Novel four-arm star oligomeric surfactants: Synthesis and tensioactive properties

Accepted Manuscript

Novel Four-arm Star Oligomeric Surfactants :Synthesis and Tensioactive Properties Ling Wang , Demin Wang , Chunde Liu , Simeng Gao , Wei Ding PII: DOI: Reference:

S2468-0230(17)30048-2 10.1016/j.surfin.2017.04.007 SURFIN 90

To appear in:

Surfaces and Interfaces

Received date: Revised date: Accepted date:

9 November 2016 8 April 2017 19 April 2017

Please cite this article as: Ling Wang , Demin Wang , Chunde Liu , Simeng Gao , Wei Ding , Novel Four-arm Star Oligomeric Surfactants :Synthesis and Tensioactive Properties, Surfaces and Interfaces (2017), doi: 10.1016/j.surfin.2017.04.007

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Novel Four-arm Star Oligomeric Surfactants :Synthesis and Tensioactive Properties Ling Wang*, Demin Wang, Chunde Liu, Simeng Gao, Wei Ding*

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Provincial Key Laboratory of Oil&Gas Chemical Technology ,Chemistry and Chemical Engineering College of Northeast Petroleum University, Daqing 163318 ,China

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ABSTRACT: Novel Four-arm-star shaped oligomeric (Tetrameric) surfactants, bearing four alkyl aryl hydrophobic chains and four anionic hydrophilic headgroups connected by pentaerythritol, were firstly designed and synthesized by four-step procedures involving the aryl oleic acid pentaerythritol ester as key intermediates for connecting amphilic moieties. The surface active properties, including the critical aggregation concentration(CAC),surface tension at the CAC(γCAC), ability of these compounds to lower surface tension of 0.02N/m (pC20), mininum surface area occupied by per surfactant molecule (Amin) and the maximum surface excessive concentration(Γmax), were studied at 25 and 45℃. It was found that the molecular architecture of these compounds strongly influences the values of physicochemical parameters. The ability to reduce surface tension is weakened with increasing the number of substituents on the aromatic ring. A-marked surfactant exhibits excellent surface activities at 45 ℃. The pC20 values at 25℃have opposite trends compared with the ones at 45℃, which were changed by temperatures for revealing the aggregation behavior of these three surfactants. Interestingly, the surfactants displays high emulsification ability to aromatic compounds even at very low concentration.

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Keywords: star-shaped oligomeric surfactants;surface tension ;tetrameric surfactants; emulsification ability

Introduction Conventional surfactants are compounds that possess a polar hydrophilic group and a nonpolar hydrophobic group in a molecule. The separation tendency caused by the repulsion or hydration of the ionic head groups makes them difficult to be arranged in the interface or the molecular aggregates, which causes the low surface activity. Although the polymeric surfactants with high molecular weight have good solubilization, increasing consistency, dispersion and flocculation, but in general it is

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difficult to form stable layer in the interface, as a result the surface activity is weak compared with traditional surfactants, the surface tension needs a long time to reach a constant value. These defects limit the application of traditional surfactants and polymeric surfactants. Therefore, our aim is to bridge the gap between them, trying to explore and synthesize the novel surfactants with high efficiency and effectiveness. It gave birth to the concept of oligomeric surfactants, which are made up of two or more amphiphilic moieties chemical connected by spacer groups. Gemini surfactants are the most simple oligomeric surfactants that have been investigated extensively[].The unique properties exhibited by gemini surfactants have stimulated the synthesis and investigation of trimer or more oligomeric surfactants. Cationic oligomeric surfactants have been the mainly reported at present[8-10], but less about anionic[11] and nonionic surfactant oligomers[12], and even not any about zwitterionic. Liu[13]synthesized four cationic trimeric surfactants, finding these compounds to be capable of adsorbing to the air/water interface and orienting

