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

Abstract

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.