Density−Viscosity Product of Small-Volume Ionic Liquid Samples Using Quartz Crystal Impedance Analysis

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• 作者：Glen McHale ; Chris Hardacre ; Rile Ge ; Nicola Doy ; Ray W. K. Allen ; Jordan M. MacInnes ; Mark R. Bown ; Michael I. Newton
• 刊名：Analytical Chemistry
• 出版年：2008
• 出版时间：August 1, 2008
• 年：2008
• 卷：80
• 期：15
• 页码：5806-5811
• 全文大小：190K
• 年卷期：v.80,no.15(August 1, 2008)
• ISSN：1520-6882

文摘

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, ρη. 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 ∼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 √(ρη) ≈ 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.