J. Phys. Chem. B 2004, 108, 19825-19830
19825
Dielectric Relaxation in Aqueous Solutions of Hydrazine and Hydrogen Peroxide: Water Structure Implications† Ayumi Minoguchi, Ranko Richert, and C. Austen Angell* Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604 ReceiVed: June 29, 2004; In Final Form: October 28, 2004
We report dielectric relaxation studies of aqueous solutions of two water-like molecules, hydrazine and hydrogen peroxide, in the neighborhood of their glass transition temperatures, Tg. These solutions behave in a rather simple manner, reminiscent of the diols and diamines of which they are the limiting cases. Their relaxations near Tg are more nearly exponential than in most other cases, and they show essentially no secondary relaxations. Supercooled hydrazine solutions are the more stable. At the composition 20 mol % N2H4, the liquid exhibits precise time-temperature-superposition (TTS) behavior. At higher N2H4 contents, a weak deviation from TTS appears. The temperature dependence of the relaxation time follows the Vogel-Fulcher-Tammann (VFT) equation, and the strength parameter, D, is similar to that of glycerol, a liquid of intermediate fragility. The VFT divergence temperature, T0, lies close to the Kauzmann temperature, TK, determined earlier from calorimetric studies implying that the thermodynamic and kinetic measures of fragility are very similar. Tg values assessed from T(τ)100s) agree well with observed calorimetric, Tg’s. Extrapolation of the relaxation time behavior to pure water would imply a Tg for water of 135-140 K; however, the dielectric behavior of amorphous solid water in the temperature range 130-160 K is completely different from that of the solutions showing no sign of the loss peak exhibited by all the solutions. Based on the solution behavior, water controversially must either remain glassy up until the temperature of crystallization or be an almost ideally strong liquid above 136 K. Having shown elsewhere how this implies glassy character up to LDA crystallization and a Tg above 160 K, we now examine the implications for water structure reorganization on dissolution of solutes, certain glycols excepted. It appears that the water in these solutions behaves like ice III rather than ice I.
Introduction The liquids hydrazine and hydrogen peroxide are very waterlike in their melting (-1.6 and 1.9 °C) and boiling (157.9 and 113.6 °C) behavior. In the range above melting, their viscosities and dielectric constants are also water-like, and it is only in the supercooled liquid range that more significant differences appear. The anomalies of supercooled water, such as the famous density maximum (at 4 °C)1 and the diverging heat capacity, viscosity, and relaxation times (at -45 °C),2 are not shared by the others nor by their binary solutions with water. In fact their binary solutions have been used as a basis for separating the “anomalous” component of water from the “normal” background component.3 Aqueous solutions of hydrazine and hydrogen peroxide, unlike water itself, vitrify quite readily on rapid cooling to liquid nitrogen temperatures.4 Their glass transition temperatures are very easy to detect because of the exceptionally large (>100%) increases of heat capacity that occur on passage through the glass transition.3 Glass transition temperatures, Tg, for vitrified solutions were reported a long time ago,4,5 and they have values in the range 135-140 K. These values are very similar to that generally attributed to water itself, largely on the basis of extrapolations of binary solution data.2,4-6 Measurements made directly on the vitreous forms of water prepared by different methods, on the other hand, have yielded ambiguous results. These have been reviewed in refs 7 and 8 †
Part of the special issue “Frank H. Stillinger Festschrift”.
and will not be revisited here. The methods of preparation, also reviewed in refs 7 and 8, range from vapor deposition,9 through hyperquenching,10 to pressure-induced amorphization,11 where only the first examples of each type are cited here. In the lowdensity forms, known variously as low-density amorphous solid water (ASW),12 hyperquenched glassy water (HQGW),13 and low-density amorphous water (LDA),11 the various preparations have very similar structures, though subtle differences in their relaxation behavior have been noted and have lead to the suggestion that there are actually distinct forms of water (waters A and B14) of the low-density amorph, even above Tg in the putative ultraviscous liquid state. Arguments intended to resolve these sources of confusion by showing that the proposed glass temperature of pure ambient pressure water actually falls above the crystallization temperature have been made15,16 but not generally accepted.17,18 Since the glass transition is a relaxation phenomenon dependent on the motion of molecules on long time scales (order of minutes at Tg), it would seem reasonable to seek resolution of phenomenological puzzles by bringing the most precise of relaxation-sensitive techniques available to bear on the problem. The most sensitive method available for the study of slow processes in condensed matter is unquestionably dielectric spectroscopy. The dielectric relaxation time of a molecular liquid can now be determined over a range of 20 orders of magnitude, from times as long as one year19 to values as short as 10-14 s.20 Although dielectic relaxation studies of N2H4-H2O and H2O2H2O solutions near ambient temperature have been made,21,22
10.1021/jp0471608 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/11/2004
19826 J. Phys. Chem. B, Vol. 108, No. 51, 2004
Minoguchi et al.
