Structural factors affecting migration

A variety of factors affect the horizontal and vertical migration of PAHs, including contaminant volume and viscosity, temperature, land contour, plant cover, and soil composition (Morgan Watkinson, 1989)- Vertical movement occurs as a multiphase flow that will be controlled by soil chemistry and structure, pore size, and water content. For example, non-reactive small molecules (i.e., not PAHs) penetrate very rapidly through dry soils and migration is faster in clays than in loams due to the increased porosity of the clays. Once intercalated, however, sorbed PAHs are essentially immobilized. Mobility of oily hydrophobic substances can potentially be enhanced by the biosurfactant-production capability of bacteria (Zajic et al., 1974) but clear demonstrations of this effect are rare. This is discussed below in more detail (see Section 5 5). 

Tlie aim of this chapter is to provide an overview of materials where fast transport of alkali metal cations and protons is observed. A general discussion of factors affecting conductivity and techniques used to study ion migration paths is followed by a review of the large number of cation conductors. Materials with large alkali ions (Na-Cs) are often isostructural and therefore examined as a group. Tire lithium conductors with unique crystal structure types and proton conductors with unique conduction mechanisms are also discussed. 

Different di- and trihydroxy bile salts and drugs that differ in lipophilicity, basicity, and structure have been compared in order to examine solvatochromic equilibria. The principle exploited in the determination of thermodynamic equilibrium constants is the indirect measurement of the capacity factor affected by the tenside concentration (in this case, the bile acid concentration). A pronounced shift in the migration times and thus effective ionic mobility... 

A Lewis acid e.g. aluminium tert-butoxide, boron trifluoride, neutral alumina) is considered to coordinate with both the 17-OH and 20-keto functions, bringing them into a cfs-relationship and effectively locking the side chain in one conformation. Product formation is then determined by the relative ease of migration of the Cps)--C(i ) and C(i6>-C(i7) bonds towards the electron-deficient C<20). The structures of the resulting ketones show that the C i3)-C(i ) bond migrates in the i7j -hydroxy compound (i), and the C(i6)-"C(i7) bond in the i7a-hydroxy isomer (2). The reason for this difference has been the subject of much speculation, and is still not clear. The factors which have been considered [202] as affecting the stability of respective transition states include ... 

The ways in which reaction parameters affect a two phase batch reaction are similar to those considered above for the three phase systems. Since there is no gas phase, agitation only serves to keep the catalyst suspended making it more accessible to the dissolved reactants so it only has a secondary effect on mass transfer processes. Substrate concentration and catalyst quantity are the two most important reaction variables in such reactions since both have an influence on the rate of migration of the reactants through the liquid/solid interface. Also of significant importance are the factors involved in minimizing pore diffusion factors the size of the catalyst particles and their pore structure. 

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