The fragile-to-strong dynamic crossover transition in confined water: nuclear magnetic resonance results

By means of a nuclear magnetic resonance experiment, we give evidence of the existence of a fragile-to-strong dynamic crossover transition (FST) in confined water at a temperature T(L)=223+/-2 K. We have studied the dynamics of water contained in 1D cylindrical nanoporous matrices (MCM-41-S) in the temperature range 190-280 K, where experiments on bulk water were so far hampered by crystallization. The FST is clearly inferred from the T dependence of the inverse of the self-diffusion coefficient of water (1D) as a crossover point from a non-Arrhenius to an Arrhenius behavior. The combination of the measured self-diffusion coefficient D and the average translational relaxation time tau(T), as measured by neutron scattering, shows the predicted breakdown of Stokes-Einstein relation in deeply supercooled water.     

Observation of fragile-to-strong liquid transition in surface water in CeO2

A quasielastic neutron-scattering experiment carried out on a backscattering spectrometer with sub-micro eV resolution in the temperature range of 200-250 K has revealed the dynamics of surface water in cerium oxide on the time scale of hundreds of picoseconds. This slow dynamics is attributed to the translational mobility of the water molecules in contact with the surface hydroxyl groups. The relaxation function of this slow motion can be described by a slightly stretched exponential with the stretch factor exceeding 0.9, which indicates almost a Debye-type dynamics. Down to about 220 K, the temperature dependence of the residence time for water molecules follows a Vogel-Fulcher-Tamman law with the glass transition temperature of 181 K. At lower temperatures, the residence time behavior abruptly changes, indicating a fragile-to-strong liquid transition in surface water at about 215 K.     

The low-temperature dynamic crossover phenomenon in protein hydration water: simulations vs experiments

A super-Arrhenius-to-Arrhenius dynamic crossover phenomenon has been observed in the translational alpha-relaxation time and in the inverse of the self-diffusion constant both experimentally and by simulations for lysozyme hydration water in the temperature range of TL = 223 +/- 2 K. MD simulations are based on a realistic hydrated powder model, which uses the TIP4P-Ew rigid molecular model for the hydration water. The convergence of neutron scattering, nuclear magnetic resonance and molecular dynamics simulations supports the interpretation that this crossover is a result of the gradual evolution of the structure of hydration water from a high-density liquid to a low-density liquid form upon crossing of the Widom line above the possible liquid-liquid critical point of water.     

Dynamics of the surface layer in cyanobiphenyl-aerosil nanocomposites with a high silica density

Composites were prepared from an aerosil and 4-n-alkyl-4'cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of ∼7 g aerosil in 1 cm³ of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10^-2-10⁹ Hz) in a large temperature range (220-370 K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel-Fulcher-Tammann law, which is found to be typical for glass-forming liquids. The quasi two-dimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd-even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface).     

Temperature divergence of the dynamics of a poly(vinyl acetate) glass: dielectric vs. mechanical behaviors

The dynamics of glass forming liquids as the glass transition temperature (T(g)) is traversed has become of special interest because of the continuing question as to whether or not the dynamics diverge towards an ideal glass transition/Kauzmann temperature or if the apparent Vogel-Fulcher-Tammann (VFT) divergence is lost as one goes below the conventional T(g) but remains in equilibrium. Here we examine the response of a poly(vinyl acetate) PVAc polymer glass-former using both dielectric and mechanical methods in the vicinity of T(g). Isothermal measurements were performed to study the aging behavior of the PVAc and to assure that the equilibrium state was achieved for temperatures as much as 16 K below the calorimetric T(g). Surprisingly, we found that the mechanical response took much longer to age into its equilibrium than did the dielectric response. Also, the temperature dependence of the time-temperature shift factors obtained from the two methods is different and the dielectric response shows a turnover to an apparent Arrhenius behavior rather than a continuation of the VFT extrapolated divergence at the lowest temperatures tested.     