extensions to low temperatures near Tg have, to the best of our knowledge, never been performed. Thus an investigation of the dielectric relaxation behavior of these water-like systems near their glass transition temperatures, particularly in relation to water content in water-rich solutions, should provide important information relevant to the understanding of water itself in the region of controversy. Quite apart from the specific problem of glassy water, these solutions offer interesting challenges. H2O2, for instance, is the simplest of the diols, which have been liquids of choice for many previous studies of dielectric relaxation.23-30 Propylene glycol (PG), for instance, not only has been studied in detail both in pure form,23-25 in confined geometries,26-29 and in solutions,30,31 but also has been one of a few selected substances on which other relaxation spectroscopies (e.g., heat capacity spectroscopy32,33) have been carried out. Notable among their features have been relatively small departures from exponential relaxation, weak secondary relaxations, and relatively nonfragile behavior. It is of interest to find the extent to which this behavior pattern carries over to the smallest molecules of this difunctional type. The comparison of diol with diamine characteristics, at the small molecule limit, is also an attractive aspect of this study.
Figure 1. Real (upper panel) and imaginary (lower panel) parts of the dielectric susceptibility of the solution (N2H4)20-(H2O)80 in the temperature range 112-154 K, within which no crystallization occurred. Curve fits are obtained using the Havriliak-Negami function, eq 1.
Experimental Section Hydrogen peroxide was purchased from Aldrich as the most concentrated aqueous solution commercially available, 50 wt % (33 mol %) H2O2. Higher concentrations may be studied34 but pose an explosion hazard. Hydrazine (98%) was purchased from Aldrich and diluted to the desired concentrations in the range 20-50 mol % hydrazine, using doubly distilled water. The cell in which the measurements were carried out is of simple form, designed for fast cooling at rates of order 10 K/s as determined elsewhere.35 To carry out the present measurements, the solutions were located between two stainless steel electrodes that define a 20 mm diameter of thickness determined by six spacer strips of 25 µm thickness Teflon. This yielded a geometric capacity of C0 ) 111.3 pF. The filled dielectric cell was immersed in liquid nitrogen to provide rapid cooling to the glassy state. The cell was then transferred to a nitrogen-gas filled cryostat the temperature of which was controlled and measured by a Novocontrol Quatro controller. The stability of the temperature reading is better than 0.05 K. Frequency-dependent impedance measurements were performed at fixed temperatures, controlled to (0.05 K, in a frequency range from 1 Hz to 1 MHz using a Solatron 1260 gain-phase analyzer equipped with a Mestec DM-1360 transimpedance amplifier. This measuring method requires a reference measurement to calibrate the frequency-dependent transimpedance, Zt(ω). For the present case, the empty capacitor was appropriate. Temperatures of the scans ranged from several tens of kelvins below Tg to a temperature where the data showed onset of time dependence due to crystallization onset. To analyze the frequency dependence of the complex permittivity, we employed the following fit function:
*(ω) ) ∞ +
σdc ∆ -i R γ 0ω [1 + (iωτHN) ]
(1)
where ∆ is the relaxation strength, τHN is the characteristic relaxation time of the Havriliak-Negami (HN) function,36 the value of which is close to the average relaxation time 〈τ〉, γ and R are the shape parameters, which represent asymmetric and symmetric broadening, respectively, ∞ is the high-frequency
Figure 2. Real (upper panel) and imaginary (lower panel) parts of the dielectric susceptibility of the solution (N2H4)36.5-(H2O)73.5 in the temperature range 110-162 K, within which no crystallization occurred. Curve fits are obtained using the Havriliak-Negami function, eq 1.
