RESUMO
We present a consistent picture of the respective role of density (rho) and temperature (T) in the viscous slowing down of glassforming liquids and polymers. Specifically, based in part upon a new analysis of simulation and experimental data on liquid ortho-terphenyl, we conclude that a zeroth-order description of the approach to the glass transition (in the range of experimentally accessible pressures) should be formulated in terms of a temperature-driven super-Arrhenius activated behavior rather than a density-driven congestion or jamming phenomenon. The density plays a role at a quantitative level, but its effect on the viscosity and the alpha-relaxation time can be simply described via a single parameter, an effective interaction energy that is characteristic of the high-T liquid regime; as a result, rho does not affect the "fragility" of the glassforming system.
RESUMO
The translational diffusion coefficient and the partition coefficient of a spin-labeled solute, di-t-butyl nitroxide, in an aqueous suspension of dipalmitoyl lecithin vesicles have been studied by electron spin resonance spectroscopy. When the lecithin is cooled through its phase transition temperature near 41 degrees C, some solute is "frozen out" of the bilayer, and the standard partial molar enthalpy and entropy of partition go more positive by a factor of 8 and 6, respectively. However, the apparent diffusion constant in the lecithin phase is only slightly smaller than that in water, both above and below the transition temperature. The fraction of bilayer volume within which solute is distributed may increase with temperature, contributing to the positive enthalpy of partition. Comparison of time constants suggests that there is a permeability barrier to this solute in the periphery of the bilayer.