Diffusion process overview

Highly sophisticated pulse sequences have been developed for the extraction of the desired information from ID and multidimensional NMR spectra [172]. The same techniques can be used for high-resolution 1-NMR, s-NMR and NQR. Pulse experiments are commonly used for the measurement of relaxation times [173], for the study of diffusion processes [174] and for the investigation of chemical reactions [175]. Davies et al. [176] have described naming and proposed reporting of common NMR pulse sequences (IUPAC task group). An overview of pulse sequence experiments has been given [177],... 

ONP (Fig. 3) in organic molecular crystals can be described as a three-step process (1) optical creation of electronic spin polarization in (transient) excited triplet states (2) polarization transfer to nearby nuclear spins and (3) accumulation of bulk nuclear spin magnetization in the (diamagnetic) ground state via spin-diffusion processes. Highly detailed theoretical descriptions and modeling of ONP processes observed in many systems can be found in the reviews mentioned above thus, only a brief overview is provided here. 

The effects of molecular order on the gas transport mechanism in polymers are examined. Generally, orientation and crystallization of polymers improves the barrier properties of the material as a result of the increased packing efficiency of the polymer chains. Liquid crystal polymers (LCP) have a unique morphology with a high degree of molecular order. These relatively new materials have been found to exhibit excellent barrier properties. An overview of the solution and diffusion processes of small penetrants in oriented amorphous and semicrystalline polymers is followed by a closer examination of the transport properties of LCP s. 

After being washed (for total process overview cf. Figure 9.3.5) the beets are sliced into strips (cossettes), which afterwards are fed into the so-called diffusers where about 98% of the sucrose is extracted. High temperatures (65-70 °C) in the diffusion apparatus denature the plant cells and open the cell membranes for improved sucrose transfer. The entire extraction of the sucrose is carried out via a counter current flow of the extraction medium, i.e. water and the denatured cossettes. 

Heat is produced by chemical reaction in a reaction zone. The heat is transported, mainly by conduction and molecular diffusion, ahead of the reaction zone into a preheating zone in which the mixture is heated, that is, preconditioned for reaction. Since molecular diffusion is a relatively slow process, laminar flame propagation is slow. Table 3.1 gives an overview of laminar burning velocities of some of the most common hydrocarbons and hydrogen. 

Many drugs have been recognized to cross the intestinal epithelial cells via passive diffusion, thus their lipophilicity has been considered important. However, as described above, recent studies have demonstrated that a number of drug transporters including uptake and efflux systems determine the membrane transport process. In this chapter, we provide an overview of the basic characteristics of major drug transporters responsible not only for absorption but also for disposition and excretion in order to delineate the impact of drug transport proteins on pharmacokinetics. 

A perspective based on kinetics leads to a better understanding of the adsorption mechanism of both ionic and nonionic compounds. Boyd et al. (1947) stated that the ion exchange process is diffusion controlled and the reaction rate is limited by mass transfer phenomena that are either film diffusion (FD) or particle diffusion (PD) controlled. Sparks (1988) and Pignatello (1989) provide a comprehensive overview on this topic. 

Part IV is organized in die following way Chapter 18 gives an overview of transport phenomena in the environment by grouping them into just two categories directed transport and random processes. While directed transport (advection, transport under the influence of gravitation, etc.) will be treated in detail in Chapter 22, the discussion in Chapter 18 focuses on transport by randomness. We start with different kinds of diffusion phenomena, discuss Fick s laws, introduce the concept... 

Situations in which either Da or Pe are much larger or much smaller than 1 indicate that in the diffusion-advection-reaction equation some of these processes are dominant while others can be disregarded. Figure 22.3 gives an overview of such cases. A first distinction is made according to the size of Da ... 

Fig. 5.1 Overview on the influence of applied potential E and diffusion or transportation processes at the electrode...

Four appendixes are included. Appendix A contains standard materials selection used by many refiners and contractors in petroleum processing equipment. Appendix B contains a rules of thumb overview of refinery materials of construction. Appendix C contains background information on hydrogen diffusion through vessel walls, and Appendix D contains a standard specification for steel line pipe. 

The next section gives a brief overview of the main computational techniques currently applied to catalytic problems. These techniques include ab initio electronic structure calculations, (ab initio) molecular dynamics, and Monte Carlo methods. The next three sections are devoted to particular applications of these techniques to catalytic and electrocatalytic issues. We focus on the interaction of CO and hydrogen with metal and alloy surfaces, both from quantum-chemical and statistical-mechanical points of view, as these processes play an important role in fuel-cell catalysis. We also demonstrate the role of the solvent in electrocatalytic bondbreaking reactions, using molecular dynamics simulations as well as extensive electronic structure and ab initio molecular dynamics calculations. Monte Carlo simulations illustrate the importance of lateral interactions, mixing, and surface diffusion in obtaining a correct kinetic description of catalytic processes. Finally, we summarize the main conclusions and give an outlook of the role of computational chemistry in catalysis and electrocatalysis. 

Today, spin diffusion is often used in a quite general way to describe multispin polarization-transfer processes, whether or not the process can actually be described by a diffusion equation. In this overview, we will interpret spin diffusion in this broad sense. If applied in the context of two-dimensional homonuclear experiments, it becomes synonymous with total through-space correlation spectroscopy (TOSSY) [2], the dipolar equivalent of the liquid-state TOCSY [3] experiment. 

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