Cyclodextrin and Termethylated Cyclodextrin Complexation of Aromatic Carboxylic Acids and Their Conjugate Bases in Aqueous Solution: the Effect of Size, Hydrophobicity and Charge

For α- cyclodextrin ( αCD ), the complexation constants (K) for the formation of binary host-guest complexes ( HA.αCD ) are 750�60, 1070�60, 27�3 and 17�4 dm3 mol-1 when the guests (HA) are benzoic, 4-methylbenzoic and (R)- and (S)-2-phenylpropanoic acids, respectively, as determined by potentiometric titration in aqueous solution at 298.2 K and I = 0.10 mol dm-3 ( KCl ). For the analogous hexakis (2,3,6-tri-O-methyl)-α- cyclodextrin complexes ( HA.TMαCD ), K = 1580�150, 2890�130, 220�10 and 207�8 dm3 mol-1, and for the heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin complexes ( HA.TMαCD ), K = 200�20, 340�30, 129�5 and 170�10 dm3 mol-1. The binary complexes formed by the corresponding carboxylates (A-) are much less stable. Ternary host-guest α- cyclodextrin complexes (HA.αCD2) are also formed. These data, together with literature data for β- cyclodextrin, are discussed in terms of the factors influencing complexation.