Article pubs.acs.org/IECR
Synthesis, Characterization, and Evaluation of Surface Properties of Cyclohexanoxycarbonylmethylpyridinium and Cyclohexanoxycarbonylmethylimidazolium Ionic Liquids Rajni Aggarwal, Sukhprit Singh,* and Geeta Hundal Department of Chemistry, UGC Sponsored Centre for Advanced StudiesI, Guru Nanak Dev University, Amritsar 143005, India S Supporting Information *
ABSTRACT: New ionic liquids have been synthesized by esterification of cyclohexanol with bromoacetic acid or chloroacetic acid to form cyclohexyl-2-bromoacetate or cyclohexyl-2-chloroacetate. The bromoacetate/chloroacetate on quaternization with heterocyclic bases (pyridine or 2-methylpyridine or 3-methylpyridine or 4-methylpyridine or N-methylimidazole) gave the respective ionic liquids. One of the ionic liquids has been subjected to ion exchange with potassium hexafluorophosphate, potassium trifluoromethanesulfonate, and sodium tetrafluoroborate to get their respective hydrophobic ionic liquids by the exchange of anions. To confirm the exchange of anions, these ionic liquids have been crystallized from acetone and their structures have been established with single crystal X-ray diffraction studies. All the ionic liquids have been found to possess good surface properties and are thermally stable up to a temperature of more than 473.15 K.
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INTRODUCTION Ionic liquids (ILs) are organic salts containing only ions with a melting point of 100 °C and below, although there is no restriction to calling salts with higher melting points ionic liquids.1 Pyridinium and imidazolium salts are an important class of ionic liquids. ILs have outstanding chemical and thermal stabilities and have negligible vapor pressures,2−4 broad liquid temperature ranges, and extended specific solvent abilities.5,6 Ionic liquids can be used in high-vacuum systems due to their nonflammability and eliminate many containment problems. Due to enhanced thermal and operational stabilities and regio- or enantioselectivities, the activities of ionic liquids are comparatively higher than those of organic solvents. They can be used as reaction media for several enzymatic reactions.7 They can be easily recycled and reused without any emission of toxic substances.8 Better reactivity and selectivity can be achieved for homogeneous and heterogeneous catalyzed reactions by ionic liquids compared to conventional organic solvents.9,10 The ionic liquid 1-butyl-3-methylimidazolium chloride has been used for separating hydrogen from ammonia borane11 and has also been investigated as a nonaqueous electrolyte medium for the recovery of uranium and other metals from spent nuclear fuel and other sources.12,13 Since the beginning of the 20th century, medicinal chemists have given considerable attention to the design and synthesis of different types of ILs because of their antimicrobial activities.14 For example, algaecides, which are useful for the control of biological contamination in industrial water systems and pools, contain pyridinium salts.15 The effective polarities and the hydrogen bonding capacities of ILs and their ability to dissolve biopolymers lead to new functionalization pathways that cannot be accessed by traditional solvents.16 Therefore, there has been an increase of interest in the synthesis of ionic liquids. Organic reactions, catalytic processes, and separation technologies require the development of alternative solvents © 2012 American Chemical Society
and technologies. The ideal solvent should be environmentally friendly, chemically and physically stable, recyclable, safe, and eventually easy to handle and inexpensive. The work of Harjani and co-workers on the pyridinium and imidazolium based ILs has demonstrated that the introduction of ester groups does increase their biodegradation17 and fulfill all the conditions required above. Evaluation of an ionic liquid for a particular application requires the knowledge of thermophysical properties. We measure the melting point and thermal decomposition temperature. Since ionic liquids have immeasurably low vapor pressure, their potential operating range could extend up to the point where they thermally decompose. Thus the thermal decomposition temperature gives an idea of the upper operating range of the ionic liquid.18 The dependence of thermal stability on the cation and anion structure has also been investigated. Since ionic liquids are defined as molten salts that melt below 100 °C, the melting point is a defining feature of an ionic liquid. The structure and chemical composition of an ionic liquid is related to its melting point. By comparing various chloride salts, the influence of the cationic component becomes obvious. Alkali metal chlorides such as sodium chloride and potassium chloride have extremely high melting points (801 and 772 °C, respectively). By replacing the metal ion with a suitable organic cation, for example, [EMIM]+ or [BMIM]+, the melting point decreases drastically, to 87 and 65 °C, respectively.19,20 The cationic factor is not only the deciding factor in the melting point of the ionic liquid. The anion also plays an important role. With the change of anion the melting point also changes. As the anion size increases, the melting point is decreased.20 Received: Revised: Accepted: Published: 1179
August 1, 2012 November 16, 2012 December 30, 2012 December 31, 2012 dx.doi.org/10.1021/ie3020473 | Ind. Eng. Chem. Res. 2013, 52, 1179−1189
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Article
EXPERIMENTAL SECTION
Scheme 1
