Solvent properties, desirable density difference

Some of these criteria are crucial, while others are desirable properties improving separation and/or making it more economical. Solvent selectivity, recoverability, and a large density difference in respect to the raffinate are essential. Some of the requirements on the solvent can be in conflict, and thus a compromise may be necessary. 

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. 

In this context the ILs can be called as "designer solvents" [30] because their physical properties (such as melting point, viscosity, density and hydrophobicity) can be modified according to the nature of the desired application by modification of their cations and anions. The range of available anion and cation combinations could provide up to 10 different ILs. 

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