Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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JIEC-1874; No. of Pages 9

Synthesis, surface active and thermal properties of novel imidazoliumcationic monomeric surfactants

Pankaj Patial a, Arifa Shaheen b,*, Ishtiaque Ahmad a,*a Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Indiab Department of Applied Science, Punjabi University, Patiala 147002, India

A R T I C L E I N F O

Article history:

Received 12 October 2013

Accepted 13 January 2014

Available online xxx

Keywords:

Long chain imidazolium cationics

cmc

Salt effect

Thermal stability

A B S T R A C T

A series of long chain water soluble cationics have been synthesized by using renewable raw materials

like fatty alcohols and epichlorohydrin. The surface activity of the molecules has been determined by

measurement of their conductance and surface tension in aqueous solution. The dynamics of surface

activity of these surfactants have also been investigated in the presence of sodium halides, NaCl and NaBr

by surface tension measurement. A series of useful parameters like critical micelle concentration (cmc),

surface tension at the cmc (gcmc), adsorption efficiency (pC20), effectiveness of surface tension reduction

(Pcmc), Gibbs free energy of the micellization (DG0 mic) and Gibbs free energy of adsorption (DG0 ads)

have been determined from the measurements obtained by surface tension and conductivity method.

Further with the application of the Gibbs adsorption isotherm, maximum surface excess concentration

(umax) and minimum surface area/molecule (Amin) at the air–water interface were also estimated.

Thermal stability of these long chain cationics has been measured by thermal gravimetric analysis under

nitrogen atmosphere. Analysis of thermal stability measurement indicated that the thermal stability of

these long chain imidazoliums increase with an increase in chain length.

� 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering

Chemistry.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

The current chemical research gives significant importance toeco-friendly products. The use of renewable feed stock for thesynthesis of surface active compounds having better surface andbiological properties has become priority in the field of surfaceand colloid science. Several new surfactants have been developedin recent past having renewable structural moieties like sugar,amino acids, fatty esters [1–3], etc. They have been found topossess better surface and biological properties compared toconventional surfactants. Pyridinium and imidazolium surfac-tants particularly are important ingredients of several cosmeticproducts [4–6]. They are often utilized as corrosion inhibitors[7,8] as well as being used in emulsion polymerization, theflotation of minerals and textile processing [9–11]. Biologicalapplications of these surface-active agents include their antimi-crobial activity, as well as their use as drug and gene deliveryagents [12–15]. Several cationic surfactants are also used in DNAextraction methods [16,17]. The overall production of cationic

* Corresponding authors. Tel.: +91 7417596876; fax: +91 183 2258820.

E-mail addresses: chemarifa@gmail.com (A. Shaheen),

chemi_ahmad@yahoo.co.in (I. Ahmad).

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1226-086X/$ – see front matter � 2014 Published by Elsevier B.V. on behalf of The Ko

http://dx.doi.org/10.1016/j.jiec.2014.01.032

surfactant amounts to 350 000–500 000 tons per annum [18–20].One major challenge that industry faces in the 21st century is toget the desired molecule not only in cost-effective manner butalso via environmentally friendly means. Furthermore, environ-mental aspects, such as the biodegradability of the surfactants,must be further improved, so that the surfactants can be easilydegraded in the environment. Cationic surfactants due to theirbiocide activity and emulsification properties are resistant, tosome extent, to biological agents and undergo slow biodegrada-tion [21,22]. In view of the above the present work deals with thesynthesis and evaluation of imidazolium based cationic surfac-tants by renewable raw materials like fatty alcohols andepichlorohydrin. Here we have chosen the greener approach tomake the process environment friendly and cost effective, too. Thepurpose of this work was to prepare and characterize the cationicimidazole based cationic monomeric surfactants from renewableraw materials and to evaluate their surface active properties.These surfactants have been found to have better surfaceproperties as compared to conventional cationic surfactants[23–25]. Further investigations by TGA established superiorthermal stability of these new surfactants. The purpose of thiswork was to prepare and characterize the cationic imidazolebased cationic monomeric surfactants from renewable rawmaterials and to evaluate their surface active properties

(2014), http://dx.doi.org/10.1016/j.jiec.2014.01.032

rean Society of Industrial and Engineering Chemistry.

P. Patial et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx2

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JIEC-1874; No. of Pages 9

2. Materials and methods

2.1. Materials

Epichlorohydrin, 1-bromo-propane, 1-chloro-propane and tet-rabutylammonium iodide were purchased from Sigma–AldrichChemical Co., USA. Lauryl alcohol (dodecyl alcohol), myristylalcohol (tetradecyl alcohol), cetyl alcohols (hexadecyl alcohol) andsilica gel for T.L.C. were purchased from S. D. Fine Chemicals Ltd;

Cl

O

+ RO H

O

OR

O

OR + N

HNOH

OR

N

N

OH

OR

N

N+

HO

RO

N

N

1-3

4-6

7-12

Overnightstirring

Overnightstirring

1 hour

60 0C

70 0C

60 0C

X

X

Mumbai, India. Imidazole and sodium hydroxide was purchasedfrom Merck, Germany.