limiting permittivity, and ω is an angular frequency. Dcconductivity is quantified by the parameter σdc. Results Results of this study will be presented in different forms as needed by our analysis, but the basic data acquired are the real and imaginary parts of the dielectric susceptibility, ′ and ′′, of the solutions. These are presented as a compendium of the frequency scans for all the isotherms for which there was no interference from sample crystallization. We studied hydrazine and water solutions of compositions 20, 26.5, and 33 mol % N2H4 and present the raw data for the first two of these in Figures 1 and 2, respectively. The 26.5% solution gave the largest temperature range of data, essentially because it had the eutectic composition and thus the smallest probability of nucleating crystals. The phase diagram for this system is given in ref 34. In Figure 3, we show the smaller data set for the least rapidly crystallizing of the solutions in the system H2O2-H2O. This solution (33% H2O2) was the only solution in the H2O-H2O2 system that could be successfully obtained as a glass with our fastest cell cooling. Again it is close to the eutectic composition in the system, according to the phase diagram for this system.37
Dielectric Relaxation in Aqueous Solutions
J. Phys. Chem. B, Vol. 108, No. 51, 2004 19827
Figure 4. Master plot for loss spectra of the liquid state of the compound (N2H4)20-(H2O)80 showing time-temperature superposition for the temperature range 146-154 K. Figure 3. Real (upper panel) and imaginary (lower panel) parts of the dielectric susceptibility of the solution (H2O2)35-(H2O)65 at the temperatures 150 and 152 K, where full spectra could be obtained without crystallization. Curve fits are obtained using the Havriliak-Negami function, eq 1.
Because this solution was not very stable against crystallization, scans were taken on a reduced frequency set basis to maximize the temperature range that could be covered. The smooth curves through the data points are based on a fit with the HavriliakNegami equation (Fourier transformed from the time domain), and it is seen that the data are represented within experimental uncertainty by this equation. The fit using the three-parameter stretched exponential relaxation function that dates back to Kohlrausch is only slightly inferior.
Figure 5. Master plots for the loss spectra of the solution (N2H4)33(H2O)67, showing a systematic narrowing with increasing temperature in the range 144-154 K.
Discussion The dielectric spectra obtained in these two systems prove to be rather unusual in their simplicity. We will discuss first the spectral shapes, that is, the relaxation function and its dependence on temperature, then the temperature dependence of the relaxation time, and finally more general aspects of the system behavior. Of particular interest are the relation of the behavior to that of pure water in the temperature range 130160 K, and the implications of these observations to the nature of water structure in aqueous solutions. It is seen in Figures 1 and 2 that the data can be fitted over the entire range of frequencies by the HN equation. There is no secondary relaxation or high-frequency wing to be identified, only the 1/f noise that is present in all systems. For the 20% solution, which corresponds to the liquid state of one of the two compounds in the system,34 the spectra can be rather precisely superposed by shifting along the log frequency axis. Thus the solution obeys TTS (time-temperature superposition), where the effect of temperature reduces to shifting the characteristic time scale of the relaxation process. The full width at half-maximum (fwhm) of the Figure 2 spectra, which are superposed on the logarithmic scale of Figure 4, correspond to a stretching exponent in the stretched exponential representation of the relaxation function for viscous liquids38,39 of β ) 0.76 ( 0.04. This is unusually close to simple exponential behavior. Exponential relaxation is occasionally seen in the case of normal alcohols,23,40,41 where it is associated with a decoupling of the dielectric process from the structural relaxation. The latter may be up to 3 decades faster. In these cases, the value of Tg predicted from the dielectric relaxation time, by the rule Tg ) T(τ)100s) is found to be higher than the value determined by standard differential scanning calorimetry (DSC) methods.42 A similar decoupling has been expected for supercooled water on the basis of the lowtemperature relaxation times measured for structure and electric
Figure 6. Arrhenius plot of the dielectric relaxation times from the present work and from the microwave studies of refs 21 and 22. In the case of H2O2 solutions, extrapolations of the data of ref 22 are required to obtained values at the composition, 33% H2O2, of the present study. The insert shows that this can be done with some reliability since the values are almost invariant with composition. The data have been fitted with VFT equations that are constrained to have preexponential constants of 10-14 s, in order that values of the Vogel temperature, T0, can be validly compared with Kauzmann temperatures obtained from calorimetric studies. In fact, the best fit values are only marginally different from the constrained best fit values, see Table 1.