Temperature dependence of anisotropy decay and solvation dynamics of coumarin 153 in gamma-cyclodextrin aggregates

Effect of temperature on the fluorescence anisotropy decay and the ultraslow component of solvation dynamics of coumarin 153 (C153) in a gamma-cyclodextrin (gamma-CD) nanocavity are studied using a picosecond set up. The steady-state anisotropy (0.13 +/- 0.01) and residual anisotropy (0.14 +/- 0.01) in fluorescence anisotropy decay in an aqueous solution containing 7 microM C153 and 40 mM gamma-CD are found to be quite large. This indicates formation of large linear nanotube aggregates of gamma-CD linked by C153. It is estimated that >53 gamma-CD units are present in each aggregate. In these aggregates with rise in temperature, the average solvation time ((obs)) decreases markedly from 680 ps at 278 K to 160 ps at 318 K. The dynamic Stokes shift is found to decrease from 800 cm(-1) at 278 K to 250 cm(-1) at 318 K. The fraction of dynamic Stokes shift (f(d)) detected in a picosecond set up is calculated using the Fee-Maroncelli procedure. The corrected solvation time ((corr) = f(d)<(tau(s)>(obs)) displays an Arrhenius type temperature dependence. From the temperature variation, the activation energy and entropy of the solvation process are determined to be 12.5 kcal M(-1) and 28 cal M(-1) K(-1), respectively. The ultraslow component and its temperature dependence are ascribed to a dynamic exchange between bound and free water molecules.     

High frequency dynamics and structural relaxation process in liquid ammonia

The dynamic structure factor S(Q,omega) of liquid ammonia has been measured by inelastic x-ray scattering in the terahertz frequency region as a function of the temperature in the range of 220-298 K at a pressure P=85 bars. The data have been analyzed using the generalized hydrodynamic formalism with a three term memory function to take into account the thermal, the structural, (alpha) and the microscopic (mu) relaxation processes affecting the dynamics of the liquid. This allows to extract the temperature dependence of the structural relaxation time (tau(alpha)) and strength (Delta(alpha)). The former quantity follows an Arrhenius behavior with an activation energy E(a)=2.6+/-0.2 kcal/mol, while the latter is temperature independent suggesting that there are no changes in the interparticle potential and arrangement with T. The obtained results, compared with those already existing in liquid water and liquid hydrogen fluoride, suggest the strong influence of the connectivity of the molecular network on the structural relaxation.     

Influence of phase transformation on the low temperature dielectric measurements in 1-cyanoadamantane

Dielectric measurements have been performed on the supercooled plastic crystal phase of cyanoadamantane under isothermal and isochronal conditions in the temperature range [170-250 K], each experiment with a new sample. The measurements performed in the course of isotherms allowed us to emphasize the effects of the transformations close to the glass transition temperature leading to a change in the dynamic behavior of the compound. The temperature dependence of the dielectric relaxation times exhibits an Arrhenius evolution on the whole metastable domain and gives at the calorimetric glass transition temperature a value of 1 s, i.e., two orders of magnitude lower than the usual value [tau(T(g)) congruent with 100 s]. These results indicate that the Vogel-Fulcher-Tammann behavior observed with other techniques does not appear in the Brillouin zone center (q=0) and that the glass transition for this glass former is not associated with the freezing out of the tumbling motion of individual molecular dipoles but to the freezing out of fluctuations of an antiferroelectric local ordering.     

Experimental evidence of fragile-to-strong dynamic crossover in DNA hydration water

We used high-resolution quasielastic neutron scattering spectroscopy to study the single-particle dynamics of water molecules on the surface of hydrated DNA samples. Both H(2)O and D(2)O hydrated samples were measured. The contribution of scattering from DNA is subtracted out by taking the difference of the signals between the two samples. The measurement was made at a series of temperatures from 270 down to 185 K. The relaxing-cage model was used to analyze the quasielastic spectra. This allowed us to extract a Q-independent average translational relaxation time of water molecules as a function of temperature. We observe clear evidence of a fragile-to-strong dynamic crossover (FSC) at T(L)=222+/-2 K by plotting log versus T. The coincidence of the dynamic transition temperature T(c) of DNA, signaling the onset of anharmonic molecular motion, and the FSC temperature T(L) of the hydration water suggests that the change of mobility of the hydration water molecules across T(L) drives the dynamic transition in DNA.     

