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Title:Orientation of adsorbate-adsorbate complexes using a novel waveguide linear dichroism technique
Author(s):Cropek, Donald Michael
Doctoral Committee Chair(s):Bohn, Paul W.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Analytical
Abstract:A novel approach to linear dichroism (LD) measurements is utilized to investigate orientations of adsorbed monolayer assemblies. Polarized radiation in integrated optical waveguide structures is employed to maximize interactions between incident radiation and adsorbed molecules at the waveguide surface. In this manner, the experimental applications and detection limits of orientation measurements have been greatly expanded.
Orientations of monophenyl silyl ether monolayers on glass substrates were initially investigated for three primary reasons: (1) the absorption cross section is small which provides a strenuous test for the applicability of this LD approach on weakly absorbing molecules, (2) the structure of the silane permitted an unambiguous relationship between the absorption transition moment and the molecular orientation, and (3) these silanes provide a surface which closely mimics a reversed-phase liquid chromatographic (RPLC) phenyl stationary phase commonly used for the separation of aromatic compounds. Successful experiments revealed an average orientation angle of the phenyl rings from the O - Si - C$\sb{\rm aromatic}$ bond plane of close to 60$\sp\circ$ in both air and combinations of a water/methanol solvent.
These monophenyl silyl ether structures were then used in an RPLC capacity to observe the orientation of polycyclic aromatic hydrocarbon (PAH) solute molecules as complexation occurs with the aromatic bonded phase. A consistent orientation behavior is seen for the five different PAH solutes studied as the concentration of methanol is increased in the methanol/water solvent bath. At small methanol concentrations, the solute molecules orient at an angle commensurate with the orientation angle of the phenyl moieties in the bonded monolayer. As the methanol concentration increases, the solute molecules move toward an alternate orientation until a methanol concentration is reached where no further absorption by the solute molecule is observed. The data appear to support a model where a face-to-face orientation between the PAH solute and the phenyl ring is favored in solvents with a large aqueous character while the solutes prefer an edge-to-phenyl ring face orientation as the solvent becomes increasingly organic in nature. As a result of these experiments, it appears that the retention mechanism in RPLC can be described with a combination of an orientation factor with the solvophobic theory which is commonly accepted.
This technique has proved to be a powerful new probe of the orientation of adsorbate-adsorbate complexes at solid-liquid interfaces. It will also prove useful for the study of other weakly absorbing monolayer structures which have eluded easy investigation because of small absorption cross sections. In addition, the simple mathematical treatment and the straightforward relationship between the obtained data and molecular orientation relieves the interpretational difficulties of other orientation measurement techniques.
Issue Date:1991
Rights Information:Copyright 1991 Cropek, Donald Michael
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9210774
OCLC Identifier:(UMI)AAI9210774

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