IR spectroscopic study of the state of water in dioxane and acetonitrile: Relationship with the thermodynamic activity of water

The absorption spectra of water in dioxane and acetonitrile were measured a t 25 degreesC by Fourier IR spectroscopy while varying the thermodynamic ac tivin, of water in organic media from 0 to 1. The stare of water in organic solvents is defined in terms of variations in the integrated intensities o f water and the contour shape of the band of OH stretching vibrations. The fraction of associated water molecules is estimated in terms of water activ ity in organic media. In both dioxane and acetonitrile, the water molecules can exist either as associated (H-bonded) molecules or as single molecules complexed with organic molecules, which depends on the thermodynamic activ ity of the water molecule. In both solvents, a transition between these sta tes of water occurs within a relatively nan-ow range of activity, brit the transition point differs between the solvents. The factors leading to the d ifferent distributions of the associated forms of water al e discussed in r elation to the thermodynamic activity of water in organic media.