Letter pubs.acs.org/JPCL
Glass Transitions in Pressure-Collapsed Ice Clathrates and Implications for Cold Water Ove Andersson*,† and Akira Inaba‡ †
Department of Physics, Umeå University, 901 87 Umeå, Sweden Research Center for Structural Thermodynamics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
‡
S Supporting Information *
ABSTRACT: Ice is known to collapse to amorphous ice upon pressurization at low temperatures and shows the unusual feature of multiple distinct solid amorphous water states, which have inspired models of liquid water’s structure and unusual properties. Here, we use heat capacity Cp measurements to show that similarly collapsed ice clathrates display identical glass behavior as amorphous ice but that crystallization above the glass transition temperature Tg of ∼140 K at 1 GPa is inhibited. This effect of the homogeneously distributed “guest molecules” in water reveals a relatively strong reversible Cp increase above Tg but no further transition before crystallization at ∼190 K. This is consistent with a glass−liquid transition of water at Tg, which suggests a new path to study an ultraviscous liquid water network and evaluate water models.
SECTION: Glasses, Colloids, Polymers, and Soft Matter
W
it is therefore important to obtain a better understanding of the two amorphous ices. This means that new possibilities to probe cold, ultraviscous, water properties are interesting for improving the knowledge of both cold and ambient water. The properties of ultraviscous water are mainly based on simulations and extrapolations because of the rapid homogeneous or heterogeneous crystallization that occurs by the normal cooling of liquid water. Crystallization can be prevented only by hyperquenching micrometer-sized water and vapor deposition onto a cold plate, which produces hyperquenched glassy water (HGW)6 and amorphous solid water (ASW),7 respectively. However, glassy water rapidly crystallizes upon heating up to the glass transition, which prevents the study of ultraviscous bulk water in a wide temperature and pressure range. To find model systems for experimental studies of cold water-like states, two commonly employed strategies to avoid crystallization have been used, (i) confinement in the form of emulsions, or solid materials with micro- or nanosized cavities, and (ii) solutions with noncrystallizing, strongly interacting liquids. However, the finding of HDA, which has given a new method to produce ultraviscous water from amorphous solid water via the crystalline phase,3,8 provides also a unique possibility to obtain new model systems. When hexagonal (or cubic) ice I is compressed to ∼1 GPa below 140 K, it collapses to HDA. Because it has been recently shown that HDA displays a weak glass transition in heat
ater has many unusual properties that have long inspired studies of cold or supercooled liquid water.1 Such studies can help resolve suggestions on the structure of water and evaluate models proposed to explain its properties. In addition, these studies may also lead to a better understanding of the biological processes that occur at temperatures substantially below 0 °C. Recently, it was suggested that the water structure is inhomogeneous and that the ambient water structure is a mixture of nanometer-sized patches of tetrahedrallike and hydrogen-bond-distorted structures, which coexist in a temperature−pressure (T−p)-dependent equilibrium.2 This model is inspired by the model of two distinct liquid phases of water, in which the two phases are supposed to coexist at a first-order T−p coexistence line that ends in a (second) critical point. Nevertheless, because the temperature range of the purported coexistence is below the homogeneous nucleation temperature TH = 235 K at 1 atm, rapid crystallization prohibits most experimental studies of pure liquid water. Instead, experimental support for liquid polymorphism is provided by the observation of two solid amorphous ices, low- and highdensity amorphous ices (LDA and HDA) at even lower temperatures and under high pressure.3,4 LDA has a local order structure similar to that of crystalline ice I (tetrahedral arrangement), whereas the structure of HDA is similar to that of ambient liquid water5 but with smaller nearest-neighbor distances consistent with its higher mass density. The LDA and HDA states have served as templates for the model of two different types of nanometer-sized patches of liquid water and are also essential in other water models.1To refute, confirm, or further develop the models for water’s structure and properties, © 2012 American Chemical Society
Received: June 19, 2012 Accepted: July 9, 2012 Published: July 9, 2012 1951
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capacity data measured at 1 GPa,8 it follows that HDA is a glassy water state, but like glassy water produced at 1 bar, it crystallizes near the glass transition temperature. There exists, however, an ideal system for producing a crystalline water state with a perfectly distributed molecular filler, which can be collapsed to a more stable amorphous state by the same procedure, and it is provided by clathrate hydrates. Clathrate hydrates (CHs) or ice clathrates9,10 are inclusion compounds in which water (host) forms a hydrogen-bonded-cage-like structure, as shown in Figure 1. Molecules of suitable size,
Figure 2. Heat capacity per unit volume plotted as a function of pressure at ∼130 K. Results for (○) THF-CH and (□) DXL-CH, which collapse with increasing pressure above 0.9 GPa, as shown by the abrupt increase in density. The collapse proceeds with time, as shown by the change during 3 h at constant temperature and pressure (arrow).
