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Synthesis and Surface Active Properties of NovelNonionic Aryl Oleic Diethanolamide SurfactantsZhigang Xu a , Dongliang Liu a , Weihong Qiao a , Zongshi Li a & Lubo Cheng aa State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, ChinaVersion of record first published: 22 Nov 2006.

To cite this article: Zhigang Xu , Dongliang Liu , Weihong Qiao , Zongshi Li & Lubo Cheng (2006): Synthesis and Surface ActiveProperties of Novel Nonionic Aryl Oleic Diethanolamide Surfactants, Petroleum Science and Technology, 24:11, 1363-1370

To link to this article: http://dx.doi.org/10.1080/10916460500292717

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Petroleum Science and Technology, 24:1363–1370, 2006Copyright © Taylor & Francis Group, LLCISSN: 1091-6466 print/1532-2459 onlineDOI: 10.1080/10916460500292717

Synthesis and Surface Active Propertiesof Novel Nonionic Aryl OleicDiethanolamide Surfactants

Zhigang Xu, Dongliang Liu, Weihong Qiao, Zongshi Li,and Lubo Cheng

State Key Laboratory of Fine Chemicals, Dalian University of Technology,Dalian, China

Abstract: A series of novel aryl oleic diethanolamide (AOD) surfactants with differenthydrophobic groups were synthesized. An aromatic ring introduced to a long alkylchain had a significant effect on reducing water surface tension. The critical micelleconcentration (CMC) and the surface tension (γcmc) at the CMC were investigated.The CMC was 4.56 × 10−8 mol/L to 5.83 × 10−6 mol/L for AOD1:1 (molar ratioof aryl oleic acid:diethanolamide = 1:1) and corresponding γcmc was 36.4 mN/m to39.3 mN/m. The CMC of AOD1:2 (molar ratio of aryl oleic acid:diethanolamide =1:2) were in 1.21 × 10−7–9.98 × 10−6 mol/L and γcmc were in 31.6–34.4 mN/m.

Keywords: aryl oleic diethanolamide, aryl alkanolamide, surface tension, critical mi-celle concentration

1. INTRODUCTION

Surfactants benign to the environment, so called “environmentally friendlysurfactants,” are currently attracting popular attention. Some of the morecommon surfactants in use today have been questioned with regard to goodbiodegradability, decontamination in hard water, frothiness, and less irritationto skin. That is why the replacement of traditional surfactants by environ-mentally friendly surfactants is one of the major topics today in the colloidalfield.

Novel aryl oleic diethanolamide, whose aryl ring is benzene, toluene, andxylene surfactants with bulky hydrophobic arylalkyl chains and hydrophilic

Address correspondence to Zhigang Xu, State Key Laboratory of Fine Chemicals,Dalian University of Technology, 158 Zhongshan Rd., Dalian 116012, China. E-mail:xuzhigangyours@yahoo.com

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diethanolamide groups, are environmentally friendly and biocompatible non-ionic surfactants. These alkanolamide surfactants are widely used as emulsi-fiers and also in pharmaceutical and cosmetic formulations (Chen et al., 2001;Wang et al., 2001; Xu et al., 2004; Li et al., 1995a, Denkinger et al., 1990).Two-step reactions of esterification and combination to improve the yield ofproducts were reported (Qing et al., 2000; Raaijmakers et al., 1993; Tian-hong et al., 1996). Although the kinds of synthesis for these reactions havebeen reported in a few cases (Kim et al., 2002; Hossain et al., 2004; Oteyand Mehltretter, 1958; Pavia et al., 1992; Sela et al., 1993; Lee et al., 1989),arylalkyl chain as hydrophobic group for the change of hydrophobic segmentsize has not been reported for the preparations of alkanolamides. The surfaceactive properties of a surfactant provide the basis for various applicationsand are usually characterized by the critical micelle concentration (CMC),surface concentration at the air/water interface, and water surface tensionreduction efficiency and effectiveness. Water surface tension as a functionof surfactant concentration has been the most widely used. In this paper,we present a report on the synthesis processes and surface active propertiesof AOD.

2. EXPERIMENTAL

2.1. Material and Methods

Oleic acid was from Tianjing Kermel Chemical Industry Co., Ltd (China).Methanesulfonic acid, diethanolamide, and other reagents were analyticalgrade.

The iodine values of both the raw materials and products were determinedby ISO 3961:1989 and the percentages of conversion were judged.

Surface tensions of aqueous solutions of the products at different con-centrations were measured at 25◦C by drop volume method (Li et al., 1995b;Zhao and Zhu, 2003). For each measurement, the solutions of proper con-centrations were left for 30 min to allow equilibration.

2.2. Preparation of Aryl Oleic Diethanolamide (AOD)

Aryl Oleic Acid

In a three-necked flask (100 ml) equipped with a thermometer and a mechan-ical stirrer, methanesulfonic acid was added to a mixture of 15.0 g oleic acidand an appropriate amount of aromatic compounds with vigorous stirring atroom temperature with oleic acid:aromatic compounds:methanesulfonic acidmolar ratio of 1:5:6. The reactants were heated to 60◦C slowly and maintainedfor 6 h at this temperature. After reaction, the oil upper layer was separated

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Synthesis and Surface Activity of Surfactants 1365

and washed to be neutral by water and then concentrated to a brown viscousresidue by distillation.

Aryl Methyl Oleate

Aryl oleic acid and methanol with molar ratio of 1:7 were added to a flaskwith an appropriate amount of oil of vitriol as catalyst. The reaction solutionwas stirred at 70◦C for 5–6 h and then concentrated to a light yellow viscousresidue by vacuum distillation.

