Article
Adsorption Kinetics at Silica Gel/Ionic Liquid Solution Interface Jolanta Flieger *, Małgorzata Tatarczak-Michalewska, Anna Groszek, Eliza Blicharska and Ryszard Kocjan Received: 3 September 2015 ; Accepted: 19 November 2015 ; Published: 10 December 2015 Academic Editor: Derek J. McPhee Department of Analytical Chemistry, Faculty of Pharmacy with Division of Medical Analytics, Medical University of Lublin, 4a Chod´zki St., Lublin PL-20093, Poland;
[email protected] (M.T.-M.);
[email protected] (A.G.);
[email protected] (E.B.);
[email protected] (R.K.) * Correspondence:
[email protected]; Tel./Fax: +48-81448-7180
Abstract: A series of imidazolium and pyridinium ionic liquids with different anions (Cl´ , Br´ , BF4 ´ , PF6 ´ ) has been evaluated for their adsorption activity on silica gel. Quantification of the ionic liquids has been performed by the use of RP-HPLC with organic-aqueous eluents containing an acidic buffer and a chaotropic salt. Pseudo-second order kinetic models were applied to the experimental data in order to investigate the kinetics of the adsorption process. The experimental data showed good fitting with this model, confirmed by considerably high correlation coefficients. The adsorption kinetic parameters were determined and analyzed. The relative error between the calculated and experimental amount of ionic liquid adsorbed at equilibrium was within 7%. The effect of various factors such as initial ionic liquid concentration, temperature, kind of solvent, kind of ionic liquid anion and cation on adsorption efficiency were all examined in a lab-scale study. Consequently, silica gel showed better adsorptive characteristics for imidazolium-based ionic liquids with chaotropic anions from aqueous solutions in comparison to pyridinium ionic liquids. The adsorption was found to decrease with the addition of organic solvents (methanol, acetonitrile) but it was not sensitive to the change of temperature in the range of 5–40 ˝ C. Keywords: ionic liquids; silica gel; sorption kinetics; pseudo-second-order equation
1. Introduction Ionic liquids (ILs) are a broad class of salts melting at or below 100 ˝ C. Over the last few years they have gained immense popularity in various fields of chemistry thanks to their environmentally friendly properties and the opportunities of matching their structure to a particular purpose. Initially, ionic liquids were used as reaction media for organic synthesis and biphasic catalysis primarily on industrial scale as an alternative to organic solvents [1–5]. So far different organic reactions like esterification, transesterification, nitration, and acetylation have been carried out using ionic liquids [6–13]. The high yields of all the above mentioned reactions indicate that ionic liquids possess huge potential in dedicated technologies of interest to the chemical industry. Currently increasing interest can also be observed in the use of ionic liquids on an analytical scale [14–16]. So far, ionic liquids have found a number of beneficial applications in electrochemistry [17–25] and separation techniques. There are examples of ionic liquid applications in the extraction of both ionic inorganic compounds, for instance metal cations [26], organic compounds [27] and biomolecules like peptides and proteins [28]. The leading role in the liquid-liquid extraction, even in a miniaturized version called liquid phase microextraction (LPME), is played by water-insoluble ionic liquids. In turn, the hydrophilic ionic liquids are used to create aqueous biphasic systems (ABS) in the presence of highly hydrated inorganic salts with kosmotropic (salting-out) properties. Such two phase systems Molecules 2015, 20, 22058–22068; doi:10.3390/molecules201219833
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Molecules 2015, 20, 22058–22068
are usually used for extractions, as an alternative to traditional liquid-liquid or liquid-solid partition systems. The resulting extraction system is especially suitable for the analysis of aqueous samples, and the use of the ABS technique for the extraction of hormones, alkaloids, vitamins, antibiotics from biological and environmental samples has been described [29–35]. The thermomorphic behavior of some ionic liquids allows carrying out the so-called Molecules 2015, 20, page–page Molecules 2015,20, 20,page–page page–page homogenous liquid-liquid extraction (HLLE), wherein the phase separation is induced by Molecules 2015, temperaturealternative changes.to traditional liquid-liquid or liquid-solid partition systems. The resulting extraction system alternativeto totraditional traditionalliquid-liquid liquid-liquidor orliquid-solid liquid-solidpartition partitionsystems. systems.The The resultingextraction extractionsystem system alternative is especially suitable for the analysis of aqueous samples, and the liquids use ofresulting the to ABSmodify techniqueadsorbents for the In recent years, suitable attempts have been made samples, to use and ionic by isespecially especially suitable forthe the analysis ofaqueous aqueous samples, and theuse useof ofthe theABS ABStechnique techniquefor forthe the isextraction for analysis of the of hormones, alkaloids, vitamins, antibiotics from biological and environmental samples immobilization onto silica or polymeric supports [36–39]. The resulting so-called supported ionic extraction of