Solvent effects on chemical processes. 9. Energetic contributions to the complexation of 4-nitroaniline with α-cyclodextrin in water and in binary aqueous-organic solvents

Abstract

Abstract The phenomenological theory of solvent effects partitions the free energy change of a process into contributions from solvent-solvent interactions (the general medium or solvophobic effect), solvent-solute interactions (the solvation effect), and solute-solute interactions (the intersolute or intrasolute effect). This approach is applied to complex formation between α-cyclodextrin and 4-nitroaniline in water and many aqueous-organic solvent mixtures; the organic solvents were methanol, ethanol, 2-propanol, ethylene glycol, dimethylsulfoxide, acetonitrile, and acetone. Complex stability constants were measured spectrophotometrically. The parameters of the phenomenological model were extracted by nonlinear regression or by graphical analysis. It is observed that there are two classes of cosolvents, distinguished by their polarity. If the pure solvent has log P < −0.3 (where P is the 1-octanol/water partition coefficient), the system behaves “normally,” with parameter values typical of nonelectrolyt...