Quartz crystal impedance analysis has been developed as, 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 viscositydensity 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 ∼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 (Gη) ≈ 18 kg m, 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.Over the past decade, the drive toward cleaner industrial processes has led to the development of ionic liquids as alternative, environmentally friendly, solvents. 16 However, the data on their physical properties as a function of chemical composition are limited, and extending the range of known data is difficult due to the expense and difficulty of producing large volumes of pure liquids for characterization.Acoustic wave microsensors, such as the quartz crystal microbalance (QCM), are widely used for studying the properties of small-volume samples of liquids, the attachment of mass from the liquid phase and in situ determination of the properties of surface coatings, such as electrodeposited polymers, during the deposition process. [17][18][19] A QCM operates by creating a highfrequency, typically 5 MHz, shear mode oscillation of the surface. When operated in a liquid environment, this surface oscillation entrains liquid and creates an oscillation, which for a Newtonian liquid decays within a penetration depth of the interface δ ) (η/ *1/2 where F and η are the density and viscosity of the liquid and f s is the resonant frequency. 20 In impedance analysis, both the resonant frequency and bandwidth, B, of the crystal are measured and are functions of the liquid properties. Bandwidth is a measure of the loss of energy and of the damping of the shear mode oscillation of the liquid close to the solid-liquid interface, and so some authors prefer to define a dissipation D ) B/f s (also equal to Q -1 ). When the liquid is Newtonian, a frequency decrease, ∆f, and a bandwidth increase, ∆B, occur in proportion to the square root of the viscosity-density product,where the specific acoustic impedance of quartz is, f o is the fundamental frequency and f s ) nf o is the overtone frequency at which the response is measured. 21,22 Thus, by verifying that changes in resonant frequency and bandwidth are correlated, such that ∆f ) -∆B/2,...
