Journal of Chromatography A, 1253 (2012) 171–176
Contents lists available at SciVerse ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Phosphonium-based ionic liquids in electrokinetic capillary chromatography for the separation of neutral analytes Susanne K. Wiedmer a,∗ , Alistair W.T. King b , Marja-Liisa Riekkola a a b
Laboratory of Analytical Chemistry, Department of Chemistry, POB 55, 00014 University of Helsinki, Finland Laboratory of Organic Chemistry, Department of Chemistry, POB 55, 00014 University of Helsinki, Finland
a r t i c l e
i n f o
Article history: Received 2 May 2012 Received in revised form 24 June 2012 Accepted 26 June 2012 Available online 2 July 2012 Key words: Electrokinetic chromatography Ionic liquids Pseudo-stationary phase
a b s t r a c t In this work we elucidated the applicability of phosphonium-based ionic liquids (ILs) as pseudostationary phase in electrokinetic capillary chromatography (EKC) with UV-detection. The phosphonium ILs studied contain bromide, chlorine, or tosylate ions, as counter ions, and alkyl side chains of variable length on the phosphorous atom. The effects of the type and concentration of the IL, pH, ionic strength, and type of background electrolyte solution on the electroosmotic ﬂow (EOF) and on the effective electrophoretic mobilities of some neutral model analytes were investigated and large variations in the migration times were observed. Especially the IL employed remarkably affected the strength and direction of the EOF Successful separations were obtained for neutral aromatic singly substituted analytes, namely benzene, toluene, phenol, and nitrobenzene. The results demonstrated the potential of capillary electromigration methods for rapid interaction studies between ILs and analytes, which is useful for the development of novel materials for sample preparation and separation purposes or for novel catalyst and chemical processing studies. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) comprise a group of solvents that have very distinct chemical and physical characteristics. The increasing interest in using ILs is as much based on their unique solvent properties as on their low volatility. The earliest known report of an IL is from 1888, when Gabriel reported the formation of the protic IL ethanolammonium nitrate, with a melting point of 52–55 ◦ C. However, the reaction between ethylamine and nitric acid, forming a protic IL with a melting point of 12 ◦ C, published by Walden in 1914, is by many considered as the ﬁrst room temperature IL (RTIL). The large number of recent reviews on the applicability of ILs for research and industrial purposes shows the ever increasing interest in the solvents [1,2]. ILs have frequently been utilized in the ﬁeld of separation science and for sample preparation [1,3–11]. For extraction purposes, the main focus has been on liquid–liquid extraction and liquid-phase and solid-phase microextraction [5,12,13]. Modiﬁcation of materials with various types of ILs has also been of particular interest, see [14,15] and refs therein. In addition to the use of ILs for sample preparation and extraction, ILs have been commonly used in biphasic ionic–neutral reactions, for the conversion of commodities into higher value
∗ Corresponding author. Tel.: +358 9 19150264; fax: +358 9 19150253. E-mail address: [email protected]
ﬁ (S.K. Wiedmer). 0021-9673/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2012.06.084
products. One important example of this, by Chauvin et al. [16,17] is the difasol process, whereby the success of the process depends on the relative solubility of monomer and dimer reactants between the ionic and neutral reactant phase. In an extension of the biphasicpartitioning concept, ILs have also attracted attention as support media for catalysts in supported IL phase-catalysis (SILP-catalysis), and in these processes the activity of the catalysts depends on the relative partitioning of reactants and products between the IL support phase and the neutral reactant phase . The most commonly reported ILs include dialkylimidazolium moieties. The reason for their popularity lies in the fact that they form low melting and low viscosity