J . Phys. Chem. 1989, 93, 4986-4990
4986
Glass-liquid Transition and the Enthalpy of Devitrification of Annealed Vapor-Deposited Amorphous Solid Water. A Comparison with Hyperquenched Glassy Water Andreas Hallbrucker, Erwin Mayer, Institut fur Anorganische und Analytische Chemie, Universitiit Innsbruck, A-6020 Innsbruck, Austria
and G. P. Johari* Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, L8S 4L7, Canada (Received: May 25, 1988; In Final Form: September 9, 1988)
The thermal behavior of vapor-deposited amorphous solid water (ASW) was investigated by differential scanning calorimetry. After annealing in vacuo, ASW shows a thermally reversible glass-liquid transition: the onset temperature is 136 f 1 K, the temperature range of the transition is 14 deg, and the increase in the heat capacity is 1.9 f 0.2 J K-'mol-'. The heat of crystallization of the annealed ASW to cubic ice is -1.29 f 0.01 kJ mol-'. These values are similar to those previously reported for hyperquenched glassy water. This similarity and the changes of the X-ray diffractograms feature indicate that ASW anneals or relaxes during heating in vacuo to a structural state approaching that of hyperquenched glassy water. Therefore, its state can be thermodynamically continuous with that of water, but its H-bonded structure is not likely to be the same as that of water at ambient temperature.
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Introduction The reports on the glass-liquid transition of vapor-deposited amorphous solid water (ASW) and the speculations on its value are bewildering. In 1965, McMillan and Losl first reported a glass transition in ASW with an onset temperature, or T,, of 139 K for a heating rate of 20 K min-'. In 1968, GhormleyZ was not liquid transition. He attributed the able to observe a glass apparent endothermic step observed by McMillan and Losl to a combination of exothermic processes in unannealed samples of ASW, because for slow rates of heating the rate of heat evolution decreases between 133 and 143 K. This decrease results in a decrease in the slope of the warming curves in a differential thermal analysis (DTA) and can be mistaken for a glass to liquid liquid transition. Ghormley2 also pointed out that a glass transition below 153 K would be masked by the irreversible exothermic processes during the first warming but would be observable in the subsequent warmings. Adiabatic calorimetric measurements on ASW by Sugisaki et aL3 showed an increase in the heat capacity that is characteristic of the glass liquid transition near 135 K. They observed an increase of 35 J K-' mol-' in the heat capacity at 135 K. In order to resolve the abovementioned contradictions, MacFarlane and Ange114,5in 1984 reinvestigated ASW by differential scanning calorimetry (DSC) under high-sensitivity conditions. In the temperature range from 110 to 150 K, they detected no evidence for an endothermic step assignable to a glass transition. They concluded that the glassliquid transition of ASW is masked by the strong devitrification exotherm and, therefore, its T , lies above 160 K. The glass liquid transition of ASW, its Tgand, its AC, value at T g are significant for understanding several aspects of ASW and liquid water, namely, (1) the thermodynamic continuity of their structural (2) the polymorphism of the various forms of the noncrystalline solid water within the restrictions of hydrogen-bonded networks,@ and (3) the nature of ASW in relation
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(1) McMillan, J. A,; Los, S . C. Nature (London) 1965, 206, 806. (2) Ghormley, J. A. J. Chem. Phys. 1968,48, 503; ibid. 1967,46, 1321. ( 3 ) Sugisaki, M.; Suga, H.; Seki, S. Bull. Chem. SOC.Jpn. 1968,41, 2591. (4) MacFarlane, D. R.; Angell, C. A. J. Phys. Chem. 1984, 88, 759. (5) Angell, C. A. Annu. Rev. Phys. Chem. 1983, 34, 1077. (6) Johari, G. P. Philos. Mag. 1977, 35, 1077. (7) For a complete review see the comprehensive article by: Sceats, M. G.; Rice, S. A. In Water, A Comprehensiue Treatise; Franks, F., Ed.; Plenum: New York, London, 1982; Vol. 7, Chapter 2. (8) Maddox, J. Nature 1987, 326, 823.
