Molecular dynamics of iso-amyl bromide by dielectric spectroscopy, and theeffects of a nonpolar solvent, 2-methylpentane, on the spectral features

To gain insight into the effects of the weakening of the electrostatic inte ractions on molecular dynamics when polar molecules are dissolved in a nonp olar solvent, the dielectric polarization and relaxation behaviors of iso-a mylbromide and its 50 mol % solution in 2-methylpentane have been studied i n detail over the frequency range, 1 mHz-1 MHz, and a temperature range app roaching their liquid to glass transition. Features of the (i) alpha-relaxa tion spectrum, (ii) the Johari-Goldstein relaxation process in the liquid s tate at low temperatures, with an asymmetric spectral shape, and (iii) the temperature dependence of the relaxation dynamics have been determined and the effects of weakening of the electrostatic interaction on these features examined. The high-frequency wing of the loss spectrum of the alpha-relaxa tion is proportional to omega(-beta). The dynamics of its alpha-relaxation follows the Arrhenius equation initially at high temperatures and thereafte r the Vogel-Fulcher-Tamman equation. Alternative equations for the change i n the relaxation rate have been discussed. A decrease in the dipole-dipole interaction and reduction in the internal field in a solution with a nonpol ar solvent leads to a remarkable change in the shape of the relaxation spec tra at high frequencies such that the dielectric loss for the alpha-relaxat ion becomes proportional to omega(-alpha beta), with alpha, beta < 1. The r elaxation spectra of iso-amyl bromide dissolved in 2-methylpentane follows the H-N function and therefore behaves similar to a polymer, whereas for pu re iso-amyl bromide follows the Davidson-Cole behavior. (C) 1999 American I nstitute of Physics. [S0021-9606(99)50748-4].