Liquid Water−Acetonitrile Mixtures at 25 °C:  The Hydrogen-Bonded Structure Studied through Infrared Absolute Integrated Absorption Intensities

The real and imaginary refractive index spectra of mixtures of water and acetonitrile over the full composition range at 25 °C were determined between 8000 and 700 cm-1 by calibrated multiple attenuated total reflection spectroscopy. Under the assumption of the Lorentz local field, the corresponding molar polarizability spectra, αm(ν̃) = α‘m(ν̃) + iα‘‘m(ν̃), were calculated and used to investigate the structure of the mixtures. The concentrations of water-bonded, acetonitrile-bonded, and non-hydrogen-bonded O−H groups, and of water-bonded and non-hydrogen-bonded acetonitrile molecules, were obtained from the integrated intensities COH and CCN, the areas under the O−H and C⋮N stretching bands in the ν̃α‘‘m spectra. The results indicate that no enhancement of the water structure (OH---O bonding) results from the addition of acetonitrile. In contrast, a monotonic decrease in the fraction of O−H groups that are bonded to oxygen is observed with increasing CH3CN content. At low acetonitrile concentration, xCH3CN ≤ 0.05, where x is the mole fraction, the total fraction of OH groups that are hydrogen bonded increases slightly with increasing CH3CN content because the formation of OH---N bonds slightly exceeds the destruction of OH---O bonds. The present results are consistent with the existence of microheterogeneity at compositions near 30−50 mol % of acetonitrile. However the fraction of OH groups that are hydrogen bonded to water is 0.50 at 50 mol % CH3CN and decreases to 0.35 at 70 mol % CH3CN. Both of these fractions are too small to support water clusters more complex than linear chains or hexagons