BROAD-BAND DIELECTRIC-SPECTROSCOPY ON CARBOXYLIC-ACID WATER MIXTURES - DEPENDENCE UPON COMPOSITION

The complex (dielectric) permittivity has been measured as a function of frequency v between 3 MHz and 70 Ghz and of mole fraction x between 0 and 1 for aqueous solutions of formic, acetic, propionic, and butyric acid at 25-degrees-C. At high water content (x < 0.5) the dielectric spectra reflect two relaxation terms. One term is characterized by a discrete relaxation time with values between 4 and 17 ps (Debye-type relation). The other term exhibits an underlying unsymmetrical relaxation time distribution (Davidson-Cole relaxation) with a remarkably broad width and with values of the principal relaxation time between 20 and 312 ps. The spectra are discussed in the framework of a model in which the mixtures are assumed to consist of two microphases, one with high and the other one with low water content. At high solute concentration (x > 0.5) the measured permittivity spectra can be well represented by one Davidson-Cole relaxation spectral function. The parameters found for this function fit to those of the microphase with low water content assumed to exist at x < 0.5.