Long-Range Diffusion in Xylitol–Water Mixtures

The Nature of the Low-Temperature Crossover of Water in Hard Confinement

The dynamics of water confined in mesoporous MIP (2-3 nm pores in size) with silica gel (secondary silica; further, the abbreviation SG will be used) and MAP (10-35 nm pores in size) without SG borosilicate glasses have been studied by broadband dielectric spectroscopy (BDS), nuclear magnetic resonance (NMR), and differential scanning calorimetry (DSC). MIP samples contain secondary silica inside the pores and provide a confinement size of about 2-3 nm, whereas MAP samples are free of secondary silica and provide a confinement size of about 10-35 nm. It is shown by BDS and NMR techniques that water exhibits a dynamic crossover of around 180 K when it is confined in MIP samples. By contrast, water confined in larger pores (MAP) does not exhibit any changes in its relaxation behavior. It is also shown that the crossover temperature depends on the hydration level (the higher the hydration level, the lower the crossover temperature). Below the crossover temperature, we find that water reorientation is isotropic (NMR) and that the temperature-dependent dielectric relaxation strength (BDS) follows the tendency expected for a solid-like material. In contrast, water reorientation is related to long-range diffusion above the crossover temperature, and the dielectric relaxation strength follows the tendency expected for a liquid-like material. Furthermore, the calorimetric results are compatible with crossing a glass transition near 180 K. Finally, the results are discussed within the Gibbs-Thomson model. In this framework, the crossover could be related to ice crystals melting.

Dynamics of aqueous peptide solutions in folded and disordered states examined by dynamic light scattering and dielectric spectroscopy

Characterizing the segmental dynamics of proteins, and intrinsically disordered proteins in particular, is a challenge in biophysics. In this study, by combining data from broadband dielectric spectroscopy (BDS) and both depolarized (DDLS) and polarized (PDLS) dynamic light scattering, we were able to determine the dynamics of a small peptide [ε-poly(lysine)] in water solutions in two different conformations (pure β-sheet at pH = 10 and a more disordered conformation at pH = 7). We found that the segmental (α-) relaxation, as probed by DDLS, is faster in the disordered state than in the folded conformation. The water dynamics, as detected by BDS, is also faster in the disordered state. In addition, the combination of BDS and DDLS results allows us to confirm the molecular origin of water-related processes observed by BDS. Finally, we discuss the origin of two slow processes (A and B processes) detected by DDLS and PDLS in both conformations and usually observed in other types of water solutions. For fully homogeneous ε-PLL solutions at pH = 10, the A-DLS process is assigned to the diffusion of individual β-sheets. The combination of both techniques opens a route for understanding the dynamics of peptides and other biological solutions.

Ultraslow Relaxation in Aprotic Double Salt Ionic Liquids

A mixture of two pure ionic liquids (ILs) or double salt ILs (DSILs) can push the limits of ILs in terms of unraveling their unique physicochemical properties and potential in clean technology. While the correlated ion dynamics and heterogeneity in the bulk of pure ILs have been reported, such a phenomenon at longer timescales in DSILs has never been elucidated. Here, a combination of temperature-dependent polarized dynamic light scattering and rheological measurements has been employed to reveal the presence of structural and ultraslow relaxation in three DSILs, each containing a 1-ethyl-3-methylimidazolium cation and two different anions. The slow relaxation caused by Brownian diffusion of cluster-like arrangements occurs at a timescale of a few to several hundred milliseconds; both the relaxation processes, nevertheless, are Arrhenius in nature. Notably, slow relaxation in the DSILs is much different compared to that in the pure ILs. The decay of intensity correlation functions (ICFs) and average hydrodynamic correlation length of the clusters and their response to temperature markedly vary with the nature of the two anions present in the DSILs. Stretched exponential analyses of the ICFs disclose the cluster-to-cluster transfer of ionic species as well as percolation dynamics among clusters. The identity of anions also governs whether the DSILs follow or violate the Stokes-Einstein relationship or not.

Brownian motion of single glycerol molecules in an aqueous solution as studied by dynamic light scattering

Aqueous solutions of glycerol are investigated by dynamic light scattering (DLS) over the whole concentration range (10-98 wt.% water) and in the temperature range 283-303 K. The measurements reveal one slow relaxation process in the geometry of polarized light scattering. This process is present in the whole concentration range, although it is very weak at the highest and lowest water concentrations and is considerably slower than the structural α relaxation, which is too fast to be observed on the experimental time scale in the measured temperature range. The relaxation time of the observed process exhibits a 1/q2 dependence, proving that it is due to long-range translational diffusion. The Stokes-Einstein relation is used to estimate the hydrodynamic radius of the diffusing particles and from these calculations it is evident that the observed relaxation process is due to the Brownian motion of single or a few glycerol molecules. The fact that it is possible to study the self-diffusion of such small molecules may stimulate a broadening of the research field used to be covered by the DLS technique.

Cause of the fragile-to-strong transition observed in water confined in C-S-H gel

In this study, the rotational dynamics of hydration water confined in calcium-silicate-hydrate (C-S-H) gel with a water content of 22 wt.% was studied by broadband dielectric spectroscopy in broad temperature (110-300 K) and frequency (10(-1)-10(8) Hz) ranges. The C-S-H gel was used as a 3D confining system for investigating the possible existence of a fragile-to-strong transition for water around 220 K. Such transition was observed at 220 K in a previous study [Y. Zhang, M. Lagi, F. Ridi, E. Fratini, P. Baglioni, E. Mamontov and S. H. Chen, J. Phys.: Condens. Matter 20, 502101 (2008)] on a similar system, and it was there associated with a hidden critical point of bulk water. However, based on the experimental results presented here, there is no sign of a fragile-to-strong transition for water confined in C-S-H gel. Instead, the fragile-to-strong transition can be explained by a merging of two different relaxation processes at about 220 K.