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to that of glassy water.1° Among these, the second is particularly important, as Rice and co-workers7have used ASW as a structural model for liquid water, in which only the static disorder is preserved. This model implied that the structure of ASW is thermodynamically connected with that of ordinary water through its region of supercooling. This means that the two noncrystalline forms of water-one made from the vapor and the second from the liquid-have the same structure. The latter has been discussed by Johari,6 who concluded that, on the basis of the T, and AC, value of ASW reported by Sugisaki et aL3, the state of ASW is thermodynamically discontinuous from that of liquid water. A discussion of the points of view taken by the various authors is found in ref 4. We have recently reportedlo that liquid water, when hyperquenched (cooling rates > lo5 K s-l), is directly converted into a vitreous state. It has a reversible glass == liquid transition with a T, of 136 f 1 K and its AC, at Tgis 1.6 f 0.1 J K-I mol-' (ref 1l), which is 1/20th of the value for ASW reported by Sugisaki et aL3 or of that anticipated by Angell and otherssJ2 from extrapolations of concentrated binary aqueous solution glasses. In order to observe such a small change in C,, it was essential to optimize the procedure for our measurement by differential scanning calorimetry. For hyperquenched glasses in general, the spontaneous enthalpy relaxation to a state of lower energy, which appears as an irreversible exotherm at T < Tgand continues up to Tg,effectively masks an endothermic step to a glass liquid transition. This can be removed by careful annealing at T C Tg. For water in particular, rapid crystallization to cubic ice at a temperature a few degrees above Tgcreates an additional difficulty. For ASW, a further difficulty in detecting a glass liquid transition arises because, as has been shown, the surface area of ASW prepared at -77 K is large, and the reduction of this surface area, which occurs as a result of sintering during heating,2*13-15 creates thermal effects of its own. This too appears as an irreversible exotherm in the DSC scans. In this paper we report on the DSC and X-ray diffraction studies of carefully annealed samples of ASW. Here a reversible
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(9) Speedy, R. J. J. Phys. Chem. 1982, 86, 982. (10) Johari, G. P.; Hallbrucker, A.; Mayer, E. Nature 1987, 330, 552. (1 1) Hallbrucker, A.; Mayer, E.; Johari, G.P. Philos. Mag. In press. (12) Angell, C. A,; Tucker, J. C. J. Phys. Chem. 1980, 84, 268. (13) Mayer, E.; Pletzer, R. Nature 1986, 319, 298. (14) Mayer, E.; Pletzer, R. J. Phys. 1987, 48, CI-581. (15) Schmitt, B.; Ocampo, J.; Klinger, J. J. Phys. 1987, 48, CI-159.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 4987
Thermal Behavior of Amorphous Solid Water
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glass liquid transition was detected and its AC, value at Tg determined, and a value for the enthalpy of devitrification to cubic ice was obtained. These results are compared with the earlier work on ASW and further with the thermal properties of glassy water made by hyperquenching of the liquid. Since both materials, ASW and hyperquenched glassy water (HGW), were investigated in the same DSC instrument and handled and transferred in the same manner, any artifacts of their behaviors arising from procedural differences are largely eliminated. Experimental Section
The samples of ASW were prepared as described before.16 Briefly, water vapors from a reservoir of water held at room temperature were admitted through a fine metering valve and a tube with 13-mm inner diameter into a high vacuum system, where the vapor condensed on a demountable Cu substrate (50-mm diameter) precooled to 77 K. The apparatus was as is depicted in Figure 2 of ref 16, except for an O-ring sealed glass flange (DN 100 ISO, Schott) which was used here instead of the greased NS 85 joint in the setup in ref 16, primarily because the grease joint tended to stick. Thus the inner diameter of the glass flange body was 100 mm. A baffle was used above the entrance tube in order to avoid supersonic flow of the water vapor and the problems arising from it.16 The distance between the baffle and the cryoplate was 170 mm. Base pressure in the vacuum system during deposition was 2 X lo4 mbar. The pressure of water vapor at source measured with a Pirani manometer as indicated in Figure 2 of ref 16 was varied between 0.050 and 0.10 mbar. In our control experiments, water vapors were condensed on a demountable X-ray low-temperature sample holder for simultaneous determination of the diffractogram and of the DSC scan on reheating. In all these experiments, ASW thus formed was examined and no reflections attributable to crystalline ice were observed. Since our X-ray technique is sensitive to a crystallinity only > 2%, we infer that our ASW was at least 98% amorphous. Samples of ASW 0.8 and 1.5 mm thick (measured with a vernier caliper under liquid nitrogen) were prepared by water vapor deposition for a period between 5 and 10 h. This corresponds