Trigonal tricationic ionic liquids: a generation of gas chromatographic stationary phases

High-Resolution Performance of Polycaprolactone Functionalized with Guanidinium Ionic Liquid for Gas Chromatography

This work firstly reported a new polycaprolactone based material functionalized with guanidinium ionic liquid (PCL-GIL) as the stationary phase with high resolution performance for capillary gas chromatography (GC). It is composed of polycaprolactone (PCL) and guanidinium ionic liquid (GIL) with amphiphilic conformation. The PCL-GIL capillary column coated by static method exhibited high column efficiency of 3942 plates/m and moderate polarity. As a result, the PCL-GIL column exhibited high-resolution capability. For a mixture of 27 analytes with a wide ranging polarity and outperformed the PCL-2OH and HP-35 columns, showing its advantageous separation capability for analytes of diverse types. Moreover, the PCL-GIL column showed high resolving capability for various positional isomers and cis-/trans-isomers, including alkylbenzenes, chlorobenzenes, naphthalenes, bromonitrobenzenes, chloronitrobenzenes, benzaldehydes, phenols, alcohols, respectively. In a word, PCL derivatized by GIL units as a new type of stationary phase has a promising future in GC separations.

Regulation of hydrogen bond acidity and its effect on separation performances

Ionic liquid bonded polysiloxanes (PILs) are a class of polysiloxanes whose side chains contain ionic liquid (IL) moieties. They not only inherit the character of "dual nature" from ILs but also inherit the excellent film-forming ability and thermal stability from polysiloxanes. In this paper, the solvation parameter model is introduced to investigate the interaction characteristics of PILs. The experimental results show that the b values of PILs occur in a wider range than those previously reported for the stationary phases. The hydrogen bond acidity can be effectively adjusted by varying the ionic liquid content or substituents. Hindering the formation of the hydrogen-bonded networks and increasing the exposed hydrogens may be intrinsic to the strong hydrogen bond acidity of PILs. Subsequently, the separation performances of these PIL stationary phases were demonstrated by separating various mixed samples of aromatic isomers, dichloroanilines, substituted alkanes, alcohols, esters, etc. The results show that the PILs with strong hydrogen bond acidity have excellent selectivity performances for aromatic position isomers, alcohols, and substituted alkanes. This study is significant for understanding the hydrogen bond acidity and broadening the range of hydrogen bond acidity of ionic liquid stationary phases.

Facile Synthesis of Tetra-Branched Tetraimidazolium and Tetrapyrrolidinium Ionic Liquids

A facile synthetic route for tetra-branched tetraimidazolium and tetrapyrrolidinium ionic liquids was developed. In contrast to the previous synthetic scheme, the new synthetic route requires only three reaction steps instead of seven. The total yield of tetracation was also improved from 17-21 to 39-41%. Using the new synthetic scheme, four kinds of tetracations were synthesized from the combination of two cationic units (imidazolium and pyrrolidinium) and two counteranions [bis(fluorosulfonyl)imide (FSI) and bis(trifluoromethanesulfonyl)imide (TFSI)]. Basic physical properties including glass transition temperature, thermal decomposition temperature, density, viscosity, and ionic conductivity were determined. The counterion exchange from TFSI to FSI resulted in lower glass transition temperature and higher ionic conductivity. Tetrapyrrolidinium exhibited higher viscosity and lower ionic conductivity than tetraimidazolium. The counterion exchange from TFSI to FSI resulted in lower viscosity in the case of tetraimidazolium, while the opposite result was obtained in the case of tetrapyrrolidinium. Tetracations composed of ethyl imidazolium units, diethylene glycol spacers, and FSI counterions exhibited the highest ionic conductivity of 3.5 × 10^-4 S cm^-1 at 25 °C under anhydrous conditions. This is the best ionic conductivity in the tetracations ever reported.

Efficiency of capillary GC columns based on phosphonium ionic liquids

Gas chromatographic columns based on ionic liquids (ILs) are very promising since the selectivity of these columns can be tuned by both the cation and the anion chemical nature. In this paper, efficiencies of capillary columns based on four phosphonium ionic liquids were studied. The performance of seven columns containing the cation trihexyl(tetradecyl)phosphonium and the anions bromide, chloride, and bis(trifluoromethylsulfonyl)imide was evaluated by measuring the solute band broadening as a function of gas velocities at three temperatures. Hence, classical height equivalent to a theoretical plate (H) against gas velocity (u) plots corresponding to those columns were generated and the data were fitted to the Golay-Guiochon equation with the aim of seeking the optimum conditions to be operated each of them. Band broadening at practical gas velocities is mainly due to poor mass transfer properties of solutes in the (viscous) liquid phases, which limits the achieved efficiencies. These H/u plots proved to be necessary to characterize the column quality at a given temperature, to interpret the band broadening phenomena and thus, to establish the lower temperature limits and the expected plate counts at that temperature.

