J . Phys. Chem. 1989, 93, 7751-7752
7751
Glass Transition in Pressure-Amorphized Hexagonal Ice. A Comparison with Amorphous Forms Made from the Vapor and Liquid Andreas Hallbrucker, Erwin Mayer,* Institut f u r Anorganische und Analytische Chemie, Universitat Innsbruck, A-6020 Innsbruck, Austria
and G . P. Johari Department of Materials Science and Engineering, McMaster University, HamiltonlOntario L8S 4L7, Canada (Received: July 17, 1989)
Amorphous ice made by uniaxial compression of hexagonal ice at 77 K was investigated by differential scanning calorimetry at a heating rate of 30 K m i d . Its low-density form shows a thermally reversible glass + liquid transition: the onset temperature (T,) is 129 h 1 K, the temperature range of the transition is =12", and the increase in the heat capacity is 2.0 f 0.2 J K-' mol-'. Crystallization to cubic ice begins at 152 f 1 K and the peak temperature is 159 f 1 K. Its T, is 7 K lower than the Tgs of both the hyperquenched glassy water or vapor-deposited amorphous ice at the same rate of heating. The kinetics of its crystallization to cubic ice also differs from that of the latter two amorphous forms. We suggest that the structure of the low-density amorphous ice made from hexagonal ice differs from the structure of the amorphous forms made either from the vapor or from the liquid.
Introduction Metastable, amorphous, or noncrystalline states of water can now be obtained directly from all of its three stable states, namely gas, liquid and crystal, by using different procedures. Water vapor deposited at G77 K yields amorphous ice' (ASW) which seems to anneal, or relax, during heating to a different structural state as inferred from X-ray diffraction experiments.* This solid shows a reversible glass liquid transition with an onset temperature (T,) of 136 K and an increase in heat capacity, AC of 1.9 J K-l mol-' in the glass transition region, both determined c y differential scanning calorimetry (DSC) at a heating rate of 30 K min-l (ref 2). (The earlier study of ASW by adiabatic calorimetry by Sugisaki et aL3 gave a Tgof 135 K and a much higher AC, value of 35 J K-' mol-'. For reasons outlined in ref 2 we believe that these values are in error.) Hyperquenching of micrometer size droplets of liquid water produces glassy water4 (HGW), which shows a similar and reversible Tg of 136 K, and a AC,, of 1.6 J K-' mol-' for the rate of 30 K min-' heating.s*6 Hexagonal ice becomes amorphous on uniaxial compression to = I O kbar at 77 K.' This ~ 2 5 %denser amorphous form (hda) converts, on heating to .=I 15 K, to another amorphous form of the same density as hexagonal ice.' Handa and Klugs have observed that the low-density amorphous form (Ida) when heated at a rate of 0.167 K min-l shows an onset of the glass liquid transition at 124 K, a value 12 K lower than the 136 K observed by DSC for ASW and HGW during heating at 30 K min-l. Also its ACp at T, of 0.7 J K-l mol-] is less than half of that observed for ASW and HGW. Because of the differences in heating rate between the two sets of experiments it is not possible to decide if Ida made from hexagonal ice on the one hand, and ASW and HGW made from the vapor and the liquid on the other, are identical with liquid transition and crystallization berespect to their glass haviors. I n this Letter we report an investigation of Ida by DSC in a manner that facilitates a direct comparison between the different amorphous forms. We show that Tgof Ida is 7 K lower than the Tgsof both annealed ASW or HGW at the same rate of heating, and that Ida differs from the latter two amorphous forms with
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(I) (2) 4986. (3) (4)
Burton, E. F.; Oliver, W. F. Proc. R. SOC.A 1935. 153, 166. Hallbrucker, A.; Mayer, E.; Johari, G. P. J . Phys. Chem. 1989, 93,
Sugisaki, M.; Suga, H.: Seki, S. Bull. Chem. SOC.Jpn. 1968,4/, 2591. Mayer, E. J. Appl. Phys. 1985, 56, 663; Cryoletters 1988, 9, 66. G. P.; Hallbrucker, A,; Mayer, E. Nature 1987, 330, 552. (6) Hallbrucker, A.; Mayer, E.; Johari, G. P. Philos. Mag. 1989.608, 179. (7) Mishima, 0.;Calvert, L. D.; Whalley, E. Nature 1984, 3 / 0 , 393; 1985, 3 / 4 , 76. (8) Handa, Y. P.; Klug, D. D.J . Phys. Chem. 1988, 92, 3323. (5) Johari,
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respect to its kinetics of crystallization to cubic ice.
