Nonexponential dielectric relaxation dynamics in supercooled liquid and glassy states of isoamyl bromide and 2-methylpentane mixtures

The dielectric spectra of mixtures of the polar solute isoamyl bromide in 2 -methylpentane have been investigated in the frequency range 1 mHz to 1 MHz and in the temperature range approaching the glass transition temperature. The results obtained from the spectra are compared with those obtained rec ently [J. Chem. Phys. 111, 10979 (1999)] on pure isoamyl bromide. It is fou nd that on increasing dilution with the nonpolar solvent, the width of the curves of the dielectric spectra increase significantly, and this is reflec ted in the increase in the nonexponential nature of the relaxation dynamics . This is found to be a consequence of the decrease in the cooperativity of the relaxation dynamics and or an increase in the heterogeneity of the sol ution. The data are found to fit the Havriliak-Negami equation extremely we ll. The data at low and high frequencies also fits the "universal law," sin ce the latter is a low and high frequencies limiting case of the Havriliak- Negami equation. The scaling parameters of this law are calculated for the 25 mol % solution of isoamyl bromide in 2-methylpentane, and these are show n to experimentally relate to the H-N parameters. The stretched exponential parameter, gamma, is estimated as a function of the temperature and is sho wn to follow the equation gamma approximate to a(T-T-0). Vogel-Fulcher-Tamm ann equation fits the data of the relaxation peak frequency as a function o f the inverse of absolute temperature for the various mixtures quite well, this being possibly a consequence of the temperature dependence of the stre tched exponential parameter. The predictions from the mode coupling theory and those by Bertrand and Souletie are verified with the exception that the exponent is found to be much greater than predicted by these theories. The relative predominance of the Johari-Goldstein process in isoamyl bromide i ncreases initially with dilution with 2-methylpentane and then disappears a s the number density of the independent relaxors increases with further dil ution. (C) 2001 American Institute of Physics.