LETTER pubs.acs.org/JPCL
Mixing It Up: Measuring Diffusion in Supercooled Liquid Solutions of Methanol and Ethanol at Temperatures near the Glass Transition Jesper Matthiesen, R. Scott Smith,* and Bruce D. Kay* Fundamental & Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352, United States
bS Supporting Information 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. SECTION: Statistical Mechanics, Thermodynamics, Medium Effects
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morphous or glassy materials play an important part in the world around us.1 Optical fibers are made of very pure amorphous silica; in pharmaceuticals, amorphous materials have shown large potential as the carrier for the active molecules, and vitrification is used for the encapsulation of nuclear waste. However, understanding the formation of a glass through the extraordinary viscous slow down which follows when a liquid is supercooled toward its glass transition, Tg, is still a major scientific challenge.2 Thus, increasing our knowledge of the properties of supercooled liquids is important in the effort to understand this phenomenon. There are a number of reports in which the translational diffusion of pure supercooled liquids has been studied, typically using spin-echo NMR3,4 or following the diffusion of probe molecules in the supercooled liquid.5-7 However, similar experimental studies of supercooled liquid mixtures are scarce, and hence, not much is known about the properties of supercooled mixtures and their dependence on composition. One recent study investigated the breakdown of the Stokes-Einstein relation for mixtures of water and glycerol at temperatures down to 1.45 Tg.8 Our group has previously studied supercooled liquid solutions of ethanol and methanol using the desorption kinetics from the composite films.9,10 In that work, initially separated amorphous layers of methanol and ethanol were deposited at 20 K and then heated above Tg, where upon the two layers formed a homogeneous solution. These experiments found that the intermixing of methanol and ethanol acts to inhibit crystallization that would have otherwise rapidly occurred in either of the pure components. Further, the desorption kinetics from the supercooled r 2011 American Chemical Society
liquid mixtures were quantitatively described by a kinetic model for desorption from an ideal solution. However, because the layers intermixed at temperatures (