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themselves through the interactions among the hydrocarbon chains. Grau[14]prepared the anionic tetrameric surfactants with multiple-ring spacers, and found that the ability of these compounds to lower surface tension is good. Furthermore, a homologous series of anionic triple-chain surfactants with three sulfonate groups were synthesized by Nakatsuji’s group[15] ,and the backbone structure on the adsorption behavior of the molecules on the surface , the surface active properties were studied, finding that the choice of the backbone structure of a triple-chain surfactant is important to its surface properties in water ,and these compounds could form pre-micelles at lower concentration. Yang and co-workers[16] synthesised nonionic trimeric surfactants alkylphenol polyoxyethylene, the compounds exhibited better surface properties of low cmc values, strong adsorption affinities and wet abilities. Most of the surfactants used in industrial fields are petroleum-based. Bio-based surfactants have attracted much attention from scientific and industrial fields due to their renewable feedstock environmentally friendly applicationand have excellent surface/ interfacial properties. In this paper, we synthesized a series of tetrameric anionic sulfonate surfactants, using renewable resources of oleic acid as the main raw material, pentaerythritol as spacer group. Drop volume method was used to evaluate surface active properties of the surfactant solution.

Experimental Section Materials.

ACCEPTED MANUSCRIPT Oleic acid, Methanesulfonic acid, p-toluenesulphonic acid were purchased from Tianjing Kermel Chemical Industry Co.Ltd(China). Benzene (toluene, xylene), pentaerythritol, Chlorine sulfonic acid and other organic solvents were purchased from Tianjing Damao Chemical Industry Co.Ltd(China). Triply distilled water was used in all experiments. Apparatus The Infrared (IR) spectra was recorded on a Bruker Vector-22 spectrometer; HNMR nuclear magnetic resonance(NMR) spectrum were measured in CDCl3 with a Bruker DRX 500MHz nuclear magnetic resonance instrument at 25oC and TMS(tetramethylsilane) was being used as internal standard.

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Gas chromatography mass spectrometry (GC-MS) was recorded on Agilent 6890N Network GC system and 5978 inter Mass Selective Detector. Synthesis

Aryl pentaerythritol oleate sulfonate was synthesized according to Schem1, with synthesis of aryl pentaerythritol oleate sulfonate as an example. Aryl oleic acid

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The synthesis of aryl oleic acid as reference literature[1]. Methanesulfonic acid( 0.6mol,57.6g) was added to a mixture of oleic acid(0.1mol,28.2g) and aromatic compounds(0.5mol) with vigorous stirring in a nitrogen atmosphere at room temperature. Then the reactants were heated to 60℃slowly and maintained for 6h at this temperature, the whole process was under nitrogen protection. After reaction, the oil upper layer was seperated and washed to be neutral by water and then concentrated to a brown viscous residue by distillation.

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Aryl pentaerythritol oleate Aryl oleic acid and pentaerythritol with molar ration of 4.4:1 were added to a flask with an appropriate amount of p-toluenesulphonic acid as catalyst, toluene as water carrier. Then the reactants were heated slowly to reflux and maintained for 3h. After the reaction, washing the product to be neutral by water, and then evaporated to a deep brown viscous product. The purification was performed by column chromatography on silica gel with petroleum ether-ethyl acetate(1/17,v/v) as eluent. It afforded aryl pentaerythritol oleate in good yields(53.2%）as oil pale yellow. 1H NMR(CDCl3): 1H NMR(CDCl3): the

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signal of Hi protons appears at δ=7.10-7.26ppm, the region of δ=4.14ppm are the signals of Hb protons, δ=2.28-2.39ppm are the signals of Hg,Hd; δ=2.03ppm are the signals of Hf, δ=1.65ppm are the signals of He, the other protons of -CH2- appears at δ=1.21-1.42ppm, δ=0.983-0.833ppm are the signals of Hj . In addition, the signal of -CH2OH disappears, that all four hydroxyl groups have been esterificated, indicating successfully synthesis the target product.

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2.03

7.16

1.65

4.14

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Fig.1 1H NMR spectrum of methyphenyl alkyl pentaerythritol oleate

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100

Mw:56

Mw:42

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intensity/%

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28

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Mw:28

Mw:44

Mw:68

42 44

116 Mw:116

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60 m/z

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120

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Fig.2 ESI HRMS spectrum of methyphenyl alkyl pentaerythritol oleate

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100 2026

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80 2965 2860 3131 2930

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862 952

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60 70

1743

1161 1068

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2927

50 4000 3500 3000 2500 2000 1500 1000

Wavenumber/cm

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-1

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1715

Transmittance%

Transmittance %

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1465

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1045 1223

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Wavenumber/cm

C-H), 2965, 2930 (-CH3 ) , 2860 (-CH2-), 1715(C=O) , 1462.87

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IR(KBr) /cm-1:3131 (

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Fig.3 FT-IR spectrum of methyphenyl alkyl pentaerythritol oleate and the sulfonate

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( ,C-C); 950-1250, multiple bands refer to the benzene area; 862 ( ,C-H ), 724 ((CH2)n ,n > 4), there is no absorption peak among 1680~1610, indicating no C=C.

