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A novel urea-functionalized surface-confined octadecylimidazolium ionic liquid silica stationary phase for reversed-phase liquid chromatography夽 Mingliang Zhang a,b , Ting Tan c , Zhan Li a , Tongnian Gu a , Jia Chen a , Hongdeng Qiu a,∗ a Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, China
a r t i c l e
i n f o
Article history: Received 30 April 2014 Received in revised form 3 August 2014 Accepted 7 September 2014 Available online xxx Keywords: Surface-confined ionic liquid Octadecylimidazolium Urea group Linear solvation energy relationships
a b s t r a c t One-pot synthesis of surface-confined ionic liquid functionalized silica spheres was proposed using N-(3aminopropyl)imidazole, ␥-isopropyltriethoxysilane and 1-bromooctadecane as starting materials. The surface modification of the silica spheres was successful with a high surface density of octadecylimidazolium, enabling the utilization of this new urea-functionalized ionic liquid-grafted silica material as stationary phase for high-performance liquid chromatography in reversed-phase mode. The long aliphatic chain combined with the multiple polar group embedded in the ligands imparted the new stationary phase fine selectivity towards PAH isomers and polar aromatics and higher affinity for phenolic compounds. The unique features of the new material, especially the effect of the urea group on the retention were elucidated by mathematic modeling. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) have been widely studied in many disciplines since their discovery in 1914. As an important member in IL family, the imidazolium cation-based ILs are receiving an avalanche of attention as novel media in organic synthesis [1,2], catalysis [3], nanomaterial etc. [4]. Various 1,3-dialkylimidazolium salts containing a great diversity of anions, such as PF6 − , BF4 − , CF3 SO3 − and NTf2 − , have been synthesized. Due to their negligible vapor pressure and non-flammability at room temperature, they are regarded as eco-friendly alternatives to traditional organic solvent. Additionally, their hydrophobicity/hydrophilicity and miscibility can be modified by changing the alkyl substituents and/or anions. Therefore, tailor-made ILs can be designed to fulfill specific demands. Many efforts have been made to explore the functionalized ILs, i.e. “task-specific” ILs [5,6]. The beneficial characteristics of ILs also facilitate their successful application in analytical chemistry [7–10], for example, they are outstanding alternatives to conventional extraction
夽 Invited paper for the Honor Issue of Professor Peichang Lu’s 90th birthday. ∗ Corresponding author. Tel.: +86 931 4968877; fax: +86 931 8277088. E-mail address: [email protected] (H. Qiu).
solvents in many sample preparation techniques [11–13], as well as remarkable application as stationary phases for gas chromatography due to their superior thermostability and capability of multiple intermolecular interactions [14–16]. In HPLC, they have been used as mobile additives in place of organic amine or other amino quenchers, in order to mitigate the negative effect of free silanol groups on separation of basic solutes [17,18]. ILs have also been used for surface modification of silica, immobilizing them to silica surface and forming surface-confined ionic liquid (SCIL) [19–21]. It must be highlighted that the increasing utilization of SCILs has been the topic of recent review articles [12,22]. As mentioned in these literatures, the tunability of ILs by employing different substituents and anions allows the application of SCIL stationary phases in various HPLC modes, such as reversed-phase (RP), normal-phase (NP), ion-exchange (IE) and hydrophilic interaction (HILIC) mode, for separation of a wide range of substances. Up to date, the synthetic strategies for SCIL stationary phases could be categorized into monomeric and polymeric approaches [22,23]. The existing monomeric approach requires 1-alkylimidazole and ␥-haloalkyltrialkoxysilane. The reaction between them leads to an “IL-silane”, which is then bonded to silica. Or the IL can be heterogeneously generated in-situ on ␥haloalkylated silica. The monomeric approach is convenient and effective; however, the bonding amount is usually not high due to
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Please cite this article in press as: M. Zhang, et al., A novel urea-functionalized surface-confined octadecylimidazolium ionic liquid silica stationary phase for reversed-phase liquid chromatography, J. Chromatogr. A (2014), http://dx.doi.org/10.1016/j.chroma.2014.09.018
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Fig. 1. Illustration of the structures of the new and referenced SCIL stationary phases.
the heterogeneity of the reaction medium. The existing polymeric approach requires ␥-mercaptopropyltrialkoxysilane and alkenylcontaining imidazolium salts, which are “task-specific” in view of their structures. Silica is firstly modified by the silane, and then alkenyl-containing imidazolium salts are polymerized on the silica via catalyzed thiol-ene click reaction. This approach can guarantee a much higher bonding density and enhanced column stability than monomeric one, yet it is a must that the imidazolium salts have alkenyl group. The effect of steric hindrance is significant in both immobilization procedures, bulky substituents or anions frequently result in lower bonding amount [24,25]. With an aim to simplify the preparation work by avoiding the synthesis of 1-octadecyllimidazole or alkenyl-functionalized octadecylimidazolium salts and to enlarge the scope of useful ILs in HPLC stationary phase, we adopted a strategy proposed in our recent work [26] for convenient and efficient immobilization of “task-specific” ILs onto silica sphere, and the recommendable performance of the resulted multifunctional materials was illustrated when used as SCIL stationary phases in RP mode. To obtain more insights into the behavior of SCIL stationary phase, the influences of the functional group (urea) and substituent within the IL on the chromatographic process was investigated by eluting multitudinous analytes and comparatively studied with another octadecylimidazolium stationary phase and commercial C18 stationary phase in the framework of linear solvation energy relationships (LSER) model. 2. Experimental 2.1. Reagents and materials 1-Bromooctadecane (99%) and ␥-isocyanatopropyltriethoxysilane (ICPTES) (98%) were purchased from Sun Chemical Technology Co., Ltd (Shanghai, China). N-(3-Aminopropyl)imidazole (APIm) (98%) were obtained from Alfa Aesar (Tianjin, China). Biphenyl, anilines and phenols of analytical grade were obtained from Sinopharm Chemical Reagents Co., Ltd (Shanghai, China). Polycyclic aromatic hydrocarbons (PAHs), including Triphenylene, o- and m-terphenyl, tetracene and n-alkylbenzenes (n-hexylbenzene, noctylbenzene, n-decylbenzene and n-dodecylbenzene) of analytical standard were supplied by J&K Chemical (Beijing, China). Phenanthrene, anthracene, chrysene, p-terphenyl and p-quaterphenyl of analytical grade were purchased from Aladdin Industrial Inc. (Shanghai, China). Ultrapure water (18.3 M at 25 ◦ C) was produced by a Milipore Direct Q 3UV unit; acetonitrile (MeCN), methanol (MeOH), ethanol (EtOH) and tetrahydrofuran (THF) of HPLC grade were used. Solutes for LSER analysis and other solvents for synthesis of analytical grade or better
were obtained from different origins. 1-n-Alkylnaphthalene (1n-hexylnaphthalene, 1-n-octylnaphthalene, 1-n-decylnaphthalene and 1-n-dodecylnaphthalene) and spherical porous silica (diame˚ surface area: 400 m2 g−1 ) were supplied ter: 5 m, pore size: 90 A, by Lanzhou Institute of Chemical Physics (Lanzhou, China). 2.2. Preparation of stationary phase ˚ and MeCN of HPLC grade was dried by molecular sieve (3 A) distilled prior to use. Upon each addition of material, the solvent was purged by nitrogen. The whole reaction was carried out in an atmosphere of nitrogen. The synthesis was carried out according to Ref. [26]. Briefly, equimolar ICPTES was added dropwise to a MeCN solution of APIm cooled at 0 ◦ C, and then the solution was stirred at room temperature overnight. Secondly, slightly excessive 1-bromooctadecane was added and refluxed for 48 h. Finally, Silica spheres was added under mechanical stirring and the slurry was refluxed for another 24 h. Modified silica (Sil-UIm-C18, Fig. 1) was obtained by filtration, subsequent washing with proper solvents and dried at 80 ◦ C under vacuum. 2.3. Apparatus The carbon, hydrogen and nitrogen contents of the silica support were determined by elemental analysis using a Vario EL III elemental analyzer (Hanau, Germany). Wettability tests of the stationary phases were carried out on a Dataphysics OCA 20 contact angle measuring and contour analysis unit (Filderstadt, Germany), the contact angle of the silica sample was measured twice to give average value. The infrared spectra were collected on a Brucker IFS120HR Fourier-transform spectrometer (Ettlingen, Germany). The stationary phase was suspended in 1,4-dioxane and slurrypacked into stainless steel column (150 × 4.6 mm I.D.) using MeOH as propellant solvent at a liquid pressure of 60 MPa for 15 min. All the HPLC tests were run on a Shimadzu Essentia system (Kyoto, Japan) composed of LC-15C binary pumps, SPD-15C UV detector, CTO-15C column oven and a Rheodyne 7725i injector with 20 L sample loop (Cotati, CA, USA). The column was evaluated at 30 ◦ C with flow rate of 1 mL min−1 , the UV detection wavelength was 254 nm. MeOH and water were filtered through 0.45 m membrane and ultrasonically degassed prior to use. The analytes were dissolved in MeCN and stored in refrigerator. A Shimadzu ˚ pore WondaSil C18-WR column (diameter: 5 m, pore size: 100 A, volume: 1.05 mL g−1 , surface area: 450 m2 g−1 , carbon loading: 14%, end-capped, 150 mm × 4.6 mm I.D.) and another SCIL column, Sil-E˚ surface area: 400 m2 g−1 , ImC18 (diameter: 5 m, pore size: 100 A„ carbon loading: 9%, octadecylimidazolium density: 0.53 mol m2 , 150 mm × 4.6 mm I.D.) [27], were used as reference columns. The
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dead time of each column was determined by the signal of pure water. The LSER study was performed by the LINEST function in Microsoft Office 2003 Suite.
