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

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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 nonelectrolyte solutes, whereas if log P > −0.3, unusual parameter values, attributed to strong interaction between the organic solvent and α-cyclodextrin, are observed. The α-cyclodextrin cavity is inferred to have a polarity corresponding to log P = −0.3. The energetic contributions to complex stability in water suggest that essentially all of the stability is derived from the general medium (hydrophobic) effect. Addition of organic cosolvents weakens the complex through both general medium and solvation effects.

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