Novel ionic liquid-type Gemini surfactants: Synthesis, surface property and antimicrobial activity

Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 116–124

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Novel ionic liquid-type Gemini surfactants: Synthesis, surface property and antimicrobial activity Hongqi Li ∗ , Chaochao Yu, Rui Chen, Juan Li, Jinxing Li Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Road, Shanghai 201620, China

a r t i c l e

i n f o

Article history: Received 6 September 2011 Received in revised form 28 November 2011 Accepted 7 December 2011 Available online 16 December 2011 Keywords: Gemini surfactants Ionic liquid Surface activity Foaming ability Antimicrobial activity

a b s t r a c t Novel quaternary ammonium Gemini surfactants, which turned out to be ionic liquids due to the intermolecular hydrogen bonding formed from hydroxyl groups in the spacers, were synthesized and characterized by IR, 1 H NMR, 13 C NMR and mass spectra. The surface properties of the Gemini surfactants were investigated by means of surface tension measurements. Surface tension parameters including surface excess concentration,  cmc , surface area demand per molecule, Acmc , efficiency in surface tension reduction, pC20 , the effectiveness of surface tension reduction,  cmc , critical micelle concentration, CMC, 0 0 and standard free energy of micellization, Gads and Gmic were obtained. The results indicated that compared with those of their corresponding conventional single-chain surfactant counterparts, these novel Gemini surfactants exhibited lower CMC values and greater efficiency in lowering the surface tension of water. The foamability and foam stability of these Gemini surfactants decreased with the increase in the length of the spacer chain. Gemini surfactants with alkyl chains of moderate length (C12 or C14 ) showed the best foamability and the highest foam stability. Some of the Gemini surfactants exhibited antimicrobial activity against Gram-negative bacteria Escherichia coli. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Recently new types of surfactant called Gemini surfactants attracted increasing attention from both industrial and academic fields. Gemini surfactants are a new class of surfactants having two hydrophobic tails and two hydrophilic head groups connected through a linkage adjacent to the hydrophilic head groups in a molecule. They display greater propensity to form micelles and can efficiently reduce surface tension compared with their corresponding conventional single-chain surfactant counterparts [1–9]. Gemini surfactants have been used as detergents [10], diesel fuel wax or dye dispersants [11,12], auxiliaries for disperse dyeing of polyester fiber [13], photosensitive vesicles or fluid [14,15], nano-templating agents [16,17], gene delivery vector [18], and antimicrobial agents [19–26]. Ionic liquid-type Gemini surfactants were also synthesized [27–29] which showed discotic liquid crystalline property [30] and were used in dispersion of multiwalled carbon nanotubes [31]. These ionic liquid-type Gemini surfactants unexceptionally are based on imidazolium unit, the typical ionic liquid core. We have found that novel quaternary ammonium Gemini surfactants with hydroxyl groups but without imidazolium also

∗ Corresponding author. Tel.: +86 21 6779 2594; fax: +86 21 6779 2608. E-mail address: [email protected] (H. Li). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.014

turn out to be ionic liquids. Herein we report the synthesis and surface and antimicrobial activity of these Gemini surfactants. 2. Experimental 2.1. Materials and instruments All materials and solvents used in this study were of analytical grade. All chemical reagents were purchased from commercial sources and used as received unless other statements. 1 H NMR and 13 C NMR spectra were recorded on a Bruker AM 400 spectrometer. CDCl3 or DMSO-D6 was used as solvent and chemical shifts recorded were internally referenced to Me4 Si (0 ppm). IR spectra were obtained on a Thermo Electron Corporation Nicolet 380 FT-IR spectrophotometer. Mass spectra were obtained on a Bruker APEX III 7.0 instrument (EI, 70 eV) or an Agilent 6000 LC-MS instrument (ESI, positive mode, 70–1000 amu). Surface tension measurements were carried out on a JK99C automatic tensiometer. 2.2. Synthesis of Gemini surfactants 2.2.1. Synthesis of compound 1 To a mixture consisting of epichlorohydrin (13.5 g, 0.15 mol), potassium hydroxide (8.4 g, 0.15 mol), tetrabutylammonium bisulfate (0.4 g, 1.18 mmol) and water (0.6 mL) was added ethylene

H. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 116–124

glycol (1.55 g, 12.5 mmol) dropwise at 0 ◦ C. The mixture was stirred for 20 min then heated to 40 ◦ C and stirred for 20 min to produce yellow paste. Methylene chloride (15 mL) was added to the reaction mixture then it was filtered. The precipitate was washed with methylene chloride then the filtrate and washings were combined and dried over anhydrous magnesium sulfate. After evaporation to remove solvent, the residue was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, 1:4, v/v) to give the target compound ethylene glycol diglycidyl ether (1a) as yellow oil (yield 2.15 g, 49%). IR (KBr): 3482, 3057, 2998, 2873, 1457, 1337, 1104, 911, 855, 759 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 2.54 (m, 2H, CH2 ), 2.72 (m, 2H, CH2 ), 3.10 (m, 2H, CH), 3.27 (m, 2H, CH2 ), 3.57 (s, 4H, CH2 ), 3.72 (m, 2H, CH2 ). 13 C NMR (100 MHz, DMSO-D6 ): ı = 46.16 (CH2 ), 53.05 (CH), 72.69 (CH2 ), 74.31 (CH2 ). Compound 1b and 1c were prepared by the same procedure. Diethylene glycol diglycidyl ether (1b): yellow oil, yield 42%. IR (KBr)  = 3454, 3059, 2999, 2874, 1639, 1455, 1352, 1253, 1101, 911, 856, 758, 536 cm−1 . 1 H NMR (400 MHz, CDCl3 ): ı = 2.60 (m, 2H, CH2 ), 2.78 (m, 2H, CH2 ), 3.15 (m, 2H, CH), 3.43 (m, 2H, CH2 ), 3.66 (m, 8H, CH2 CH2 ), 3.78 (m, 2H, CH2 ). Triethylene glycol diglycidyl ether (1c): yellow oil, yield 33%. IR (KBr)  = 3507, 3057, 2998, 2873, 1643, 1456, 1350, 1253, 1101, 911, 856, 769, 524 cm−1 . 1 H NMR (400 MHz, CDCl3 ): ı = 2.61 (m, 2H, CH2 ), 2.80 (m, 2H, CH2 ), 3.16 (m, 2H, CH), 3.44 (m, 2H, CH2 ), 3.67 (m, 12H, CH2 CH2 ), 3.78 (m, 2H, CH2 ). 2.2.2. Synthesis of compound 2 To a flask containing compound 1a (1.74 g, 0.01 mol) was added 33% aqueous dimethylamine (27.3 g, 0.2 mol) dropwise at room temperature. Then the mixture was stirred for another 72 h. After reaction the mixture was subjected to evaporation and the residue was purified by column chromatography on silica gel (methanol/ethyl acetate, 1:5, v/v) to give the target compound ␣,␻-bis(dimethylaminomethyl)trioxyethylene glycol (2a) as yellow oil (yield 1.1 g, 42%). IR (KBr): 3380, 2943, 2863, 2824, 1654, 1460, 1263, 1121, 1037, 933, 837, 668 cm−1 . 1 H NMR (400 MHz, CDCl3 ): ı = 2.25 (m, 2H, CH2 ), 2.28 (s, 12H, CH3 ), 2.43 (m, 2H, CH2 ), 3.45 (m, 2H, OCH2 ), 3.54 (m, 2H, OCH2 ), 3.68 (s, 4H, CH2 ), 3.88 (m, 2H, CH). 13 C NMR (100 MHz, CDCl3 ): ı = 45.68 (CH3 ), 62.04 (NCH2 ), 67.01 (CH), 70.70 (OCH2 ), 74.00 (OCH2 ). MS (EI, 70 eV): m/z (%) 206 (M+ −Me2 NCH2 , 18), 162 (14), 88 (6%), 58 (Me2 NCH2 + , 100). Compound 2b and 2c were synthesized by the same procedure. ␣,␻-Bis(dimethylaminomethyl)tetraoxyethylene glycol (2b): yellow oil, yield 45%. IR (KBr): 3444, 2942, 1635, 1568, 1457, 1416, 1114, 1035, 566 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 2.22 (s, 12H, CH3 ), 2.26 (m, 2H, CH2 ), 2.33 (m, 2H, CH2 ), 3.30 (m, 2H, OCH2 ), 3.37 (m, 2H, OCH2 ), 3.52 (m, 8H, CH2 CH2 ), 3.72 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D6 ): ı = 48.33 (CH3 ), 65.04 (NCH2 ), 69.80 (CH), 72.54 (OCH2 ), 72.86 (OCH2 ), 76.71 (OCH2 ). ␣,␻-Bis(dimethylaminomethyl)pentaoxyethylene glycol (2c): yellow oil, yield 50%. IR (KBr): 3432, 2944, 1637, 1400, 1384, 1099, 597 cm−1 . 1 H NMR (400 MHz, CDCl3 ): ı = 2.32 (s, 12H, CH3 ), 2.35 (m, 2H, CH2 ), 2.48 (m, 2H, CH2 ), 3.45 (m, 2H, OCH2 ), 3.53 (m, 2H, OCH2 ), 3.65 (m, 8H, CH2 ), 3.90 (m, 2H, CH), 4.12 (s, 4H, CH2 ). 13 C NMR (100 MHz, CDCl3 ): ı = 45.47 (CH3 ), 61.97 (NCH2 ), 66.92 (CH2 ), 70.42 (CH2 ), 70.45 (CH2 ), 70.65 (CH2 CH2 ), 74.04 (CH). 2.2.3. Synthesis of Gemini surfactants 3 (n-EOm -n) To a flask containing 1-bromooctane (3.54 g, 9.17 mmol) and absolute ethanol (50 mL) was added compound 2a (1.1 g, 4.17 mmol) at room temperature. The mixture was refluxed for 48 h. After reaction the mixture was evaporated and the yellow oily residue was repeatedly washed with anhydrous diethyl ether to afford the target Gemini surfactant ␣,␻-bis(octyldimethylammoniomethyl)trioxyethylene glycol