salts that are thermally stable, even up to 400 ◦ C in some cases. They are also acid stable and are relatively easy to synthesize. These are the archetypical IL structures and as such, have received much attention in the literature and have a proven track record for several applications. They are, however, unstable in the presence of bases and some have thermal stabilities as low as 175 ◦ C. Phosphonium-based ILs have shown a great importance in a vast number of industrial and pharmaceutical applications, including their use for extracting sulphur-rich compounds, as lubricants, electrolytes, paramagnetic ﬂuids, and media for metal deposition, to mention a few [19–22]. Phosphonium-based ILs have several advantages over archetypical ILs, such as drastically improved chemical and thermal stabilities. They are both acid and alkaline stable to a much higher degree than imidazolium-based ILs and can
S.K. Wiedmer et al. / J. Chromatogr. A 1253 (2012) 171–176
thus afford much harsher reaction conditions and therefore allow wider kinetic and thermodynamic control of reactions. In addition, they are typically much more ‘hydrophobic’ than other IL classes, due to the presence of the long alkyl chains on the quaternary phosphorous atom. This also affords them different general properties, in comparison to other ILs, such as insolubilities in water for some structures, higher viscosities, surfactant properties, etc. Consequently more hydrophobic materials are miscible with some of these ILs (e.g. even hexane or toluene) and they are perfect for being tuned for liquid–liquid phase-separation or as media for biphasic reactions. Phosphonium-based ILs have found much use in GC as stationary phases . In separation science one important beneﬁt of phosphonium-based ILs, over imidazolium-based ILs, is the low UV absorption at low wavelengths. In practical terms this means that direct UV absorption detection is feasible. Regarding the use of imidazolium-based ILs, as chromophores in indirect CE, the study on neutral carbohydrates by Vaher et al.  is worth mentioning. Phosphonium-based ILs have been applied in CE for suppression and reversal of EOF for improved separation of charged ions [25,26] and as pseudostationary phase in electrokinetic capillary chromatography (EKC) [27–31]. The aim of this study was to further investigate the applicability of phosphonium-based ILs as pseudo-stationary phases in EKC with UV-detection for interaction studies with uncharged aromatic compounds, representative of some of today’s petrochemical-based commodities. Up to now focus in the ﬁeld of CE techniques has been on the use of ILs for modiﬁcation and/or suppression of the electroosmotic ﬂow (EOF), and to a lesser extent as pseudo-stationary phases in EKC. Details on interactions between analytes and ILs are essential for the development of new phosphonium-based IL materials for extraction, partitioning, and separation purposes. 2. Experimental 2.1. Chemicals All four tested ILs, i.e., tributyltetradecylphosphonium chloride ([P14444 ]Cl; CYPHOS 3453 W; CAS no. 81741-28-8), tributyl(hexadecyl)phosphonium bromide ([P16444 ]Br; CYPHOS IL 162; CAS no. 14937-45-2), tetrabutylphoshonium chloride ([P4444 ]Cl; CYPHOS 443 W; CAS no. 2304-30-5), and triisobutylmethylphosphonium tosylate ([P1444 ][OTs]; CAS no. 344774-05-6) were from Cytec Industries (Woodland Park, NJ, USA). ortho-Phosphoric acid (85%), phenol, and pH solutions (7 and 10) used for calibrating the pH meter were purchased from Merck (Darmstadt, Germany). Sodium hydroxide (1.0 M) was from FF-Chemicals (YliIi, Finland) and potassium hydroxide from J.T. Baker (Mallinckrodt, Baker, Deventer, the Netherlands). Toluene and dimethylsulfoxide (DMSO) were from Lab-Scan (Gliwice, Poland). Benzene, nitrobenzene, and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were purchased from Sigma (Darmstadt, Germany). Distilled water was further puriﬁed with a Millipore waterpuriﬁcation system (Millipore, Molsheim, France). 2.2. Buffer and sample preparation Phosphate buffers were prepared from phosphoric acid (85%, corresponding to 15 M), adjusted to desired pH with 1 M sodium hydroxide or 1 M potassium hydroxide. HEPES buffer was prepared from the sodium salt of the buffer, with the pH adjusted with sodium hydroxide. The ionic strengths of all the background electrolytes (BGEs) used were 10 mM (calculated with Peakmaster 5.2 program). The concentration of IL in the BGE varied between 2.5 and 50 mM. The concentrations of the ILs were calculated from the
