ORIGINAL ARTICLE

Effect of a Spacer Group on Surface Activity, Salinityand Hardness Tolerance, Mimic Oil Washing Efficiencyof Monododecyl Diaryl Disulfonate

Fengmei Xing • Jinping Niu • Xiaochen Liu •

Xiaoyu Wang

Received: 14 November 2012 / Accepted: 11 February 2013

� AOCS 2013

Abstract Three anionic surfactants of the monododecyl

diaryl disulfonate type (MDDADS-n, n = 0, 1, 2) were

synthesized. The structural characters of MDDADS-n

surfactants were verified by electrospray ionization/mass

spectrometry. The effect of the spacer group on the surface

activity, salinity and hardness tolerance and mimic oil

washing efficiency were investigated. The results showed

that the critical micelle concentration (CMC), surface

tension at CMC, C20 and the minimum area per molecule

of the anionic surfactants increased when the spacer group

length increased. There was less effect on mimic oil

washing efficiency of MDDADS-n when the spacer group

changed; meanwhile, they displayed higher mimic oil

washing efficiency in salt solution (NaCl or CaCl2) than

that of sodium dodecylbenzenesulfonate (SDBS); all

MDDADS-n surfactants showed much lower sensitivity to

water hardness than SDBS, what would be beneficial to

enhancing oil recovery in a high salinity oil field.

Keywords Monododecyl diaryl disulfonate � Surface

activity �Mimic oil washing efficiency �Anionic surfactants �Salinity and hardness tolerance � Spacer group �Critical micelle concentration

Abbreviations

CMC Critical micelle concentration

Amin Minimum area per molecule

Cmax Surface excess concentration

SDBS Sodium dodecylbenzenesulfonate

MDDADS-n Monododecyl diaryl disulfonate

ESI-MS Electrospray ionization-mass spectrometry

Introduction

Gemini surfactants represent a new class of surfactants that

are made up of two amphiphilic moieties connected at the

level of the head groups or very close to the head groups by

a spacer group. They generally exhibited superior proper-

ties contrasted with those of their single-chain analogue

surfactants with a similar chain length and head group

[1–4]. The fact that the properties of gemini surfactants

could differ greatly from those of conventional surfactants

is related to the distribution of distances between head

groups in micelles formed by these two types of surfactants

[5]. The spacer influence on the properties of gemini sur-

factants has been widely studied [6–9], and it was found

that the spacer played an important role in the properties.

Although the influence of the spacer on the physico-

chemical properties of symmetrical surfactants was thor-

oughly studied, there was little knowledge of its effect on

their dissymmetric counterparts. In this paper, we mainly

investigated the influence of the spacer length on the sur-

face activity, salinity and hardness tolerance and mimic oil

washing efficiency properties of MDDADS-n surfactants.

Experimental Procedures

Materials

Lauryl alcohol, decane, methanol, tetrahydrofuran and

1,2-dichloroethane were obtained from Tianjin Kemiou

Chemical Reagent Co., Ltd. in China. Diphenyl methane

F. Xing (&) � J. Niu � X. Liu � X. Wang

China Research Institute of Daily Chemical Industry,

34 Wenyuan Street, Taiyuan 030001, Shanxi Province,

People’s Republic of China

e-mail: njp6611@163.com; Meini05huaxue@yahoo.com.cn

123

J Surfact Deterg

DOI 10.1007/s11743-013-1459-z

was available from Shanghai Licheng Chemical Co., Ltd.,

China. Diphenylethane was purchased from Shandong

Shouguang Luyuan Salt Chemical Co., Ltd., China, and

biphenyl was obtained from Tianjin Fine Chemical

Research Institute, China. Silica gel powder (H 60–80

type) was from a branch of Qingdao Haiyang Chemical

Co., Ltd., China. The oil content percentage of modified

silica gel power by liquid paraffin was 41.2 wt% (mass

fraction). Note that the chemicals listed above were of

analytical grade and used as received. Double-distilled

deionized water was used in all experiments.

