LAUSR

Abstract The driving force involved in the partition of benzocaine (BZ) and procaine (PR) in macromolecule + electrolyte + water aqueous biphasic systems (ABSs) was investigated. Enthalpy changes using isothermal titration calorimetry and the partition coefficients (K) of both solutes were studied as function of the pH, macromolecule hydrophobicity, and tie-line length of the ABS. The BZ and PR molecules presented distinct partition behavior, and the KBZ and KPR values in poly(ethylene oxide)11–poly(propylene oxide)17–poly(ethylene oxide)11 ABS were as high as 5.14 × 103 and 5.12, respectively. The hydrophobic interactions between BZ and the components of the macromolecule-rich phase (MRP) contributed to the more negative standard enthalpy change of transfer, Δ tr H θ , values of this solute, which ranged from −38.8 to −24.6 kJ mol−1 while the Δ tr H θ values of PR ranged from −10.5 to −8.33 kJ mol−1. At lower pH values, KBZ and KPR decreased from 2.37 × 103 and 354, respectively, at pH = 12.0, to 1.01 × 103 and 2.98, respectively, at pH = 1.0, owing to the increase in electrostatic repulsive interactions between solutes in ionized form and the positive charge species that formed in the MRP. Hydrophobic forces were the main contributors to the partition of the BZ and PR molecules.