molar masses given by the supplier. Before use in electrophoresis, the IL BGEs were ﬁltered through a 0.45-m syringe ﬁlter (Gelman Sciences, Ann Arbor, MI, USA). The model sample mixture contained 0.05% (v/v) of DMSO (as the EOF marker), 200 g/mL of benzene, 100 g/mL of nitrobenzene, 35 g/mL of phenol, and 125 g/mL of toluene, all in water. The sample concentrations were selected to achieve high UV responses for all model analytes at 200 nm. Another good wavelength for all analytes was 380 nm. At 245, 254 and 280 nm only nitrobenzene gave a good UV response. 2.3. Instrumentation EKC was performed using a Hewlett Packard 3D Capillary Electrophoresis system (Agilent, Waldbronn, Germany). Uncoated fused-silica capillaries were from Polymicro Technologies (Phoenix, AZ, USA). Dimensions of the capillaries were 50 m I.D. (375 m O.D.) with a length of 40/48.5 cm (length to the detector/total length). A new capillary was taken into use when changing the type of IL. The new capillary was preconditioned by rinsing at a pressure of 940 mbar for 10 min with 0.1 M sodium hydroxide, for 10 min with water and ﬁnally for 10 min with the IL-BGE. Running conditions were as follows: voltage (negative polarity) −30 kV (ﬁeld strength of −619 V/cm); temperature 25 ◦ C; sample injection 10 s at 10 mbar; UV-detection at 200 nm. The electrophoretic runs were repeated 6–10 times. 0.05% (v/v) DMSO was used the EOF marker. The currents never exceeded −24 A with any of the IL-BGEs used (maximum IL concentration during electrophoresis was 20 mM). Critical aggregation (micelle) concentrations (CMC) were determined at 25 ◦ C by conductometric titration using an electrode and conductivity meter from Radiometer (Copenhagen, Denmark). 3. Results and discussion The applicability of phosphonium-based ILs as pseudostationary phases in EKC for the separation of neutral aromatics was examined. First we wanted to investigate the dynamic adsorption of ILs to the fused silica capillary simply by rinsing the capillary with an IL-containing buffer and performing runs with buffers without IL. Next four different ILs, shown in Fig. 1, were compared when applied as pseudo-stationary phase for the separation of uncharged aromatic compounds, used as model analytes. The EOF suppression caused by the ILs was compared. The optimal IL was selected and the effect of the concentration of the IL in the BGE on the separation of the compounds was evaluated. The inﬂuence of the pH over the range 6–9 on the separation was studied using two different buffers. In addition, the effect of the phosphate buffer cation was evaluated. 3.1. Suppression of EOF In CE techniques ILs have been used both in dynamic and static coatings of fused silica capillaries, and to some extent as additives in the background electrolyte solution. By far the most frequently used ILs have been imidazolium-based ILs . In this work it was of interest to examine if phosphonium-based ILs can easily be dynamically adsorbed onto the fused silica capillary and employed as semi-permanent coatings in CE to affect the EOF. A 50 mM IL dispersion of [P14444 ]Cl in sodium phosphate buffer at pH 8.9 (ionic strength of 10 mM) was selected for this purpose. The capillary was preconditioned with sodium hydroxide and water, after which the capillary was rinsed at high pressure (940 mbar) for 15 min with the 50 mM IL-BGE solution. This rinsing step was followed by a short rinse with a BGE without IL, and the EOF was determined in consecutive runs using only sodium phosphate buffer
S.K. Wiedmer et al. / J. Chromatogr. A 1253 (2012) 171–176
Fig. 1. Structures of investigated phosphonium-based ILs. (A) tributyltetradecylphosphonium chloride ([P14444 ]Cl); (B) tributyl(hexadecyl)phosphonium bromide ([P16444 ]Br); C) tetrabutylphoshonium chloride ([P4444 ]Cl); and (D) triisobutylmethylphosphonium tosylate ([P1444 ][OTs]).
