Relaxation Time Of Molecules Research Articles

Еffect of fullerene C60 on the diffusion of organic solvents' molecules

The results of studying the dynamics of molecules of solutions of fullerene C60 in benzene and toluene are presented. The studies were carried out by the method of quasi-elastic scattering of slow neutrons. The following were determined: the total self-diffusion coefficient D, its collective Dcoll and single-particle Ds-p contributions, and molecule relaxation time τ0 before and after the dissolution of fullerene. It is necessary to pay attention to the fact that in our case a kind of neutron contrast experiment takes place. The significant difference (more than ten times) in the cross-sections for an incoherent scattering of neutrons by hydrogen and carbon nuclei made it possible to obtain diffusion parameters that characterize the dynamics of solvent molecules exclusively. It turned out that the intensity of diffusion movements of solvent molecules is almost ten times higher than the mobility of fullerene molecules.

Frequency Dependence of Dynamic Properties of Polyethylene Glycol Molecules on Oscillating Solid–Liquid Interface

Frequency dependence of the resonant length of polyethylene glycol (PEG) on a solid–liquid interface oscillating at megahertz was studied by a quartz crystal microbalance (QCM). A QCM frequency and a number-average molecular weight (Mn) of PEG were changed systematically. This attempt revealed that, in all of the frequencies used, the series-resonant frequency shift, ΔF, of the QCM against the square root of the density-viscosity product of the PEG solutions was linear and had intercept. In addition, the systematical analysis made it clear that the ΔF slope was explainable by the Debye process, and the ΔF intercept became constant above the resonant length. These results led to the novel findings that the resonant length of the PEG molecules formed by physisorption was equal to that in the region of the viscous penetration depth and that the relationship between resonant length and QCM frequency was able to be explained by τ ∝ Mnν, where τ is a relaxation time of molecule and ν is a constant. In particula...

Dielectric Properties of Diesel and Gasoline by Terahertz Spectroscopy

In this study we have investigated the dielectric properties of diesel and gasoline in the Terahertz (THz) spectral region. We present frequency dependent absorption coefficients, refractive indices, and dielectric constants calculated from the transient measurements of the fuel oils between 0.1 and 1.1 THz. Observed weak absorption coefficient of fuel oils is explained by transient dipole moments induced by collisions between individual molecules. Fuel oils were modeled successfully with Debye model to investigate the relaxation dynamics after interaction with the electric field. Significant differences in relaxation times of molecules in diesel and gasoline are attributed to the differences in their intermolecular forces. Dispersion forces are much greater in diesel due to the longer hydrocarbon chains (C8-C40) compared to that (C4-C12) of the gasoline. This leads to a comparably faster relaxation right after THz electric field is applied. Clear differences in optical properties offer a simple yet effective way to discriminate fuel oils from each other by using THz spectroscopy without any danger of combustion or decomposition of the samples. Such an approach may also be used for the quality determination of either fuels. The study presents the great potential of THz spectroscopy to study very complex mixtures like fuel oils by the use of instantaneous THz wave/matter interactions and relaxation dynamics of the constituent molecules.

Nuclear Magnetic Resonance Studies on Microstructure of Cement Pastes

For water filled porous materials, the difference between the relaxation time of molecules at the pore surface and the relaxation time of molecules in the bulk fluid can be interpreted by a fast diffusion model. With this model, the nuclear magnetic resonance (NMR) can be applied to investigate the microstructure of cement pastes. The cement pastes we tested are two series, one is of same water to cement ratio (w/c=0.4) at different curing time (7d, 28d and 90d), the other is of different water to cement ratio (w/c=0.3, 0.4 and0.5, respectively) at the same curing day. Comparing results by NMR method with those by mercury intrusion porosimetry (MIP) shows that NMR is a convenient and nondestructive way to probe pore distribution of cement pastes.

