Colloidal systems spectroscopy

In order to achieve a true comparison between both catalytic systems, colloidal and molecular, which display very different reaction rates, a series of experiments were carried out with the homogeneous molecular system, decreasing the catalyst concentration in the studied allylic alkylation reaction. The reaction evolution is monitored taking samples at different reaction times and analysing each of them by NMR spectroscopy (to determine the conversion) and HPLC chromatography with chiral column (to determine the enantioselectivity of I and II). For molecular catalyst systems, the Pd/substrate ratio was varied between 1/100 and 1/10,000. For the latter ratio, the initial reaction rate was found comparable to that of the colloidal system (Figure 2a), but interestingly the conversion of the substrate is quasi complete after ca. 100 h in... 

Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-a-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecular-level probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. 

This chapter reviews the wide range of colloidal systems amenable to investigation by FT - IR spectroscopy. Molecular level information about die interactions of amphiphilic substances in aggregates such as micelles, bilayers, and gels can be obtained and related to the appearance and stability of the various phases exhibited. The interactions of polymers, surfactants and proteins with interfaces, which substantially modify the solid - liquid or liquid - air interface in many important industrial and natural processes, can also be monitored using FT - IR. 

Y. Feldman, T. Skodvin, and J. Sjoblom, Dielectric spectroscopy on emulsion and related colloidal systems—A review, in Encyclopedic Handbook of Emulsion Technology, Marcel Dekker, New York, 2001, pp. 109-168. 

Dielectric Spectroscopy on Emulsion and Related Colloidal Systems—A Review... 

In the following we combine dielectric spectroscopy and colloidal systems. We belive this to be a fiiritful eombina-tion of two highly important and current topies. Dieleetrie measurements have roots from over a hundred years ago in the belinning of eourse only simple eapacitanee measurements were made. However, the findings made especially... 

Dielectric measurements have developed Ifom cumbersome Wheatstone-bridge measurements to an efficient, precise, and rapid spectroscopic technique. The new technique dielectric spectroscopy soon found interesting applications within the field of eolloid chemistry. The technique can be applied as a preeision method in a thorough mapping of the static and dynamic properties of colloidal systems. Industrially, dielectric measurements ean be utilized in the online eharaeterization of sueh eomplex systems. 

In the writing of this review we have not sought to cover every aspect of the dielectric properties of colloidal systems. Our aim has rather been to demonstrate die usefidness of dielectric spectroscopy for such systems, using die application to selected systems as illustrations. 

The electro-optical responses of colloidal systems were detected by reflection spectroscopy and by the variations of scattered light intensity. Induced variations in the reflection spectra of colloidal crystals turned out to be very sensitive to electroacoustic effects, while the experimental setup of the second method permits direct comparison with numerous data on anisometric colloids. Investigations into reflection spectroscopy were carried out by Okubo and coworkers at Gifu University (Japan), and the light scattering studies by Stoimenova, Okubo, and coworkers at the Bulgarian Academy of Sciences. The essential parts of the results are due to our joint research. 

Unlike most other characterization techniques introduced in this chapter that only allow conclusions to be drawn on the bulk level, self-diffusion NMR gives detailed information on colloidal systems by focussing on the molecular level. This complex technique will therefore be introduced in detail. NMR spectroscopy, based on physical properties of the molecular spin, is a very powerful method for the measurement of self-diffusion of small molecules in complex solution [67] with direct insight into general aspects of the solution structure [68]. 

At first, an overview of the experimental methods which are suitable to characterize the CNT aggregation state in general, and thus are suitable to monitor CNT debundling in aqueous medium and in presence of surfactant, will be reviewed. Three main streams will be presented (i) "direct imaging of the CNT dispersions by m icroscopic techniques (ii] spectroscopic techniques, such as Raman spectroscopy, which exploit the difference in electronic properties between bundled and individualized CNTs and (iii] depolarized dynamic light scattering which is commonly used to characterize colloidal systems. 

In any given material, the relaxation modulus will reflect the response of the material on different timescales. To make a measurement, materials are deformed under a periodic load with frequency w. Then, G and G are measured across a wide range of frequencies (typically three to four decades). Measurements of G and G" can be used to characterize the mechanical properties of soft materials, including polymer networks and colloidal systems. The technique is also known as mechanical spectroscopy. In a viscoelastic material, the elastic modulus will cross over the viscous modulus at the transition point from viscous to elastic bulk behavior and indicates a possible sol-gel transition or the onset of rubbery behavior in a polymer network. 

A second ensemble technique is DLS (Nicoli et al, 1991) - also called quasi-elastic light scattering (QELS) or photon correlation spectroscopy (PCS) — which is based on analysis of the temporal fluctuations in the scattered intensity caused by Brownian motion, or diffusion, of the particles. In recent years, it has become a popular technique for characterizing many submicron colloidal systems, including polymer lattices, because of its large size range (roughly 1 nm to 5 p,m) and approximate independence from optical properties. 

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