2.2. Methods

IR spectra were recorded as a thin film on KBr Pellet on aShimadzu 8400s FT-IR (Kyoto, Japan) instrument. Mass spectrawere recorded on Waters Q-T of Micro mass using ESI as an ionsource at sophisticated analytical instrumentation facility (SAIF),***Panjab University, Chandigarh. 1H, DEPT, Cozy and 13C NMRspectra were recorded on a JEOL AL-300 (JEOL, Japan) system as asolution in CDCl3, using tetramethylsilane (TMS) as an internalstandard.

2.2.1. Synthesis of cationic imidazolium surfactants

The preparation of compounds (1–3 and 4–6) is done in ourprevious work [26]. In continuation of our work the compound (4–6)on further treatment with 1-chloropropane and 1-bromopropane(1:1) at 60 8C for 1 h leads to the preparation of imidazoliumsurfactants (7–12). In each case the resulting crude product wascrystallized with ether and subsequently recrystallized in coldacetone to get the pure compounds (7–12) which were character-ized on the basis of IR, 1H, 13C NMR, DEPT, COSY and mass spectral

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analysis as 1-(3-(hexadecyloxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-ium chloride (7), 1-(3-(tetradecyloxy)-2-hydroxypro-pyl)-3-propyl-1H-imidazol-3-ium chloride (8), 1-(3-(dodecy-loxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-ium chloride(9), 1-(3-(hexadecyloxy)-2-hydroxypropyl)-3-propyl-1H-imida-zol-3-ium bromide (10), 1-(3-tetradecyloxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-ium bromide (11), 1-(3-(dodecyloxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-ium bromide (12). Theschemes of the reactions are shown below.

2.2.1.1. Spectral results. 1-(3-(Hexadecyloxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-ium chloride (7). White solid, Yield,92.5%. Melting point 117 8C. The IR (KBr Pellet, cm�1): 1126, 1564,2949, 3047, and 3343. 1H NMR (d ppm, CDCl3): 0.81 (t, 3H, CH3),1.25 (d, chain 26H, (–CH2–)13, 1.49 (m, 2H, CH2 next to terminalmethyl groups), 2.17 (t, 3H, N+CH2CH2CH3), 2.71 (d, 1H, OH), 3.36(d, 2H, OCH2), 3.50 (t, 2H, CH2O), 3.89 (m, 1H, CHOH), 4.18 (m, 2H,N+CH2CH2CH3), 4.34 (t, 2H, NCH2), 4.45 (d, 2H, N+CH2), 7.28 (d, 1H,a CH to +ve Nitrogen of imidazole), 7.32 (d, 1H, b CH to +veNitrogen of imidazole), 9.32 (s, H, sandwiched proton between tonitrogen of imidazole). 75 MHz 13C/DEPT NMR (d ppm, CDCl3):14.45 (terminal methyl carbons), 22.26 (CH3CH2), 26.07(OCH2CH2), 29.22 (chain methylene carbons), 29.68(N+CH2CH2CH3), 31.61 (N+CH2CH2CH3), 48.24 (NCH2), 50.02(N+CH2), 67.38 (secondary carbon attached to hydroxyl group),71.00 (OCH2), 71.87 (CH2O), 103.54 (NCH2CH2N), 123.21 (sand-wiched carbon between to nitrogen of imidazole). ESI-MS positiveions at m/z (relative intensity %) 407.3 (100%) {(M�2Cl)�1)}+,408.3 (40%) (M�2Cl)+.

1-(3-(Tetradecyloxy)-2-hydroxypropyl)-3-propyl-1H-imida-zol-3-ium chloride (8). White solid, Yield, 92.5%. Melting point112 8C. The IR (KBr Pellet, cm�1): 1129, 1560, 2941, 3041, 3341. 1H

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NMR (d ppm, CDCl3): 0.81 (t, 3H, CH3), 1.18 (d, chain 22H, (–CH2–)11, 1.49 (m, 2H, CH2 next to terminal methyl groups), 2.14 (t, 3H,N+CH2CH2CH3) 2.59 (d, 1H, OH), 3.36 (d, 2H, 2XOCH2), 3.49 (t, 2H,2XCH2O), 3.53 (m, 1H, CHOH), 4.16 (m, 2H, N+CH2CH2CH3), 4.34 (t,2H, NCH2), 4.45 (d, 2H, N+CH2), 7.39 (d, 1H, a CH to +ve Nitrogen ofimidazole), 7.45 (d, 1H, b CH to +ve Nitrogen of imidazole), 9.29 (s,1H, sandwiched proton between to nitrogen of imidazole). 75 MHz13C/DEPT NMR (d ppm, CDCl3): 14.14 (terminal methyl carbons),22.63 (CH3CH2), 26.13 (OCH2CH2), 29.29 (chain methylenecarbons), 29.61 (N+CH2CH2CH3), 31.59 (N+CH2CH2CH3), 46.78(NCH2), 53.42 (N+CH2), 67.38 (secondary carbon attached tohydroxyl group), 71.28 (OCH2), 71.84 (CH2O), 123.54 (NCH2CH2N),137.21 (sandwiched carbon between to nitrogen of imidazole).ESI-MS positive ions at m/z (relative intensity %) 379.4 (100%){(M�2Cl)�1)}+, 380.2 (40%) (M�2Cl)+.

