title: Density-viscosity product of small-volume ionic liquid samples using quartz crystal impedance analysis.
abstract: Quartz crystal impedance analysis has been developed as a technique to assess whether room-temperature ionic liquids are Newtonian fluids and as a small-volume method for determining the values of their viscosity-density product, rho eta. Changes in the impedance spectrum of a 5-MHz fundamental frequency quartz crystal induced by a water-miscible room-temperature ionic liquid, 1-butyl-3-methylimiclazolium trifluoromethylsulfonate ([C4mim][OTf]), were measured. From coupled frequency shift and bandwidth changes as the concentration was varied from 0 to 100% ionic liquid, it was determined that this liquid provided a Newtonian response. A second water-immiscible ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C4mim][NTf2], with concentration varied using methanol, was tested and also found to provide a Newtonian response. In both cases, the values of the square root of the viscosity-density product deduced from the small-volume quartz crystal technique were consistent with those measured using a viscometer and density meter. The third harmonic of the crystal was found to provide the closest agreement between the two measurement methods; the pure ionic liquids had the largest difference of approximately 10%. In addition, 18 pure ionic liquids were tested, and for 11 of these, good-quality frequency shift and bandwidth data were obtained; these 12 all had a Newtonian response. The frequency shift of the third harmonic was found to vary linearly with square root of viscosity-density product of the pure ionic liquids up to a value of square root(rho eta) approximately 18 kg m(-2) s(-1/2), but with a slope 10% smaller than that predicted by the Kanazawa and Gordon equation. It is envisaged that the quartz crystal technique could be used in a high-throughput microfluidic system for characterizing ionic liquids.
authors.affiliation: School of Science & Technology, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK. glen.mchale@ntu.ac.uk
authors.forename: Glen
authors.initials: G
authors.lastname: McHale
authors.forename: Chris
authors.initials: C
authors.lastname: Hardacre
authors.forename: Rile
authors.initials: R
authors.lastname: Ge
authors.forename: Nicola
authors.initials: N
authors.lastname: Doy
authors.forename: Ray W K
authors.initials: RW
authors.lastname: Allen
authors.forename: Jordan M
authors.initials: JM
authors.lastname: MacInnes
authors.forename: Mark R
authors.initials: MR
authors.lastname: Bown
authors.forename: Michael I
authors.initials: MI
authors.lastname: Newton