Dynamics of water in supercooled aqueous solutions of glucose and poly(ethylene glycol)s as studied by dielectric spectroscopy

Nanoscale water-polymer interactions tune macroscopic diffusivity of water in aqueous poly(ethylene oxide) solutions

The separation and anti-fouling performance of water purification membranes is governed by both macroscopic and molecular-scale water properties near polymer surfaces. However, even for poly(ethylene oxide) (PEO) - ubiquitously used in membrane materials - there is little understanding of whether or how the molecular structure of water near PEO surfaces affects macroscopic water diffusion. Here, we probe both time-averaged bulk and local water dynamics in dilute and concentrated PEO solutions using a unique combination of experimental and simulation tools. Pulsed-Field Gradient NMR and Overhauser Dynamic Nuclear Polarization (ODNP) capture water dynamics across micrometer length scales in sub-seconds to sub-nanometers in tens of picoseconds, respectively. We find that classical models, such as the Stokes-Einstein and Mackie-Meares relations, cannot capture water diffusion across a wide range of PEO concentrations, but that free volume theory can. Our study shows that PEO concentration affects macroscopic water diffusion by enhancing the water structure and altering free volume. ODNP experiments reveal that water diffusivity near PEO is slower than in the bulk in dilute solutions, previously not recognized by macroscopic transport measurements, but the two populations converge above the polymer overlap concentration. Molecular dynamics simulations reveal that the reduction in water diffusivity occurs with enhanced tetrahedral structuring near PEO. Broadly, we find that PEO does not simply behave like a physical obstruction but directly modifies water's structural and dynamic properties. Thus, even in simple PEO solutions, molecular scale structuring and the impact of polymer interfaces is essential to capturing water diffusion, an observation with important implications for water transport through structurally complex membrane materials.

Glass transition phenomena and dielectric relaxations in supercooled d-lyxose aqueous solutions

Differential scanning calorimeter and broadband dielectric spectroscopy in a broad range of temperatures (150-300 K) were employed to study the d-lyxose aqueous mixture at different hydration levels. Two relaxation processes were observed in all investigated d-lyxose aqueous mixtures. A relaxation process (process-I) usually known as the primary relaxation mode which is accountable for the collective motion of d-lyxose aqueous solution, was observed above the glass transition temperature (Tg). Below Tg, another process designated as process-II was found which is mainly related to the water molecule relaxation inside the d-lyxose matrix. The average relaxation times as a function of temperature and dielectric strengths of both observed relaxation processes (I & II) were analyzed for all hydration levels in d-lyxose. It was identified that the relaxation amplitude of process-II in the d-lyxose aqueous mixture was increased drastically and their activation energies were found to be approximately independent of the content of water above critical concentration, xc = 0.28. This suggests that the dynamical process observed above xc was dominated by the presence of water clusters. In the current aqueous mixture, the critical content of water (xc) is slightly higher as compared to previously reported aqueous mixtures, indicating a more cooperative nature of water molecules with a d-lyxose matrix. Additionally, the Tg of pure water was estimated at 128 ± 5.8 K from the extrapolation of DSC Tg data of the d-lyxose aqueous solution by using the well-known Gordon-Taylor equation. Our current result gives further support to the well-accepted glass transition (Tg) of pure water.

Dynamics of hydrated proteins and bio-protectants: Caged dynamics, β-relaxation, and α-relaxation

BACKGROUND

The properties of the three dynamic processes, α-relaxation, ν-relaxation, and caged dynamics in aqueous mixtures and hydrated proteins are analogous to corresponding processes found in van der Waals and polymeric glass-formers apart from minor differences.

METHODS

Collection of various experimental data enables us to characterize the structural α-relaxation of the protein coupled to hydration water (HW), the secondary or ν-relaxation of HW, and the caged HW process.

RESULTS

From the T-dependence of the ν-relaxation time of hydrated myoglobin, lysozyme, and bovine serum albumin, we obtain Ton at which it enters the experimental time windows of Mössbauer and neutron scattering spectroscopies, coinciding with protein dynamical transition (PDT) temperature Td. However, for all systems considered, the α-relaxation time at Ton or Td is many orders of magnitude longer. The other step change of the mean-square-displacement (MSD) at Tg_alpha originates from the coupling of the nearly constant loss (NCL) of caged HW to density. The coupling of the NCL to density is further demonstrated by another step change at the secondary glass temperature Tg_beta in two bio-protectants, trehalose and sucrose.

CONCLUSIONS

The structural α-relaxation plays no role in PDT. Since PDT is simply due to the ν-relaxation of HW, the term PDT is a misnomer. NCL of caged dynamics is coupled to density and show transitions at lower temperature, Tg_beta and Tg_alpha.

GENERAL SIGNIFICANCE

The so-called protein dynamical transition (PDT) of hydrated proteins is not caused by the structural α-relaxation of the protein but by the secondary ν-relaxation of hydration water. "This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo".

