Glycerol dielectric relaxation

G. R. Fleming In simple liquids such as methanol and ethanol there is no evidence for relaxation times slower than expected from dielectric measurements. Glasses at room temperature clearly show time scales that are infinite on our measurement time scale. In complex liquids such as glycerol-water mixtures and ethylene glycol, we may observe time scales that are longer than dielectric relaxation, but further studies are required to confirm this. 

Davidson, D., and R. Cole Dielectric relaxation in glycerol, propylene glycol and n-propanol. J. Chem. Phys. 19, 1484—1490 (1951). 

The VFT behavior of supercooled glycerol is well known from studies of liquid and supercooled glycerol [3,186-190], while the Arrhenius dependence of the dielectric relaxation time is more relevant for crystals. For example, the temperature dependence of the dielectric relaxation time of ice I also obey the Arrhenius law with the activation energy about 60 kJ moF1 [198,199]. 

EFFECT OF PRESSURE ON THE VISCOSITY AND DIELECTRIC RELAXATION TIME IN GLYCEROL. 

STUDY OF THE PROCESSES OF DIELECTRIC RELAXATION OF GLYCEROL AND ITS ESTERS WITH ACETIC ACID AS A FUNCTION OF PRESSURE. 

D. W. Davidson and R. H. Cole [1951] Dielectric Relaxation in Glycerol, Propylene Glycol, and -propanol,... 

Figure 2 includes curves for and " calculated using equation 4 and the KWW function for = 0.50. The loss curve is broad and nonssrmmetrical, with a total half-width A1/2 2.2 (cf 1.14 for the SRT process). The dielectric data for glycerol conform approximately to the KWW function (fi increases from about 0.6 to 0.95 as temperature is increased). Thus dielectric relaxation in polymers and other materials is characterized by the shapes of the b and b" curves in addition to Ab and (t). 

As mentioned above, the frequency dependence of the complex dielectric permittivity (e ) of the main relaxation process of glycerol [17,186] can be described by the Cole-Davidson (CD) empirical function [see (21) with a = 1, 0 < Pcd < 1], Now Tcd is the relaxation time which has non-Arrhenius type temperature dependence for glycerol (see Fig. 23). Another well-known possibility is to fit the BDS spectra of glycerol in time domain using the KWW relaxation function (23) < )(t) (see Fig. 24) ... 

Figure 24. The imaginary parts of the dielectric spectrum for anhydrous glycerol in the supercooled state at 196 K [186]. The dotted and dashed line show descriptions of the main relaxation process by CD [Eq. (21)] with tcd = 2.61 s, Ae = 63.9, and Pq, = 0.51) and KWW [Eq. (23)] with iK — 1.23 s, As = 62.0, and (3 = 0.69) functions, respectively. (The half-width of the loss curve were fixed for both CD and KWW functions.) (Reproduced with permission from Ref. 208. Copyright 2005, American Chemical Society.)...

Figure 41. Typical dielectric spectra of 20 mol% of glycerol—water mixtures at (a) 185 K (supercooled state) and (b) 218 K (frozen state), where solid and dashed curves show the real and imaginary parts of complex dielectric permittivity. Each relaxation process in the frozen state was fitted by (114) and by Cole-Cole and Debye relaxation functions, respectively, in order to separate the main process, the process due to interfacial water, and the process due to ice. (Reproduced with permission from Ref. 244. Copyright 2005, American Chemical Society.)...

Figure 37a presents 2H 7) data of glycerol-. A typical 7) minimum is observed at T>Tg, while below Tg the temperature-dependence of 7) is weak (cf. also Fig. 52). In order to test whether NMR relaxation probes similar relaxation processes as DS, 7) was calculated utilizing the dielectric spectra (cf. Fig. 37b), assuming that the NMR spectral density Si ( o>) can be approximated...

Figure 46. Linearized plots of the three parameters xa, vmin, and %min, determined from the MCT analyses of the relaxation spectrum in the high-temperature regime. Plotted are the scaling law amplitude (SLA) as indicated (a) from the dielectric spectra of glycerol (cf. Fig. 18a) (adapted from Ref. 136) (b) from the light scattering spectra of 2-picoline (cf. Fig. 18b) (from Ref. 183).

Although in some cases a consistent analysis of LS or DS spectra was carried out by applying the asymptotic laws of MCT, there are strong indications that these features are not completely appropriate to quantitatively describe, for example, DS as well as LS spectra. As discussed above, this is by now well known for PC and glycerol, at least. In order to tackle the problem of different experimental probes in a more realistic fashion, several MCT approaches have been published [265,380,400]. In a two-correlator schematic model, in which the dynamics of some probe (e.g., molecular reorientation in a dielectric experiment) is coupled to the overall structural relaxation in a simple manner, a simultaneous description of LS, DS, and NS spectra was possible even below Tc. Some of the results are... 

Figure 20. Dielectric loss data of glycerol and threitol at various combinations of temperature and pressure as indicated to demonstrate the departure of invariance of the dispersion of the a-relaxation at constant loss peak frequency or equivalently at constant a-relaxation time ra.

Similar heterogeneous model has been used to develop a relaxation function by Chamberlin and Kingsbury (1994), who consider the localized normal modes to be involved in the relaxation process. Localized (domains) regions are assumed to be present between Tg and T. They are described as dynamically correlated domains (DCD). A Gaussian distribution of the domain sizes has been assumed, with each domain characterized by a Debye relaxation time. Expressions for the dielectric susceptibility have been derived and used to fit the experimental susceptibilities of salol, glycerol and many other substances with remarkable agreement over 13 decades of frequency (even when only one adjustable parameter is employed). 

The dielectric behaviour of viscous molecular liquids has been studied since the early 1930 s. Baker and Smyth (48) had observed relaxation in supercooled isoamyl- and isobutyl bromide in the kHz region and had noted the similarity to the relaxations of liquid glucose (49) and glycerol (50). Similar... 

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