J. Phys. Chem. 1995, 99, 5161-5165
5161
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Glass Liquid Transition and Devitrification of LiCk11H20 Solution and of Hyperquenched and Vapor-Deposited Water Erwin Mayer,* Andreas Hallbrucker, and Giinter Sartor Institut f i r Allgemeine, Anorganische und Theoretische Chemie, Leopold-Franzens-Universitat Innsbruck, A-6020 Innsbruck, Austria
G. P. Johari Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario U S 4L7, Canada Received: June 4, 1994; In Final Form: October 31, 1994@
Because of Angell’s (J. Phys. Chem. 1993, 97, 6339) report of remarkably enhanced kinetic stability of amorphous water on heating above its glass transition temperature in comparison with that of 8.33 mol % LiCl solution and of the conclusions drawn, we have reinvestigated the thermal behavior of glassy 8.33 mol % aqueous LiCl solution and compare it with those of hyperquenched glassy water (HGW) and vapor-deposited amorphous solid water (ASW). For a heating rate of 30 K min-’, the thermal effects observed for 8.33 mol % LiCl solution (for HGW) give 138 K (136 K) for the onset temperature of glass liquid transition and e140.5 K (e142 K) for the midpoint temperature, e 5 deg (e12 deg) for the width, 0.97 (0.089) J K-’ g-l for the increase in the heat capacity, and e l 4 9 K for the beginning of crystallization for both. The temperature span from the onset of glass liquid transition to the beginning of crystallization is el 1 deg ( e 1 3 deg) for 8.33 mol % LiCl solution (for HGW), but from the midpoint temperature it is %8.5 deg ( e 7 deg). A comparison between the temperature spans from the midpoint temperature is more appropriate because of the large differences in widths of the glass liquid transition. These analyses show that HGW on heating above its glass liquid transition is less stable toward crystallization than 8.33 mol % aqueous LiCl solution, and it removes the claimed evidence for HGW being a “strong” liquid, or a new phase. Any meaningful deduction on the characteristics and structure of ASW must include also the spurious effects of gas inclusion on its thermal behavior.
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Introduction Angell and colleagues’-3 recently compared the calorimetric behavior of glassy 8.33 mol % aqueous LiCl solution on heating with those of hyperquenched glassy water (HGW) and vapordeposited amorphous solid water (ASW) and inferred that the viscosity-temperature relationship of water near but above the onset temperature of its glass liquid transition (T,) is that of a “strong” liquid, like Si02. Although the viscosity of ASW or HGW is not measured yet, they concluded that their inference supports Speedy’s4 conjecture that water above its Tg is a separate and distinct phase of water, and should be called “water II”. Their experimentalevidence, presented for example by two curves drawn on different vertical scales in Figure 1 of ref 1, is that ASW, despite a lower T, than that of 8.33 mol % LiCl solution, survives to a higher temperature before the beginning of crystallization ( T J , Le., in their Figure 1 Tc is 152 K for 8.33 mol % LiCl solution but e160 K for ASW. We note that this Figure 1 of ref 1 is originally Figure 3 of MacFarlane and AngelF who had looked for a Tg of ASW by differential scanning calorimetry (DSC) but failed to find it and concluded that glass transition is nonexistent in the ASW. Angell states that the “warm-up behavior of hyperquenched liquid water is essentially the same as that of the vapor deposit” (ref 1, caption to Figure l), and presumably because of that the DSC scan or heating curve of ASW could be compared (in place of that of HGW) against that of the LiCl solution. Our experiments6,’ had shown that this is true only for ASW samples which have been heated in vacuo up to ~ 1 1 K3 prior to their first exposure to air. Otherwise ASW’s crystallization kinetics is altered by a combination of the effects of supersonic flow
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‘Abstract published in Advance ACS Abstracts, March 1 , 1995.
0022-365419512099-5161$09.0010
during deposition and subsequent gas encl~sure,~ and this thermal treatment needed to remove the effects from ASW was apparently not done in the studies of refs 1 and 5 . That ASW formed by deposition at e 5 7 8 K is a highly microporous amorphous which on heating can enclose considerable amounts of gas such as N2 or 0 2 in its micropores has been independently shown by several investigators.8-’8Once enclosed, the gas cannot be removed by pumping at low temperatures but escapes gradually during the warming of the sample up to 273 K. These enclosed gases retard the crystallization of ASW to cubic ice on further heating and lead to crystallization in several distinct steps at temperatures which depend on the type of enclosed gas. Most important in the context here is the fact that crystallization of ASW can be shifted by the enclosed gas to much higher temperature than that of the annealed ASW.7 It is significant that the phenomenon of gas enclosure has now been used for developing a new method for making clathrate hydrates with guest molecules such as 0 2 , Nz, CO, Ar, and and that these special gas adsorption and inclusion properties of ASW form the basis for a suggestion to use deposited samples as a sort of ~ r y o p u m p . ~ ~ It is important to note that ASW deposits are highly microporous even when formed at very slow rates of deposition where molecular flow of the water vapor occurs. This is shown in ref 8 where Nz-adsorption isotherms of ASW samples deposited at only -3 8, s-’ are reported. This deposition rate is lower than that of -5-6 8, SKI(Le., -2 pm h-I film thickness) used by MacFarlane and Angell,5 or that of -8 8, s-’ (Le., -1 mm thickness in 2 weeks) used by Narten et aLZ5 for preparing ASW for X-ray diffraction studies. Micropores are defined as pores of
