In this thesis, thermodynamic investigation of salt effect in aqueous systems based on ionic liquids (ILs) – amino acid and solid–liquid equilibria measurements of aqueous carboxylic acid solutions and carboxylic acid – carboxylic acid binary systems were systematically studied in two sections. In the first section, in order to achieve further experimental evidence for better understanding of the molecular mechanisms responsible for the soluting-out phenomena in the IL – amino acid aqueous systems, systematic studies on the vapor−liquid, liquid−liquid and solid-liquid equilibrium behavior of aqueous solutions of several ILs were carried out in the presence of a range of amino acids. All the investigated ternary IL – amino acid aqueous systems show the negative deviations from the semi–ideal behavior and therefore the soluting-out effects have been seen in these systems. In the case of [C4mim][CF3SO3] –amino acids aqueous systems in which the IL is more hydrophobic than the investigated amino acids, the IL is soluted-out by the amino acids and in these systems the soluting-out effect appears by aqueous biphasic systems (ABS) formation (liquid-liquid equilibria). For these systems, the liquid-liquid equilibria properties such as the binodal curves and tie-lines were measured at 298.15 K. The results indicate that the ability of the amino acids to form ABS with [C4mim][CF3SO3] increases by decreasing temperature and follows the order: serine ≈ glycine > alanine > proline. Finally, the partition coefficients of caffeine, selected as a model biomolecule, were measured in the studied ABS and the obtained results show that its partition coefficient increased by increasing the tie line length. However, for the [C4mim]Cl and [C6mim]Cl containing systems, in which the amino acids are more hydrophobic than the ILs, the amino acids are soluted-out by the ILs and in these systems the soluting-out effect appears by precipitation of the amino acids from the solution (solid-liquid
