Dynamics of water in supercooled aqueous solutions of poly(propylene glycol) as studied by broadband dielectric spectroscopy and low-temperature FTIR-ATR spectroscopy.

Binary water mixtures usually display a water relaxation (process II) which can be studied by broadband dielectric spectroscopy (BDS) at subzero temperatures. In a large collection of binary water mixtures, a slight increase of the relaxation strength is observed for low water concentration, whereas a faster increase is seen above a critical concentration. The assumption behind this result is that at high water concentration self-associations of water molecules are present in the solutions. In this work, we have studied poly(propylene glycol) water solutions by means of broadband dielectric spectroscopy and Fourier transform infrared spectroscopy (FTIR) using the attenuated total reflectance method (ATR) in the temperature range of 120-300 K. By combining both techniques, we found a critical water concentration x(w) = 0.20 above which the relaxation strength of the water relaxation (process II) increases more rapidly than at low water concentration indicating the self-association of water molecules.