IR Spectroscopic Study of the State of Water in Dioxane and Acetonitrile: Relationship with the Thermodynamic Activity of Water at 278-318 K

The absorption spectra of water in dioxane and acetonitrile were measured at 25°C by Fourier IR spectroscopy while varying the thermodynamic activity of water in organic media from 0 to 1. The state of water in organic solvents is defined in terms of variations in the integrated intensities of water and the contour shape of the band of OH stretching vibrations. The fraction of associated water molecules is estimated in terms of water activity 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 activity of the water molecule. In both solvents, a transition between these states of water occurs within a relatively narrow range of activity, but the transition point differs between the solvents. The factors leading to the different distributions of the associated forms of water are discussed in relation to the thermodynamic activity of water in organic media.