The JG β-relaxation in water and impact on the dynamics of aqueous mixtures and hydrated biomolecules

Abstract

Although by now the glass transition temperature of uncrystallized bulk water is generally accepted to manifest at temperature Tg near 136 K, not much known are the spectral dispersion of the structural α-relaxation and the temperature dependence of its relaxation time τ α,bulk (T). Whether bulk water has the supposedly ubiquitous Johari-Goldstein (JG) β-relaxation is a question that has not been answered. By studying the structural α-relaxation over a wide range of temperatures in several aqueous mixtures without crystallization and with glass transition temperatures Tg close to 136 K, we deduce the properties of the α-relaxation and the temperature dependence of τ α,bulk (T) of bulk water. The frequency dispersion of the α-relaxation is narrow, indicating that it is weakly cooperative. A single Vogel-Fulcher-Tammann (VFT) temperature dependence can describe the data of τ α,bulk (T) at low temperatures as well as at high temperatures from neutron scattering and GHz-THz dielectric relaxation, and hence, there is no fragile to strong transition. The Tg-scaled VFT temperature dependence of τ α,bulk (T) has a small fragility index m less than 44, indicating that water is a "strong" glass-former. The existence of the JG β-relaxation in bulk water is supported by its equivalent relaxation observed in water confined in spaces with lengths of nanometer scale and having Arrhenius T-dependence of its relaxation times τ conf (T). The equivalence is justified by the drastic reduction of cooperativity of the α-relaxation in nanoconfinement and rendering it to become the JG β-relaxation. Thus, the τ conf (T) from experiments can be taken as τ β,bulk (T), the JG β-relaxation time of bulk water. The ratio τ α,bulk (Tg)/τ β,bulk (Tg) is smaller than most glass-formers, and it corresponds to the Kohlrausch α-correlation function, exp[−(t/τ α,bulk ) 1−n ], having (1−n) = 0.90. The dielectric data of many aqueous mixtures and hydrated biomolecules with Tg higher than that of water show the presence of a secondary ν-relaxation from the water component. The ν-relaxation is strongly connected to the α-relaxation in properties, and hence, it belongs to the special class of secondary relaxations in glass-forming systems. Typically, its relaxation time τν(T) is longer than τ β,bulk (T), but τν(T) becomes about the same as τ β,bulk (T) at sufficiently high water content. However, τν(T) does not become shorter than τ β,bulk (T). Thus, τ β,bulk (T) is the lower bound of τν(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is τ β,bulk (T) but not τα(T) that is responsible for the dynamic transition of hydrated globular proteins.

Supercooled water confined in a metal-organic framework

“…45, based on an estimate of the -relaxation time of bulk water using the coupling model, 101,102 which leads to values consistent with those experimentally found for hydrous MCM-41 at low temperatures, T < T cr . 45 The above rationalization of the crossover at T cr is supported by the fact that both the VFT and the Arrhenius regions of the (1/T) curve can be consistently described assuming the same attempt frequency, 1/(2 0 ), as demonstrated in the inset of Fig. 3.…”

“…2(b)], lies well within the region often considered for the "true" water dynamics of supercooled water within the no-man's land. 19,45 For example, the peak frequencies are located at similar frequencies as for the "ideal" confinement host 19,45 MCM-41 with 2.1 nm pore size (cf., e.g., Fig. 3 of Ref.…”

“…46 However, one should be aware that in several works the much slower main confined-water relaxation, corresponding to process 3 of the present work, was also suggested to reflect the JG relaxation of bulk water. 19,45,79 In Ref. 46 this apparent discrepancy was resolved by proposing two different  relaxations for confined water.…”

“…Different scenarios were proposed to explain the numerous, partly contradicting experimental findings for confined supercooled water, speaking both in favor or against the suggested liquid-to-liquid transition (see Refs. 19,45, and 46 for examples). Obviously, despite the vast database on confined water, drawing unequivocal conclusions that would lead to a consensus in the community, is difficult.…”

“…19 water confined in the porous amorphous silica MCM-41, which contains very regular cylindrical pores, was suggested to represent an "ideal model system of confined water". In a very recent work, 45 relating the dynamics of confined water for various host systems to a secondary relaxation, the molecular water dynamics in MCM-41 also was considered to effectively represent the intrinsic bulk-water dynamics.…”