Ion jelly conductive properties using dicyanamide-based ionic liquids

The thermal behavior and transport properties of several ion jellys (IJs), a composite that results from the combination of gelatin with an ionic liquid (IL), were investigated by dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC), and pulsed field gradient nuclear magnetic resonance spectroscopy (PFG NMR). Four different ILs containing the dicyanamide anion were used: 1-butyl-3-methylimidazolium dicyanamide (BMIMDCA), 1-ethyl-3-methylimidazolium dicyanamide (EMIMDCA), 1-butyl-1-methylpyrrolidinium dicyanamide (BMPyrDCA), and 1-butylpyridinium dicyanamide (BPyDCA); the bulk ILs were also investigated for comparison. A glass transition was detected by DSC for all materials, ILs and IJs, allowing them to be classified as glass formers. Additionally, an increase in the glass transition temperature upon dehydration was observed with a greater extent for IJs, attributed to a greater hindrance imposed by the gelatin matrix after water removal, rendering the IL less mobile. While crystallization is observed for some ILs with negligible water content, it was never detected for any IJ upon thermal cycling, which persist always as fully amorphous materials. From DRS measurements, conductivity and diffusion coefficients for both cations (D+) and anions (D-) were extracted. D+ values obtained by DRS reveal excellent agreement with those obtained from PFG NMR direct measurements, obeying the same VFTH equation over a large temperature range (ΔT ≈ 150 K) within which D+ varies around 10 decades. At temperatures close to room temperature, the IJs exhibit D values comparable to the most hydrated (9%) ILs. The IJ derived from EMIMDCA possesses the highest conductivity and diffusion coefficient, respectively, ∼10(-2) S·cm(-1) and ∼10(-10) m(2)·s(-1). For BMPyrDCA the relaxational behavior was analyzed through the complex permittivity and modulus formalism allowing the assignment of the detected secondary relaxation to a Johari-Goldstein process. Besides the relevant information on the more fundamental nature providing physicochemical details on ILs behavior, new doorways are opened for practical applications by using IJ as a strategy to produce novel and stable electrolytes for different electrochemical devices.

Broadband dielectric spectroscopic, calorimetric, and FTIR-ATR investigations of D-arabinose aqueous solutions

The dielectric relaxation behavior of D-arabinose aqueous solutions at different water concentrations is examined by broadband dielectric spectroscopy in the frequency range of 10(-2) -10(7) Hz and in the temperature range of 120-300 K. Differential scanning calorimetry is also performed to find the glass transition temperatures (T(g)). In addition, the same solutions are analyzed by Fourier transform infrared (FTIR) spectroscopy using the attenuated total reflectance (ATR) method at the same temperature interval and in the frequency range of 3800-2800 cm(-1). The temperature dependence of the relaxation times is examined for the different weight fractions (x(w)) of water along with the temperature dependence of dielectric strength. Two relaxation processes are observed in the aqueous solutions for all concentrations of water. The slower process, the so-called primary relaxation process (process-I), is responsible for the T(g) whereas the faster one (designated as process-II) is due to the reorientational motion of the water molecules. As for other hydrophilic water solutions, dielectric data for process-II indicate the existence of a critical water concentration above which water mobility is less restricted. Accordingly, FTIR-ATR measurements on aqueous solutions show an increment in the intensity (area) of the O-H stretching sub-band close to 3200 cm(-1) as the water concentration increases.

Dynamics of the slow mode in the family of six-carbon monosaccharides monitored by dielectric spectroscopy

Broadband dielectric measurements performed on D-glucose, L-sorbose, D-fructose and D-galactose revealed that, except for the structural relaxation process, one can detect in the liquid phase of these carbohydrates a much slower relaxation mode. Recently we have demonstrated that in D-glucose this relaxation mode might be related to the long range correlation of density fluctuations (LRCDF), also called Fischer clusters (FC). Based on the dielectric data obtained for the four monosaccharides we were able to make a more general conclusion about the characteristic dielectric features of the slow mode in the whole family of carbohydrates. We found out that the timescale separation between structural and considered relaxation reaches up to six decades at the glass transition temperature and the dielectric strength decreases significantly with lowering temperature. Another very interesting feature of the slow process is that it can be described by an almost exponential response function. We have found out that the fragility of the slow process lies within the range m = 44-50. Finally, we have also shown that there is a close link between structural and slow relaxation.

Universal features of water dynamics in solutions of hydrophilic polymers, biopolymers, and small glass-forming materials

A systematic investigation by dielectric spectroscopy of 18 different water-rich mixtures with very different hydrophilic substances shows universal features for the water dynamics. The temperature dependence of the relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior at the T(g) range of the mixtures. Furthermore, the temperature dependence of the relaxation times presents a universal behavior both above and below the crossover temperature. We also show that these features suggest that the observed crossover is associated with the emergence of confinement effects.

