Cooperativity and heterogeneity of the dynamics in nano-confined liquids

We have measured the time resolved Stokes shift of the probe molecule quinoxaline in glass-forming solvents of different polarities under the condition of geometrical confinement, for which this solvation dynamics technique is particularly suitable. While solvation probes the local dielectric relaxation in polar liquids, it is associated with the local mechanical responses in non-polar systems. The effect of porous glasses on the relaxations of supercooled liquids is strongly dependent on the surface chemistry and can be rationalized on the basis of the cooperativity concept. We also present evidence for the heterogeneous nature of relaxation dynamics in nano-confined liquids. In a further experiment, in which the chromophores are selectively positioned at the liquid/solid interface, we observe that the relaxation in the immediate vicinity of the glass surface can be qualitatively similar to the bulk behaviour.