Absorption of Water by Room-Temperature Ionic Liquids: Effect of Anions on Concentration and State of Water

Abstract

Near-infrared (NIR) spectrometry was successfully used for the noninvasive 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]⁺[BF₄]⁻, [BuMIm]⁺[bis((trifluoromethyl)sulfonyl)amide, or Tf₂N]⁻ and [BuMIm]⁺[PF₆]⁻, 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, [BF₄]⁻ provides the strongest interactions and [PF₆]⁻ the weakest. In 24 hours, [BuMIm]⁺[BF₄]⁻ can absorb up to 0.320 M of water, whereas [BuMIm]⁺[PF₆]⁻ can only absorb 8.3 × 10⁻² 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 × 10⁻³ M.