On the importance of the metastable liquid state and glass transition phenomenon to transport and structure studies in ionic liquids. I. Transport properties

An attempt is made to show that progress in the understanding of ionic liquid transport properties and structure has been impeded by a lack of information on behavior in the "low-temperature" region of the liquid state, which largely involves the metastable super-cooled liquid state. Insight into the nature of this lower region provided by molecular dynamics machine calculations is discussed in relation to the relevance of structural distinctions between crystallizing and noncrystallizing ionic liquids. Associated with the low-temperature region are systematic departures from Arrhenius behavior in the temperature dependence of transport which are not explained by conventional transition state theory. Two theories which lead to the observed form of the temperature dependence are reviewed. The recent Adam-Gibbs theory seems the more promising approach and leads to the recognition of a corresponding temperature scale based on isoentropic states (states of equal configurational entropy) which can be used to define the low-temperature region and which may have interesting applications in correlating liquid structural properties. Comparison of this theory with the previous transition state theory approach suggests that the notion that volume and potential energy ("jumping") terms in the temperature dependence can be separated by constant volume measurements may be in error. A tentative explanation of existing constant volume data is offered.