An electrostatic interpretation of structure-property relationships in ionic liquids

Abstract

Room-temperature ionic liquids represent a novel and intriguing class of materials, but little is understood about the relationship between their ionic structure and their macroscopic physical properties. One of the most uniquitous concepts in molecular liquid theory, that of a molecular dipole moment, is in fact ill-defined for ionic materials and useless in the interpretation of their behavior. We derive an alternative electrostatic multipole description of a molecular ion and show how it can be used to interpret the macroscopic dynamic properties of an ionic liquid. Key to this analysis is the derivation of a quantity we call the "charge arm," which is readily calculable for any ion and permits comparison of ions of both similar and disparate chemical structure. The principles advanced are useful in the interpretation of ionic liquid behavior and in the design of novel ionic liquids with desirable properties.