Journal of Structural Chemistry, Vol. 41, No. 6, 2000 IR SPECTROSCOPIC STUDY OF THE STATE OF WATER IN DIOXANE AND ACETONITRILE: RELATIONSHIP WITH THE THERMODYNAMIC ACTIVITY OF WATER V. A. Sirotkin, B. N. Solomonov, UDC 541.123.22 D. A. Faizullin, and V. D. Fedotov 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. INTRODUCTION Investigating the relationship between the structural and thermodynamic characteristics of the state of water in organic media is a matter of considerable interest since the aqueous solutions of nonelectrolytes have found wide use as mixed solvents and specific media for chemical reactions, and also as model systems for studies of processes involving biological macromolecules [1, 2]. Aqueous organic mixtures are typical examples of nonideal systems where the thermodynamic characteristics of the state of water differ for the same molar ratio of components. To apply a correction for nonideality of a system, the concentrations of the substances are recalculated into activity coordinates. The thermodynamic activity is related to the chemical potential of the solute as [3] μ i = μ i 0 + RT ln a i , (1) where μ i 0 is the standard chemical potential, and a i is the activity of the ith component in solution. Accordingly, the variation of the Gibbs energy (∆G) for the transfer of one mole of water from the pure state to an aqueous organic medium with a mole fraction of water x w equals ∆G = RT ln a i . (2) Therefore using the thermodynamic activity of water permits one to normalize the concentration dependences of the characteristics of the effects under study with respect to the Gibbs energy of water transfer from pure liquid to an aqueous organic medium. This technique was previously employed to study DNA transition into A-form in various triple 0022-4766/00/4106-0997$25.00 © 2001 Plenum Publishing Corporation 997 Kazan State University. Institute of Biochemistry and Biophysics, Kazan Scientific Center, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 41, No. 6, pp. 1205-1212, November-December, 2000. Original article submitted November 14, 1999.