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

Spectrophotometric studies of the complexation of Methyl Orange (MO - ) and Tropaeolin 000 No. 2 (TR - ) anions by dimeric N,N′-bis(6 A -deoxy-6 A -β-cyclodextrin)urea (βCD) 2 ur and its oxalamide and succinamide analogues, (βCD) 2 ox and (βCD) 2 su, respectively, are consistent with the predominant formation of complexes of the general formulae (βCD) 2 x·MO - characterized by stability constants K 1 =(1.05±0.04)×10 5 , (1.92±0.04)×10 5 and (2.50±0.02)×10 4 dm3 mol -1 and (βCD) 2 x·TR - characterized by K 1 =(1.39±0.03)×10 4 , (7.4±0.1)×10 3 and (4.60±0.05)×10 3 dm3 mol -1 , in aqueous phosphate buffer at pH 9.0 and 5.5 and 298.2 K. These values are significantly greater than K 1 =2160 and 710 dm3 mol -1 for the β-cyclodextrin complexes, βCD·MO - and βCD·TR - and are indicative of cooperative binding in (βCD) 2 x·MO - and (βCD) 2 x·TR - . The factors affecting complex stability are discussed and comparisons are made with related systems.