Dynamics of nano-confined water in Portland cement - comparison with synthetic C-S-H gel and other silicate materials.

The dynamics of water confined in cement materials is still a matter of debate in spite of the fact that water has a major influence on properties such as durability and performance. In this study, we have investigated the dynamics of water confined in Portland cement (OPC) at different curing ages (3 weeks and 4 years after preparation) and at three water-to-cement ratios (w/c, 0.3, 0.4 and 0.5). Using broadband dielectric spectroscopy, we distinguish four different dynamics due to water molecules confined in the pores of different sizes of cements. Here we show how water dynamics is modified by the evolution in the microstructure (maturity) and the w/c ratio. The fastest dynamics (processes 1 and 2, representing very local water dynamics) are independent of water content and the degree of maturity whereas the slowest dynamics (processes 3 and 4) are dependent on the microstructure developed during curing. Additionally, we analyze the differences regarding the water dynamics when confined in synthetic C-S-H gel and in the C-S-H of Portland cement.

Structure of Aqueous Trehalose Solution by Neutron Diffraction and Structural Modeling.

The molecular structure of an aqueous solution of the disaccharide trehalose (C12H22O11) has been studied by neutron diffraction and empirical potential structure refinement modeling. Six different isotope compositions with 33 wt % trehalose (corresponding to 38 water molecules per trehalose molecule) were measured to ensure that water-water, trehalose-water, and trehalose-trehalose correlations were accurately determined. In fact, this is the first neutron diffraction study of an aqueous trehalose solution in which also the nonexchangeable hydrogen atoms in trehalose are deuterated. With this approach, it was possible to determine that (1) there is a substantial hydrogen bonding between trehalose and water (∼11 hydrogen bonds per trehalose molecule), which is in contrast to previous neutron diffraction studies, and (2) there is no tendency of clustering of trehalose, in contrast to what is generally observed by molecular dynamics simulations and experimentally found for other disaccharides. Thus, the results give the structural picture that trehalose prefers to interact with water and participate in a hydrogen-bonded network. This strong network character of the solution might be one of the key reasons for its extraordinary stabilization effect on biological materials.

The Role of Trehalose for the Stabilization of Proteins.

Understanding of how the stabilization mechanism of trehalose operates on biological molecules against different types of environmental stress could prove to gain great advancements in many different types of conservation techniques, such as cryopreservation or freeze-drying. Many theories exist that aim to explain why trehalose possesses an extraordinary ability to stabilize biomolecules. However, all of them just explain parts of its mechanism and a comprehensive picture is still lacking. In this study, we have used differential scanning calorimetry (DSC) and viscometry measurements to determine how the glass transition temperature Tg, the protein denaturation temperature Tden, and the dynamic viscosity depend on both the trehalose and the protein concentration in myoglobin-trehalose-water systems. The aim has been to determine whether these physical properties are related and to gain indirect structural insights from the limits of water crystallization at different concentration ratios. The results show that for systems without partial crystallization of water the addition of protein increases Tg, most likely due to the fact that the protein adsorbs water and thereby reduces the water content in the trehalose-water matrix. Furthermore, these systems are generally decreasing in Tden with an increasing protein concentration, and thereby also an increasing viscosity, showing that the dynamics of the trehalose-water matrix and the stability of the native structure of the protein are not necessarily coupled. We also infer, by analyzing the maximum amount of water for which ice formation is avoided, that the preferential hydration model is consistent with our experimental data.

Dynamics of aqueous binary glass-formers confined in MCM-41.

