Mixing It Up: Measuring Diffusion in Supercooled Liquid Solutions of Methanol and Ethanol at Temperatures near the Glass Transition

Abstract

How do liquid mixtures cooled to temperatures below their freezing point behave? We address this question using nanoscale films of methanol and ethanol supercooled liquid solutions of varying composition (0−100% methanol) at temperatures near their glass transition, Tg. The permeation of Kr through these films is used to determine the diffusivities of the supercooled liquid mixtures. We find that the temperature-dependent diffusivities of the mixtures are well-fit by a Vogel−Fulcher−Tamman equation, indicating that the mixtures exhibit fragile behavior at temperatures just above their Tg. Further, for a given temperature, the composition-dependent diffusivity is well-fit by a Vignes-type equation, that is, the diffusivity of any mixture can be predicted using an exponential weighting of the pure methanol and ethanol diffusivities. These results show that deeply supercooled liquid mixtures can be used to provide valuable insight into the properties of normal liquid mixtures.