How many molecules form a liquid?

Abstract

Broad-band dielectric spectroscopy is employed to study the molecular dynamics of (dielectrically active) glass-forming liquids which are confined to (dielectrically inactive) zeolites and nanoporous glasses. For the H-bond-forming liquid ethylene glycol (EG) embedded in zeolites of different sizes and topologies one observes a sharp transition from a single-molecule dynamics (with an Arrhenius-type temperature dependence) to that of a liquid (with a temperature dependence of the mean relaxation rate following a Vogel-Fulcher-Tammann (VFT) law): while EG in silicalite (showing a single-molecule relaxation) has a coordination number of four, EG in zeolite beta or -5 has a coordination number of five and behaves like a bulk liquid. For the H-bonded liquid propylene glycol confined to (uncoated and silanized) nanopores (pore sizes: 2.5 nm, 5.0 nm and 7.5 nm), a molecular dynamics is observed which is comparable to that of the bulk liquid. Due to surface effects in uncoated nanopores, the relaxation time distribution is broadened on the long-term side and the mean relaxation rate is decreased by about half a decade. This effect can be counterbalanced by lubricating the inner surfaces of the pores. That causes the molecular dynamics of the molecules inside the pores to decouple from the solid walls and the resulting relaxation rate becomes slightly faster compared to that for the bulk liquid. For the `quasi'-van der Waals liquid salol confined to silanized nanopores, the molecular dynamics is completely different to that for the H-bonded systems: over a wide temperature range, the dynamics of the confined system is identical to that of the bulk liquid. But with decreasing temperature, a sharp pore-size-dependent transition is found from a VFT-type to an Arrhenius-type temperature dependence. This reflects the inherent length scale of cooperativity of the dynamic glass transition.