1-(3-(Dodecyloxy)-2-hydroxypropyl)-3-propyl-1H-imida-zol-3-ium chloride (9). White solid, Yield, 92.5%. Melting point102 8C. The IR (KBr Pellet, cm�1): 1125, 1544, 2949, 3042, 3341. 1HNMR (d ppm, CDCl3): 0.80 (t, 3H, CH3), 1.28 (d, chain 18H, (–CH2–)9,1.48 (m, 2H, CH2 next to terminal methyl groups), 2.11 (t, 2H,N+CH2CH2CH3) 2.99 (d, 1H, OH), 3.29 (d, 2H, OCH2), 3.36 (t, 2H,CH2O), 3.49 (m, 1H, CHOH), 4.15 (m, 2H, N+CH2CH2CH3), 4.38 (t, 2H,NCH2), 4.55 (d, 2H, N+CH2), 7.43 (d, 2H, a CH to +ve Nitrogen ofimidazole), 7.99 (d, 1H, b CH to +ve Nitrogen of imidazole), 9.56 (s,1H, sandwiched proton between to nitrogen of imidazole). 75 MHz13C/DEPT NMR (d ppm, CDCl3): 13.49 (terminal methyl carbons),22.45 (CH3CH2), 26.23 (OCH2CH2), 29.09 (chain methylenecarbons), 29.24 (N+CH2CH2CH3), 31.60 (N+CH2CH2CH3), 46.78(NCH2), 53.42 (N+CH2), 67.38 (secondary carbon attached tohydroxyl group), 71.28 (OCH2), 71.84 (CH2O), 123.54 (NCH2CH2N),137.21 (sandwiched carbon between to nitrogen of imidazole).ESI-MS positive ions at m/z (relative intensity %) 351.5 (100%){(M�2Cl)�1)}+, 352.3 (40%) (M�Cl)+.

1-(3-(Hexadecyloxy)-2-hydroxypropyl)-3-propyl-1H-imida-zol-3-ium bromide (10). White solid, Yield, 92.5%. Melting point124 8C. The IR (KBr Pellet, cm�1): 1127, 1540, 2949, 3045, and3340. 1H NMR (d ppm, CDCl3): 0.87 (t, 3H, CH3), 1.29 (d, chain 26H,(–CH2–)13, 1.54 (m, 2H, CH2 next to terminal methyl groups), 2.51(t, 3H, N+CH2CH2CH3), 2.73 (d, 1H, OH), 3.36 (d, 2H, OCH2), 3.43 (t,2H, CH2O), 3.59 (m, 1H, 2XCHOH), 4.21 (m, 2H, N+CH2CH2CH3), 4.43(t, 2H, NCH2), 4.63 (d, 2H, N+CH2), 7.38 (d, 2H, a CH to +ve Nitrogenof imidazole), 7.84 (d, 1H, b CH to +ve Nitrogen of imidazole), 9.64(s, 1H, sandwiched proton between to nitrogen of imidazole).75 MHz 13C/DEPT NMR (d ppm, CDCl3): 13.35 (terminal methylcarbons), 22.63 (CH3CH2), 26.13 (OCH2CH2), 29.29 (chain methy-lene carbons), 29.83 (N+CH2CH2CH3), 31.24 (N+CH2CH2CH3), 31.86(NCH2), 46.78 (N+CH2), 67.38 (secondary carbon attached tohydroxyl group), 71.28 (OCH2), 71.84 (CH2O), 123.54 (NCH2CH2N),137.21 (sandwiched carbon between to nitrogen of imidazole).ESI-MS positive ions at m/z (relative intensity %) 407.3 (100%){(M�2Br)�1)}+, 408.3 (40%) (M�2Br)+.

1-(3-Tetradecyloxy)-2-hydroxypropyl)-3-propyl-1H-imida-zol-3-ium bromide (11). White solid, Yield, 92.5%. Melting point116 8C. The IR (KBr Pellet, cm�1): 1126, 1564, 2930, 3030, and3340. 1H NMR (d ppm, CDCl3): 0.81 (t, 3H, CH3), 1.29 (d, chain 22H,2X (–CH2–)11, 1.49 (m, 2H, CH2 next to terminal methyl groups),2.17 (t, 3H, N+CH2CH2CH3), 2.71 (d, 1H, OH), 3.36 (d, 2H, OCH2),3.50 (t, 2H, CH2O), 3.89 (m, 1H, CHOH), 4.18 (m, 2H, N+CH2CH2CH3),4.34 (t, 2H, NCH2), 4.45 (d, 2H, N+CH2), 7.72 (d, 1H, a CH to +veNitrogen of imidazole), 7.44 (d, 1H, b CH to +ve Nitrogen ofimidazole), 9.21 (s, 1H, sandwiched proton between to nitrogen ofimidazole). 75 MHz 13C/DEPT NMR (d ppm, CDCl3): 13.49 (terminalmethyl carbons), 22.45 (CH3CH2), 26.28 (OCH2CH2), 29.22 (chainmethylene carbons), 29.68 (N+CH2CH2CH3), 31.61 (N+CH2CH2CH3),46.78 (NCH2), 53.42 (N+CH2), 66.88 (secondary carbon attached tohydroxyl group), 71.28 (OCH2), 71.84 (CH2O), 121.51 (NCH2CH2N),

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136.21 (sandwiched carbon between to nitrogen of imidazole).ESI-MS positive ions at m/z (relative intensity %) 379.3 (100%){(M�2Br)�1)}+, 380.4 (40%) (M�2Br)+.

