SOLVENT EFFECTS ON CHEMICAL PROCESSES .2. BINDING CONSTANTS OF METHYL-ORANGE WITH ALPHA-CYCLODEXTRIN IN BINARY AQUEOUS ORGANIC-SOLVENTS

The standard free energy change for complex formation is written as a sum of effects arising from solvent-solvent interactions (the general medium effect), solvent-solute interactions (the solvation effect), and solute-solute interactions (the intersolute effect). The general medium effect is given by g-DELTA-A (gamma-gamma(o)), where g is a curvature correction factor to the solvent surface tension gamma, DELTA-A is the change in surface area as the two solvent cavities containing the substrate (Methyl Orange) and ligand (alpha-cyclodextrin) collapse into a single cavity containing the complex, and gamma(o) is the value of surface tension at which there is no net solvophobic interaction; gamma is defined to be the value appropriate to the equilibrium mean solvation shell composition. The solvation effect is modeled by equilibrium stoichiometric formation of solvated species. All data are related to the fully aqueous system to give delta(M)DELTA-G-degrees, the solvent effect on the free energy change, as an explicit function of solvent composition. Complex stability data (obtained spectrophotometrically) on seven aqueous-organic cosolvent systems were fitted to this relationship to obtain estimates of g-DELTA-A and K1, the solvation exchange constant.