Figure 1. Cages made of hydrogen-bonded water molecules in type II clathrate hydrates: pentagonal dodecahedron (D, left) and hexakaidecahedral (H, right) with a guest molecule. The H cage formed by 12 pentagonal and 4 hexagonal faces can accommodate large molecules. The type II unit cell consists of 16 D and 8 H cages (136 water molecules and 8 guest molecules or a molar ratio of 17:1).
“guest molecules”, are confined in these cages and are therefore equidistantly dispersed in the crystalline clathrate structure. Ice clathrate with a type II structure9 can accommodate large guest molecules such as tetrahydrofuran (THF) or 1,3-dioxolane (DXL) molecules with an ideal stoichiometry of M·17H2O, where M is the guest molecule. It has been shown that a THFCH undergoes a transition similar to the collapse of ice I11−16 and that the subsequent crystallization upon heating at low16 and high14 pressures is sluggish. Thus, the collapsed THF-CH system is a state corresponding to that of HDA with a perfectly dispersed molecular filler. In this study, we have performed heat capacity measurements under extreme conditions to show that two different collapsed type II CHs display identical glass transition behavior as pure bulk water.8 Figure 2 shows the heat capacity per unit volume c, that is, the product of density ρ and specific heat cp, plotted against the pressure at 130 K. (We use cp for J kg−1 K−1 and Cp for J mol−1 K−1.) The small slope in the 0.1−0.9 GPa range shows that increasing ρ with pressure is compensated for by decreasing cp. Above 0.9 GPa, c increases as the CHs collapse, the molecular structure changes, and a high-density structure forms. Thus, the increase in c is mainly, or entirely, due to the increase in density. When the THF-CH sample was annealed at a constant pressure of approximately 1.0 GPa at 130 K, c continued to increase with time (∼6% increase in c during 3 h), as shown by the arrow in Figure 2. The transformation is similar to that of hexagonal ice, which collapses to HDA near 1 GPa in temperature-, pressure-, and time-dependent manners,17,18 and the state formed lacks the diffraction features of a crystal.13 Figure 3 shows the plots of c against temperature obtained upon heating the collapsed THF-CH and DXL-CH at 1 GPa. The plots show that their c values are the same within experimental error. For the collapsed THF-CH, c measured by continuous heating at an average rate of 0.2 K min−1 shows a
Figure 3. Heat capacity per unit volume plotted as a function of temperature. Results for (Δ) collapsed THF-CH, (□) collapsed DXLCH, (solid line) HDA8 at 1 GPa, and (○) as-made THF-CH measured at 0.05 GPa. The dashed line above 250 K represents c of a supercooled THF−water solution at 1 atm calculated from cp measurements of Tombari et al.21 and the density near room temperature (0.997 g/cm3).