Chemicals. Cyclohexanol, chloroacetic acid, and bromoacetic acid were purchased from Central drug house (New Delhi, India). Pyridine, 2-methylpyridine, 3-methylpyridine, 4methylpyridine, and 3-methylimidazoline were purchased from Qualigens Fine Chemicals (Mumbai, India). Potassium hexafluorophosphate and sodium tetrafluoroborate were purchased from Acros Organics (Morris Plains, NJ, USA). Potassium trifluoromethanesulfonate was purchased from Sigma-Aldrich (St. Louis, MO, USA). Instrumentation. The IR spectra of all ionic liquids were recorded for pellets in KBr on a Shimadzu 8400s FT-IR (Kyoto Japan) instrument. Mass spectra of ionic liquids were recorded on a Waters Q-Tof Micromass using electrospray ionization (ESI) as ion source. 1H, 13C distortionless enhancement by polarization transfer (DEPT), correlation spectroscopy (COSY), and heteronuclear shift correlation (HETCOR) NMR spectra of the products were recorded on a Bruker Avance II (Switzerland) FT-NMR 400 MHz system as solution in CDCl3, using tetrametylsilane (TMS) as an internal standard. Thermal analysis (Tonset, °C) was performed on an SDT Q600 simultaneous thermogravimetric analysis (TGA)/ differential scanning calorimetrtic (DSC) analyzer, and CHNS analysis was recorded by a Flash EA 1112 series elemental analyzer (Thermo Electron Corp., U.K). X-ray data were collected on a Bruker Apex-II CCD diffractometer using Mo Kα (λ = 0.710 69 Å). The data were corrected for Lorentz and polarization effects, and empirical absorption corrections were applied using SADABS from Bruker. For complex 15a a total of 7782 reflections were measured, of which 2096 were independent and 1425 were observed [I > 2σ(I)] for θ 28°; for 15b a total of 41 666 reflections were measured, of which 11 308 were independent and 6280 were observed [I > 2σ(I)] for θ 32°; for 15c a total of 5238 reflections were measured, of which 1839 were independent and 720 were observed [I > 2σ(I)] for θ 25°. The crystals of compound 15c showed a poor diffraction with a low unique/expected ratio of reflections (0.66) and ratio of observed/unique reflection (39%) which did not improve on measuring the data twice on different crystals. Consequently, the R1 and wR values obtained are rather high: 0.23 and 0.55, respectively. The structures were solved by direct methods using SIR-9221 and refined by full-matrix least-squares refinement methods based on F2, using SHELX-97.22 The bond lengths of the cyclohexane group were restrained to the ideal values in the case of 15c. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were fixed geometrically with their Uiso values 1.2 times those of the phenylene and methylene carbons. All calculations were performed using the WinGX package.23 General Procedures. Preparation of Cyclohexyl 2Haloacetates. Bromoacetic acid (2a; 2.78 g; 20 mmol) or chloroacetic acid (2b; 1.89g; 20 mmol) was added to cyclohexanol (1.2 g; 20 mmol) at 80 °C in the presence of concentrated sulfuric acid (Scheme 1). The reaction mixture was stirred for 4 h at 80 °C without any additional solvent. The progress of the reaction was monitored by thin layer chromatography (Silica gel G coated (0.25 mm thick) glass plates, using hexane:ethyl acetate (95:5) as mobile phase, the spots were visualized by iodine), and the equilibrium stage arrived in 4 h. The crude reaction mixture was cooled to room
temperature and then transferred to a separating funnel using 50 mL of chloroform. The reaction mixture was washed four times with water:methanol (80:20) mixture, followed by saturated solution of sodium carbonate until the pH of the solution became 7. The solution was dried over anhydrous sodium sulfate and filtered and the chloroform was distilled to give cyclohexyl 2-bromoacetate (3a; 3.76 g; 17 mmol) or cyclohexyl 2-chloroacetate (3b; 2.93 g; 16.6 mmol). Preparation of Ionic Liquids. Cyclohexyl 2-bromoacetate (3a; 3.31 g; 15 mmol) or cyclohexyl 2-chloroacetate (3b; 2.65 g; 15 mmol) was stirred with pyridine (4; 1.18 g; 15 mmol), 2methylpyridine (5; 1.40 g; 15 mmol), 3-methylpyridine (6; 1.41 g; 15 mmol), 4-methylpyridine (7; 1.41 g; 15 mmol), or 3methylimidazole (8; 1.23 g; 15 mmol) at room temperature (25−27 °C). The progress of the reaction was monitored by thin layer chromatography, and reaction was completed in 15− 20 min (Scheme 1). The crude reaction mixture was suspended in 25 mL of diethyl ether and stirred at room temperature. The suspended material was filtered to remove unreacted ester or pyridine. The process of suspension of the product was repeated in a mixture of acetone and ethyl acetate to obtain pure white product (Scheme 1). The ionic liquids 13a and 13b are liquid at room temperature, while 15b and 15c are ionic liquids with melting points lower than 100 °C. Other than these ionic liquids, all are crystalline solids with melting points above 100 °C. The product was then vacuum dried at 80 °C to obtain the pure pyridinium or imidazolium salt. N-(Cyclohexanoxycarbonylmethyl)pyridinium Bromide (9a). Yield 2.87 g (64%) as a white solid, Tm = 132 °C, IR stretching for CO (1738 cm−1), C−O (1223, 1201 cm−1) (two bands), CC (aromatic ring) 1431 cm−1, CN (aromatic ring) 1633 cm−1, C−N (aromatic ring) 1354 cm−1. 1 H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.19−1.90 (m, 10H, 5 × CH2), 4.82−4.89 (m, 1H, −CH−O−), 6.23 (S, 2H, 1180
dx.doi.org/10.1021/ie3020473 | Ind. Eng. Chem. Res. 2013, 52, 1179−1189
