Scaling the transport properties of molecular and ionic liquids

Abstract

There are many forms of scaling for the transport properties of liquids and liquid solutions. This review concentrates on the application of models based on (a) hard-sphere molecular dynamics results for dense molecular fluids, (b) Stokes-Einstein-Sutherland (SES) scaling of experimental transport properties with the viscosity for both molecular and ionic liquids, and (c) thermodynamic or density scaling, again for both types. An alternative, but similar, approach to SES scaling for ionic liquids employs so-called “distinct diffusion coefficients” incorporating both conductivity and self-diffusion terms in Walden-type plots. In ionic liquid families, related compounds are found to scale on to common curves in some cases. Recently thermodynamic scaling, using (TVγ) as the scaling function has been introduced. For molecular liquids, γ can be related to the exponent of the repulsive part of the potential, but its meaning is less clear for ionic liquids that consist of large charged ions and have both significant van der Waals and Coulombic interactions. The further development of scaling offers promise for the future in relating the behaviour of ionic liquids to those of other liquid classes.