Dielectric and Mechanical Relaxation of Glass-Forming Liquids in Nanopores

We have measured the time resolved phosphorescence of different probe molecules in glassforming solvents under the condition of geometrical confinement in porous glasses. This solvation dynamics technique probes the local dielectric relaxation in the case of a dipolar chromophore in polar liquids. In the absence of dipolar interactions, the observed Stokes shifts reflect the local density or mechanical responses. Therefore, both orientational and translational modes of molecular motions can be measured for liquids imbibed in porous silica glasses. The effect of confinement on the relaxations of supercooled liquids is strongly dependent on the surface chemistry and can be rationalized on the basis of the cooperativity concept. As in the bulk case, we find that the relaxations in nano-confined liquids display heterogeneous dynamics. The density relaxation turns out to be more sensitive to the thermal history relative to the orientational features of molecular motion. By selectively positioning the chromophores at the liquid/solid interface, we observe also that the structural relaxation of the liquid in the immediate vicinity of the glass surface is slowed down but not entirely blocked.