Dielectric permittivity measurements were performed on water solutions of propylene glycol (PG) and propylene glycol monomethyl ether (PGME) confined in 21 Å pores of the silica matrix MCM-41 C10 in wide frequency (10(-2)-10(6) Hz) and temperature (130-250 K) ranges. The aim was to elucidate how the formation of large hydrogen bonded structural entities, found in bulk solutions of PGME, was affected by the confined geometry, and to make comparisons with the dynamic behavior of the PG-water system. For all solutions the measurements revealed four almost concentration independent relaxation processes. The intensity of the fastest process is low compared to the other relaxation processes and might be caused by both hydroxyl groups of the pore surfaces and by local motions of water and solute molecules. The second fastest process contains contributions from both the main water relaxation as well as the intrinsic ß-relaxation of the solute molecules. The third fastest process is the viscosity related α-relaxation. Its concentration independency is very different compared to the findings for the corresponding bulk systems, particularly for the PGME-water system. The experimental data suggests that the surface interactions induce a micro-phase separation of the two liquids, resulting in a full molecular layer of water molecules coordinating to the hydrophilic hydroxyl groups on the surfaces of the silica pores. This, in turn, increases the geometrical confinement effect for the remaining solution even more and prevents the building up of the same type of larger structural entities in the PGME-water system as in the corresponding bulk solutions. The slowest process is mainly hidden in the high conductivity contribution at low frequencies, but its temperature dependence can be extracted for the PGME-water system. However, its origin is not fully clear, as will be discussed.

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.

Different behavior of water in confined solutions of high and low solute concentrations.

Water-glycerol solutions confined in 21 Å pores of the silica matrix MCM-41 C10 have been studied using differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). The results suggest a micro-phase separation caused by the confinement. Likely the water molecules coordinate to the hydroxyl surface groups of the pores, leaving most of the glycerol molecules in the centre of the pores. This makes the dynamics of glycerol almost concentration independent up to water concentrations of about 85 wt%. However, at higher water concentrations no substantial clustering of glycerol molecules should occur and the glass transition related dynamics exhibit an anomalous behaviour. Instead of a common plasticization effect of water, as for the corresponding bulk solutions (when no ice is formed), it is evident that water acts as an anti-plasticizer in the confinement at high water concentrations. We propose that the increased water concentration slows down the glass transition related dynamics in the deeply supercooled regime due to that a rigid hydrogen bonded network structure of water molecules is formed at low temperatures and low glycerol concentrations. This is in contrast to the situation in a homogenously mixed bulk solution of a high solute concentration where the water molecules will be less hydrogen bonded, and therefore are typically more mobile than the surrounding solute molecules. An almost complete hydrogen bonded network of water molecules may, even in confinements, be sufficiently rigid to slow down the relaxation of embedded solute molecules. It can also be expressed the other way around, i.e. small amounts of glycerol act as a plasticizer for water, due to its breaking up of the nearly tetrahedral network structure. From the here observed concentration dependent behaviour of the deeply supercooled bulk and confined solutions it seems, furthermore, evident that the Tg value of bulk water cannot be estimated from extrapolations of aqueous solutions.

Calorimetric and relaxation properties of xylitol-water mixtures.

We present the first broadband dielectric spectroscopy (BDS) and differential scanning calorimetry study of supercooled xylitol-water mixtures in the whole concentration range and in wide frequency (10(-2)-10(6) Hz) and temperature (120-365 K) ranges. The calorimetric glass transition, T(g), decreases from 247 K for pure xylitol to about 181 K at a water concentration of approximately 37 wt. %. At water concentrations in the range 29-35 wt. % a plentiful calorimetric behaviour is observed. In addition to the glass transition, almost simultaneous crystallization and melting events occurring around 230-240 K. At higher water concentrations ice is formed during cooling and the glass transition temperature increases to a steady value of about 200 K for all higher water concentrations. This T(g) corresponds to an unfrozen xylitol-water solution containing 20 wt. % water. In addition to the true glass transition we also observed a glass transition-like feature at 220 K for all the ice containing samples. However, this feature is more likely due to ice dissolution [A. Inaba and O. Andersson, Thermochim. Acta, 461, 44 (2007)]. In the case of the BDS measurements the presence of water clearly has an effect on both the cooperative α-relaxation and the secondary ß-relaxation. The α-relaxation shows a non-Arrhenius temperature dependence and becomes faster with increasing concentration of water. The fragility of the solutions, determined by the temperature dependence of the α-relaxation close to the dynamic glass transition, decreases with increasing water content up to about 26 wt. % water, where ice starts to form. This decrease in fragility with increasing water content is most likely caused by the increasing density of hydrogen bonds, forming a network-like structure in the deeply supercooled regime. The intensity of the secondary ß-relaxation of xylitol decreases noticeably already at a water content of 2 wt. %, and at a water content above 5 wt. % it has been replaced by a considerably stronger water (w) relaxation at about the same frequency. However, the similarities in time scale and activation energy between the w-relaxation and the ß-relaxation of xylitol at water contents below 13 wt. % suggest that the w-relaxation is governed, in some way, by the ß-relaxation of xylitol, since clusters of water molecules are rare at these water concentrations. At higher water concentrations the intensity and relaxation rate of the w-relaxation increase rapidly with increasing water content (up to the concentration where ice starts to form), most likely due to a rapid increase of small water clusters where an increasing number of water molecules interacting with other water molecules.

