Thermodynamic investigations of methyl tert-butyl ether and water mixtures

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 degrees C at infinite dilution has Delta H degrees = -17.0 +/- 0.6 kJ mol(-1); Delta S degrees = -80 +/- 2 J mol(-1) K-1; Delta C-p = + 332 +/- 15 J mol(-1) K-1; Delta V degrees = -18 +/- 2 cm(3) 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 Delta H degrees = + 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 Delta H-H degrees = 50 +/- 1 kJ mol(-1) and Delta S-H degrees = 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.