to the rate of deposition between 100 and -200 pm h-', which are about 2 orders of magnitude greater than those used by MacFarlane and Angel14 or those recommended by Sceats and Rice7 to ensure a fully amorphous deposit. Since our X-ray diffraction measurements ascertained that our samples were completely amorphous, we believe that the difference between our observations and those in the literature4q7are due to the differences between the dimensions of our apparatus that was used for preparing ASW and theirs. We used a fairly voluminous highvacuum system whereas in ref 7 the drawing of the apparatus for in situ preparation indicates an arrangement with less internal volume. In a separate experiment we had observed that, for the same rate of deposition (but in an arrangement with less internal volume), crystalline ice is more likely to form, possibly due to the problems in obtaining a low base pressure. In order to reduce the surface area of the deposit, the cryoplate and the sample together were warmed in vacuo to 113 K within -0.5 h, after the deposition. These were thereafter cooled to 77 K by filling the cold finger with liquid nitrogen. This procedure, which is equivalent to an annealing treatment, did not increase the crystallinity in the amorphous samples as their X-ray diffractograms taken in the preannealed and annealed state showed. The vacuum system was then brought to near 1 bar with nitrogen, and the cryoplate was unscrewed from the bottom plate of the cold finger under liquid nitrogen. Samples of ASW for the DSC scans were taken from this cryoplate. A Perkin-Elmer DSC-4 instrument computerized with the TADS system was used. Curvature was reduced with the S A Z function, which subtracts during scanning a base line obtained with empty sample pans. Water, cyclohexane and n-heptane (Merck, Uvasol quality) were used for calibration. Calorimetric accuracy of their low-temperature phase transitions was within
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(16) Mayer, E.; Pletzer, R. J. Chem. Phys. 1W4,80, 2939.
3 0'
20"
4 0'
50'
ze
6 0"
Figure 1. X-ray diffractograms (Cu Ka,90 K) of (1) unannealed ASW, (2) ASW annealed in vacuo to -113 K, and (3) hyperquenched glassy water. Deposition rate for samples used for curves 1 and 2 was 100 Fm h-l. The peak at 28 52 deg in curves 1 and 2 is from the sample holder, and the weak peak at 28 40' in curve 3 is from a small amount of crystalline ice. N
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specification, i.e., i l % ; the transition temperatures were accurate to better than f l K. ASW samples were handled and transferred into the DSC instrument in the same manner as described in ref 17 for H G W samples. The high-pressure cells available for the DSC-4 instrument were used because these cells could be filled and closed under liquid nitrogen, and their comparatively high mass prevented excessive warming during rapid transfer from liquid nitrogen to the precooled head of the DSC instrument. Condensation of water vapor onto the cold cell during the transfer was negligible in our procedure, which involved rapid flushing of the glovebox with dry air and maintaining 3.5-bar pressure of dry helium for flushing the instrument head. As reported before,17 the mass of the samples could not be accurately determined by weighing, as loss due to evaporation occurred during the transfer of the cell from the instrument to the balance. This mass was therefore determined by measuring the sample's heat of melting and scaling it with the standard value of 6.012 W mol-'. The sample's mass was between 17 and 26 mg in the different measurements. The X-ray diffractograms were obtained at -90 K at atmospheric pressure on a Kristalloflex 4 (Siemens), using a vertically held flat sample holder. Results
Effects of Annealing in Vacuum. On heating ASW from 77 K, a rapid decrease in its surface area occurs and, as sintering proceeds, any pores in it become closed and isolated from each other. If this initial heating is carried out in the presence of N2 or other gases, the adsorbed gases become enclosed in the pores during sintering. Once enclosed, the gas cannot be removed by pumping at low temperatures but is given off gradually during the warming of the sample up to 273 K.l3-I4 This evolution of gas causes the appearance of additional features in the DSC scans, which become further modified due to the different thermal conductivity of the gas that was used for flushing the instrument's head. Since it was necessary to minimize the exothermic effects due to the reduction of surface area on initial heating, ASW samples were first heated in vacuo up to 113 K. This procedure ensured that (i) the apparent surface area decreased from several hundred m2 g-I after deposition at 77 K to -40 m2 g-' after heating (annealing) to 1 13 Kl3*l4and that a further reduction in the surface area on heating from 113 K was less in the annealed than in the unannealed ASW, and (ii) no gas was enclosed in the pores during the sintering of the microporous material. All ASW
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(17) Hallbrucker, A,; Mayer, E. J. Phys. Chem. 1987, 91, 503.
