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  • Title: The JG β-relaxation in water and impact on the dynamics of aqueous mixtures and hydrated biomolecules.
    Author: Capaccioli S, Ngai KL, Ancherbak S, Bertoldo M, Ciampalini G, Thayyil MS, Wang LM.
    Journal: J Chem Phys; 2019 Jul 21; 151(3):034504. PubMed ID: 31325935.
    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 τα,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 τα,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 τα,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 τα,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 τconf(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 τconf(T) from experiments can be taken as τβ,bulk(T), the JG β-relaxation time of bulk water. The ratio τα,bulk(Tg)/τβ,bulk(Tg) is smaller than most glass-formers, and it corresponds to the Kohlrausch α-correlation function, exp[-(t/τα,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 τβ,bulk(T), but τν(T) becomes about the same as τβ,bulk(T) at sufficiently high water content. However, τν(T) does not become shorter than τβ,bulk(T). Thus, τβ,bulk(T) is the lower bound of τν(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is τβ,bulk(T) but not τα(T) that is responsible for the dynamic transition of hydrated globular proteins.
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