Dielectric loss factors measured at a frequency of are represented as lines. For one concentration, the actual data are additionally shown to see how many temperatures were actually measured. (a) For large alcohol concentrations , the Debye response is the most prominent. The contribution of a single-exponential relaxation is indicated by the dotted line. (b) Dielectric loss factors covering the entire concentration range.
(a) Dielectric constant and (b) dielectric loss of a sample with are shown as a function of the frequency . The existence of two different relaxation processes is clearly seen. The solid lines are fits using Eq. (1).
Strengths (a) of the Debye-like process and (b) of the relaxation as estimated from the maxima in the loss factor (filled symbols, scale on the right hand side) at the specified frequencies. The relaxation strengths (open symbols, scale on the left hand side) are shown for at which both dispersion steps are within the experimental frequency window for all concentrations. Data for pure BuOH from Murthy et al. (Ref. 14) obtained at are included (open triangle). Rough agreement between both measures for the susceptibility is obtained except at large alcohol concentrations where partial crystallization leads to a decrease in . The solid lines emphasize the approximately linear concentration dependence of the relaxation strengths.
(a) Isothermal concentration dependence of the relaxation time . The time constants do not depend on the alcohol concentration for larger than about 0.6. For smaller concentrations, decreases by more than an order of magnitude. The scaling in (a) shows that this behavior is independent of temperature. Frame (b) reveals that and the width parameter are compatible with the same critical concentration. For large alcohol concentrations, the Debye process is monodispersive, but for smaller it continuously broadens. The time constants for the process, scaled by and plotted in frame (c), exhibit an approximately exponential concentration dependence. The lines are drawn as guides for the eyes.
Relaxation maps for the and for the Debye-like processes. The solid lines are fits using the Vogel–Fulcher expression given in the text. The traces (frame a) show a large degree of overall similarity; i.e., the traces corresponding to different alcohol concentrations are relatively close to each other. Those for (frame b) reveal that the primary relaxation is characterized by a more pronounced concentration dependence.
The relaxation time of the process, , plotted vs that of the Debye-like response, . The dotted lines serve to estimate the relative separation of the two processes. The decoupling is the smallest for the pure alcohol and increases up to almost a factor of in the dilute limit.
(a) vs for mixtures of 2-methyl-1-butanol (MBOH) with 2-ethyl-1-hexanol (EHOH) (Ref. 22) and of BuBr with BuOH (present work). Note that for the MBOH:EHOH system, the OH fraction does not change and refers to the EHOH concentration. (b) For the latter system the exponent is plotted vs . The lines are guides for the eye.
Normalized relaxation strengths of the Debye-like process in several mixtures, partly taken from Ref. 17. The alcohols are 2-ethyl-1-hexanol (EHOH), 1-butanol (BuOH), and 1-pentanol (PeOH). 2-Methylpentane (2MP) represents a nonpolar substance. The polar additives are the salt lithium perchlorate , the primary amine 2-ethyl-hexylamine (EHA), and the alkyl halide 1-bromobutane (BuBr). The lines are drawn as guides for the eyes.
Boiling points , melting points , calorimetric glass transition temperatures , (Refs. 12 and 14), and electrical dipole moments (Ref. 47). The temperature at which dielectric relaxation times deduced from the present study reach is designated as .
Article metrics loading...
Full text loading...