at pH 8.9 (ionic strength of 10 mM) as the BGE solution. Fig. S1 demonstrates that [P14444 ]Cl is strongly attached onto the inner wall of the fused silica capillary. The stabilization of the dynamic coating was apparently slow and the coating reached ca. 50% of its original value after 60 injections. These results, however, demonstrate the suitability of tetraalkylphosphonium additives, with at least one long alkyl chain, as semi-permanent dynamic coatings in CE. The next step was to use the ILs as pseudostationary phases in EKC and for this we ﬁrst wanted to know the effect of the IL-BGE on the EOF. The pre-treated capillary was rinsed with different phosphonium-based IL-BGE solutions containing 10 mM (concentration) IL in sodium phosphate buffer at pH 8.0 (ionic strength of 10 mM) and the same dispersion was used as BGE during electrophoresis. The results in Fig. 2 show that the EOF
Electroosmotic flow (m2s-1V-1)
6E-08 4E-08 2E-08 0 -2E-08 -4E-08 -6E-08
Fig. 2. EOFs in fused silica capillaries ﬁlled with BGEs containing 10 mM (ionic strength) sodium phosphate at pH 8 in addition to 10 mM IL ([P14444 ]Cl; [P16444 ]Br; [P4444 ]Cl; [P1444 ][OTs]; no IL added). Running conditions: capillary, 40/48.5 cm; temperature, 25 ◦ C; voltage, −30 kV (−619 V/cm); injection for 10 s at 10 mbar; UV-detection at 200 nm.
was strongly suppressed and reversed with the long-chain ILs, i.e., [P14444 ]Cl and [P16444 ]Br. This was much expected and similar results have been reported for imidazolium-based ILs [7,8]. When [P14444 ]Cl was used as an EOF modiﬁer for the improved chiral separation of propranolol, EOF reversal was observed at both pH 3 and pH 10 at low IL concentration (<5 mM). In our study the highest EOF suppression was obtained with [P16444 ]Br, giving an EOF of −4.8 × 10−8 m2 V−1 s−1 (anodic direction). The same phosphonium-based IL has previously been used as an EOF suppressant and BGE additive for the separation of inorganic and organic anions . In that work the IL was dispersed in 20 mM (concentration) borate buffer at pH 9.2 and the EOF was approx −6.6 × 10−8 m2 V−1 s−1 . The slightly different EOF obtained in our study can be explained by the use of a different buffer ion and concentration, knowing the strong inﬂuence of ionic strength on the EOF. In general, our results agree well with previous studies on EOF modiﬁers, showing that compounds with one long alkyl chain reverse the EOF very effectively [25,32]. Because bulkier compounds with shorter alkyl chains have typically weaker effect on the EOF, the adsorption of ILs on the fused silica wall is related both to the head group (in our case not an issue because all investigated ILs where phosphonium-based ILs) and to the length of the alkyl chain. As discussed above, more effective adsorption has been observed for ILs with irregular shapes (with only one long alkyl chain). In our study we included one symmetrical IL, i.e., tetrabutylphoshonium chloride ([P4444 ]Cl), and one irregular IL with tri-isobutyl alkyl groups, i.e., triisobutylmethylphosphonium tosylate ([P1444 ]OTs). These two ILs had a weaker effect on the EOF as could be expected. The EOF was only slightly suppressed and it remained positive (cathodic direction). No reversal of EOF was seen whatsoever. Our data agree quite well with other published data. Namely, only a small EOF reduction has been observed in a recent study where [P4444 ]Cl was added to the BGE at pH 10, while a stronger effect has been observed at pH 3.0 . In addition, only a moderate EOF suppression with [P4444 ]Br in 20 mM borate pH 9.2 buffer (EOF value of approx. 6 × 10−8 m2 V−1 s−1 ) has been reported in another study . The differences in the values obtained in our study and in the previous
S.K. Wiedmer et al. / J. Chromatogr. A 1253 (2012) 171–176