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−CO−CH2−), 8.15−8.18 (t, J = 4.5 Hz, 2H, 1 × H-3,5 of C5H5N+), 8.63−8.66 (t, J = 4.8 Hz, 1H, 1 × H-4 of C5H5N+), 9.53−9.55 (d, J = 3.6 Hz, 2H, 1 × H-2,6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 23.57−31.37 (−ve, hexyl ring CH2), 61.22 (−ve, py+ −CH2−), 76.61 (+ve, −O−CH−), 125.75 (+ve, C-2 and C-4 of C 5H 5 N + ), 146.22(+ve, C-3 of C5H5N+), 146.66 (+ve, C-1 and C-5 of C 5 H 5 N + ), 165.10 (−CO). MS (ESI): m/z, 220.1(C13H18NO2+), 138.0 (C7H7NO2+). Calculated C% 52.01, H % 6.04, N% 4.67; found C% 52.04, H% 6.03, N% 4.70. 2-Methyl-N-(cyclohexanoxycarbonylmethyl)pyridinium Bromide (10a). Yield 3.65 g (78%), Tm = 152 °C, IR stretching for CO (1734 cm−1), C−O (1223, 1156 cm−1) (two bands), CC (aromatic ring) 1446 cm−1, CN (aromatic ring) 1623 cm−1, C−N (aromatic ring) 1366 cm−1. 1H NMR δ ppm (400 MHz; CDCl3; Me4Si) 1.30−1.91 (m,10H, 5 × CH2), 2.92 (S, 3H, α to C5H5N+), 4.85−4.90 (m, 1H, −CH−O−), 6.08 (S, 2H, −CO−CH2−), 7.95−7.98 (q, 2H, 1 × H-3,5 of C5H5N+), 8.44−8.48 (t, J = 4.7 Hz, 1 × H-4 of C5H5N+), 9.70−9.71 (d, J = 3.6 Hz, 1H, 1 × H-6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 21.15 (+ve, −CH3, α to C5H5N+), 23.55−31.37 (−ve, hexyl ring CH2), 59.43 (−ve, py+−CH2−), 76.56 (+ve, −O−CH−), 125.88 (+ve, C-5 of C6H7N+), 129.86 (+ve, C-3 of C6H7N+), 146.20 (+ve, C-6 of C6H7N+), 147.94 (+ve, C-4 of C6H7N+), 156.11 (C-2 of C6H7N+), 165.36 (−CO). MS (ESI): m/z, 234.1 (C14H20NO2+), 152.0 (C8H9NO2+). Calculated C% 53.34, H% 6.71, N% 4.44; found C% 53.58, H% 6.65, N% 4.23. 3-Methyl-N-(cyclohexanoxycarbonylmethyl)pyridinium Bromide (11a). Yield 3.40 g (72.5%), Tm = 130 °C, IR stretching for CO (1740 cm−1), C−O (1230, 1166 cm−1) (two bands), CC (aromatic ring) 1448 cm−1, CN (aromatic ring) 1637 cm−1, C−N (aromatic ring) 1354 cm−1. 1 H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.30−1.93 (m, 10H, 5 × CH2), 2.63 (S, 3H, β to C5H5N+), 4.81−4.88 (m, 1H, −CH−O−), 6.14 (S, 2H, −CO−CH2−), 7.96−8.00 (q, 1H, 1 × H-5 of C5H5N+), 8.30−8.32 (d, J = 5 Hz, 1 × H-4 of C5H5N+), 9.24−9.26 (t, J = 4 Hz, 2H, 1 × H-2,6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 18.65 (+ve, −CH3, β to C5H5N+), 23.57−31.37 (−ve, hexyl ring CH2), 60.94 (−ve, py+−CH2−), 76.52 (+ve, −O−CH−), 127.11 (+ve, C-4 of C6H7N+), 138.87 (C-3 of C6H7N+), 143.83 (+ve, C-4 of C6H7N+), 146.20 (+ve, C-6 of C6H7N+), 146.57 (+ve, C-2 of C6H7N+), 165.22 (−CO). MS (ESI): m/z, 234.1 (C14H20NO2+), 152.0 (C8H9NO2+). Calculated C% 53.34, H% 6.71, N% 4.44; found C% 53.47, H% 6.61, N% 4.47. 4-Methyl-N-(cyclohexanoxycarbonylmethyl)pyridinium Bromide (12a). Yield 2.35 g (50.3%), Tm = 180 °C, IR stretching for CO (1741 cm−1), C−O (1232, 1168 cm−1) (two bands), CC (aromatic ring) 1445 cm−1, CN (aromatic ring) 1641 cm−1, C−N (aromatic ring) 1352 cm−1. 1 H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.29−1.91 (m, 10H, 5 × CH2), 2.70 (S, 3H, γ to C5H5N+), 4.82−4.87 (m, 1H, −CH−O−), 6.12 (S, 2H, −CO−CH2−), 7.84−7.86 (d, J = 3.8 Hz, 2H, 1 × H-3,5 of C5H5N+), 9.23−9.25 (d, J = 4.0 Hz, 2H, 1 × H-2,6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 22.52 (+ve, −CH3, γ to C5H5N+), 23.61−31.40 (−ve, hexyl ring CH2), 60.49 (−ve, py+−CH2−), 76.47 (+ve, −O−CH−), 128.11 (+ve, C-3 and C-5 of C6H7N+), 145.70 (+ve, C-2 and C-6 of C6H7N+), 160.01 (C-4 of C6H7N+), 165.36 (−CO). MS (ESI): m/z, 234.1(C14H20NO2+), 152.0 (C8H9NO2+). Calcu-
lated C% 53.34, H% 6.71, N% 4.44; found C% 53.23, H% 6.52, N% 4.47. 3-Methyl-N-(cyclohexanoxycarbonylmethyl)imidazolium Bromide (13a). Yield 2.96 g (65.5%), liquid state, IR stretching for CO (1743 cm−1), C−O (1214, 1180 cm−1), (2360, 2428 cm−1) (overtone) (two bands), CC (aromatic ring) (1571, 1450 cm−1), CN (aromatic ring) (1614 cm−1), C−N (aromatic ring) 1358 cm−1. 1H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.30−1.89 (m, 10H, 5 × CH2), 4.10 (S, 3H, −CH3−N−), 4.82−4.86 (m, 1H, −CH−O−), 5.40 (S, 2H, −CO−CH2−), 7.60−7.65 (m, 2H, 1 × H-4,5 of C4H6N2+), 10.00 (s, 1 × H-2 of C4H6N2+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 23.60−31.39 (−ve, hexyl ring CH2), 36.93 (+ve, N−CH3), 50.47 (−ve, imid+−CH2−), 76.04 (+ve, −O−CH−), 123.17 (+ve, C-4 of C4H6N2+), 123.79 (+ve, C-5 of C4H6N2+), 138.20 (+ve, C-2 of C 4 H 6 N 2 + ), 165.54 (−CO). MS (ESI): m/z, 223.1 (C12H19N2O2+), 141.1 (C8H13O2+). Calculated C% 47.54, H % 6.32, N% 9.24; found C% 47.33, H% 6.15, N% 9.16. N-(Cyclohexanoxycarbonylmethyl)pyridinium Chloride (9b). Yield 2.5 g (65.3%), Tm = 135 °C, IR stretching for CO (1741 cm−1), C−O (1220, 1158 cm−1) (two bands), CC (aromatic ring) 1450 cm−1, CN (aromatic ring) 1634 cm−1. 1H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.25−1.90 (m, 10H, 5 × CH2), 4.81−4.87 (m, 1H, −CH−O−), 6.27 (S, 2H, −CO−CH2−), 8.11−8.15 (t, J = 4.6 Hz, 2H, 1 × H-3,5 of C5H5N+), 8.56−8.60 (t, J = 4.8 Hz, 1 × H-4 of C5H5N+, 9.57− 9.58 (d, J = 3.6 Hz, 2H, 1 × H-2,6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 23.57−31.37 (−ve, hexyl ring CH2), 61.22 (−ve, py+ CH2), 76.61 (+ve, −O−CH), 125.75 (+ve, C-2 and C-4 of C5H5N+), 146.22 (+ve, C-3 of C5H5N+), 146.66 (+ve, C-1 and C-5 of C5H5N+), 165.10 (−CO). MS (ESI): m/z, 220.1 (C13H18NO2+), 138.0 (C7H7NO2+). Calculated C% 61.05, H% 7.09, N% 5.48; found (C% 61.09, H% 7.19, N% 5.28. 2-Methyl-N-(cyclohexanoxycarbonylmethyl)pyridinium Chloride (10b). Yield 3.21 g (79.3%), Tm = 110 °C, IR stretching for CO (1739 cm−1), C−O (1222, 1169 cm−1) (two bands), CC (aromatic ring) 1447 cm−1, CN (aromatic ring) 1631 cm−1, C−N (aromatic ring) 1359 cm−1. 1 H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.30−1.91 (m, 10H, 5 × CH2), 2.92 (S, 3H, α to C5H5N+), 4.85−4.90 (m, 1H, −CH−O−), 6.08 (S, 2H, −CO−CH2−), 7.92−7.97 (q, 2H, 1 × H-3,5 of C5H5N+), 8.40−8.44 (t, J = 4.8 Hz, 1 × H-4 of C5H5N+), 9.72−9.73 (d, J = 3.75 Hz, 1H, 1 × H-6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 21.15 (+ve, −CH3, α to C5H5N+), 23.55−31.37 (−ve, hexyl ring CH2), 59.43 (−ve, py+−CH2−), 76.56 (+ve, −O−CH−), 125.88 (+ve, C-5 of C6H7N+), 129.86 (+ve, C-3 of C6H7N+), 146.20 (+ve,C-6 of C6H7 N+), 147.94 (+ve, C-4 of C6H7 N+), 156.11 (C-2 of C6H7N+), 165.36 (−CO). MS (ESI): m/z, 234.1 (C14H20NO2+), 152.0 (C8H9NO2+). Calculated C% 62.33, H% 7.47, N% 5.19; found C% 62.38, H% 7.43, N% 5.12. 3-Methyl-N-(cyclohexanoxycarbonylmethyl)pyridinium Chloride (11b). Yield 2.87 g (71%), hygroscopic, IR stretching for CO (1740 cm−1), C−O (1228, 1169 cm−1) (two bands), CC (aromatic ring) 1450 cm−1, CN (aromatic ring) 1639 cm−1, C−N (aromatic ring) 1381 cm−1. 1H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.30−1.93 (m, 10H, 5 × CH2), 2.63 (S, 3H, β to C5H5N+), 4.81−4.85 (m, 1H, −CH−O−), 6.22 (S, 2H, −CO−CH2−), 7.95−7.99 (q, 1H, 1 × H-5 of C5H5N+), 8.27−8.29 (d, J = 4.9 Hz 1 × H-4 of C5H5N+), 9.28−9.32 (t, J = 4.0 Hz, 2H, 1 × H-2,6 of C5H5N+). 1181
