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Effect of Organic Additives and Crude Oil Fractions onInterfacial Tensions of Alkylbenzene SulfonatesRong-hua Zhao a b , Hai-yao Huang a , Hong-yan Wang c , Ji-chao Zhang c , Lei Zhang a , LuZhang a & Sui Zhao aa Technical Institute of Physics and Chemistry, Chinese Academy of Sciences , Beijing , P. R.Chinab Graduate University of Chinese Academy of Sciences , Beijing , P. R. Chinac Geological Scientific Research Institute of Shengli Oilfield Co. Ltd., SINOPEC , Shandong ,P. R. ChinaAccepted author version posted online: 18 May 2012.

To cite this article: Rong-hua Zhao , Hai-yao Huang , Hong-yan Wang , Ji-chao Zhang , Lei Zhang , Lu Zhang & Sui Zhao (2013)Effect of Organic Additives and Crude Oil Fractions on Interfacial Tensions of Alkylbenzene Sulfonates, Journal of DispersionScience and Technology, 34:5, 623-631, DOI: 10.1080/01932691.2012.685844

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Effect of Organic Additives and Crude Oil Fractions onInterfacial Tensions of Alkylbenzene Sulfonates

Rong-hua Zhao,1,2 Hai-yao Huang,1 Hong-yan Wang,3 Ji-chao Zhang,3

Lei Zhang,1 Lu Zhang,1 and Sui Zhao11Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, P. R. China2Graduate University of Chinese Academy of Sciences, Beijing, P. R. China3Geological Scientific Research Institute of Shengli Oilfield Co. Ltd., SINOPEC,Shandong, P. R. China

GRAPHICAL ABSTRACT

The dynamic interfacial tension (IFT) behavior of two alkylbenzene sulfonates systems have beenresearched with the addition of organic additives to pure alkane oil phase, n-Nonane. The resultsshowed that IFTs increased with increasing chain length and concentration of carboxylic acidsand alcohols due to the deviation of hydrophilic-lipophilic balance. Moreover, the IFT minimumappears in long-chain alkane by adding organic components in n-alkane model oil phase in thelight of the matching of the alkane carbon number (ACN) for the minimum IFT (nmin) and equiva-lent alkane carbon number (EACN) values. In addition, four crude oils were separated into satu-rates, aromatics, resins, asphaltenes, and acidic fractions. We have found that acidic fractions incrude oil is the dominant factor affecting the stable value of IFT and antagonism has beenobserved when EACN/nmin value is far from unity by adding acidic fractions in jet fuel. Theresults presented here suggest that organic additives in oil phase and acidic fractions in crudeoil can influence the partition of surfactants between the oil phase and aqueous phase, which resultedin the change of IFTs.

Keywords Acidic fraction, antagonism, crude oil, EACN, interfacial tension, organicadditive, synergism

1. INTRODUCTION

Producing ultra-low interfacial tension (IFT) is one of themost important mechanisms relating to alkali-surfactant-polymer (ASP) flooding and surfactant-polymer (SP) flood-ing for enhanced oil recovery. The study focus is to searchfor a flooding system that will be high-efficiency, low-cost,and safe for the environment. The key of that is efficient

Received 26 March 2012; accepted 29 March 2012.The authors thank financial supports from the National

Science & Technology Major Project (2011ZX05011-004),National High Technology Research and Development Program(2008AA092801), and the Knowledge Innovation Program ofthe Chinese Academy of Sciences (KJCX1-YW-21-03) of China.

Address correspondence to Lu Zhang and Sui Zhao, TechnicalInstitute of Physics and Chemistry, Chinese Academy of Sciences,Beijing 100190, P. R. China. E-mail: luyiqiao@hotmail.com andzhaosui@mail.ipc.ac.cn

Journal of Dispersion Science and Technology, 34:623–631, 2013

Copyright # Taylor & Francis Group, LLC

ISSN: 0193-2691 print=1532-2351 online

DOI: 10.1080/01932691.2012.685844

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utilization of natural surface-active components in crude oil.So, the investigation on the fraction of crude oil should be apart of study that aims at the problems related to SP andASP flooding technique.[1,2]

It is of practical significance to study the IFTs betweensurfactant solutions and crude oil, however, many fieldtests failed due to the complicated compositions of crudeoil. Although there exist different kinds of natural activecomponents in crude oil, organic acids are confirmed theprimary surface-active components,[3–5] which has beenrecognized as contributing in lowering IFT of EOR sys-tems in numerous investigations of both experimentaland theoretical character.[6–12] In addition, asphaltenes,resins, wax, hydroxybenzene, and ester are also recognizedto be active components in crude oil.[13–15] Therefore, it isdifficult to identify the reasons for producing low IFTsbetween surfactant solutions and crude oil.