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Figure 2. Glass-liquid transition and devitrification of annealed (Le., warmed in vacuo to 1 13 K) ASW as seen in the DSC scans. The same sample was used for all scans, and it was heated or cooled at a rate of 30 K min-' in all cases. Curve 1 is for a 26.0" sample heated from 103 to 129 K, and curve 2 is for the sample on second heating to 130 K. Curve 3 is obtained after annealing at 130 K for 90 min, cooling to 103 K, and thereafter heating to 149 K. For curve 4 the sample was cooled immediately after the procedure for curve 3 to 130 K, annealed again at 130 K for 90 min, cooled to 103 K, and then heated finally to 250 K. Curve 5 is on a reduced scale ('/*th) to show the devitrification to cubic ice. Tg is the glass transition temperature, Tmdpis the midpoint temperature of the glass liquid transition endotherm, and T, is the crystallization temperature. The temperature scale is not corrected for the
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thermal lag of the instrument.
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samples studied here were, therefore, first heated (annealed) from 77 K to 1 13 K in vacuo prior to their exposure to any gaseous environment. The X-ray diffractograms (Cu Ka),of the unannealed ASW taken at 90 K, and of the annealed in vacuo ASW at 113 K are shown in curves 1 and 2, respectively, of Figure 1. The diffractograms show that the samples were completely amorphous. But more importantly, the full width at half-maximum (fwhm) 24 deg is decreased of the first broad diffraction peak at 20 on the heat treatment. This value is -7 deg for the unannealed ASW and - 5 deg for the annealed or heat treated ASW sample. Heating of the unannealed ASW to 130 K at 1 bar, when contained in the X-ray apparatus, had no effect on its fwhm, as the diffractograms taken at this temperature remained essentially the same as those taken at 90 K. We conclude that once N, gas is contained in the microporous ASW during sintering at T < 113 K, its fwhm remains -2 deg higher than that of the ASW which was sintered in vacuo. A diffractogram of HGW at -90 K was also obtained. This is also included as curve 3 in Figure 1 for comparison. The position of the first peak in the diffractogram of HGW is within the instrumental uncertainty of the positions observed for ASW, but its fwhm is comparable with that of the annealed ASW (shown by curve 2). This suggests an approximate structural similarity between the annealed, or heat-treated, (in vacuo) ASW and HGW.
