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 <=n/GHz <=89 reveals that in the alcohol-rich region of ~0.3<=XA<=1.0, XA is the mole fraction of alcohol, a three-step relaxation model is most appropriate for the description of the spectra whereas at low XAthe intermediate process becomes too small to be resolved. The dominating low-frequency Cole-Cole 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 timescale is largely governed by the number density of H-bond acceptor and donor sites but steric effects also contribute. Two additional Debye terms (j=2 and j=3) with relaxation times of t2 ~10-20ps and t3 ~1-2ps 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 De2 and the simultaneous decrease of the (effective) dipole-dipole correlation factor with decreasing XAin ~0.5<=XA<=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 t2-mode.
The excess activation free energy,DGE , enthalpy, DHE, and entropy, DSE, of the cooperative relaxation time, t1, and their partial molar quantities, DGiE, DHiE, and DSiE, DHiE (i=alcohol, A, or water, W) are discussed. Above the boundary concentration Xb(MW: Xb ~0.30; EW: 0.18; 1PW: 0.14; 2PW: 0.15), DHAE and DSAE 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 DHAE and DSAE in the water-rich region at X1 and X2 (MW: X1~0.045; EW: 0.04; 1PW: 0.03; 2PW: 0.03; MW: X2~0.12; EW: 0.08, 1PW: 0.06; 2PW: 0.07) are connected with the hydrophobic hydration of alcohol monomers (X1) and small multimers (X2) predominating at these concentrations.