1-(3-(Dodecyloxy)-2-hydroxypropyl)-3-propyl-1H-imida-zol-3-ium bromide (12). White solid, Yield, 92.5%. Melting point108 8C. The IR (KBr Pellet, cm�1): 1130, 1570, 2930, 3047, 3345. 1HNMR (d ppm, CDCl3): 0.80 (t, 3H, CH3), 1.18 (d, chain 18H, (–CH2–)9,1.49 (m, 2H, CH2 next to terminal methyl groups), 2.16 (t, 3H,N+CH2CH2CH3), 2.71 (d, 1H, OH), 3.21 (d, 2H, OCH2), 3.36 (t, 2H,2XCH2O), 3.51 (m, 1H, CHOH), 4.18 (t, 2H, N+CH2CH2CH3), 4.38 (t,2H, NCH2), 4.55 (d, 2H, N+CH2), 7.33 (d, 1H, a CH to +ve Nitrogen ofimidazole), 7.43 (d, 1H, b CH to +ve Nitrogen of imidazole), 9.46 (s,1H, sandwiched proton between to nitrogen of imidazole). 75 MHz13C/DEPT NMR (d ppm, CDCl3): 14.45 (terminal methyl carbons),22.26 (CH3CH2), 26.07 (OCH2CH2), 29.22 (chain methylenecarbons), 29.60 (CH2CH3), 31.42 (CH2CH3), 48.24 (NCH2), 50.02(N+CH2), 67.38 (secondary carbon attached to hydroxyl group),71.00 (OCH2), 71.87 (CH2O), 103.54 (NCH2CH2N), 123.21 (sand-wiched carbon between to nitrogen of imidazole). ESI-MS positiveions at m/z (relative intensity %) 351.5 (100%) {(M�2Br)�1)}+,352.3 (40%) (M�2Br)+.

2.2.2. Conductivity measurements [27,28]

The critical micelle concentrations (cmc) of these surfactants(7–12) were determined by the conductivity method. Theconductance as a function of surfactant concentration wasmeasured at 25 8C. Measurements were performed with anEquiptronic Conductometer (Auto temperature conductivity metermodel E.Q.661) with stirring to control the temperature. Thesolutions were thermostated in the cell at 25 8C. For each series ofmeasurements, an exact volume of 25 ml Millipore water(resistivity 18 MV) was introduced into the vessel, and thespecific conductivity of the water was measured. For thedetermination of cmc, adequate quantities of concentrated stocksurfactant solutions were added in order to change the surfactantconcentration from concentrations well below the critical micelleconcentration (cmc) and repeated to verify our results. The curve ofconductivity versus surfactant concentration was taken as the cmc.The degree of counterion binding (b) was calculated as (1 � a),where a = Smicellar/Spremicellar, i.e., ratio of the slope before and aftercmc.

2.2.3. Surface tension measurements [29]

Surface tension values were used to calculate cmc using a CSCDu Nouy interfacial tensiometer (Central scientific Co., Inc.)equipped with platinum–iridium ring (circumference 5.992 cm)at 25 8C. The tensiometer was calibrated using triple distilledwater. For the determination of cmc and surface tension, adequatequantities of a concentrated stock solution were used. The data ofthis determination is presented in Table 1.

2.2.4. Thermal stability measurements

The thermal stability of the present monomeric surfactants wasmeasured with SDT Q600 Thermal Gravimetric Analyzer (TGA),using a nitrogen atmosphere. All samples were run in aluminumpans under a nitrogen atmosphere at a heating rate of 10 8C perminute.

3. Results and discussion

3.1. Characterization of cationic imidazolium surfactants

The structures of these cationic imidazolium surfactants (7–12)have been established by IR, 1H, 13C NMR, DEPT, COSY and massspectral data. The IR spectra of the cationic imidazoliumsurfactants (7–12) showed the absorption bands in the region at

(2014), http://dx.doi.org/10.1016/j.jiec.2014.01.032

Table 1cmc(a): by conductivity measurements, cmc(b): by surface tension measurement, degree of counter ion binding (b) and surface tension properties of cationic surfactants (7–

12).

S. no cmc(a) (mM) a (%) b (%) cmc(b)

(mM)

g(mN/m)

Pcmc

(mN/m)

C20 (M) cmc/C20 106 Gmax

(mol/m2)

Amin (nm2) DGomic (kJ/mol) DGoads (kJ/mol)

7 0.073 46 54 0.067 54.23 17.97 0.011 6.72 1.11 0.88 �21.06 �38.68

8 0.184 42 58 0.169 31.51 40.69 0.047 3.82 1.55 1.13 �19.48 �40.05

9 0.411 40 60 0.398 34.88 37.32 0.179 2.36 1.87 1.59 �18.06 �40.78

10 0.061 52 48 0.053 50.46 21.74 0.016 3.75 1.18 0.93 �19.49 �38.08

11 0.166 45 55 0.149 35.88 36.32 0.049 3.48 1.66 1.14 �17.99 �41.34

12 0.273 42 58 0.254 27.71 44.49 0.149 1.89 1.93 1.55 �17.11 �41.75

(7)

2

3

4

5

6

7

0 0.2 0. 4 0.6 0.8 1

Concentration in mM

cmc

k(m

Scm

-1)

(8)

0

5

10

15

20

0 0.5 1 1.5

Concentration in mM

cmc

k(m

Scm

-1)

(9)

0

10

20

30

40

50

0 0.05 0.1 0.15 0.2 0.2 5

cmc

Concentration in mM

k(m

Scm

-1)

Fig. 1. Specific conductivity vs. concentration plot of surfactants 7, 8 and 9.

