Dielectric relaxation mechanism and dynamical structures of the alcohol/water mixtures

Abstract

The complex dielectric permittivity of methanol/water (MW), ethanol/water (EW), 1-propanol/water (1PW), and 2-propanol/water (2PW) mixtures was determined using Time Domain Reflctometry (TDR) for the entire concentration range 0≤ X(mmolar fraction of alcohol)≤1.0 in 0.1–25GHz at 20°C–30°C. The dielectric relaxation mechanism of the mixtures is discussed based of the dielectric behaviors of pure water and alcohols reported by Barthel et al.[4,5] and Buchner et al.[6]. In the alcohol-rich region of ∼0.4≤ X≤1.0, the spectra can be well described by the two process model, the superposition of the Cole-Cole ( j=1) and the additional Debye ( j=2) relaxations. We found that the relaxation amplitude for the high frequency process, Δϵ 2, is kept small at the order of 2 even at maximum. The processes are validly attributed to the cooperative dynamics of the H-bond system ( j=1) and a rotation of singly H-bonded alcohol monomers at the ends of chainlike cluster ( j=2). The excess activation free energy, enthalpy, and entropy of the alcohol/water mixtures, ΔG E , ΔH E , and ΔS E , and their partial molar quantities, ΔG i E , ΔH i E , and ΔS i E ( i=alcohol (A) and water (W)) have been calculated. Above the boundary concentration, X b, where X b∼0.30 (MW), 0.18 (EW), 0.14 (1PW), and 0.15 (2PW), ΔH A E and ΔS A E become nearly zero, indicating that a state of alcohol molecules in the mixtures is not very different from that in pure liquid. In the water-rich region, two maxima of ΔH A E and ΔS A E attributed to hydrophobic hydration appear at X 1 and X 2, where X 1∼0.045 (MW), 0.08 (EW), 0.03 (1PW), and 0.03 (2PW), and X 2∼0.12 (MW), 0.08 (EW), 0.06 (1PW), and 0.065 (2PW). The phenomena suggest the formation of two types of saturated hydration structure and their transition.