Kinetic and equilibrium studies of crystal violet–cyclodextrin inclusion complexes

Abstract

The equilibrium and temperature-jump ultraviolet–visible spectra of the cationic form of the dye crystal violet (CV) in the presence of γ-cyclodextrin (γCD) exhibit variations which are consistent with the formation of two distinct inclusion complexes: CV·γCD and (CV)₂·γCD. The formation equilibria are CV +γCD⇌CV·γCD fast (K) [graphic omitted] where K₁=(4.63 ± 0.07)× 10² dm³ mol^–1, K₂=(1.03 ± 0.09)× 10⁶ dm³ mol^–1, k₂=(1.73 ± 0.08)× 10⁹ dm³ mol^–1 s^–1 and k_–2=(1.68 ± 0.07)× 10³ s^–1 as derived from temperature-jump spectrophotometric data for 1.00 mol dm^–3 aqueous solutions of sodium chloride at pH 6.5 and 298.2 K. Equilibrium spectrophotometric data are also consistent with this equilibrium scheme. Under similar conditions K_d≈ 6 × 10² dm³ mol^–1 for the equilibrium [graphic omitted] and temperature-jump studies show that k_d≈ 9 × 10⁵ dm³ mol^–1 s^–1 and k_–d≈ 1.5 × 10³ s^–1. Thus the increase in stability of the crystal violet dimer in (CV)₂·γCD over that observed in the absence of γCD is a consequence of k₂≈ 10³k_d.

The high stability of the dimer inclusion complex (CV)₂·γCD compared with the monomer inclusion complex CV·γCD appears to be related to the annular radius of γCD. This, in conjunction with the observation that αCD forms no inclusion complex with CV and that βCD probably only forms CV·βCD, demonstrates a substantial degree of selectivity in the formation of cyclodextrin inclusion complexes consistent with the importance of dispersion forces in inclusion complex stability.