dx.doi.org/10.1021/ie3020473 | Ind. Eng. Chem. Res. 2013, 52, 1179−1189
Industrial & Engineering Chemistry Research
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C NMR, δ ppm (normal/DEPT-135; CDCl3) 18.65 (+ve, −CH3, β to C5H5N+), 23.57−31.37 (−ve, hexyl ring CH2), 60.94 (−ve, py+−CH2−), 76.52 (+ve, −O−CH−), 127.11 (+ve, C-4 of C6H7N+), 138.87 (C-3 of C6H7N+), 143.83 (+ve, C-4 of C6H7N+), 146.20 (+ve, C-6 of C6H7N+), 146.57 (+ve, C-2 of C6H7N+), 165.22 (−CO). MS (ESI): m/z, 234.1 (C14H20NO2+), 152.1 (C8H9NO2+). Calculated C% 62.33, H% 7.47, N% 5.19; found C% 62.45, H% 7.61, N% 5.63. 4-Methyl-N-(cyclohexanoxycarbonylmethyl)pyridinium Chloride (12b). Yield 1.96 g (48.5%), Tm = 180 °C, IR stretching for CO (1741 cm−1), C−O (1224, 1201 cm−1) (two bands), CC (aromatic ring) 1473 cm−1, CN (aromatic ring) 1640 cm−1, C−N (aromatic ring) 1359 cm−1. 1 H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.29−1.91 (m, 10H, 5 × CH2), 2.70 (S, 3H, γ to C5H5N+), 4.82−4.87 (m, 1H, −CH−O−), 6.12 (S, 2H, −CO−CH2−), 7.86−7.87 (d, J = 3.9 Hz, 2H, 1 × H-3,5 of C5H5N+), 9.31−9.33 (d, J = 4.1 Hz, 2H, 1 × H-2,6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 22.52 (+ve, −CH3 γ to C5H5N+), 23.61−31.40 (−ve, hexyl ring CH2), 60.49 (−ve, py+−CH2−), 76.47 (+ve, −O−CH−), 128.11 (+ve, C-3 and C-5 of C6H7N+), 145.70 (+ve, C-2 and C-6 of C6H7N+), 160.01 (C-4 of C6H7N+), 165.36 (−CO). MS (ESI): m/z, 234.1 (C14H 20 NO2 +), 152.0 (C8 H 9NO2 +). Calculated C% 62.33, H% 7.47, N% 5.19; found C% 62.23, H% 7.43, N% 5.09. 3-Methyl-N-(Cyclohexanoxycarbonylmethyl)imidazolium Chloride (13b). Yield 2.46 g (63.5%), liquid state, IR stretching for CO (1742 cm−1), C−O (1215, 1180 cm−1), (2360, 2430 cm−1) (overtone) (two bands), CC (aromatic ring) 1571, 1450 cm−1, CN (aromatic ring) 1615 cm−1, C−N (aromatic ring) 1358 cm−1. 1H NMR, δ ppm (400 MHz; CDCl3; Me4Si) 1.30−1.89 (m, 10H, 5 × CH2), 4.10 (S, 3H, CH3−N−), 4.82− 4.86 (m, 1H, −CH−O−), 5.40 (S, 2H, −CO−CH2−), 7.60− 7.65 (m, 2H, 1 × H-4,5 of C4H6N2+), 10.02 (s, 1 × H-2 of C4H6N2+). 13 C NMR, δ ppm (normal/DEPT-135; CDCl3) 23.60−31.39 (−ve, hexyl ring CH2), 36.93 (+ve, N−CH3), 50.47 (−ve, imid+−CH2−), 76.04 (+ve, −O−CH−), 123.17 (+ve, C-4 of C4H6N2+), 123.79 (+ve, C-5 of C4H6N2+), 138.20 (+ve, C-2 of C 4 H 6 N 2 + ), 165.54 (−CO). MS (ESI): m/z, 223.1 (C12H19N2O2+), 141.1 (C8H13O2+). Calculated C% 55.70, H % 7.40, N% 10.83; found C% 55.54, H% 7.28, N% 10.69. Exchange of Anion. Saturated aqueous solution of potassium hexafluorophosphate (14a; 1.84 g, 10 mmol), potassium trifluoromethanesulfonate (14b; 1.88 g, 10 mmol), or sodium tetrafluoroborate (14c; 1.10 g, 10 mmol) was stirred with aqueous solution of N-(cyclohexanoxycarbonylmethyl)pyridinium bromide (9a; 3 g, 10 mmol) in an ice bath for 15 min, which resulted in the precipitation of the water-insoluble ionic liquids with the exchange of halide anions. The separated solid was filtered, dried under vacuum, and recrystallized from acetone (Scheme 2). 1-(2-(Cyclohexyloxy)-2-oxoethyl)pyridin-1-ium Hexafluorophosphate (15a). Yield 3.05 g (83.56%) as a white crystalline solid, Tm = 105 °C, 1H NMR, δ ppm (DMSO-d6) 1.39−1.79 (m, 10H, 5 × CH2), 4.80−4.87 (m, 1H, −CH−O−), 5.54 (S, 2H, −CO−CH2−), 8.21−8.23 (q, 2H, 1 × H-3,5 of C5H5N+), 8.68−8.70 (t, 1 × H-4 of C5H5N+), 8.96−9.03 (t, 2H, 1 × H2,6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135, DMSO-d6) 22.95− 30.73 (−ve, hexyl ring CH2), 60.26 (−ve, py+−CH2−), 74.64 (+ve, −O−CH−), 127.91 (+ve, C-2 and C-4 of C5H5N+), 13
Scheme 2
146.24 (+ve, C-3 of C5H5N+), 146.26 (+ve, C-1 and C-5 of C5H5N+), 165.76 (−CO). Theoretical C% 42.75, H% 4.97, N% 3.83; experimental C% 42.73, H% 4.97, N% 3.80. 1-(2-(Cyclohexyloxy)-2-oxoethyl)pyridin-1-ium Trifluoromethylsulfonate (15b). Yield 2.95 g (80.8%) as a white crystalline solid, Tm = 76 °C, 1H NMR, δ ppm (CDCl3) 1.112− 1.866 (m, 10H, 5 × CH2), 4.842−4.903 (m, 1H, −CH−O−), 5.588 (S, 2H, −CO−CH2−), 8.027−8.075 (q, 2H, 1 × H-3, 5 of C5H5N+), 8.51−8.553 (t, 1 × H-4 of C5H5N+), 8.89−8.91 (t, 2H, 1 × H-2, 6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135) (CDCl3) 23.46− 31.13 (−ve, hexyl ring CH2), 61.086 (−ve, py+ −CH2−), 76.522 (+ve, −O−CH−), 127.988 (+ve, C-2 and C-4 of C5H5N+), 146.263 (+ve, C-3 of C5H5N+), 146.386 (+ve, C-1 and C-5 of C5H5N+), 164.988 (−CO). Theoretical C% 45.53, H% 4.91, N% 3.79, S% 8.68; experimental C% 45.42, H % 4.88, N% 3.76, S% 8.65. 1-(2-(Cyclohexyloxy)-2-oxoethyl)pyridin-1-ium Tetrafluoroborate (15c). Yield 2.75 g (89.6%) as a transparent crystalline solid, Tm = 65 °C, 1H NMR, δ ppm (CDCl3) 1.09−1.84 (m, 10H, 5 × CH2), 4.80−4.89 (m, 1H, −CH−O−), 5.41 (S, 2H, −CO−CH2−), 7.95−8.0 (q, 2H, 1 × H-3,5 of C5H5N+), 8.44− 8.49 (t, 1 × H-4 of C5H5N+), 8.68−8.70 (t, 2H, 1 × H-2,6 of C5H5N+). 