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dibromide (8-EO1 -8) as yellow oil (yield 5.15 g, 86%). IR (KBr): 3416, 2926, 2857, 1633, 1468, 1256, 1116, 1032, 986, 906, 723, 567 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.86 (t, 6H, CH3 ), 1.28 (m, 20H, CH2 ), 1.76 (m, 4H, CH2 ), 3.11 (s, 12H, NCH3 ), 3.34 (m, 16H, CH2 ), 4.19 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D6 ): ı = 16.73 (CH3 ), 23.97 (CH2 ), 24.57 (CH2 ), 28.81 (CH2 ), 28.56 (CH2 ), 31.25 (CH2 ), 33.94 (CH2 ), 53.88 (NCH3 ), 54.00 (NCH3 ), 66.73 (NCH2 ), 67.02 (CH), 68.58 (NCH2 ), 72.79 (OCH2 ), 75.75 (OCH2 ). MS (ESI): m/z 671.7 (M+Na)+ (Calc. 671.3), 563.6 (M−C6 H13 )+ . Other Gemini surfactants were synthesized by the same procedure. ␣,␻-Bis(decyldimethylammoniomethyl)trioxyethylene glycol dibromide (10-EO1 -10): yellow oil, yield 81%. IR (KBr): 3442, 2925, 2854, 1638, 1466, 1120, 721, 564 cm−1 . 1 H NMR (400 MHz, DMSOD6 ): ı = 0.86 (t, 6H, CH3 ), 1.28 (m, 28H, CH2 ), 1.76 (m, 4H, CH2 ), 3.09 (s, 12H, NCH3 ), 3.34 (m, 16H, CH2 ), 4.19 (m, 2H, CH). MS (ESI): m/z 729.7 (M+Na)+ (Calc. 729.7), 564.5 (M−C10 H22 )+ . ␣,␻-Bis(dodecyldimethylammoniomethyl)trioxyethylene glycol dibromide (12-EO1 -12): yellow oil, yield 83%. IR (KBr): 3442, 2924, 2853, 1637, 1466, 1120, 721, 564 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.85 (t, 6H, CH3 ), 1.28 (m, 36H, CH2 ), 1.76 (m, 4H, CH2 ), 3.12 (s, 12H, NCH3 ), 3.40 (m, 16H, CH2 ), 4.19 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D ): ı = 16.69 (CH ), 21.29 (CH ), 28.59 6 3 2 (CH2 ), 31.32 (CH2 ), 31.48 (CH2 ), 31.66 (CH2 ), 31.75 (CH2 ), 31.77 (CH2 ), 31.81 (CH2 ), 34.06 (CH2 ), 35.02 (CH2 ), 53.85 (NCH3 ), 53.98 (NCH3 ), 66.71 (NCH2 ), 67.01 (CH), 68.53 (NCH2 ), 72.80 (OCH2 ), 75.75 (OCH2 ). MS (EI, 70 eV): m/z (%) 168 (C12 H24 + , 20), 57 (C4 H9 + , 100). ␣,␻-Bis(tetradecyldimethylammoniomethyl)trioxyethylene glycol dibromide (14-EO1 -14): yellow oil, yield 74%. IR (KBr): 3422, 2923, 2853, 1638, 1466, 1118, 721, 567 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.83 (t, 6H, CH3 ), 1.24 (m, 44H, CH2 ), 1.76 (m, 4H, CH2 ), 3.09 (s, 12H, NCH3 ), 3.35 (m, 16H, CH2 ), 4.18 (m, 2H, CH). MS (ESI): m/z 561.6 (M−C7 H15 Br2 )+ . ␣,␻-Bis(hexadecyldimethylammoniomethyl)trioxyethylene glycol dibromide (16-EO1 -16): yellow oil, yield 65%. IR (KBr): 3440, 2918, 2850, 1639, 1467, 1416, 1114, 721, 562 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.84 (t, 6H, CH3 ), 1.24 (m, 52H, CH2 ), 1.76 (m, 4H, CH2 ), 3.07 (s, 12H, NCH3 ), 3.36 (m, 16H, CH2 ), 4.18 (m, 2H, CH). MS (ESI): m/z 517.6 (M−C14 H31 Br2 )+ , 266.3 (M−2C16 H33 Br)+ . ␣,␻-Bis(octyldimethylammoniomethyl)tetraoxyethylene glycol dibromide (8-EO2 -8): yellow oil, yield 90%. IR (KBr): 3441, 2925, 2857, 1638, 1467, 1114, 987, 723, 566 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.85 (t, 6H, CH3 ), 1.27 (m, 20H, CH2 ), 1.76 (m, 4H, CH2 ), 3.10 (s, 12H, NCH3 ), 3.62 (m, 20H, CH2 ), 4.20 (m, 2H, CH). MS (ESI): m/z 715.6 (M+Na)+ (Calc. 715.3), 421.5 (M−C8 H19 Br2 )+ . ␣,␻-Bis(decyldimethylammoniomethyl)tetraoxyethylene glycol dibromide (10-EO2 -10): yellow oil, yield 91%. IR (KBr): 3417, 2924, 2855, 1637, 1466, 1119, 993, 902, 721, 565 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.86 (t, 6H, CH3 ), 1.28 (m, 28H, CH2 ), 1.76 (m, 4H, CH2 ), 3.10 (s, 12H, NCH3 ), 3.76 (m, 20H, CH2 ), 4.19 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D6 ): ı = 16.72 (CH3 ), 24.55 (CH2 ), 24.86 (CH2 ), 28.56 (CH2 ), 31.28 (CH2 ), 31.43 (CH2 ), 31.60 (CH2 ), 31.67 (CH2 ), 34.05 (CH2 ), 53.89 (NCH3 ), 54.03 (NCH3 ), 66.71 (NCH2 ), 66.99 (CH), 68.56 (NCH2 ), 72.46 (OCH2 ), 72.93 (OCH2 ), 75.71 (OCH2 ). MS (ESI): m/z 771.7 (M+Na)+ (Calc. 771.4), 463.5 (M−C9 H19 Br2 )+ . ␣,␻-Bis(dodecyldimethylammoniomethyl)tetraoxyethylene glycol dibromide (12-EO2 -12): yellow oil, yield 88%. IR (KBr): 3437, 2923, 2853, 1639, 1466, 1115, 721, 564 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.85 (t, 6H, CH3 ), 1.28 (m, 36H, CH2 ), 1.76 (m, 4H, CH2 ), 3.09 (s, 12H, NCH3 ), 3.35 (m, 20H, CH2 ), 4.19 (m, 2H, CH). MS (ESI): m/z 637.5 (M−C12 H25 )+ . ␣,␻-Bis(tetradecyldimethylammoniomethyl)tetraoxyethylene glycol dibromide (14-EO2 -14): yellow oil, yield 86%. IR (KBr): 3427, 2923, 2853, 1639, 1466, 1119, 721, 566 cm−1 . 1 H NMR (400 MHz,