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JIEC-1874; No. of Pages 9

3340–3345 cm�1 indicating the presence of hydroxyl groups. Theabsorptions at 2950–2930 cm�1 indicate the presence of C–Hstretchings whereas other absorptions at 3047–3030 cm�1 indi-cate the presence of C–H in aromatic ring. The band at 1570–1540 cm�1 very well established, the presence of aromatic C55C ofall products (7–12). The band at 1126–1130 cm�1 very wellestablished, the presence of aromatic C–N group of all products Theterminal methyl protons of these monomeric imidazoliumsurfactants (7–12) are observed as a distorted triplet at d 0.80–0.87 in their 1H NMR spectra. Broad doublet in (7–12) is observedat d 1.18–1.29 accountable for methylene protons of chain.Multiplet signals are observed at d 1.48–1.54 due to protons nextto terminal methyl group. The proton of (N+CH2CH2CH3) isobserved as triplet at d 2.11–2.51. The proton of hydroxyl groupis observed as multiplet at 2.59–2.99. A doublet is observed at3.21–3.36 due to OCH2. Multiplets are observed at d 3.36–3.50 dueto methylene protons attached to ether linkage. Other Multipletsare observed at d 3.49–3.89 due to a proton of secondary carbon.Other multiplets are observed at 4.15–4.18 due to methyleneprotons of N+CH2CH2CH3. Another type of multiplets is observed atd 4.34–4.43 due to NCH2 protons. Other multiplets are observed atd 4.45–4.63 due to methylene protons attached to positivenitrogen of imidazole. The three sets of ring protons of imidazolering are observed as a doublet at d 7.28–7.72 and d 7.32–7.99 and asinglet 9.32–9.64. 13C/DEPT NMR spectra displayed sp3 carbon ofterminal methyl group at d 13.49–14.45. The carbons next toterminal methyl groups are observed in the range of d 22.26–22.63.The carbons (OCH2CH2) are observed at 26.07–26.28. The chaincarbons are observed at 29.22–29.29. The methylene carbons, i.e.(N+CH2CH2CH3) are observed at 29.24–29.83. The methylenecarbons, i.e. (N+CH2CH2CH3) are observed at 31.24–31.61. Themethylene carbon of NCH2 is observed at 46.78–48.24. Themethylene carbons attached to imidazole nitrogen are observedat d 50.02–53.42. Other signals are observed at d 67.88–67.67 dueto secondary carbons. Other signal is observed at 71.00–71.28 dueto methylene carbon attached to ether linkage. A signal is observedat 71.84–71.87 due to (CH2O). Other structure revealing signals areobserved at d 103.54–123.54 due to (NCH2CH2N). Other structurerevealing signals are observed at d 123.21–137.21 due to(sandwiched carbon between to nitrogen of imidazole). On allthese accounts the structures of (7–12) are deduced as 1-(3-(hexadecyloxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-iumchloride (7), 1-(3-(tetradecyloxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-ium chloride (8), 1-(3-(dodecyloxy)-2-hydroxypro-pyl)-3-propyl-1H-imidazol-3-ium chloride (9), 1-(3-(hexadecy-loxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-ium bromide(10), 1-(3-tetradecyloxy)-2-hydroxypropyl)-3-propyl-1H-imida-zol-3-ium bromide (11), 1-(3-(dodecyloxy)-2-hydroxypropyl)-3-propyl-1H-imidazol-3-ium bromide (12). The structures of thesesurfactants (7–12) are further consolidated by ESI-MS (positiveion) mass spectral data. Important peaks in these spectra are foundat m/z 407.3, 408.3, 379.4, 380.2, 351.5, 352.3, 407.3, 408.3, 379.3,380.4, 351.5, 352.3. These ion peaks account for the loss of protonand one chloride/bromide ions from the molecule leading to the

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formation of positively charged parent ion {(M�Cl/Br)�1)}+ anddirect loss of Chloride/bromide ions from the molecule leading toformation of (M�Cl/Br)+ positively charged ions.

(2014), http://dx.doi.org/10.1016/j.jiec.2014.01.032

0

5

10

15

20

0.250.20.150.10.050

Concentration in mM

cmc

k(m

Scm

-1)

(10)

0

10

20

30

40

50

60

70

10.80.60.40.20

cmc

Concentration in mM

k(m

Scm

-1)

(11)

0

10

20

30

40

50

10.80.60.40.20

cmc

Concentration in mM

k(m

Scm

-1)

(12)

Fig. 2. Specific conductivity vs. concentration plot of surfactants 10, 11 and 12.