Tropospheric multiphase chemistry of 2,5- and 2,6-dimethylphenols: determination of the mass accommodation coefficients and the Henry's law constants

The uptake of 2,5-dimethylphenol and 2,6-dimethylphenol on aqueous surfaces was measured between 279 and 293 K, using the wetted-wall flow tube technique coupled with UV absorption spectroscopic detection. For both compounds, the uptake coefficients gamma were found to be independent of the KOH scavenger concentration in the range of 0.01 to 1 M (pH > pK(a)) and of the liquid-gas contact times. In addition, the uptake coefficients and the derived mass accommodation coefficients alpha show a negative temperature dependence in the investigated temperature range. The mass accommodation coefficients decrease from 1.1 x 10(-3) to 1.1 x 10(-4), and from 5.4 x 10(-4) to 6.4 x 10(-5) for 2,5-dimethylphenol and 2,6-dimethylphenol, respectively. These results are used to discuss the incorporation of these species into the liquid using the nucleation theory. Henry's law constants (HLC) of both compounds were directly measured using a dynamic equilibrium system based on the water/air equilibrium at the interface within the length of a microporous tube. The measurements were conducted over the range 278-293 K in both deionized water and 35 g L(-1) solution of NaCl. At 293 K and in pure water, HLC were found to be equal to (in units of M atm(-1)): 2,5-dimethylphenol, HLC = (1270 +/- 240); 2,6-dimethylphenol, HLC = (250 +/- 80). All of the values for HLC in 35 g L(-1) salt solution were 5-55% lower than the corresponding values in deionized water, depending on the compound and the temperature. These data (mass accommodation coefficients and Henry's law constants) were then used to estimate the partitioning of these phenolic compounds between gaseous and aqueous phases and the corresponding atmospheric lifetimes under clear sky (tau(gas)) and cloudy conditions (tau(multiphase)) have then been derived. The calculated multiphase lifetimes (in units of hours) are lower than those in gas phase at a cumulus temperature of 283 K (in parentheses): 2,5-dimethylphenol, 2.2 (3.5); 2,6-dimethylphenol, 3.8 (4.2).     

Influence of hydration on protein dynamics: combining dielectric and neutron scattering spectroscopy data

Combining dielectric spectroscopy and neutron scattering data for hydrated lysozyme powders, we were able to identify several relaxation processes and follow protein dynamics at different hydration levels over a broad frequency and temperature range. We ascribe the main dielectric process to protein's structural relaxation coupled to hydration water and the slowest dielectric process to a larger scale protein's motions. Both relaxations exhibit a smooth, slightly super-Arrhenius temperature dependence between 300 and 180 K. The temperature dependence of the slowest process follows the main dielectric relaxation, emphasizing that the same friction mechanism might control both processes. No signs of a proposed sharp fragile-to-strong crossover at T approximately 220 K are observed in temperature dependences of these processes. Both processes show strong dependence on hydration: the main dielectric process slows down by six orders with a decrease in hydration from h approximately 0.37 (grams of water per grams of protein) to h approximately 0.05. The slowest process shows even stronger dependence on hydration. The third (fastest) dielectric relaxation process has been detected only in samples with high hydration ( h approximately 0.3 and higher). We ascribe it to a secondary relaxation of hydration water. The mechanism of the protein dynamic transition and a general picture of the protein dynamics are discussed.     

Signatures of fragile-to-strong transition in a binary metallic glass-forming liquid

Classical molecular dynamics investigations of the evolution of the microscopic structure and atomic dynamics are found to provide signatures of fragile-to-strong transition in a Cu-Zr bulk metallic glass forming alloy. Present study reveals that (i) the alloy exhibits a non-monotonic decoupling of the self-diffusion coefficient D and the relaxation time τ as observed in case of supercooled water despite the difference in the intermolecular interactions compared to this system, (ii) the temperature dependence of D and τ suggests a crossover from non-Arrhenius to Arrhenius behavior near mode-coupling transition temperature T(C), and (iii) the alloy exhibits a crossover from Stokes-Einstein ((D ~ (τ/T)(-1)) to fractional Stokes-Einstein (D ∝ (τ/T)(-ζ)) with exponent ζ ≈ 0.6. A weak first-order transition, associated with the fragile-to-strong transition, has also been observed in the undercooled region. These findings are in accordance with the growing idea of fragile-to-strong crossover having larger generality than the traditional classification of the glass-forming liquids as fragile and strong.     