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The Glass-Liquid Transition
Figure 2 shows several steps of the DSC scans of the ASW, heated or cooled in all cases at a rate of 30 K min-I. This heating rate was chosen to accentuate the weak endothermic effect. The scans were made with the same sample for obtaining a consistent effect of thermal history. On initial heating from 103 to 129 K, a pronounced decrease in the heat output occurs, as seen in curve 1 of Figure 2. This is due to enthalpy relaxation and/or further sintering. The sample was immediately cooled again to 103 K, and on second heating a positive slope, as is seen in curve 2, is observed in the beginning of the DSC scan. This is due to the steady increase of the heat capacity of the sample with rising temperature. After annealing at 130 K for 90 min and cooling to 103 K, the DSC scan in curve 3 showed on third heating to
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160
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180
T/K
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Figure 3. Comparison of DSC scans of annealed ASW (in vacuo to 113 K) with those of hyperquenched glassy water. Curves 1 and 3 are for hyperquenched glassy water, and curves 2 and 4 are for annealed ASW. Heating rate for scans 1 and 2 was 10 K min-' and for scans 3 and 4, 30 K min-I. Curves 3 and 4 were obtained after annealing the samples at 130 K for 90 min and are shown at higher sensitivity. Curves 1-4 are normalized with respect to the areas of their melting endotherms. The temperature axis is not corrected for the thermal lag of the instru-
ment. 149 K the beginning of the glass transition endotherm at 138.5 K. In order to ascertain its reversibility, the sample was cooled immediately to 130 K, annealed again at 130 K for 90 min, cooled to 103 K, and finally heated to 250 K (curve 4). The S-shaped endotherm is nearly identical with that in curve 3. The strong exotherm following the endothermic step is due to crystallization to cubic ice. Curve 5 is the same scan as curve 4 but plotted on a reduced scale ('/$h). Three more samples, two from the same batch and one from a different batch, showed a similar behavior in their DSC scans. The reproducibility of their onset temperatures was within 0.3 K. For the four ASW samples investigated, the onset temperature of the endotherm in the DSC scan denoted in Figure 2 by Tg,and determined as marked in curve 4, is 138 f 1 K. The midpoint temperature of the transition endotherm, Tmdp,is 145 f 1 K, and the crystallization to cubic ice begins at 152 f 1 K. After correction for the instrument's thermal lag, these values are 136, 143, and 150 K, respectively, and are accurate within f l K. The temperature width of the transition is 14 deg. During the entire procedure outlined above very little crystallization to cubic ice occurred. This was also confirmed by comparing the area of the crystallization exotherm with the area of the melting endotherm, as described earlier." Several samples of the unannealed ASW were also heated under identical conditions, and their DSC scans also showed a glass liquid transition endotherm with an onset temperature of 138 f 1 K, the same as for the annealed ASW (in vacuo). The Increase in Heat Capacity at TB. When measuring the heat capacity of the annealed samples of ASW (in vacuo) by using the procedure developed by Perkin-Elmer for the DSC-4 instrument, we encountered the same problems as for HGW described elsewhere." These were interpreted in terms of the DSC artifacts, as discussed by Suzuki and Wunderlich.ls Therefore, only AC, in the glass liquid transition region could be reliably determined from the normal DSC scans. Here again the heating rate was 30 K/min. The height of the endothermic step was maximized by annealing the samples at 130 K for 90 min, which removed most of the exothermic effect. These annealing conditions were found to be optimal for samples of HGW and therefore were also used here. For curve 4 of Figure 2 , AC, is 1.8 J K-' mol-'. Four ASW samples were investigated, and these gave a AC, value of 1.9 f
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(18) Suzuki, H.; Wunderlich, B. J . Therm. Anal. 1984, 29, 1369.
Thermal Behavior of Amorphous Solid Water 0.2 J K-I mol-'. Because of the onset of crystallization immediately following the Tgendotherm, the AC, value may be somewhat lower than in the absence of crystallization, But, the large width of the transition endotherm indicates that it is unlikely to be much lower, as discussed before.I0 The Phase Transition to Cubic Ice. Curve 2 in Figure 3 shows the DSC scan of a sample of annealed ASW heated at 30 K m i d with the prominent exotherm due to the phase transition to cubic ice, For comparison, the DSC scan of a sample of HGW, heated at the same rate as ASW, is shown as curve 1. Heating rates of 10 K min-' were then used for determining the temperature and the enthalpy of the phase transition for three samples of ASW taken from three different batches. The peak minimum temperatures for these were reproducible within 0.4 deg. The heat of their transition to cubic ice, which was determined by comparing peak areas of crystallization exotherms with those of melting endotherms, varied to within 2% for different samples. The enthalpy of the phase transition of the annealed ASW to cubic ice was determined as -1.29 f 0.01 kJ mol-'. Their crystallization temperature and the temperature of the minimum were 150 and 162 K, respectively, both within an accuracy of at least 1 K. The crystallization of the unannealed ASW samples were also studied under the same conditions as those of the annealed ASW (in vacuo). The crystallization and peak minimum temperatures of the exotherm due to ASW ICphase transition were in general 2-3 deg lower than those observed for the annealed ASW.
