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ABSTRACT
The aim of this study was to develop models for rigorous analysis of phase solubility diagrams, particularly the descending portion, in order to obtain individual thermodynamic complex formation and solubility product constants. Additionally, the effect of varying the initial solute concentration St in excess of the optimal solubility Sm, on the general shape of the plateau and descending portions of the solubility diagram was investigated. The solubilities of propylpapraben (Seq) were measured against initial β-cyclodextrin concentrations (Lt) at different temperatures and different St Equilibrium concentrations (Leq) were also measured. The only effect observed was a broadening of the plateau region with increase in St with no discernible effect on Sm. Simultaneous rigorous analysis of the rising as well as the descending portions of the phase diagram could only be interpreted in terms of formation of two soluble complexes: SL and S2L. Rigorous analysis of the descending portion allowed the determination of individual formation constants K11 (SL) and K21 (S2L) in addition to the solubility product of the less soluble complex Ks11 (SL). Thermodynamic analysis of the individual solubility of propylparaben and β-cyclodextrin was carried out in water at different temperatures. In aqueous β-cyclodextrin solutions, however, the solubility of propylparaben is enhanced due to the formation of both SL (δG°11 = -26.4 kJ/mol) and S2L (δG°21 = -46.4 kJ/mol) soluble complexes. Formation of the SL-complex is favored both by enthalpy (δH°11 = -20.7 kJ/mol) and a slight increase in entropy (δS°11 = 18.6 J/mol · K). Formation of S2L from SL apparently involves stronger solute-SL binding (δH°21 = -45.9 kJ/mol) yet is retarded by a net decrease in entropy (δS°21 = -86.3 J/mol · K). The solubility of the SL-complex is highly endothermic (δH°S11 = 55.8 kJ/mol), and although is accompanied by much randomization (δS°S11 = 101.8 J/mol · K), its solubility remains quite low (δG°S11 = 25.5 kJ/mol). Molecular mechanical modeling of propylparaben/β-cyclodextrin interactions in water revealed that the S2L-complex formation is energetically more favored.