3. Results and discussion 3.1. Preparation and surface chemistry of urea-functionalized octadecylimidazolium stationary phase In our previous work [26], isocyanate-functionalized silane and N-aminopropylimidazole were employed for the synthesis of a series of imidazolium-grafted spherical silica materials bearing various substituents. And this time the focus was on the octadecylimidazolium-modified material, on account of preliminary chromatographic evaluations, which exhibited its favorable column efficiency, high selectivity towards polar aromatics as well as balanced hydrophobicity/hydrophilicity. In the whole synthetic process, IL was produced homogeneously during the synthesis, eliminating any isolation procedure and sharply elevating the conversion rate of imidazole and bonding density (octadecylimidazolium density: 1.98 mol m2 ) compared to heterogeneous conditions [23]. The produced urea group was expected to undergo – stacking with analytes like amide group [28–31]. The proposed one-pot synthesis was efficient and effective for the surface modification of silica. The proposed method was also applicable to engineer hydrophilic surface using hydrophilic substituents. The column performance was recorded by eluting ethylbenzene and naphthalene in mobile phase composed of 80% MeOH in water. The column pressure was 58 bar. The theoretical plate number for each solute was 32000 and 31500, respectively. The peak tailing factor was 1.04 and 1.11, respectively.
3.2. Hydrophobic and aromatic selectivity In consideration of the plural moieties capable of – interaction within the IL ligand, the difference in hydrophobic (methylene) and aromatic selectivities of Sil-UIm-C18 was expected to be evident, which was verified by separation of n-alkylbenzenes and condensed-ring PAHs. In view of the additional urea group contained in the entire IL ligand of Sil-UIm-C18, the reference imidazolium stationary phase, Sil-E-C18, which appeared in our earlier work containing only octadecylimidazolium cores and some unreacted chloropropyl chains, as well as commercial C18 stationary phase were chosen to perceive the selectivity difference emanating from the various embedded polar groups capable of – stacking. It would be better to mention that Sil-E-C18 showed preferential selectivity towards condensed-ring PAHs, because of the – interaction between PAH solute and imidazolium. From Fig. 2, it could be seen Sil-UIm-C18 exhibited excellent methylene selectivity and enhanced aromatic selectivity, the slope for condensed-ring PAHs exceeded that for alkylbenzenes to a much greater extent than SilE-C18, while on C18 alkylbenzenes had a greater slope (an inversion between slopes). Baseline separations of seven alkylbenzenes and isomeric butylbenzenes were given in Fig. 3. Compared to alkylbenzenes, the selectivity towards condensed-ring PAHs increased notably with the increase of -electron density. The comparison between two probe sets suggested that for a given log P value (lipophilicity), retention was exceptionally enhanced by – interactions. The contribution of urea group to – interaction was visualized in Fig. 2A, as Sil-UIm-C18 displayed steeper slopes for the two sets of PAHs than those in Fig. 2B and 2C. This discrepancy could be deemed an evidence for the additional electronic interactions
Fig. 2. Log k vs. log P plots for Sil-UIm-C18 (A), Sil-E-ImC18 (B) and C18 (C). Solutes: benzene (1), ethylbenzene (2), n-butylbenzene (3), n-hexylbenzene (4), n-octylbenzene (5), n-decylbenzene (6), n-dodecylbenzene (7), naphthalene (9), anthracene (10), tetracene (11).
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Table 1 Retention factors (k) and selectivity factors (˛) for PAH isomers on Sil-UIm-C18, Sil-E-ImC18 and C18 a Solute
cis-Stilbene trans-Stilbene Phenanthrene Anthracene o-Terphenyl Diphenylmethane Fluorene o-Terphenyl m-Terphenyl p-Terphenyl Triphenylene Chrysene Tetracene a
Sil-UIm-C18
Sil-E-ImC18
WondaSil C18
k
˛
k
˛
k
˛
3.07 5.10 5.74 6.08 3.98 2.63 4.16 3.98 8.13 9.03 15.49 16.28 19.17
(1) 1.66 1.13 1.06 0.65 (1) 1.58 (1) 2.04 1.11 1.72 1.05 1.18
2.15 3.23 3.59 3.77 4.56 2.10 2.42 4.56 5.24 5.35 5.83 6.06 6.79
(1) 1.50 1.11 1.05 1.21 (1) 1.15 (1) 1.15 1.02 1.09 1.04 1.12
6.89 7.21 6.21 6.76 10.21 5.26 6.02 10.21 12.52 14.54 13.56 13.56 16.05
(1) 1.05 0.86 1.09 1.51 (1) 1.14 (1) 1.23 1.16 0.93 1.00 1.18
Mobile phase: MeOH/water = 80/20 (v/v).
Anthracene was retained more than o-terphenyl on Sil-UIm-C18. This retention inversion was identical to that observed between benzo[a]pyrene (BaP) and tetrabenzonaphthalene (TBN), the linear anthracene was more retained on polymeric phase, non-planar o-terphenyl on monomeric one [35]. The planarity recognition was further tested by trans/cis-stilbene/phenanthrene and diphenylmethane/fluorene. Greater selectivity factor was obtained for each pair on Sil-UIm-C18, and again different elution orders of trans/cisstilbene/phenanthrene were observed on both stationary phases. The results indicated the preferential retention of the more planar solute. The linearity recognition was evaluated by phenanthrene/anthracene and triphenylene/chrysene/tetracene. Sil-UIm-C18 could discriminate these isomers to an almost equal extent to C18, and there was once again an inversion of elution order of p-terphenyl/triphenylene, which could be ascribed to the enhanced retention of triphenylene (log P = 5.49) on SilUIm-C18 due to its high -electron density, while the retention of p-terphenyl (log P = 6.03) was conclusively governed by its hydrophobicity on C18. This dissimilarity between two columns mirrored the superior – interaction took place on Sil-UIm-C18. Comparative chromatograms of linear and planar PAHs were given in Fig. 4. Fig. 3. Separation of alkylbenzenes (A) containing toluene (1), ethylbenzene (2), iso-propylbenzene (3), tert-butylbenzene (4), n-butylbenzene (5), n-pentylbenzene (6) and n-hexylbenzene (7); of isomeric butylbenzenes (B) containing tertbutylbenzene (1), sec-butylbenzene (2), iso-butylbenzene (3) and n-butylbenzene (4) on Sil-UIm-C18. Mobile phase: MeOH/water = 70/30 (v/v).
