Multiple relaxation processes versus the fragile-to-strong transition in confined water

""Broadband dielectric spectroscopy data on water confined in three different environments, namely. at the surface of a globular protein or inside the small pores of two silica substrates, in the. temperature range 140 K r T r 300 K, are presented and discussed in comparison with previous. results from different techniques. It is found that all samples show a fast relaxation process,. independently of the hydration level and confinement size. This relaxation is well known in the. literature and its cross-over from Arrhenius to non-Arrhenius temperature behavior is the object. of vivid debate, given its claimed relation to the existence of a second critical point of water. We. find such a cross-over at a temperature of B180 K, and assign the relaxation process to the layer. of molecules adjacent and strongly interacting with the substrate surface. This is the water layer. known to have the highest density and slowest translational dynamics compared to the average:. its apparent cross-over may be due to the freezing of some degree of freedom and survival of very. localized motions alone, to the onset of finite size effects, or to the presence of a calorimetric glass. transition of the hydration shell at B170 K. Another relaxation process is visible in water. confined in the silica matrices: this is slower than the previous one and has distinct temperature. behaviors, depending on the size of the confining volume and consequent ice nucleation.. I. Introduction. Pure bulk water, as most compounds, can be kept liquid below. its melting temperature, where is said supercooled. This. metastable phase extends down to about 235 K: below this. temperature water crystallizes, unless hyperquenching (a cooling. rate of the order of 107 K s1) is applied to get a glassy form of. water.1 There is experimental evidence2–4 for devitrification of. hyperquenched glassy water at about Tg = 136 K (that is said. the glass transition temperature), and existence of an ultraviscous. liquid phase, which eventually crystallizes above. 150 K. Thus, below the thermodynamic region of existence. of the stable liquid, there are two metastable phases of liquid. water, which are separated by a region where only crystalline. polymorphs of water exist: this region is traditionally called. ‘‘the no-man’s land’’,5 and only theoretical conjectures or. computer simulations can be used to infer what should be. the behavior of liquid water if it could enter this forbidden. region.6–8 When the ‘‘no man’s land’’ is approached from. higher temperatures, several thermodynamic response functions. increase so fast that seem to diverge at some point. It is. object of debate whether these functions do really diverge or. experience a maximum and, consequently, which theoretical. scenario or conjecture best""