Solvent properties, desirable solute selectivity

The chemical properties of solvents have obviously a strong bearing on their applicability for various purposes. The solvents function by selectively dissolving desired solutes, by remaining inactive in the chemical reactions undergone by the solutes, and by solvating (selectively), reactants, transition-state intermediates, and products (Marcus, 1998a). 

The chemical properties of solvents have obviously a strong bearing on their applicability for various purposes. The solvents should selectively dissolve the desired solutes and not some others, they should be inactive in the chemical reactions undergone by the solutes, but solvate, again selectively, reactants, transition states, intermediates, and products. These aspects of the behaviour can be achieved by the proper blend of the chemical properties of structuredness, polarity, electron-pair and hydrogen bond donation and acceptance ability, softness, acidity and basicity, hydrophilicity or hydrophobicity, and redox properties, among others. Such chemical characteristics can often be derived from physical properties, but in other cases must be obtained from chemical interactions, for instance by the use of chemical probes ( indicators ). 

Chemical properties of solvents affect their usefulness in various applications. The solvent should selectively dissolve the desired solutes, should be inactive/inert in the chemical reactions, and solvate the transition states and products really well. This can be achieved by the proper blend of chemical properties such as solvency, polarity, hydrogen bond donation or acceptance ability, acidity or basicity, hydrophilicity, and redox properties. 

For a supercritical fluid to be suitable as a solvent in extraction, a high solubility of the solute is required, tf the objective is to separate components, the solvent should also have selective dissolution properties. Moreover, the pressure effect on the solubility is a factor. High compression costs may be incurred if the conditions for desirable solubility require excessively elevated pressures. The critical temperature of a potential solvent is also important. If the solvent is to be around its critical point for optimal performance, it is preferred that its critical temperature not be too far from ambient temperature. 

During the extraction of otganic species, it may be desirable to modify the solvent. An inert paraffinic compound or mixture may be blended with a suitable modifier (e.g., a species that hydrogen bonds) to enhance the solvent properties. Such properties might include viscosity, density, surface tension, or attraction for the solute. In these cases, the mutual solubility curve may appear as in Fig. 7.2-4 when the solvent mixture is plotted at one vertex. Reasons for solvent blending may include improved solvent selectivity, interiacial tension, reduced solvent phase viscosity, and increased density differences between the two phases. A solvent that forms stable etnulstotte when mixed with the diluent phase, for example, may be suitable for use when it is modified with a suitable inert paraffinic material. 

Mixture property Define the model to be used for liquid activity coefficient calculation, specify the binary mixture (composition, temperature, pressure), select the solute to be extracted, the type of phase equilibrium calculation (VLE or LLE) and finally, specify desired solvent performance related properties (solvent power, selectivity, etc.)... 

Theta solvents. Selection of a poor solvent for a polymer is desirable when making solution property measurements because it permits the use of higher concentrations and minimizes the effects of nonideality. The most suitable choice is a theta solvent (73). Table 12 lists the theta solvents and the corresponding theta temperatures which have been found for PTHF. 

Because of cost factors, solvent extraction applied to large scale hydrometallurgical processes, such as the recovery of copper from acidic ore leach solutions, is carried out with the most selective reagent for e.g., copper versus iron, which is not itself a liquid solvent, in a petroleum diluent that confers on the mixture the desired physical properties. For the particular case of copper recovery, commercial hydroxyoxime reagents have been used on a very large scale, but their discussion is outside the scope of this book. 

Interest in the use of SC solvents as a reaction media is founded upon recent advances in our understanding of their unique thermo-physical and chemical properties. Worthy of special note are those thermophysical properties (6) which can be manipulated as parameters to selectively direct the progress of desirable chemical reactions. These properties include the solvent s dielectric constant (7), ion product (8,9), electrolyte solvent power (10,11), transport properties"[viscosity (12), diffusion coefficients (13) and ion mobilities (14)], hydrogen bonding characteristics (15), and solute-solvent "enhancement factors" (6). All these properties are strongly influenced by the solvent s density P in the supercritical state. 

Biochemical substances usually are present in only small concentrations in the source materials and to be prepared in usable form, they must be concentrated and also separated from a variety of other ingredients. The properties of these substances make them amenable to purification by adsorption processes, but the exact procedure to be employed varies from one case to another. In a few cases, it is possible to selectively adsorb the desired biochemical substance and leave most of the impurities in solution. The carbon cake is then eluted with a solvent that preferentially extracts the desired compound. Generally, the procedure is much more involved as it is seldom possible to obtain an adsorbent that will strongly adsorb the desired substance and simultaneously exclude the many other compounds present in the source materials. 

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