field.43 We may test for a similar origin of the approximate exponentiality of relaxation in the present case by using a short extrapolation of the dielectric relaxation time data to find the temperature at which τD ) 100 s and comparing with the measured value of Tg. To obtain such values, the relaxation times obtained from the peak frequencies of the Figures 1-3 spectra (used to produce the master plots of Figures 4 and 5) are shown in Arrhenius form in Figure 6. Included in the plot are data from the microwave studies of Verstakov et al.21 and Lyashchenko et al.22 for near-ambient temperature solutions of N2H4 and H2O2, repectively. The expected Vogel-Fulcher-Tammann behavior is seen, and short extrapolations to log τ ) 2.0 yield “dielectric Tg’s”,
19828 J. Phys. Chem. B, Vol. 108, No. 51, 2004
Minoguchi et al. study is the recent F1/2 fragility, assessed from Figure 6 by the relation47-49
Tg F1/2 ) 2 -1 T1/2
Figure 7. Dielectric Tg’s, using Tg ) T(τD)100s), compared with calorimetric values from refs 3 and 39. Corresponding values for the case of propylene glycol-H2O solutions (ref 30) are included for later discussion. The upward curving plots with question mark is one way of rationalizing the tan δ findings of Figure 8.
TABLE 1: Parameters Characterizing Relaxation Function and Relaxation Time Temperature Dependence for the Solutions of This Studya A. Free VFT Fitting τ0, s
D
T 0, K
Tg/T0
F1/2
β
3.16 × 5.01 × 10-14 3.16 × 10-14 1.2 × 10-14
11.71 10.55 11.00 10.19
102.5 104.3 105 109.5
1.33 1.30 1.31 1.28
0.58 0.60 0.60 0.64
0.72 0.8 0.75 0.72
x, mol % solute 20.0 26.5 33.0 33.0
N2H4 N2H4 N2H4 H2O2
10-14
B. Constrained VFT Fitting (τ0 ) 10-14 s) x, mol %
solute
D
20.0 26.5 33.0 33.0
N2H4 N2H4 N2H4 H2O glycerol46-48
T0, K
14.2 97.5 13.5 98 13.0 101 12.5 103 12850
TK, K 5 Tg/T0 Tg/TK
F1/2
102.5 104 105
0.56 0.58 0.59 0.60 0.55
13550
1.39 1.37 1.35 1.34 1.51
1.32 1.29 1.30 1.43
a
The different compositions are identified by their mole fractions x. Tg is taken as T for τHN ) 100 s.