hormones, alkaloids, vitamins, antibiotics from biological and environmental samples extraction of hormones, alkaloids, vitamins, antibiotics from biological and environmental samples has been described [29–35]. has(SILPs) beendescribed described [29–35]. has been [29–35]. liquid phases are used as sorptive materials in allows solid-phase techniques. The thermomorphic behavior of some ionic liquids carrying extraction out the so-called homogenousThe first Molecules 2015, 20, page–page The thermomorphicbehavior behaviorof ofsome someionic ionicliquids liquidsallows allowscarrying carryingout outthe theso-called so-calledhomogenous homogenous The thermomorphic adsorbent subjected toextraction modification silica gel with immobilized liquid-liquid (HLLE),was wherein the phase separation is induced 1-butyl-3-methylimidazolium by temperature changes. liquid-liquid extraction (HLLE), wherein the phase separation is induced by temperature changes. liquid-liquid (HLLE), wherein phase separation is by temperature changes. to traditional liquid-liquid orthe liquid-solid partition systems. resulting extraction system Inalternative recentextraction years, attempts have been made to use ionic liquids toinduced modify adsorbents byaqueous immobilization hexafluorophosphate, which wasforfurther applied toionic the isolation ofThemetals from media [40]. Inrecent years, attempts have been made touse use ionic liquids tomodify modify adsorbents byimmobilization immobilization isrecent especially suitable the analysis of aqueous samples, andto the use of adsorbents the ABS technique for the In years, attempts have been made to liquids by onto silica or polymeric supports [36–39]. The resulting so-called supported ionic liquid phases (SILPs) The aim of this research is the study of the adsorption process of imidazolium and pyridinium ontosilica silica orpolymeric polymeric supports [36–39]. Theresulting resulting so-called supported ionicliquid liquidphases phases (SILPs) extraction of hormones, alkaloids, vitamins, antibiotics from biological and environmental samples onto or supports The so-called supported ionic (SILPs) are used as sorptive materials in[36–39]. solid-phase extraction techniques. The first adsorbent subjected to ´ , PF ´ ´ , BFextraction hasas been described [29–35].in are used as sorptive materials in´ solid-phase extraction techniques. The first adsorbent subjected to kind of are used sorptive materials solid-phase techniques. The first adsorbent subjected to ) on silica gel. The influence of the ionic liquids with different anions (Cl , Br 6 4 modification was silica gel behavior with immobilized 1-butyl-3-methylimidazolium hexafluorophosphate, The thermomorphic of some ionic liquids allows carrying out the so-called homogenous modification was was silica silica gel gel with with immobilized immobilized 1-butyl-3-methylimidazolium 1-butyl-3-methylimidazolium hexafluorophosphate, hexafluorophosphate, modification solvent, temperature, and the kind of anion cation onaqueous adsorption efficiencychanges. were all examined. which liquid-liquid was further applied to the isolation of from media [40]. (HLLE), whereinand themetals phase separation is induced by temperature whichwas wasfurther furtherextraction appliedto to theisolation isolation of metals fromaqueous aqueous media [40]. which applied the of metals from media [40]. The aim of this research is the study of the adsorption process of imidazolium and pyridinium In recent years, attempts have been made to use ionic liquids to modify adsorbents by immobilization The adsorptionThe mechanism of the examined ionic liquids with anions of different chaotropicity was The aimof ofthis thisresearch research thestudy study ofthe the adsorption process ofimidazolium imidazolium andpyridinium pyridinium aim isisthe adsorption process of and −, Br−of onto silica or different polymericanions supports [36–39]. The so-called supported ionic liquid phases (SILPs) ionic liquids with (Cl , BF 4−resulting , PF 6−) on silica gel. The influence of the kind of solvent, −, BF−4−, PF−6−) on silica gel. The influence of the kind of solvent, −,−,Br −model. studied with a pseudo-second-order kinetic ionic liquids with different anions (Cl Br ionic liquids with different anions (Cl , BF 4 , PF 6 ) on silica gel. The influence of the kind of solvent, are used as sorptive materials in solid-phase extraction techniques. The first adsorbent subjected to 2.
temperature, and the kind of anion and cation on adsorption efficiency were all examined. The adsorption temperature, andthe the kind ofanion anion andimmobilized cationon on adsorption efficiency wereall allexamined. examined.The Theadsorption adsorption modification was silica gel with 1-butyl-3-methylimidazolium hexafluorophosphate, temperature, and kind of and cation mechanism of the examined ionic liquids withadsorption anions ofefficiency different were chaotropicity was studied with a mechanism of the examined ionic liquids with anions ofdifferent different chaotropicity wasstudied studiedwith withaa whichof was further applied to the isolation of metals from aqueous media [40]. Results mechanism and Discussion the examined ionic liquids with anions of chaotropicity was pseudo-second-order kinetic model. The aim of this research is the study of the adsorption process of imidazolium and pyridinium pseudo-second-order kinetic model. pseudo-second-order kinetic model. ionic liquids with different anions (Cl−, Br−, BF4−, PF6−) on silica gel. The influence of the kind of solvent,