involving urea group, which would be cross-verified in the LSER analysis (See later). 3.3. Separation of PAHs In view of the typical RP behavior and enhanced aromatic selectivity, the shape selectivity of Sil-UIm-C18 was evaluated by PAHs including linear, planar and trans/cis isomers. Selectivity factors in Table 1 demonstrated that Sil-UIm-C18 offered different selectivity from C18, and Sil-UIm-C18 was more capable of distinguishing planar isomer than C18. The planar selectivity towards isomeric terphenyls was remarkably enhanced, where selectivity factor as high as 3.90 was obtained for oterphenyl/triphenylene, which was only 1.42 on C18. According to literature, such high selectivity factor implied significant solute planarity recognition (˛ ≥ 2) [32,33] and “polymeric-like” nature of the stationary phase (˛ > 3) [34]. Furthermore, different elution orders of anthracene/o-terphenyl were observed on Sil-UIm-C18 and C18.
3.4. Separation of polar aromatics To assert the influence of the multiple polar groups contained in the SCIL ligand on the separation process, positional isomers of aromatic derivatives with amino and hydroxyl groups were selected. The data for different isomers on Sil-UIm-C18, Sil-E-ImC18 and C18 were listed in Table 2. Based on the retention factors, it could be observed that Sil-UIm-C18 showed higher affinity for those solutes with electron-donating groups, especially hydroxyl group and favored the retention of naphthalene derivatives. The higher affinity for phenolic compounds was attributed to the ion-dipole interaction between imidazolium and polar-substituted benzene [36] and presumably the proton-accepting nature of urea group (described later in LSER analysis). Separation of several phenols is offered in Fig. 5. Compared to more hydrophobic o-tert-butyphenol, naphthalene derivatives were more retained on Sil-UIm-C18, symbolizing the predominant – interaction beween naphthyl ring and imidazolium/carbonyl group. The selectivity factor higher or comparable to that of C18 indicated Sil-UIm-C18 was able to differentiate the positional isomers. It was revealed by the factors below 1 that selectivity changes occurred for some solute pairs, defying the rule of hydrophobicity. The difference of dipolarity was unable to explain the different
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Fig. 4. Separation of PAHs containing benzene (1), naphthalene (2), biphenyl (3), o-terphenyl (4), phenanthrene (5), anthracene (6), m-terphenyl (7), pterphenyl (8), triphenylene (9), chrysene (10) and tetracene (11). Mobile phase: MeOH/water = 80/20 (v/v). *: impurity from tetracene.
elution orders of o-, m-, p-chloroaniline and nitroaniline on SilUIm-C18, which probably suggested the subtle variance of chlorine and nitro group on the retention, for the former was intuitively electron-withdrawing but electron-donating through resonance, the latter was purely electron-withdrawing. Another change was among hydroquinone and phloroglucinol, the latter was preferentially retained on Sil-UIm-C18. The reason could be the superior hydrogen bonding interaction between the triple hydroxyl groups and the stationary phase over hydrophobic interaction.
Table 2 Retention factors (k) and selectivity factors (˛) for polar aromatics on Sil-UImC18,Sil-E-ImC18 and C18. Sil-UIm-C18
Solute
a
p-Nitroaniline m-Nitroaniline o-Nitroaniline 2-Naphthylamine a 1-Naphthylamine p-Chloroaniline a m-Chloroaniline o-Chloroaniline Hydroquinone b Resorcinol Catechol Phloroglucinol* 2-Naphthol b 1-Naphthol p-Phenylenediamine c m-Phenylenediamine o-Phenylenediamine p-Aminophenol c m-Aminophenol o-Aminophenol a b c *
Sil-E-ImC18
WondaSil C18
k
˛
k
˛
k
˛
2.22 2.08 2.45 3.54 3.66 2.17 2.43 2.11 0.63 2.15 2.49 2.38 9.55 10.79 0.08 0.58 1.14 0.65 1.70 2.13
(1) 0.93 1.18 (1) 1.03 (1) 1.12 0.87 (1) 3.41 1.16 3.78 (1) 1.13 (1) 7.31 1.98 (1) 2.62 1.25
1.78 1.70 1.94 2.53 2.55 1.66 1.79 1.99 0.34 0.62 0.63 0.51 3.77 4.15 0.13 0.43 0.74 0.45 0.86 0.94
(1) 0.95 1.14 (1) 1.01 (1) 1.08 1.11 (1) 1.82 1.02 1.50 (1) 1.10 (1) 3.31 1.73 (1) 1.92 1.09
0.52 0.73 1.03 1.29 1.35 1.07 1.13 1.37 0.21 0.44 0.44 0.09 2.22 2.65 0.17 0.18 0.57 0.24 0.37 0.66
(1) 1.40 1.42 (1) 1.04 (1) 1.05 1.22 (1) 2.10 1.01 0.42 (1) 1.19 (1) 1.03 3.19 (1) 1.51 1.80
Mobile phase: MeOH/water = 70/30 (v/v). Mobile phase: MeOH/water = 65/35 (v/v). Mobile phase: MeOH/water = 60/40 (v/v). using hydroquinone as reference.
Fig. 5. Separation of phenolic compounds containing p-aminophenol (1), maminophenol (2), phenol (3), m-nitrophenol (4), p-cresol (5), p-chlorophenol (6), 2-naphthol (7), o-tert-butylphenol (8) and 1-naphthol (9). Mobile phase: MeOH/water = 60/40 (v/v).
It was noteworthy that in addition to separation of aniline isomers, Sil-UIm-C18 was also capable of separating mixture set of anilines with diverse substituents [26]. 3.5. LSER analysis The above analyses reflected the existence of multiple interactions between solutes and Sil-UIm-C18. This was understandable since there were diverse functional groups residing in the ligand. As for the magnitude of individual interaction, it was necessary to apply mathematic approach to obtain insight. Herein, we resorted to LSER model, which has been extensively employed in correlating retention with fundamental solute/mobile phase and solute/stationary phase interactions on a wide range of RP stationary phases, such as conventional alkyl [37,38] and phenyl [39,40] phase, as well as some homemade phases [41–43] including SCIL phase [27,44,45]. The form of the LSER model is expressed as follows: log k = c + eE + sS + aA + bB + vV
(1)
where c is system constant, E the excess molar refraction, S solute dipolarity/polarizability, A and B the solute overall hydrogen bond donor acidity and solute hydrogen bond acceptor basicity respectively and V the McGowan characteristic volume. The coefficients c, e, s, a, b and v are characteristics of the HPLC system, i.e. a particular stationary phase with a specified composition of mobile phase, they are extracted from multiple linear regression analysis of the retention data set, each of them is a reflection of the difference of a specific interaction of the solutes between stationary phase and mobile phase. A positive value for the system coefficient indicates a more intensive interaction between solute and stationary phase; similarly a negative value signifies a more favorable interaction between solute and mobile phase.
