Viscosity Measurements on Ionic Liquids: A Cautionary Tale

Abstract

The vibrating-wire viscometer has proven to be an exceedingly effective means of determining the viscosity of liquids over a wide range of temperature and pressure. The instrument has a long history but a variety of technological and theoretical developments over a number of years have improved its precision and most recently have enabled absolute measurements of high accuracy. However, the nature of the electrical measurements required for the technique has inhibited its widespread use for electrically conducting liquids so that there have been only a limited number of measurements. In the particular context of ionic liquids, which have themselves attracted considerable attention, this is unfortunate because it has meant that one primary measurement technique has seldom been employed for studies of their viscosity. In the last 2 years systematic efforts have been made to explore the applicability of the vibrating-wire technique by examining a number of liquids of increasing electrical conductivity. These extensions have been successful. However, in the process we have had cause to review previous studies of the viscosity and density of the same liquids at moderate temperatures and pressures and significant evidence has been accumulated to cause concern about the application of a range of viscometric techniques to these particular fluids. Because the situation is reminiscent of that encountered for a new set of environmentally friendly refrigerants at the end of the last decade, in this paper the experimental methods employed with these liquids have been reviewed which leads to recommendations for the handling of these materials that may have

Tris(2-ethylhexyl) trimellitate (TOTM) a potential reference fluid for high viscosity. Part I: Viscosity measurements at temperatures from (303 to 373) K and pressures up to 65 MPa, using a novel vibrating-wire instrument

“…A new vibrating wire sensor for measuring viscosity at high pressures and at temperatures up to 373 K was designed, based on the vibrating wire sensor validated by Caetano et al [47], which was used for accurate viscosity measurements up to 500 mPa s, with molecular [8,19,47] and ionic liquids [33,45,46], after calibration with the viscosity primary reference, water at 293.15 K and 0.1 MPa. However, the later sensor was designed for atmospheric pressure measurements.…”

“…[8]. The experimental setup and procedure were previously described by Diogo et al [33]. The temperature of the thermostatic oil was measured, with a 100 V platinum resistance thermometer, calibrated by EIA (Portugal) with an overall estimated uncertainty of AE0.05 K. The capillary was immersed in a silicone oil thermostatic bath (Schott-Geräte CT1445).…”

“…The temperature was stable within AE0.01 K during a series of measurements. Based on previous sensitivity studies [9,33,45], the overall maximum uncertainty of these viscosity measurements was estimated as AE1.5%.…”

“…However, a number of difficulties, mainly related to the experimental handling of the liquids, and the experimental procedures of the measurement techniques, have shown to be very difficult to achieve very accurate and even reproduce viscosity results of ionic liquid samples [33], due to the significant effects on viscosity caused by contaminations, mainly halides or water. It has been shown that contaminations of the order of hundreds or tens of parts per million can cause important effects on viscosity [30,31,33,34].…”

On the viscosity of two 1-butyl-1-methylpyrrolidinium ionic liquids: Effect of the temperature and pressure

“…It is worth pointing out that viscosity values of ILs reported in literature show discrepancies higher than the uncertainty claimed by authors [26]. Although the water content and other impurities present in samples affect significantly viscosity values and other physical properties [27], the experimental techniques are sometimes not adequate to measure the viscosity, and many authors have recommended the use of primary viscosimetric techniques [26,28] to get more reliable data.…”