13 C NMR, δ ppm (normal/DEPT-135, CDCl3) 24.08−31.51 (−ve, hexyl ring CH2), 61.62 (−ve, py+−CH2−), 76.62 (+ve, −O−CH−), 128.44 (+ve, C-2 and C-4 of C5H5N+), 146.43 (+ve, C-3 of C5H5N+), 146.86 (+ve, C-1 and C-5 of C5H5N+), 166.45 (−CO). Theoretical C% 50.84, H% 5.91, N% 4.56; experimental C% 50.68, H% 5.76, N% 4.48. Surface Tension Measurements. The critical micelle concentration (cmc) and surface tension attained at the cmc were determined using a Du Noüy tensiometer (CSC Scientific Co.) with a platinum−iridium ring at 25.0 ± 0.1 °C. The tensiometer was calibrated using triple-distilled water. For the determination of the cmc, an adequate quantity of a concentrated ionic liquid solution was added to triple-distilled water in order to change the concentration well below the cmc up to more than 2 times the cmc. Specific Conductivity Measurements. The specific conductivity was measured using a conductometer with a probe whose cell constant is 0.98 cm−1. The conductometer was calibrated with triple-distilled water. The change of specific conductivity for aqueous solutions of the ionic liquids as a function of the ionic liquid concentration was investigated. The specific conductivity values fit into two straight lines of different slopes. The location of the abrupt changes of the slopes corresponds to the critical aggregation concentration.24
■
RESULT AND DISCUSSION Characterization. The structures of the ionic liquids synthesized by the quaternization of cyclohexyl-2-haloacetates (2a/2b and 3a/3b) with various heterocyclic bases (Scheme 1) 1182
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141.1 for 13a. These signals account for the loss of cyclohexanol moiety from 9a leading to the formation of (C7H7NO2+) ion, while the same in the case of 13a can be attributed to the loss of imidazolium ring from the molecule leading to the formation of (C8H13O2+) ion; a similar trend has also been observed in the cases of other ionic liquids related to the above two structures of 9a and 13a. X-ray Crystallography. The availability of a large set of ionic liquids offers a great toolbox for crystal engineering. It is well-known that ionic liquids are highly structured in their liquid states and can crucially influence the results of organic reactions26 carried out using them as solvents. Although a large number of ionic liquids have been reported in the literature, there are only a few reports on the crystal structures of ionic liquids.27 Thus, it is interesting to investigate the single crystal X-ray diffraction of ionic liquids to find out the existence of inter- and intramolecular nonbonding interactions responsible for the structured liquid states of these ionic liquids. In our present study we selected one of the ionic liquids (9a) and subjected it to an ion exchange for its anion (with potassium hexafluorophosphate, potassium trifluoromethanesulfonate, and sodium tetrafluoroborate, Scheme 2) to get relatively hydrophobic salts (15a−15c). These salts were crystallized from their solution in acetone to form a crop of crystals. One of the selected crystals of these ionic liquids was subjected to single crystal X-ray diffraction analysis. Molecular Structures of 15a−15c. The molecular structure of the compound 15a (Figure 1) has a plane passing through
have been confirmed by various spectral methods. In the infrared spectra of these ionic liquids the carbonyl stretch (C O) of ester appeared in the range of 1741−1738 cm−1. The bands for the C−O stretches have been observed in the range 1230−1166 cm−1. The CC stretches for the aromatic pyridinium ring have been observed in the range 1473−1431 cm−1, while in the case of imidazolium ionic liquids 13a and 13b the two bands were observed at 1571 and 1450 cm−1. The CN stretching frequency for the pyridinium and imidazolium ionic liquids has been observed in the range 1640−1631 cm−1. The stretching frequency for the carbon directly attached to the positively charged nitrogen of pyridinium and imidazolium ring for all ionic liquids appeared in the range 1366−1352 cm−1. The structures of ionic liquids have further been established by 1H, 13C DEPT (distortionless enhanced polarization transfer), two-dimensional (2D) HETCOR (heteronuclear chemical shift correlation), and 2D COSY (correlation spectroscopy) experiments. The protons of cyclohexyl ring methylenes of ionic liquids 9a and 9b were observed as a multiplet between δ 1.19 and 1.90 ppm; however, this signal appeared as a multiplet between δ 1.29 and 1.91 ppm for all other ionic liquids. The proton present at C-1 of cyclohexyl ring appeared as a multiplet at δ 4.82−4.90 ppm for all the ionic liquids. The methylene protons directly attached to pyridinium ring appeared as a singlet at δ 6.23 ppm for 9a, δ 6.27 ppm for 9b, δ 5.63 ppm for 15a, δ 5.58 for 15b, and δ 5.41 for 15c, while the same protons for other pyridinium salts have been observed at δ 6.08 ppm. In the cases of imidazolium ionic liquids 13a and 13b the methyl protons directly attached to imidazolium ring have been observed at δ 5.40 ppm. All the pyridinium ring protons were observed between δ 7.95 and 9.55 ppm. The methyl protons directly attached to the positively charged pyridinium ring have been observed as a singlet at δ 2.92 ppm for 10a and 10b, δ 2.63 ppm for 11a and 11b, and δ 2.70 ppm for 12a and 12b. All the imidazolium ring protons were observed between δ 7.60 and 10.00 ppm. The signal for −NCHN− proton has been observed as a singlet at δ 10.00 ppm. This chemical shift is typical for −NCHN− proton.25 Signals for −NCHCHN− protons appeared between δ 7.60 and 7.65 ppm. The signal of the methyl group directly attached to imidazolium moiety was observed as a singlet at δ 4.10 ppm. The 13C DEPT NMR spectra helped in the assignment of the chemical shift of δ 23.61−31.61 ppm to sp3 carbons of cyclohexyl ring for all ionic liquids. The carbon of the methyl group directly attached to a positively charged pyridinium or imidazolium ring was observed at δ 21.15 ppm for 10a and 10b, δ 18.65 ppm for 11a and 11b, δ 22.52 ppm