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(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.