Characterizations for Vinylimidazolium Based Ionic Liquid Polymer Stationary Phases for Capillary Gas Chromatography

Anomalous Thermal Characteristics of Poly(ionic liquids) Derived from 1-Butyl-2,3-dimethyl-4-vinylimidazolium Salts

The synthesis of 1-butyl-2,3-dimethyl-4-vinylimidazolium triflate, its polymerization, and ion exchange to yield a trio of 1-butyl-2,3-dimethyl-4-vinylimidazolium polymers is described. Irrespective of the nature of the anion, substitution at the 2-position of the imidazolium moiety substantially increases the distance between the anion and cation. The methyl substituent at the 2-position also served to expose the importance of H-bonding for the attractive potential between imidazolium moiety and anions in polymers without a methyl group at the 2-position. The thermal characteristics of poly(1-butyl-2,3-dimethyl-4-vinylimidazolium) salts and corresponding poly(1-ethyl-3-methyl-4-vinylimidazolium) salts were evaluated. While the mid-point glass transition temperatures, T_g-mid, for 1-ethyl-3-methyl-4-vinylimidazolium polymers with CF₃SO₃⁻, (CF₃SO₂)₂N⁻ and PF₆⁻ counterions, were 153 °C, 88 °C and 200 °C, respectively, the T_g-mid values for 1-butyl-2,3-dimethyl-4vinylimidazolium polymers with corresponding counter-ions were tightly clustered at 98 °C, 99 °C and 84 °C, respectively. This dramatically reduced influence of the anion type on the glass transition temperature was attributed to the increased distance between the center of the anions and cations in the 1-butyl-2,3-dimethyl-4-vinylimidazolium polymer set, and minimal H-bonding interactions between the respective anions and the 1-butyl-2,3-dimethyl-4-vinylimidazolium moiety. It is believed that this is the first observation of substantial independence of the glass transition of an ionic polymer on the nature of its counterion.

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.

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.

Recent advances of ionic liquids and polymeric ionic liquids in capillary electrophoresis and capillary electrochromatography

Ionic liquids (ILs) and polymeric ionic liquids (PILs) with unique and fascinating properties have drawn considerable interest for their use in separation science, especially in chromatographic techniques. In this article, significant contributions of ILs and PILs in the improvement of capillary electrophoresis and capillary electrochromatography are described, and a specific overview of the most relevant examples of their applications in the last five years is also given. Accordingly, some general conclusions and future perspectives in these areas are discussed.

Ionic liquids as silica deactivating agents in gas chromatography for direct analysis of primary amines in water

Analysis of primary amines in aqueous samples remains a challenging analytical issue. The preferred approach by gas chromatography is hampered by interactions of free silanol groups with the highly reactive amine groups, resulting in inconsistent measurements. Here, we report a method for direct analysis of aliphatic amines and diamines in aqueous samples by gas chromatography (GC) with silanol deactivation using ionic liquids (ILs). ILs including trihexyl(tetradecyl)phosphonium bis 2,4,4-(trimethylpentyl)phosphinate (Cyphos IL-104), 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide [pmim][Tf(2)N] and N″-ethyl-N,N,N',N'-tetramethylguanidinium tris(pentafluoroethyl)trifluorophosphate [etmg][FAP] were tested as deactivating media for the GC liner. Solutions of these ILs in methanol were injected in the system prior to the analysis of primary amines. Butane-1,4-diamine (putrescine, BDA) was used as a reference amine. The best results were obtained using the imidazolium IL [pmim][Tf(2)N]. With this deactivator, excellent reproducibility of the analysis was achieved, and the detection limit of BDA was as low as 1mM. The applicability of the method was proven for the analysis of two different primary amines (C4-C5) and pentane-1,5-diamine.

[Evaluation of two hydroxyl-terminated monocationic ionic liquid stationary phases with high thermal stability for capillary gas chromatography]

Novel hydroxyl-terminated ionic liquids of 1-(6-hydroxyhexyl)-3-butyl imidazolium bis(trifluoromethyl) sulfonylimide (HHBIM-NTf2) and 1-(8-hydroxyoctyl) -3-butyl imidazolium bis(trifluoromethyl) sulfonylimide (HOBIM-NTf2) were synthesized as stationary phases for capillary gas chromatography (GC). Besides, 1-octyl-3-butyl imidazolium bis(trifluoromethyl) sulfonylimide (OBIM-NTf2) was used as the reference. Selectivities and thermal stabilities of HHBIM-NTf2, HOBIM-NTf2 and OBIM-NTf2 were evaluated by means of the separation of Grob test mixture and positional isomers after the columns were conditioned up to 160 degrees C, 250 degrees C and 300 degrees C, respectively. As a result, baseline separations of Grob test mixture and alcohol mixture and better peak shapes were observed with HHBIM-NTf2 and HOBIM-NTf2. Importantly, baseline separation of dimethoxybenzene isomers was also achieved with HHBIM-NTf2 and HOBIM-NTf2. Satisfactory selectivity of HHBIM-NTf2 still remained even after conditioned at 300 degrees C for 8 h. The results for thermal stability showed that as GC stationary phases, HHBIM-NTf2 and HOBIM-NTf2 were stable at least up to 300 degrees C and 250 degrees C, respectively. The hydroxyl-terminated monocationic ionic liquids possess excellent selectivity and thermal stability and are alternative candidates for GC stationary phases.

Application of ionic liquids in high performance reversed-phase chromatography

Ionic liquids, considered “green” chemicals, are widely used in many areas of analytical chemistry due to their unique properties. Recently, ionic liquids have been used as a kind of novel additive in separation and combined with silica to synthesize new stationary phase as separation media. This review will focus on the properties and mechanisms of ionic liquids and their potential applications as mobile phase modifier and surface-bonded stationary phase in reversed-phase high performance liquid chromatography (RP-HPLC). Ionic liquids demonstrate advantages and potential in chromatographic field.