Microwave dielectric relaxation of methyl and ethylcellosolve

Abstract

The complex dielectric permittivity of methyl and ethylcellosolve at 25°C and in the frequency range 1–90 GHz are reported. The real and imaginary coefficients of the permittivity show a relaxation profile with frequency that can be interpreted either by a Cole-Davidson distribution function or by the sum of two discrete Debye-relaxation processes. Preference for the latter description is given not on the basis of an optimum of a numerical fit, but rather on the proposal that the two Debye processes are related to H-bond breaking, as for the alcohols, and to alkoxy and/or molecular tumbling respectively. Evidence of the above hypothesis is given by reporting the complex permittivity of methylcellosolve-dimethoxyethane mixtures in the whole composition range. It is shown that the relaxation contribution ε₀-ε_∞1, attributed to the lower Debye relaxation process (and assigned to the H-bond breaking process) is proportional to the molarity of methylcellosolve up to the pure liquid. This shows that by substituting the OH group by the methoxy group, the effect attributed to (ε₀-ε_∞1) decreases and disappears when no -OH groups are present as for pure dimethoxyethane. Further, by taking the position that the contribution ε₀-ε_∞1 is to be dealt with as in liquid mixtures, the apparent dipole moment μ∼3 Debyes is calculated by the Böttcher theory. This figure is comparable to the values of μ calculated by the Onsager theory for the alcohols. The above seems to suggest that although the Cole-Davidson distribution function may fit the relaxation profile numerically, it eludes (by its own nature) the molecular description of the dielectric relaxation processes.