Gas-phase electronic properties of tri-cationic imidazolium-based ionic liquids in comparison with mono- and di-cationic ionic liquids

The optimized geometries, electronic structures, and gas-phase properties of widely applicable non-linear trigeminal tri-cationic ILs (TT-X₃) were investigated using density functional theory (DFT) calculations and compared with mono- (M-X) and di-cationic (D-X₂) ionic liquids. The studied ILs are based on the imidazolium cation containing halide (X^‾) anions, where X^‾ = Cl^‾, Br^‾ and I^‾. Inter-molecular hydrogen bonds were studied by atoms in molecules (AIM) and natural bond orbital (NBO) analyses. Accordingly the most significant cation-anion charge transfer is related to C₁-H₁ … X (X = Cl, Br, I) interaction which strongly occurs in TT-X₃ ILs and especially in TT-Cl₃. Among ILs under investigation, TT-Cl₃ has the strongest cation-anion interaction. Also M - I IL has the largest and D-Cl₂ has the smallest electrical dipole moment.

Room-temperature-ionic-liquid coated graphitized carbons for selective preconcentration of polar vapors

Most adsorbent materials used for preconcentrating and thermally desorbing volatile and semi-volatile organic compounds (S/VOCs) in portable or "micro" gas chromatographic (GC/µGC) instruments preferentially capture non-polar or moderately polar compounds relative to more polar compounds. Here, we explore the use of a known trigonal-tripyramidal room-temperature ionic liquid (RTIL) as a surface modifier for the graphitized carbons, Carbopack B (C-B) and Carbopack X (C-X), with the goal of enhancing their capacity and selectivity for polar S/VOCs. Breakthrough tests were performed by challenging tubes packed with ∼2.5 mg of C-B or RTIL-coated C-B (RTIL/C-B) with 13 individual S/VOCs, including several organophosphorus compounds and reference alkyl and aromatic hydrocarbons of comparable vapor pressures, at concentrations ranging from 14 to 130 mg/m³. The 10% breakthrough volume, V_b10, was used as the measure of capacity. For the RTIL/C-B, the V_b10 values of the five organophosphorus vapors tested were consistently ∼2.5 times larger than those for the untreated C-B, and V_b10 values of the four non-polar reference vapors were 11-26 times smaller for the RTIL/C-B than for the untreated C-B. For compounds of similar vapor pressure the capacity ratios for polar vs. non-polar compounds with the RTIL/C-B ranged from 1.8 to 34. Similar results were obtained with C-X and RTIL/C-X on a smaller set of compounds. Tests at 70% relative humidity or with a binary mixture of a polar and non-polar compound had no effect on the capacity of the RTIL/C-B, and there were no changes in V_b10 values after several months of testing that included cycling from 25 to 250 °C. Capacity was strongly correlated with vapor pressure. Attempts to reconcile the selectivity using models based on linear-solvation-energy relationships were only partially successful. Nonetheless, these results indicate that RTIL coating of carbon adsorbents affords a simple, reliable means of rendering them selective for polar S/VOCs.

Ionic liquids as gas chromatographic stationary phases: how can they change food and natural product analyses?

The volatile fraction of natural products often consists of complex mixtures of isomeric and/or homologous components with similar structural and physical characteristics (e.g. mono- and sesquiterpenoids) that are not easy to separate simultaneously with conventional GC stationary phases, even when used with multidimensional systems. The introduction of ionic liquids (ILs) as stationary phases has opened up new perspectives in this field as their unique solvation properties result in uncommon selectivity, which is completely different to that of classic polydimethylsiloxane (PDMS)- and polyethyleneglycol (PEG)-based columns. Because of their peculiar selectivity and high inertness, IL-based columns have already found several applications in the natural product field in mono- and multidimensional GC and preparative GC, leading to the exhaustive analysis of complex samples (including aqueous solutions), and the separation of challenging pair(s) of compounds. This article provides an overview of how IL-based columns can be exploited in the fields of food and natural products, explores the wide range of applications that have already been developed and highlights the main features of these promising stationary phases, which are expected to be further extended in the near future, in particular, for routine use. Graphical abstract.