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Table 3 Comparison among different stationary phase-mobile phase systems. Stationary phase
Sil-UIm-C18
Sil-E-ImC18 a
Nucleosil 5-C18 b
Mobile phase (MeOH/water = v/v)
70%/30 60%/40 50%/50 70%/30 60%/40 50%/50 70%/30 60%/40 50%/50
System coefficients
Statistics
c
e
s
a
b
v
R
F
SE
n
−0.44 −0.38 −0.30 −0.59 −0.19 −0.43 0.09 0.10 0.12
0.23 0.27 0.37 0.17 0.15 0.26 0.16 0.20 0.12
−0.14 −0.21 −0.25 −0.11 −0.18 −0.29 −0.32 −0.42 −0.51
0.38 0.39 0.39 −0.06 −0.04 −0.07 −0.33 −0.40 −0.44
−1.24 −1.55 −1.88 −1.08 −1.36 −1.59 −0.96 −1.31 −1.62
0.97 1.27 1.52 1.02 1.23 1.65 0.95 1.34 1.75
98.0% 99.0% 98.5% 99.1% 97.6% 98.8% 99.2% 99.4% 99.4%
135 282 187 300 164 228 343 455 502
0.035 0.032 0.048 0.025 0.054 0.046 0.040 0.040 0.050
20 20 20 20 20 20 35 35 34
R: overall correlation coefficient; F: statistic; SE: standard error; n: number of solutes. a Values obtained from Ref. [25]. b Values obtained from Ref. [44,45].
The solute set (Table S1) used in this work was the same as in our previous work [27]. To get a better concept of the intermolecular interaction offered by the new SCIL stationary phase/methanolic mobile phase system, the system coefficients for Sil-UIm-C18 were juxtaposed with those for Sil-E-ImC18 and a set of data for another commercial C18 column taken from literature [46,47] (Table 3). The suitability of LSER model for chemical interpretation of Sil-UIm-C18 was justified by experimental log k (log k (exp) ) versus calculated log k (log k (calc) ) plot (Fig. 6) in all mobile phase compositions. None of the solutes appeared to be outliers and the correlation coefficient was high (R ≥ 0.99). From Table 3, we could observe that Sil-UIm-C18 displayed typical RP features like the alkyl-bonded phase, as reflected by the positive r and v and negative s and b [39,40], but with peculiar character in the aspect of a parameter. The v parameter was an indication of the chromatographic system’s tendency to interact with solute through hydrophobic interaction. A positive v value meant a stronger interaction between solute and stationary phase than mobile phase. Due to the long aliphatic chain, Sil-UIm-C18 was more hydrophobic than mobile phase, but less hydrophobic than pure C18-bonded phase, resulting from the multiple hydrophilic moieties (imidazolium and urea). And the magnitude of v increased with the decrease of methanol content (˚methanol ) in mobile phase, since methanol was more hydrophobic than water.
Fig. 6. Log k (exp) versus log k (calc) plots for Sil-UIm-C18.
The r parameter was a reflection of the system’s tendency to interact with solute through – interaction. Sil-UIm-C18 had the largest r parameter, signifying its highest capability of – stacking and behavior similar to phenyl-bonded phase [39,40]. This behavior can be attributed to the -electron of imidazolium [45] and urea group (carbonyl). The difference of r values between Sil-UImC18 and Sil-E-ImC18 could be attributed to the dissimilarity of their ligand, which would confirm that urea group facilitated the – interaction. When ˚methanol increased, the magnitude of r decreased. Because of the lower refractive index of methanol than water, rmobile phase increased slightly as ˚water increased. However, rstationary phase must increase faster than rmobile phase so that an elevated r was observed. The preferential sorption of methanol onto the solvated stationary phase and desorption of water when ˚methanol increased could explain this variation [39]. The s parameter highlighted the difference in dipolarity/polarizability of stationary phase and mobile phase. The negative s values for all the stationary phase were obtained, indicating the higher dipolarity/polarizability of mobile phase than that of stationary phase, this was reasonable since methanol and water were much more polar than the stationary phases. Uplift of ˚water increased smobile phase , thus the overall s value became even negative. However, SCIL stationary phases had a less negative s values than C18, due to the presence of imidazolium of high polarizability. It could be foretold that an increase in dipolarity/polarizability of solute would decrease its retention less on SCIL stationary phases, compared with C18. The a parameter was related to the difference of hydrogenbonding acceptor properties of stationary phase and mobile phase. Interestingly, Sil-UIm-C18 exhibited positive a values in all mobile phase compositions, similar to the cases of diol, amine and cyano columns characterized in NP mode [48]. This would probably indicate the hydrogen-bonding acceptance ability of the additional urea group. Moreover, it was already clear that Sil-UIm-C18 showed higher affinity for phenols, which could be seen as hydrogenbonding donors. The change of a parameter for Sil-UIm-C18 was insensitive to mobile phase compositions. This suggested that astationary phase was strongly correlated with amobile phase [49]. When ˚methanol decreased, amobile phase would decrease too, astationary phase must decrease by to the same degree to maintain a almost constant. This was consistent with a decrease in sorption of methanol in the stationary phase as ˚methanol declined [50]. The b coefficient represents the hydrogen bond acidity of the chromatographic system, vis. the capability of SP and MP to donate a proton to solute. According to literature [51], imidazolium showed little hydrogen bond acidity, which prevented it from participating in hydrogen-bonding donation with the solute. Hydrogen-bonding donation was essentially the contribution of residual surface silanols. A more negative b meant either intensified
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proton donation from MP or weakened donation from SP. For all the SCIL stationary phase, the b values were similar to or more negative than that for C18 (endcapped). It was believable that Sil-UIm-C18 had an effective coverage of silanol. Another reason was the effect of imidazolium on reducing the silanol activity (SA) [52]. To be more precise, the individual case of aniline and phenol were examined in 60% MeOH, which were test compounds in the following equation of Galushko test [53]: SAG = 1 + 3[(kaniline /kphenol )-1]. SAG for Sil-UIm-C18 was calculated as -0.46, as aniline was eluted before phenol, suggesting that the residual silanols were well shielded and they could not completely interact with the amine. Under the same conditions, the commercial C18 columns showed positive SAG and usually reversed elution order [54]. In all the investigated mobile phase compositions, b value tended to be more negative with the increase of ˚water , owing to the strongest hydrogen-bonding donating ability of water among all the solvents. In conclusion, Sil-UIm-C18 could provide typical RP retention mechanisms with distinctive characteristics, such as its behavior similar to phenyl-bonded stationary phases in strengthened – interaction, effective silanol shielding and drastically enhanced hydrogen-bonding acceptance ability. 4. Conclusions A novel surface-confined octadecylimidazolium silica stationary phase bearing urea functional group was prepared based on onepot synthetic strategy employing ␥-isocyanatopropyltriethoxysilane, N-(3-aminopropyl)imidazole and 1-bromooctadecane. The resulted hydrophobic material was rich in polar groups, enabling its participation in multiple interactions with different solutes. Towards PAH isomers, Sil-UIm-C18 demonstrated “polymeric-C18” behavior and enhanced planar selectivity. Towards polar aromatics, such as phenols, Sil-UIm-C18 offered unique selectivity and intensified retention compared to commercial C18 column. The merits of urea group and imidazolium cation were elucidated by LSER analysis. Predominantly, the – stacking (imidazolium cation and urea) and hydrogen-bonding acceptor nature (urea) of Sil-UIm-C18 were quite beneficial for separations of solutes of high -electron density and of hydrogen-bonding donor nature, respectively. Acknowledgments The authors express their thanks to the support of the “Hundred Talents Program” of Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chroma. 2014.09.018. References [1] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2084. [2] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley Online Library, 2008. [3] H. Olivier-Bourbigou, L. Magna, Ionic liquids: perspectives for organic and catalytic reactions, J. Mol. Catal. A: Chem. 182 (2002) 419–437. [4] M. Tunckol, J. Durand, P. Serp, Carbon nanomaterial–ionic liquid hybrids, Carbon 50 (2012) 4303–4334. [5] S.G. Lee, Functionalized imidazolium salts for task-specific ionic liquids and their applications, Chem. Commun. (2006) 1049–1063. [6] R. Giernoth, Task-specific ionic liquids, Angew. Chem. Int. Ed. 49 (2010) 2834–2839. [7] J. Liu, G. Jiang, J. Liu, J.Å. Jönsson, Application of ionic liquids in analytical chemistry, Trends Anal. Chem. 24 (2005) 20–27. [8] P. Sun, D.W. Armstrong, Ionic liquids in analytical chemistry, Anal. Chim. Acta 661 (2010) 1–16.
