Composition dependence of the Adam-Gibbs cooperative relaxation parameters in glycerol aqueous solutions

Cooperative relaxation of glycerol and its four aqueous solutions (60%, 70%, 80% and 90% by mass) has been investigated in terms of the nonlinear Adam-Gibbs (AG) enthalpy relaxation theory using differential scanning calorimetry (DSC). The AG parameters were obtained using curve-fitting method. The results indicated that the relaxation time of glycerol/water mixtures is water-sensitive. With the changing of water content, regular trend was found in both the equilibrium and the glassy state. The fitting results were used to estimate the microscopic parameters of the cooperative rearranging region (CRR), in particular the size of the CRR (z*) and the configurational state available to it (W*). The results showed that the W* recommended for polymers led to physically meaningless z* for glycerol and its aqueous solutions. Johari's method, which still based on the AG theory, yielded three to four molecules in the CRR. But the W* is anomalistically higher than those of polymers. With the changing of the water content, the size of CRR estimated using Donth formula seemed to be reasonable according to the analysis of the apparent activation energy (Δh*), the distribution parameter the of relaxation times (β). But it is difficult to reconcile the Adam-Gibbs’ z* with the results obtained using Donth's formula.     

Restricted orientational motion of nitroxides in molecular glasses: direct estimation of the motional time scale basing on the comparative study of primary and stimulated electron spin echo decays

A comparative study of anisotropic relaxation in two-pulse primary and three-pulse stimulated electron spin echo decays provides a direct way to distinguish fast (correlation time tau(c)<10(-6) s) and slow (tau(c)>10(-6) s) motions. Anisotropic relaxation is detected as a difference of the decay rates for different resonance field positions in anisotropic electron paramagnetic resonance spectra. For fast motion anisotropic relaxation influences the primary echo decay and does not influence the stimulated echo decay. For slow motion it is seen in both two-pulse echo and three-pulse stimulated echo decays. For nitroxide spin probes dissolved in glassy glycerol only fast motion was found below 200 K. Increase of temperature above 200 K results in the appearance of slow motion. Its amplitude increases rapidly with temperature increase. While in glycerol glass slow motion appears above glass transition temperature T(g), in ethanol glass it is observable below T(g). The scenario of motional dynamics in glasses is proposed which involves the broadening of the correlation time distribution with increasing temperature.     

Viscosity dependent radiationless relaxation rate of cyanine dyes. A picosecond laser spectroscopy study

The viscosity dependent radiationless relaxation of several cyanine dyes has been studied by picosecond laser spectroscopy. It was found that the relaxation rate is proportional to η^?α. The value of α, however, is not constant for a certain dye molecule, but is strongly dependent on the kind of solvent used. In n-alcohols for instance α is typically about 1. In glycerol/methanol or glycerol/water mixtures on the other hand α ≈ 0.5. A comparison is made with literature data on orientational relaxation lifetimes of some dyes in similar solvents. It is shown that the radiationless relaxation of cyanine dyes and the orientational relaxation of for instance xanthene dyes changes in roughly the same way as the solvent is changed. This is taken as proof of the proposal that a torsional motion of the heterocyclic quinolyl rings is the main course of the viscosity dependent relaxation of the cyanine dyes studied.     