Synthesis

Diphenyl methane (0.4 mol) was dissolved in 60 mL

n-decane; and 0.2 mol dodecyl alcohol and 20 g activated

clay (catalyst) were added to the mixture. Then the solution

was heated and constantly stirred for 60 min. When the

reaction had finished, the solution was filtered to remove

the catalyst. A light yellow liquid product was obtained,

and then distilled off under vacuum [10]. The intermediate

product obtained (0.03 mol) was dissolved in 20 mL

1,2-dichloroethane. Then sulfur trioxide (0.13 mol) dis-

solved in 20 mL dichloroethane was dropped into the

above-mentioned mixture at temperatures between 20 and

30 �C. The solution was heated up to 50 �C and this was

maintained for 40 min, and then it was neutralized with

30 % aqueous sodium hydroxide to a pH value of 8 at room

temperature [11]. The raw products were desalted and

deoiled in anhydrous ethanol and petroleum ether,

respectively; then the pure product would generate after

drying. And the same synthesis process was suitable for

synthesis of the MDDADS-n (n = 0, 2). The route of the

synthesis reaction is outlined in Scheme 1.

Measurements

ESI-MS Analysis

The ESI-MS (negative) analysis was carried out using an

Agilent 1100 series LC-MSD Trap SL mass spectrometer

with an electrospray interface (ESI). The mass spectra were

recorded in negative mode. The parameters were as fol-

lows: drying gas flow rate, 4.0 L min-1; drying gas tem-

perature, 180 �C; nebulizer, 0.03 MPa; HV capillary

voltage, 3,500 V. For full scan MS analysis, the spectra

were recorded in the range of m/z 100–1,300. MS data were

acquired in the automatic data-dependant mode.

Surface Tension Measurements

The surface tensions of aqueous solutions of MDDADS-n

were measured with a KRUSS K12 Processor Tensiometer

by the Wilhelmy plate technique at 25 ± 0.1 �C. Solutions

were prepared with deionized doubly distilled water. The

length of the platinum was 19.9 mm, and the thickness was

0.2 mm. The dipping distance was 2 mm.

Salinity and Hardness Tolerance

The salinity and hardness tolerance experiments were

conducted in 25 mL glass tubes. The steps of experiment

were as follows: 10 mL 0.3 wt% MDDADS-n solutions

were added into the glass tubes; next gradually put the salt

(NaCl or CaCl2) into the solution; and then, the solution

were stirred for 5 min at the room temperature (about

25 �C), finally, observed if there was new phase precipi-

tated out.

Scheme 1 The synthesis route

of MDDADS-n

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Mimic Oil Washing Efficiency in Lab

To mimic oil washing efficiency measurement in lab, the

silica gel powder was soaked in liquid paraffin for 24 h,

then was filtrated and dried in the water bath. The pro-

cessed powder should not agglomerate, and continued to be

dried until the weight became constant. The oil (liquid

paraffin) content was calculated from the weight change

before and after the oil soaking [12].

The Washburn glass tube whose length was 15 cm and

diameter was 0.8 cm was used in this experiment. To

prevent the oil spilling from the bottom, the filter should be

plugged. The modified silica gel powder was filled in the

washburn glass tube, and was soaked with surfactant

solution (3 g L-1) at a temperature of 60 �C in water bath.

The oil climbed the modified silica gel powder bed and

formed an oil column as shown in Fig. 1.

Results and Discussion

Characterization of MDDADS-n

MDDADS-n was in line with the result of the ESI-MS

analysis. The m/z corresponded to either [M–Na]- or

[M–2Na]2-. The mass of MDDADS-n was calculated to be

526, 540 and 554, respectively (n = 0, 1, 2). ESI-MS

(negative): for MDDADS-0, m/z 240.2 [M–2Na]2-; for

MDDADS-1, m/z 517.22 [M–Na]-, m/z 247.11

[M–2Na]2-; for MDDADS-2, m/z 531.3 [M–Na]-,

m/z 254.2 [M–2Na]2- and m/z 509.3 [M–Na ? H]-. The

characterizations by ESI-MS were consistent with the

structures of MDDADS-n surfactants molecules. All

quoted the m/z values were monoisotopic (Fig. 2).

