Dielectric relaxation and thermodynamic studies of methanol–ethylene glycol and ethanol–ethylene glycol binary mixtures using time domain reflectometry

Dielectric relaxation behavior and thermodynamic properties of methanol–ethylene glycol (MeOH–EG) and ethanol–ethylene glycol (EtOH–EG) binary mixtures were investigated at 25, 20, 15 and 10 °C over the frequency range 10 MHz–30 GHz using the time domain reflectometry technique. The complex permittivity spectra were analyzed using the Cole–Davidson model to evaluate dielectric parameters such as static dielectric permittivity (ε0), high-frequency dielectric constant (ε∞), relaxation time (τ), excess dielectric permittivity (ε0E), excess inverse relaxation time (1/τ)E, effective Kirkwood correlation factor (geff), and theoretical Bruggeman factor (fB). For both binary systems, ε′ decreased systematically with increasing frequency, while ε″ exhibited broad relaxation peaks, confirming non-Debye type relaxation behavior. The static dielectric permittivity increased with increasing EG concentration and decreasing temperature, whereas relaxation time exhibited systematic composition and temperature-dependent variation. The excess dielectric parameters showed both positive and negative deviations, indicating non-ideal dielectric behavior arising from intermolecular interactions within the mixtures. The geff values remained greater than unity throughout the investigated composition range, indicating correlated dipolar orientation and cooperative interaction effects. The Arrhenius plots of log (Tτ) versus 1000/T exhibited approximately linear behavior for all compositions, confirming thermally activated relaxation dynamics. The thermodynamic parameters, namely enthalpy change (ΔH) and entropy change (ΔS), exhibited positive values over most of the composition range, indicating endothermic and non-ideal mixing behavior. Comparative analysis revealed comparatively larger composition-dependent variations in dielectric and thermodynamic parameters for the EtOH–EG system than for the MeOH–EG system. The present study provides useful insight into dielectric relaxation dynamics, dipolar correlation behavior, and intermolecular interaction effects in hydrogen-bonded alcohol–EG binary mixtures.