Computational experiments on the intramolecular energy flow in macromolecules

The microscopic details of the flow of energy in a single chain of polyethylene containing 300 atoms is discussed. The intramolecular dynamics of the polyethylene molecule is studied as a function of CH stretch excitation, temperature, and pressure. The rate of energy flow from CH stretching modes is found to be very rapid and irreversible, occurring on a timescale of less than 0.5 ps at low temperatures, and increases with temperature. A general characteristic two-phase energy flow behavior is observed, where there is initially a very rapid flow (due to the decay of the initial excitation) followed by a slower flow (due to energy redistribution throughout the system). The mechanism for the initial facile energy flow is shown to involve strong resonant pathways. In particular, a CH stretch/HCH bend Fermi (1:2) resonance is shown to dominate the short-time dynamics and facilitates the overall process of energy redistribution. The increase in the rate of energy flow as a function of the backbone temperature is found to be due to the increase in the density of the bath states for energy redistribution which subsequently results in the formation of new low-order resonant interactions (1:1, etc). The long-time dynamics, associated to complete redistribution of the initial CH stretch energy with all of the 894 available vibrational modes, occurs within a time of 2 ps. This timescale corresponds to the time for intramolecular redistribution. A comparison of the intramolecular redistribution time to that of intermolecular redistribution (redistribution in the condensed or solid phase as opposed to a single chain) is also made. A preliminary study of energy flow in a crystal of polyethylene (system containing 19 polyethylene chains) shows that the energy flow exhibits two very different time behaviors. The first is for the intramolecular redistribution as in the single chain study and the second is for intermolecular (chain-to-chain) redistribution. The timescale for intermolecular redistribution is found to be on the order of 0.2 ns at room temperature and pressure, about two orders of magitude larger than the intramolecular timescale.     

Study of dynamics and crystallization kinetics of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile at ambient and elevated pressure

The organic liquid ROY, i.e., 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, has been a subject of detailed study in the last few years. One interest in ROY lies in its polymorph-dependent fast crystal growth mode below and above the glass transition temperature. This growth mode is not diffusion controlled, and the possibility that it is enabled by secondary relaxation had been suggested. However, a previous study by dielectric relaxation spectroscopy had not been able to find any resolved secondary relaxation. The present paper reports new dielectric measurements of ROY in the liquid and glassy states at ambient pressure and elevated pressure, which were performed to provide more insight into the molecular dynamics as well as the crystallization tendency of ROY. In the search of secondary relaxation, a special glassy state of ROY was prepared by applying high pressure to the liquid state, from which secondary relaxation was possibly resolved. Thus, the role of secondary relaxation in crystallization of ROY remains to be clarified. Notwithstanding, the secondary relaxation present is not necessarily the sole enabler of crystallization. In an effort to search for possible cause of crystallization other than secondary relaxation, we also performed crystallization kinetics studies of ROY at different T and P combinations while keeping the structural relaxation time constant. The results show that crystallization of ROY speeds up with pressure, opposite to the trend found in the crystallization of ibuprofen studied up to 1 GPa. The dielectric relaxation and thermodynamic properties of ROY with phenolphthalein dimethylether (PDE) are similar in many respects, but PDE does not crystallize. Taking all the above into account, besides the secondary relaxation, the specific chemical structure, molecular interactions and packing of the molecules are additional factors that could affect the kinetics of crystallization found in ROY.     

Molecular dynamics study on the solvent dependent heme cooling following ligand photolysis in carbonmonoxy myoglobin

The time scale and mechanism of vibrational energy relaxation of the heme moiety in myoglobin was studied using molecular dynamics simulation. Five different solvent models, including normal water, heavy water, normal glycerol, deuterated glycerol and a nonpolar solvent, and two forms of the heme, one native and one lacking acidic side chains, were studied. Structural alteration of the protein was observed in native myoglobin glycerol solution and native myoglobin water solution. The single-exponential decay of the excess kinetic energy of the heme following ligand photolysis was observed in all systems studied. The relaxation rate depends on the solvent used. However, this dependence cannot be explained using bulk transport properties of the solvent including macroscopic thermal diffusion. The rate and mechanism of heme cooling depends upon the detailed microscopic interaction between the heme and solvent. Three intermolecular energy transfer mechanisms were considered: (i) energy transfer mediated by hydrogen bonds, (ii) direct vibration-vibration energy transfer via resonant interaction, and (iii) energy transfer via vibration-translation or vibration-rotation interaction, or in other words, thermal collision. The hydrogen bond interaction and vibration-vibration interaction between the heme and solvent molecules dominates the energy transfer in native myoglobin aqueous solution and native myoglobin glycerol solutions. For modified myoglobin, the vibration-vibration interaction is also effective in glycerol solution, different from aqueous solution. Thermal collisions form the dominant energy transfer pathway for modified myoglobin in water solution, and for both native myoglobin and modified myoglobin in a nonpolar environment. For native myoglobin in a nonpolar solvent solution, hydrogen bonds between heme isopropionate side chains and nearby protein residues, absent in the modified myoglobin nonpolar solvent solution, are key interactions influencing the relaxation pathways.     

