Tran Chieu D, Challa Santhosh, Franko Mladen
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881, USA.
Anal Chem. 2005 Nov 15;77(22):7442-7. doi: 10.1021/ac0512913.
The use of ionic liquids (ILs) as a solvent for thermal lens measurements has been investigated. It was found that ILs provide a better medium for thermal lens measurements than water. Specifically, not only the ILs offer at least 20 times higher sensitivity than water but that the enhancement can be appropriately adjusted by changing either the cation or the anion of the ILs. For example, the sensitivity in [BMIm]+[Tf2N]- is approximately 26 times higher than in water. It can be increased up to 31 times by changing the anion to [PF6]- (i.e., [BMIm]+[PF6]-) or to 35 times by changing the cation to [OMIm]+ (i.e., [OMIm]+[Tf2N]-). In fact, the sensitivity of thermal lens measurements in ILs is comparable to those in volatile organic solvents such as benzene, carbon tetrachloride, and hexane. However, the ILs are more desirable as they have virtually no vapor pressure. Furthermore, additional sensitivity enhancement (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 tc and thermal lens strength theta, it is evident that the observed sensitivity enhancement by the ILs is due to their relatively better thermooptical properties. More specifically, the enhancement is due not to the relatively modest lowering of the thermal conductivity but rather to the substantial increase in their dn/dT values. Because of the relationship between dn/dT and drho/dT, it is expected that ILs can serve as an attractive and superior solvent not only for thermal lens measurements but also for other photothermal and photoacoustic techniques as well. Also equally important is the fact that the thermal lens technique in particular and photothermal techniques, in general, 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 lacking but is of extreme importance for implementing ILs as a solvent in various industrial applications.
人们已经研究了将离子液体（ILs）用作热透镜测量的溶剂。研究发现，离子液体为热透镜测量提供了比水更好的介质。具体而言，离子液体不仅比水提供至少20倍的更高灵敏度，而且可以通过改变离子液体的阳离子或阴离子来适当地调节这种增强效果。例如，[BMIm]+[Tf2N]-中的灵敏度比水中的灵敏度高约26倍。通过将阴离子改为[PF6]-（即[BMIm]+[PF6]-），灵敏度可提高到31倍；或者将阳离子改为[OMIm]+（即[OMIm]+[Tf2N]-），灵敏度可提高到35倍。事实上，离子液体中热透镜测量的灵敏度与苯、四氯化碳和己烷等挥发性有机溶剂中的灵敏度相当。然而，离子液体更具优势，因为它们几乎没有蒸气压。此外，只需向离子液体中添加表面活性剂，就可以实现额外的灵敏度增强（比水中高42倍）。根据由测量的热时间常数tc和热透镜强度θ计算出的热导率（k）和dn/dT值，很明显，离子液体观察到的灵敏度增强归因于它们相对较好的热光性质。更具体地说，增强不是由于热导率相对适度的降低，而是由于它们的dn/dT值大幅增加。由于dn/dT与drho/dT之间的关系，预计离子液体不仅可以作为热透镜测量的有吸引力且优越的溶剂，而且还可以用于其他光热和光声技术。同样重要的是，特别是热透镜技术以及一般的光热技术，可以提供一种独特的方法来确定离子液体的热光和热物理性质（例如热导率、热扩散率和相变温度）。目前缺乏这类数据，但对于在各种工业应用中将离子液体用作溶剂来说至关重要