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Elemental analysis. found(calc’d): C,80.51(80.59); H,10.77(10.90); O,8.72(8.51)

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GC-MS/(m/z, % rel. int.): (116, 40.5)

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(56 , 100) (28, 65)

; (42, 40.5)

; (68, 54.1) ; (44,40)

; ;

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Aryl pentaerythritol oleate sulfonate Pure aryl pentaerythritol oleate and solvent dichloromethane were added to a flask with vigorous stirring below 5℃, then dropping chlorine sulfonic acid and dichloromethane mixed solution slowly to ensure the system temperature is lower than 5℃. After titration, continue stirring for 2 hours below 5℃, then stirring the reactants at room temperature for 2 hours without HCl generated. With a

ACCEPTED MANUSCRIPT mechanical stirrer, an appropriate amount of sodium hydroxide(20%) was added slowly into the flask, the mixture was neutralized with sodium hydroxide to pH 7–8.

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After the reaction, evaporated the product to yellow viscous liquid .The product was dissolved with ethanol and filtered to remove inorganic salts. Water was added to obtain sulfonate solution with ethanol-water as solvent. Petroleum ether with a boiling range of 60-90℃was used repeatedly to remove the non sulfonated ester. The separated ethanol-aqueous layer was evaporated so as to obtain the sample of aryl pentaerythritol ester sulfonate. IR (KBr) /cm-1: 1223(-SO3

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antisymmetric vibration)、1045(-SO3 symmetric vibration)

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Schem1 The synthetic route of tetrameric surfactants

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Surface Tension Measurement

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benzene toluene xylene

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benzene toluene xylene

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 (mN/m)

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 (mN/m)

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Air-water surface tensions were measured at 25 and 45℃by the drop volume method. The critical aggregation concentrations were determined using a series of aqueous solutions at different concentrations. Values of CAC and surface tension at CAC(γCAC) were estimated from the intersection of the two straight lines drawn in low and high concentration regions of each surface tension versus concentration(on log scale) curve using a linear regression analysis method. The values of surface tension as a function of concentration are shown in Fig1.

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45 40 35

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Temperature 25℃

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Temperature 45℃

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Fig.1. Surface tension of solutions of A-C compounds as a function of concentration

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Results and discussion

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The critical aggregation concentration(CAC)，surface tension at the CAC(γCAC), ability of these compounds to lower surface tension by 0.02N/m(C20 and pC20), the mininum suface area per surfactant molecule (Amin) and the maximum surface

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excessive concentration(Γmax) at 25 and 45℃are summarized in Table1.

Table 1 Surface active properties of tetrameric surfactants A-C, at 25 and 45℃. Amin T

Compound

γo /

(℃) mN/m

CAC/

γCAC /

mol/L

mN/m

πCAC mN/m

lgCAC dγ/dlgc

Γmax / 10-10mol/cm2

/nm2

pC20

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72.16 3.80E-04

31.65

40.51

-3.420

-10.51

0.920

1.80

3.581

C105H176Na4O20S4(B)

25

72.16 3.71E-04

35.23

35.93

-3.431

-6.91

0.605

2.75

3.824

C109H188Na4O20S4(C)

25

72.16 3.63E-04

36.41

35.75

-3.440

-6.57

0.575

2.89

3.891

C101H164Na4O20S4(A) 45

72.16 2.39E-04

25.68

46.48

-3.622

-8.20

0.673

2.47

4.567

C105H176Na4O20S4(B)

45

72.16 2.34E-04

32.98

39.18

-3.631

-6.80

0.558

2.98

4.300

C109H188Na4O20S4(C)

45

72.16 2.29E-04

34.03

38.13

-3.640

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C101H164Na4O20S4(A) 25

-6.67

0.547

3.03

It can be seen from Fig1 and Table1 that the aggregations of these three

tetrameric surfactants take place at 10-4mol/L range, which is a distinctive feature of

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oligomeric surfactants. Furthermore, with the increase of substituents on the

aromatic ring, the CAC values slightly decrease and the surface tensions at CAC significantly increase. These changes can be explained in terms of hydrophobic interactions as follows. The more substituents which may become part of the aggregate hydrophobic core on the aromatic ring, the more stronger the

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hydrophobic characteristics. More substituents can decrease free energy of the molecule, making it need less free energy to aggregate and resulting in a lower CAC.