Experimental Section A piston-cylinder apparatus with an 8 mm diameter piston was used for the compression of hexagonal ice by means of a hydraulic press at 77 K. The displacement of the piston was measured to IO pm by a dial gauge as a function of pressure both during the compression and decompression parts of the cycle. This indicated the occurrence of phase transformation. The pressure was raised to 18 kbar, which is well above the transformation pressure of -10 kbar, before unloading. Indium capsules were not used and the transformed ice was pushed out at 1 bar into a container filled with liquid nitrogen, where the sample was stored until used for experiments. The displacement-pressure curves were similar to those observed by Mishima et al.,' Johari and Jones,9 and Bizid et a1.,I0 except for a somewhat smaller displacement because of some nontransformed ice, and are therefore not included here. 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, n-heptane, and cyclopentane (Merck, Uvasol quality) were used for calibration. Calorimetric accuracy of the enthalpy of their low-temperature phase transitions was within specification, Le., f 1%, and the transition temperatures were accurate to better than f l K. The 20-26-mg samples from a batch recovered from the pressure vessel were contained in stainless steel capsules at 1 bar. Details for handling and transfer of the capsules in and out of the DSC apparatus are given in ref 2, 5, and 6.
Results and Discussion Figure 1 shows the several steps of the DSC scans of Ida, 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 and further to enable a comparison with the Tgs of ASW and HGW which were studied mainly at the same rate of heating. On initial heating from 103 to 143 K, an exothermic peak is observed with a peak temperature of 131 K, as seen in curve 1 of Figure 1. By analogy with previous studies,lOJ'the exothermic peak, although weaker than observed in ref 1 I , is attributed to conversion from hda to Ida phase. The sample was immediately cooled to 103 K, and heated again to 148 K. Its DSC scan on second heating shows (9) Johari, G. P.; Jones, S. J. Philos. Mag. 1986, 54B,31 I . (IO) Bizid, A.; Bosio, L.; Defrain, A.; Oumezzine, M. J . Chem. Phys. 1987, 87, 2225. ( 1 1 ) Handa, Y. P.: Mishima, 0.;Whalley, E. J . Chem. Phys. 1986, 84, 2766.
0 1989 American Chemical Society
7752
Letters
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989
width of the transition is ~ 1 2 ' . The heat evolved during crystallization of Ida to cubic ice was between 0.95 and 1.15 kJ mol-' of sample. A value of between 1.3 and 1.4 kJ mol-' has been reported for this irreversible phase Therefore, by using transition of the various amorphous the value of 1.4 kJ mol-' as the heat evolved on crystallization of 100% amorphous solid, we estimate that 68-85% of hexagonal ice was originally transformed under pressure up to 18 kbar. We believe that the transformation was not close to 100% because of a pressure gradient in the ==lo-mm column of pressurized hexagonal ice. The increase in the heat capacity in the glass transition region, which was determined by correcting for the nontransformed hexagonal ice, is 2.0 f 0.2 J K-' mol-'. This value of AC, is about 3 times the value reported by Handa and Klug.* A remarkable feature seen in curve 4 of Figure 1 is that the glass transition region is clearly separated from the crystallization exotherm by a plateau region of ==IO K. This was also observed in and confirmed by DSC scans for two separate samples of Ida not shown here. This plateau region was not observed by the previous calorimetric study of Ida at a much lower rate of heating of 0.167 K min-' (ref 8). It was also not observed in the DSC scans of both annealed ASW and HGW investigated at a heating rate of 30 K min-] in earlier studies of ref 2, 5 , and 6. The results clearly indicate that T, of pressure-amorphized hexagonal ice is 7 K lower than that of ASW or HGW for the same rate of heating. Strictly interpreted, it means that the thermally activated transitions between the various configurational states in the structure of Ida require less (thermal) energy than between those in the structures of either annealed ASW or HGW. Alternatively stated, the viscosity of the fluid state obtained from Ida is lower (or its relaxation time shorter) than that of ASW or HG W at the same temperature. The Ida was also found to differ from both ASW and HGW in that annealing for different periods at several temperatures below T , had no detectable effect on the height of the endothermic step, as is seen by a comparison