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A novel urea-functionalized surface-confined octadecylimidazolium ionic liquid silica stationary phase for reversed-phase liquid chromatography夽 Mingliang Zhang a,b , Ting Tan c , Zhan Li a , Tongnian Gu a , Jia Chen a , Hongdeng Qiu a,∗ a Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, China
a r t i c l e
i n f o
Article history: Received 30 April 2014 Received in revised form 3 August 2014 Accepted 7 September 2014 Available online xxx Keywords: Surface-confined ionic liquid Octadecylimidazolium Urea group Linear solvation energy relationships
a b s t r a c t One-pot synthesis of surface-confined ionic liquid functionalized silica spheres was proposed using N-(3aminopropyl)imidazole, ␥-isopropyltriethoxysilane and 1-bromooctadecane as starting materials. The surface modification of the silica spheres was successful with a high surface density of octadecylimidazolium, enabling the utilization of this new urea-functionalized ionic liquid-grafted silica material as stationary phase for high-performance liquid chromatography in reversed-phase mode. The long aliphatic chain combined with the multiple polar group embedded in the ligands imparted the new stationary phase fine selectivity towards PAH isomers and polar aromatics and higher affinity for phenolic compounds. The unique features of the new material, especially the effect of the urea group on the retention were elucidated by mathematic modeling. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) have been widely studied in many disciplines since their discovery in 1914. As an important member in IL family, the imidazolium cation-based ILs are receiving an avalanche of attention as novel media in organic synthesis [1,2], catalysis [3], nanomaterial etc. [4]. Various 1,3-dialkylimidazolium salts containing a great diversity of anions, such as PF6 − , BF4 − , CF3 SO3 − and NTf2 − , have been synthesized. Due to their negligible vapor pressure and non-flammability at room temperature, they are regarded as eco-friendly alternatives to traditional organic solvent. Additionally, their hydrophobicity/hydrophilicity and miscibility can be modified by changing the alkyl substituents and/or anions. Therefore, tailor-made ILs can be designed to fulfill specific demands. Many efforts have been made to explore the functionalized ILs, i.e. “task-specific” ILs [5,6]. The beneficial characteristics of ILs also facilitate their successful application in analytical chemistry [7–10], for example, they are outstanding alternatives to conventional extraction
夽 Invited paper for the Honor Issue of Professor Peichang Lu’s 90th birthday. ∗ Corresponding author. Tel.: +86 931 4968877; fax: +86 931 8277088. E-mail address: [email protected] (H. Qiu).
solvents in many sample preparation techniques [11–13], as well as remarkable application as stationary phases for gas chromatography due to their superior thermostability and capability of multiple intermolecular interactions [14–16]. In HPLC, they have been used as mobile additives in place of organic amine or other amino quenchers, in order to mitigate the negative effect of free silanol groups on separation of basic solutes [17,18]. ILs have also been used for surface modification of silica, immobilizing them to silica surface and forming surface-confined ionic liquid (SCIL) [19–21]. It must be highlighted that the increasing utilization of SCILs has been the topic of recent review articles [12,22]. As mentioned in these literatures, the tunability of ILs by employing different substituents and anions allows the application of SCIL stationary phases in various HPLC modes, such as reversed-phase (RP), normal-phase (NP), ion-exchange (IE) and hydrophilic interaction (HILIC) mode, for separation of a wide range of substances. Up to date, the synthetic strategies for SCIL stationary phases could be categorized into monomeric and polymeric approaches [22,23]. The existing monomeric approach requires 1-alkylimidazole and ␥-haloalkyltrialkoxysilane. The reaction between them leads to an “IL-silane”, which is then bonded to silica. Or the IL can be heterogeneously generated in-situ on ␥haloalkylated silica. The monomeric approach is convenient and effective; however, the bonding amount is usually not high due to
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Fig. 1. Illustration of the structures of the new and referenced SCIL stationary phases.
the heterogeneity of the reaction medium. The existing polymeric approach requires ␥-mercaptopropyltrialkoxysilane and alkenylcontaining imidazolium salts, which are “task-specific” in view of their structures. Silica is firstly modified by the silane, and then alkenyl-containing imidazolium salts are polymerized on the silica via catalyzed thiol-ene click reaction. This approach can guarantee a much higher bonding density and enhanced column stability than monomeric one, yet it is a must that the imidazolium salts have alkenyl group. The effect of steric hindrance is significant in both immobilization procedures, bulky substituents or anions frequently result in lower bonding amount [24,25]. With an aim to simplify the preparation work by avoiding the synthesis of 1-octadecyllimidazole or alkenyl-functionalized octadecylimidazolium salts and to enlarge the scope of useful ILs in HPLC stationary phase, we adopted a strategy proposed in our recent work [26] for convenient and efficient immobilization of “task-specific” ILs onto silica sphere, and the recommendable performance of the resulted multifunctional materials was illustrated when used as SCIL stationary phases in RP mode. To obtain more insights into the behavior of SCIL stationary phase, the influences of the functional group (urea) and substituent within the IL on the chromatographic process was investigated by eluting multitudinous analytes and comparatively studied with another octadecylimidazolium stationary phase and commercial C18 stationary phase in the framework of linear solvation energy relationships (LSER) model. 2. Experimental 2.1. Reagents and materials 1-Bromooctadecane (99%) and ␥-isocyanatopropyltriethoxysilane (ICPTES) (98%) were purchased from Sun Chemical Technology Co., Ltd (Shanghai, China). N-(3-Aminopropyl)imidazole (APIm) (98%) were obtained from Alfa Aesar (Tianjin, China). Biphenyl, anilines and phenols of analytical grade were obtained from Sinopharm Chemical Reagents Co., Ltd (Shanghai, China). Polycyclic aromatic hydrocarbons (PAHs), including Triphenylene, o- and m-terphenyl, tetracene and n-alkylbenzenes (n-hexylbenzene, noctylbenzene, n-decylbenzene and n-dodecylbenzene) of analytical standard were supplied by J&K Chemical (Beijing, China). Phenanthrene, anthracene, chrysene, p-terphenyl and p-quaterphenyl of analytical grade were purchased from Aladdin Industrial Inc. (Shanghai, China). Ultrapure water (18.3 M at 25 ◦ C) was produced by a Milipore Direct Q 3UV unit; acetonitrile (MeCN), methanol (MeOH), ethanol (EtOH) and tetrahydrofuran (THF) of HPLC grade were used. Solutes for LSER analysis and other solvents for synthesis of analytical grade or better
were obtained from different origins. 1-n-Alkylnaphthalene (1n-hexylnaphthalene, 1-n-octylnaphthalene, 1-n-decylnaphthalene and 1-n-dodecylnaphthalene) and spherical porous silica (diame˚ surface area: 400 m2 g−1 ) were supplied ter: 5 m, pore size: 90 A, by Lanzhou Institute of Chemical Physics (Lanzhou, China). 2.2. Preparation of stationary phase ˚ and MeCN of HPLC grade was dried by molecular sieve (3 A) distilled prior to use. Upon each addition of material, the solvent was purged by nitrogen. The whole reaction was carried out in an atmosphere of nitrogen. The synthesis was carried out according to Ref. [26]. Briefly, equimolar ICPTES was added dropwise to a MeCN solution of APIm cooled at 0 ◦ C, and then the solution was stirred at room temperature overnight. Secondly, slightly excessive 1-bromooctadecane was added and refluxed for 48 h. Finally, Silica spheres was added under mechanical stirring and the slurry was refluxed for another 24 h. Modified silica (Sil-UIm-C18, Fig. 1) was obtained by filtration, subsequent washing with proper solvents and dried at 80 ◦ C under vacuum. 2.3. Apparatus The carbon, hydrogen and nitrogen contents of the silica support were determined by elemental analysis using a Vario EL III elemental analyzer (Hanau, Germany). Wettability tests of the stationary phases were carried out on a Dataphysics OCA 20 contact angle measuring and contour analysis unit (Filderstadt, Germany), the contact angle of the silica sample was measured twice to give average value. The infrared spectra were collected on a Brucker IFS120HR Fourier-transform spectrometer (Ettlingen, Germany). The stationary phase was suspended in 1,4-dioxane and slurrypacked into stainless steel column (150 × 4.6 mm I.D.) using MeOH as propellant solvent at a liquid pressure of 60 MPa for 15 min. All the HPLC tests were run on a Shimadzu Essentia system (Kyoto, Japan) composed of LC-15C binary pumps, SPD-15C UV detector, CTO-15C column oven and a Rheodyne 7725i injector with 20 L sample loop (Cotati, CA, USA). The column was evaluated at 30 ◦ C with flow rate of 1 mL min−1 , the UV detection wavelength was 254 nm. MeOH and water were filtered through 0.45 m membrane and ultrasonically degassed prior to use. The analytes were dissolved in MeCN and stored in refrigerator. A Shimadzu ˚ pore WondaSil C18-WR column (diameter: 5 m, pore size: 100 A, volume: 1.05 mL g−1 , surface area: 450 m2 g−1 , carbon loading: 14%, end-capped, 150 mm × 4.6 mm I.D.) and another SCIL column, Sil-E˚ surface area: 400 m2 g−1 , ImC18 (diameter: 5 m, pore size: 100 A„ carbon loading: 9%, octadecylimidazolium density: 0.53 mol m2 , 150 mm × 4.6 mm I.D.) [27], were used as reference columns. The
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dead time of each column was determined by the signal of pure water. The LSER study was performed by the LINEST function in Microsoft Office 2003 Suite.