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DMSO-D6 ): ı = 0.83 (t, 6H, CH3 ), 1.24 (m, 44H, CH2 ), 1.76 (m, 4H, CH2 ), 3.09 (s, 12H, NCH3 ), 3.35 (m, 20H, CH2 ), 4.18 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D ): ı = 14.41 (CH ), 22.25 (CH ), 22.55 6 3 2 (CH2 ), 26.26 (CH2 ), 28.99 (CH2 ), 29.01 (CH2 ), 29.17 (CH2 ), 29.31 (CH2 ), 29.42 (CH2 ), 29.45 (CH2 ), 29.48 (CH2 ), 29.52 (CH2 ), 31.75 (CH2 ), 51.57 (NCH3 ), 51.72 (NCH3 ), 64.40 (NCH2 ), 64.69 (CH), 66.27 (NCH2 ), 70.15 (OCH2 ), 70.62 (OCH2 ), 73.41 (OCH2 ). MS (ESI): m/z 727.8 (M−C4 H8 Br)+ . ␣,␻-Bis(hexadecyldimethylammoniomethyl)tetraoxyethylene glycol dibromide (16-EO2 -16): yellow waxy solid, yield 70%. IR (KBr): 3432, 2918, 2850, 1638, 1467, 1415, 1110, 1021, 721, 564 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.84 (t, 6H, CH3 ), 1.24 (m, 52H, CH2 ), 1.76 (m, 4H, CH2 ), 3.07 (s, 12H, NCH3 ), 3.32 (m, 20H, CH2 ), 4.18 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D6 ): ı = 16.72 (CH3 ), 24.27 (CH2 ), 24.86 (CH2 ), 28.57 (CH2 ), 30.91 (CH2 ), 30.99 (CH2 ), 31.11 (CH2 ), 31.30 (CH2 ), 31.47 (CH2 ), 31.54 (CH2 ), 31.62 (CH2 ), 31.66 (CH2 ), 31.76 (CH2 ), 31.81 (CH2 ), 34.06 (CH2 ), 54.04 (NCH3 ), 66.76 (NCH2 ), 67.01 (CH), 68.62 (NCH2 ), 72.46 (OCH2 ), 72.93 (OCH2 ), 75.63 (OCH2 ). MS (ESI): m/z 561.6 (M−C14 H31 Br2 )+ . ␣,␻-Bis(octyldimethylammoniomethyl)pentaoxyethylene glycol dibromide (8-EO3 -8): yellow oil, yield 85%. IR (KBr): 3405, 2924, 2857, 1634, 1467, 1350, 1252, 1112, 988, 935, 721, 565 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.85 (t, 6H, CH3 ), 1.27 (m, 20H, CH2 ), 1.76 (m, 4H, CH2 ), 3.10 (s, 12H, NCH3 ), 3.38 (m, 24H, CH2 ), 4.20 (m, 2H, CH). MS (ESI): m/z 465.5 (M−C8 H17 Br2 )+ . ␣,␻-Bis(decyldimethylammoniomethyl)pentaoxyethylene glycol dibromide (10-EO3 -10): yellow oil, yield 82%. IR (KBr): 3419, 2924, 2855, 1637, 1467, 1351, 1250, 1113, 994, 949, 722, 570 cm−1 . 1 H NMR (400 MHz, DMSO-D ): ı = 0.86 (t, 6H, CH ), 1.28 (m, 28H, 6 3 CH2 ), 1.76 (m, 4H, CH2 ), 3.10 (s, 12H, NCH3 ), 3.38 (m, 24H, CH2 ), 4.19 (m, 2H, CH). MS (ESI): m/z 493.5 (M−C10 H21 Br2 )+ . ␣,␻-Bis(dodecyldimethylammoniomethyl)pentaoxyethylene glycol dibromide (12-EO3 -12): yellow oil, yield 56%. IR (KBr): 3444, 2923, 2854, 1638, 1415, 1344, 1245, 1111, 721, 565 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.85 (t, 6H, CH3 ), 1.28 (m, 36H, CH2 ), 1.76 (m, 4H, CH2 ), 3.09 (s, 12H, NCH3 ), 3.38 (m, 24H, CH2 ), 4.19 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D6 ): ı = 13.94 (CH3 ), 21.74 (CH2 ), 22.04 (CH2 ), 25.07 (CH2 ), 28.50 (CH2 ), 28.69 (CH2 ), 28.81 (CH2 ), 28.93 (CH2 ), 29.00 (CH2 ), 30.68 (CH2 ), 31.27 (CH2 ), 51.08 (NCH3 ), 51.23 (NCH3 ), 63.91 (NCH2 ), 64.15 (CH), 65.75 (NCH2 ), 69.63 (OCH2 ), 69.78 (OCH2 ), 70.12 (OCH2 ), 72.87 (OCH2 ). MS (ESI): m/z 647.6 (M–C3 H7 Br2 )+ . ␣,␻-Bis(tetradecyldimethylammoniomethyl)pentaoxyethylene glycol dibromide (14-EO3 -14): yellow waxy solid, yield 68%. IR (KBr): 3427, 2923, 2853, 1637, 1467, 1352, 1112, 721, 564 cm−1 . 1 H NMR (400 MHz, DMSO-D ): ı = 0.83 (t, 6H, CH ), 1.24 (m, 6 3 44H, CH2 ), 1.76 (m, 4H, CH2 ), 3.09 (s, 12H, NCH3 ), 3.36 (m, 24H, CH2 ), 4.18 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D6 ): ı = 13.93 (CH3 ), 21.74 (CH2 ), 22.07 (CH2 ), 25.75 (CH2 ), 28.50 (CH2 ), 28.69 (CH2 ), 28.70 (CH2 ), 28.81 (CH2 ), 28.93 (CH2 ), 29.00 (CH2 ), 29.04 (CH2 ), 30.68 (CH2 ), 31.27 (CH2 ), 51.08 (NCH3 ), 51.24 (NCH3 ), 63.86 (NCH2 ), 64.15 (CH), 65.73 (NCH2 ), 69.62 (OCH2 ), 69.78 (OCH2 ), 70.12 (OCH2 ), 72.86 (OCH2 ). MS (ESI): m/z 703.7 (M−C3 H7 Br2 )+ . ␣,␻-Bis(hexadecyldimethylammoniomethyl)pentaoxyethylene glycol dibromide (16-EO3 -16): yellow waxy solid, yield 65%. IR (KBr): 3419, 2918, 2850, 1634, 1467, 1350, 1245, 1110, 721, 562 cm−1 . 1 H NMR (400 MHz, DMSO-D6 ): ı = 0.84 (t, 6H, CH3 ), 1.24 (m, 52H, CH2 ), 1.76 (m, 4H, CH2 ), 3.07 (s, 12H, NCH3 ), 3.33 (m, 24H, CH2 ), 4.18 (m, 2H, CH). 13 C NMR (100 MHz, DMSO-D6 ): ı = 13.92 (CH3 ), 21.74 (CH2 ), 22.06 (CH2 ), 25.76 (CH2 ), 28.50 (CH2 ), 28.69 (CH2 ), 28.70 (CH2 ), 28.75 (CH2 ), 28.81 (CH2 ), 28.85 (CH2 ), 28.93 (CH2 ), 29.00 (CH2 ), 29.04 (CH2 ), 30.66 (CH2 ), 31.26 (CH2 ), 51.08 (NCH3 ), 51.32 (NCH3 ), 63.95 (NCH2 ), 64.29 (CH), 65.81 (NCH2 ), 69.62 (OCH2 ), 69.71 (OCH2 ), 70.14 (OCH2 ), 72.88 (OCH2 ). MS (ESI): m/z 551.7 (M−C18 H37 Br2 )+ .