1H NMR relaxation in glycerol solutions of nitroxide radicals: effects of translational and rotational dynamics

(1)H spin-lattice relaxation rates in glycerol solutions of selected nitroxide radicals at temperatures between 200 K and 400 K were measured at 15 MHz and 25 MHz. The frequency and temperature conditions were chosen in such a way that the relaxation rates go through their maximum values and are affected by neither the electron spin relaxation nor the electron-nitrogen nucleus hyperfine coupling, so that the focus could be put on the mechanisms of motion. By comparison with (1)H spin-lattice relaxation results for pure glycerol, it has been demonstrated that the inter-molecular electron spin-proton spin dipole-dipole interactions are affected not only by relative translational motion of the solvent and solute molecules, but also by their rotational dynamics as the interacting spins are displaced from the molecular centers; the eccentricity effects are usually not taken into account. The (1)H relaxation data have been decomposed into translational and rotational contributions and their relative importance as a function of frequency and temperature discussed in detail. It has been demonstrated that neglecting the rotational effects on the inter-molecular interactions leads to non-realistic conclusions regarding the translational dynamics of the paramagnetic molecules.     

Anharmonicity in a fragile glass-former probed by inelastic neutron scattering

Within the overall understanding of the glass transition, the relationship between microscopic dynamics and fragility is still to be clarified. Decalin is an organic glass former, for which a cis/trans mixture exhibits the highest known degree of fragility in a molecular system. It is therefore an ideal system for the investigation of microscopic dynamics in fragile systems. In the present study, the microscopic dynamics of pure cis-decalin has been measured by inelastic and quasi-elastic incoherent neutron scattering, giving the single particle self-correlation function. The fast relaxation dynamics and low-frequency vibrational modes are reported here. Both in the glass and in the crystal the vibrations show strong anharmonic behavior. In the glass phase, the short time microscopic dynamics evolve rapidly with temperature, however do not exhibit any significant change around the glass transition temperature T(g). The elastic intensity provides a measure of the mean square displacements which are comparable to those measured in other fragile glass formers, in particular, the archetypical fragile glass former orthoterphenyl. It appears that the microscopic relaxation gets unfrozen, relative to T(g), at much lower temperature than in other fragile systems.     

The effect of memory in the stochastic master equation analyzed using the stochastic Liouville equation of motion. Electronic energy migration transfer between reorienting donor-donor, donor-acceptor chromophores

This paper discusses the process of energy migration transfer within reorientating chromophores using the stochastic master equation (SME) and the stochastic Liouville equation (SLE) of motion. We have found that the SME over-estimates the rate of the energy migration compared to the SLE solution for a case of weakly interacting chromophores. This discrepancy between SME and SLE is caused by a memory effect occurring when fluctuations in the dipole-dipole Hamiltonian (H(t)) are on the same timescale as the intrinsic fast transverse relaxation rate characterized by (1/T(2)). Thus the timescale critical for energy-transfer experiments is T(2) approximately 10(-13) s. An extended SME is constructed, accounting for the memory effect of the dipole-dipole Hamiltonian dynamics. The influence of memory on the interpretation of experiments is discussed.     