Surface Activity Properties

The equilibrium surface tensions of the MDDADS-n

aqueous solutions at different concentrations were

Fig. 1 The diagram of the surfactant solution penetration process in a

washburn glass tube. a During the process, b end of the process. The

meaning of the symbols a the MDDADS-n solution, b the filter at the

bottom of the tube, c the modified silica gel powder, d the interface

which indicated the rising surfactant solution level, j the surfactant

solution column, f the replaced oil column, g the washburn glass tube,

H the height of the modified silica gel powder

Fig. 2 Negative ionic ESI-MS mass spectrums of the MDDADS-n

(n = 0, 1, 2)

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measured and are plotted in Fig. 3. At first, the surface

tension of the solutions sharply decreased with the increase

in the surfactant concentration, but its variation became less

obvious when the concentration reached a certain value. On

the basis of these plots of surface tension versus logarithm

of surfactant molar concentration, some solution parameters

of the MDDADS-n surfactants could be interpreted. The

results were listed in Table 1, and the corresponding solu-

tion parameters of the conventional single-chain surfactant

SDBS were compared with the MDDADS-n.

The critical micelle concentration (CMC) is one of the

major performance parameters of a surfactant, it can be

directly interpreted from the transition point indicated in

Fig. 3. The CMC data are shown in Table 1; it can be seen

that all the CMC data of MDDADS-n were lower than that

of the single-chain surfactant SDBS. This suggests the

excellent micelle-forming ability of the MDDADS-n in

water. Among the MDDADS-n surfactants, the one which

had the fully rigid spacer (MDDADS-0) showed the lowest

CMC value; the insertion of the flexible methylene unit

into the fully rigid spacer showed a significantly higher

CMC value than that of MDDADS-0; one more introduc-

tion of the flexible methylene unit had less effect on the

CMC compared with MDDADS-1. It was reported that

symmetric sulfonate surfactants with a flexible spacer had a

lower CMC value [14–16]. However, our data indicated

that a long rigid, especially a semi-rigid spacer and dis-

symmetric molecular structure had an excellent ability to

form micelles. The CMC values increased with the

increasing of the spacer length. The equilibrium distance of

the electrostatic repulsion of the head groups also increased

with the spacer group lengthening. The insertion of flexible

methylene group would increase the free energy in the

micelle and result in a higher CMC value.

The surface tension at the CMC (cCMC) can also be read

out directly from Fig. 3 and the data are listed in Table 1. It

is clear that the cCMC values of MDDADS-n were 34.78,

38.43, 38.59 mN/m. And comparing the three dissymmetric

anionic surfactants, we could conclude that the longer the

spacer was, the larger the cCMC value was. It was due to the

increasing distance between the head groups, which caused

loose packing of the MDDADS-n surfactants molecules on

the solution surface layer.

The studies of the adsorption of the surfactants at the air/

solution interface aimed to assess the efficiency and

effectiveness of the surfactants in reducing the surface

tension of water. These measurements also aimed to

measure the Cmax, Amin occupied by one surfactant at the

air/water interface and C20. Cmax (in mol/m2) and Amin

(nm2) were calculated using the following Gibbs adsorp-

tion isotherm equation [17, 18]:

Cmax ¼ �1

2:303nRT

dcd log C

� �T

ð1Þ

Amin ¼1018

NCmax

ð2Þ

where dc/dlogC was the slope of the surface tension c(mN/m) versus logC dependence below the CMC, and it

can be read off from Fig. 3. The parameter n was the

number of ionic species absorbed at the air/solution

interface whose concentration changes with surfactant

concentration. In our present work, n was taken as 3 for an

anionic surfactant made up of a divalent surfactant ion

and two univalent counterions in the absence of addedFig. 3 Surface tensions versus log C of MDDADS-n in aqueous

solution at 25 �C

Table 1 Surface property of

MDDADS-n and SDBS in

aqueous solutions

Surfactants cCMC

(mN m-1)