2.1. HPLC Conditions for Discussion Ionic Liquids Determination 2. Results and temperature, and the kind of anion and cation on adsorption efficiency were all examined. The adsorption Resultsand andDiscussion Discussion 2.2.Results
mechanism of the examined ionic liquids with anions of different chaotropicity was studied with a
There exist only a few papers dealing with high-performance liquid chromatography methods 2.1. HPLC Conditions for Ionic Liquids Determination pseudo-second-order kinetic model. 2.1.HPLC HPLCConditions Conditionsfor forIonic IonicLiquids LiquidsDetermination Determination 2.1. suitable for IL quantification [41–44]. Cations derived from ionic liquids can be analyzed separately in There existand only a few papers dealing with high-performance liquid chromatography methods 2. Results Discussion There exist only afew fewpapers papers dealingphases. withhigh-performance high-performancewhen liquidchromatography chromatography methods There exist only a dealing with liquid methods reversed-phase mode on different stationary using conventional octadecyl suitable for IL quantification [41–44]. Cations derivedHowever, from ionic liquids can be analyzed separately suitablefor forIL ILquantification quantification[41–44]. [41–44].Cations Cationsderived derivedfrom fromionic ionicliquids liquidscan canbe beanalyzed analyzedseparately separately suitable 2.1. HPLC Conditions for Ionic Liquids Determination in reversed-phase mode on different stationary phases. However, when using conventional octadecyl bonded phases with two component organic-aqueous mobile phases, theconventional efficiency octadecyl and separation inreversed-phase reversed-phasemode modeon ondifferent differentstationary stationaryphases. phases.However, However,when whenusing usingconventional in octadecyl bonded phases with two acomponent organic-aqueous mobile phases, efficiency and separation There exist only few papers dealing with high-performance liquid the chromatography methods selectivity tend to be poor. bonded phases with two component organic-aqueous mobile phases, the efficiency and separation bonded phases with two component organic-aqueous mobile phases, the efficiency and separation selectivity tend poor. suitable fortoILbe quantification [41–44]. Cations derived from ionic liquids can be analyzed separately selectivitytend tendto tobe bepoor. poor. selectivity in reversed-phase mode on different stationary phases. However, when using conventional octadecyl
1.Table Structures of the investigated ionicliquids. liquids. bonded phasesTable with two component organic-aqueous mobile phases, the efficiency and separation 1. Structures of the investigated ionic Table1.1.Structures Structuresof ofthe theinvestigated investigatedionic ionicliquids. liquids. Table selectivity tend to be poor. Table 1. Structures of the investigated ionic liquids.
BMIM PF6 BMIMPF PF66 BMIM
[PF6]−− [PF6]6]− [PF
[PF6 ]´
BMIM PF6
BMIM PF6
[PF6]−
BMIM Cl BMIMCl Cl BMIM
[Cl]−− [Cl]− [Cl]
´ [Cl]− [Cl]
BMIM Cl BMIM Cl
EMIM PF6 EMIMPF PF6 EMIM EMIM 6PF6
[PF6]−− [PF6]−
6] [PF 6]− [PF
[PF6 ]´
EMIM PF6
EMPyr Br EMPyr EMPyr BrBr EMPyr Br
[Br]−
− [Br] −− [Br] [Br]
[Br]´
EMPyr Br
EPyr EPyr BFBF 4 4 EPyrBF BF44 EPyr
4]− [BF 4]− [BF
[BF4]4]−− [BF
[BF4 ]´
EPyr BF4 2
2 22
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It was proved that a significant improvement of peak shape and selectivity can be achieved by addition of acidic buffers and small amounts of a chaotropic salt to the mobile phase. The investigated Molecules 2015, (Table 20, 22058–22068 ionic liquids 1) have been analyzed on a Zorbax Extend-C18 (150 mm × 4.6 mm I.D., 5 μm) column using multicomponent mobile phases. The composition of eluent systems has been chosen according to the IL cation structure (polarity). The mobile phase components together with obtained peak It was proved that a significant improvement of peak shape and selectivity can be achieved by parameters are collected in Table 2. addition of acidic buffers and small amounts of a chaotropic salt to the mobile phase. The investigated ionicTable liquids (Table 1) have analyzed on a Zorbax Extend-C18 (150 mm ˆionic 4.6 mm I.D., 2. The mobile phasebeen components suitable for HPLC analysis of appropriate liquids on 5a µm) column using multicomponent mobile phases. The composition of eluent systems has been chosen Zorbax Extend-C18 column. according to the IL cation structure (polarity). The mobile phase components together with obtained Ionic Liquid Mobile RT (min) As N (EUP) λmax k peak parameters areThe collected inPhase Table Composition 2. 15%MeOH, 30 mM phosphate buffer, 3.87 1.98 1.73 38,480 220 mM NaBFsuitable 4 Table 2. The mobile phase30 components for HPLC analysis of appropriate ionic liquids on a 15%MeOH, Zorbax Extend-C18 column.30 mM phosphate buffer, BMIM Cl 3.92 2.02 1.11 26,233 220 30 mM NaBF4 50 mM phosphate buffer, RT (min) Ionic Liquid 5%MeOH, The Mobile Phase Composition k As N (EUP) λmax EMIM PF6 3.20 1.46 1.32 12,673 220 30 mM NaPF 6 15%MeOH, 30 mM phosphate buffer, BMIM PF6 3.87 1.98 1.73 38,480 220 30 mM NaBF4 8%MeOH, 30 mM phosphate buffer, EMPyr Br 4.24 2.26 1.36 21,626 255 15%MeOH, 30 mM phosphate buffer, 30 mM NaPF6 3.92 2.02 1.11 26,233 220 BMIM Cl 30 mM NaBF4 5%MeOH, 50 mM phosphate buffer, 5%MeOH, 50 mM phosphate buffer, EPyr EMIM BF4 PF 2.61 1.461.011.32 1.34 12,673 20,300220 255 3.20 6 30 30 mM mMNaPF NaPF66