of curve 2 with curve 3 in Figure I . We, therefore, suggest that the amorphous form obtained on heating pressure-amorphized ice has a structural state that does not allow as readily a buildup of enthalpy deficiency as that of ASW or HGW. In the interpretation of glass relaxation, it means that the former has a much lower fictive temperature than the latter two. This difference in the structure also becomes apparent in the kinetics of its crystallization to cubic ice. The crystallization of Ida to cubic ice begins at a temperature about 2 K higher than that of annealed ASW or HGW for the same rate of heating, and the peak temperature is lowered by about 6 K. The combined effect of a 7 K lowering of Tgand the retardation of crystallization by about 2 K produces a remarkable plateau region above the glass-liquid transition where fluid water is thermally stable. For ASW or HGW, the endothermic step was followed almost immediately by the crystallization exotherm. Thus despite the presence of 15-32% untransformed hexagonal ice in the Ida, its fluid state above T, is substantially stable against crystallization. I n this study pressure-amorphized hexagonal ice was annealed at a somewhat lower temperature than HGW or ASW have been in previous s t u d i e s * ~in~ *order ~ to have a comparable annealing history. It is important to note that the Tgvalues of Ida, HGW, and ASW do not depend on annealing: for the same rate of heating, samples of Ida (compare curves 2 and 3 of Figure I ) , HGW2-5.6and ASW2 gave the same T, values as samples annealed at various temperatures and/or times. Only the height of the endothermic step at T, was influenced by annealing in the case of HGW and ASW but not of Ida, as pointed out above.
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Figure 1. The DSC thermograms of pressure-amorphized hexagonal ice during several stages of calorimetric changes, heated or cooled at a rate of 30 K min-' in all cases. Curve 1 is for a 26.5-mg sample heated from 103 to 143 K and curve 2 for the sample on second heating to 148 K . Curve 3 is obtained after annealing at 127 K for 60 min, cooling to 103 K, and thereafter heating to 148 K. Curve 4 is for a new sample of 26.6 mg annealed at 127 K for 90 min, cooled to 103 K, and heated to 260 K; curve 5 is curve 4 plotted on a reduced (1/7th) scale. Vertical scale has been arbitrarily shifted to accommodate the various thermograms. Curves 1-3 are drawn on the same scale. The temperature scale is not corrected for the thermal lag of the instrument.
an endothermic step with an onset temperature of 131 K, as is seen in curve 2. In order to ascertain the reversibility of the endothermic step, and to determine the influence of annealing, the sample was cooled immediately from 148 to 127 K, annealed at 127 K for 60 min. cooled to 103 K, and heated again to 148 K. Its DSC scan, shown as curve 3, is similar to the previous scan of curve 2. The shape of the endothermic step observed is of course liquid transition, and its reversibility characteristic of glass further confirms the correctness of its assignment. The same sample was thermally cycled two more times between 103 and 148 K . Its DSC scans are not shown here as these were similar to those shown in curves 2 and 3 with respect to both T, and AC,. In samples thermally cycled several times between 103 and 148 K, the beginning of crystallization shifted to a somewhat lower temperature. Curve 4 shows the DSC scan of a new Ida sample which was annealed at 127 K for 90 min, cooled to 103 K, and heated to 260 K . Curve 5 is curve 4 plotted on a reduced scale ( 1 /7th) to show the complete crystallization exotherm to cubic ice. Crystallization to cubic ice starts at 154 K, and the peak temperatures for the phase transition to cubic and hexagonal ice are 161 and 234 K, respectively. Two more samples from the same batch showed a similar behavior in their DSC scans. Four Ida samples were investigated, and their Tgswere determined as indicated in curve 4 of Figure 1. The mean value of T, is I3 1 K. Its reproducibility for different samples was within 0.5 K . The midpoint temperature of the transition endotherm, Tmdpr is 137 f 1 K, and the crystallization to cubic ice begins at 154 f 1 K. After correction for the thermal lag of the instrument, these values are 129, 135, and 152 K, respectively. and are accurate within fl K. The temperature
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Acknowledgment. We are grateful for financial support by the "Forschungsforderungsfonds" of Austria. We thank Dr. D. D. Klug for reading the manuscript.