Dynamics of glass-forming materials confined in thin films

By considering the effect of confinement of the collective motion of glass-forming molecules, we show how the relaxation time of molecules is expected to vary across a thin film and as a function of the film thickness. The results of these calculations are compared with available experimental data.

Shear rheology of polymer solutions near the critical condition for elastic instability

The use of constant viscosity, highly elastic polymer solutions, so called Boger fluids, has been remarkably successful in elucidating the behavior of polymeric materials under flowing conditions. However, the behavior of these fluids is still complicated by many different physical processes occurring within a narrow window of observation time and applied shear rate. In this study, we investigate the long-time shear behavior of an ideal Boger fluid: a well characterized, athermal, dilute, binary solution of high molecular weight polystyrene in oligomeric polystyrene. Rheological measurements show that under an applied steady shear flow, this family of polymer solutions undergoes a transient decay of normal stresses on a timescale much longer than the polymer molecule's relaxation time. Rheological and flow visualization results demonstrate that the observed phenomenon is not caused by polymer degradation, phase separation, viscous heating, or secondary flows from elastic instabilities. Although the timescale is much shorter than that associated with polymer migration in the same solutions (MacDonald and Muller, 1996), the appearance of this phenomenon only at the rates where migration has been observed suggests that it may be a prerequisite for observing migration. In addition, we note that through sufficient preshearing of the sample, the normal stress decrease suppresses the elastic instability. These results show that there is considerable uncertainty in choosing the appropriate measure of the fluid relaxation time for consistently modeling the critical condition for the elastic instability, the decay of normal stresses, and the migration of polymer species.

Thin-cell dielectric response of a ferroelectric liquid crystal.

The results of a study of the dielectric response of the liquid crystal BOH8-MPOOBC in the frequency range 5 Hz to 13 MHz, in the vicinity of the phase transition to the ferroelectric phase, in an aligned cell of 2.7 \ensuremath{\mu}m thickness, are presented. At small ac probe fields, critical-like behavior is observed in the dielectric permittivity and the characteristic relaxation time of molecules in the bulk of the sample. At high ac probe fields, a second relaxation appears due to the reorientation of molecules at the cell surfaces. The dielectric strength of the bulk relaxation grows linearly with the spontaneous polarization. The viscosities of the surface and bulk processes, as determined from measured characteristic times and spontaneous polarization, increase in a thermally activated manner for temperatures within the ferroelectric phase. Comparison of frequency-dependent optical response and dielectric response measurements at small applied fields shows that both are due to the same bulk molecular reorientation process.

Electronic damping mechanism for vibrations, rotations, and translations of adsorbates on metal surfaces

Abstract Further theoretical evidence for the importance of the electronic mechanism for the energy exchange between an adsorbate and a metallic substrate is provided by first-principle calculations of the damping of vibrational modes of chemisorbed H and H 2 on free-electron-like metal surfaces and of the friction coefficient of an approaching H atom. In reactive situations, i.e. when a resonance in the adsorbate-induced electron structure crosses the Fermi level of the substrate conduction electrons, enhanced damping is exhibited. With rough estimates for rotational lifetimes added, relevant relaxation times for molecules approaching metal surfaces are briefly discussed.

Structural Studies on Polypeptide Hormones: II. POLARIZATION OF FLUORESCENCE

The rotational characteristics of polypeptides containing a tryptophanyl residue have been evaluated by polarization of fluorescence measurements in concentrated glycerol solutions. A curve has been obtained showing the dependence of relaxation time on chain size based on the behavior of two series of peptides, i.e. Glyx−Trp−Glyy and adrenocorticotropic hormone and some of its fragments. If these molecules are structureless polypeptides then it can be ascertained, from the relaxation times of molecules with unknown structures, whether the indole group possesses unrestricted or restricted rotational freedom. In the latter case, one can conclude that there exist some elements of structure in the molecule. Several hormones and polypeptides (gastrin, glucagon, parathyroid hormone, and mellitin) and some of their fragments have been evaluated for tertiary interactions by this procedure.