Water's second glass transition

Abstract

The glassy states of water are of common interest as the majority of H2O in space is in the glassy state and especially because a proper description of this phenomenon is considered to be the key to our understanding why liquid water shows exceptional properties, different from all other liquids. The occurrence of water's calorimetric glass transition of low-density amorphous ice at 136 K has been discussed controversially for many years because its calorimetric signature is very feeble. Here, we report that high-density amorphous ice at ambient pressure shows a distinct calorimetric glass transitions at 116 K and present evidence that this second glass transition involves liquid-like translational mobility of water molecules. This "double Tg scenario" is related to the coexistence of two liquid phases. The calorimetric signature of the second glass transition is much less feeble, with a heat capacity increase at Tg,2 about five times as large as at Tg,1. By using broadband-dielectric spectroscopy we resolve loss peaks yielding relaxation times near 100 s at 126 K for low-density amorphous ice and at 110 K for high-density amorphous ice as signatures of these two distinct glass transitions. Temperature-dependent dielectric data and heating-rate-dependent calorimetric data allow us to construct the relaxation map for the two distinct phases of water and to extract fragility indices m = 14 for the low-density and m = 20-25 for the high-density liquid. Thus, low-density liquid is classified as the strongest of all liquids known ("superstrong"), and also high-density liquid is classified as a strong liquid.

Keywords: dielectric relaxation; differential scanning calorimetry; polyamorphism; supercooled water.

Fig. 1.

Two possible scenarios to explain the metastable phase behavior of deeply supercooled water based on experimental literature T_g data for LDA (filled black triangle) (–6) and HDA (filled black circles) (14) and compared with glass transition temperatures from computer simulations (lines) (17). The single T_g scenario, in A indicated by the dash-dotted line, is qualitatively similar to the situation found in water simulations using the SPC/E model (17), whereas the double T_g scenario in B is qualitatively similar to the situation found for the ST2 model (17). The dashed line corresponds to the T_g,2 line for HDA, whereas the dotted line corresponds to the predicted T_g,1 line for LDA. The observation of a second T_g (open circle) at ambient pressure reported in this work rules out the possibility of a single T_g scenario.

Fig. 2.

DSC scans of 20.3 mg eHDA. The samples were first heated to 123 K (trace 1, light gray), followed by cooling at 30 K/min to 90 K and subsequent heating to 123 K (trace 2, dark gray), followed by cooling at 30 K/min to 90 K and subsequent heating to 145 K, at which the temperature was kept for 10 min at 145 K (trace 3, blue). The exotherm in trace 3, blue, indicates transformation to LDA. After cooling at 30 K/min to 90 K, the sample (now LDA) is heated to 253 K (trace 4, black). The exotherm in trace 4 indicates crystallization of the sample to cubic ice. The glass transition onset temperatures of LDA and eHDA are marked by T_g,1 and T_g,2, respectively, and the corresponding increase in heat capacity by Δc_p,1 and Δc_p,2. Baseline was corrected by subtracting a scan of hexagonal ice. All curves are shifted vertically for clarity. The inset shows the shift in T_g,2 in a heating scan after changing the cooling rate from 30 K/min to 1 K/min. The heating scans displayed in the inset were obtained using the same protocol. The blue and gray heating scans were measured using the protocol also used for the heating scans 2 and 3 in the main figure, whereas the red and yellow heating scans were both measured after cooling the sample 1 K/min. In the inset the blue/gray pair of traces and also the red/yellow pair are shown without vertical shift to demonstrate the exact match of two subsequent heating scans when using the same cooling/heating rate combination. The rate-dependent shift in T_g upon changing the cooling rate is a characteristic feature of the glass transition. Heating rate is always 10 K/min.

Fig. 3.

Dielectric loss spectra of (A) HDL and (B) LDL are plotted as connected open symbols for several temperatures. (A) At 126 K the plusses reflect measurements acquired while HDL transforms to LDL. The crosses demonstrate that the relaxation in LDL is slower than in HDL. For comparison, the dielectric loss spectrum of ultraviscous glycerol (at T = 196 K, loss divided by 60) (44) is added (dashed line). (B) The transformation of LDL to cubic ice (stars) takes place above 151 K and is recognized from a shift of the spectra to lower frequencies.

Fig. 4.

Relaxation map of H₂O phases obtained from eHDA. Blue circles, diamonds, and squares refer to dielectric measurements. The numbered green arrows indicate the thermal history of our samples—that is, the temperature program used. The filled symbols were determined directly from peak frequencies, while the crossed symbols were obtained by applying time temperature superposition. The dash-dotted lines correspond to temperatures at which transitions occur; the dotted line marks a time scale of 100 s, which is usually associated with the glass transition temperature. Red triangles correspond to the calorimetric relaxation times τ_cal, calculated using Eq. 1, for heating rates of q = 5, 10, and 30 K/min, which agree excellently with the dielectric time constants.