Thermal phase behavior of triethylamine–water mixtures studied by near-infrared spectroscopy: band shift of the first overtone of the C–H stretching modes and the phase diagram

Thermal phase behavior of triethylamine (TEA)–water mixtures with various concentrations has been studied by use of near-infrared (NIR) spectroscopy. Bands due to the first overtones of C–H stretching bands (6000–5500 cm−1) of TEA in the NIR spectra of one-phase region of the mixtures show significant shifts to a lower wavenumbers during heating process. It has been revealed that the magnitude of the band shift for the critical mixture (32.1 wt%) is markedly prominent in comparison with that of the off-critical mixtures. Since the C–H band shifts to the lower wavenumber are accompanied by the condensation of TEA, these results suggest that the microscopic association of non-polar moieties acts as a trigger for the LCST-type phase separation.