Dynamics of solvent and rotational relaxation of glycerol in the nanocavity of reverse micelles

The dynamics of solvent and rotational relaxation of Coumarin 480 and Coumarin 490 in glycerol containing bis-2-ethyl hexyl sulfosuccinate sodium salt (AOT) reverse micelles have been investigated with steady-state and time-resolved fluorescence spectroscopy. We observed slower solvent relaxation of glycerol confined in the nanocavity of AOT reverse micelles compared to that in pure glycerol. However, the slowing down in the solvation time on going from neat glycerol to glycerol confined reverse micelles is not comparable to that on going from pure water or acetonitrile to water or acetonitrile confined AOT reverse micellar aggregates. While solvent relaxation times were found to decrease with increasing glycerol content in the reverse micellar pool, rotational relaxation times were found to increase with increase in glycerol content.     

Solvation dynamics of electron produced by two-photon ionization of liquid polyols. III. Glycerol

The solvation dynamics of excess electrons in glycerol have been measured by the pump-probe femtosecond laser technique at 333 K. The electrons are produced by two-photon absorption at 263 nm. The change in the induced absorbance is followed up to 450 ps in the spectral range from 440 to 720 nm. The transient signals of electron solvation have been analyzed by two kinetic models: a stepwise mechanism and a continuous relaxation model, using a Bayesian data analysis method. The results are compared with those previously published for ethylene glycol (J. Phys. Chem. A 2006, 110, 175) and for propanediols (J. Phys. Chem. A 2007, 111, 4902). From the comparison, it is pointed out that solvation dynamics in glycerol is very fast despite its high viscosity. This is interpreted as the existence of efficient traps for the electrons in glycerol with low potential energy. The small shift of the absorption band of the excess electron indicates that the potential of these traps is very close to that corresponding to the fully solvated electron.     

Dielectric properties of glycerol/water mixtures at temperatures between 10 and 50 degrees C

At six temperatures T between 10 and 50 degrees C and at mole fractions x(g) of glycerol (0相似文献   

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.     

A Surrogate for Debye-Waller Factors from Dynamic Stokes Shifts

We show that the short-time behavior of time-resolved fluorescence Stokes shifts (TRSS) are similar to that of the intermediate scattering function obtained from neutron scattering at q near the peak in the static structure factor for glycerol. This allows us to extract a Debye-Waller (DW) factor analog from TRSS data at times as short as 1 ps in a relatively simple way. Using the time-domain relaxation data obtained by this method we show that DW factors evaluated at times ≥ 40 ps can be directly influenced by α relaxation and thus should be used with caution when evaluating relationships between fast and slow dynamics in glassforming systems.     

What is the time scale for α-helix nucleation?

Helix formation is an elementary process in protein folding, influencing both the rate and mechanism of the global folding reaction. Yet, because helix formation is less cooperative than protein folding, the kinetics are often multiexponential, and the observed relaxation times are not straightforwardly related to the microscopic rates for helix nucleation and elongation. Recent ultrafast spectroscopic measurements on the peptide Ac-WAAAH(+)-NH(2) were best fit by two relaxation modes on the ～0.1-1 ns time scale, (1) apparently much faster than had previously been experimentally inferred for helix nucleation. Here, we use replica-exchange molecular dynamics simulations with an optimized all-atom protein force field (Amber ff03w) and an accurate water model (TIP4P/2005) to study the kinetics of helix formation in this peptide. We calculate temperature-dependent microscopic rate coefficients from the simulations by treating the dynamics between helical states as a Markov process using a recently developed formalism. The fluorescence relaxation curves obtained from simulated temperature jumps are in excellent agreement with the experimentally determined results. We find that the kinetics are multiphasic but can be approximated well by a double-exponential function. The major processes contributing to the relaxation are the shrinking of helical states at the C-terminal end and a faster re-equilibration among coil states. Despite the fast observed relaxation, the helix nucleation time is estimated from our model to be 20-70 ns at 300 K, with a dependence on temperature well described by Arrhenius kinetics.     