work can be explained by the effects described above, in addition to a different counter ion in the IL (chloride vs. bromide). 3.2. Phosphonium-based ILs as pseudo-stationary phases for separation of neutral aromatics The next step was to evaluate the ILs as pseudo-stationary phases in EKC. Several applications have been reported and detailed lists on other applications of ILs as pseudo-stationary phases in EKC can be found in the recent reviews, listed in the Introduction. To mention some, imidazolium-based ILs have been used in microchip EKC for the separation of proteins , chiral ILs have been used in combination with cyclodextrins for the enantioseparation of drugs [34,35], and IL has been used as an additive in micellar electrokinetic chromatography (MEKC), using sodium dodecyl sulfate as the pseudostationary phase, for the separation of ﬂuoroquinolones . Moreover, IL-EKC with electrochemiluminescence detection has been demonstrated , and IL-type cationic surfactants have been used in sweeping-MEKC mode for the concentration of benzodiazepines . Furthermore, as mentioned in the introduction, imidazolium-based ILs have been utilized in CE with indirect UV for the detection of underivatized neutral carbohydrates, where the effect of up to 50 mM IL concentrations were investigated . More recent applications of ILs in EKC include the use of IL-in-water microemulsions  and IL-coated multi-walled nanotubes  as pseudo-stationary phases. There are only a few studies on the use of ILs as pseudostationary phases in EKC for the separation of neutral analytes, such as seven neutral phenolic compounds using 1-hexadecyl-3methylimidazolium bromide dispersed in phosphate buffer at pH 4.8 . The IL was found to be superior over the single chain surfactant cetyl trimethylammonium bromide. Acidic pH values (4.8–6.1) were investigated, although the buffering capacity of the phosphate buffer was unfortunately not valid in the range used. 1Tetradecyl-3-methylimidazolium chloride has been used earlier as pseudo-stationary phase in EKC for the separation of neutral benzene derivatives , but severe peak tailing was observed in the analyses. In that work 15 mM IL dispersed in 30 mM Tris buffer at pH 7.0 was used as the BGE solution. One highly relevant work for the present study is the work by Schnee et al.  where N-alkyl-Nmethylpyrrolidinium ILs are employed as pseudostationary phases. In that study the successful separation of some neutral aromatic compounds was demonstrated. In our work the goal was to explore the applicability of phosphonium-based ILs in EKC for the separation of neutral analytes. The sample mixture in our study contained benzene and single-substituted benzene-derivatives, namely toluene, phenol, and nitrobenzene [27,30]. Preliminary investigations on the applicability of the selected ILs as pseudo-stationary phase at pH 8 were performed. No separation of the tested analyte mixture was achieved using up to 50 mM of [P4444 ]Cl or [P1444 ][OTs] in the BGE. On the other hand, good separation was achieved with both [P14444 ]Cl and [P16444 ]Br. The effective electrophoretic mobilities of the analytes were similar (Fig. S2), despite very different EOF values (see Fig. 2). The corresponding electropherograms are seen in Fig. 3. The migration order was benzene, nitrobenzene, phenol, and toluene. The separation did not follow the log Po/w values of the analytes; benzene 2.177 ± 0.154, nitrobenzene 1.921 ± 0.166, phenol 1.540 ± 0.185, and toluene 2.720 ± 0.168, proving that hydrophobic behavior (as expressed as the octanol/water partitioning) does not solely explain the migration of the compounds. When the IL forms micelles and other distinct phases, there are different regions that may take part in interaction with the neutral compounds. All compounds and in particular benzene and toluene are expected to be soluble in the inner hydrophobic region of these phases. Nitrotoluene and phenol with more ‘polar’ nature have also
Fig. 3. Electropherograms of benzene and benzene-derivatives using BGEs comprising 10 mM (ionic strength) sodium phosphate at pH 8 and 10 mM [P14444 ]Cl or [P16444 ]Br. The running conditions were as in Fig. 2. Analytes: (1) benzene, (2) nitrobenzene, (3) phenol, and (4) toluene.
