Polarity of Some [NR₁R₂R₃R₄]⁺[Tf₂N]⁻ Ionic Liquids in Ethanol: Preferential Solvation versus Solvent–Solvent Interactions

Abstract

The polarity of the ionic liquids [N 4111 ][Tf 2 N], [N 4441 ][Tf 2 N], and [choline][Tf 2 N], as well as their binary mixtures with ethanol has been investigated using solvatochromic dyes and expressed in terms of the Reichardt parameter, E T N , and the Kamlet−Taft parameters (π*, α, and β), at 298.15 K. The synergetic behavior revealed essentially by the ethanol + [N 4441 ][Tf 2 N] system for the acidity of the solvent shows the potential importance of this solvent media for some chemical applications. Even though the preferential solvation model of Bosch and co-workers has been applied with some statistical success to the whole composition range of EtOH + [N 4111 ]-[Tf 2 N] and EtOH + [N 4441 ][Tf 2 N] binary mixtures, the interpretation of the results is somehow difficult due to the competition of molecular interactions in the evaluation of transition energies for the pairs: dye−solvent (dye−cation and/or dye−anion, dye− EtOH, dye−complex entity) and cation−anion, depending on the composition range. Different patterns of solvent polarity behavior are shown along the entire ionic liquid (IL) composition. ■ INTRODUCTIONAlthough molecular solvents have been the usual media for physical and chemical processes in solution, 1 in the past decade, room temperature ionic liquids (RTILs) have shown to be very useful because of their very particular properties, such as nonflammability, thermal stability, very low volatility at ambient conditions, and high solubility in organic and inorganic compounds with a wide range of polarity. These characteristics make ionic liquids (ILs) promising compounds for use in industry and with a recent emphasis in the pharmaceutical industry. 2−8 When compared with molecular solvents, ILs often present higher dipolarity/polarizability characteristics which make them valuable "effective polarity" solvents. 9−11 In this perspective it will be crucial to obtain information regarding the relative polarity of ILs and to study the change in physicochemical properties produced by their addition to molecular solvents. It has been already identified that, when an IL is mixed with an other solvent such as water or an alcohol, the polarity properties of the system can be tunable depending on the combination of the cation and anion and on the careful choice of the cosolvent. 12−17 The solvent polarity is defined as "its overall solvation capability" depending on the action of all possible, specific (hydrogen bonds, electron pair donor/electron pair acceptor), and nonspecific (van-der-Waals and Coulombic interactions) intermolecular solute−solvent forces. 1,18 The inadequate way of characterizing the solvent polarity, merely based on macroscopic physical parameters, gave rise to the appearance of solvatochromic parameters. These parameters are based on molecular probes, the spectroscopic properties of which are strongly solvent-dependent and serve as appropriate model processes for the study of other solvent effects. 19−24 The purpose of this work is to measure the polarity of the ILs, butyl-trim...

Hydrogen-bond acidity of ionic liquids: an extended scale

“…and S3 in the ESI, † respectively) and might also be explained by the factors previously mentioned. It should however be pointed out that the solvatochromic parameters a (1.33) and p* (1.84) for [C 2 C 1 im][NTf 2 ] reported by Fujita and co-workers,29 for [C 8 C 1 pyrr][NTf 2 ] (a = 1.00) reported by Lee and Prausnitz,48 for [C 4 C 1 pip][NTf 2 ] (a = 0.93) reported by Rai and Pandey,24 and for [N 4111 ][NTf 2 ] (a = 1.273) reported by Moita and co-workers51 clearly deviate from the remaining data (cf. Fig.S4to S15 in the ESI †).In general, and from the data provided in Table1, the a values of the studied [NTf 2 ]-based ILs vary significantly from 0.235 (for [P 666,14 ][NTf 2 ]) to 0.820 (for [C 1 C 1 im][NTf 2 ]), while the b and p* values are less divergent.…”

“…13,15,17,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][47][48][49][50][51] It should be highlighted that the hydrogen-bond acceptor strength of an IL is dominated by its anion while the hydrogen-bond donor ability is essentially controlled by the cation and only slightly depends upon the anion of the IL. 18,19 Thus, and since we were focused on developing an extended series for the hydrogen-bond acidity of ILs, ILs comprising a fixed anion ([NTf 2 ] − ) and a wide range of cations (imidazolium-, pyridinium-, piperidinium-, pyrrolidinium-, ammonium-, sulfonium-, and phosphonium-based) were selected.…”

“…Due to some inconsistencies in the literature regarding the KT solvatochromic parameters for ILs, we firstly attempted their experimental determination for a wide range of bis(trifluoromethylsulfonyl)imide-based ILs (henceforth abbreviated as [NTf 2 ]-based ILs), and properly compared our data with those previously reported in the literature. 13,15,17,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][47][48][49][50][51] It should be highlighted that the hydrogen-bond acceptor strength of an IL is dominated by its anion while the hydrogen-bond donor ability is essentially controlled by the cation and only slightly depends upon the anion of the IL. 18,19 Thus, and since we were focused on developing an extended series for the hydrogenbond acidity of ILs, ILs comprising a fixed anion ([NTf 2 ] À ) and a wide range of cations (imidazolium-, pyridinium-, piperidinium-, pyrrolidinium-, ammonium-, sulfonium-, and phosphoniumbased) were selected.…”

Solvatochromic Absorbance and Fluorescence Probe Behavior within Ionic Liquid + γ-Butyrolactone Mixture

“…The preferential solvation model proposed by Buhvestov et al and Skwierczynski and Connors has been applied to delineate the solvation behavior of absorbance and fluorescence probes in various binary solvents system including supercritical fluids and ILs. ,− Here, a simple model has been used to explore the molecular-level understanding of absorbance and fluorescence solvatochromic probes (Betaine dye 33, 4-nitroanisole, 4-nitroaniline, ANS, PRODAN, and pyrene). The correlation parameters that were recovered by this simple model hint both synergistic and nonsynergistic effects and shed light on the solute/solvent and/or solvent–solvent interactions present in these binary systems.…”

“…The solvation sphere near the cybotactic region may be different from the bulk composition, which causes different spectral behaviors of indicators. Therefore, it is an utmost need to calculate the local mole fraction of solvent species around the probe, and it will further give information regarding structural changes occurring at the molecular level in solution. − Figure shows the local mole fraction ( X 1 L , X 2 L , and X 12 L ) of the IL, GBL, and IL–GBL complex, respectively, near the probe molecules such as Betaine dye 33, 4-nitroaniline, 4-nitroanisole, ANS, PRODAN, and Py in the [bmim]­[BF 4 ] + GBL binary mixture. The local mol fractions X 1 L , X 2 L , and X 12 L have been calculated according to the equations described in our earlier reports. , For all probes, a similar type of behavior is observed (Figure ) and indicates the formation of a new solvent–solvent complex species in a binary mixture.…”