for 12a and 12b (all attached to pyridinium ring), and δ 36.93 ppm for 13a and 13b (attached to imidazolium ring). The carbon directly attached to the positively charged nitrogen of pyridinium and imidazolium was observed in the range δ 50.47−61.47 ppm. The C-1 carbon of the cyclohexyl ring which is attached to the oxygen of the ester group was observed at δ 76.56 ppm. The signal for sp2 hybridized carbonyl carbon was observed at δ 165.10 ppm. The structures of these ionic liquids have further been confirmed by ESI−MS (positive ion) mass spectroscopy. The parent ion peaks for the pyridinium (9a/9b) and imidazolium (13a/13b) ionic liquids have been observed at m/z 220.1and 223.1, respectively. These signals account for the loss of bromide ion and chloride ion from the molecule leading to the formation of positively charged parent ion M+−Br or M+−Cl. The base ion peak was observed at m/z 138.0 for 9a and m/z
Figure 1. ORTEP diagram of compound 15a showing labeling scheme.
the center of the cyclohexane and pyridine rings and through the connecting −CH2COO group. It also passes through the PF6 anion, specifically through F1−P1−F3 atoms. Thus the asymmetric unit contains half the cation and the anion molecules. The pyridine ring is placed almost perpendicular with respect to the −CH2COO group (dihedral angle 89.5(2)°), giving it a “spade”-like conformation. The C1− O1−C5−O2, C1−O1−C5−C6, O1−C5−C6−N1, and O2− C5−C6−N1 torsion angles are 0.0, 180.0, 180.0, and 0.0°, respectively. Thus the pyridine ring is pointing to the same side as the carbonyl oxygen O2 (Figure 2). The cyclohexane ring is in the chair conformation in all three compounds. The molecular structure is stabilized by ionic and intramolecular H-bonding interactions between methylene C3 with F2 and cyclohexane C1 with carbonyl O2 (Table 11 in the Supporting Information) atoms (Figure 2). The molecular structure of 15b has two crystallographically independent molecules, molecule 1 (C1 to C13) and molecule 1183
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Figure 2. Conformations and intramolecular H-bonding in 15a−15c.
Figure 3. ORTEP diagram of compound 15b showing two crystallographically independent units and the labeling scheme used.
Figure 4. ORTEP diagram of compound 15c and the labeling scheme used.
Figure 5. Showing formation of zigzag chains due to H-bonding interactions in 15a.
in both molecules and quite different from those found for 15a especially for the O(ester)−C−C−N(py) and O(carbonyl)− C−C−N(py) angles which are cis and trans, respectively, in 15b but trans and cis, respectively, in 15a. Thus the pyridine ring is turned toward the carbonyl oxygen in 15a, whereas it is
2 (C15 to C27), in the unit cell which have slightly different conformations and bond parameters (Figure 3). The pyridine rings are rotated with respect to the −CH2COO group by 70.9(1) and 74.7(1)°, respectively, in these cationic molecules. The torsion angles around the ester groups are slightly different 1184
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Figure 6. Packing diagram of 15b, showing intramolecular (red) and intermolecular H-bonds (blue) forming a 2D network, down the a axis.
rotated toward the ester oxygen in both molecules of 15b (Figure 2). This inversion may be due to the steric factors for the accommodation of a bigger triflate group. The molecular structure (Figure 4) of 15c is similar to that of 15a with a spade-like conformation. The dihedral angle between −CH2COO and the pyridine ring is 72.7(5)°. The torsion angles around the −CH2COO group again suggest a similar conformation as found for 15a with pyridine pointing to the same side as the carbonyl oxygen O1 in this case (Figure 2). There is intramolecular H-bonding between C8−H8···O1 and the C4−H4···F4 atom of the BF4 anion in 15c. Crystal Structures of 15a−15c. The crystal structure of 15a is dominated by the C−H···F and C−H···O type of weak Hbonding interactions between the cation and the anion. C6− H6A···F4 and C8−H8···F3 H-bonding interactions form zigzag chains running parallel to the b axis (Figure 5). The crystal structure of 15b is supported by both intra- and intermolecular H-bonding between the cations and anions to form a three-dimensional (3D) H-bonded network. Various intramolecular H-bonds (Table 11 in the Supporting Information) and intermolecular H-bonding of methylene carbons C8 and C22 with carbonyl oxygen O2 and sulfonate oxygen O4, respectively, as well as pyridine carbons C23 and C24 with sulfonate oxygens O3 and O4 form a linear tape running parallel to the c axis in the bc plane. The successive linear tapes are held to each other with the help of the cyclohexane C20−H20B···F3 interaction to give a 2D network in the bc plane (Figure 6). This 2D network is extended into a 3D one due to various other intermolecular Hbonding interactions such as cyclohexane and pyridine carbons with triflate groups (Figure S3, Supporting Information) and methylene carbon C8 and triflate oxygen O10 and methylene C22 and carbonyl O7 atoms. There is extensive C−H···O and C−H···F type of intermolecular H-bonding interactions in 15c, between the cation and the anion, due to which the crystal packing shows formation of helical chains parallel to the b axis in the bc plane (Figure 7). Formation of helical chains in ionic liquids has also been reported earlier.28 It has been reported that a stronger hydrogen bond acceptor favors the formation of helical chains. Further, it has also been observed that the self-organization of matter in ionic liquids is influenced by only a small change in the cation29 and the anion.28
Figure 7. Crystal packing in 15c showing formation of a helical chain parallel to the b axis.
