Spectroscopic techniques, hydrodynamic

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

A spectroscopic technique that probes membrane fluidity can either directly measure mobility and order parameters for membrane constituents (NMR) or use probes (ESR, fluorescence). Some fluorescent and ESR probes are shown in Fig. 4. The connection between the rotational correlation time of a membrane embedded probe and the membrane fluidity can be illustrated using the example of a simple isotropic liquid, in which fluidity is merely a reciprocal viscosity ri and the rotational correlation time Xc for a molecule with a hydrodynamic volume V is given by the well-known Debye-Stokes-Einstein relation Xc = r VlkT, where k is the Boltzmann constant and T is the... 

An armoury of powerful electrochemical methods is available. Potential step techniques such as differential pulse DP or square-wave SW voltammetry offer advantages in sensitivity and resolution. Hydrodynamic techniques involving use of rotating disc or rotating ring-disc electrodes allow the chemical steps of the electrode process to be separated from mass transport. Electrochemical transformations may be monitored optically with spectroelectrochemical methods. Even the electrode interface itself is amenable to study by in situ spectroscopic techniques. Detailed descriptions of these methods are to be found in appropriate texts [1-4]. 

The kinetic interactions between solvent and solute molecules in free solution determine their rotational and translational diffusion characteristics. Fluorescence polarization is a spectroscopic technique that allows the determination of motional preferences of reporter molecules in fluids with respect to both the rate of motion and the orientational restriction of that motion [1,2], For spherical molecules in isotropic fluids at low concentrations, these motions can be described by the Stokes-Einstein and Perrin relationships, and if these motions have an equal probability of occurring in any dimension they are referred to as isotropic. However, when a fluid displays structure, or anisotropy, the motion of diffusing molecules may be restricted, generally to different extents in different dimensions, and these motions are said to be anisotropic. New approaches must then be taken in order to describe the probe s hydrodynamic behavior. By measuring the hydrodynamic properties of a fluorescent probe in solution, it is possible to extract valuable information on the physical structure and properties of a fluid. Knowledge of the physical structure and properties of food fluids and matrices is essential for solving practical problems in food research. 

The physics of molecular liquids has been described through many different approaches and models, reported widely in the literature. The general framework of the liquid physics can be found in the following references [50-53], which includes the more recent developments on complex liquids. The basic hydrodynamic point of view of the liquid phase can be found in [54,55]. Finally the fundamental spectroscopic techniques devoted to the liquid investigations are reported in [42,56,57]. 

Several attempts were undertaken to correlate the volume changes observed by direct densimetry with the results of other physical techniques for example, hydrodynamic methods (viscometry and analytical ultfacentrifugation), spectroscopic techniques (UV absorption, fluorescence, circular dichroism), scattering techniques (light and small-angle scattering), measurement of sonic speed. 

With the development of new instrumental techniques, much new information on the size and shape of aqueous micelles has become available. The inceptive description of the micelle as a spherical agglomerate of 20-100 monomers, 12-30 in radius (JJ, with a liquid hydrocarbon interior, has been considerably refined in recent years by spectroscopic (e.g. nmr, fluorescence decay, quasielastic light-scattering), hydrodynamic (e.g. viscometry, centrifugation) and classical light-scattering and osmometry studies. From these investigations have developed plausible descriptions of the thermodynamic and kinetic states of micellar micro-environments, as well as an appreciation of the plurality of micelle size and shape. 

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

Before discussing details of their model and others, it is useful to review the two main techniques used to infer the characteristics of chain conformation in unordered polypeptides. One line of evidence came from hydrodynamic experiments—viscosity and sedimentation—from which a statistical end-to-end distance could be estimated and compared with values derived from calculations on polymer chain models (Flory, 1969). The second is based on spectroscopic experiments, in particular CD spectroscopy, from which information is obtained about the local chain conformation rather than global properties such as those derived from hydrodynamics. It is entirely possible for a polypeptide chain to adopt some particular local structure while retaining characteristics of random coils derived from hydrodynamic measurements this was pointed out by Krimm and Tiffany (1974). In support of their proposal, Tiffany and Krimm noted the following points ... 

This whole volume shows the advantages of combining electrochemistry and spectroscopy. In certain cases, it is valuable to link the electrode and the spectroscopic detection by a controlled hydrodynamic system. Concentration patterns can then be calculated and the spectroscopic signal can be quantitatively interpreted to yield kinetic data for the reaction mechanism. An example of this technique is our work [14, 22, 23] and that of Waller and Compton [24] using channel electrodes in an ESR spectrometer. 

One of the advantages of flow-through electrodes over other types of hydrodynamic electrodes is that they involve no moving parts. They are therefore ideal for use in conjunction with both spectroscopic and microscopic techniques. Whilst both channel and tubular electrodes can be used in... 

The helix-coil transition can be demonstrated by polarization of fluorescence techniques, and the results may be compared with spectroscopic measurements to correlate the change in the hydrodynamic properties of the molecule with the change of its conformational structure. It is clear that the hydrodynamic and conformational changes do not necessarily parallel one another. Ion adsorption, breaks in the helix, and changes in helical type can occur without being reflected in the optical rotation parameters. 

Other techniques for studying protein molecnles in solntion are less infln-enced by these microscopic effects. Square-wave voltammetry is widely used due to its great sensitivity, and even a low density of productive sites on the electrode may give rise to a sharp and analyzable response [28]. The electrode may also be rotated to achieve forced convection and hydrodynamic control of solution redox species, while amperometric (and coulometric) measurements—where the current (or charge) is recorded following a potential step—enable the time and potential domains to be deconvoluted [28,29]. These options complement each other to provide a detailed picture of the thermodynamics and kinetics of redox processes. Finally, bulk electrolytic methods enable samples of a particular redox state to be prepared quantitatively for spectroscopic examination, at precise electrode potentials that may lie outside the range of conventional chemical titrants. 

Many of the physical properties of polynucleotides are changed upon alteration of their structure this allows for the application of a wide range of techniques in such studies. Hydrodynamic properties such as viscosity, sedimentation coefficients, and buoyant density can all be readily monitored. Light and low-angle X-ray scattering are also well understood. Changes in spectroscopic properties are also conveniently studied. These have all been summarized in Physical Chemistry of Nucleic Acids [28], and the major effects which are discussed in this book are briefly reviewed below. 

The confrontation of this elegant prediction by experiment has taken many years to complete because all experimental techniques available at that time did not have the proper spatial resolution. Spectroscopic methods such as EPR, NMR, IR are only sensitive to the fraction of monomers attached to the wall. Ellipsometry measures the first moment of the monomer distribution and vields an average length called the ellipsometric thickness Hydrodynamic methods, based on flow restric-tion in capillaries covered by the adsorbed layer, yield another average length related to the maximum extension of the adsorbed layer To study concentration profiles at the solid-liquid interface, one has thus to devise special tools. 

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