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JIEC-1874; No. of Pages 9

3.2. Critical micelle concentration

cmc and degree of counter ion binding of these newimidazolium amphiphiles have been determined by conductivitymethod. These new monomeric imidazolium amphiphiles havelow cmc values. It has been found that the cmc of thesemonomeric imidazolium amphiphiles decreases with increase inchain length. The values of cmc and degree of counter ion bindingare given in Table 1. The graph of the concentration vs.conductivity has been plotted in Figs. 1 and 2. It is found that theimidazolium cationic monomeric surfactants having short chainlength have low cmc value than surfactants having long chainlength. Also it is found that the surfactants having bromine as acounter ion has low cmc as compared to surfactants havingchlorine as a counter ion.

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3.3. The degree of counterion binding (b)

The ratio of the slopes of the conductivity vs. concentrationcurve above and below cmc gives degree of counterion dissociationa (i.e., a = Smicellar/Spremicellar) and (1 � a) gives the degree ofcounterion binding, b. It is an important parameter because itmanifests the counterions that are contained in the Stern layer tocounterbalance the electrostatic force that opposes micelleformation. Quagliotto et al. [30] determined the b value for aseries of gemini bispyridinium bromides having different spacerswhere they had shown that different spacer is responsible fordifferent b value. The b value signifies the ability of counter ion tobind micelles. We in our study on new series of monomericimidazolium surfactants (7–12) have found that b value decreaseswith increase in chain length (Table 1).

3.4. Surface tension measurements

The cmc of new imidazolium surfactants were calculated byusing surface tension measurements. The increase in the surfacetension for series of imidazolium surfactants with increasinghydrophobic alkyl chain length can be explained on the basis ofcmc/C20 ratio observed for these amphiphiles. The affinity of aparticular surfactant to reduce surface tension of solvent dependsupon cmc/C20 ratio, greater the observed value greater is thetendency of the amphiphile to reduce surface tension of the system[31]. Thus, imidazolium surfactants 7 and 10 has maximum abilitywhile amphiphile 9 and 12 has minimum ability to reduce surfacetension of aqueous system in the series of amphiphile beingreported. Two important parameters of surface active surfactantsi.e. the effectiveness of surface tension reduction (Pcmc) and theadsorption efficiency (pC20) were obtained from the surfacetension plots. The former parameter, Pcmc is the surface pressureat the cmc and is defined as:

Pcmc ¼ g0 � gcmc

where g0 is the surface tension of pure solvent and gcmc is themeasured surface tension at cmc. The maximum reduction insurface tension caused by the dissolution of amphiphilic moleculeshas been indicated by Pcmc and as a result Pcmc becomes a measurefor the effectiveness of the amphiphile to lower the surface tensionof the water [31]. Imidazolium surfactants synthesized in presentwork (7, 8, 9, 10, 11 and 12) have greater ability to reduce surfacetension of aqueous system. The adsorption efficiency (pC20) isdetermined by using the following equation [31]:

pC20 ¼ �log C20

In this equation C is the molar concentration of surfactant and C20stands for the concentration required to reduce the surface tensionof the pure solvent by 20 mN m�1 [31]. Thus C20 becomes ameasure of adsorption efficiency of amphiphilic molecules at theair–water interface. The results from Table 1 indicate that thecompounds 9 and 12 have higher adsorption efficiency among sixlong chain imidazolium surfactants. The values of these twoparameters obtained for the six surfactants are listed in Table 1along with their cmc and gcmc values. The maximum surface excessconcentration (umax) was estimated by applying Gibbs adsorptionisotherm [31] to the surface tension data:

umax ¼ �12nRT

@g

@ ln C

� �T

where R is the gas constant (8.314 J/mol K), T is the absolutetemperature, and C is the surfactant concentration. The value of n is

(2014), http://dx.doi.org/10.1016/j.jiec.2014.01.032

50

55

60

65

-1-1.2-1.4-1.6-1.8-2-2.2-2.4

cmcSurf

ace

tens

ion

(mN

/m)

Log C

(7)

30

35

40

45

50

55

60

65

-0.5-1-1.5-2-2.5

Surf

ace

tens

ion

(mN

/m)

Log C

cmc

(8)

30

35

40

45

50

55

60

65

0-0.5-1-1.5-2

cmc

Surf

ace

tens

ion

(mN

/m)

Log C

(9)

Fig. 3. Plot of surface tension vs. log of surfactant concentration of surfactants 7, 8

and 9.

50

55

60

65

70

-0.8-1-1.2-1.4-1.6-1.8-2-2.2-2.4Log C

Surf

ace

tens

ion

(mN

/m)

cmc

(10)

35

40

45

50

55

60

65

-0.5-1-1.5-2-2.5

Log C

Surf

ace

tens

ion

(mN

/m)

cmc

(11)

20

30

40

50

60

70

0-0.5-1-1.5-2

Surf

ace

tens

ion

(mN

/m)

Log C

cmc

(12)

Fig. 4. Plot of surface tension vs. log of surfactant concentration of surfactants 10, 11

and 12.