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Discussion
Our observation of a reversible glass * liquid transition in the annealed ASW is in contrast with the recent conclusion obtained from a DSC study by MacFarlane and Angell! But, we feel that in some sense our observations complement their study by indicating that the change in C, at Tgis small and can be detected only by a careful procedure and one in which the DSC technique is optimized to obtain information such as that on hyperquenched glassy and dilute aqueous solutions. In particular, our method is an improvement over the earlier DSC study4 in five ways, namely, (1) in the use of an increased amount of sample, up to five times as much as in other studies, (2) reducing the masking effect of the structural relaxation on the endotherm of liquid transition, (3) accentuating the endotherm by glass increasing the heating rate to 30 K min-I, (4) optimizing the annealing temperature and time in order to build up an enthalpy deficiency, and (5) using a higher S/N ratio in our computerized system with automatic base-line subtraction than in the DSC-2 instrument. Comparison of the thermal behavior of ASW with that of HGW is the most important aspect of this work for several reasons, as outlined briefly in the Introduction. In Figure 2, the DSC scans of the annealed ASW, curves 2 and 4, are compared with those of the corresponding curves 1 and 3 obtained for HGW. The close correspondence in the thermal behavior of both types of amorphous materials is remarkable. The endothermic steps due to their glass liquid transitions, which are compared in curves 3 and 4 at high gain, are identical within experimental error and have almost the same values of Tgand AC,. Their phase transitions to cubic ice, which are compared in curves 1 and 2, are also very similar. These values for ASW and HGW are collected in Table I. There are admittedly small differences between the AC, values, the temperature widths of the glass transition, and the heats of devitrification, but within the experimental error, these differences are probably insignificant. Thus the ASW afer annealing in vacuo acquires the same kinetic stability against crystallization as HGW. It is important to note that this comparison with annealed ASW applies only to the optimally quenched samples of liquid water and that it does not apply to liquid water quenched under conditions that are believed to give lower rates of cooling. The latter has been shown to devitrify to cubic ice in two distinct steps.l9 This phenomenon had been related to the anomalies of supercooled water19 but is at present not fully understood.
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(19) Mayer, E. J. Microsc. 1985, 140, 3.
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 4989 TABLE I: Comparison of Thermal Effects on Reheating Annealed Vapor-Deposited Amorphous Solid Water with Those in Hyperquenched Glassy Water
annealed vapor-deposited hyperquenched amorphous solid watef glassy waterb Glass ==Liquid Transitionc T.. K
136 143
Trod,,
AC.. J mol-' K-l wi&h, deg
1.9
* 0.2
14
136 142 1.6 12
* 0.1
Phase Transition to Cubic IceC Tc, K
K Aq, kJ mol-I
150 162 -1.29 f 0.01
150 163 -1.33 f 0.02
'This work. bValues from ref 10 and 11. ' T S and Tmdpare the onset and midpoint temperatures of the glass transition endotherm, and AC, is the change in heat capacity. Tc characterizes the beginning of crystallization. The temperatures are corrected for the thermal lag of the DSC instrument and are accurate to better than *l K. For annealed ASW the heat of devitrification is clearly smaller than the values reported in the literature for unannealed samples, namely,-1.8 f0.2,2-1.64,'and-1.5 kO.1 kJmol-l.4 Theselarger values most likely contain exothermic contributions due to enthalpy relaxation and/or reduction of surface area. The glass liquid transitions reported by McMillan and Los' and by Sugisaki et al.3 for unannealed ASW show a much greater change in heat capacity than oberved in this work. We believe that the fairly intense endothermic step reported by McMillan and Los' could be due to the inclusion in the DTA trace of the effects pointed out by Ghormley2 and mentioned in the Introduction here. The large endothermic effect observed by Sugisaki et al.,3 near 135 K with a AC, value of 35 J K-' mol-', is more difficult to reconcile with the value of 1.9 J K-' mol-' found in this work. The special problems associated with adiabatic calorimetry, and the resulting difference in thermal history of a sample warmed in small steps from that of a sample scanned