Melting Point (Tm, °C). The melting points of most of the ionic liquids being reported in the present study were observed to be greater than 100 °C, which is the dividing line between ionic liquids and salts. Factors which are responsible for lowering the melting points of ionic liquids are cations with low symmetry,5 weak intermolecular hydrogen bonding, and a good distribution of charge in the cations.30 Melting point also depend upon the size of the anion. The melting point generally decreases with increasing anion radius except for PF6. This is because the larger anion radius induces weaker electrostatic interaction with imidazolium or pyridinium cation. However, ILs with PF6 have strong hydrogen bonds for the sake of an F atom and their melting points are comparatively higher.31 In addition to 13a and 13b, which are liquid at room temperature, 15b and 15c may also be classified as ionic liquids, because their melting points are 76 and 65 °C, respectively. From our results it has been found that the cationic component is not the only deciding factor in the melting point of the system. The anion also plays an important role. By keeping the cation constant, it can be seen that altering the size of the anion while 1185
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Table 1. Onset and Start Temperatures for Thermal Decomposition of ILs compound Tstart (°C) Tonset (°C)
9a
10a
11a
12a
13a
9b
10b
11b
12b
13b
15a
15b
15c
177.9 212.9
177.7 201.3
176.6 201.1
178.8 203.1
196.6 225.5
167.2 182.7
161.3 181.8
176.3 200.0
179.0 202.2
167.7 203.1
218.0 238.8
201.3 209.7
187.5 200.0
Figure 8. (a) Start and onset temperatures of degradation of 9a and 9b. (b) TGA graph of 15a, N-(cyclohexanoxycarbonylmethyl)pyridinium hexafluorophosphate, showing the start and onset temperatures of degradation and percent mass loss.
Table 2. Surface Properties of Ionic Liquids ILs 9a 9b 10a 10b 11a 11b 12a 12b 13a 13b a
cmc (mM) a
b
15.3 (13.4) 17.5a (18.2)b 15.0a (13.80)b 15.2a(15.84)b 13.33a (12.59)b 15.01a (13.77)b 16.66a (15.48)b 16.68a (15.84)b 12.5a (13.18)b 13.0a (13.54)b
γcmc (mN/m)
α
β
Γmax (106) (mol/m2)
Amin (nm2/molecule)
Πcmc (mN/m)
48.5 52.17 51.34 53.47 50.03 54.22 49.95 52.92 52.13 54.45
0.46 0.48 0.33 0.43 0.40 0.45 0.40 0.41 0.38 0.45
0.54 0.52 0.66 0.56 0.59 0.55 0.60 0.59 0.62 0.55
1.386 1.103 1.322 1.157 1.482 1.103 1.413 1.263 1.347 1.058
1.197 1.504 1.255 1.434 1.119 1.503 1.174 1.313 1.231 1.567
24.10 20.43 21.26 19.13 22.57 18.38 22.65 19.68 20.47 18.15
cmc determined from electrical conductivity measurements. bcmc determined from surface tension measurements.
as decomposition occurs.33 The start temperature (Tstart) is the temperature at which the decomposition of the sample begins. The method to determine the onset and start temperatures is shown in Figure 8a. The onset and start temperatures for the present ILs are listed in Table 1. The thermal stabilities of ionic liquids have been shown to be limited by the strength of their heteroatom−carbon and their heteroatom−hydrogen bonds.34 In the cases of the ionic liquids being reported in the present study, not only the anion but also the ester linkage may have contributed to the relatively moderate thermal stabilities. Figure 8a shows that the ionic liquid with bromo as the counterion has higher thermal stability compared to the ionic liquid with chloro as the counterion. The results are in consonance with recently reported β-hydroxy-γ-alkyloxy-N-methylimidazolium surfactants.35 Ionic liquid containing hexafluorophosphate (PF6−) as counterion was found to have more thermal stability compared to other ionic liquids, because the PF6 anion forms a stronger hydrogen bond for the sake of an F atom.31 TGA results obtained for the ionic liquid 15a (1-(2-(cyclohexyloxy)-
maintaining the same charge alters the melting point. For example, 9a having bromide as an anion has a melting point of 135 °C, but the melting point decreased when the Br anion was replaced with PF6 (105 °C), CF3SO3 (76 °C), and BF4 (65 °C). Thermal Stability. The thermal stabilities of these ionic liquids were tested in a nitrogen atmosphere between 50 and 500 °C at a heating rate of 10 °C/min using an SDT Q600 simultaneous TGA (thermogravimetric analyzer). All the ionic liquids synthesized in the present study exhibited medium thermal stabilities. Most of these ionic liquids started decomposing in the temperature range 204−250 °C (the onset temperature; Table 1). The decomposition at lower temperature may be attributed to the decomposition of cyclohexylate ester group.32 Figure 8a shows thermal decomposition graphs of the ILs 9a and 9b. The observed onset temperature (Tonset) is the intersection of the baseline weight from the beginning of the experiment and the tangent of the weight vs temperature curve 1186
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Figure 9. Surface tension vs log concentration plot of ILs (a) 9a−13a and (b) 9b−13b.
chloride ion for screening of the electrostatic repulsion among the polar head groups, since Br− has a larger affinity to the interface than Cl−. As a result, the respective cations are adsorbed at the air−water interface more densely, and accordingly, the (γcmc) value becomes smaller for bromo derivatives compared to the chloro derivatives. Thus, these results indicate that, for surface active compounds with the same hydrophilic head, the counterion has an effect on the (γcmc) value. The effectiveness of surface tension reduction (the surface pressure at the cmc, Πcmc) was determined by eq 1.