3. Results and discussion 3.1. Preparation and surface chemistry of urea-functionalized octadecylimidazolium stationary phase In our previous work [26], isocyanate-functionalized silane and N-aminopropylimidazole were employed for the synthesis of a series of imidazolium-grafted spherical silica materials bearing various substituents. And this time the focus was on the octadecylimidazolium-modified material, on account of preliminary chromatographic evaluations, which exhibited its favorable column efficiency, high selectivity towards polar aromatics as well as balanced hydrophobicity/hydrophilicity. In the whole synthetic process, IL was produced homogeneously during the synthesis, eliminating any isolation procedure and sharply elevating the conversion rate of imidazole and bonding density (octadecylimidazolium density: 1.98 mol m2 ) compared to heterogeneous conditions [23]. The produced urea group was expected to undergo – stacking with analytes like amide group [28–31]. The proposed one-pot synthesis was efficient and effective for the surface modification of silica. The proposed method was also applicable to engineer hydrophilic surface using hydrophilic substituents. The column performance was recorded by eluting ethylbenzene and naphthalene in mobile phase composed of 80% MeOH in water. The column pressure was 58 bar. The theoretical plate number for each solute was 32000 and 31500, respectively. The peak tailing factor was 1.04 and 1.11, respectively.
3.2. Hydrophobic and aromatic selectivity In consideration of the plural moieties capable of – interaction within the IL ligand, the difference in hydrophobic (methylene) and aromatic selectivities of Sil-UIm-C18 was expected to be evident, which was verified by separation of n-alkylbenzenes and condensed-ring PAHs. In view of the additional urea group contained in the entire IL ligand of Sil-UIm-C18, the reference imidazolium stationary phase, Sil-E-C18, which appeared in our earlier work containing only octadecylimidazolium cores and some unreacted chloropropyl chains, as well as commercial C18 stationary phase were chosen to perceive the selectivity difference emanating from the various embedded polar groups capable of – stacking. It would be better to mention that Sil-E-C18 showed preferential selectivity towards condensed-ring PAHs, because of the – interaction between PAH solute and imidazolium. From Fig. 2, it could be seen Sil-UIm-C18 exhibited excellent methylene selectivity and enhanced aromatic selectivity, the slope for condensed-ring PAHs exceeded that for alkylbenzenes to a much greater extent than SilE-C18, while on C18 alkylbenzenes had a greater slope (an inversion between slopes). Baseline separations of seven alkylbenzenes and isomeric butylbenzenes were given in Fig. 3. Compared to alkylbenzenes, the selectivity towards condensed-ring PAHs increased notably with the increase of -electron density. The comparison between two probe sets suggested that for a given log P value (lipophilicity), retention was exceptionally enhanced by – interactions. The contribution of urea group to – interaction was visualized in Fig. 2A, as Sil-UIm-C18 displayed steeper slopes for the two sets of PAHs than those in Fig. 2B and 2C. This discrepancy could be deemed an evidence for the additional electronic interactions
Fig. 2. Log k vs. log P plots for Sil-UIm-C18 (A), Sil-E-ImC18 (B) and C18 (C). Solutes: benzene (1), ethylbenzene (2), n-butylbenzene (3), n-hexylbenzene (4), n-octylbenzene (5), n-decylbenzene (6), n-dodecylbenzene (7), naphthalene (9), anthracene (10), tetracene (11).
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Table 1 Retention factors (k) and selectivity factors (˛) for PAH isomers on Sil-UIm-C18, Sil-E-ImC18 and C18 a Solute
cis-Stilbene trans-Stilbene Phenanthrene Anthracene o-Terphenyl Diphenylmethane Fluorene o-Terphenyl m-Terphenyl p-Terphenyl Triphenylene Chrysene Tetracene a
Sil-UIm-C18
Sil-E-ImC18
WondaSil C18
k
˛
k
˛
k
˛
3.07 5.10 5.74 6.08 3.98 2.63 4.16 3.98 8.13 9.03 15.49 16.28 19.17
(1) 1.66 1.13 1.06 0.65 (1) 1.58 (1) 2.04 1.11 1.72 1.05 1.18
2.15 3.23 3.59 3.77 4.56 2.10 2.42 4.56 5.24 5.35 5.83 6.06 6.79
(1) 1.50 1.11 1.05 1.21 (1) 1.15 (1) 1.15 1.02 1.09 1.04 1.12
6.89 7.21 6.21 6.76 10.21 5.26 6.02 10.21 12.52 14.54 13.56 13.56 16.05
(1) 1.05 0.86 1.09 1.51 (1) 1.14 (1) 1.23 1.16 0.93 1.00 1.18
Mobile phase: MeOH/water = 80/20 (v/v).
Anthracene was retained more than o-terphenyl on Sil-UIm-C18. This retention inversion was identical to that observed between benzo[a]pyrene (BaP) and tetrabenzonaphthalene (TBN), the linear anthracene was more retained on polymeric phase, non-planar o-terphenyl on monomeric one [35]. The planarity recognition was further tested by trans/cis-stilbene/phenanthrene and diphenylmethane/fluorene. Greater selectivity factor was obtained for each pair on Sil-UIm-C18, and again different elution orders of trans/cisstilbene/phenanthrene were observed on both stationary phases. The results indicated the preferential retention of the more planar solute. The linearity recognition was evaluated by phenanthrene/anthracene and triphenylene/chrysene/tetracene. Sil-UIm-C18 could discriminate these isomers to an almost equal extent to C18, and there was once again an inversion of elution order of p-terphenyl/triphenylene, which could be ascribed to the enhanced retention of triphenylene (log P = 5.49) on SilUIm-C18 due to its high -electron density, while the retention of p-terphenyl (log P = 6.03) was conclusively governed by its hydrophobicity on C18. This dissimilarity between two columns mirrored the superior – interaction took place on Sil-UIm-C18. Comparative chromatograms of linear and planar PAHs were given in Fig. 4. Fig. 3. Separation of alkylbenzenes (A) containing toluene (1), ethylbenzene (2), iso-propylbenzene (3), tert-butylbenzene (4), n-butylbenzene (5), n-pentylbenzene (6) and n-hexylbenzene (7); of isomeric butylbenzenes (B) containing tertbutylbenzene (1), sec-butylbenzene (2), iso-butylbenzene (3) and n-butylbenzene (4) on Sil-UIm-C18. Mobile phase: MeOH/water = 70/30 (v/v).
