High-sensitivity Raman Spectroscopy of Supercritical water and methanol over a wide range of density

High-sensitivity Raman vibrational spectroscopic equipment was developed to study the hydrogen-bonding structure in supercritical fluids over a wide density range. Supercritical water was investigated to the very dilute region of 0.1 MPa (3 x 10(-4) g cm(-3)) at 400 degrees C, and the spectroscopic profile at the isolated state was observed. For comparison, supercritical methanol was also investigated, and the effect of the alkyl group on the hydrogen bonding is elucidated. For both water and methanol, the OH stretching peak shifted toward a lower frequency with an increase in the density as a result of hydrogen-bond formation. The red shift relative to the isolated value was not always proportional to the density. In the case of water at 400 degrees C, the shift was nonlinear over a wide density range; it was almost linearly dependent on the density between 0.6 and 0.2 g cm(-3), whereas at lower densities, the dependence became steeper. For methanol, a nonlinear density dependence was similarly observed at a corresponding reduced temperature. The density dependence then became more linear at higher temperatures. The density dependence of the spectroscopic profile is interpreted in terms of the matrix of force constants affected by the formation of hydrogen bonding.