stretched sigmoidal increase with the onset at ∼139 K, which is followed by exothermic crystallization upon heating to above ∼190 K.19 (At crystallization, c shows an anomalous increase; see the Supporting Information.) DXL-CH shows identical behavior but with a lower crystallization temperature. The stretched sigmoidal increases of the CHs occur at virtually the same temperatures and appear to be of similar size, as shown by HDA ice. However, HDA crystallizes at ∼153 K, just after the c increase begins to level off. In contrast, DXL-CH was temperature-cycled up to 181 K at 1 GPa, and the results upon cooling agreed with those upon heating, showing that the results are reversible up to at least this temperature.20 Moreover, it appears feasible that an extrapolation to higher temperatures connects to c of the solution.21 Figure 4b shows results upon temperature cycling at 1 GPa obtained in experiments on pressure-collapsed DXL-CH, where c was measured using two different time scales, the normal with a 1.4 s heating pulse and another 10 times longer, as discussed in more detail in the Supporting Information. Although the data scatter significantly more for the long-time measurements, the results upon both heating and cooling differ systematically 1952
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at ∼310 K and ∼0.8 GPa. However, it is an irreversible process, that is, it cannot explain the reversible sigmoidal change and the reversible increase in c up to ∼180 K. The results do not exclude an irreversible sluggish separation into water-rich and THF-rich phases but show that a separated state must have identical c values and show the features similar to those of the homogeneous solution. Moreover, as pure water and solutions with less than 5.6 mol % solute crystallize near 153 K,8 the separation cannot proceed to such an extent and produce large domains of pure water. Instead, separation must mainly occur above 180 K and near the crystallization temperature. We can conclude that the origin of the sigmoidal c increase must be due to a quasi-reversible change in cp, and we can identify the following possibilities for this change: (i) a glass− liquid transition, (ii) an orientational glass transition or the onset of proton mobility, and (iii) a second-order phase transition. In principle, this behavior could be due to a reversible phase transition (case (iii)). Because no enthalpy of transformation is detected, it must be a second-order transition. However, the collapsed clathrate is amorphous,13 which prevents a reversible second-order phase transition. Instead, an amorphous state is the prerequisite for glass transition, and one of the archival characteristics is the sigmoidal change in cp observed upon heating and cooling due to the kinetic unfreezing and freezing, respectively, of the disordered state. This kinetic process of a polar substance is also detected by maxima in the dielectric loss, and this is indeed observed in both THF-CH14 and HDA,22,23 as shown in Figure 4a. These results, which pertain to the measurement frequency f of 0.5 Hz, mean that the maximum in the loss corresponds to a dielectric relaxation time τd of approximately 0.3 s (τd = (2πf) −1), which is in good agreement with the calorimetric relaxation time of approximately 0.2 s at the c increase (Supporting Information). A key feature of the cp increase in the glass transition range is the shift to lower temperatures with an increasing experimental time scale, which is verified by the results in Figure 4. The shift of about 7 K in the transition range is of the size expected for a glass transition when the measurement time is increased from 1.4 to 14 s. Thus, the dielectric results of collapsed THF-CH and HDA show that molecular motions are present on time scales typical near a glass transition and in correspondence with the characteristic cp increase at the glass transition temperature Tg. In principle, this could be a purely reorientational (proton) motion, which would result in an orientational glass transition upon cooling.25−27 However, such transitions have been observed only in crystalline CHs and crystalline ices. As it is common, we derive Tg from the intercept of the baseline with the tangential line at the maximum slope of cp versus T and find (138 ± 1) and (139 ± 1) K for THF-CH and DXL-CH, respectively, as summarized in Table S1 (Supporting Information). The CHs show an increase in cp that is slightly more gradual than that in HDA with Tg = 140 K at 1 GPa,8 and the glass transition range is approximately 17 K. For comparison, Tg of ASW7 and HGW6,28 by heating at 30 K min−1 at 1 bar is (136 ± 1) K. In both ASW and HGW, the transition is broad with an approximate range of 14 K for the stretched sigmoidal cp increase. Because the density changes smoothly and typically much less than 1% in the glass transition range, we can estimate the Cp increases at Tg for the collapsed CHs from the measured data for c. Detailed analyses of the results in Figure 4c show that the increases are (3.9 ± 0.4) and (3.7 ± 0.4) J (H2O-
Figure 4. Glass transition of collapsed CHs and HDA. (a) Normalized dielectric loss at the measurement frequency of 0.5 Hz plotted against temperature: (□) THF-CH and (○) HDA.14,22,23 (b) Heat capacity per unit volume plotted against temperature for DXL-CH upon heating and subsequent cooling using 1.4 (dashed line) and 14 s (full line) heating pulses. (c) Excess heat capacity plotted against temperature for DXL-CH (dashed−dotted line) and THF-CH (short dashed line), water solutions with 0.3 mol % THF (short dash) and 2.9 mol % THF (filled circle), and HDA (dashed line),8 which crystallizes at 153 K. The full line shows results for DXL-CH using 14 s long heating pulses. The vertical line indicates Tg in the experiments using short heating pulses.
