| Abstract
| - The use of ionic liquids (ILs) as a solvent for thermal lensmeasurements has been investigated. It was found thatILs provide a better medium for thermal lens measurements than water. Specifically, not only the ILs offer atleast 20 times higher sensitivity than water but that theenhancement can be appropriately adjusted by changingeither the cation or the anion of the ILs. For example, thesensitivity in [BMIm]+[Tf2N]- is ∼26 times higher thanin water. It can be increased up to 31 times by changingthe anion to [PF6]- (i.e., [BMIm]+[PF6]-) or to 35 timesby changing the cation to [OMIm]+ (i.e., [OMIm]+[Tf2N]-).In fact, the sensitivity of thermal lens measurements inILs is comparable to those in volatile organic solventssuch as benzene, carbon tetrachloride, and hexane.However, the ILs are more desirable as they have virtuallyno vapor pressure. Furthermore, additional sensitivityenhancement (up to 42 times higher than that in water)can be achieved by simply adding surfactants into the ILs.Based on the thermal conductivity (k) and dn/dT values,calculated from the measured thermal time constant tcand thermal lens strength θ, it is evident that the observedsensitivity enhancement by the ILs is due to their relativelybetter thermooptical properties. More specifically, theenhancement is due not to the relatively modest loweringof the thermal conductivity but rather to the substantialincrease in their dn/dT values. Because of the relationship between dn/dT and dρ/dT, it is expected that ILscan serve as an attractive and superior solvent not onlyfor thermal lens measurements but also for other photothermal and photoacoustic techniques as well. Alsoequally important is the fact that the thermal lens technique in particular and photothermal techniques, ingeneral, can offer a unique means to determine themooptical and thermal physical properties of the ILs (e.g.,thermal conductivity, thermal diffusivity, and phase transition temperatures). This type of data is currently lackingbut is of extreme importance for implementing ILs as asolvent in various industrial applications.
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