152 Volume 57, Number 2, 2003 APPLIED SPECTROSCOPY 0003-7028 / 03 / 5702-0152$2.00 / 0 q 2003 Society for Applied Spectroscopy Absorption of Water by Room-Temperature Ionic Liquids: Effect of Anions on Concentration and State of Water CHIEU D. TRAN,* SILVIA H. D E PAOLI LACERDA, and DANIEL OLIVEIRA Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin 53201 Near-infrared (NIR) spectrometry was successfully used for the non- invasive and in situ determination of concentrations and structure of water absorbed by room-temperature ionic liquids (RTILs). It was found that RTILs based on 1-butyl-3-methylimidazolium, namely, [BuMIm] 1 [BF 4 ] 2 , [BuMIm] 1 [bis((triuoromethyl)sulfonyl)amide, or Tf 2 N] 2 and [BuMIm] 1 [PF 6 ] 2 , are hydroscopic and can quickly absorb water when they are exposed to air. Absorbed water interacts with the anions of the RTILs, and these interactions lead to changes in the structure of water. Among the RTILs studied, [BF 4 ] 2 provides the strongest interactions and [PF 6 ] 2 the weakest. In 24 hours, [Bu- MIm] 1 [BF 4 ] 2 can absorb up to 0.320 M of water, whereas [Bu- MIm] 1 [PF 6 ] 2 can only absorb 8.3 3 10 22 M of water. It seems that higher amounts of water can be absorbed when the anion of the RTIL can strongly interact and hence stabilize absorbed water molecules by forming hydrogen bonds with them or inducing hydrogen bonds among water molecules. More importantly, the NIR technique can be sensitively used for the noninvasive, in situ determination of absorbed water in RTILs, without any pretreatment, and at limits of detection as low as 3.20 3 10 23 M. Index Headings: Room-temperature ionic liquids; Green chemistry; Near-infrared; Acousto-optic tunable lter. INTRODUCTION Room-temperature ionic liquids (RTILs) are a group of organic salts that are liquid at room temperature. 1,2 They have unique chemical and physical properties in- cluding air and moisture stablity, high solubility power, and virtually no vapor pressure. Because of these prop- erties, they can serve as a ‘‘green’’ recyclable alternative to the volatile organic compounds that are traditionally used as industrial solvents. 1–4 In fact, RTILs have been successfully used as solvents in many applications in- cluding organic and inorganic syntheses, 5–7 chemical sep- arations, 8–16 and electrochemistry. 17,18 Room-temperature ionic liquids are known to be hy- droscopic and can absorb signicant amounts of water from the atmosphere. 19–22 Properties of RTILs, including their solubility, polarity, viscosity, and conductivity, are not only changed by but also are dependent on the amount of absorbed water. 1,19–22 As a consequence, ab- sorbed water may alter rates of chemical reactions and efciencies of various processes in RTILs. It is, therefore, of particular importance that a method capable of deter- mining concentrations and molecular states of absorbed water be developed. Such considerations have prompted a variety of studies including those based on the use of FT-IR to determine molecular states of water in RTILs and on uorescence techniques to determine the effect of water on the polarity of RTILs. 19–22 Unfortunately, these studies are not suited for the direct determination of ab- Received 17 July 2002; accepted 2 October 2002. * Author to whom correspondence should be sent. sorbed water. This may be due to the lack of a suitable technique that has noninvasive and in situ capabilities without any pretreatment of samples. Near-infrared (NIR) spectrometry can offer a solution to this problem. Near-infrared spectrometry has been used extensively in recent years for chemical analysis and characteriza- tion 23 because it has many advantages, including wide applicability, noninvasiveness, nondestructiveness, and real-time and on-line capabilities. The NIR region covers the overtone and combination transitions of the C–H, O– H, and N–H groups, and since all organic and most in- organic compounds possess at least one or more of these groups, the technique can, in principle, be used for anal- ysis of all organic and most inorganic compounds. Ad- ditionally, because it has real-time and on-line capabili- ties that satisfy one of twelve principles of Green Chem- istry, 24 the NIR technique is suited as a Green Method. Unfortunately, in spite of its potentials, to date, the NIR technique has not been used for the determination of wa- ter absorbed by RTILs. The limitation is probably due to the fact that both water and RTILs absorb NIR light, and hence, there is extensive overlap among their spectra. High-quality NIR spectra are, therefore, required for sub- sequent data treatment including background subtraction, deconvolution, and analysis by multivariate methods. A high-performance NIR spectrometer with high sensitivity, high light throughput, high stability, and no drift is need- ed to measure such spectra. Such a spectrometer was suc- cessfully constructed in our laboratory using the acousto- optic tunable lter. 25–27 An acousto-optic tunable lter (AOTF) is an all-solid- state, electronic dispersive device that is based on the diffraction of light by acoustic waves in an anisotropic crystal. 28–30 The wavelength of the diffracted light is de- pendent on the frequency and the power of the applied acoustic wave. The scanning speed of the AOTF is de- ned by the speed of the acoustic wave in the crystal, which is on the order of microseconds. As a consequence, compared to conventional gratings, the AOTFs have such advantages as being all solid state (contains no moving parts), having rapid scanning ability (ms), wide spectral tuning range, and high throughput, and giving high res- olution (,1 nm). 28–30 The lters can also provide a unique means to maintain the intensity of the light source (by controlling either the frequency or the power of the ap- plied radio frequency (RF) signal through a feed-back loop). 28–30 As a consequence of this development, an NIR spectrophotometer based on the AOTF is very sensitive, has high light throughput, high stability, and no drift. In fact, we recently developed an AOTF-based NIR spec- trometer and successfully used it for a variety of spectro- scopic and analytical measurements, including the deter-