Probing the molecular dynamics of azeotropes

Azeotropes are constant boiling mixtures that cannot be separated by fractional distillation. Components in an azeoptropic mixture evaporate at a boiling point different to that of each pure substance. [1] The presence of azeotropes in industrial processing causes many environmental and economic challenges due to the difficult and costly nature of the separation process. Their common occurrence contrasts with the lack of understanding about their molecular dynamics. To improve the understanding of the molecular dynamics of azeotropes, self-diffusion coefficients of a range of aqueous solutions of a variety of alcohols (with mole fractions, χOH, 0" 1), including a series with structure making/breaking salts MgCl2 and CaCl2. were measured using Nuclear Magnetic Resonance (NMR). Previous NMR diffusion studies of the ethanol-water system suggest the formation of a single diffusing entity between ethanol and water at the azeotropic point.[2] However, results obtained in this study indicate that the diffusion coefficients do not converge at the azeotropic point. Furthermore, the addition of MgCl2/CaCl2 caused the diffusion coefficient of >0.5 χOH solutions to converge. Additional research predicting the diffusion coefficient from viscosity and comparing it to experimental values and the addition of stronger structure making/breaking salts will help unravel the molecular-level origin of azeotropic behaviour.