Thermodynamic investigations of methyl tert-butyl ether and water mixtures

Abstract

Interactions between methyl tert-butyl ether (MTBE) and water have been investigated by scanning calorimetry, isothermal titration calorimetry, densitometry, IR-spectroscopy, and gas chromatography. The solubilization of MTBE in water at 25 °C at infinite dilution has ΔH° = −17.0 ± 0.6 kJ mol⁻¹; ΔS° = −80 ± 2 J mol⁻¹ K⁻¹; ΔC_p = +332 ± 15 J mol⁻¹ K⁻¹; ΔV° = −18 ± 2 cm³ mol⁻¹. The signs of these thermodynamic functions are consistent with hydrophobic interactions. The occurrence of hydrophobic interaction is further substantiated as IR absorption spectra of MTBE–water mixtures show that MTBE strengthens the hydrogen bond network of water. Solubilization of MTBE in water is exothermic whereas solubilization of water in MTBE is endothermic with ΔH° = +5.3 ± 0.6 kJ mol⁻¹. The negative mixing volume is explained by a large negative contribution due to size differences between water and MTBE and by a positive contribution due to changes in the water structure around MTBE. Henry's law constants, K_H, were determined from vapor pressure measurements of mixtures equilibrated at different temperatures. A van't Hoff analysis of K_H gave ΔH_H° = 50 ± 1 kJ mol⁻¹ and ΔS_H° = 166 ± 5 J mol⁻¹ K⁻¹ for the solution to gas transfer. MTBE is excluded from the ice phase water upon freezing MTBE–water mixtures.