Dielectrical Relaxation Measurements in Binary Liquid Mixtures with an Upper Critical Point

The complex (dielectric) permittivity of the systems isobutyric acid/water and butyric acid/water is measured as function of frequency (0.001 GHz ≤ v ≤ 38 GHz), concentration (1 mol · dm−3 ≤ c ≤ 5 mol · dm−3) and temperature (25°C ≤ T ≤ 35°C). Both systems have an upper critical point (isobutyric acid/water: ccrit = 4.36 mol · dm−3, Tc = 26.30°C; butyric acid/water: ccrit = 4.33 mol · dm−3, Tc = ‐ 3.0°C).—The temperature dependence of permittivity of isobutyric acid/water mixtures of critical composition shows no critical contribution near the critical temperature (0.01 K ≤ (T–Tc) ≤ 9 K) within the accuracy of the measurements (±1%).—Different relaxation functions based on different models of the solution are fitted to the measured dielectric spectra. The results of this analysis can be interpreted by assuming that butyric acid and isobutyric acid molecules form associates in solution even well below the critical concentration. On the basis of this assumption it is possible to understand: (a) the concentration dependence of the main relaxation time of water in the Cole‐Cole‐distribution function; (b) the concentration dependence of the number of water molecules per molecule of solute having relaxation times different from that of pure water; (c) the concentration dependence of the relaxation time of hydration water.