SUM-FREQUENCY VIBRATIONAL SPECTROSCOPY AT THE LIQUID AIR INTERFACE OF METHANOL - WATER SOLUTIONS

The vibrational spectrum of surface methanol molecules is investigated in the CH3 stretching region in a large concentration range corresponding to CH3OH mole fractions of the bulk of 0.011 less-than-or-equal-to x less-than-or-equal-to 1. Striking spectral changes are observed for the water-methanol surface network with excess number densities of methanol and increasing molecular ordering upon dilution. For neat methanol (x = 1) the methyl groups are found to be oriented with an average tilt angle theta0 < 40-degrees of the symmetry axis with respect to the surface normal. For the diluted system, x less-than-or-equal-to 0.1, our data suggest that the molecules are polar oriented with theta0 < 16-degrees.