Contribution of relaxation processes to adhesive-joint strength of viscoelastic polymers

Relaxation processes accompany all stages of the lifetime of viscoelastic pressure-sensitive polymer adhesives, which can form strong adhesive joints with substrates of various chemical natures under application of a slight external pressure to the adhesive film for a few seconds. This review deals with comparison of the adhesion and relaxation properties of a number of typical pressure-sensitive adhesives based on polyisobutylene, butyl rubber, styrene-isoprene-styrene triblock copolymers, alkyl acrylate copolymers, and silicone adhesives as well as pressure-sensitive adhesives based on blends of high-molecular-mass polyvinylpyrrolidone with oligomeric poly(ethylene glycol). Within all three stages of the lifetime of adhesive joints (under adhesive-bond-forming pressure, upon withdrawal of contact pressure in the course of relaxation of the adhesive material, and under the force detaching an adhesive film from the substrate surface), the strength of adhesive joints has been shown to be controlled by large-scale relaxation processes, which are characterized by long relaxation times in the range 150–800 s. All examined pressure-sensitive adhesives can be arbitrarily divided into two groups. The first group is composed of fluid adhesives that relax comparatively fast and exhibit no residual (unrelaxed) stress. The second group includes elastic adhesives capable storing mechanical energy in the course of deformation that are characterized by appreciably longer relaxation times and display residual stress after relaxation. Conditions of adhesive debonding (e.g., strain amplitude and deformation velocity) significantly affect the relaxation process.     

Ionic Mobility and Dielectric Relaxation in Glass-Forming Mixtures of Sodium Chloride and Glycerol

Measurements of electrical conductivities of liquid and supercooled liquid NaCl–glycerol solutions were carried out between +42° and ?87°C. Time-domain measurements of dielectric relaxation in pure glycerol and in NaCl–glycerol solutions between ?78 and ?89.9°C are described. The transient response data were fitted to the empirical Davidson–Cole response model. The specific conductivity data show a non-Arrhenius behavior near the glass-transition temperature and is well described by the Vogel–Tammann–Fulcher (VTF) law, which also fits the dielectric relaxation time data. The Vogel–Fulcher divergence temperature is consistent with the Kauzmann temperature. The dielectric relaxation time is increased significantly by the addition of sodium chloride, whereas the static relative permittivity decreases linearly with concentration, indicating that NaCl has a “structure-making” effect on the structure of glycerol.     

Determination of free glycerol in biodiesel at a platinum oxide surface using potential cycling technique

A novel, inexpensive and fast method based on the electrooxidation of glycerol on platinum electrodes by the potential cycling technique has been designed for the determination of free glycerol in biodiesel. A wide range of linearity was achieved between 15 and 150 mg L⁻¹ (0.16 and 1.6 mmol L⁻¹), which corresponds to concentrations ranging between 56 and 560 mg kg⁻¹ (glycerol:biodiesel) for an extraction using 2 g biodiesel. A method for the fast extraction of glycerol from biodiesel with water followed by elimination of organic interferents has also been developed, so that the novel determination method can be applied to various biodiesel samples. The excellent repeatability allows determination of glycerol in numerous samples, with no need for recalibration.     

Energy partitioning in the reaction O(1D) + H2O → OH + OH. V. Rotational relaxation of OH(X2Π, ν″, J″)

The rotational relaxation of OH(X²Π, ν″, J″) in ν″, = 0, 1, and 2 produced from the reaction of O (¹D) with H₂O has been studied as a function of H₂O vapor pressure and added argon. Water molecules are extremely efficient in bringing about relaxation and the experiments performed indicate that, on the average, the high temperature distribution is relaxed to nearly room temperature at a gas kinetic rate. This observation is rationalized by assumming a collision complex between OH and H₂O having a quasichemical interaction similar to weak hydrogen bonding. The nascent OH internal energy distribution does not depend upon the translational energy of the O(¹D) reactant. Translational relaxation of the nascent OH product by H₂O is fast, as fast as rotational relaxation.