The hypothesis that aqueous, rather than pure, ionic liquids provide substantially altered physicochemical properties and versatile/increased solute solvation is addressed. Solvatochromic absorbance probes are utilized to gather information on aqueous solutions of a completely water-miscible ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4). A simplistic solvation model suggests possible preferential solvation of a water-soluble betaine dye by bmimBF4. The lowest-energy electronic absorption band of this dye is further used to interpret dipolarity/polarizability and hydrogen-bond donating (HBD) acidity of aqueous bmimBF4. On the basis of the responses of two other probes, N,N-diethyl-4-nitroaniline and 4-nitroaniline, dipolarity/polarizability, HBD acidity, and hydrogen-bond accepting basicity are separately assessed.
Ionic liquids (ILs), despite some of their crucial disadvantages, have been established to be the apt and relevant replacement of the volatile organic compounds (VOCs) in the industrial and academic sectors as solvents. Recent investigations on thriving multifaceted applications of ionic liquids have unleashed that they are really among beneficial “environmentally-benign” solvents as far as their impact on the ecosystem is concerned. This caused them to be an exciting and lucrative domain to explore in a wider fashion and many of the leading research groups are involved in the manifestation of their inherent undisclosed legacy. While exploring the efficacy of ILs, it was found that ILs or IL-based mixed solvent systems were very good alternative of conventional solvents and could act as mobile/stationary phases or additives in gas chromatography (GC), multidimensional gas chromatography (MDGC), inverse gas chromatography (IGC), high performance liquid chromatography (HPLC) and capillary electrophoresis (CE) very efficiently. Their (IL’s) efficacy in improvising and synchronizing the mode and nature of the chromatographic separation is well-established, undoubtedly commendable and unique in many ways. In this review we have put our concentration on the recent, beneficial and some less-explored aspects of the implementation of ILs in the above said chromatographic separation domain.
In this work, an array of molecular-level solvent features—including solute−solvent/solvent−solvent interactions, dipolarity, heterogeneity, dynamics, probe accessibility, and diffusion—were investigated across the entire composition of ambient mixtures containing the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4], and pH 7.0 phosphate buffer, based on results assembled for nine different molecular probes utilized in a range of spectroscopic modes. These studies uncovered interesting and unusual solvatochromic probe behavior within this benchmark mixture. Solvatochromic absorbance probes—a water-soluble betaine dye (betaine dye 33), N,N-diethyl-4-nitroaniline, and 4-nitroaniline—were employed to determine ET (a blend of dipolarity/polarizability and hydrogen bond donor contributions) and the Kamlet−Taft indices π* (dipolarity/polarizability), α (hydrogen bond donor acidity), and β (hydrogen bond acceptor basicity) characterizing the [bmim][BF4] + phosphate buffer system. These parameters each showed a marked deviation from ideality, suggesting selective solvation of the individual probe solutes by [bmim][BF4]. Similar conclusions were derived from the responses of the fluorescent polarity-sensitive probes pyrene and pyrene-1-carboxaldehyde. Importantly, the fluorescent microfluidity probe 1,3-bis(1-pyrenyl)propane senses a microviscosity within the mixture that significantly exceeds expectations derived from simple interpolation of the behavior in the neat solvents. On the basis of results from this probe, a correlation between microviscosity and bulk viscosity was established; pronounced solvent−solvent hydrogen-bonding interactions were implicit in this behavior. The greatest deviation from ideal additive behavior for the probes studied herein was consistently observed to occur in the buffer-rich regime. Nitromethane-based fluorescence quenching of pyrene within the [bmim][BF4] + phosphate buffer system showed unusual compliance with a "sphere-of-action" quenching model, a further manifestation of the microheterogeneity of the system. Fluorescence correlation spectroscopic results for both small (BODIPY FL) and macromolecular (Texas Red-10 kDa dextran conjugate) diffusional probes provide additional evidence in support of microphase segregation inherent to aqueous [bmim][BF4].
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