with fairly high rates, have been discussed by MacFarlane and AngelL4 We want to point out an additional artifact as an explanation for this intense endotherm. ASW was prepared by Sugisaki et aL3in their apparatus by in situ condensation of water vapor. In our experience, it is very difficult to maintain during deposition a low base pressure at the substrate because of the small size of inlet and exit tubes and of the dimensions of the apparatus. As pointed out by Ghormley2 and other^,'^-'^,^^ any gas-including even hydrogen-present during the deposition of water vapor to ASW will be enclosed in the pores and will be given off gradually during the warm-up. Ghormley2 has shown that a peak in the rate of gas evolution appears in the same temperature range in which phase transition to cubic ice occurs. Such an effect is expected to be observable in adiabatic calorimetry as an irreversible endotherm prior to crystallization, but such an endotherm would not be thermally reversible. Therefore, it could be distinguished from a reversible glass transition on thermal cycling prior to crystallization. Sugasaki et al.3 have not looked for this thermal reversibility, and it is conceivable that on thermal cycling this endothermic effect would have been considerably reduced in magnitude in the adiabatic calorimetry. The small increase in C,, which is comparable with that observed in Ge02, S O 2 , and BeF2 and discussed elsewhere," implies that there is very little change in the number of entropically different accessible configurations when a tetrahedrally bonded network undergoes a structural relaxation. This aspect has also been discussed elsewhere." It is important to consider the X-ray diffraction of ASW during its annealing in vacuo. The X-ray diffractograms of Figure 1 seem to indicate that the structural state of ASW relaxes during heating or annealing in vacuo to a state approaching that of HGW, because the diffractograms of annealed ASW and the HGW shown as curves 2 and 3, respectively, are very similar, whereas that of
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(20) Brackmann, R.T.;Fite, W. L.J . Chem. Phys. 1961, 34, 1572.
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the unannealed ASW in curve 1 is clearly different with respect to fwhm and the relative height of the first peak at 28 24 deg. This interpretation is subject to revision because unannealed ASW could not be investigated in situ at 77 K, but had to be transferred under liquid nitrogen, and the effect of nitrogen adsorbed or that enclosed in the pores of the microporous ASW on the diffractogram is difficult to ascertain. We do not know of other work where X-ray diffraction of ASW has been measured in situ at T > 77 K for investigating the annealing effects. It would be important to confirm by in situ measurements the temperature dependence of the diffraction patterns at T > 77 K. The uncorrected diffractograms of ASW reported by Dowell and Rinfret2’ and by Dubochet et aL2*resemble that of annealed ASW shown in curve 2 of Figure 1. A comparison of our results with the detailed X-ray study of Narten et aLZ3is not possible because their data are reported after correction and only as structure functions, while our instrument did not allow an accurate determination of the latter. However, it is conceivable that the adsorbed and/or enclosed gases reduce the extent of sintering that occurs during heating and, thus, preserve the structure of ASW as it exists after deposition at -77 K. This observation is therefore consistent with that of previous studies made by differential thermal analysis16 and by adsorption measurement^.'^-'^ In an earlier paper, one of us6 had pointed out that when Sugisaki et al.’s3data on the heat capacity of ASW are analyzed, it becomes difficult to find a thermodynamic continuity of states between ASW and water (at T > 273 K) without violating the third law of thermodynamics, and further that a possibility of polymorphism in the noncrystalline states of water is a reasonable consideration. Several forms of noncrystalline solid water have since been prepared by additional procedures. These are by (1) rapid quenching of the jet-produced micrometer-size droplets in a ~ r y o m e d i u m (2) , ~ ~rapid quenching of water as I-pm-thick film contained in electron microscope grade carbon grids in a of micrometer-size droplets ~ r y o m e d i u m ,(3) ~ ~ hyperquenching *~~ on a ~ r y o p l a t e (4) , ~ ~uniaxial compression of hexagonal ice,2sv29 and (5) supercooling of emulsified water in a ternary system with octane after a special thermal treatment.30 These forms do not seem to be interconvertible. Furthermore, it is not certain whether or not any one of these forms has the same H-bonded structure as liquid water at T > 273 K . The evidence for polymorphism in the noncrystalline solid forms of water, which is now available, makes the structural similarity between the ASW and liquid water less convincing and conclusions based on this similarity less obvious. Even if a continuous thermodynamic path between liquid water and ASW is found,