Pressure effects on the alpha and alpha' relaxations in polymethylphenylsiloxane

In some polymers, in addition to the usual structural alpha relaxation, a slower alpha' relaxation is observed with a non-Arrhenius temperature dependence. In order to understand better the molecular origin of this alpha' relaxation in poly(methylphenylsiloxane) (PMPS) we have studied, for the first time, the pressure dependence of its relaxation time, together with the usual temperature dependence, by means of dynamic light scattering (DLS). For the same material the alpha relaxation was also studied by means of DLS and dielectric spectroscopy (DS) in broad temperature and pressure ranges. We find that the temperature dependence of both alpha and alpha' relaxation times, at all pressures studied, can be described by a double Vogel-Fulcher-Tammann (VFT) law. The pressure dependence of the characteristic temperatures Tg (glass transition temperature) and T0 (Vogel temperature) as well as the activation volumes for both alpha and alpha' processes are very similar, indicating, that both relaxation processes originate from similar local molecular dynamics. Additionally, for both alpha and alpha' relaxations the combined temperature and pressure dependences of the relaxation times can be described using a parameter Gamma=rhon/T with the same value of the exponent n.     

Molecular dynamics study of tryptophylglycine: a dipeptide nanotube with confined water

To investigate the mechanism of structural changes of a peptide nanotube and water confined inside the channel, the helical peptide tryptophylglycine monohydrate (WG.H2O) was studied by molecular dynamics (MD) simulations using the three-dimension parallel MD program ddgmq (software package) and a consistent force field. Simulations were performed on both the water-containing system and a model system without water molecules. The details of the structural behavior with temperature are investigated for the entire simulated temperature range. Phase transitions were obtained at 115, 245, 270, 310, and 385 K, due to the contributions of both the peptide and the confined water subsystems. The crystalline, amorphous, liquidlike, liquid, and superheated phases of water were observed in the temperature ranges 40-115, 115-245, 245-310, 310-385, and >385 K, respectively. At 300 K, the diffusion constant of the confined water is 0.46 x 10-5 cm2 s-1, a value comparable to that of other peptide nanotubes. The empty peptide system melts at 440 K. Mechanisms of the negative thermal expansion (NTE) along the tube axis were investigated for different temperature ranges. The contraction of the crystalline water (or amorphous water) draws also the tube walls in and leads to NTE below 245 K. The other NTEs appear to be connected to the collapse of the ice network or the solid peptide network between 245 K and room temperature or from 310 to 440 K, respectively.     

Separable cooperative and localized translational motions of water confined by a chemically heterogeneous environment

We report quasi-elastic neutron scattering experiments at two resolutions that probe timescales of picoseconds to nanoseconds for the hydration dynamics of water, confined in a concentrated solution of N-acetyl-leucine-methylamide (NALMA) peptides in water over a temperature range of 248 K to 288 K. The two QENS resolutions used allow for a clean separation of two observable translational components, and ultimately two very different relaxation processes, that become evident when analyzed under a combination of the jump diffusion model and the relaxation cage model. The first translational motion is a localized beta-relaxation process of the bound surface water, and exhibits an Arrhenius temperature dependence and a large activation energy of approximately 8 kcal mol(-1). The second non-Arrhenius translational component is a dynamical signature of the alpha-relaxation of more fluid water, exhibiting a glass transition temperature of approximately 116 K when fit to the Volger Fulcher Tamman functional form. These peptide solutions provide a novel experimental system for examining confinement in order to understand the dynamical transition in bulk supercooled water by removing the unwanted interface of the confining material on water dynamics.     