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taken as 2 as there is one counter ion associated with each cationichead group. The minimum area occupied by a single amphiphilicmolecule at the air–water interface (Amin) was also obtained byapplying Gibbs adsorption isotherm to the surface tension data:

Amin ¼1

NA� Tmax ð�1023Þ

where NA is the Avogadro constant. All imidazolium surfactantshave lower Amin values (Table 1). The lowest Amin values ofimidazolium surfactants (7 and 10) can be attributed to tighterpacking of the longer chains at the interface [32]. A part frompositively charged imidazolium cation all these imidazoliumsurfactants contain free hydroxyl group. The presence of hydroxylgroup in addition to positively charged center plays important role

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in their aggregation behavior. The energy required for the closerpacking of hydrocarbon alkyl chain length at interface may comefrom the energy released upon hydrogen bond formation betweenthe hydroxyl group present close to positively charged imidazo-lium center and water molecule [33]. The Gibbs free energy of themicellization (DG0 mic) was calculated by use of following equation[34].

DG0 mic ¼ ð2 � aÞRT ln Xcmc

where Xcmc is the mole fraction at the cmc and a is the extent ofcounter ion dissociation. The micellization free energy indicatesnegative sign because thermodynamically stable micelles areformed spontaneously. The results from Table 1 indicate that thedriving force for micellizatiom becomes large as DG0 mic becomes

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Table 2Critical micelle concentration of 7, 8, 9, 10, 11 and 12 in sodium halide solutions at

298 K.

Salt Conc. (M) 7 8 9 10 11 12

NaBr 0.001 0.060 0.115 0.190 0.031 0.105 0.168

NaCl 0.001 0.063 0.125 0.194 0.057 0.112 0.122

6 8

6 9

7 0

7 1

7 2

7 3

-2 .2 - 2 - 1 .8 - 1 .6 -1 .4 -1 .2 -1 - 0 .8 -0 .6Log C

Surf

ace

tens

ion

(mN

/m)

C M C

(10)

6 46 66 87 07 27 4

ensi

on (m

N/m

)

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more negative. The standard Gibbs free energy of adsorption(DG0 ads) was obtained from the following relationship [35].

DG0 ads ¼ DG0 mic � Pcmcumax

Here, pcmc denotes the surface pressureat the cmc (pcmc = g0 � gcmc, where g0 and gcmc are the surfacetensions of water and the surfactant solution at the cmc,respectively). The free energy of adsorption (DG0 ads) representsthe free energy of transfer of 1 mol of surfactant in solution to thesurface, and the free energy of micellization (DG0 mic) representsthe work done to transfer the surfactant molecules from themonomeric form at the surface to the micellar phase [36]. Thestandard free energy of micellization (DG0 mic) and adsorption(DG0 ads) is always negative, indicating tendencies to form micellesin solution and to adsorb at the air/water interface [37]. If the value

60

62

64

66

68

70

72

-1-1.5-2

Surf

ace

tens

ion

(mN

/m)

CMC

(7)

60

65

70

75

-0.5-1-1.5-2Log C

Log C

Surf

ace

tens

ion

(mN

/m)

CMC

(8)

354045505560657075

-0.5-1-1.5-2Log C

Surf

ace

tens

ion

(mN

/m)

CMC

(9)

Fig. 5. Plot of surface tension vs. log of surfactant concentration of surfactants 7, 8

and 9 in the presence of NaBr (0.001 mol/L) at 298 K.

5 86 06 2

-2 .2 -2 -1 .8 -1 .6 -1 .4 -1 .2 -1 -0 .8 -0 .6

Log CSu

rfac

e t

C M C

(11)

56586062

6466687072

-2 -1 .5 -1 -0 .5 0

Log C

Surf

ace

tens

ion

(mN

/m)

CM C

(12)

Fig. 6. Plots of surface tension vs. log of surfactant concentration of surfactants 10,

11 and 12 in the presence of NaBr (0.001 mol/L) at 298 K.

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of (DG0 ads) is more negative and greater than the differencebetween (DG0 ads) and (DG0 mic), then the adsorption of surfactantmolecules at the interface becomes more favorable because of thegreater freedom of motion of hydrocarbon chains at the planar air/aqueous solution interface than in the interior of the micelle.However, if the energy difference is small, then less work has to bedone to transfer surfactant molecules from the monomeric form atthe surface to the micellar phase. When the difference in the freeenergies is small, the surfactant undergoes aggregation morereadily than when the difference in the free energies is large. This isevident from the results obtained by Yeshimua et al. [38]. The(DG0 mic) and (DG0 ads) values of imidazolium surfactants aresummarized in Table 1. The difference in the free energy gap issmall for imidazolium surfactants therefore; these surfactantshave a greater tendency to aggregate in solution as compared toother surfactants.

The graphs of the surface tension vs. concentration are shownfor surfactants (7–12). A clear break is observed in all the

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67

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72

73

-2.2 -2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6

Log C

Surf

ace

tens

ion

(mN

/m)

cmc

(7)

586062646668707274

-2.2 -2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6

Log C

Surf

ace

tens

ion

(mN

/m)

cmc

(8)

56

58

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62

64

66

68

70

72

-2 -1.5 -1 -0.5 0Surf

ace

tens

ion

(mN

/m)

Log C

cm c

(9)

Fig. 7. Plots of surface tension vs. log of surfactant concentration of surfactants 7, 8

and 9 in the presence of NaCl (0.001 mol/L) 298 K.