BMIM PF6
8%MeOH, 30 mM phosphate buffer, The following theoretical to255 USP 4.24 2.26 plates 1.36 (N) according 21,626 EMPyr Br equation was used to calculate the number of 30 mM RTNaPF is the6 actual full retention time of the appropriate peak, w is the standards: N = 16(RT/w)2, where 5%MeOH, 50 mM phosphate buffer, BF4obtained by drawing tangents to each side of 2.61the peak 1.01and1.34 20,300 255 peak EPyr width calculating the distance 30 mM NaPF6 between the two points where the tangents meet the baseline. The tailing factor (As) is based on the The following equation was used to calculate the number of theoretical plates (N) according to USP standards: measurement the half-width parameters A and of the peak calculated N = 16(RT/w)2of , where RT is the actual full retention time B of at the5% appropriate peak,height, w is the and peak is width obtainedas to each side of was the peak calculating the between to thethe tworecorded points where the 1/2(1 + tangents B/A). The detection set and at wavelength (λdistance max) according spectra. Asby= drawing tangents meet the baseline. The tailing factor (As ) is based on the measurement of the half-width parameters The retention factor k is expressed as: (RT − t0)/t0 where t0 is the retention time of void volume marker. A and B at 5% of the peak height, and is calculated as A = 1/2(1 + B/A). The detection was set at wavelength s
(λmax ) according to the recorded spectra. The retention factor k is expressed as: (RT ´ t0 )/t0 where t0 is the time of void volume Asretention it can be seen, there is nomarker. significant difference in the retention times between ILs differing only
in the kind anion (cf. BMIM Cl and BMIM PF6). The difference in retention times (3.92 − 3.87 = 0.04 min) As itthecan be seen,inthere is no significant difference in the retention times between ILs is within uncertainty the measurements. Therefore, in subsequent figures (Figure 1A,B), only a differing only was in the kind anion (cf. BMIM Cl and BMIM PF6 ). The difference in retention times kind of cation illustrated. (3.92 The ´ 3.87 = 0.04 min) is within in the measurements. in subsequent detection of the peaks the wasuncertainty set at an appropriate wavelength Therefore, chosen according to the figures (Figure a kind cation recorded spectra1A,B), in theonly range fromof 220 nm towas 400illustrated. nm illustrated in Figure 1B. The detection of the peaks was set at an appropriate wavelength chosen according to the recorded spectra in the range from 220 nm to 400 nm illustrated in Figure 1B.
(A) Figure 1. Cont. Figure 1. Cont.
3
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(B) Figure 1. 1. (A) of of peaks: a—EMPyr Br, b—BMIM PF6, c—EMIM PF6, d—EPyr BF4 obtained Figure (A)Comparison Comparison peaks: a—EMPyr Br, b—BMIM PF6, c—EMIM PF6, d—EPyr BF4 on a Zorbax Extend-C18 column using the mobile phases listed in Table 2; (B) UV spectra obtained on a Zorbax Extend-C18 column using the mobile phases listed in Table 2; (B) UV obtained spectra 6, EMIM PF6, EPyr BF4. for the investigated ionic liquids: BMIM obtained for the investigated ionic EMPyr liquids:Br, EMPyr Br,PF BMIM PF6 , EMIM PF6 , EPyr BF4 .
2.2.Conditions Conditionsfor forIL ILQuantification Quantification 2.2. The quantitative analysis of the examined ionic liquids was performed by the use of an external The quantitative analysis of the examined ionic liquids was performed by the use of an external standard method applying the chromatographic system described in Section 2.1. A 20 μL sample of standard method applying the chromatographic system described in Section 2.1. A 20 µL sample of each dilution was injected in triplicate. The mean peak areas were taken for the construction of the each dilution was injected in triplicate. The mean peak areas were taken for the construction of the calibration curves. The data were analyzed by a linear regression least squares model. The equation calibration curves. The data were analyzed by a linear regression least squares model. The equation parameters for the regression lines are collected in Table 3. parameters for the regression lines are collected in Table 3. Table 3. Linearity (y = ax + b), LOD, LOQ parameters for the investigated ionic liquids. Table 3. Linearity (y = ax + b), LOD, LOQ parameters for the investigated ionic liquids. Ionic Ionic Liquid Liquid BMIM PF6 BMIM PF6 BMIM Cl BMIM Cl EMIM PF6 EMIM PF6 EMPyr EMPyrBrBr EPyrBF BF EPyr 4 4
Conc. Range: a ± SD b ± SD R2 −1) Conc. Range: (µg·mL a ˘ SD b ˘ SD R2 (µg¨ mL´1 ) 6824.04 8521.04 0.5–50 0.9984 (±98.07) 6824.04 (±2735.60) 8521.04 0.5–50 0.9984 (˘98.07) 18027.91 (˘2735.60) 8110.84 2.5–50 0.9981 8110.84 (±3908.27) 18027.91 (±145.70) 2.5–50 0.9981 (˘145.70) 2386.59 (˘3908.27) 6482.15 5–50 0.9983 6482.15 2386.59 0.9983 5–50 (±108.17) (±3372.33) (˘108.17) (˘3372.33) 17059.98 10599.09 17059.98 10599.09 5–50 0.9977 5–50 0.9977 (±333.56) (˘333.56) (±10398.61) (˘10398.61) 11531.70 −4670.56 ´4670.56 11531.70 0.9993 5–50 5–50 (˘126.90) (±3956.14) (˘3956.14) 0.9993 (±126.90)
s s
5397.32 5397.32 6575.45 6575.45
LOQ LOD LOD (µg·mL−1) (µg·mL−1) LOQ n F (µg¨ mL´1 ) (µg¨ mL´1 ) 4842.17 0.0474 0.1436 4842.17 0.0474 0.1436 8 3098.93 0.0593 0.1796 3098.93 0.0593 0.1796 6 F
