RELAXATION PHENOMENA IN MIXED ISOMERIC ALCOHOLS BY MANDELSTAM-BRILLOUIN SCATTERING

Mandelstam-Brillouin scattering data in mixed isomeric alcohols n-pentanol (nPe-OH) and 2-methyl-2-butanol (2Me-2BuOH) are presented. The hypersonic velocity and normalized absorption are measured as a function of the scattering angle, in the temperature range from - 15-degrees-C to + 45-degrees-C, and as a function of n-PeOH molar fraction going from the pure n-PeOH to the pure 2Me-2BuOH. The experimental results confirm the existence of a shear relaxation phenomenon in the GHz region, that has been previously detected in pure liquids. The temperature dependence of the relaxation time tau-s and of the shear modulus G-infinity evaluated within viscoelastic liquid models, support the existence of open heterostructures in the 2-methyl-2-butanol rich phase. A detailed analysis is also carried out for the measured shear viscosity eta-s, whose temperature dependence becomes more and more non-Arrhenian by adding normal pentanol (n-PeOH) to its stereoisomer (2Me-2BuOH). A comparison between the shear flow mean activation enthalpy and the high frequency shear relaxation enthalpy allows us to demonstrate the more `fragile' behaviour of 2Me-2BuOH and of its more concentrated mixtures with respect to that of n-PeOH, confirming that the observed non-ideal behaviour in the mixing process can be associated with the existence of H-bond imposed structural aggregates that are in dynamic equilibrium.