Mechanistic insight into the structure and dynamics of entangled and hydrated λ-phage DNA

Intrinsic dynamics of DNA plays a crucial role in DNA-protein interactions and has been emphasized as a possible key component for in vivo chromatin organization. We have prepared an entangled DNA microtube above the overlap concentration by exploiting the complementary cohesive ends of λ-phage DNA, which is confirmed by atomic force microscopy and agarose gel electrophoresis. Photon correlation spectroscopy further confirmed that the entangled solutions are found to exhibit the classical hydrodynamics of a single chain segment on length scales smaller than the hydrodynamic length scale of single λ-phage DNA molecule. We also observed that in 41.6% (gm water/gm DNA) hydrated state, λ-phage DNA exhibits a dynamic transition temperature (T(dt)) at 187 K and a crossover temperature (T(c)) at 246 K. Computational insight reveals that the observed structure and dynamics of entangled λ-phage DNA are distinctively different from the behavior of the corresponding unentangled DNA with open cohesive ends, which is reminiscent with our experimental observation.     

Correlation between thermodynamic and kinetic fragilities in nonpolymeric glass-forming liquids

A phenomenological relationship between reduced excess heat capacity of supercooled liquid DeltaC(p)(exc)(T(g))DeltaS(m) at the glass transition temperature T(g), fragility index m, and reduced glass transition temperature T(rg)=T(g)T(m), where T(m) is the melting (liquidus) temperature, was derived for fragile nonpolymeric glass-forming liquids under the assumptions that the fragile behavior of these liquids is described by the Vogel-Fulcher-Tammann (VFT) equation; the excess heat capacity of liquid is inversely proportional to the absolute temperature and the VFT temperature T(0) is equal to the Kauzmann temperature T(K). It was found that DeltaC(p)(exc)(T(g))DeltaS(m) is a composite function of m and T(rg), which indicates that the empirical correlation DeltaC(p)(exc)(T(g))DeltaS(m)=0.025m recently identified by Wang et al. [J. Chem Phys. 125, 074505 (2006)] is probably valid only for liquids which have nearly the same values of T(rg).     

1-丁腈-3-甲基咪唑双三氟甲基磺酸亚胺的性质

制备了功能化离子液体1-丁腈-3-甲基咪唑双三氟甲基磺酸亚胺。在T为283.15-353.15 K温度范围内,测定了该功能化离子液体的密度、动力粘度、电导率及折光率。讨论了亚甲基的增减对该类功能化离子液体的密度、动力粘度、电导率及折光率等性质的影响,并与传统咪唑类、吡啶类离子液体物理化学性质的变化趋势进行了对比。通过经验方程计算了该功能化离子液体的热膨胀系数、分子体积、标准摩尔熵及晶格能等热力学性质参数。讨论了Vogel-Fulcher-Tamman (VFT)方程和Arrhenius方程的适用性,得出VFT方程适用于该功能化离子液体,而Arrhenius方程并不适用。有关研究对新型离子液体的合成及其工业化的应用具有十分重要的意义。     

A 13C NMR study of the molecular dynamics and phase transition of confined benzene inside titanate nanotubes

This work investigated the nanoconfinement effect on the molecular dynamics and phase transition of confined benzene inside titanate nanotubes with a uniform inner diameter of approximately 5.3 nm. For 13C-enriched organics, the 13C nuclear spin-spin relaxation was demonstrated as a sensitive tool to differentiate molecular translational motion and reorientation and, thus, was shown to be advantageous over the commonly employed 1H and 2H NMR for studying complex phase diagram, specifically, for separating the phase behavior of translational motion and the phase behavior of molecular reorientation. In such an approach, the melting of translational motion of confined benzene was explicitly observed to take place in a broad temperature range below the bulk melting temperature. The abrupt change of the 13C nuclear spin-spin relaxation time of the confined liquid benzene at about 260 K suggested that nanoconfinement induced two topologically distinct liquid phases.     

Broadband dielectric investigation on poly(vinyl pyrrolidone) and its water mixtures

Broadband dielectric spectroscopy and differential scanning calorimetry measurements have been performed to study the molecular dynamics poly (vinyl pyrrolidone) and its water solutions in a wide range of concentrations (0 wt %20 wt % suggesting that this dynamical process is dominated by water-water interactions. In addition, the temperature dependence of the water relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior during cooling throughout the glass transition range, which has been interpreted as due to the constrains imposed by the rigid polymer matrix on the water molecules dynamics.