The inclusion of pyronine Y by β- and γ-cyclodextrin. A Kinetic and equilibrium study

Abstract

Temperature-jump spectrophotometric studies show the dye pyronine Y (PY) in its monovalent cationic form to be included by γ-cyclodextrin (γCD) in a fast step to form a 1:1 complex (PY·γCD, which is followed by the slower formation of the 2:1 complex (PY)₂·γCD, in which pyronine Y dimerizes. There is also evidence for the formation of a 2:2 complex (PY)₂·(γCD)₂ in a third and fast step. The temperature-jump relaxation data obtained at 298.2 K from 1.0 mol dm^–3 NaCl aqueous solutions at pH 6.1 yielded the apparent stability constants K₁=(1.0±0.1)× 10³ dm³ mol^–1, K₂=(1.2±1.1)× 10⁵ dm³ mol^–1 and K₃= 52±22 dm³ mol^–1 for these complexes, and the rate constants k₂=(1.7±1.0)× 10)× 10⁹ dm³ mol^–1s^–1 and k_–2=(1.4±0.5)× 10⁴s^–1 for the second complex. In the presence of βCD the 1:1 PY·βCD and 1:2 PY ·(βCD)₂ complexes are formed, but no complexation of PY by αCD was detected.