Depolarized

Photoexcited fluorescence from spread monolayers may be studied [158,159] if the substance has both a strong absorption band and a high emission yield as in the case for chlorophyll [159]. Gaines and co-workers [160] have reported on the emission from monolayers of Ru(bipyridine)3, one of the pyridine ligands having attached C g aliphatic chains. Ruorescence depolarization provides information about the restriction of rotational diffusion of molecules in a monolayer [161], Combining pressure-area... 

ELL Ellipsometry [194, 195] Depolarization of reflected Thickness of adsorbed film... 

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

Additional mfomiation about the vibration ean be obtained tln-ough the depolarization ratio. This is the ratio of the intensity of seattered light that is polarized in a plane perpendieular to the ineident radiation relative to that the seattered light that is polarized parallel to the ineident polarization, p For totally synnnetrie... 

B1.3.2.5 THE MICROSCOPIC HYPERPOLARIZABILITY TENSOR, ORIENTATIONAL AVERAGING, THE KRAMERS-HEISENBERG EXPRESSION AND DEPOLARIZATION RATIOS... 

Thus one predicts that the depolarization ratio for excitation with natural light should be... 

In analogy with the depolarization ratio for linearly polarized light, the ratio of the two above quantities is known as the reversal coefficient, R(Q, given by... 

Tao T 1969 Time-dependent fluorescence depolarization and Brownian rotational diffusion coefficients of macromolecules Biopolymers 8 609-32... 

Rahman T S, Knox R S and Kenkre V M 1979 Theory of depolarization of fluorescence in molecular pairs Chem. Phys. 44 197-211... 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

The second ring vibration gives rise to a very weak infrared absorption band at 467 cm and to a weak and depolarized Raman line at 470 cm (202, 203) (Table 1-23). 

The frequencies classified in suites IX and X belong to depolarized Raman lines and correspond to vibrations-rotation bands of the C type. They can be assigned to oscillations of A" symmetry. 

In contrast to spectrophotometry, hght-scattering experiments are generally conducted at constant wavelength. Mercury vapor lamps are the most widely used light sources, since the strong lines at 436 and 546 nm are readily isolated by filters to allow monochromatic illumination. Polarizing filters are also included for both the incident and scattered beams so that depolarization can... 

Photomultipliers are used to measure the intensity of the scattered light. The output is compared to that of a second photocell located in the light trap which measures the intensity of the incident beam. In this way the ratio [J q is measured directly with built-in compensation for any variations in the source. When filters are used for measuring depolarization, their effect on the sensitivity of the photomultiplier and its output must also be considered. Instrument calibration can be accomplished using well-characterized polymer solutions, dispersions of colloidal silica, or opalescent glass as standards. 

The Raman spectrum can be used to give additional information regarding the symmetry properties of vibrations. This information derives from the measurement of the depolarization ratio p for each Raman band. The quantity p is a measure of the degree to which the polarization properties of the incident radiation may be changed after scattering... 

Taste-active chemicals react with receptors on the surface of sensory cells in the papillae causing electrical depolarization, ie, drop in the voltage across the sensory cell membrane. The collection of biochemical events that are involved in this process is called transduction (15,16). Not all the chemical steps involved in transduction are known however, it is clear that different transduction mechanisms are involved in different taste quaUties different transduction mechanisms exist for the same chemical in different species (15). Thus the specificity of chemosensory processes, ie, taste and smell, to different chemicals is caused by differences in the sensory cell membrane, the transduction mechanisms, and the central nervous system (14). 

Several aspects affect the extent and character of taste and smell. People differ considerably in sensitivity and appreciation of smell and taste, and there is lack of a common language to describe smell and taste experiences. A hereditary or genetic factor may cause a variation between individual reactions, eg, phenylthiourea causes a bitter taste sensation which may not be perceptible to certain people whose general abiUty to distinguish other tastes is not noticeably impaired (17). The variation of pH in saUva, which acts as a buffer and the charge carrier for the depolarization of the taste cell, may influence the perception of acidity differently in people (15,18). Enzymes in saUva can cause rapid chemical changes in basic food ingredients, such as proteins and carbohydrates, with variable effects on the individual. 

Lithium fluoride is an essential component of the fluorine cell electrolyte 1% LiF in the KF 2HF electrolyte improves the wettability of the carbon anodes and lowers the tendency of the cells to depolarize (18). Thermoluminescent radiation dosimeters used in personnel and environmental monitoring and in radiation therapy contain lithium fluoride powder, extmded ribbons, or rods (19). 

It is experimentally easy to generate Raman spectra using polarized light and to observe the partial depolarization of the spectra. Bands of totally symmetric vibrations are strongly polarized in Hquid or solution spectra. AH other bands in Hquid or solution are depolarized. Polarization effects are essential to elucidate stmctures, but are usuaHy ignored in most other appHcations. Details of the theory and experimental procedure can be found in the Hterature (15,16). 

Red mercuric oxide, identical chemically to the yellow form, is somewhat less reactive and more expensive to produce. An important use is ia the Ruben-MaHory dry cell, where it is mixed with graphite to act as a depolarizer (see Batteries). The overall cell reaction is as follows ... 

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