The slow dielectric Debye relaxation of monoalcohols in confined geometries.

Broadband dielectric relaxation measurements have been performed on monoalcohols confined in the quasi-two-dimensional space between clay platelets and the quasi-one-dimensional pores of approximately 10 Å diameter in a molecular sieve. Interestingly, the results show that the slow Debye-like process is present even in these severe confinements, proving that structural models that are based on two-dimensional or three-dimensional cluster formations as the structural origin of the Debye-like process can be excluded. Rather, the insensitivity of its time-scale to confinements suggests that it is of local character and in some way related to the lifetime or breaking and reformation of hydrogen bonds.

Role of solvent for the dynamics and the glass transition of proteins.

For the first time, a systematic investigation of the glass transition and its related dynamics of myoglobin in water-glycerol solvent mixtures of different water contents is presented. By a combination of broadband dielectric spectroscopy and differential scanning calorimetry (DSC), we have studied the relation between the protein and solvent dynamics with the aim to better understand the calorimetric glass transition, T(g), of proteins and the role of solvent for protein dynamics. The results show that both the viscosity related α-relaxation in the solvent as well as several different protein relaxations are involved in the calorimetric glass transition, and that the broadness (ΔT(g)) of the transition depends strongly on the total amount of solvent. The main reason for this seems to be that the protein relaxation processes become more separated in time with decreasing solvent level. The results are compared to that of hydrated myoglobin where the hydration water does not give any direct contribution to the calorimetric T(g). However, the large-scale α-like relaxation in the hydration water is still responsible for the protein dynamics that freeze-in at T(g). Finally, the dielectric data show clearly that the protein relaxation processes exhibit similar temperature dependences as the α-relaxation in the solvent, as suggested for solvent-slaved protein motions.

Hidden slow dynamics in water.

It is well known that the structural and dynamical properties of water are of central importance for life on our planet. However, despite this knowledge its structural and dynamical properties are still far from fully understood. In this Letter we show for the first time that water exhibits an anomalously slow relaxation process, which is about 4 orders of magnitude slower than the viscosity-related structural main relaxation. This slow Debye-like process has previously only been observed in monoalcohols and more recently also in polyalcohols, and due to its slowness it is generally believed to be caused by some kind of collective motion of hydrogen-bonded structures. The new finding has important structural and dynamical implications for water.

Slow Debye-type peak observed in the dielectric response of polyalcohols.

Dielectric relaxation spectroscopy of glass forming liquids normally exhibits a relaxation scenario that seems to be surprisingly general. However, the relaxation dynamics is more complicated for hydrogen bonded liquids. For instance, the dielectric response of monoalcohols is dominated by a mysterious Debye-like process at lower frequencies than the structural alpha-relaxation that is normally dominating the spectra of glass formers. For polyalcohols this process has been thought to be absent or possibly obscured by a strong contribution from conductivity and polarization effects at low frequencies. We here show that the Debye-like process, although much less prominent, is also present in the response of polyalcohols. It can be observed in the derivative of the real part of the susceptibility or directly in the imaginary part if the conductivity contribution is reduced by covering the upper electrode with a thin Teflon layer. We report on results from broadband dielectric spectroscopy studies of several polyalcohols: glycerol, xylitol, and sorbitol. The findings are discussed in relation to other experimental observations of ultraslow (i.e., slower than the viscosity related alpha-relaxation) dynamics in glass formers.

A unified model of protein dynamics.