which are plotted in Figure 7 along with data from the calorimetric studies of refs 3 and 44. It is seen that, except for the most water-rich case of the hydrazine solutions, the two Tg’s agree within experimental uncertainty. Thus the near-exponential behavior of the dielectric relaxation spectra would not seem to find an explanation in the presence of some slowly relaxing dielectrically dominant entity. Rather it must be correlated with the homogeneity of the structure. X-ray data for these solutions are not available to judge (from the prominence of a first sharp diffraction peak) the extent of intermediate range order that might exist. However, to the extent that intermediate range order is related to low fragility, some evidence is available from the parameters of the VFT equation fits of Figure 6. These are provided in Table 1, along with the Kauzmann temperatures obtained from the calorimetric studies of Oguni and co-workers45,46 for the compound compositions of 20% and 50% N2H4 and for the compound containing 33% H2O2 in the hydrogen peroxide-water system. As noted elsewhere,44 the parameter D of the modified VFT equation
τ ) τ0 exp
[ ] DT0 T - T0
(2)
is linearly related to the ratio Tg/T0, and either may be used as a measure of the “strength” of the liquid. According to this metric, none of the solutions is fragile; in fact, the Tg/T0 values are very similar to those of glycerol, an “intermediate” liquid, see Table 1. The least ambiguous measure of fragility from our
(3)
where T1/2 is the temperature where τ ) 10-6 s. It is seen in Table 1 that the values of the F1/2 fragility are close to that for glycerol. However, the relaxation is more nearly exponential than that for glycerol, and glycerol does not exhibit TTS. Also noteworthy in Table 1 is the proximity of the Vogel temperature, T0, to the Kauzmann temperature, TK. The latter may be assessed unambiguously for congruently melting compounds by the usual extrapolated vanishing of the excess entropy of liquid over crystal. It may also be assessed with equal rigor for cases of incongruently melting crystals.46 With greater uncertainty, it can be obtained from the enthalpy of fusion of a eutectic mixture, assuming that the entropy of mixing makes no significant contribution in the assessment of the Kauzmann temperature. The linear composition dependence of the values of TK assessed for the hydrazine solutions suggests that this is the case in the present instance.45,46 The ratio TK/T0 in each case in Table 1 is close to unity, providing a further example of this coincidence.44,50 As emphasized elsewhere,51 the coincidence means that thermodynamic fragility and kinetic fragility (Tg/TK and Tg/T0, respectively) are the same or very similar. These quantities are characteristic of liquids exhibiting intermediate fragility. As intermediate liquids, the fraction of the entropy of fusion that is residual at Tg is quite large (about 40%), and this highlights the difference between these solutions and the amorphous state of water in which the excess entropy of the amorphous state proves to be closer to that of the crystal than in most other cases.43 Evidently, the cooperative rush to the open network configurational ground state that characterizes water52,53 has been lost as the solution is formed. We now examine the behavior of the present solutions relative to that of water itself, both for its intrinsic interest and as a part of the further assessment of the high-frequency (normally the slow secondary relaxation and “high-frequency wing”) behavior of the present systems. It is well-known, since the pioneering work of Johari and Goldstein,54 that the presence of a β relaxation is most clearly revealed by plotting the dielectric loss, measured at a fixed frequency, against temperature. This is because (a) the temperature variable causes rapid change of relaxation time and so many decades of frequency change can be compressed into a small interval of temperature change and (b) the β relaxation separates rapidly from the R relaxation as temperature approaches and falls below the glass temperature. A plot with most of the same advantages is that of the ratio of imaginary-to-real parts of the susceptibility, known variously as tan δ, the “loss tangent”, or D, the dissipation factor. This quantity has the special advantage that it does not depend on the cell constant. This is the reason that it is often used for the presentation of dielectric data of samples in thin film or geometrically complex configurations. The outstanding case, for our present purposes, of its use is in the presentation of data on amorphous water obtained by vapor deposition ASW.55,56 It will also be useful when high-density amorphous water (HDA) is studied dielectrically in thin films obtained by high-pressure transformation of ice. We therefore present, as tan δ vs T in Figure 8, the data for the present solutions. We have recently57 used this plot to demonstrate that pure amorphous water is apparently nonrelaxing in the temperature
Dielectric Relaxation in Aqueous Solutions
Figure 8. Plots of the loss tangent, ′′/′, at constant f ) 1 kHz vs temperature for 33% solutions of N2H4 and H2O2 in H2O (open symbols) showing the sharp rise to the maximum at the temperature where the relaxation time (2πf)-1 is 1.6 × 10-4 s. Solid symbols are the data for “sintered” ASW from ref 54. Note that the change in slope at ∼135 K occurs when the dissipation due to the R relaxation rises in strength above the 1/f noise level and is not an indicator of the glass transition. If any one of the frequency, distribution of relaxation times, or fragility were to be different, the slope change would occur at a different temperature. For instance, in comparable 1 kHz studies of various molecular liquids described in ref 58, this change of slope occurred well below Tg.57
