Cooperative and molecular dynamics of alcohol/water mixtures: the view of dielectric spectroscopy

Abstract

This contribution reviews our recent investigations into the dielectric relaxation behavior of methanol/water (MW), ethanol/water (EW), 1-propanol/water (1PW), and 2-propanol/water (2PW) mixtures. The analysis of the complex permittivity spectra measured in the frequency range 0.1≤ ν/GHz≤89 reveals that in the alcohol-rich region of ∼0.3≤ X A≤1.0, where X A is the mole fraction of alcohol, a three-step relaxation model is most appropriate for the description of the spectra whereas at low X A the intermediate process becomes too small to be resolved. The dominating low-frequency Cole–Cole (CC) dispersion ( j=1) is assigned to the cooperative dynamics of the H-bond system where the motions of alcohol and water molecules cannot be distinguished. Its time scale is largely governed by the number density of H-bond acceptor and donor sites but steric effects also contribute. Two additional Debye (D) terms ( j=2 and j=3) with relaxation times of τ 2∼10–20 ps and τ 3∼1–2 ps are required to reproduce the high-frequency part of the spectrum. These small-amplitude dispersion steps can be assigned to the motion of singly H-bonded alcohol monomers at the ends of the chain structure ( j=2) and to the flipping motion of free OH ( j=3). The increase of the amplitude Δ ɛ 2 and the simultaneous decrease of the (effective) dipole–dipole correlation factor with decreasing X A in ∼0.5≤ X A≤1.0 suggests insertion of water molecules into the zigzag structure of H-bonded alcohol chains inducing a reduction of the average chain length and an increase of the number of end-standing alcohol molecules that can contribute to the τ 2-mode. The excess activation free energy, Δ G E, enthalpy, Δ H E, and entropy, Δ S E, of the cooperative relaxation time, τ 1, and their partial molar quantities, Δ G i E, Δ H i E, and Δ S i E ( i=alcohol, A, or water, W) are discussed. Above the boundary concentration X b (MW: X b∼0.30; EW: 0.18; 1PW: 0.14; 2PW: 0.15), Δ H A E and Δ S A E remain nearly zero, indicating that alcohol molecules in the mixtures already form a zigzag chain structure similar to pure liquids but branched by inserted water molecules. The two pertinent maxima of Δ H A E and Δ S A E in the water-rich region at X 1 and X 2 (MW: X 1∼0.045; EW: 0.04; 1PW: 0.03; 2PW: 0.03; MW: X 2∼0.12; EW: 0.08, 1PW: 0.06; 2PW: 0.07) are connected with the hydrophobic hydration of alcohol monomers ( X 1) and small multimers ( X 2) predominating at these concentrations.