2.3. Surface tension measurements The surface tension of aqueous solutions was measured at 20 ◦ C by the du Nouy ring method. Calibration was performed against a range of standard liquids to obtain an excellent agreement with the reference values. The surface tension was measured three times for each sample with a 40-min interval between each reading to ensure equilibrium data. The critical micelle concentration (CMC) values were determined using a series of aqueous solutions at various concentrations, and estimated from the break point of each surface tension versus concentration on (log scale) curves. The ability of the surfactants to lower surface tension at the CMC ( cmc ) was calculated therefrom. The pC20 value of each surfactant is defined to be the surfactant concentration where a decrease in the surface tension of 20 mN m−1 from pure water is recorded (pC20 = –log C20 ), and therefore, this value is indicative of an efficiency in lowering the surface tension. The occupied area per molecule at the cmc (Acmc ) was estimated from the surface excess concentration estimated at the cmc,  cmc . The latter one was calculated by applying the Gibbs adsorption isotherm equation. It has been addressed by Thomas et al. and Eastoe et al. that the implication of a constant gradient for the pre-CMC tension data is that the adsorption is constant too. Generally it is not the case because the existence of impurities in the surfactants [32–34]. In most cases  cmc could be calculated without a significant error by applying the Gibbs adsorption isotherm equation:  cmc = –(1/n 2.303 RT) (d/d log C), where R is the gas constant and T is the absolute temperature,  is the equilibrium surface tension measured at the surfactant concentration of C and n equals 3 for the Gemini surfactants [35–39]. The area per molecule at the interface was calculated from Acmc = 1/N  cmc where N is Avogadro’s number [25,37–39]. The standard free energy of micel0 , could be approximated lization that occurs in bulk solution, Gmic 0 without a significant error by the equation Gmic = RT ln(cmc/ω), proposed for ionic Gemini surfactants when the cmc is less than 10−2 mol dm−3 [40], where ω is the molarity of water. The standard free energy of adsorption to the air–aqueous solution interface, 0 , was calculated by the equation G0 = G0 −  Gads cmc /cmc ads mic where cmc represents the surface pressure at the cmc (= 0 –  cmc ;  0 and  cmc represent the surface tension of water and surfactant solution at the cmc, respectively) [40–44]. Results are summarized in Table 1. 2.4. Foamability and foam stability measurements A calibrated 100 mL glass cylinder with a stopper was used for the measurement of foam stability and foamability. Twenty milliliters of surfactant solution (0.1 wt%) was poured into the calibrated cylinder. The solution was shaken vigorously for 10 s, and the height and volume of the foam were monitored and recorded at different times (5 min, 30 min, 1 h, and 24 h). The initial foam height was reported as the foamability. The foam stability was determined to be the foam height at different times [45]. All measurements were performed at 15 ◦ C. 2.5. Antimicrobial assay Antimicrobial activity of the surfactants against the bacteria strain Escherichia coli was measured using diffusion disc method. The compounds tested were dissolved in water in the concentration of 1.0 or 0.3 mg mL−1 and no precipitate was observed at the higher concentration of the surfactants. A sterilized disc from a filter paper saturated with a measured quantity of the sample was placed on a plate containing a solid bacterial medium (nutrient agar broth) which had been heavily seeded with the spore suspension of the tested organism. After incubation, the diameter of the clear zone of

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Table 1 Parameters obtained from the plots of surface tension of n-EOm -n (n = 8, 10, 12, 14 or 16; m = 1, 2 or 3) as a function of concentration. Surfactant

CMC (mmol dm−3 )

8-EO1 -8 1.02 0.859 10-EO1 -10 12-EO1 -12 0.711 0.243 14-EO1 -14 0.631 16-EO1 -16 1.822 8-EO2 -8 1.70 10-EO2 -10 12-EO2 -12 1.239 0.333 14-EO2 -14 16-EO2 -16 0.389 0.701 8-EO3 -8 0.649 10-EO3 -10 0.607 12-EO3 -12 14-EO3 -14 0.502 0.398 16-EO3 -16 C12 H25 N(CH3 )3 Br [8] 14.75 42

 cmc (mN m−1 )

 cmc (␮mol m−2 )

55.2 32.55 30.57 34.50 37.00 49.08 46.96 28.54 34.81 36.51 52.22 48.28 33.78 33.61 43.92

0.802 1.466 3.00 0.947 1.314 0.519 0.417 1.06 0.813 0.785 0.519 0.827 1.40 1.04 0.925

2.9

0.57

inhibition surrounding the sample was taken as a measure of the inhibition power of the sample against the particular test organism [46–48]. 3. Results and discussion 3.1. Synthesis and characterization of Gemini surfactants We have designed a series of Gemini quaternary ammonium surfactants in the light of several reports that Gemini bis-ammonium surfactants with a pendant hydroxyl group exhibited mesomorphic or thermotropic liquid crystalline properties due to the intermolecular hydrogen bonding [49–51]. The novel Gemini surfactants were synthesized by a three-step protocol, as showed in Fig. 1. Diepoxy compounds 1a–c were synthesized from epichlorohydrin and glycols by a modified phase transfer catalysis reaction as reported [52]. Treatment of diepoxy compounds 1a–c with dimethylamine gave the important intermediates bis(dimethylaminomethyl)oligooxyethylene glycol 2a–c in moderate yields. These amination products were reacted with 1bromohydrocarbons in the next quaternization step to afford novel quaternary ammonium Gemini surfactants 3 (n-EOm -n, m = 1, 2 or 3; n = 8, 10, 12, 14 or 16) in good to excellent yields. The chemical structures of the Gemini surfactants n-EOm -n were characterized by 1 H NMR, 13 C NMR, IR and mass spectrum. Most of the novel quaternary ammonium Gemini surfactants turned out to be ionic liquids at room temperature apart from 16-EO2 -16 and n-EO3 -n (n = 14 or 16) which were yellow waxy solid at room temperature and easily melted upon heating. Several ionic liquid-type Gemini surfactants have been reported and all of them are based on imidazolium moiety as showed in Fig. 2 [27–31]. Ionic liquid-type Gemini surfactant without an imidazolium unit has not been reported yet. Most cationic Gemini surfactants with an alkylene or oligooxyethylene spacer turn out to be solids [2–5,8,9,42,45,53]. If the spacer in the surfactants bears a hydroxyl group, intermolecular hydrogen bonding is expected and can affect the state and property of the surfactants. It has been found that for the Gemini surfactant 2-hydroxylpropanediyl-␣,␻-bis(dimethyldodecylammonium bromide), abbreviated as 12-3(OH)-12, the intermolecular hydrogen bonding between the molecules adsorbed at the air–water interface makes their arrangement tighter and thus produces lower  cmc compared with unsubstituted Gemini surfactant propanediyl-␣,␻bis(dimethyldodecylammonium bromide) (referred to as 12-3-12). Moreover, the effect of intermolecular hydrogen bonding promotes growth of 12-3(OH)-12 micelles and leads to dissociation