n 8 6
4833.93 3590.78 0.0551 4833.93 3590.78 0.0551
0.1669 0.1669 6
6
14905.46 2615.88 2615.88 0.0645 0.0645 14905.46
0.1954 6 0.1954
6
5670.77 5670.77 8257.59 8257.59 0.0363 0.0363
0.1100 0.1100 6
6
Eight or or six relationships werewere of excellent linearity, as expressed by the correlation Eight sixpoint pointcalibration calibration relationships of excellent linearity, as expressed by the 2 2 coefficientscoefficients (R ) higher than high values of F—Fisher’s limit of detection (LOD) correlation (R ) 0.9977 higherand than 0.9977 and high valuestest. of The F—Fisher’s test. The limitand of quantification (LOQ) were based on the calibration curves. The standard deviation of intercepts detection (LOD) and quantification (LOQ) were based on the calibration curves. The standardof regressionoflines was used as the standard ICH requirements, LOD can deviation intercepts of regression lines deviation was used (SD). as theAccording standard to deviation (SD). According to be calculated as 3.3LOD SD ofcan regression line/slope and as 10 SD of regression line/slope ICH requirements, be calculated as 3.3 SDLOQ of regression line/slope and LOQ as[45]. 10 SD of regression line/slope [45]. 2.3. Influence of Ionic Liquid Kind and Concentration on Adsorption Efficiency 2.3. Influence of Ionic Liquid Kind and Concentration on Adsorption Efficiency This study indicates that the absolute adsorption is higher for imidazolium ionic liquids in comparison to pyridinium onesthe (Figure 2). With increasing concentrations of ionic liquid aqueous This study indicates that absolute adsorption is higher for imidazolium ionic in liquids in −1 −1 solutions from 10 to 50 μg·mL imidazolium and from 5concentrations to 50 μg·mL for liquids comparison to pyridinium ones for (Figure 2). With increasing of pyridinium ionic liquid ionic in aqueous ´1 for imidazolium ´1 for their adsorption decreases constantly almost half of entire value. Coating the silica gel solutions from 10efficiency to 50 µg¨ mL and from 5 the to 50 µg¨ mL pyridinium ionic surface by ionic liquids ions is definitely enhanced by chaotropic anions. Summarizing, the order of ionic liquids regarding the percentage of their adsorption on silica gel increases from BMIM PF6 > EMIM 22061 4
Molecules 2015, 20, 22058–22068
liquids their adsorption efficiency decreases constantly almost half of the entire value. Coating the silica gel surface by ionic liquids ions is definitely enhanced by chaotropic anions. Summarizing, Molecules 20, page–page the order of2015, ionic liquids regarding the percentage of their adsorption on silica gel increases from BMIM PF6 > EMIM PF6 > BMIM Cl to the remaining pyridinium cations: EMPyr > EPyr. In the 6 > BMIM the remaining pyridinium cations: EMPyr > EPyr. In the case of pyridinium ionic PFMolecules 2015,Cl 20, to page–page case of pyridinium ionic liquids, the kind of anion is less significant in terms of adsorption capacity. liquids, the kind of anion is less significant in terms of adsorption capacity. Considering the fact that Considering the fact that the ionichave liquids at the beginning the imidazolium cation ionic but different 6 > BMIM Clattothe the remaining pyridinium cations: EMPyrbut >have EPyr. In the case of pyridinium PFionic the liquids beginning the imidazolium cation different anions, their adsorption ability liquids, the kind of anion is less significant in terms of adsorption capacity. Considering the fact that anions, their adsorption ability would be affected mostly by the nature of anions. would be affected mostly by the nature of anions. the Hexafluorophosphates ionic liquids at the beginning have the imidazolium but different anions, their adsorption ability Hexafluorophosphates (∆G kJ/mol)cation are characterized a more positive Gibbs (ΔG hyd== ´214 −214 kJ/mol) are characterized by abymore positive Gibbs free free hyd would be affected mostly by the nature of anions. energy of hydration of the ions (∆G ) )inincomparison (∆Ghydhyd = ´347 kJ/mol) favoring energy of hydration of the ions (ΔG comparison to chlorides chlorides (ΔG = −347 kJ/mol) favoring hydhyd Hexafluorophosphates (ΔGhyd = −214 kJ/mol) are characterized by a more positive Gibbs free electrostatic interactions aqueoussolution. solution. Furthermore, viscosity B coefficients of the electrostatic interactions in in aqueous Furthermore,ionic ionic viscosity B coefficients ofJones the Jones energy of hydration of the ions (ΔGhyd) in comparison to chlorides (ΔGhyd = −347 kJ/mol) favoring equation (more positive chlorides) differ differ significantly ififcomparing anions [46].[46]. ThusThus the the DoleDole equation (more positive forforchlorides) significantly comparing anions electrostatic interactions in aqueous solution. Furthermore, ionic viscosity B coefficients of the Jones trend for the adsorption ability of these ionic liquids is in agreement with the order of the ΔG hyd values trend for adsorption ability for of these ionic liquids is in agreement with the[46]. order ofthe the ∆Ghyd Dolethe equation (more positive chlorides) differ significantly if comparing anions Thus and viscosity the associated counterions. the of adsorption of these ionic liquids is in agreement with the order of the ΔGhyd values valuestrend and for viscosity of the ability associated counterions. and viscosity of the associated counterions.
