VITRIFIED LIQUID WATER : A COMPARISON WITH VAPOR DEPOSITED AMORPHOUS SOLID WATER

Abstract

A new method for the vitrification of pure liquid water and dilute aqueous solutions is described which works as the only one without liquid cryomedium for heat transfer : rapid cooling of aqueous aerosol droplets on a solid cryoplate (1). The method can be applied in diverse fields such as cryobiology, cryomicroscopy and low temperature spectroscopy of water and aqueous solutions or suspensions to avoid the formation of crystalline ice during quenching to cryogenic temperatures, the elimination of a liquid cryomedium being particularly helpful for in situ spectroscopic investigations. Until now vitrified liquid water prepared by this method has been investigated by infrared spectroscopy (2) and by differential thermal analysis (DTA) (3), and its properties compared with those of vapor deposited amorphous solid water, H2O (as). A comparison of infrared spectra shows two differences : first, in the vitrified aerosol the peak maximum of the decoupled O-D stretching transition at 20 K is at lower wavenumbers, at 2416 cm-l, whereas in H2O (as) it is at 2440 cm-l. Second, in the coupled O-H stretching band region of vitrified liquid water an additional shoulder is observed at approximately 3120 cm-l. These spectral differences are probably related to structural differences, implying that the amorphous forms of water made from the liquid and the vapor have different structures. The devitrification of quenched aerosol droplets as observed by DTA occurs in two steps, with peak maximum temperatures of 150 and 156 K. This stepwise devitrification has been observed previously for samples quenched in liquid cryomedium by high-pressure jet-freezing (4). To my knowledge water is the only vitrified pure liquid with this behaviour. The two exotherms are interpreted in terms of two fractions of vitrified liquid water formed in the cooling process with different properties, which devitrify during reheating at slightly different temperatures, and this interpretation is related to the anomalous properties of supercooled liquid water. A comparison with the devitrification of H2O (as) establishes a further difference, because H2O (as) prepared this way devitrifies as expected in one step, the DTA curve showing only one exotherm.