from the short-time measurements only in the transition range, and then with a shift of approximately 7 K. Figure 4a depicts the results for the dielectric loss of the collapsed THF-CH and HDA at the measurement frequency of 0.5 Hz, which we have extracted from previous measurements.14,22,23 The increase in c on the short time scale agrees well with the maximum in the loss. In order to better display the sigmoidal change in the heat capacity, we have used data for density (Supporting Information) and calculated the excess Cp data (ΔCp) by subtraction of a linear function fitted to the data below 135 K, and we plotted these in Figure 4c. This plot also shows the corresponding results for HDA8 and water solutions with 0.3 and 2.9 mol % THF, that is, solutions with less solute than that of the ideal composition (5.6 mol %). The results for ΔCp and its onset are about the same for all samples, but only the collapsed CHs remain stable at temperatures well above the sigmoidal increase in Cp. The c results shown in Figure 3 reveal a stronger temperature-induced increase in cp above the sigmoidal increase than below. The reversibility of the temperature cycling shows that both this and the sigmoidal increase must be inherent features of the collapsed states, at least up to ∼180 K. A gradual change, for example, slow crystallization or separation into water-deficient and water-rich domains, can both be accompanied by densification and thus an increase in c. A separation process into a heterogeneous state is also likely on the basis of the phase diagram of the THF−water system,24 which shows the coexistence of immiscible water-rich and THF-rich liquids 1953
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mol)−1 K−1 in DXL-CH and THF-CH, respectively, or about two times that observed in ASW (1.9 ± 0.2 J mol−1 K−1)7 and HGW (1.6 ± 0.2 J mol−1 K−1)6 at 1 bar. Moreover, the increases are approximately the same or slightly larger than that estimated in HDA (3.7 ± 0.4) J mol−1 K−1.8 We can also compare our result for the Cp increase with that at Tg for the pure unfreezing of the proton mobility. The value reported for THF-CH is 1.1 J (H2O-mol)−1 K−1 at Tg = 85 K.25 For hexagonal ice, the behavior in the vicinity of Tg of approximately 95 K significantly depends on the thermal history.26,27 We have extracted a heat capacity jump of slightly less than 0.5 J mol−1 K−1 from the original values26 of a sample annealed for 624 h at 89.4 K; however, the jump is less pronounced and occurs at higher temperatures for samples with less or no annealing. For example, we estimate a jump of ∼0.3 J mol−1 K−1 near 105 K for a sample “quenched” using a cooling rate of 1 K min−1. We can conclude that the stretched sigmoidal Cp increase observed here upon heating and cooling collapsed CHs at 1 GPa is characteristic of a glass−liquid transition. The temperature of the increase agrees well with that expected on the basis of dielectric results, and the Cp increase shifts to lower temperatures with increasing probe time, which substantiates the validity of the findings. Moreover, the significant Cp step at the transition and the lack of further transitions up to 190 K suggests that the system remains in an ultraviscous state in a wide temperature range. The behavior exactly mimics the behavior of water and water solutions with 0.3 and 2.9 mol % THF, but the crystallization shifts upward by ∼40 K. The perfectly distributed guest molecules in CHs apparently obstruct crystallization in an ultraviscous system under high pressure by preventing the formation of crystalline nuclei larger than the critical size required for crystallization. There is a possibility of hydrogen bonding between the solute molecules and water, but the insignificant change in the glass transition properties suggests that the solute molecules do not take significant part in, or strongly disrupt, the hydrogen-bonded water network. The stabilizing feature of the solute may therefore be used to gain further information about the properties of cold water, and the results obtained here reveal a relatively strong Cp increase above Tg. The effect may also be valuable to improve the stability of other unstable ultraviscous systems without the use of strongly interacting liquids or confinements. Finally, we note that the finding of glass transitions in collapsed CHs and water provides an opportunity to determine the effect of soft confinements, such as those in emulsions, on the slow, glassy, dynamics of water.
upon crystallization.) The transient hot-wire method (see the Supporting Information) was used for measuring the heat capacity per unit volume c with an estimated experimental inaccuracy of ±5%.
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ASSOCIATED CONTENT
S Supporting Information *
A detailed description of the method, explanation of the anomalous increase in c at crystallization, details for the experiments using longer heating pulses, calculation of the relaxation time at Tg, a summary of the glass transition properties (Table S1), and estimation of the density of the CHs at 1 GPa. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported financially by the Faculty of Science and Technology, Umeå University. We thank G. P. Johari for helpful discussions.
