Effect of Pressure on Dielectric Relaxation in Isomeric Octanols

The extent of H bonding in alcohols may be reduced by sterically hindering its OH group. This technique is used here for investigating the reasons for the prominent Debye-type dielectric relaxation observed in monohydroxy alcohols ͓Kudlik et al., Europhys. Lett. 40, 549 ͑1997͒; Hansen et al., J. Chem. Phys. 107, 1086 ͑1997͒; Kalinovskaya and Vij, ibid. 112, 3262 ͑2000͔͒, and broadband dielectric spectroscopy of supercooled liquid and glassy states of 1-phenyl-1-propanol is performed over the 165-238 K range. In its molecule, the steric hindrance from the phenyl group and the existence of optical isomers reduce the extent of intermolecular H bonding. The equilibrium permittivity data show that H-bonded chains do not form in the supercooled liquid, and the total polarization decays by three discrete relaxation processes, of which only the slower two could be resolved. The first is described by the Cole-Davidson-type distribution of relaxation times and a Vogel-Fulcher-Tammann-type temperature dependence of its average rate, which are characteristics of the ␣-relaxation process as in molecular liquids. The second is described by a Havriliak-Negami-type equation, and an Arrhenius temperature dependence, which are the characteristics of the Johari-Goldstein process of localized molecular motions. The relaxation rate's non-Arrhenius temperature dependence has been examined qualitatively in terms of the Dyre theory, which considers that the apparent Arrhenius energy itself is temperature dependent, as in the classical interpretations, and quantitatively in terms of the cooperatively rearranging region's size, without implying that there is an underlying thermodynamic transition in its equilibrium liquid. The relaxation rate also fits the power law with the critical exponent of 13.4, instead of 2-4, required by the mode-coupling theory, thereby indicating the ambiguity of the power-law equations.

Section: B the Nature Of The Dielectric Relaxation Processesmentioning

confidence: 87%

Effects of induced steric hindrance on the dielectric behavior and H bonding in the supercooled liquid and vitreous alcohol

Johari

Kalinovskaya

Vij

2001

“…This is the same mechanism as was used for interpreting the relaxation in monohydroxy alcohols. 51 Evidently, hydrogen bonding is unnecessary for the observation of a Debye relaxation in an ultraviscous liquid. In relation to our study, it is clear that hydrogen bonding does not necessarily produce a Debye relaxation in some alcohols, and there seems to be no a priori need for attributing the Debye relaxation to micelles or other structure in pure monohydroxy alcohols.…”

Section: -7mentioning

confidence: 99%

Relaxations and nano-phase-separation in ultraviscous heptanol-alkyl halide mixture

Power¹,

Vij²,

Johari³

2007

To gain insight into the effects of liquid-liquid phase separation on molecular relaxation behavior we have studied an apparently homogeneous mixture of 5-methyl-2-hexanol and isoamylbromide by dielectric spectroscopy over a broad temperature range. It shows two relaxation regions, widely separated in frequency and temperature, with the low-frequency relaxation due to the alcohol and the high-frequency relaxation due to the halide. In the mixture, the equilibrium dielectric permittivity s of the alcohol is 41% of the pure state at 155.7 K and s of isoamylbromide is ϳ86% of the pure state at 128.7 K. The difference decreases for the alcohol component with decreasing temperature and increases for the isoamylbromide component. The relaxation time of 5-methyl-2-hexanol in the mixture at 155.7 K is over five orders of magnitude less than in the pure state, and this difference increases with decreasing temperature, but of isoamylbromide in the mixture is marginally higher than in the pure liquid. This shows that the mixture would have two T g 's corresponding to its of 10 3 s, with values of ϳ121 K for its 5-methyl-2-hexanol component and ϳ108 K for its isoamylbromide component. It is concluded that the mixture phase separates in submicron or nanometer-size aggregates of the alcohol in isoamylbromide, without affecting the latter's relaxation kinetics, while its own s and decrease markedly.

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Work at the time was hampered by the long and tedious manual procedures required to make dielectric measurements for different pressures at different temperatures. More recently, however automated data collection techniques as well as the data analysing techniques 14,15 have made such studies more revealing of the effects observed at pressures especially exceeding 500 MPa (5 kbar).…”

Section: Introductionmentioning

confidence: 99%

Effect of high hydrostatic pressure on the dielectric relaxation in a non-crystallizable monohydroxy alcohol in its supercooled liquid and glassy states

Pawlus

Paluch

Nagaraj

et al. 2011

The complex relative permittivity of a non-crystallizable secondary alcohol, 5-methyl-2-hexanol, is measured over a wide range of temperatures and pressures up to 1750 MPa (17.5 kbar). The data at atmospheric pressure (P = 0.101 MPa) are analyzed in terms of three processes, and the results are in complete agreement with that of O. E. Kalinovskaya and J. K. Vij [J. Chem. Phys. 112, 3262 (2000)]. Process I is of the Debye type and process II is of the Davidson-Cole type, whereas process III is identified as the Johari-Goldstein relaxation process. For pressures of ∼500 MPa and higher, processes I and II are seen to merge into each other to form a single dominant process which unambiguously cannot be resolved into more than one process. The dielectric relaxation strength of process I decreases slightly initially with pressure and when the two processes have merged at elevated pressures, the total relaxation strength increases with increase in pressure. Process III is better resolvable at higher pressures especially above T g in the supercooled liquid state for the reason that the separation in the time scales between the dominant and the JG relaxation process increases at elevated pressures. Surprisingly we find a change in the slope in the plot of log τ JG vs. 1/T for P = 1750 MPa. The results for the relaxation time of alcohols are compared with the Kirkwood correlation factor, g, and it is found that higher is the g, lower is the relaxation time for process I, and it is more of the Debye type. On a reduction in g brought about by an increase in pressure at lower temperatures, the dominant process becomes non-Debye though extensive hydrogen bonding is still present. The dielectric strength of the merged processes increases with increase in pressure. The values of the steepness index, m = |d log τ /d(T g /T)| T = Tg for processes I and II are different for P = 0.1 MPa. However the value of m, for the composite process, which is a merger of processes I and II, for P = 1750 MPa is almost the same for process II at P = 0.1 MPa. From the results of the activation volume, activation enthalpy, and a comparison of the relaxation times with the g factor, we conclude that both processes I and II are significantly affected by hydrogen bonding and both contribute to the structural relaxation.