Investigating the effect of ligand and cation on the properties of metal fluorinated acetylacetonate based magnetic ionic liquids

The effect of chemical structure on various physiochemical properties including thermal stability, solvent miscibility, magnetic susceptibility and viscosity is studied for acetylacetone based magnetic ionic liquids.

A flexible and convenient strategy for synthesis of ionic liquid bonded polysiloxane stationary phases

Ionic liquid bonded polysiloxanes (PILs) are a class of polysiloxanes whose side chains contain ionic liquid (IL) moieties. Considering their excellent selectivity and thermo-stability, PILs have great potentials in the development of polar stationary phases for gas chromatography. In this paper, a novel synthesis strategy for PILs is proposed to diversify PIL stationary phases and also facilitate the study on relationships between stationary phase structure and separation performances. The polysiloxane with imidazole groups at the side chains was synthesized firstly, and then these imidazole groups further reacted with halogenated compounds to produce various IL groups. Upon this, fifteen PIL stationary phases differing in IL content, IL group or combination of different IL groups have been synthesized and used to prepare capillary columns through static coating method. These columns have quite different general polarity indexes (the average value of all Rohrschneider-McReynolds constants in this paper) falling in a broad range from 218 to 717, and most columns have column efficiency values over 3000 plates/m. In addition, IL content, structure of the IL and combination of different IL groups have noticeable influences on interaction features of the stationary phases. After that, the separation performances of these PIL stationary phases were demonstrated by separating various mixed samples of aliphatic esters, dichloro-anilines, alcohols, aromatic amines, substituted alkanes, and so on. In order to reveal the relationship of interaction characteristics and separation performances, a set of indexes of contribution rates (CRs) is proposed. Based on CRs, the separation selectivity of the PIL stationary phases has been discussed in detail. The results indicate that there are significant differences in the separation selectivity not only between PILs and conventional polar stationary phases, but also among different PILs. All of these imply a family of practical PILs with special selectivity could be constructed upon this synthesis strategy.

Novel chiral ionic liquids stationary phases for the enantiomer separation of chiral acid by high-performance liquid chromatography

Novel chiral ionic liquid stationary phases based on chiral imidazolium were prepared. The ionic liquid chiral selector was synthesized by ring opening of cyclohexene oxide with imidazole or 5,6-dimethylbenzimidazole, and then chemically modified by different substitute groups. Chiral stationary phases were prepared by bonding to the surface of silica sphere through thioene "click" reaction. Their enantioselective separations of chiral acids were evaluated by high-performance liquid chromatography. The retention of acid sample was related to the counterion concentration and showed a typical ion exchange process. The chiral separation abilities of chiral stationary phases were greatly influenced by the substituent group on the chiral selector as well as the mobile phase, which indicated that, besides ion exchange, other interactions such as steric hindrance, π-π interaction, and hydrogen bonding are important for the enantioselectivity. In this report, the influence of bulk solvent components, the effects of varying concentration, and the type of the counterion as well as the proportion of acid and basic additives were investigated in detail.

Probing the tricationic ionic liquid/vacuum interface: insights from molecular dynamics simulations

The surface properties of three symmetric linear tricationic ionic liquids (LTILs) with the common anion, bis(trifluoromethylsulfonyl)imide ([NTf₂]⁻), were studied using atomistic molecular dynamics simulation and identification of the truly interfacial molecules (ITIM) analysis.

Recent advances on ionic liquid uses in separation techniques

The molten organic salts with melting point below 100°C, commonly called ionic liquids (ILs) have found numerous uses in separation sciences due to their exceptional properties as non molecular solvents, namely, a negligible vapor pressure, a high thermal stability, and unique solvating properties due to polarity and their ionic character of molten salts. Other properties, such as viscosity, boiling point, water solubility, and electrochemical window, are adjustable playing with which anion is associated with which cation. This review focuses on recent development of the uses of ILs in separation techniques actualizing our 2008 article (same authors, J. Chromatogr. A, 1184 (2008) 6-18) focusing on alkyl methylimidazolium salts. These developments include the use of ILs in nuclear waste reprocessing, highly thermally stable ILs that allowed for the introduction of polar gas chromatography capillary columns able to work at temperature never seen before (passing 300°C), the use of ILs in liquid chromatography and capillary electrophoresis, and the introduction of tailor-made ILs for mass spectrometry detection of trace anions at the few femtogram level. The recently introduced deep eutectic solvents are not exactly ILs, they are related enough so that their properties and uses in countercurrent chromatography are presented.

Basic Overview on Gas Chromatography Columns

The main purpose of this chapter is to give a basic overview on the basic components of stationary phases/columns in gas chromatography ( GC ) for the benefit of students and beginners in the field of GC .