Acmc (nm2 mol−1 ) 2.07 1.33 0.554 1.75 1.264 3.20 3.98 1.57 2.04 2.116 3.20 2.01 1.19 1.60 1.796 –50.61

0 Gads (kJ mol−1 )

0 Gmic (kJ mol−1 )

pC20

CMC/pC20

–47.5 –53.9 –37.1 –69.6 –55.0 –80.7 –85.3 –67.1 –75.0 –75.1 –83.0 –62.5 –55.1 –65.2 –60.0

–26.6 –27.0 –27.5 –30.0 –27.7 –25.2 –25.3 –26.1 –29.3 –28.9 –27.5 –27.7 –27.8 –28.3 –28.9

– 3.863 3.64 4.727 4.140 3.417 3.222 4.124 4.630 4.710 – 3.539 4.000 4.389 5.910

– 0.222 0.21 0.0514 0.152 0.533 0.527 0.300 0.072 0.083 – 0.183 0.152 0.114 0.067

–25.31

of the counterions on the aggregate surfaces [51]. Surfactant 12-3(OH)-12 shows the zero-shear viscosity far higher than that of 12-3-12 and forms longer wormlike micelles than 12-3-12 due to the role of the intermolecular hydrogen bonding which occurs between the hydroxyl substituted spacers of 12-3(OH)-12 [4]. Wei and colleagues have reported that Gemini surfactants [Cn H2n+1 N+ (CH3 )2 CH2 CH(OH)CH2 N+ (CH3 )2 Cn H2n+1 ] have relatively low melting points and form thermotropic mesophases over a broad temperature range, as compared to those without a hydroxyl group at the spacer, which is attributed to the intermolecular hydrogen bonding [49]. Heterogemini surfactants [Cm H2m+1 OCH2 CH(OH)CH2 N(CH3 )2 Cn H2n+1 Br] exhibit thermotropic liquid crystalline property because the hydrogen bonding between the hydroxyl and Br− counterion is beneficial to stabilizing the liquid crystal phase [50]. The phenomenon that the novel quaternary ammonium Gemini surfactants n-EOm -n (m = 1, 2 or 3; n = 8, 10, 12, 14 or 16) turn out to be ionic liquids may be attributed to the existence of hydroxyl groups in the spacers which leads to the intermolecular hydrogen bonding. 3.2. Surface activity of Gemini surfactants Surface tension of aqueous solutions of the Gemini surfactants 3 (n-EOm -n) was measured by the du Nouy ring method. Parameters obtained from the plots of surface tension of n-EOm -n (n = 8, 10, 12, 14 or 16; m = 1, 2 or 3) as a function of concentration were summarized in Table 1. It can be seen that the CMC values of the Gemini surfactants are lower than that of the conventional cationic surfactant C12 H25 N(CH3 )3 Br [8] by 1–2 grade, indicating that the linkage of two monomeric cationic surfactants by the oligooxyethylene spacer effectively decreases the electrostatic repulsion between the two head groups thus enables the Gemini surfactants to be more likely to form micelles. The occupied area per molecule at the cmc (Acmc ) values of the Gemini surfactants were larger than that of C12 H25 N(CH3 )3 Br [8], perhaps due to the existence of the large head groups which somewhat affect the molecular arrangement of the surfactants at the surface. The standard free energy of micel0 , and the standard free lization that occurs in bulk solution, Gmic 0 , energy of adsorption to the air–aqueous solution interface, Gads of the Gemini surfactants were negative, indicating that the divalent cationic surfactants can adsorb to the gas–liquid interface and form micelles spontaneously. The influence of the length of alkyl chains on the surface property of the Gemini surfactants n-EOm -n was similar to those observed in the conventional surfactants, namely, the CMC values decrease with the increase in the length of the hydrophobic

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Cl +

O

O

KOH, Bu4NHSO4

HO R OH

O R O O 1a. R = CH2CH2, 49% 1b. R = CH2CH2OCH2CH2, 42% 1c. R = CH2CH2OCH2CH2OCH2CH2, 33%

OH NH

O R O

N

CnH2n+1Br

N OH

2a. R = CH2CH2, 42% 2b. R = CH2CH2OCH2CH2, 45% 2c. R = CH2CH2OCH2CH2OCH2CH2, 50% OH Br

N

O R O

N OH

CnH2n+1

Br CnH2n+1

3: n-EOm-n 2Br 8-EO1-8: R = CH2CH2, n = 8, 86% 10-EO1-10: R = CH2CH2, n = 10, 81% 12-EO1-12: R = CH2CH2, n = 12, 83% 14-EO1-14: R = CH2CH2, n = 14, 74% 16-EO1-16: R = CH2CH2, n = 16, 65% 8-EO2-8: R = CH2CH2OCH2CH2, n = 8, 90% 10-EO2-10: R = CH2CH2OCH2CH2, n = 10, 91% 12-EO2-12: R = CH2CH2OCH2CH2, n = 12, 88% 14-EO2-14: R = CH2CH2OCH2CH2, n = 14, 86% 16-EO2-16: R = CH2CH2OCH2CH2, n = 16, 70% 8-EO3-8: R = CH2CH2OCH2CH2OCH2CH2, n = 8, 85% 10-EO3-10: R = CH2CH2OCH2CH2OCH2CH2, n = 10, 82% 12-EO3-12: R = CH2CH2OCH2CH2OCH2CH2, n = 12, 56% 14-EO3-14: R = CH2CH2OCH2CH2OCH2CH2, n = 14, 68% 16-EO3-16: R = CH2CH2OCH2CH2OCH2CH2, n = 16, 65% Fig. 1. Synthetic route to Gemini surfactants 3 (n-EOm -n).