55 55
Adsorption Adsorption (%)(%)
45 45
BMIM PF6
35
BMIM PF6
BMIM Cl
35
BMIM Cl
EMIM PF6
25
EMIM PF6
25
EMPyr Br
EMPyr Br
15
EPyr BF4
15
EPyr BF4
5
5
0
0
10
10
20
20
30
30
40
40
Concentration of IL (µg/mL)
50
50
Concentration of IL (µg/mL)
Figure 2. Influence concentrationonon adsorption efficiency. Figure 2. Influenceofofionic ionic liquid liquid concentration adsorption efficiency. Figure 2. Influence of ionic liquid concentration on adsorption efficiency.
2.4. Influence of Solvent Kind and Concentration onAdsorption Adsorption Efficiency 2.4. Influence ofofSolvent Kind and Concentration Adsorption Efficiency 2.4. Influence Solvent Kind and Concentration on Efficiency Different solvents were investigated: pure water and water mixed with organic additives Different solvents were investigated: pure and water mixed with organic additives Different solvents were investigated: purewater water and water mixed with organic additives (methanol, acetonitrile). The adsorption efficiency was the highest for pure water and decreases (methanol, acetonitrile). The adsorption efficiency was the highest for pure water and decreases (methanol, acetonitrile). The adsorption efficiency was the highest for pure water and decreases constantlyafter addition of an organic organic solvent. Generally ofof 5%5% of of organic to water constantly addition an Generallyaddition addition to water constantly afterafter addition ofofan organicsolvent. solvent. Generally addition oforganic 5% solvent ofsolvent organic solvent to causesananadsorption adsorption efficiency efficiency decrease was adopted as solvent in in causes decrease of of about about5%, 5%,sosopure purewater water was adopted as solvent water further causesexperiments. an adsorption efficiency decrease of about 5%, so pure water was adopted as solvent further experiments.
Adsorption Adsorption(%) (%)
in further experiments.
ACN
ACN
MeOH
MeOH
H2O
H2O
Concentration of organic solvent (%)
Concentration of organic solvent (%) 3. Influenceofof solvent solvent kind andand concentration on adsorption efficiency of 20 μg BMIM FigureFigure 3. Influence kind concentration on adsorption efficiency of PF 206 on µg BMIM 0.02 g3. ofInfluence silica gel. of solvent kind and concentration on adsorption efficiency of 20 μg BMIM PF6 on Figure PF6 on 0.02 g of silica gel.
0.02 g of silica gel.
5
5
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Molecules 2015, 20, 22058–22068 Molecules 2015, 20, page–page
2.5. Influence of Temperature on Adsorption Efficiency 2.5. Influence of Temperature on Adsorption Efficiency
It is common knowledge that temperature can be an important parameter influencing adsorption It is common knowledge that temperature can be an important parameter influencing adsorption processes. Here, two (BMIM BMIM Cl) were as representative 6 and processes. Here, twoimidazolium imidazolium derivatives derivatives (BMIM PFPF 6 and BMIM Cl) were used used as representative ionic liquids theeffect effectof of temperature onadsorption the adsorption effectiveness 4). ionic liquidstoto evaluate evaluate the temperature on the effectiveness (Figure 4).(Figure The Thepercentage percentage of adsorption was determined in the range from 5 tofound 90 ˝ C, and to be in of adsorption was determined in the range from 5 to 90 °C, and to be in found the range the of range of measurement for both liquids 40 ˝temperature, C. At higherlowering temperature, lowering of measurement errors for errors both liquids up to 40 °C.up At to higher of adsorption capacity was observed for ionic liquid with polyfluorinated anions indicating its possible decomposition. adsorption capacity was observed for ionic liquid with polyfluorinated anions indicating its possible Simultaneously,Simultaneously, this reflects a huge rolereflects of this anion in the adsorption process. results decomposition. this a huge role of this anion inThe theobtained adsorption process. indicate thatclearly in the indicate temperature of: 5–40 °C, therange adsorption of ionic liquids is ˝ C, the Theclearly obtained results thatininthe therange temperature in the of: 5–40 adsorption not sensitive to the temperature of the system. Therefore, the adsorption can be performed at room of ionic liquids is not sensitive to the temperature of the system. Therefore, the adsorption can be temperature, which is important in practice. performed at room temperature, which is important in practice.
Adsorption (%)
60 50 40 30
BMIMPF6
20
BMIMCl
10 0 5
10 15 20 25 30 40 50 60 70 80 90
Temperature (°C) Figure 4. 4. Influence onadsorption adsorption efficiency of BMIM PF6BMIM and BMIM Cl. Figure Influenceof oftemperature temperature on efficiency of BMIM PF6 and Cl.
