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 X<0.3, 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.