involving urea group, which would be cross-verified in the LSER analysis (See later). 3.3. Separation of PAHs In view of the typical RP behavior and enhanced aromatic selectivity, the shape selectivity of Sil-UIm-C18 was evaluated by PAHs including linear, planar and trans/cis isomers. Selectivity factors in Table 1 demonstrated that Sil-UIm-C18 offered different selectivity from C18, and Sil-UIm-C18 was more capable of distinguishing planar isomer than C18. The planar selectivity towards isomeric terphenyls was remarkably enhanced, where selectivity factor as high as 3.90 was obtained for oterphenyl/triphenylene, which was only 1.42 on C18. According to literature, such high selectivity factor implied significant solute planarity recognition (˛ ≥ 2) [32,33] and “polymeric-like” nature of the stationary phase (˛ > 3) [34]. Furthermore, different elution orders of anthracene/o-terphenyl were observed on Sil-UIm-C18 and C18.
3.4. Separation of polar aromatics To assert the influence of the multiple polar groups contained in the SCIL ligand on the separation process, positional isomers of aromatic derivatives with amino and hydroxyl groups were selected. The data for different isomers on Sil-UIm-C18, Sil-E-ImC18 and C18 were listed in Table 2. Based on the retention factors, it could be observed that Sil-UIm-C18 showed higher affinity for those solutes with electron-donating groups, especially hydroxyl group and favored the retention of naphthalene derivatives. The higher affinity for phenolic compounds was attributed to the ion-dipole interaction between imidazolium and polar-substituted benzene [36] and presumably the proton-accepting nature of urea group (described later in LSER analysis). Separation of several phenols is offered in Fig. 5. Compared to more hydrophobic o-tert-butyphenol, naphthalene derivatives were more retained on Sil-UIm-C18, symbolizing the predominant – interaction beween naphthyl ring and imidazolium/carbonyl group. The selectivity factor higher or comparable to that of C18 indicated Sil-UIm-C18 was able to differentiate the positional isomers. It was revealed by the factors below 1 that selectivity changes occurred for some solute pairs, defying the rule of hydrophobicity. The difference of dipolarity was unable to explain the different
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Fig. 4. Separation of PAHs containing benzene (1), naphthalene (2), biphenyl (3), o-terphenyl (4), phenanthrene (5), anthracene (6), m-terphenyl (7), pterphenyl (8), triphenylene (9), chrysene (10) and tetracene (11). Mobile phase: MeOH/water = 80/20 (v/v). *: impurity from tetracene.
elution orders of o-, m-, p-chloroaniline and nitroaniline on SilUIm-C18, which probably suggested the subtle variance of chlorine and nitro group on the retention, for the former was intuitively electron-withdrawing but electron-donating through resonance, the latter was purely electron-withdrawing. Another change was among hydroquinone and phloroglucinol, the latter was preferentially retained on Sil-UIm-C18. The reason could be the superior hydrogen bonding interaction between the triple hydroxyl groups and the stationary phase over hydrophobic interaction.
Table 2 Retention factors (k) and selectivity factors (˛) for polar aromatics on Sil-UImC18,Sil-E-ImC18 and C18. Sil-UIm-C18
Solute
a
p-Nitroaniline m-Nitroaniline o-Nitroaniline 2-Naphthylamine a 1-Naphthylamine p-Chloroaniline a m-Chloroaniline o-Chloroaniline Hydroquinone b Resorcinol Catechol Phloroglucinol* 2-Naphthol b 1-Naphthol p-Phenylenediamine c m-Phenylenediamine o-Phenylenediamine p-Aminophenol c m-Aminophenol o-Aminophenol a b c *
Sil-E-ImC18
WondaSil C18
k
˛
k
˛
k
˛
2.22 2.08 2.45 3.54 3.66 2.17 2.43 2.11 0.63 2.15 2.49 2.38 9.55 10.79 0.08 0.58 1.14 0.65 1.70 2.13
(1) 0.93 1.18 (1) 1.03 (1) 1.12 0.87 (1) 3.41 1.16 3.78 (1) 1.13 (1) 7.31 1.98 (1) 2.62 1.25
1.78 1.70 1.94 2.53 2.55 1.66 1.79 1.99 0.34 0.62 0.63 0.51 3.77 4.15 0.13 0.43 0.74 0.45 0.86 0.94
(1) 0.95 1.14 (1) 1.01 (1) 1.08 1.11 (1) 1.82 1.02 1.50 (1) 1.10 (1) 3.31 1.73 (1) 1.92 1.09
0.52 0.73 1.03 1.29 1.35 1.07 1.13 1.37 0.21 0.44 0.44 0.09 2.22 2.65 0.17 0.18 0.57 0.24 0.37 0.66
(1) 1.40 1.42 (1) 1.04 (1) 1.05 1.22 (1) 2.10 1.01 0.42 (1) 1.19 (1) 1.03 3.19 (1) 1.51 1.80
Mobile phase: MeOH/water = 70/30 (v/v). Mobile phase: MeOH/water = 65/35 (v/v). Mobile phase: MeOH/water = 60/40 (v/v). using hydroquinone as reference.
Fig. 5. Separation of phenolic compounds containing p-aminophenol (1), maminophenol (2), phenol (3), m-nitrophenol (4), p-cresol (5), p-chlorophenol (6), 2-naphthol (7), o-tert-butylphenol (8) and 1-naphthol (9). Mobile phase: MeOH/water = 60/40 (v/v).
It was noteworthy that in addition to separation of aniline isomers, Sil-UIm-C18 was also capable of separating mixture set of anilines with diverse substituents [26]. 3.5. LSER analysis The above analyses reflected the existence of multiple interactions between solutes and Sil-UIm-C18. This was understandable since there were diverse functional groups residing in the ligand. As for the magnitude of individual interaction, it was necessary to apply mathematic approach to obtain insight. Herein, we resorted to LSER model, which has been extensively employed in correlating retention with fundamental solute/mobile phase and solute/stationary phase interactions on a wide range of RP stationary phases, such as conventional alkyl [37,38] and phenyl [39,40] phase, as well as some homemade phases [41–43] including SCIL phase [27,44,45]. The form of the LSER model is expressed as follows: log k = c + eE + sS + aA + bB + vV
(1)
where c is system constant, E the excess molar refraction, S solute dipolarity/polarizability, A and B the solute overall hydrogen bond donor acidity and solute hydrogen bond acceptor basicity respectively and V the McGowan characteristic volume. The coefficients c, e, s, a, b and v are characteristics of the HPLC system, i.e. a particular stationary phase with a specified composition of mobile phase, they are extracted from multiple linear regression analysis of the retention data set, each of them is a reflection of the difference of a specific interaction of the solutes between stationary phase and mobile phase. A positive value for the system coefficient indicates a more intensive interaction between solute and stationary phase; similarly a negative value signifies a more favorable interaction between solute and mobile phase.
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Table 3 Comparison among different stationary phase-mobile phase systems. Stationary phase
Sil-UIm-C18
Sil-E-ImC18 a
Nucleosil 5-C18 b
Mobile phase (MeOH/water = v/v)
70%/30 60%/40 50%/50 70%/30 60%/40 50%/50 70%/30 60%/40 50%/50
System coefficients
Statistics
c
e
s
a
b
v
R
F
SE
n
−0.44 −0.38 −0.30 −0.59 −0.19 −0.43 0.09 0.10 0.12
0.23 0.27 0.37 0.17 0.15 0.26 0.16 0.20 0.12
−0.14 −0.21 −0.25 −0.11 −0.18 −0.29 −0.32 −0.42 −0.51
0.38 0.39 0.39 −0.06 −0.04 −0.07 −0.33 −0.40 −0.44
−1.24 −1.55 −1.88 −1.08 −1.36 −1.59 −0.96 −1.31 −1.62
0.97 1.27 1.52 1.02 1.23 1.65 0.95 1.34 1.75
98.0% 99.0% 98.5% 99.1% 97.6% 98.8% 99.2% 99.4% 99.4%
135 282 187 300 164 228 343 455 502
0.035 0.032 0.048 0.025 0.054 0.046 0.040 0.040 0.050
20 20 20 20 20 20 35 35 34
R: overall correlation coefficient; F: statistic; SE: standard error; n: number of solutes. a Values obtained from Ref. [25]. b Values obtained from Ref. [44,45].
