Solvent design criteria

These precursor delivery methods have been described in more detail in other texts and are not discussed in further detail here.14 The consequence of the availability of these methods is that the problems of thermal decomposition, low volatility, and prolonged heating of a precursor in a bubbler can be circumvented to some extent. However, this imposes another design criterion on the precursor, which is that it must have significant solubility in a solvent that does not play a detrimental role in film deposition. 

Another aspect of cost reduction would be solvent economy. The need to preferentially select inexpensive solvents and employ the minimum amount of solvent per analysis would be the third performance criteria. Finally, to conserve sample and to have the capability of determining trace contaminants, the fourth criterion would be that the combination of column and detector should provide the maximum possible mass sensitivity and, thus, the minimum amount of sample. The performance criteria are summarized in Table 1. Certain operating limits are inherent in any analytical instrument and these limits will vary with the purpose for which the instrument was designed. For example, the preparative chromatograph will have very different operating characteristics from those of the analytical chromatograph. 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

Summarising, to optimise the partition coefficient P of a solute i, P, should be maximised by mixing three solvents in the correct proportions. The use of mixture design statistical techniques with the natural logarithm of the partition coefficient as response criterion is a valid way to achieve this. 

The second point is that solvents, by design, should be relatively inert substances. This means that they are not easily broken down by the body or in nature. Lastly, the criterion of easy removal means that most organic solvents are volatile organic compounds (VOCs). Thus, they have low vapor pressures and are easily inhaled. 

Convenient rates for most glycosides are only obtained with approximately molar concentrations of acid even at moderately elevated temperatures (50-100 °C). Much early work was focused on trying to determine whether solvent was involved as a nucleophile, using as a criterion the dependence of rate on h, a parameter designed to measure the ability of the solution to protonate a neutral base. " In dilute aqueous solution, it was identical with... 

As discnssed immediately above, an important criterion for solvent selection is solubility. The solvent mnst accommodate a reasonable amonnt of the species to be removed from the gas while still maintaining selectivity between the solnte(s) and carrier gas. When the carrier is air or an inert gas, it is easy to designate solnte vs. carrier in other cases, it is a matter of relative solnbility. Other criteria for solvent selection inclnde the following ... 

In the high polymer domain there remains, therefore, only the direct method of using semi-permeable membranes, which has actually been developed successfully by Herzog and Spurlin and a number of observers in recent years. In this way we may use either the static elevation methody in which equilibrium between osmotic and hydrostatic pressure serves as the criterion, or a dynamic method which was first designed by Van Campen and later developed by C. G. Boissonnas, Obogi and Broda, and K. H. Meyer. In the latter the osmotic pressure, which causes a streaming of the pure solvent through the semi-permeable membrane is compensated by a suitable counter-pressure in such a way that the motion of the liquid practically ceases. 

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