alkyl chains. Plots of surface tension of Gemini surfactants n-EO1 n (n = 8, 10, 12, 14 or 16) as a function of concentration on log scale were shown in Fig. 3. It can be seen that the CMC values of the Gemini surfactants n-EO1 -n (n = 8, 10, 12, 14 or 16) decrease with n varies from 8 to 14. The except that 16-EO1 -16 shows a larger CMC value than 14-EO1 -14 may be interpreted by the ocurrence of self-coiling or premicellization of the surfactant 16-EO1 -16 molecules, which generally appears only when the carbon atoms in hydrophobic chains are more than 14 [37]. In the other two series of surfactants n-EO2 -n (n = 8, 10, 12, 14 or 16) and n-EO3 -n (n = 8, 10, 12, 14 or 16) the CMC values essentially decrease with n varies from 8 to 14, with an exception observed for

Br C12H25

N

N

CH2

s

s = 2, 4, 6

N

N

Br C12H25

16-EO2 -16. The results are in good accordance with that has been found for (oligooxa)-␣,␻-bis(m-alkylbenzene sulfonate) anionic Gemini surfactants (Cn-E4 -Cn, n = 8, 10 or 12), for which the CMC values decrease with an increase in the alkyl chain length from 8 to 12 [54]. Effect of spacer length on the surface property of the Gemini surfactants n-EOm -n can be seen from Table 1 and Fig. 4. The latter shows the plots of surface tension of Gemini surfactants 12-EOm -12 (m = 1, 2 or 3) as a function of concentration on log scale. The surface tension of Gemini surfactants n-EOm -n generally does not show marked difference with change in the spacer length (m = 1, 2 or 3) and the CMC values decrease for the Gemini

Br R

N

N

CH2

4

N

N

Br R

R = C10H21, C12H25, C14H29

Fig. 2. Typical structures of ionic liquid-type Gemini imidazolium surfactants.

H. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 116–124

121

Table 2 Surface tension parameters of Gemini surfactant 14-EO1 -14 at different temperatures. Temperature (◦ C)

CMC (mmol dm−3 )

 cmc (mN m−1 )

pC20

CMC/pC20

20 30 40

0.243 0.380 0.437

34.50 29.36 27.73

4.727 5.15 5.27

0.0514 0.0849 0.0721

Fig. 3. Plots of surface tension of Gemini surfactants n-EO1 -n (n = 8, 10, 12, 14 or 16) as a function of concentration on log scale.

surfactants n-EO3 -n compared to their counterparts with a shorter spacer length (n-EO1 -n and n-EO2 -n). The results may be attributed to two reasons. On one hand, the spacer group is restricted to the micelle–water (or air–water) interface for short spacer groups [55,56], leading to increased rigidity of the molecule which hinders the preferential location of the spacer group at or near the surface in the micelles. However, the rigidity will diminish as the length of the spacer increases (to m = 3). On the other hand, the oxygen atoms in the spacer can form hydrogen bonds with water or hydroxyl groups, and the additional hydration at the level of the spacer chain should mitigate the Coulombic repulsion between the head groups. Therefore the electrostatic repulsions between the positively charged head groups will diminish when the spacer length increases. These contributions lead to the result that the Gemini surfactants with a long spacer aggregate at a lower concentration. This is in consistent with the results obtained for other Gemini surfactants [54,57] though contrast observations have also been reported for 12-CH2 CH2 (EO)S -12 series of Gemini surfactants, for which the CMC values are found to increase as the number of oxyethylene segments increases possibly due to the spacer’s flexible character and Coulombic repulsion between the head groups [53,56].

Fig. 5. Plots of surface tension of Gemini surfactant 14-EO1 -14 at different temperatures.

Effect of temperature on the surface property of the Gemini surfactants n-EOm -n has been investigated. Surface tension parameters including CMC,  cmc , pC20 and CMC/pC20 of Gemini surfactant 14-EO1 -14 at different temperatures are listed in Table 2 and plots of surface tension of Gemini surfactant 14-EO1 -14 at different temperatures are shown in Fig. 5. It can be seen that the surface tension of the Gemini surfactant decreases and the pC20 values increase with the increase of temperature from 20 ◦ C to 40 ◦ C, indicating that the efficiency in lowering the surface tension of the Gemini surfactants increases with the increase of temperature. 3.3. Foamability and foam stability of the Gemini surfactants

65

12-EO 1 -12

12-EO 2 -12

60

12-EO 3 -12

surface tension (mN/m)

55

The foaming behavior of the Gemini surfactants n-EOm -n has been investigated. Fig. 6 shows the foam height of aqueous Gemini surfactants n-EO1 -n (n = 8, 10, 12, 14 or 16) at different times (0 min, 5 min, 30 min, 1 h, and 24 h). It is obvious that the Gemini surfactants with a short alkyl chain (n = 8 or 10) or the longest alkyl chain (n = 16) show very weak foamability and foam

50 45 40 35 30 25 -5.0 -4.8 -4.6 -4.4 -4.2 -4.0 -3.8 -3.6 -3.4 -3.2 -3.0 -2.8 -2.6 -2.4 -2.2 log C

Fig. 4. Plots of surface tension of Gemini surfactants 12-EOm -12 (m = 1, 2 or 3) as a function of concentration on log scale.

Fig. 6. Foam height of aqueous Gemini surfactants n-EO1 -n (n = 8, 10, 12, 14 or 16) at different times (0 min, 5 min, 30 min, 1 h, and 24 h).

