Complexation of Methyl Orange and Tropaeolin 000 No. 2 by β-cyclodextrin dimers

Abstract

Spectrophotometric studies of the complexation of Methyl Orange (MO-) and Tropaeolin 000 No. 2 (TR-) anions by dimeric N,N′-bis(6A-deoxy-6A-β-cyclodextrin)urea (βCD)2ur and its oxalamide and succinamide analogues, (βCD)2ox and (βCD)2su, respectively, are consistent with the predominant formation of complexes of the general formulae (βCD)2x·MO- characterized by stability constants K1 = (1.05 ± 0.04) × 105, (1.92 ± 0.04) × 105 and (2.50 ± 0.02) × 104 dm3 mol-1 and (βCD)2x·TR- characterized by K1 = (1.39 ± 0.03) × 104, (7.4 ± 0.1) × 103 and (4.60 ± 0.05) × 103 dm3 mol-1, in aqueous phosphate buffer at pH 9.0 and 5.5 and 298.2 K. These values are significantly greater than K1 = 2160 and 710 dm3 mol-1 for the β-cyclodextrin complexes, βCD·MO- and βCD·TR- and are indicative of cooperative binding in (βCD)2x·MO- and (βCD)2x·TR-. The factors affecting complex stability are discussed and comparisons are made with related systems.