2-oxoethyl)pyridin-1-ium hexafluorophosphate) (Figure 8b) show the percent mass and the rate of mass loss as a function of the temperature. The thermal degradation of the compound is observed as a large weight loss (72.82%) at 258.77 °C due to loss of ester linkage with the anion. Our experimental results also show that the introduction of methyl group at the 2-, 3-, or 4-position in the pyridine nucleus shows slight changes in the degradation temperature, i.e., Tonset, of the ionic liquids. The N-methylimidazolium core containing ionic liquids have higher thermal degradation temperatures (thermally more stable) compared to the corresponding pyridinium ionic liquids. Self-Aggregation in Aqueous Solution. The aggregation properties of synthesized ionic liquids in aqueous media were determined by means of different experimental techniques involving measurement of surface tension and conductivity (Table 2). It is well-known that a sharp decrease of the surface tension followed by a plateau reveals the amphiphilic characteristic of the cation, which is because of the possible micelle formation.36 Introduction of an ester in the alkyl chain lowers the critical aggregation number when compared to ionic liquids without additional functional groups. The decrease may be attributed to the introduction of an ester in the hydrophobic chain close to the polar head group leading to an increased H-bonding in the head group region and making them more favorable for aggregation and less favorable for adsorption at the interface.37 Figure 9 represents the surface tension (γ) versus log concentration (C) plot obtained for the solution of 10 surface active ILs at 298 K. As can be seen, for each IL, the surface tension gradually decreases with the increase in IL concentration up to a plateau region, above which a nearly constant value is obtained. Recently it has been proved that the cmc values when evaluated by two different techniques may differ significantly depending upon the self-aggregation behavior of the IL or surfactant under consideration.38 Our experimental results show that the introduction of a methyl group to the 2-, 3-, or 4-position of the pyridinium core did not give rise to a major change in the cmc values of the ionic liquids. On the other hand, when the chloro derivative (9b/13b) and bromo derivative (9a/13a) of these ionic liquids were compared, the bromide ion was more effective than the
Πcmc = γo − γcmc
(1)
where γo is the surface tension of pure solvent and γcmc is the measured surface tension at the critical micelle concentration. This parameter measures the effect of surfactant on the surface tension of pure solvent, i.e., water. The values of the parameter obtained for the 10 ionic liquids have been listed in Table 2 together with the cmc and γcmc values. Higher Πcmc values of 9a, 11a, and 12a indicate that these ionic liquids are more effective than the other surface active ILs in the reduction of surface tension. The maximum surface excess concentration, Γmax, at the air/water interface has been determined by applying the Gibbs adsorption isotherm39 (eq 2). Γmax = −1/2.303nRT (∂γ /∂ ln C)T
(2)
where R is the gas constant, T is the absolute temperature, and C is the surfactant concentration. Γmax, i.e., the amount of surfactant adsorbed per unit area at the air−water interface after complete monolayer formation, is calculated from the slope of the log cmc vs surface tension plot when the concentration is approaching the cmc. The higher value of Γmax of cationic bromide could be attributed to the weak hydration of its counterion [Br]−, which causes a more effective screening of the electrostatic repulsion among the polar head groups and hence results in enhanced adsorption at the air−water interface. The minimum area occupied by a single surfactant molecule at the air−water interface, Amin, was also estimated using, eq 3. A min = 1/(NA Γmax)
(3) 2
where NA is Avogadro’s number and Amin is in nm /molecule. Our experimental results show slightly lower Amin values for the ionic liquid containing bromo as counterion. Further, when 1187
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compared with some conventional surfactants, such as noctyltrimethylammonium bromide and n-decyltrimethylammonium bromide, the cmc values of these ester containing ionic liquids are less.40 These ionic liquids also have smaller cmc values than ionic liquid 1-decyl-3-methylimidazolium chlorides reported by El Seoud et al.41 and 1-decyl-3-methylimidazolium bromides and 1-octyl-3-methylimidazolium bromides reported by Cornellas et al.15 Thus, this shows that these ILs have better surface activities and micelle formation capabilities compared to conventional ionic liquids. The aggregation properties have also been determined by measuring the change in specific conductance of the aqueous solution of the ionic liquids with change in concentration. In the low concentration range the rise of specific conductivity is due to the increase of free cations and anions in the aqueous media. Above the aggregation value the micelles can contribute to the charge transport to a lesser extent than free ions owing to their lower mobility and there is an effective loss of ionic charge due to binding of a fraction of counterions to the micellar surface.42 This result gives a break point in the concentration vs specific conductance curve of the solution of amphiphiles. The concentration corresponding to the break point is the critical micelle concentration (cmc). The cmc determined by this method has also been recorded in Table 2. These cmc values coincide well with the cmc values determined by another method, i.e., surface tension measurements. The degree of counterion dissociation, α, was obtained from the ratio of the slope above and below the break indicative of the cmc, and the degree of counterion binding to micelles, β is equal to 1 − α.43 The larger value of α, i.e., the smaller value of β, means a weaker ability of counterion binding to micelles. The experimental results shown in Table 2 correlate with the Hofmeister (lyotropic) series of anions for the cationic surfactants, Br− > Cl− > F−. The ionic liquid containing bromide as counterion has lower cmc and α when compared to ionic liquid containing chloride as counterion. The larger repulsion of chloride anions from the interface than bromide anions is caused by their difference in excess polarizability and the van der Waals interaction with the interface. The decrease in the size of hydrated anion is accompanied by an increase in polarizability. A largely polarized anion can easily bind at the micellar surface and cause a decrease in the electrostatic repulsion between the cationic head groups of ILs, accompanied by the increase in a tendency to aggregate, thus resulting in the decrease of both cmc and α. In the present study cmc values of imidazolium ionic liquids were found to be lower compared to their pyridinium analogues. This trend is in accordance with the recently reported values of cmc for imidazolium and pyridinium ionic liquids by Cornellas et al.15
decrease in cmc values compared to simple alkyl chain containing ionic liquids. The thermal stabilities of these ILs were measured by using TGA. Thermal stability data of ILs have also established that the increase of counterion size and introduction of a methyl group at the pyridinium core causes the increase of thermal stability. The experimental results also show that these surface active ionic liquids are thermally stable up to 250 °C. In brief, the studies of these ILs showed that the ester containing ILs have better surface properties compared to other ILs without ester functionality.
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ASSOCIATED CONTENT
S Supporting Information *
1
H, 13C NMR spectra, 13C DEPT, 2D COSY and 2D HETCOR, mass spectra, IR spectra, and X-ray crystallographic data of ionic liquids studied in this research. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors are thankful to the DST (Department of Science and Technology), India, for providing the research grant (SR/ S1/OC-35/2010) for this work. R.A. is also thankful for the financial support by the UGC SAP in the form of a research fellowship. We are thankful to the Sophisticated Analytical Instrumentation Facility (SAIF) at Panjab University (Chandigarh, India) for the 1H, 13C DEPT, 2D COSY, HETCOR, and mass spectral analysis of the ILs.
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CONCLUSIONS These ionic liquids have been synthesized without using high temperature, long reaction hours, and complex purification techniques such as column chromatography. Therefore, these ionic liquids have been synthesized using energy-saving and cost-effective methodology. The surface activities of these ionic liquids were determined by surface tension and conductivity measurements. By comparing cmc values with those of other similar ionic liquids, without ester functionality, i.e., [C8Pyr]Br,15 [C8MIm]Br,15 and [C8MIm]Cl,15 it has been concluded that ester containing ionic liquids exhibited a nearly 10−15-fold 1188
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Industrial & Engineering Chemistry Research
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