Water in confined geometries has obvious relevance in biology, geology, and other areas where the material properties are strongly dependent on the amount and behavior of water in these types of materials. Another reason to restrict the size of water domains by different types of geometrical confinements has been the possibility to study the structural and dynamical behavior of water in the deeply supercooled regime (e.g., 150-230 K at ambient pressure), where bulk water immediately crystallizes to ice. In this paper we give a short review of studies with this particular goal. However, from these studies it is also clear that the interpretations of the experimental data are far from evident. Therefore, we present three main interpretations to explain the experimental data, and we discuss their advantages and disadvantages. Unfortunately, none of the proposed scenarios is able to predict all the observations for supercooled and glassy bulk water, indicating that either the structural and dynamical alterations of confined water are too severe to make predictions for bulk water or the differences in how the studied water has been prepared (applied cooling rate, resulting density of the water, etc.) are too large for direct and quantitative comparisons.

No abstract

2H NMR is used to study the mechanisms for the reorientation of protein hydration water. In the past, crossovers in temperature-dependent correlation times were reported at Tx1 approximately 225 K (X1) and Tx2 approximately 200 K (X2). We show that neither X1 nor X2 are related to a fragile-to-strong transition. Our results rule out an existence of X1. Also, they indicate that water performs thermally activated and distorted tetrahedral jumps at T < Tx2, implying that X2 originates in an onset of this motion, which may be related to a universal defect diffusion in materials with defined hydrogen-bond networks.

The microscopic details of local particle dynamics is studied in a glass-forming one component supercooled liquid modeled by a Dzugutov potential developed for simple metallic glass formers. Our main goal is to investigate particle motion in the supercooled liquid state, and to ascertain the extent to which this motion is cooperative and occurring in quasi-one-dimesional, string-like paths. To this end we investigate in detail the mechanism by which particles move along these paths. In particular, we show that the degree of coherence--that is, simultaneous motion by consecutive particles along a string--depends on the length of the string. For short strings, the motion is highly coherent. For longer strings, the motion is highly coherent only within shorter segments of the string, which we call "microstrings." Very large strings may contain several microstrings within which particles move simultaneously, but individual microstrings within a given string are temporally uncorrelated with each other. We discuss possible underlying mechanism for this complex dynamical behavior, and examine our results in the context of recent work by Garrahan and Chandler [Phys. Rev. Lett. 89, 035704 (2002)] in which dynamic facilitation plays a central role in the glass transition.

(2)H NMR reveals two dynamic crossovers of supercooled water in nanoscopic (∼2 nm) confinement. At ∼225 K, a dynamic crossover of liquid water is accompanied by formation of a fraction of solid water. Therefore, we do not attribute the effect to a liquid-liquid phase transition but rather to a change from bulk-like to interface-dominated dynamics. Moreover, we argue that the α process and β process are observed in experiments above and below this temperature, respectively. Upon cooling through a dynamic crossover at ∼175 K, the dynamics of the liquid fraction becomes anisotropic and localized, implying solidification of the corresponding water network, most probably, during a confinement-affected glass transition.

We study the Johari–Goldstein β process of organic glass formers by one- (1D) and two-dimensional (2D) H2 nuclear magnetic resonance (NMR). In particular, we compare systems with pronounced secondary relaxation in dielectric spectroscopy, namely toluene-d5 and polybutadiene-d6 (PB), with compounds which do not exhibit a distinct β peak, i.e., glycerol-d5 and polystyrene-d3 (PS). Choosing large interpulse delays in the applied echo pulse sequences we increase the sensitivity on small angle rotational jumps. This way, we are able to probe clearly the β process of toluene and PB in the line shape of 1D 2H NMR spectra and in the orientational correlation functions of 2D 2H NMR in time domain which is not possible when using the conventional techniques. Below the glass transition temperature Tg, the secondary relaxation of both glass formers is caused by a highly restricted reorientation of essentially all molecules. Comparing our results with simulations we estimate that the reorientation of most toluene molecules and PB monomeric units is characterized by an amplitude χ&lt;10°. This amplitude is approximately unchanged below Tg, but strongly increases above the glass transition. Closer investigating the 1D 2H NMR line shape for large interpulse delays we moreover demonstrate that the reorientation involved in the β process takes place step-by-step via many elementary rotational jumps. On the other hand, for glycerol and PS, hardly any effects are observed in 1D and 2D 2H NMR experiments below Tg when applying comparable experimental parameters. We conclude that reorientations with an amplitude χ&gt;1° do not occur on a time scale of μs−ms for the majority of molecules in glassy glycerol and PS.

2 H NMR spin-lattice relaxation and line-shape analyses are performed to study the temperaturedependent dynamics of water in the hydration shells of myoglobin, elastin, and collagen. The results show that the dynamical behaviors of the hydration waters are similar for these proteins when using comparable hydration levels of h = 0.25 − 0.43. Since water dynamics is characterized by strongly nonexponential correlation functions, we use a Cole-Cole spectral density for spin-lattice relaxation analysis, leading to correlation times, which are in nice agreement with results for the main dielectric relaxation process observed for various proteins in the literature. The temperature dependence can roughly be described by an Arrhenius law, with the possibility of a weak crossover in the vicinity of 220 K. Near ambient temperatures, the results substantially depend on the exact shape of the spectral density so that deviations from an Arrhenius behavior cannot be excluded in the hightemperature regime. However, for the studied proteins, the data give no evidence for the existence of a sharp fragile-to-strong transition reported for lysozyme at about 220 K. Line-shape analysis reveals that the mechanism for the rotational motion of hydration waters changes in the vicinity of 220 K. For myoglobin, we observe an isotropic motion at high temperatures and an anisotropic largeamplitude motion at low temperatures. Both mechanisms coexist in the vicinity of 220 K.13 C CP MAS spectra show that hydration results in enhanced elastin dynamics at ambient temperatures, where the enhancement varies among different amino acids. Upon cooling, the enhanced mobility decreases. Comparison of 2 H and 13 C NMR data reveals that the observed protein dynamics is slower than the water dynamics.

The potential energy landscape (PEL) of supercooled binary Lennard-Jones (BLJ) mixtures exhibits local minima, or inherent structures (IS), which are organized into meta-basins (MB). We study the particle rearrangements related to transitions between both successive IS and successive MB for a small 80:20 BLJ system near the mode-coupling temperature TMCT . The analysis includes the displacements of individual particles, the localization of the rearrangements and the relevance of string-like motion. We find that the particle rearrangements during IS and MB transitions do not change significantly at TMCT . In particular, an onset of single particle hopping on the length scale of the inter-particle distance is not observed. Further, it is demonstrated that IS and MB dynamics are spatially heterogeneous and facilitated by string-like motion. To investigate the mechanism of string-like motion, we follow the particle rearrangements during suitable sequences of IS transitions. We find that most strings observed after a series of transitions do not move coherently during a single transition, but subunits of different sizes are active at different times. Several findings suggest that, though string-like motion is of comparable relevance when the system explores a MB and when it moves from one MB to another, the occurrence of a successful string enables the system to exit a MB. Moreover, we show that the particle rearrangements during two consecutive MB transitions are basically uncorrelated. Specifically, different groups of particles are highly mobile during subsequent MB transitions. We further find the positions of strings during successive MB transitions weakly but positively correlated supporting the idea of dynamic facilitation. Finally, the relation between the features of the PEL and the relaxation processes in supercooled liquids is discussed.