Nature of Water’s Second Glass Transition Elucidated by Doping and Isotope Substitution Experiments

Abstract

Based on calorimetry and dielectric spectroscopy, the influence of dopants as well as H/D-isotope substitution on the dynamics and thermodynamics of expanded high-density amorphous ice (eHDA) is studied. We find that dopants do not significantly alter the phase behavior, the dielectric relaxation times, and the calorimetric glass transition of eHDA. These observations starkly contrast those made for crystalline ices such as ice I h , ice V, ice VI, and ice XII, where suitable dopants enhance the dielectric dynamics by several orders of magnitude and can trigger hydrogen order-disorder transitions, then taking place below the orientational glass transition temperature of undoped samples. This conspicuous contrast to the behavior of crystalline ices strongly argues against point-defect dynamics in amorphous ices and against a previously suggested "crystallinelike" nature of the amorphous ices. Furthermore, H/D substitution also does not affect the calorimetric glass transition in eHDA much, whereas for crystalline ices, the heat capacity increase at the glass transition is roughly halved. In addition, the H/D-isotope shift of the glass transition onset is much larger for crystalline ices than it is for amorphous ices. This observation favors the notion of eHDA's glass transition as a glass-to-liquid transition and is evidence against a mere molecular-reorientation unfreezing at water's second glass transition. Comparing the isotope effect on activation energies for dielectric relaxation with ice V suggests that in amorphous ice water molecules move translationally above T g. Thus, the present work strongly supports that above this glass transition, water does indeed exist in its contested high-density liquid state.

Deeply supercooled aqueous LiCl solution studied by frequency-resolved shear rheology

“…These findings agree with the orientational glass transition temperature of ice VI (≈128 K) being slightly lower than the ones of ice V and ice XII (≈131 K); see Table II in Ref. 46. However, based on its calorimetric orientational glass transition temperature (≈140 K), the time scale for ice IV would be expected to be significantly longer than for all other high-pressure crystalline ices compiled in Ref.…”

“…50 These (and other nominally pure) crystalline ices are well known to display orientational glass transitions. 46,[68][69][70] During such a transition, the hydrogen degrees of freedom freeze upon cooling, giving rise to a calorimetrically detectable signature, while the crystalline center-of-mass lattice remains unaffected. In dielectric spectroscopy, the freezing transition of the crystalline ices is typically accompanied by a thermally activated slow-down of the dipolar degrees of freedom.…”

“…40 Previous dielectric studies concerned with the dynamics of HGW or ASW were either conducted as a function of temperature at a few frequencies, [9][10][11]41 or the samples were studied after pore collapse (with information on impurities trapped from the background gas between resulting lamellae or in remaining micropores not available). 42,43 Effects of doping on the dynamics of amorphous 16,[44][45][46] and crystalline [47][48][49] ices were also intensively examined. From dielectric as well as from calorimetric work, it has become clear that the implantation of defects into the H 2 O network can make a distinctive difference.…”

“…48,49 Conversely, for the numerous dopants tested so far, such enhancements appear to be absent in the amorphous ices. 16,[44][45][46] In order to explore the extent to which various undoped ice forms can be distinguished on the basis of dynamical properties, here we report on broadband dielectric spectra for HGW and compare them with dielectric results obtained for the amorphous ices ASW, 11,42,43 LDA, and HDA. 53 Further comparison is made with dielectric data reported for various crystalline ices, again, without intentional impurity doping.…”

Glass polymorphism in TIP4P/2005 water: A description based on the potential energy landscape formalism

“…23,[29][30][31][32][33]. However, the true nature of LDA and HDA [34,35] and their relationship with the liquid state are still a matter of debate [36][37][38][39][40][41][42].…”