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Influence of substituents on surface tension at the CAC is clear, as the hydrophobic property becomes stronger, the ability to reduce the surface tension weakened, it decreases the amount of the saturation of adsorption for the oligomers at the

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air-water interface, resulting in higher γCAC, this behavior is more evident at 45℃.

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It is clearly shown that the values of the minimum surface area per surfactant molecule Amin increases from A to C. That's because with the volume of aromatic ring increasing, the size of the alkyl chain increases, single molecule occupied area at the air-water interface increases, molecules in the interface arranged more loosely, therefore resulting in bigger Amin and smaller Γmax . The Amin(ranging from1.80- 2.89 nm2,at 25℃) values are less than four times the values compared with the corresponding single-chain surfactants, indicating that the tetrameric surfactant molecules are not arranged side by side as the single-chain reference compound but staggered somewhat more closely at the air-solution interface .Values of Amin for B and C don't change much upon increasing the temperature from 25℃to 45℃. However, the minimum surface area per molecule of surfactant A significantly increased, this maybe due to the greater flexibility of the hydrophobic chains of A compared to those of B and C.

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The pC20 values reveal the ability of these surfactants to be absorbed at the surface and the efficiency to reduce the surface tension, which improves with increasing temperature as reported[19]. But the pC20 values of these compounds with different substituents at 25℃have different trends compared at 45℃. The pC20 values increase from surfactant A to surfactant C at 25℃, in accordance with the fact that the efficiency of surfactant in aqueous solution increases with its hydrophobicity enhanced.

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However with opposite trends at 45℃, this can be explained as the molecules with relatively more free energy are more likely to migrate to the surface from the bulk phase.

Table 2 Emulsibility of tetrameric surfactants A-C at 45℃

Water

Concentratio n

Diversion Time/s

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toluene

xylene

decane

(mM/L)

tetradecane

0.005

3342

3284

15

10

C105H176Na4O20S4(B)

0.005

3021

2983

53

58

C109H188Na4O20S4(C)

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2974

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Emulsification Ability of these three surfactants. The ultra low CAC concentration of these three surfactants compels us to study its efficiency in forming emulsions with oil

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at the low concentration far below CAC. Water diversion method was used to

evaluate emulsion stability of these three surfactants. The surfactants solution of

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0.005 mM is used to emulsify several different oil phases, including toluene, xylene, decane and tetradecane. It is observed that the O/W type emulsions are formed quickly after vigorous shaking, and the surfactants/toluene and surfactants/xylene

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emulsions are stabilized , but the surfactants/decane and surfactants/tetradecane mixtures nearly cannot form emulsion. Besides, the emulsion stability was reduced with structure of aromatic ring became more complex.

Conclusions Three long chain tetrameric alkyl aryl sulfonate surfactants were successfully synthesized with the renewable oleic acid as the main raw material, and their

ACCEPTED MANUSCRIPT molecule structures were confirmed by IR, 1HNMR and GC-MS. Their surface active properties in aqueous solution were investigated and the effect of structure difference on behavior of these oligomeric surfactants were discussed. Values of CAC, γCAC, Amin, Γmax and pC20 are reported at 25 and 45℃. By comparing these parameters, it is found that with the increase of the substituents on the aromatic ring, values of CAC、Γmax decrease,γCAC and Amin increase. But the pC20 values at 25 ℃have opposite trends compared with 45 ℃, it is manifested that the aggregation

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behavior of these three surfactants may change at different temperature. In this experiment, surfactant A at 45 ℃exhibits much better surface activities. Especially, the surfactants shows a high efficiency in forming emulsions with aromatic

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compounds at a very low concentration, which may have wide potential applications.

Acknowledgment

We are grateful for financial support from National Nature Science Foundation of China（51474068); National Science and Technology fund major project (2011ZX 05011-004); Heilongjiang province Nature Science Foundation(ZJG0507).

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