Mechanism of the Major Orientation Polarization in Alcohols, and the Effects of Steric Hindrance-, and Dilution-Induced Decrease on H-Bonding

Abstract

To gain insight into the effects of intermolecular H-bond association on the changes in permittivity and relaxation characteristics of supercooled alcohols, two techniques were used here: (i) introducing a C6H5 group in 2-propanol to obtain 1-phenyl-2-propanol, and thus increasing the steric hindrance to H-bonding, and (ii) dissolving the latter in 2-methylpentane and thus decreasing the extent of H-bonding by separating molecules in a nonpolar solvent. Broad-band dielectric spectroscopy studies of supercooled liquid 1-phenyl-2-propanol and its 1:1 (mol:mol) mixture in 2-methylpentane were performed over the 188−238 K range. These show that ∼94% of the total polarization decays according to the Davidson−Cole distribution of relaxation times and that the equilibrium permittivity decreases when phenyl group is substituted in 2-propanol. Analysis in terms of the statistical theories of dielectric behavior shows that the decrease is due to a decrease in the orientation correlation factor, and that this also occurs in the mixture with 2-methylpentane. The induced steric hindrance reduces the extent of intermolecular H-bonding in comparison with that of 2-propanol. The relaxation rate follows the non-Arrhenius temperature dependence. It has been examined qualitatively in terms of the Dyre theory which considers that the apparent Arrhenius energy itself is temperature-dependent, as in the classical interpretations, and quantitatively in terms of the cooperatively rearranging region's size, without implying that there is an underlying thermodynamic transition in its equilibrium liquid. The relaxation rate also fits the power law with the critical exponent of 14.52 for 1-phenyl-2-propanol and 12.9 for the mixture, instead of 2 to 4, usually required by the mode-coupling theory. This indicates the ambiguity of the power-law equations. The excess dielectric loss observed at high frequencies may indicate Nagle's “wing”, or else a merged Johari−Goldstein relaxation.