Protein functions require conformational motions. We show here that the dominant conformational motions are slaved by the hydration shell and the bulk solvent. The protein contributes the structure necessary for function. We formulate a model that is based on experiments, insights from the physics of glass-forming liquids, and the concepts of a hierarchically organized energy landscape. To explore the effect of external fluctuations on protein dynamics, we measure the fluctuations in the bulk solvent and the hydration shell with broadband dielectric spectroscopy and compare them with internal fluctuations measured with the Mössbauer effect and neutron scattering. The result is clear. Large-scale protein motions are slaved to the fluctuations in the bulk solvent. They are controlled by the solvent viscosity, and are absent in a solid environment. Internal protein motions are slaved to the beta fluctuations of the hydration shell, are controlled by hydration, and are absent in a dehydrated protein. The model quantitatively predicts the rapid increase of the mean-square displacement above approximately 200 K, shows that the external beta fluctuations determine the temperature- and time-dependence of the passage of carbon monoxide through myoglobin, and explains the nonexponential time dependence of the protein relaxation after photodissociation.

Relaxation processes in supercooled confined water and implications for protein dynamics.

We show that the viscosity-related main (alpha) relaxation of confined water vanishes at a temperature where the volume required for the cooperative alpha relaxation becomes larger than the size of the geometrically confined water cluster. This occurs typically around 200 K, implying that above this temperature we observe a merged alpha-beta relaxation, whereas below it only a local (beta) relaxation remains. This also means that such confined supercooled water does not exhibit any true glass transition, in contrast to other liquids in similar confinements. Furthermore, it implies that deeply supercooled water in biological systems, such as membranes and proteins, generally shows only a local beta relaxation, a finding of importance for low temperature properties of biological materials.

Relation between solvent and protein dynamics as studied by dielectric spectroscopy.

We present results obtained by dielectric spectroscopy in wide frequency (10(-2)-10(9) Hz) and temperature ranges on human hemoglobin in the three different solvents water, glycerol, and methanol, at a solvent level of 0.8 g of solvent/g of protein. In this broad frequency region, there are motions on several time-scales in the measured temperature range (110-370 K for water, 170-410 K for glycerol, and 110-310 K for methanol). For all samples, the dielectric data shows at least four relaxation processes, with frequency dependences that are well described by the Havriliak-Negami or Cole-Cole functions. The fastest and most pronounced process in the dielectric spectra of hemoglobin in glycerol and methanol solutions is similar to the alpha-relaxation of the corresponding bulk solvent (but shifted to slower dynamics due to surface interactions). For water solutions, however, this process corresponds to earlier results obtained for water confined in various systems and it is most likely due to a local beta-relaxation. The slowing down of the glycerol and methanol relaxations and the good agreement with earlier results on confined water show that this process is affected by the interaction with the protein surface. The second fastest process is attributed to motions of polar side groups on the protein, with a possible contribution from tightly bound solvent molecules. This process is shifted to slower dynamics with increasing solvent viscosity, and it shows a crossover in its temperature dependence from Arrhenius behavior at low temperatures to non-Arrhenius behavior at higher temperatures where there seems to be an onset of cooperativity effects. The origins of the two slowest relaxation processes (visible at high temperatures and low frequencies), which show saddlelike temperature dependences for the solvents water and methanol, are most likely due to motions of the polypeptide backbone and an even more global motion in the protein molecule.

Dielectric and calorimetric studies of hydrated purple membrane.

Purple membranes (PM) from halobacteria were hydrated to approximately 0.4 and approximately 0.2 g H(2)O/g of PM and studied by dielectric spectroscopy and differential scanning calorimetry between 120 and 300 K. The dielectric process, attributed to a local (beta) relaxation of the confined supercooled water, shows an Arrhenius temperature behavior at low temperatures. In the case of the most hydrated PM a small deviation from the Arrhenius behavior occurs at 190-200 K together with a pronounced endothermic process and an increased activation energy. The observed crossover is accompanied by a reduction of the interlayer spacing due to the partial loss of the intermembrane water. All these effects at approximately 200 K are consistent with a scenario where the local relaxation process merges with a nonobservable alpha-relaxation of the interlayer water, giving rise to a more liquid-like behavior of the interfacial water. For the less hydrated sample the effects are less pronounced and shift to a slightly higher temperature.