Effects of induced steric hindrance on the dielectric behavior and H bonding in the supercooled liquid and vitreous alcohol

Abstract

The extent of H bonding in alcohols may be reduced by sterically hindering its OH group. This technique is used here for investigating the reasons for the prominent Debye-type dielectric relaxation observed in monohydroxy alcohols [Kudlik et al., Europhys. Lett. 40, 549 (1997); Hansen et al., J. Chem. Phys. 107, 1086 (1997); Kalinovskaya and Vij, ibid. 112, 3262 (2000)], and broadband dielectric spectroscopy of supercooled liquid and glassy states of 1-phenyl-1-propanol is performed over the 165–238 K range. In its molecule, the steric hindrance from the phenyl group and the existence of optical isomers reduce the extent of intermolecular H bonding. The equilibrium permittivity data show that H-bonded chains do not form in the supercooled liquid, and the total polarization decays by three discrete relaxation processes, of which only the slower two could be resolved. The first is described by the Cole–Davidson-type distribution of relaxation times and a Vogel–Fulcher–Tammann-type temperature dependence of its average rate, which are characteristics of the α-relaxation process as in molecular liquids. The second is described by a Havriliak–Negami-type equation, and an Arrhenius temperature dependence, which are the characteristics of the Johari–Goldstein process of localized molecular motions. The relaxation rate’s non-Arrhenius temperature dependence 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 13.4, instead of 2–4, required by the mode-coupling theory, thereby indicating the ambiguity of the power-law equations.