Kinetics AdsorptionProcess Process 2.6. 2.6. Kinetics of of Adsorption phenomenonofofadsorption adsorption at interface plays a crucial role in processes TheThe phenomenon at the the solid/liquid solid/liquid interface plays a crucial role in processes applied on an industrial scale. The study of this phenomenon consists in analyzing the state the of the applied on an industrial scale. The study of this phenomenon consists in analyzing the of state adsorption equilibrium. Kinetic studies were conducted under optimum conditions determined in adsorption equilibrium. Kinetic studies were conducted under optimum conditions determined in the preliminary experiments (initial concentration of ionic liquid 20 μg·mL−1, solution volume 2 mL, the preliminary experiments (initial concentration of ionic liquid 20 µg¨ mL´1 , solution volume 2 mL, adsorbent mass 20 mg, temperature 25 °C). For the purpose of evaluating the effect of time on the ˝ adsorbent mass 20 mg, the temperature For thewas purpose evaluating of time on the adsorption efficiency, time range 25 fromC). 0–30 min. tested. of Figure 5 showsthe thateffect the adsorption adsorption time range tested. 5 shows that the effect adsorption efficiencyefficiency, gradually the increased up to 5from min. 0–30 In themin. regionwas from 5 to 30Figure min. a type of saturation efficiency gradually increased to 5 min. In theinregion fromwith 5 to time 30 min. type of saturation effect was observed, where no otherup significant changes adsorption wereaobserved. Description of no kinetic provides or semi-empirical equations such as was observed, where otherprocesses significant changesempirical in adsorption with time were observed. pseudo-first-order or pseudo-second-order. The pseudo-second-order equation which best fits the Description of kinetic processes provides empirical or semi-empirical equations such as experimental data has been proposed by Ho et al. [47,48] and Blanchard [49]: pseudo-first-order or pseudo-second-order. The pseudo-second-order equation which best fits the
experimental data has been proposed by Blanchard [49]: dqHo (t ) et al. [47,48] and 2
= k 2 ( qe − q (t ))
dt
dqptq “ k2 pqe ´ qptqq2 Assuming q(t = 0) = 0, the linearized form dt of the above equation is the following one:
t t 1 Assuming q(t = 0) = 0, the linearized form = of the +above equation is the following one: q (t )
k 2 qe
2
qe
t 1 t “ adsorbed ` −1 liquid) where qe is the amount of the solute (ionic qptq k2 qe 2 qate equilibrium (mg/g), k2 (g·mg ·min.) is
the equilibrium rate constant of pseudo-second-order model. The uptake of the adsorbate at time t,
qt (mg/g) wasamount calculated by the following where qe is the of the solute (ionicequation: liquid) adsorbed at equilibrium (mg/g), k2 (g¨ mg´1 ¨ min.) is the equilibrium rate constant of pseudo-second-order model. The uptake of the adsorbate at time t, qt (mg/g) was calculated by the following equation: 6
qt “ V
c0 ´ c t m
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c −c qt = V c00 − ctt qt = V m m
Molecules 2015, 20, 22058–22068
Adsorption (%)(%) Adsorption
where ct is the concentration of the ionic liquid in the solution at time t. The qe and k2 values were theintercept ionic liquid the solution time t. 6The e and k2and values were where wherectctisisthe theconcentration concentration of the ionic in the at time t. qThe k2 values determined from the slope andofthe ofliquid theincurves of solution t/q vs.att. Figure shows theqelinearized form determined from the slope and the intercept of the curves of t/q vs. t. Figure 6 shows the linearized form were determined from the slope and the intercept of the curves of t/q vs. t. Figure 6 shows of the pseudo-second-order kinetic model. The determined kinetic parameters are shown in Table 4. of the pseudo-second-order kinetic model. The kineticThe parameters are kinetic shown in Table 4. the form the pseudo-second-order model. determined parameters As itlinearized can be seen, theof correlation coefficients (R2determined ), kinetic are considerably high, reinforcing the applicability 2), are considerably high, reinforcing 2 As it can be seen, the correlation coefficients (R the applicability arepseudo-second-order shown in Table 4. kinetic As it can be seen, the correlation coefficients ), are considerably high, of model. Furthermore, the calculated and (R experimental q values were of pseudo-second-order kinetic model. Furthermore, the calculated and experimental q values were reinforcing the applicability of pseudo-second-order kinetic model. Furthermore, the calculated very close to each other, giving Δq (%) smaller than 7%. All these confirm the pseudo-second-order very to each other, giving (%) smaller than 7%. All these confirm the pseudo-second-order and close experimental q values wereΔqvery close toindicating each other, giving (%) smaller than 7%. All these model of ionic liquids adsorption on silica gel the strong∆q physisorption as dominating the model of ionic liquids adsorption on silica gel indicating the strong physisorption as dominating the confirm themechanism. pseudo-second-order model of ionic liquids adsorption on silica gel indicating the strong adsorption adsorption mechanism. physisorption as dominating the adsorption mechanism. 50 50 45 45 40 40 35 35 30 30 25 25 20 20 15 15 10 10 5 5 0 0 0 0
BMIM PF6 BMIM PF6 BMIM Cl BMIM Cl EMIM PF6 EMIM PF6 EMPyr Br EMPyr Br EPyr BF4 EPyr BF4 10 10
20 20 Time (min) Time (min)
30 30
40 40
t/qt/q (min/mg g-1g] -1] (min/mg
Figure Figure5.5.Effect Effectof oftime timeon onadsorption adsorptionefficiency. efficiency. Figure 5. Effect of time on adsorption efficiency.