CMC/10-4

(mol L-1)

Cmax/10-6

(mol m-2)

Amin

(nm2)

C20/10-5

(mol L-1)

MDDADS-0 34.78 0.52 1.87 0.88 1.42

MDDADS-1 38.43 0.91 1.62 1.02 2.62

MDDADS-2 38.59 1.11 1.21 1.36 2.89

SDBS [13] 35.10 17.80 3.26 0.51 19.10

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electrolyte [19]. And R = 8.314 J mol-1 K-1, N was the

Avogadro’s constant, T was the absolute temperature. The

values are listed in Table 1, which also includes those of

SDBS for comparison.

Cmax measured how much the air/solution interface was

changed maximally by surfactant adsorption. It depended

on the surfactant molecular structure. Amin reflecting the

packing densities of the surfactants at the air/solution

interface was important for the interpretation of the surface

activity of the surfactants. In Table 1, it is clear that all

Amin values of MDDADS-n surfactants were higher than

that of SDBS. Among MDDADS-n, MDDADS-0 with a

fully rigid spacer had the lowest Amin, whereas introducing

a flexible methylene unit increased the Amin value from

0.88 to 1.02 nm2. The further increase in the flexible unit

from methylene to ethylene increased the Amin value again.

As the spacer length increased, it had an important influ-

ence on the adsorption; Cmax decreased and Amin increased

consequently. These results were in accordance with those

obtained for the symmetric counterparts [20, 21]. The

dominant factors in determining the variation in Amin with

the spacer length were the balance of the attractive inter-

actions between the hydrophobic groups, the repulsive

interactions between the head groups, and the conforma-

tional entropy of the spacer [21, 22]. For the Cmax and Amin

changes of MDDADS-n surfactants, the reason may show

that the gradual introduction of flexible CH2 unit induced

the distance of the head groups to increase and an

enhancement of the conformational entropy.

According to Table 1, another important parameter of

surfactants, C20, could also be interpreted. C20 represented

the surfactant concentration required to reduce the surface

tension of water by 20 mN/m, and reflected the capability of

surfactants to adsorb onto the air/solution interface [15], and

the lower the C20 value was, the greater was the absorbing

efficiency. From Table 1, it could be seen clearly that all C20

values of MDDADS-n were one order of magnitude lower

than those for the corresponding conventional surfactant

SDBS. The phenomenon indicated that the SDBS molecules

could not cover the water surface as effectively as

MDDADS-n. Besides, the C20 data (in Table 1) increased

with the n value variation of MDDADS-n from 0 to 2. For the

C20 of MDDADS-1, increasing the flexible unit CH2 (from

methylene to ethylene) caused the C20 to increase, whereas

removing a flexible methylene into the spacer group led to an

obvious reduction of C20 from 2.62 to 1.42. These results

indicated that extending the spacer length could increase the

C20 value. This phenomenon was explained tentatively as

follows: the fully rigid spacer of MDDADS-0 could contact

with the water surface intimately, while the introduction of

flexible units in the rigid spacer enhanced the effect of steric

hindrance caused difficulty in coming into contact with the

water surface intimately.

Salinity and Hardness Tolerance

Salinity and hardness tolerance are prerequisite properties

in several applications such as enhancing oil recovery,

surfactant-based separation processes and detergency.

