Thermodynamic analysis of methyl orange anion association with α-cyclodextrin using a conductometric approach

Abstract

The electrical conductivities of aqueous solutions of the azo dye methyl orange (sodium salt) were measured at 25.0 °C in the concentration range between 2.103 × 10^–4 and 9.255 × 10^–4 mol L⁻¹. Molar conductivity values fit the Debye–Hückel–Wager equation for a symmetric electrolyte. The estimated value of the molar conductivity of the sodium salt of methyl orange at infinite dilution is found to be 77.93 ± 0.38 S cm² mol⁻¹. The calculated ionic conductivity at infinite dilution of the anion of methyl orange is 27.82 S cm² mol⁻¹. Using the same methodology, a thermodynamic analysis of the association between methyl orange anion and α-cyclodextrin was conducted at 20.0, 25.0, 32.0, and 40.0 °C. The measured molar conductivities decreased as the mole ratio of α-cyclodextrin to methyl orange went below 3. The conductivity measurements were analysed using a model 1:1 stoichiometry at the four different temperatures. The values of the thermodynamic quantities ∆H° and ∆S° for the inclusion process were calculated by using Van’t Hoff plot, their values are − 27.35 kJ mol⁻¹ and − 9.70 J K⁻¹ mol⁻¹ respectively. For this case of the studied inclusion process this inclusion was disfavored through entropy change and favored through enthalpy change.