the ability to act as hydrogen bond acceptor and donor, respectively. The IL phase has positive charges, localized on one atom, and anions where these more polar compounds are likely to interact by hydrogen-bonding, in comparison to the van der Waals interactions that are likely to occur for toluene and benzene. – interactions, that are possible with imidazolium-based cations, are not involved using phosphonium-based cations. The role of the anion buffer in the BGE solution however is undetermined. If the anion is retained around the IL-phases, it is likely that it plays some kind of role in hydrogen-bond formation depending on the character of analytes. Water may also be involved in the hydrogen-bonding interactions. The large difference in the electrophoretic behavior of toluene and benzene, in comparison to the more polar species was unexpected based on their hydrophobicity (the log Po/w values). Further understanding of the micellar structure of ILs is required to fully understand the interactions between IL and neutral analytes. The migration behavior might then be predicted by a more complex set of parameters, reﬂecting the partitioning between the BGE, aliphatic and ionic regions. The background noise was much higher using [P16444 ]Br, as well as the standard deviations of the migration times (as seen from the error bars in Fig. S2). In addition, there were many spikes and disturbing peaks right before the EOF marker DMSO in the electropherogram using [P16444 ]Br. Although the analyte
S.K. Wiedmer et al. / J. Chromatogr. A 1253 (2012) 171–176
effective electrophoretic mobility (m2s-1V-1)
b Electroosmotic flow (m2s-1 V-1)
-3E-08 -3,5E-08 -4E-08
Fig. 4. Effect of pH and buffer (sodium phosphate or HEPES) in the 10 mM (ionic strength) BGE solution containing 10 mM [P14444 ]Cl on (a) the effective electrophoretic mobilities of benzene and benzene-derivatives and (b) the electroosmotic ﬂow values. The running conditions were as in Fig. 2.
concentrations in the sample for injection were the same, clear differences were seen in the UV responses in the electropherograms. Because of these reasons, [P14444 ]Cl was selected as the IL in the BGE for further experiments. 3.3. Inﬂuence of BGE pH on the separation The inﬂuence of the pH of the BGE on the separations was tested over the pH range 6–9 with 10 mM [P14444 ]Cl added to the BGE solution. Phosphate, with a pKa2 at pH 7.21, was used at pHs 6–8 and HEPES, with a pKa of 7.5, was used at pHs 7–9. The ionic strength of the BGE was 10 mM. The tested analytes benzene, toluene, and nitrobenzene were uncharged at the pH values investigated. The pKa value of phenol is 9.86 ± 0.13, which means that it was ∼11% dissociated at pH 9 and only ∼1% dissociated at pH 8 (around 0.01% at pH 7). Accordingly, the slight peak tailing of phenol at pH 8 (Fig. 3) can partly be explained by electrostatic interactions between the positively charged silica wall and the negatively charged analyte. The same phenomenon was observed in the work by Borissova et al.  using 1-tetradecyl-3-methylimidaxolium chloride in Tris pH 7 buffer as EKC pseudo-stationary phase. The effective
electrophoretic mobilities using 10 mM [P14444 ]Cl in sodium phosphate or HEPES buffers are shown in Fig. 4a and b presents the corresponding EOF values. The analytes had the highest effective electrophoretic mobilities in IL-phosphate at pH 6 buffer, and the lowest values in IL-phosphate at pH 8. Fig. S3 shows the electropherograms using 10 mM [P14444 ]Cl dispersed in sodium phosphate or HEPES buffers at pH 8. Due to high background noise and some irregularities in the base line in HEPES BGE, the following study was conducted with sodium phosphate as the buffer. 3.4. Effect of IL concentration on the separation The IL [P14444 ]Cl concentration in the BGE solution was varied between 1 and 20 mM and the corresponding effective electrophoretic mobilities and electropherograms are demonstrated in Figs. 5 and S4, respectively. An increase in the IL concentration resulted in improved separation. Even at 1 mM IL concentration the analytes were already partly separated. The separation is based on the partitioning of the analytes into [P14444 ]Cl aggregates in the pseudo-stationary phase as well as with [P14444 ]Cl adsorbed onto the fused silica wall. The aggregation behavior of ILs has
S.K. Wiedmer et al. / J. Chromatogr. A 1253 (2012) 171–176
mobilities of the analytes. As could be expected the type of cation (sodium or potassium) in the IL-BGE had a rather strong inﬂuence on the EOF. This work shows the great possibility of utilizing CE for the understanding of the nature of ILs, and in particular for the clariﬁcation of the interactions between ILs used and model analytes. The information obtained can be exploited in developing new types of IL materials for IL-neutral biphasic reactions, new supported IL phase catalysts, and for separation and sample preparation purposes. Acknowledgements Ms Karina Moslova is acknowledged for performing the conductometric titrations. Funding from the University of Helsinki Research Funds (SKW: University of Helsinki project no. 2105060) is acknowledged. Appendix A. Supplementary data Fig. 5. Effective electrophoretic mobilities of benzene and benzene-derivatives using BGEs comprising 10 mM (ionic strength) sodium phosphate at pH 8 and 1–20 mM [P14444 ]Cl. The running conditions were as in Fig. 2.