Numerous laboratory studies and several field tests have

demonstrated the salinity and hardness tolerance capacity

of these surfactants [23]. Anionic surfactants tend to pre-

cipitate with cations such as Na? and Ca2?. The surface

activity of the surfactants is reduced when precipitation of

the surfactants occurs. In this paper, tolerance of salinity

and hardness were studied on the effect of the spacer group

in MDDADS-n. The salinity and hardness tolerance data of

MDDADS-n are listed in Table 2, including those of

SDBS. From Table 2, it can be seen that the salinity and

hardness tolerance of all MDDADS-n surfactants were

much stronger than the single-chain SDBS. And among

MDDADS-n surfactants in the surfactant solution with

Na?, the longer the spacer group of surfactants, the lower

the salinity tolerance; while in Ca2? surfactant solution, the

hardness tolerance increased as the spacer group length

increased. Besides, these MDDADS-n surfactants showed

remarkable salinity and hardness tolerance, which was

beneficial to efficient applications, such as enhancing oil

recovery in high salinity oilfields.

Mimic Oil Washing Efficiency

The oil washing efficiency is a key factor in petroleum

production, as not all oil can be readily drawn from the

reservoir of an oilfield. In this paper, a mimic oil washing

efficiency experiment was carried out in different surfac-

tants salt solutions. A series of MDDADS-n aqueous

solutions were used as mimic flooding solutions, liquid

paraffin as oil and modified silica gel powder as mimic oil

reservoir. When surfactant solutions were used to penetrate

and moved up into the modified silica gel bed, the oil

attached to the silica gel surface could be replaced. In the

end, the oil was displaced and converged on the top of the

silica gel powder bed, an oil column was finally formed.

Then reading the volume of the oil column allowed cal-

culation of the efficiency at washing the oil. As shown in

Table 3, the MDDAD-n surfactants displayed better oil

washing efficiency than SDBS; what was more, the appli-

cability of the MDDADS-n in higher salt content solutions

was better than SDBS. According to the comparison of

Table 2 Salinity and hardness tolerance of MDDADS-n and SDBS

Surfactants MDDADS-0 MDDADS-1 MDDADS-2 SDBS

NaCl (wt%) 23.18 22.15 20.72 0.94

CaCl2 (wt%) 16.95 17.01 17.40 0.0040

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MDDADS-n surfactants, it was found that the spacer group

variation had little effect on the oil washing efficiency.

Acknowledgments The support of the Lanzhou Institute of

Chemical Physics, Chinese Academy of Sciences is gratefully

acknowledged.

References

1. Menger FM, Littau CA (1991) Gemini surfactants: synthesis and

properties. J Am Chem Soc 113:1451–1452

2. Menger FM, Littau CA (1993) Gemini surfactants: a new class of

self-assembling molecules. J Am Chem Soc 115:10083–10090

3. Rosen M, Geminis J (1993) A new generation of surfactants.

Chem Technol 23:30–33

4. Rosen MJ, Tracy DJ (1998) Gemini Surfactants. J Surf Deterg

1:547–554

5. Danino D, Kaplun A, Talmon Y, Zana R (1994) Structure and

flow in surfactant solutions. In: Herb CA, Prud’homme RK (eds)

