Dielectric relaxation spectroscopy of 2-propanol–water mixtures

The complex dielectric spectra of 2-propanol–water mixtures were determined at seven molar fractions of 2-propanol, X=0.03, 0.065, 0.14, 0.3, 0.5, 0.7, and 0.9 at 25 °C in the frequency range 0.1⩽ν/GHz⩽89 with the help of time domain reflectometry in 0.1⩽ν/GHz⩽25 and waveguide interferometry in 13⩽ν/GHz⩽89. In the alcohol-rich region of 0.3⩽X⩽1.0, a description of the ε*(ν) spectra requires the superposition of the three relaxation processes. The dominating low-frequency dispersion (j=1) follows a Cole–Cole equation. Additionally, two Debye equations (j=2 and 3) with the relaxation times of τ2∼10–20 ps and τ3∼1–2 ps are required to fit the high-frequency part of the spectrum. The three processes are assigned to the cooperative dynamics of the H-bond system (j=1), a rotation of singly H-bonded alcohol monomers at the ends of chainlike structure (j=2), possibly connected to the formation of bifurcate hydrogen bonds, and a flipping motion of free OH group (j=3). In the region of X0.3, the intermediate alcohol monomer process becomes inseparable. Here, a two process model with a Cole–Cole equation for the main dispersion and a high-frequency Debye process for the fast switching mode gives the best fit. Based on the dielectric relaxation mechanism of the pure constituents proposed in the literatures J. Barthel et al., Chem. Phys. Lett. 165, 369 (1990), and R. Buchner et al., Chem. Phys. Lett. 306, 57 (1999), a composition-dependent relaxation behavior of the mixtures is discussed.