Behavior of rifampicin in association with β-cyclodextrin in aqueous media: a spectroscopic and conductometric study

Abstract

The behavior of rifampicin (D) with β-cyclodextrin (β-CyD) in aqueous media (W) has been examined by means of UV-vis spectroscopy and conductivity measurements over the temperature range 15–30 °C. The UV-vis study has been used to characterize the systems. The estimated molar absorption coefficient ε for D/CyD/W system was 10757±280 M⁻¹ cm⁻¹ in comparison to the value of 6133±99 M⁻¹ cm⁻¹ for D/W system. The conductivity was measured (i) as a function of [D] for binary D/W systems, (ii) as a function of [CyD], keeping the concentration of drug constant, for D/CyD/W system, and (iii) as a function of [D] in the presence of a constant cyclodextrin concentration. Two transition points were observed for D/CyD/W system at constant [CyD], which were assigned as cac-1 and cac-2. The stoichiometry of the association was estimated from the conductivity data. This was obtained from [drug] value at which the change in slope of κ occurs. The standard free energy change, $\Delta {\text{G}}^{{\text{o}}}_{{\text{a}}}$ of aggregation was also calculated from the critical concentration data. An attempt has also been made to estimate the stoichiometry of β-cyclodextrin:rifampicin association.