Viscosity-induced errors in the density determination of room temperature ionic liquids using vibrating tube densitometry

The effect of sample viscosity in the determination of the density of room temperature ionic liquids by means of vibrating tube densitometry was analysed in detail. By measuring the densities and viscosities of density standards from low to high viscosity, the viscosity-induced or damping errors were quantified. This allowed us to correctly determine the density of 1-butyl-3-methyl-imidazolium tetrafluoroborate [bmim][BF4], 1-hexyl-3-methyl-imidazolium tetrafluoroborate [hmim][BF4] and 1-octyl-3-methyl-imidazolium tetrafluoroborate [omim][BF4]. Measurements were carried out at atmospheric pressure and within the temperature interval (288.15–323.15) K using an Anton-Paar DSA-48 vibrating tube densimeter and an Anton-Paar AMVn viscometer, respectively. From these data, the effect of viscosity on the isobaric thermal expansivity was evaluated. In the case of the density, it is found that the viscosity correction can be as high as 0.0007 g cm−3 and it decreases with temperature. In the case of the isobaric thermal expansivity, it is found that the viscosity correction passes through a maximum as temperature is increased. It is observed that αp does not increase with T for the studied room temperature ionic liquids (RTILs), thereby clarifying previous contradictory results.