EXP 11 Article

Effect of β-Cyclodextrin on Bleaching of Triarylmethane Dye

Authors and Publication

• Pierre-Henri d’Hausen, Christopher N. Tait, D. Martin Davies

• Division of Chemistry, School of Applied and Molecular Sciences, University of Northumbria at Newcastle, UK

• Received: 26th October 2001; Accepted: 7th December 2001; First Published: 17th January 2002

Introduction

• Background: Research focused on oxidation remediation of polluted waters using β-cyclodextrins.

• Purpose: To explore how complexation of Green S by cyclodextrin influences its reactivity with hydrogen peroxide and alkali.

• Dye Characteristics: Green S is a triarylmethane dye, relevant for understanding oxidative reactions in environmental remediation.

• Cyclodextrin Description: β-Cyclodextrin is an oligosaccharide formed from starch, with seven glucopyranose units and a conical shape beneficial for complexation.

Key Findings

• Hydrogen Peroxide Bleaching:

  • Initial acceleration in bleaching followed by a slowdown with increasing cyclodextrin concentration.

  • Two reaction pathways involved: complexation with one and two cyclodextrin molecules.

  • Kinetic data indicates critical involvement of cyclodextrin in altering reaction rates.

• Alkali Bleaching:

  • Reaction also involves nucleophilic attack at the central carbon.

  • Showed acceleration by cyclodextrin but no observable maxima or minima in reaction rates.

  • Suggests stabilization by the second cyclodextrin molecule to the dye and bleaching product.

Mechanistic Insights

• Nucleophilic Attack: Both reactions show nucleophilic attack as central to their mechanisms, influenced by the presence of nearby hydroxyl groups on the dye.

• Complex Formation: Insufficient binding constant characterization of the cyclodextrin-dye complexes led to a method employing curve fitting based on presumed binding constants.

Transition State Theory Application

• Adoption of a pseudo-equilibrium constant approach allowed interpretation of kinetic data for both bleaching paths.

  • Kinetic Definitions: Transition states are stabilized differently depending on whether one or two molecules of cyclodextrin are binding.

• Field Effects: The study discusses how cyclodextrin affects both the reactivity of Green S and the stability of reaction intermediates.

Experimental Setup

• Kinetic Analysis:

  • Monitored by changes in absorbance using UV-visible spectroscopy during bleaching.

  • Various pH and cyclodextrin concentrations were tested, particularly focused on pH 10.8 for alkali bleaching.

• Equations of Interest:

  • Specific equations governed the calculation of rate constants and equilibrium states between dye and hydrogen peroxide, facilitating comparisons across cyclodextrin concentrations.

Discussions on Data Interpretation

• Binding Constants Analysis: Attempted characterization of binding constants provided crucial insights into the nature of the interactions.

• Rate Observations: Curve fitting of kinetic data indicated a relationship between cyclodextrin concentration and reaction rates, elucidating both accelerating and inhibiting behaviors across experiments.

Conclusion

• The dual influence of β-cyclodextrin on the bleaching efficiency offers vital insights into its role in oxidative degradation processes.

• Results underscore β-cyclodextrin's practical applicability in environmental remediation, potentially enhancing reactions involving non-biodegradable pollutants.

Acknowledgements

• Acknowledgement to Dr. Michael E. Deary for contributions to the study.