Structural dynamics and thermodynamic behavior of alcohol-polypropylene glycol binary systems: A dielectric spectroscopy study

The present work reports the use of microwave dielectric spectroscopy to investigate the molecular dynamics of binary mixtures of four primary alcohols (methanol, ethanol, propanol, and butanol) with polypropylene glycols (PPG-1000 and PPG-2000) of varying chain length. Molecular interactions in terms of chain coiling and elongation in solution have been studied under various conditions, such as varying numbers of PPG monomer units and solvent environments. The Cole-Cole model is used to describe the complex permittivity spectra. The structural dynamics for the system under study have been carried out through dielectric parameters such as the static dielectric constant (ε₀), average relaxation time (τ₀), effective dipole moment (μ_eff), and Kirkwood correlation factor (g) over the wide frequency range of 0.01 GHz to 30 GHz and at temperatures of 278.15 K, 283.15 K, 288.15 K, 293.15 K, and 298.15 K. A comparison of the results of these binary mixtures implies that the dielectric properties of these solutions were affected by the number of monomer units of PPG molecules and also the solvent environment. The values of the effective dipole moment (μ_eff) decrease towards higher concentration of solute molecules due to an increase in effective dipole-dipole antiparallel alignment in all the studied cases. It was further confirmed by the values of the Kirkwood correlation factor (g). In addition, the thermodynamic properties of the systems were also calculated. The free energy of activation (ΔG) was found to be increasing with an increase in the chain length of PPG.