Mechanism of inclusion-compound formation for binding of organic dyes, ions and surfactants to α-cyclodextrin studied by kinetic methods based on competition experiments

Abstract

This kinetic and thermodynamic study is concerned with a comparison of binding of simple anions, azo-dyes and long-chain surfactants to α-cyclodextrin. Two competition reactions have been employed. (i) In order to obtain information on the association of simple anions and surfactants with α-cyclodextrin, the competitive binding of an azo-dye with α-cyclodextrin has been used in stopped-flow experiments. (ii) In order to study the binding of the azo-dye pyridine-2-azo-p-dimethylaniline (PADA) to α-cyclodextrin, the competition reaction used is that between PADA and aquo-metal ions. Ni²⁺(aq) and Zn²⁺(aq) are used as competitors for PADA in stopped-flow and Joule-heating temperature-jump experiments. It is confirmed that when azo-dyes interact with α-cyclodextrin, they do so by a two-step mechanism. The first step involves the formation of an intermediate complex in a fast pre-equilibrium step. The second step, which is rate-determining, results in the formation of the final stable inclusion complex. Studies with I^–(aq) as a competing ion suggest that the intermediate is associated with dye binding to the inside rather than the outside of the cyclodextrin. Small anions in general have low stability constants for binding to α-cyclodextrin. In marked contrast, surfactants of general formula C_nH_2n+1SO^– ₄(where n > 8) interact strongly with α-cyclodextrin, and inclusion compounds are formed in which one surfactant binds two cyclodextrin molecules.