14 research outputs found
An Analysis of Radical Diffusion in Ionic Liquids in Terms of Free Volume Theory
The Heisenberg spin exchange – dipole-dipole (HSE-DD) separation method was used to measure the translational diffusion coefficients of the 14N-labeled perdeuterated 2, 2, 6, 6-tetramethyl-4-oxopiperidine-1-oxyl (14N-pDTEMPONE) nitroxide spin probe as a function of temperature in two methylimidazolium ionic liquid series, one based on the tetrafluoroborate (BF4) anion and another one on the bis(trifluoromethane)sulfonimide (TFSI, Tf2N) anion. The obtained translational diffusion coefficients of 14N-pDTEMPONE were analyzed in terms of the Cohen–Turnbull free volume theory. It was found that the Cohen-Turnbull theory describes, exceptionally well, the translational diffusion of 14N-pDTEMPONE in all the ionic liquids in the measured temperature range. Also, the Cohen-Turnbull theory was applied to the viscosity and self-diffusion coefficients of the cation and anion - taken from literature - in the same ionic liquids. The critical free volume for the self-diffusion of the cation and anion in a given ionic liquid is the same, which suggests that the diffusion of each ionic pair is coordinated. The critical free volumes for the 14N-pDTEMPONE diffusion, self-diffusion, and viscosity for a given cation were about twenty percent greater in the TFSI based ionic liquids than in the BF4 based ionic liquids. It appears that the ratio of the critical free volumes for a given cation between the two series correlates with the ratio of their densities
Low temperature electron-spin relaxation in the crystalline and glassy states of solid ethanol
X-band electron paramagnetic resonance (EPR) spectroscopy was used to study
the spectral properties of a nitroxide spin probe in ethanol glass and
crystalline ethanol, at 5 - 11.5 K. The different anisotropy of molecular
packing in the two host matrices was evidenced by different rigid limit values
for maximal hyperfine splitting in the signal of the spin probe. The
significantly shorter phase memory time, , for the spin probe dissolved in
crystalline ethanol, as compared to ethanol glass, was discussed in terms of
contribution from spectral diffusion. The effect of low-frequency dynamics was
manifested in the temperature dependence of and in the difference between the
data measured at different spectral positions. This phenomenon was addressed
within the framework of the slow-motional isotropic diffusion model [S. Lee,
and S. Z. Tang, Phys. Rev. B 31, 1308 (1985)] predicting the spin probe
dynamics within the millisecond range, at very low temperatures. The shorter
spin-lattice relaxation time of the spin probe in ethanol glass was interpreted
in terms of enhanced energy exchange between the spin system and the lattice in
the glass matrix due to boson peak excitations.Comment: 16 pages, 4 figures, 36 reference
Electron spin-lattice relaxation in solid ethanol: the effect of nitroxyl radical hydrogen bonding and matrix disorder
The electron spin-lattice relaxation of TEMPO and TEMPONE was measured at
temperatures between 5 and 80 K in crystalline and glassy ethanol using X-band
electron paramagnetic resonance spectroscopy. The experimental data at the
lowest temperatures studied were explained in terms of electron-nuclear dipolar
interaction between the paramagnetic center and the localized excitations,
whereas at higher temperatures low-frequency vibrational modes from the host
matrix and Raman processes should be considered. The strong impact of hydrogen
bonding between the dopant molecule and ethanol host on the spin relaxation was
observed in ethanol glass whereas in crystalline ethanol both paramagnetic
guest molecules behaved similarly.Comment: 13 pages, 2 figures, 32 reference
Study of Nanostructural Organization of Ionic Liquids by Electron Paramagnetic Resonance Spectroscopy
The
X-band electron paramagnetic resonance spectroscopy (EPR) of
a stable, spherical nitroxide spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl
(pDTO) has been used to study the nanostructural organization of a
series of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids
(ILs) with alkyl chain lengths from two to eight carbons. By employing
nonlinear least-squares fitting of the EPR spectra, we have obtained
values of the rotational correlation time and hyperfine coupling splitting
of pDTO to high precision. The rotational correlation time of pDTO
in ILs and squalane, a viscous alkane, can be fit very well to a power
law functionality with a singular temperature, which often describes
a number of physical quantities measured in supercooled liquids. The
viscosity of the ILs and squalane, taken from the literature, can
also be fit to the same power law expression, which means that the
rotational correlation times and the ionic liquid viscosities have
similar functional dependence on temperature. The apparent activation
energy of both the rotational correlation time of pDTO and the viscous
flow of ILs and squalane increases with decreasing temperature; in
other words, they exhibit strong non-Arrhenius behavior. The rotational
correlation time of pDTO as a function of η/<i>T</i>, where η is the shear viscosity and <i>T</i> is
the temperature, is well described by the Stokes–Einstein–Debye
(SED) law, while the hydrodynamic probe radii are solvent dependent
and are smaller than the geometric radius of the probe. The temperature
dependence of hyperfine coupling splitting is the same in all four
ionic liquids. The value of the hyperfine coupling splitting starts
decreasing with increasing alkyl chain length in the ionic liquids
in which the number of carbons in the alkyl chain is greater than
four. This decrease together with the decrease in the hydrodynamic
radius of the probe indicates a possible existence of nonpolar nanodomains