Dynamics of Water in Partially Crystallized Polymer/Water Mixtures Studied by Dielectric Spectroscopy

The dielectric relaxation process of water was investigated for polymer/water mixtures containing poly(vinyl methyl ether), poly(ethyleneimine), poly(vinyl alcohol), and poly(vinylpyrrolidone) with a polymer concentration of up to 40 wt % at frequencies between 10 MHz and 10 GHz in subzero temperatures down to -55 degrees C. These polymer/water mixtures have a crystallization temperature TC of water at -10 to -2 degrees C. Below TC, part of the water crystallized and another part of the water, uncrystallized water (UCW), remained in a liquid state with the polymer in an uncrystallized phase. The dielectric relaxation process of UCW was observed, and reliable dielectric relaxation parameters of UCW were obtained at temperatures of -26 to -2 degrees C. At TC, the relaxation strength, relaxation time, and relaxation time distribution change abruptly, and their subsequent changes with decreasing temperature are larger than those above TC. The relaxation strength of UCW decreases, and the relaxation time and dynamic heterogeneity (distribution of relaxation time) increase with decreasing temperature. These large temperature dependences below TC can be explained by the increase in polymer concentration in the uncrystallized phase C(p,UCP) with decreasing temperature. C(p,UCP) is independent of the initial polymer concentration. In contrast to the relaxation times above TC, which vary with the chemical structure of the polymer and its concentration, the relaxation times of UCW are independent of both of them. This indicates that the factor determining whether the water forms ice crystals or stays as UCW is the mobility of the water molecules.

The Johari-Goldstein beta-relaxation of water

There is a plethora of experimental data on the dynamics of water in mixtures with glycerol, ethylene glycol, ethylene glycol oligomers, poly(ethylene glycol) 400 and 600, propanol, poly(vinyl pyrrolidone), poly(vinyl methylether), and other substances. In spite of the differences in the water contents, the chemical compositions, and the glass transition temperatures Tg of these aqueous mixtures, a faster relaxation originating from the water (called the nu-process) is omnipresent, sharing the following common properties. The relaxation time tau(nu) has Arrhenius temperature dependence at temperatures below Tg of the mixture. The activation energies of tau(nu) all fall within a neighborhood of 50 kJ/mol. At the same temperature where mixtures are all in their glassy states, the values of tau(nu) of several mixtures are comparable. The Arrhenius temperature dependence of tau(nu) does not continue to higher temperatures and instead it crosses over to a stronger temperature dependence at temperatures above Tg. The dielectric relaxation strength of the nu-process, Deltaepsilon(nu)(T), has a stronger temperature dependence above Tg than below, mimicking the change of enthalpy, entropy, and volume when crossing Tg. These general property of the nu-process (except for the magnitude of the activation energy) had been found before in the secondary relaxation of the faster component in several binary nonaqueous mixtures. Other properties of the secondary relaxation in these nonaqueous mixtures have helped to identify it as the Johari-Goldstein (JG) secondary relaxation of the faster component. The similarities in properties lead us to conclude that the nu-processes in water mixtures are the JG secondary relaxations of water. The conclusion is reinforced by the processes behaving similarly to the nu-process found in 6 A thick water layer (two molecular layers) in fully hydrated Na-vermiculite clay, and in water confined in molecular sieves, silica hydrogels, and poly(2-hydroxyethyl methacrylate) hydrogels.

Broadband Dielectric Study of Dynamics of Polymer and Solvent in Poly(vinyl pyrrolidone)/Normal Alcohol Mixtures

Broadband dielectric measurements of poly(vinyl pyrrolidone) (PVP)-monohydroxyl alcohol mixtures of various normal alcohols with the number of carbon atoms per molecule ranging from 1 to 9 were made in the frequency range of 20 Hz to 20 GHz at 25 degrees C. Two relaxation processes due to the reorientation of dipoles on the PVP and alcohol molecules were observed. The relaxation process at frequencies higher than 100 MHz is the primary process of alcohols, and that at frequencies lower than 10 MHz is attributed to the local chain motion of PVP. For mixtures of alcohol molecules that are smaller than propanol, the relaxation time of the alcohol increases with increasing PVP concentration, whereas for mixtures of alcohol molecules larger than butanol, the relaxation time of the alcohol decreases with increasing PVP concentration. The increase in the density of hydrogen-bonding sites upon the addition of PVP reduces the relaxation time of alcohol in the mixture, and vice versa. The relaxation time of the local chain motion of PVP increases with PVP concentration and solvent viscosity. Different time scales of the molecular motions of polymer and solvent coexist in homogeneous mixtures with hydrogen-bonded polar solvent and polymer.

The True Johari−Goldstein β-Relaxation of Monosaccharides

Broadband isothermal dielectric relaxation measurements of anhydrous fructose, glucose, galactose, sorbose, and ribose were made at ambient pressure in their liquidus and glassy states. We found a new secondary relaxation in fructose and glucose that is slower than those seen before by others. This new secondary relaxation also appears in the dielectric spectra of galactose, sorbose, and ribose, and hence it is a general feature of the relaxation dynamics of the monosaccharides. Dielectric measurements at elevated pressure of fructose and ribose show that the new secondary relaxation shifts to lower frequencies with applied pressures, mimicking the behavior of the alpha-relaxation. In contrast, the faster secondary relaxation remains stationary on applying pressure. These results together with other inferences indicate that the slower secondary relaxation bears relations to the alpha-relaxation, and hence, it is the true Johari-Goldstein secondary relaxation of the monosaccharides.