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Table 3 Antimicrobial activity of Gemini surfactants n-EOm -n on Escherichia coli. Compound

Concentration (mg mL−1 )

8-EO1 -8 10-EO1 -10 12-EO1 -12 14-EO1 -14 8-EO2 -8 10-EO2 -10 12-EO2 -12 14-EO2 -14 8-EO3 -8 10-EO3 -10 12-EO3 -12 14-EO3 -14

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Inhibition zone diameter (mm) 0 0 0 0 0 10.7 6.5 0 8.2 9.7 7.1 8.6

stability. Gemini surfactants with an alkyl chain of modest length (n = 12 or 14) show good foamability and high foam stability. Surfactant 12-EO1 -12 shows the best foamability while the highest foam stability is observed for Gemini surfactant 14-EO1 -14. The result may be explained in terms of the surface tension equilibration of the surfactants [58]. Gemini surfactants with a hydrophobic chain of moderate length (C12 or C14 ) tend to reach fast surface tension equilibration while it is difficult for the Gemini surfactants with a short alkyl chain (less than C10 ) to form stable monolayers which may give rise to stable foams. Attachment of an excessively long alkyl chain (C16 or longer) to the Gemini surfactants may result in the decrease in solubility and thereof the poor foamability and weak foam stability. The result is in consistent with that was reported by Ikeda and colleagues who synthesized a series of Gemini surfactants with different hydrophobic tails and a three-carbon spacer unit with a hydroxyl group at the central carbon and found that both the foamability and the foam stability of the Gemini surfactants were much higher than that of the monomeric surfactants, with the dodecyl chains being optimal in respect of both foamability and foam stability [59]. In addition, dynamic surface tension and interficial reology play a major role in foaming processes. According to previous findings by Yoshimura and colleagues [41], the reduction in the dynamic surface tension depends on the hydrocarbon chain length, the chain numbers, and the concentration. An increase in the concentration and a decrease in the chain length and chain numbers results in a faster decay. The Gemini surfactants exhibit slow adsorption at the air–water interface when the hydrocarbon chain length increases. Therefore the foamability and foam stability of the Gemini surfactants increase with the increase in the hydrocarbon chain length [41]. Effect of the spacer length on the foamability and foam stability of the Gemini surfactants n-EOm -n has also been investigated. The foam heights of aqueous Gemini surfactants 12-EOm -12 (m = 1, 2 or 3) at different times (0 min, 5 min, 30 min, 1 h, and 24 h) are shown in Fig. 7. It is observable that all of the three Gemini surfactants 12-EOm -12 (m = 1, 2 or 3) exhibit good foamability and high foam stability. The influence of the spacer length on the foamability and foam stability of the Gemini surfactants n-EOm -n is far less than that of the hydrophobic alkyl chain. With the increase in the spacer length the foamability and foam stability of the Gemini surfactants decrease gradually. This may be attributed to the molecular arrangement of the surfactants in water. The increase in the spacer length causes twisted molecular structures which hinder the formation of tight films for the Gemini surfactants. Therefore the foamability and foam stability of the surfactants decrease. This result is supported by the observations that were reported by Tehrani-Bagha and Holmberg for cationic ester-containing Gemini surfactants [45].

Concentration (mg mL−1 )

Inhibition zone diameter (mm)

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

0 6.8 0 0 0 6.8 6.1 0 7.2 6.9 6.5 0

Fig. 7. Foam height of aqueous Gemini surfactants 12-EOm -12 (m = 1, 2 or 3) at different times (0 min, 5 min, 30 min, 1 h, and 24 h).

3.4. Antimicrobial activity of the Gemini surfactants The Gemini surfactants n-EOm -n were tested to evaluate their antibacterial property against Gram-negative bacteria strain E. coli. The antimicrobial efficacy was measured by the bacterial growth inhibition expressed as inhibition zone diameter (mm at 1.0 and 0.3 mg mL−1 concentration of the sample). The results were listed in Table 3 from which it could be found that some of the Gemini surfactants n-EOm -n had antimicrobial activity against E. coli while some others including 16-EOm -16 and n-EO1 -n (n = 8, 12 or 14) did not yield any antibacterial inhibition zone. The difference in activity depends on the length of hydrophobic chains, the length of the spacer and the concentration of the Gemini surfactants. Decrease in concentration results in the significant decrease in the antimicrobial activity of the surfactants. The Gemini surfactants bearing long alkyl chains (16-EOm -16) or a short spacer chain (n-EO1 -n) do not have antibacterial activity. Gemini surfactants with a moderate length of hydrophobic chain (10-EOm -10) exhibit the strongest antibacterial activity. Gemini surfactant 10-EO3 -10 is the most efficient among these compounds. 4. Conclusions Novel quaternary ammonium Gemini surfactants were synthesized and characterized by IR, 1 H NMR, 13 C NMR and mass spectrum. Results of surface tension measurements showed that compared with those of their corresponding conventional

H. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 116–124

single-chain surfactant counterparts, these novel Gemini surfactants had lower CMC values. The foamability and foam stability of these Gemini surfactants decreased with the increase of the spacer length. Gemini surfactants with alkyl chains of moderate length (C12 or C14 ) showed the best foamability and the highest foam stability. Some of the Gemini surfactants exhibited antimicrobial activity against Gram-negative bacteria E. coli. Unlike most cationic Gemini surfactants with an alkylene or oligooxyethylene spacer which are solids with relatively high melting points [2–5,8,9,42,45,53], the novel quaternary ammonium Gemini surfactants turn out to be ionic liquids and most of them even turn out to be room temperature ionic liquids. This may be attributed to the existence of hydroxyl groups in the spacer which lead to the intermolecular hydrogen bonding. Effect of the intermolecular hydrogen bonding on the state and property of the surfactants is also observed in several reports [4,49–51]. It has been found that quaternary ammonium Gemini surfactants with a hydroxyl group in the spacer have relatively low melting points and form thermotropic mesophases over a broad temperature range, as compared to those without a hydroxyl group at the spacer, due to the intermolecular hydrogen bonding [49]. Heterogemini surfactants [Cm H2m+1 OCH2 CH(OH)CH2 N(CH3 )2 Cn H2n+1 Br] have been found to exhibit thermotropic liquid crystalline property because the hydrogen bonding between the hydroxyl and Br− counterion is beneficial to stabilizing the liquid crystal phase [50]. However, ionic liquidtype Gemini surfactant without an imidazolium unit has not been reported yet. The new ionic liquid-type Gemini surfactants may be used as new catalysts or versatile functional materials such as multifunctional finishing agents in textile chemistry. Further investigation on the catalytic and biological activity of the Gemini surfactants is in progress.

Acknowledgment The authors acknowledge the support by “the Fundamental Research Funds for the Central Universities”.

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