Aryl Oleic Diethanolamide

Aryl methyl oleate, diethanolamide, and KOH as catalyst with molar ratioof 1:1:0.005 and 1:2:0.005 were, respectively, added to flasks under vacuum.The reaction solution was stirred at 80–120◦C for 1.5–2 h and then a lightyellow viscous residue could be obtained.

3. RESULTS AND DISCUSSION

In a previous article, we reported on the synthesis of aryl oleic acid witharomatic rings introducted to a long chain (Xu et al., 2004; Tuvell et al.,1978; Kravetx et al., 1982). Since the reaction of amine with ester is selective,it is not necessary to protect the hydroxyl groups in the diethanolamide.Following this procedure, we prepared a series of nonionic alkanolamidewith different hydrophilic segment lengths and hydrophobic chain sizes. Thechemical structure and designation for each nonionic surfactant synthesized isshown in Figure 1. The surfactants can be divided into two groups accordingto their structure. The first group of surfactants has the same hydrophilicdiethanolamide segments, but different hydrophobic arylalkyl segments, whilethe second group of surfactants has the same hydrophobic arylalkyl segment,but different hydrophilic diethanolamide segments. The properties of the firstgroup can be compared to quantify the effect of hydrophobic chain length,while the properties of the second group can be compared to quantify theeffect of hydrophilic diethanolamide size.

In order to find the percentage of conversion, iodine values of both rawmaterials and products are determined according to the change of doublebonds in oleic acid. Iodine value of oleic acid is 73.27. The iodine values ofthe synthesized products are shown in Table 1.

CH3(CH2)nC|Ar

H(CH2)mCH2COOH m + n = 14

As shown by this equation, the alkylation is an electrophilic reaction. Thearomatic rings would be attacked by the positively charged carbon atom on the

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Figure 1. Chemical structure and designation of aryl oleic diethanolamide nonionicsurfactants.

long alkyl chain. As the electronegativity of the aromatic ring is intensified,conversion obviously increases, which is due to the electron-donating effectof the methyl group of the benzene ring. The negatively charged benzenering would confer to higher reactivity and higher conversions.

3.1. Surfactant Structure

The selective reaction of ester with amine assures a linear structure for thesurfactants which were characterized by HP1100-MS for mass spectroscopy.

The peaks of 387.5 and 423.5 in Figure 2 corresponding to the peaks of388.3, 410.2, and 426.2 (M + H+, Na+, K+) in Figure 3, respectively, arethe molecules’ ion peaks of oleic acid after reaction with xylene. In Figure 3the peaks of 476.5, 498.3, and 514.2 (M + H+, Na+, K+) belong to the

Table 1. Iodine values, conversion, and content of aryl oleicacid with different aromatic rings

Ar Iodine value Conversion % Content %

Benzene 17.01 76.78 80.85Toluene 8.89 87.87 90.57Xylene 6.02 91.78 93.89

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Synthesis and Surface Activity of Surfactants 1367

Figure 2. Mass spectrogram of API-ES negative for the xylene-AOD1:1.

molecules’ ion peaks of xylene-AOD1:1 and the peak of 581.5 belongs toxyllene-AOD1:2.

3.2. Surface Active Properties

The critical micelle concentration (CMC) and surface tension (γcmc) at theCMC of the surfactant are the major performance parameters. Water surface

Figure 3. Mass spectrogram of API-ES positive for the xylene-AOD1:1.

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Figure 4. Plots of surface tension of aqueous solution against logarithm of surfactantconcentration for AOD1:1 at 25◦C.

tension plotted against the logarithm of surfactant concentration in aqueoussolution is shown for the novel aryl alkanolamide surfactants in Figures 4and 5.

The surface tension value of deionized water is 71.15 mN/m under thesame conditions. Surface tension was not measured for xylene-AOD1:1 be-cause of its poor solubility in water. The results follow classic surfactantbehavior. Water surface tension decreases linearly with the increasing loga-rithm of surfactant concentration, and then levels off. The inflection point,

Figure 5. Plots of surface tension of aqueous solution against logarithm of surfactantconcentration for AOD1:2 at 25◦C.

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Synthesis and Surface Activity of Surfactants 1369

Table 2. Surface activity properties of aryl oleicacid diethanolamide surfactants

CMC γcmcSurfactants (mol/L) (mN/m)

Benzene-AOD1:1 5.83 × 10−6 36.4Toluene-AOD1:1 4.56 × 10−8 39.3Benzene-AOD1:2 1.21 × 10−7 32.7Toluene-AOD1:2 5.17 × 10−6 34.4Xylene-AOD1:2 9.98 × 10−6 31.6

which corresponds to the CMC, is observed for all surfactants. The CMCresults for the alkanolamide surfactants are summarized in Table 2.

Increasing alkyl chain size decreases the CMC for AOD1:1, while thereis a big change in CMC for AOD1:2, as indicated by the results for CMCfrom 1.21 × 10−7 to 9.98 × 10−6 mol/L. The CMC results indicate that notonly the sizes of the hydrophobic segments, but also the kinds of hydrophilicgroups were shown to be important in reducing water surface tension.

4. CONCLUSIONS

In this report, we have shown that AOD surfactants can be designed in whichhydrophilic and hydrophobic segment sizes can be readily controlled. Resultsfor the AOD surfactants show that AOD1:1 and AOD1:2 exhibit differenttrends in surface active behavior. Increasing aromatic rings to alkyl chainsfrom benzene to xylene for AOD1:1 decreases the critical micelle concentra-tion and decreases the efficiency and effectiveness of reducing water surfacetension, while contrary phenomena appeared in AOD1:2. Not only the sizesof the hydrophobic segments, but also the kinds of hydrophilic groups areshown to be important in reducing water surface tension.

ACKNOWLEDGMENT

We would like to thank the 973 National Key Basic Research DevelopmentProgram for financial support.

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