500 500 450 450 400 400 350 350 300 300 250 250 200 200 150 150 100 100 50 50 0 0 0 -50 0 -50
BMIM PF6 BMIM PF6 BMIM Cl BMIM Cl EMIM PF6 EMIM PF6 EMPyr Br EMPyr Br EPyr BF4 EPyr BF4
5 5
10 10
15 15 Time (min) Time (min)
20 20
25 25
30 30
Figure 6. The linearized form of the pseudo-second-order kinetic equation. Figure 6. The linearized form of the pseudo-second-order kinetic equation. Figure 6. The linearized form of the pseudo-second-order kinetic equation.
7 7 22064
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Table 4. Kinetic parameters for ionic liquids adsorption onto silica gel at 25 ˝ C. Ionic Liquid
Slope
BMIM PF6 BMIM Cl EMIM PF6 EMPyr Br EPyr BF4
2.1272 7.4047 5.8594 8.7030 15.0048
Intercept 1.1717 7.3303 1.1926 6.2926 5.3759 d
1
∆qp%q “
R2
qe
k2
∆q (%) 1
er (%) 2
qexp
0.9989 0.9950 0.9905 0.9934 0.9855
0.470 0.135 0.170 0.114 0.066
3.862 7.479 28.788 12.036 41.880
5.1 7.0 6.2 3.2 6.1
7.3 9.9 9.3 4.6 8.7
0.232 0.121 0.160 0.215 0.069
ˇ ˇqexp ´ qcal | rpqexp ´ qcal q{qexp s2 x100; 2 er p%q “ 100 . N´1 qexp
3. Materials and Methods 3.1. Reagents Investigated compounds (Table 1) were obtained from Sigma (St. Louis, MO, USA) except for 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIM PF6), which was from Fluka (Sigma-Aldrich Group, Lausanne, Switzerland). HPLC gradient-grade acetonitrile (ACN) and methanol (MeOH) were purchased from Merck (Darmstadt, Germany). Silica gel (LiChrospher Si 1000, mean particle size 10 µm) used as adsorbent was obtained from Merck. Prior to the adsorption process, the adsorbent was washed with distilled water to eliminate impurities, dried at 120 ˝ C for 2 h. HPLC water was obtained from a Barnstead Deionising System (Dubuque, IA, USA). All mobile phases were buffered by the phosphate buffer (pH: 2.9–3.0). Its concentration was 30 or 50 mmol¨ L´1 in the whole mobile phase. The eluents were prepared by mixing the buffer solution, organic solvent and appropriate amounts of sodium hexafluorophosphate, sodium tetrafluoroborate. 3.2. Calibration Solutions The stock solutions of ionic liquids at concentration of 1.0 mg¨ mL´1 and the calibration solutions were prepared gravimetrically and stored in darkness at 4 ˝ C in glass vials. The calibration curves representing the dependence of the peak area on the concentration were used to perform quantitative analysis. 3.3. HPLC Quantification Experiments were performed using a Merck Hitachi LaChrom HPLC (Merck) model equipped with a diode array detector, L-7350 column oven and L-7612 solvent degasser. The columns (250 mm ˆ 4.6 mm I.D.) were packed with 5-µm Zorbax Eclipse XDB C18 (Agilent Technology, Waldbronn, Germany) pore size: 80 Å, surface area: 189 m2 /g; with void volume determined by the injection of thiourea. Retention data were recorded at a flow-rate of 1 mL¨ min´1 . The column was thermostated at 25 ˘ 0.1 ˝ C. The detection was set at wavelength chosen accordingly with the recorded spectra. Typical injection volumes were 20 µL. 3.4. Adsorption Experiments Batch adsorption experiments were carried out by an accurately weighed amount of adsorbent (0.02 g). Known weight of adsorbent was added to 5 mL centrifugal tube containing 2 mL of ionic liquid solution. The following conditions of the adsorption experiments were applied: temperature in the range 5–90 ˝ C, time in the range 0–30 min., IL concertation from 5 to 50 µg/mL. The tubes were shaken in a temperature-controlled shaker (Gallenkamp Orbital Incubator, Loughborough, UK) at a constant speed of 180 rpm. After that the mixture was centrifuged at 9000ˆ g. An aliquot of the supernatant was further analysed by a HPLC procedure.
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4. Conclusions In this work, solid-liquid equilibria were determined and analyzed for systems composed of imidazolium and pyridinium ionic liquids and silica gel. It was found that imidazolium ionic liquids with a longer alkyl chain (BMIM) and a chaotropic anion (PF6 ´ ) with lower Gibbs free energy of hydration exhibited stronger adsorption ability in comparison to cations with shorter alkyl substituents: EMIM, EMPyr, EPyr and less chaotropic anions: Cl´ , Br´ , BF4 ´ . Adsorption data fitting to Ho and Blanchard’ linear relationship: t/q(t) vs. t [37–39] enabled the selection of a pseudo-second-order kinetic model (PSO). Developed relationships could be used to extrapolate the kinetic data and estimate the values of qe with a relative error of no more than 10%. Under the optimized conditions adsorption processes were not sensitive to the temperature in the range 5–40 ˝ C, thus in practice they should be very effective media for the effective and economical recovery of ionic liquids from water at room temperature. Author Contributions: J.F., R.K. designed research, analyzed data, participated in the discussion of the obtained results and wrote this manuscript. M.T-M. performed research, contributed to discussion of results. A.G. performed research, analyzed the data. E.B. performed research, contributed to discussion of results. All authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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