range 130-160 K and hence is most likely a glass up to crystallization (in contradiction of the general view of the past 2 decades). Here we use the plot to emphasize the absence of secondary relaxations, or even high-frequency wing behavior, in the present systems, which accordingly seem to have particularly simple dynamics. Evidently, the solutions commence their R relaxations without any preliminary loosening of their structures: the R relaxation rises directly out of the background 1/f noise that characterizes all condensed phases.58 In this respect, they are very water-like. The difference between the solutions and pure water must lie either in a much higher characteristic Tg value for water than is suggested by the simplest extrapolation of the solution values (see Figure 7) or in the occurrence of a dramatic increase in the “strength” of the liquid as the solute is removed.57,59 The previous belief that the behavior of water between 136 and 150 K is that of an unexceptional fragile liquid with Tg of 136 K60,61 is ruled out, since even the present rapidly relaxing solutions (of Tg ) 136140 K) are not fragile. What must now be recognized is the surprising indication that a Tg value of 136 K may still be very relevant to water, but to water in the HDA, not the LDA, form. Indeed, this rather ironical twist provides the basis for an important part of our discussion, as follows. In a recent, very instructive study of the warm-up characteristics of pressure amorphized water in emulsion form with macroscopic (1-10 µm) droplet sizes, Mishima showed62 by comparison with another good glass-forming aqueous solution, bulk LiCl‚11H2O6 (and also for lower concentration solutions rendered glass-forming by the increased pressure63,64), that an endothermic effect detected during the decompression of HDA at constant temperature was best interpreted as a glass transition immediately preceding crystallization. By studying the phenomenon at different temperatures, Mishima was able to obtain the pressure dependence of the Tg of HDA. The Tg deduced for HDA was found to be essentially the same as that observed for the LiCl solutions, for which the Tg was always unambiguously determined. Like the H2O2 solutions of the present study, aqueous LiCl has a constant value of Tg over a rather wide range of compositions. Both the value for the LiCl solution and the value for pure HDA extrapolated to 136 K at ambient pressure! (Since some of the isotherms studied
J. Phys. Chem. B, Vol. 108, No. 51, 2004 19829 lay as high as 170 K, it seems likely that Mishima’s findings refer also to the more stable very high density amorphous water (VHDA), which is a configurationally relaxed form of HDA.7 VHDA should have the same value of Tg as HDA but should be better defined.) Mishima stressed that the phenomenon he observed was strongly endothermic, unlike the weak effect induced in LDA by annealing65 and assigned recently66 to a “shadow” transition. Thus, just as in the cases of BeF2 and SiO2, Tg for the highdensity form of the liquid lies below the value for the lowdensity form. This Tg relationship for the polyamorphs of water had, in fact, been suggested earlier (on the basis of the behavior of BeF2, where the difference in Tg values is very large).7 However, it had not been anticipated that the value for the highdensity form would be the same as that extrapolated from so many aqueous solutions.2-6 The significance of this observation was not missed by Mishima, who remarked “that the emulsified HDA is the solvent water of the high concentration electrolyte solution”.62 We wish to extend this observation to the great majority of ambient pressure aqueous solutions the Tg values of which extrapolate to the value 136 K, including of course the present highly hydrogen-bonded solutions, and discuss its significance in more detail. It has long been recognized that the effect of adding solutes to water is correlated with the effect of pressure. For instance, at low temperatures where the structure of water is well developed both decrease the viscosity.1,67 A pressure equivalent of the solute concentration could be assigned. An advantage of studying processes near the “bottom” of the liquid state, that is, near Tg, is that structural effects are then seen in their most pronounced form. It is therefore significant that almost all Tg extrapolations indicate the same Tg, not some value intermediate between the >165 K now assigned to LDA and 136 K, as would be reasonable to expect if each solute affected the water structure in a different way. We see the observed behavior as very consistent with the existence of two distinct polyamorphic forms and note that this information is being obtained in the ergodic, not the glassy, state. Evidently the free energies of water assemblages (clusters, fluctuations) with bond angles intermediate between those of ice I (LDA) and ice III (HDA or VHDA) are higher than that of some combination of different amounts of the separate polyamorphs so that, even locally, water exists in one of the two preferred forms. What the findings of Mishima, supported by the additional considerations advanced here, imply is that the open network structure of LDA is very sensitive to disturbance and in the presence of almost any solute will tip over into the HDA-like configuration. We are of course, just adding additional, low-temperature confirmation to the “two-state” structural models often advanced for the structure of aqueous solutions, particularly those that emphasize the disordered ice III-like character for the dense state. As in the other cases, we are also failing to make any statement about the length scale for the ice III-like groups. However, we do suggest that, in the great majority of nondilute solutions, the only water arrangements that occur are those with ice III-like angular relations. A reasonable structural model would be one in which the length scale for the water configurations grows continuously with increasing water content via the development of some sort of “wormhole” structure,6 which has the local H-bond angles of VHDA (or ice III). Likewise, in pure water at low temperatures, a “wormhole” structure involving bicontinuous structures of ice I- and ice III-like nature would be a reasonable proposition.