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-0.6-0.8-1-1.2-1.4-1.6-1.8-2-2.2

Surf

ace

tens

ion

(mN

/m)

cmc

Log C

(10)

55

60

65

70

75

-0.5-1-1.5-2-2.5

Log C

Surf

ace

tens

ion

(mN

/m)

cmc

(11)

35

40

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50

55

60

65

70

75

-0.5-1-1.5-2-2.5

Surf

ace

tens

ion

(mN

/m)

cmc

Log C

(12)

Fig. 8. Plots of surface tension vs. log of surfactant concentration of surfactants 10,

11 and 12 in the presence of NaCl (0.001 mol/L) at 298 K.

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imidazolium surfactants Figs. 3 and 4. It is observed from the graphsthat imidazolium surfactants having short chain length have lowcmc values as compared to the imidazolium surfactants having longchain length. The cmc values are reported in Table 1 for all thesurfactants. The values for both the conductivity methods andsurface tension method correspond well with each other.

4. Surface activity measurement of cationic surfactants inpresence of sodium halides

The addition of salt could easily modify the surface activity ofcationic surfactants. The surface tension of aqueous solution ofsurfactants was measured in the presence of sodium halides (NaCl,NaBr). The surface tension as well as the cmc of (7–12) was reducedby both sodium halides. The effect of NaBr on gcmc is moreprominent than effect of NaCl. The cmc values of 7, 8, 9, 10, 11 and12 in the presence of these sodium halides are listed in Table 2. Thedecrease in the cmc value of surfactants in salt-system can beexplained on the basis of model given by Davis and Rideal [36] andwhich is further modified by Borwankar and Wasan [37]. Thismodel gives an idea that the amphiphilic ions are adsorbed in theStern layer while counter ions exist in the diffuse part of the

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electric double layer without penetrating into the Stern layer.Hence, added halogen anion induces the screening effect andaccount for lowering cmc of surfactants in the salt-system. Thespecificity of the ion interactions can also account for thedifference among anion species in their effect on the criticalmicelle concentration of cationic surfactants [37]. The anionsdecrease the cmc of surfactants, in the order Br� > Cl�, since thehydration of these ions increases in the order Br� < Cl� and themore hydrated anion is less effective in neutralizing the chargesaround the micellar surface (Figs. 5–8).

5. Thermal stability measurements

Thermal stability measurement shows that these long chaincationic surfactants are stable up to 370 8C. Figure shows acharacteristic curve for the decomposition of the cationicsurfactants as measured by thermal gravimetric analyzer. Theonset temperature (TONSET) is the intersection of the baselineweight, either from the beginning of the experiment and the

(2014), http://dx.doi.org/10.1016/j.jiec.2014.01.032

Table 3Onset and start temperatures for thermal decomposition of imidazolium

surfactants.

Temperature (8C) 7 8 9 10 11 12

TONSET 282.3 278.2 265.6 288.9 283.9 273.6

TSTART 260.9 248.4 227.0 263.7 262.6 232.1

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tangent of the weight vs. temperature curve as decompositionoccurs [39]. The start temperature (TSTART) is the temperature atwhich the decomposition of the sample begins. The example of theonset and start temperatures is shown in figure. The onset and starttemperatures for present imidazolium cationic surfactants arelisted in Table 3. Thermal stability measurements designated thatthese surfactants have better thermal stability. Thermal stability ofthese cationic surfactants increases as chain length increases. Alsofrom Table 3 it is found that cationic imidazolium surfactantshaving bromine as a counter ion is more thermally stable thansurfactants having chlorine as a counter ion. The larger size ofanion account for increased thermal stability of respectivesurfactant [40].

Figure representing the thermal decomposition curve ofsurfactant (09) determined by TGA, indicating the Start (TSTART)and onset (TONSET) temperature.

6. Conclusion

In the present study we have described a new protocol for thesynthesis of renewable raw material based imidazolium cationicmonomeric surfactants through environmental friendly process.All the cationic surfactants (7–12) are produced in excellent yieldsand these surfactants have been examined and are found to havegood surface active properties. The results show that cationicimidazolium surfactants with longer hydrophobic chains have alower cmc value. It is found that cationic imidazolium surfactantshaving bromide as a counter ion have low cmc values as comparedto surfactants having chloride as a counter ion. Further resultsshow that these surfactants have good thermal properties. Thermalproperties of these surfactants increase with increase in chainlength. Also from Table 3 it is found that cationic imidazolium

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surfactants having bromine as a counter ion is more thermallystable than surfactants having chlorine as a counter ion. Besidesthese imidazolium cationic monomeric surfactants may showgood antimicrobial properties, DNA binding capability if testedproperly.

Acknowledgments

One of us (Pankaj Patial) is thankful to UGC (University grantcommission India) for providing the research grant for this workand Sophisticated Analytical Instrumentation Facility (SAIF),Panjab University, Chandigarh for the mass spectral analysis ofthe compounds. We are thankful to Dr. Satindar Kaur and Ms.Charanjeet Kaur for their help and helpful discussion.

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