The solute set (Table S1) used in this work was the same as in our previous work [27]. To get a better concept of the intermolecular interaction offered by the new SCIL stationary phase/methanolic mobile phase system, the system coefficients for Sil-UIm-C18 were juxtaposed with those for Sil-E-ImC18 and a set of data for another commercial C18 column taken from literature [46,47] (Table 3). The suitability of LSER model for chemical interpretation of Sil-UIm-C18 was justified by experimental log k (log k (exp) ) versus calculated log k (log k (calc) ) plot (Fig. 6) in all mobile phase compositions. None of the solutes appeared to be outliers and the correlation coefficient was high (R ≥ 0.99). From Table 3, we could observe that Sil-UIm-C18 displayed typical RP features like the alkyl-bonded phase, as reflected by the positive r and v and negative s and b [39,40], but with peculiar character in the aspect of a parameter. The v parameter was an indication of the chromatographic system’s tendency to interact with solute through hydrophobic interaction. A positive v value meant a stronger interaction between solute and stationary phase than mobile phase. Due to the long aliphatic chain, Sil-UIm-C18 was more hydrophobic than mobile phase, but less hydrophobic than pure C18-bonded phase, resulting from the multiple hydrophilic moieties (imidazolium and urea). And the magnitude of v increased with the decrease of methanol content (˚methanol ) in mobile phase, since methanol was more hydrophobic than water.
Fig. 6. Log k (exp) versus log k (calc) plots for Sil-UIm-C18.
The r parameter was a reflection of the system’s tendency to interact with solute through – interaction. Sil-UIm-C18 had the largest r parameter, signifying its highest capability of – stacking and behavior similar to phenyl-bonded phase [39,40]. This behavior can be attributed to the -electron of imidazolium [45] and urea group (carbonyl). The difference of r values between Sil-UImC18 and Sil-E-ImC18 could be attributed to the dissimilarity of their ligand, which would confirm that urea group facilitated the – interaction. When ˚methanol increased, the magnitude of r decreased. Because of the lower refractive index of methanol than water, rmobile phase increased slightly as ˚water increased. However, rstationary phase must increase faster than rmobile phase so that an elevated r was observed. The preferential sorption of methanol onto the solvated stationary phase and desorption of water when ˚methanol increased could explain this variation [39]. The s parameter highlighted the difference in dipolarity/polarizability of stationary phase and mobile phase. The negative s values for all the stationary phase were obtained, indicating the higher dipolarity/polarizability of mobile phase than that of stationary phase, this was reasonable since methanol and water were much more polar than the stationary phases. Uplift of ˚water increased smobile phase , thus the overall s value became even negative. However, SCIL stationary phases had a less negative s values than C18, due to the presence of imidazolium of high polarizability. It could be foretold that an increase in dipolarity/polarizability of solute would decrease its retention less on SCIL stationary phases, compared with C18. The a parameter was related to the difference of hydrogenbonding acceptor properties of stationary phase and mobile phase. Interestingly, Sil-UIm-C18 exhibited positive a values in all mobile phase compositions, similar to the cases of diol, amine and cyano columns characterized in NP mode [48]. This would probably indicate the hydrogen-bonding acceptance ability of the additional urea group. Moreover, it was already clear that Sil-UIm-C18 showed higher affinity for phenols, which could be seen as hydrogenbonding donors. The change of a parameter for Sil-UIm-C18 was insensitive to mobile phase compositions. This suggested that astationary phase was strongly correlated with amobile phase [49]. When ˚methanol decreased, amobile phase would decrease too, astationary phase must decrease by to the same degree to maintain a almost constant. This was consistent with a decrease in sorption of methanol in the stationary phase as ˚methanol declined [50]. The b coefficient represents the hydrogen bond acidity of the chromatographic system, vis. the capability of SP and MP to donate a proton to solute. According to literature [51], imidazolium showed little hydrogen bond acidity, which prevented it from participating in hydrogen-bonding donation with the solute. Hydrogen-bonding donation was essentially the contribution of residual surface silanols. A more negative b meant either intensified
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proton donation from MP or weakened donation from SP. For all the SCIL stationary phase, the b values were similar to or more negative than that for C18 (endcapped). It was believable that Sil-UIm-C18 had an effective coverage of silanol. Another reason was the effect of imidazolium on reducing the silanol activity (SA) [52]. To be more precise, the individual case of aniline and phenol were examined in 60% MeOH, which were test compounds in the following equation of Galushko test [53]: SAG = 1 + 3[(kaniline /kphenol )-1]. SAG for Sil-UIm-C18 was calculated as -0.46, as aniline was eluted before phenol, suggesting that the residual silanols were well shielded and they could not completely interact with the amine. Under the same conditions, the commercial C18 columns showed positive SAG and usually reversed elution order [54]. In all the investigated mobile phase compositions, b value tended to be more negative with the increase of ˚water , owing to the strongest hydrogen-bonding donating ability of water among all the solvents. In conclusion, Sil-UIm-C18 could provide typical RP retention mechanisms with distinctive characteristics, such as its behavior similar to phenyl-bonded stationary phases in strengthened – interaction, effective silanol shielding and drastically enhanced hydrogen-bonding acceptance ability. 4. Conclusions A novel surface-confined octadecylimidazolium silica stationary phase bearing urea functional group was prepared based on onepot synthetic strategy employing ␥-isocyanatopropyltriethoxysilane, N-(3-aminopropyl)imidazole and 1-bromooctadecane. The resulted hydrophobic material was rich in polar groups, enabling its participation in multiple interactions with different solutes. Towards PAH isomers, Sil-UIm-C18 demonstrated “polymeric-C18” behavior and enhanced planar selectivity. Towards polar aromatics, such as phenols, Sil-UIm-C18 offered unique selectivity and intensified retention compared to commercial C18 column. The merits of urea group and imidazolium cation were elucidated by LSER analysis. Predominantly, the – stacking (imidazolium cation and urea) and hydrogen-bonding acceptor nature (urea) of Sil-UIm-C18 were quite beneficial for separations of solutes of high -electron density and of hydrogen-bonding donor nature, respectively. Acknowledgments The authors express their thanks to the support of the “Hundred Talents Program” of Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chroma. 2014.09.018. References [1] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2084. [2] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley Online Library, 2008. [3] H. Olivier-Bourbigou, L. Magna, Ionic liquids: perspectives for organic and catalytic reactions, J. Mol. Catal. A: Chem. 182 (2002) 419–437. [4] M. Tunckol, J. Durand, P. Serp, Carbon nanomaterial–ionic liquid hybrids, Carbon 50 (2012) 4303–4334. [5] S.G. Lee, Functionalized imidazolium salts for task-specific ionic liquids and their applications, Chem. Commun. (2006) 1049–1063. [6] R. Giernoth, Task-specific ionic liquids, Angew. Chem. Int. Ed. 49 (2010) 2834–2839. [7] J. Liu, G. Jiang, J. Liu, J.Å. Jönsson, Application of ionic liquids in analytical chemistry, Trends Anal. Chem. 24 (2005) 20–27. [8] P. Sun, D.W. Armstrong, Ionic liquids in analytical chemistry, Anal. Chim. Acta 661 (2010) 1–16.
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