A Fundamental Study of the Transport Properties of Aqueous Superacid Solutions

Abstract

An extensive investigation of the transport properties of aqueous acid solutions was undertaken. The acids studied were trifluoromethanesulfonic (CF3SO3H), bis(trifluoromethanesulfonyl)imide [(CF3SO2)2NH], and para-toluenesulfonic (CH3C6H4SO3H), of which the first two are considered superacids. NMR measurements of self-diffusion coefficients (D), spin?lattice relaxation times (T1), and chemical shifts, in addition to ionic conductivity (?), viscosity (?), and density measurements, were performed at 30 �C over the concentration range of 2?112 water to acid molecules. Results showed broad maxima in ? for all three acids in the concentration range of 12?20 water to acid molecules. This coincided with minima in anion Ds and is attributed to a local molecular ordering, reduced solution dielectric permittivity, and increased ionic interactions. The location of the maxima in ? correlates with what is observed for hydrated sulfonated perfluoropolymers such as Nafion, which gives a maximum in ionic transport when the ratio of water to acid molecules is about 15?20. Of the three acids, bis(trifluoromethanesulfonyl)imide was found to be the least dependent on hydration level. The occurrence of the anticorrelation between the ionic conductivity maximum and the anion self-diffusion minimum supports excess proton mobility in this region and may offer additional information on the strength of hydrogen bonding in aqueous media as well as on the role of high acid concentration in the Grotthuss proton transport mechanism.