Geometrical confinement and cooperativity in supercooled liquids studied by solvation dynamics.

Abstract

The molecular dynamics of supercooled liquids confined to the mesopores of sol-gel glasses are interpreted in terms of the spatial competition between the average length scale of cooperativity assumed to increase with time and the geometrical confinement on scales between 2.5 and 7.5 nm. Opposed to relating the fast and slow relaxation components to spatially distinct regimes, the picture of a mesoscopically uniform but cooperative relaxation as stimulated by the theoretical work of J\"ackle is proposed. It will be demonstrated that rationalizing the data along these lines leads to a consistency with the theory while relaxing some of the conceptual problems encountered in previous interpretations. The data supports the idea that the relaxation in restricting geometries proceeds like in the bulk liquid until the length scale \ensuremath{\xi}(t)\ensuremath{\propto}log(t) of cooperativity reaches the pore size leading to nonergodic behavior within the experimental time scale. For a characteristic cooperativity length ${\ensuremath{\xi}}_{\mathit{c}}$ the data analysis yields ${\ensuremath{\xi}}_{\mathit{c}}$(T)\ensuremath{\propto}T and ${\ensuremath{\xi}}_{\mathit{c}}$(${\mathit{T}}_{\mathit{g}}$)\ensuremath{\approxeq}3 nm. The solvation dynamics results under study are compared to analogous dielectric relaxation data. \textcopyright{} 1996 The American Physical Society.