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REFERENCES
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EXPERIMENTAL SECTION Tetrahydrofuran (THF; puriss. p.a., 99.9%, less than 0.005% water) and 1,3-dioxolane (DXL; 99.8%, ∼75 ppm BHT as inhibitor) were purchased from Sigma-Aldrich Chemicals. A total of four water solutions were required to accumulate the results for the CHs. The solutions in pure water (Milli-Q Ultrapure WaterSystems or tissue-culture grade water supplied by Sigma) were prepared by weighing THF·16.65H2O, THF·16.34H2O, DXL·16.34H2O, and DXL·16.40H2O, which upon freezing produced type II clathrates with a slightly less than ideal stoichiometry of M·17H2O (5.6 mol % solute), where M is the guest molecule. In addition, two solutions, with 0.3 and 2.9 mol % THF, were studied to investigate the behavior at lower solute concentrations. (At higher concentrations, a solution phase separates into pure solute and CH 1954
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(14) Andersson, O.; Johari, G. P. Nature of the Pressure-Induced Collapse of an Ice Clathrate by Dielectric Spectroscopy. J. Chem. Phys. 2008, 129, 234505. (15) Andersson, O.; Johari, G. P. Collapse of an Ice Clathrate under Pressure Observed via Thermal Conductivity Measurements. Phys. Rev. B 2008, 78, 174201. (16) Bauer, M.; Többens, D. M.; Mayer, E.; Loerting, T. PressureAmorphized Cubic Structure II Clathrate Hydrate: Crystallization in Slow Motion. Phys. Chem. Chem. Phys. 2011, 13, 2167−2171. (17) Andersson, O.; Johari, G. P. Time-Dependent Amorphization of Ice at 0.8−0.9 GPa. J. Chem. Phys. 2004, 121, 3936−3938. (18) Johari, G. P.; Andersson, O. Mechanisms for Pressure- and Time-Dependent Amorphization of Ice under Pressure. Phys. Rev. B 2004, 70, 184108. (19) As the collapsed states are metastable, the crystallization temperature depends on the heating rate. The heating rate in this study was about the same (∼0.2−0.3 K min−1) as that used previously in a study of HDA (ref 8). (20) The hot-wire method used here is different from the more commonly used differential scanning calorimetry, which shows a significant hysteresis in the glass transition range between heat capacity values measured upon cooling and heating, as discussed in the Supporting Information. (21) Tombari, E.; Presto, S.; Salvetti, G.; Johari, G. P. Heat Capacity of Tetrahydrofuran Clathrate Hydrate and of Its Components, and the Clathrate Formation from Supercooled Melt. J. Chem. Phys. 2006, 124, 154507. (22) Andersson, O. Relaxation Time of Water’s High-Density Amorphous Ice Phase. Phys. Rev. Lett. 2005, 95, 205503. (23) Andersson, O.; Inaba, A. Dielectric Properties of High-Density Amorphous Ice under Pressure. Phys. Rev. B 2006, 74, 184201. (24) Manakov, A. Yu.; Goryainov, S. V.; Kurnosov, A. V.; Likhacheva, A. Yu.; Dyadin, Y. A.; Larionov, E. G. Clathrate Nature of the HighPressure Tetrahydrofuran Hydrate Phase and Some New Data on the Phase Diagram of the Tetrahydrofuran−Water System at Pressures up to 3 GPa. J. Phys. Chem. B 2003, 107, 7861−7866. (25) Yamamuro, O.; Oguni, M.; Matsuo, T.; Suga, H. Calorimetric Study of Pure and KOH-Doped Tetrahydrofuran Clathrate Hydrate. J. Phys. Chem. Solids 1988, 49, 425−434. (26) Haida, O.; Matsuo, T.; Suga, H.; Seki, S. Relaxational Proton Ordering and Glassy Crystalline State in Hexagonal Ice. Proc. Jpn. Acad. 1972, 48, 489−494. (27) Haida, O.; Matsuo, T.; Suga, H.; Seki, S. Calorimetric Study of the Glassy State X. Enthalpy Relaxation at the Glass-Transition Temperature of Hexagonal Ice. J. Chem. Thermodyn. 1974, 6, 815− 825. (28) Capaccioli, S.; Ngai, K. L. Resolving the Controversy on the Glass Transition Temperature of Water? J. Chem. Phys. 2011, 135, 104504.
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