been studied to some extent with tensiometry (surface tension measurements) , UV–vis spectroscopy , and ﬂuorescence spectroscopy . In our study the CMC of [P14444 ]Cl in sodium phosphate at pH 8 determined by conductometric titration was 0.6 ± 0.05 mM (n = 3). The intersection was determined by a linear least-square ﬁtting to the data points in the linear portions in the low and high concentration regions. Because the low CMC values proved that there were already micelles in the 1 mM IL solution, the separation was mainly based on the interactions between the analytes and the moving IL aggregates in the BGE. The type of counter ions in BGEs may have a great impact on the EOF in fused silica capillaries [40–42]. Generally it has been observed that when the concentration of the mono- or di-valent cations is increased in the buffer, the EOF is decreased and this decrease is related to the size of the cation, e.g. the EOF is lower using potassium than sodium rich buffers. To evaluate the effect of the BGE cation in the solutions, two different IL-containing BGEs were prepared at pH 8; one with sodium phosphate and another with potassium phosphate. The ionic strength of the buffer was 10 mM and the concentration of [P14444 ]Cl was kept at 20 mM. Fig. S5 demonstrates that after changing sodium ions with potassium ions no big inﬂuence on the effective electrophoretic mobilities of the analytes was seen. However, the EOFs were different in the two systems. Using the IL-sodium phosphate buffer the EOF was −3.39 × 10−8 (±4.67 × 10−10 ) m2 s−1 V−1 , while using the IL-potassium buffer the corresponding value was −4.96 × 10−8 (±2.49 × 10−9 ) m2 s−1 V−1 . Potassium ions were strongly attached to the semi-permanent IL layers, resulting in a higher anodic EOF than using sodium ions as buffer counter ions. The results prove that higher anodic ﬂows can easily be obtained simply by substituting the cation in the buffer. 4. Conclusions The applicability of phosphonium-based ILs as pseudostationary phase in EKC for the separation of neutral benzene and benzene-derivatives was demonstrated. The EOF was strongly inﬂuenced by the type of phosphonium-based IL used, and the strongest impact on the EOF was achieved with tetraalkylammoniumphosphonium ILs with one long alkyl chain (C14 or C16 ). Separation of the model analytes was achieved even at very low IL concentrations (close to the CMC of the IL). Successful separation was achieved at pH values 6–9 and the buffer component in the BGE had a small inﬂuence on the effective electrophoretic
