in the vibrational relaxation time as suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analyses of the SFG-VS spectral features can help the assignment of the SFG-VS spectra peaks to different interfacial species. These analyses concluded that the narrow 3693 cm(-1) and broad 3550 cm(-1) peaks belong to C(infinityv) symmetry, while the broad 3250 and 3450 cm(-1) peaks belong to the symmetric stretching modes with C2v symmetry. Thus, the 3693 cm(-1) peak is assigned to the free OH, the 3550 cm(-1) peak is assigned to the singly hydrogen-bonded OH stretching mode, and the 3250 and 3450 cm(-1) peaks are assigned to interfacial water molecules as two hydrogen donors for hydrogen bonding (with C2v symmetry), respectively. Thirdly, analysis of the SFG-VS spectra concluded that the singly hydrogen-bonded water molecules at the air/water interface have their dipole vector directed almost parallel to the interface and is with a very narrow orientational distribution. The doubly hydrogen-bonded donor water molecules have their dipole vector pointing away from the liquid phase.

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PubMed2005

C-H stretching vibrations of methyl, methylene and methine groups at the vapor/alcohol (N = 1-8) interfaces.

In IR and Raman spectral studies, the congestion of the vibrational modes in the C-H stretching region between 2800 and 3000 cm(-1) has complicated spectral assignment, conformational analysis, and structural and dynamics studies, even with quite a few of the simplest molecules. To resolve these issues, polarized spectra measurement on a well aligned sample is generally required. Because the liquid interface is generally ordered and molecularly thin, and sum frequency generation vibrational spectroscopy (SFG-VS) is an intrinsically coherent polarization spectroscopy, SFG-VS can be used for discerning details in vibrational spectra of the interfacial molecules. Here we show that, from systematic molecular symmetry and SFG-VS polarization analysis, a set of polarization selection rules could be developed for explicit assignment of the SFG vibrational spectra of the C-H stretching modes. These polarization selection rules helped assignment of the SFG-VS spectra of vapor/alcohol (n = 1-8) interfaces with unprecedented details. Previous approach on assignment of these spectra relied on IR and Raman spectral assignment, and they were not able to give such detailed assignment of the SFG vibrational spectra. Sometimes inappropriate assignment was made, and consequently misleading conclusions on interfacial structure, conformation and even dynamics were reached. With these polarization rules in addition to knowledge from IR and Raman studies, new structural information and understanding of the molecular interactions at these interfaces were obtained, and some new spectral features for the C-H stretching modes were also identified. Generally speaking, these new features can be applied to IR and Raman spectroscopic studies in the condensed phase. Therefore, the advancement on vibrational spectra assignment may find broad applications in the related fields using IR and Raman as vibrational spectroscopic tools.

2-Propanol↗

PubMed2005

Determination of structure and energetics for gibbs surface adsorption layers of binary liquid mixture 1. Acetone + water.

The orientation, structure, and energetics of the vapor/acetone-water interface are studied with sum frequency generation vibrational spectroscopy (SFG-VS). We used the polarization null angle (PNA) method in SFG-VS to accurately determine the interfacial acetone molecule orientation, and we found that the acetone molecule has its C=O group pointing into bulk phase, one CH3 group pointing up from the bulk, and the other CH3 group pointing into the bulk phase. This well-ordered interface layer induces an antiparallel structure in the second layer through dimer formation from either dipolar or hydrogen bond interactions. With a double-layer adsorption model (DAM) and Langmuir isotherm, the adsorption free energies for the first and second layer are determined as deltaG degrees (ads,1) = - 1.9 +/- 0.2 kcal /mol and deltaG degrees (ads,2) = - 0.9 +/- 0.2 kcal /mol, respectively. Since deltaG degrees (ads,1) is much larger than the thermal energy kT = 0.59 kcal /mol, and deltaG degrees (ads,2) is close to kT, the second layer has to be less ordered. Without either strong dipolar or hydrogen bonding interactions between the second and the third layer, the third layer should be randomly thermalized as in the bulk liquid. Therefore, the thickness of the interface is not more than two layers thick. These results are consistent with previous MD simulations for the vapor/pure acetone interface, and undoubtedly provide direct microscopic structural evidences and new insight for the understanding of liquid and liquid mixture interfaces. The experimental techniques and quantitative analysis methodology used for detailed measurement of the liquid mixture interfaces in this report can also be applied to liquid interfaces, as well as other molecular interfaces in general.

Journal Article↗

PubMed2005

Determination of structure and energetics for gibbs surface adsorption layers of binary liquid mixture 2. Methanol + water.

Vapor/methanol and vapor/methanol-water mixture interfaces have been among the benchmark liquid interfaces under extensive experimental and theoretical investigation. In this report, we studied the orientation, structure and energetics of the vapor/methanol-water interface with newly developed techniques in sum frequency generation vibrational spectroscopy (SFG-VS). Different from the interpretations in previous SFG-VS studies for a more disordered interface at higher bulk methanol concentrations, we found that the methanol-water mixture interface is well ordered in the whole concentration region. We are able to do so because direct polarization null angle (PNA) measurement allowed us to accurately determine the CH3 orientation at the interface and to separate the orientational and interface density contributions to the SFG-VS signal. We found that the CH3 groups at the interface pointed out almost perpendicularly from the interface. We further found that this well-ordered vapor/methanol-water mixture interface has an antiparallel structure. With the double layer adsorption model (DAM) and Langmuir isotherm, the adsorption free energies for the first and second layer are obtained as -1.7 +/- 0.1 kcal/mol and 0.5 +/- 0.4 kcal/mol, respectively. Therefore, the second layer adsorption is slightly negative, and this means that replacement of the second layer water molecule with methanol molecule is energetically unfavorable. Comparing this interface with the vapor/acetone-water mixture interface reported previously, we are able to correlate the second layer adsorption free energy with the work of self-association using the pairwise self- and mutual interaction energies between the water and solute molecules. These results provided detailed microscopic structural evidences for understanding of liquid interfaces.

Journal Article↗

PubMed2002

Superconductivity in dense lithium.

Superconductivity in compressed lithium is observed by magnetic susceptibility and electrical resistivity measurements. A superconducting critical temperature (Tc) is found ranging from 9 to 16 kelvin at 23 to 80 gigapascals. The pressure dependence of Tc suggests multiple phase transitions, consistent with theoretical predictions and reported x-ray diffraction results. The observed values for Tc are much lower than those theoretically predicted, indicating that more sophisticated theoretical treatments similar to those proposed for metallic hydrogen may be required to understand superconductivity in dense phases of lithium.

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