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

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 0.1less than or equal tov/GHzless than or equal to89 reveals that in the alcohol-rich region of similar to0.3less than or equal toX(A)less than or equal to1.0, 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 tau(2)similar to10-20 ps and tau(3)similar to1-2 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 Deltaepsilon(2) and the simultaneous decrease of the (effective) dipole-dipole correlation factor with decreasing XA in similar to0.5less than or equal toX(A)less than or equal to1.0 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 tau(2)-mode. The excess activation free energy, DeltaG(E), enthalpy, DeltaH(E), and entropy, DeltaS(E), of the cooperative relaxation time, tau(1), and their partial molar quantities, DeltaG(i)(E), DeltaH(i)(E), amd DeltaS(i)(E) (i=alcohol, A, or water, W) are discussed. Above the boundary concentration X-b (MW: X-b similar to0.30; EW; 0.18; 1PW: 0.14; 2PW: 0.15), DeltaH(A)(E) and DeltaS(A)(E) remain nearly zero, indicating that alcohol molecules in the mixtures already fon-n a zigzag chain ASE structure similar to pure liquids but branched by inserted water molecules. The two pertinent maxima of DeltaH(A)(E) and DeltaS(A)(E) in the water-rich region at X-1 and X-2 (MW: X(1)similar to0.045; EW: 0.04; 1PW: 0.03; 2PW: 0.03; MW: X(2)similar to0.12; 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. (C) 2004 Published by Elsevier B.V.