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.chroma.2012.06.084. References  P. Sun, D.W. Armstrong, Anal. Chim. Acta 661 (2010) 1.  N. Muhammad, Z. Man, M. Bustam Khalil, Eur. J. Wood Prod. 70 (2012) 125.  V. Pino, M. Germán-Hernández, A. Martín-Pérez, J.L. Anderson, Sep. Sci. Technol. 47 (2012) 264.  J.-F. Liu, G.-B. Jiang, J.Å. Jönsson, Trends Anal. Chem. 24 (2005) 20.  L. Vidal, M.-L. Riekkola, A. Canals, Anal. Chim. Acta 715 (2012) 19.  Y. Xu, E. Wang, J. Chromatogr. A 1216 (2009) 4817.  M. López-Pastor, B.M. Simonet, B. Lendl, M. Valcárcel, Electrophoresis 29 (2008) 94. ´  B. Buszewski, S. Studzinska, Chromatographia 68 (2008) 1.  E.M. Martinis, P. Berton, R.P. Monasterio, R.G. Wuilloud, Trends Anal. Chem. 29 (2010) 1184.  W. Bi, J. Zhou, K.-H. Row, Sep. Sci. Technol. 47 (2011) 360.  V. Pino, A.M. Afonso, Anal. Chim. Acta 714 (2012) 20.  Y. Fan, S. Zhang, Eur. J. Chem. 2 (2011) 282.  M.A. Malik, M.A. Hashim, F. Nabi, Chem. Eng. J. 171 (2011) 242.  L. Vidal, J. Parshintsev, K. Hartonen, A. Canals, M.-L. Riekkola, J. Chromatogr. A 1226 (2012) 2.  H. Han, J. Li, X. Wang, X. Liu, S. Jiang, J. Sep. Sci. 34 (2011) 2323.  Y. Chauvin, B. Gilbert, I. Guibard, J. Chem. Soc. Chem. Commun. (1990) 1715.  Y. Chauvin, H. Olivier-Bourbigou, Chem. Technol. 25 (1995) 26.  M.H. Valkenberg, C. deCastro, W.F. Hölderich, Appl. Catal. A-Gen. 215 (2001) 185.  L.J. Weng, X.Q. Liu, Y.M. Liang, Q.J. Xue, Tribol. Lett. 26 (2007) 11.  K. Tsunashima, M. Sugiya, Electrochem. Commun. 9 (2007) 2353.  R.E. Del Sesto, T.M. McCleskey, A.K. Burrell, G.A. Baker, J.D. Thompson, B.L. Scott, J.S. Wilkes, P. Williams, Chem. Commun. (2008) 447.  S.Z. El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Chemphyschem 7 (2006) 1535.  C.F. Poole, S.K. Poole, J. Sep. Sci. 34 (2011) 888.  M. Vaher, M. Koel, J. Kazarjan, M. Kaljurand, Electrophoresis 32 (2011) 1068.  T. Kˇríˇzek, Z.S. Breitbach, D.W. Armstrong, E. Tesaˇrová, P. Coufal, Electrophoresis 30 (2009) 3955.  A. Mendes, L.C. Branco, C. Morais, A.L. Simplício, Electrophoresis 33 (2012) 1182.  J. Niu, H. Qiu, J. Li, X. Liu, S. Jiang, Chromatographia 69 (2009) 1093.  S.A.A. Rizvi, S.A. Shamsi, Anal. Chem. 78 (2006) 7061.  V.P. Schnee, G.A. Baker, E. Rauk, C.P. Palmer, Electrophoresis 27 (2006) 4141.  M. Borissova, K. Palk, M. Koel, J. Chromatogr. A 1183 (2008) 192.  H.-L. Su, M.-T. Lan, Y.-Z. Hsieh, J. Chromatogr. A 1216 (2009) 5313.  T. Takayanagi, E. Wada, S. Motomizu, Analyst 122 (1997) 57.  Y. Xu, J. Li, E. Wang, J. Chromatogr. A 1207 (2008) 175.  B. Wang, J. He, V. Bianchi, S.A. Shamsi, Electrophoresis 30 (2009) 2812.  B. Wang, J. He, V. Bianchi, S.A. Shamsi, Electrophoresis 30 (2009) 2820.  Z. Chen, Z. Zhong, Z. Xia, F. Yang, X. Mu, Chromatographia 75 (2012) 65.  Y. Xu, L. Fang, E. Wang, Electrophoresis 30 (2009) 365.  J. Cao, H. Qu, Y. Cheng, Electrophoresis 31 (2010) 3492.  J. Cao, P. Li, L. Yi, J. Chromatogr. A 1218 (2011) 9428.  Q. Qu, X. Tang, D. Mangelings, C. Wang, G. Yang, X. Hu, C. Yan, J. Chromatogr. B 853 (2007) 31.  I.Z. Atamna, C.J. Metral, G.M. Muschik, H.J. Issaq, J. Liq. Chromatogr. 13 (1990) 2517.  M.X. Zhou, J.P. Foley, Anal. Chem. 78 (2006) 1849.