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(21) Dowell, L. G.; Rinfret, A. P. Nuture 1960, 188, 1144. (22) Dubochet, J.; Adrian, M.; Vogel, R. H. Cryo-Lett. 1983, 4, 233. (23) Narten, A. H.; Venkatesh, C. G.; Rice, S. A. J . Chem. Phys. 1976, 64, 1106. (24) Briiggeller,P.; Mayer, E. Nature 1980,288, 569; ibid. 1982,298, 715. (25) Dubochet, J.; McDowell, A. W. J . Microsc. 1981, RP3-RP4. (26) Dubochet, J.; Lepault, J.; Freeman, R.; Berriman, J. A,; Homo, J. U. J . Microsc. 1982, 128, 219. (27) Mayer, E. J . Appl. Phys. 1985, 58, 663; J . Phys. Chem. 1985, 89, 3474; J. Microsc. 1985, 140, 3; J . Phys. Chem. 1986, 90, 4455. (28) Mishima, 0.; Calvert, L. D.; Whalley, E. Nature 1984,310,393;ibid. 1985, 314, 16. (29) Johari, G.P.; Jones, S. J. Philos. Mug. 1986, 543, 3 1 1 . ( 3 0 ) Angell, C. A,; Choi, Y . J . Microsc. 1986, 141, 251.
Hallbrucker as is suggested by the results here, the structure of the noncrystalline solid would represent a spatial disorder in a liquid at the temperature below that of the implied A-type a n ~ m a l and y~~~~~ not that above 273 K. Nevertheless, our recent studylo and this work show that among the five noncrystalline solid forms prepared by the above-mentioned procedures, and the sixth obtained by heating the high-density form,28HGW and ASW show a reversible glass + liquid transition with almost the same values of their ACp and Tg. In a separate article” on the behavior of HGW, we have redrawn Figure 4 from Johari6 to show that, with our measured value of AC, of HGW, at least two qualitatively different thermodynamic paths can be followed by the C, of supercooled water between 136 and 273 K. One of these two paths allows the occurrence of a A-type transition and the second does not. A discussion of the thermodynamic continuity of state for ASW would be similar to that of HGW published elsewhere, particulary since their AC, and Tg,given in Table I, are closely similar. Obviously, the low value of Cpof the state of ASW heated to 146 K ensures that the loss of entropy on supercooling the liquid from 273 to 146 K is much smaller than Sugisaki et al.’s3 results had originally suggested.6 It is gratifying to now see the new results remove the thermodynamic paradox pointed out by one of u s 6 The greater stability of liquid water at 146 K than at temperatures near the A-type anomaly is significant both from the theoretical and experimental points of view. It suggests that critical 240 K fluctuations that make its liquid state unstable at T become less important near 146 K, and it allows the use of pulsed dielectric and fast Fourier transform spectroscopic techniques for future investigations of its structure at a temperature beyond the experimental limits of its supercooling. A provocative account of this behavior has been recently given by Angell.” After this paper was submitted for publication, Handa and KIug3*reported glass transition in the low-density form obtained by heating the high-density form. They found a Tgof 124 K at a heating rate of -0.17 K min-I.
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Conclusion The amorphous state of water obtained by vapor deposition anneals or relaxes during heating in vacuo to a structural state which seems similar to that of hyperquenched glassy water. On further heating it shows a reversible glass liquid transition with Tg = 136 h 1 K, temperature range of transition 14 K, and the increase in C, of 1.9 f 0.2 J K-’ mol-’. After removal of the exothermic effects due to structural relaxation and sintering, the heat of crystallization of ASW to cubic ice is -1.29 f 0.01 kJ mol-‘. The results suggest a continuity of structural states between bulk water at 273 and at 136 K, both with or without the postulated h-type transition on supercooling the liquid.
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Acknowledgment. Financial support by the “Forschungsforderungsfonds” of Austria is gratefully acknowledged. Registry No. H,O, 7732-18-5. (31) Angell, C. A. Nature 1988, 331, 206. (32) Handa, Y . P.; Klug, D.D.J . Phys. Chem. 1988, 92, 3323.
