Molecular dynamics and glass-transition of ethylene glycol adsorbed in zeolites:: Influence of surface-molecule interactions, topology, and loading degree

By means of broadband dielectric spectroscopy the molecular dynamics of glass-forming ethylene glycol (EG) adsorbed in zeolites is investigated by varying the loading degree, the type of zeolites, and the Si/Al ratio. We only concentrate to the frequency and temperature range where relaxation processes may be observed which allow conclusions about the glass-transition. Although EG molecules in the bulk liquid show a Vogel-Fulcher-Tammann (VFT) type of activation at lower temperatures, measurements for EG molecules adsorbed in NaX zeolites always follow Arrhenius-type of behaviour independent of the loading degree. This effect is explored in more detail by measuring the dielectric relaxation rate for EG adsorbed in zeolite beta, EG/beta, with different Si/Al ratios. For the case of zeolite beta with a very large Si/Al ratio, a glass-transition is observed, i.e., a VFT-type of activation is detectable. For the system EG/beta with Si/Al ratio of 56, however, a clear Arrhenius-type of activation is observed. Obviously, due to a higher number of adsorption sites, surface-molecule interactions are of greater influence and suppress the glass-transition. For EG/beta with a Si/Al ratio between 56 and infinity, there is subtle interplay between "cooperative" and "local" motions, i.e., between VFT- and Arrhenius-type of activation behaviour. (c) 2006 Elsevier Inc. All rights reserved.