ACS symposium series 578, Chap. 6. American Chemical Soci-

ety, Washington, DC, p 105

6. Alami E, Levy H, Zana R (1993) Alkanediyl-a,x-bis

(dimethylalky1ammonium bromide) surfactants. 2. Structure of

the lyotropic mesophases in the presence of water. Langmuir

9:940–944

7. Menger FM, Keiper JS, Azov V (2000) Gemini surfactants with

acetylenic spacers. Langmuir 16:2062–2067

8. Wettig SD, Verrall RE (2001) Thermodynamic studies of aque-

ous m–s–m Gemini surfactant systems. J Colloid Interface

Sci 235:310–316

9. Wettig SD, Nowak P, Verrall RE (2002) Thermodynamic and

aggregation properties of Gemini surfactants with hydroxyl

substituted spacers in aqueous solution. Langmuir 18:5354–5359

10. Coleman GH, Perkins RP, Mich M (1939) Alkylated diphenyl

oxide. U.S. Patent 2,170,809

11. Steinhauer AF, Mich M (1958) Method of making alkyl diphenyl

ether sulfonates. U.S. Patent 2,854,477

12. Bi ZC, Qi LY, Liao WS (2005) Dynamic surface properties,

wettability and mimic oil recovery of ethanediyl-a,b-bis

(cetyldimethylammonium bromide) on dodecane modified silica

powder. J Materials Sci 40:2783–2788

13. Zhu DY, Cheng F, Chen Y, Jiang SC (2012) Preparation, char-

acterization and properties of anionic Gemini surfactants with

long rigid or semi-rigid spacers. Colloids Surf A 397:1–7

14. Zhu S, Cheng F, Wang J, Yu J (2006) Anionic gemini surfactants:

synthesis and aggregation properties in aqueous solutions.

Colloids Surf A 281:35–39

15. Zhu S, Liu L, Cheng F (2010) Influence of spacer nature on the

aggregation properties of anionic gemini surfactants in aqueous

solutions. J Surf Deterg 14:221–225

16. Liu XP, Feng J, Zhang L, Gong QT, Zhao S, Yu JY (2010)

Synthesis and properties of a novel class of anionic gemini

surfactants with polyoxyethylene spacers. Colloids Surf A

362:39–46

17. Alami E, Beinert G, Marie P (1993) Alkanediyl-alpha,omega

bis(dimethylalkylammonium bromide) surfactants. 3. Behavior at

the air–water interface. Langmuir 9:1465–1467

18. Song LD, Rosen MJ (1996) Surface properties, micellization, and

premicellar aggregation of Gemini surfactants with rigid and

flexible spacers. Langmuir 12:1149–1153

19. Li X, Hu ZY, Zhu HL, Zhao SF, Cao DL (2010) Synthesis and

properties of novel alkyl sulfonate Gemini surfactants. J Surf

Deterg 13:353–359

20. Alami E, Beinert G, Marie P, Zana R (1993) Alkanediyl-a,

x-bis(dimethylalkylammonium bromide) surfactants. 3. Behavior

at the air-water interface. Langmuir 9:1465–1467

21. Diamant H, Andelmann D (1994) Dimeric surfactants: spacer

chain conformation and specific area at the air/water interface.

Langmuir 10:2910–2916

22. Diamant H, Andelmann D (1995) Dimeric surfactants: a simpli-

fied model for the spacer chain. Langmuir 11:3605–3606

23. Wu B, Harwell JH, Sabatini DA, Bailey JD (2000) Alcohol-free

diphenyl oxide disulfonate middle-phase microemulsion systems.

J Surf Deterg 3:465–474

Author Biographies

Fengmei Xing is a postgraduate student at the China Research

Institute of Daily Chemical Industry, Taiyuan, Shanxi Province, PR

China. Her main research field is the synthesis and the study of

properties of diaryl sulfonate surfactants.

Jinping Niu is a professor of Applied Chemistry at the China

Research Institute of Daily Chemical Industry. She mainly studies the

sulfonation or sulfation of organic materials.

Xiaochen Liu graduated from Hebei University of Science and

Technology, and earned his master’s degree in applied chemistry

from the China Research Institute of Daily Chemical Industry. His

research interest is the synthesis and investigation of novel sulfonates.

Xiaoyu Wang graduated from Shanxi University. Her major field is

physical chemistry and her research interest is the synthesis and

application of surfactants.

Table 3 The mimic oil washing efficiency of modified silica gel with

different surfactants in salt solution

Surfactants Mimic oil washing efficiency (wt%)

NaCl solution CaCl2 solution

MDDADS-0 65.28 58. 80

MDDADS-1 64.73 56.18

MDDADS-2 61.04 53.70

SDBS 43.14 43.25

The mimic oil washing efficiency of MDDADS-n was carried out in

250 g L-1 NaCl solution and 10 g L-1 CaCl2 solution, respectively;

and that of SDBS was measured in 5 g L-1 NaCl solution and

0.03 g L-1 CaCl2 solution, separately

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