19830 J. Phys. Chem. B, Vol. 108, No. 51, 2004 It should be noted that, with any single glass or glass-forming liquid, there is not a single structure to consider but rather a continuous range of structures depending on the pressure under which the system is examined. It is therefore not possible to assign a single density to LDA or VLDA, as emphasized by recent simulation results.68 Rather, a continuum exists. However, for solutions in the two-state region, the effect of changing pressure may be different, primarily changing the proportions of the contributing states rather than changing the structure of either one. Thus structural studies of solutions at low temperatures across the pressure range of the two-state equilibrium might prove interesting. Near the glass-forming range boundary, at any pressure, it may be possible, with freeze fracture electron microscopy or perhaps atomic force microscopy, to observe these structures in nanoscopic phase-separated form, where the separated structure now has the ice I angular relations. It is likely that it is the sudden conversion of HDA-like structures to LDA-like structures that is involved in the explosive devitrification of aqueous solutions near the glass-forming limit, particularly if the glass has been formed at high pressure from compositions that are not normally glass-forming. Concluding Remarks The structures that water adopts in propylene glycol, PG, solutions are evidently not of the type that have been discussed above, if we judge by the behavior of Tg in aqueous PG solutions seen in Figure 7. Since the theme of -OH groups spaced by alkyl and other hydrocarbon groups is a common theme in biological structures, this raises the issue of which are the preferred water arrangements in biological structures and whether switches between the two, provoked by temperature or pressure changes, may have a biological role to play. It is often observed that with decreasing water content in aqueous protein solutions the value of Tg stops changing once it has reached 160 K, with obvious implications. This question will be the subject of systematic work in the future. Acknowledgment. We are grateful for the support of the NSF under Solid State Chemistry Grant No. 0082535. References and Notes (1) Franks, H. In Water: A ComprehensiVe Treatise; Franks, F., Ed.; Plenum Press: London, 1982. (2) Angell, C. A. In Water: A ComprehensiVe Treatise; Franks, F., Ed.; Plenum Press: London, 1982; pp 215-338. (3) Oguni, M.; Angell, C. A. J. Chem. Phys. 1980, 73, 1948. (4) Ghormley, J. A. J. Am. Chem. Soc. 1957, 79, 1862. (5) Angell, C. A.; MacFarlane, D. R.; Oguni, M. Ann. N. Y. Acad. Sci. 1986, 484, 241. (6) Angell, C. A.; Sare, E. J. J. Chem. Phys. 1970, 52, 1058. (7) Angell, C. A. Chem. ReV. 2002, 102, 2627. (8) Angell, C. A. Annu. ReV. Phys. Chem. 2004, 55, 559. (9) (a) Burton, E. F.; Oliver, W. F. Nature 1935, 135, 505. (b) Burton, E. F.; Oliver, W. F. Proc. R. Soc. London 1935, A153, 166. (10) Mayer, E. J. Appl. Phys. 1985, 58, 663; J. Microsc. 1985, 140, 3; J. Microsc. 1986, 141, 269. (11) (a) Mishima, D.; Calvert, L. D.; Whalley, E. Nature 1984, 310, 393. (b) Mishima, D., Calvert, L. D.; Whalley E. Nature 1985, 314, 76. (12) Sceats, M. G.; Rice, S. A. In Water: A ComprehensiVe Treatise; Franks, F., Ed.; Plenum Press: London, 1982; pp 83-211. (13) Hallbrucker, A.; Mayer, E. J. Phys. Chem. 1987, 91, 503.
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