Picosecond dynamics of hydrogen bond rearrangements during phase separation of a triethylamine and water mixture

Abstract

The earliest stages of phase separation in a liquid triethylamine (TEA)–water mixture were observed using a picosecond IR laser pulse to produce a temperature jump and ultrafast Raman spectroscopy. Raman spectral changes in the water OH stretching region showed that the temperature rise induced by IR pulses equilibrated within a few tens of picoseconds. Amplitude changes in the TEA CH-stretching region of difference Raman spectra consisted of an initial faster and a subsequent slower process. The faster process within 100 ps is attributed to hydrogen bond weakening caused by the temperature rise. The slower process attributed to phase separation was observed for several nanoseconds, showing the number of hydrogen bond between TEA and water gradually decreased with time. The kinetics of hydrogen bond scission during phase separation indicated a linear growth of the phase-separated component, as observed previously on the nanosecond time scale, rather than the more usual exponential growth. A peak blueshift was observed in the difference Raman spectra during phase separation. This shift implies that hydrogen bond scission of TEA–water aggregates involving very few water molecules took place in the initial stage of phase separation (up to 2 ns), and then was followed by the breaking of TEA–water pairs surrounded by water molecules. This effect may be a result from spatial inhomogeneities associated with the phase separation process: aggregates or clusters existing naturally in solution even below the lower critical soluble temperature.