Although the composition of crude oil are verycomplex, previous work has proved that the behaviorof simple model systems using pure hydrocarbon oilacidified with an oil-soluble carboxylic acid are closelyrepresentative of systems involving real crude oil, parti-cularly in regard to the dynamic character of the IFTobserved.[8]

The theory of equivalent alkane carbon number (EACN)introduced by Cayias[16] in 1970s is accepted a useful tool inthe research referring to IFTs. Doe et al.[17–19] studied theIFTs of surfactant systems against n-octane, and it had beenproved that the substitution of n-alkanes for crude oilsbrought about low errors. Since then most studies have beenreported on synergism for lowering IFTs in acidic model oil=alkali=surfactant systems.[6–12] Rudin and Wasan[6] examinedboth equilibrium and dynamic IFT in acidic model oil=alkali=petroleum sulfonate system. They proposed that the union-ized acid species was the dominant factor affecting IFT,and appeared to be the key element in the synergistic processtaking place between the added surfactant and the ionizedacid species. The unionized acid species partitioned petroleumsulfonate molecules out of the aqueous phase, and the mini-mum in IFT occurred when the partition coefficient wasabout unity. Rudin et al.[7] also investigated the influence ofvarious acid species present on both dynamic and equilibriumIFT as a function of pH and ionic strength in bufferedsurfactant-enhanced alkaline flooding=crude oil systems.They suggested that the dominant mechanism responsiblefor added surfactant reducing IFT was the formation ofmixed micelles with the ionized acid, though ionic strengthwould affect interfacial behavior. Touhami et al.[8] investi-gated the IFT between acidic oil and surfactant-enhancedalkaline solutions, and then they found that the dynamicIFT behavior was a function of acid concentration in oilphase, alkali concentration, and added surfactant concen-tration in the aqueous phase. They suggested the unionizedacid contributed to the lowering of dynamic IFT through

adsorbing simultaneously onto the interface with ionized acidand added surfactant.

In our earlier studies, we reported that antagonism forlowering IFT was also a general result in acidic oil=pet-roleum sulfonate=alkali or salt systems, especially whenthe added surfactants could produce an ultra-low IFTvalue with the oil phase.[20–23] From the experimentalresults, we concluded that carboxylic acids in model oiland acidic fractions in crude oil would not only decreasethe EACN value of the oil phase, but also change the par-tition of surfactants in oil phase, aqueous phase, and inter-face. In this part, we investigated dynamic IFTs in modeloil=pure surfactant=brine systems. Thereby, alkanes andcrude oils should be involved simultaneously to performthese studies by comparison, from the point of view of bothpractical application and theoretical study. The effects oflong-chain carboxylic acids and alcohols on the IFTs ofalkylbenzene sulfonates systems were studied by addingcarboxylic acids and alcohols into n-Nonane. The purposesof this article include investigating the mechanisms respon-sible for acidic components in affecting ultra-low IFTsand providing suggestions on designing formulations forpractical application.

2. EXPERIMENTAL

2.1. Materials

In this study, two alkylbenzene sulfonates synthesizedby our laboratory, 1, 2-dihexyl-4-propylbenzene (366)and 1, 4-dibutyl-2-nonylbenzene (494), were used.[24] Thepurities of the synthesized surfactants were more than99.0%. Double-distilled water was used in the preparationof the aqueous solutions. The hydrocarbons and carboxylicacids and alcohols were of 99þmol% purity. Sodiumchloride was used as the analytical reagent.

A chromatography column, rotary vacuum evaporator,and vacuum drying oven were used in experiment. The fourcrudes under investigation were Shengli Gudong crude oil,Shengli Gudao crude oil, Shengli offshore crude oil, andShengli outer piped oil.

2.2. Separation of Fractions From Crude Oil

The crude oil sample was subjected to SARA fraction-ation (Chinese Standard Analytical Method for Petroleumand Natural Gas Industry: SY=T 5119-2008). Separationof asphaltenes from the sample was carried out by adding30mL of hexane to 50mg of sample. The mixture was stir-red, allowed to sit for 12 hours, and then filtered. A piece ofcotton-wool was placed at the mount of the funnel to filterthe asphaltenes from the mixture. The asphaltenes on thecotton wool were washed with hexane until the filtratewas colorless. The asphaltenes were dissolved and elutedby washing with dichloromethane. The maltenes were

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adsorbed onto a packed column consisting of a 10mminner diameter column packed with 3 g of preactivatedsilica gel (80–100 mesh, activated at 150�C for 4 hours)and 2 g of neutral alumina (100–200 mesh, activated at400�C for 4 hours and added 1wt% water). A total of25mL of n-hexane was used to elute the saturate fraction,and a 15mL solvent mixture of n-hexane=dichloromethane(1:1, v=v) was used to elute the aromatic fraction. A solventmixture consisting of 10mL of ethanol and 15mL ofchloroform was used to elute the resin fraction. At last,the crude oil fractions in investigation were dissolved injet fuel as model oils respectively.

2.3. Apparatus and Methods

The spinning drop technique was employed to measuredynamic IFTs. The standard spinning-drop tensiometershad been modified by the addition of video equipmentand an interface to a personal computer. The computerhad been fitted with a special video board and amenu-driven image enhancement and analysis program.The video board can ‘‘capture’’ a droplet image forimmediate analysis. Analysis usually consists of measure-ment of drop length and drop width.[25]

The volumetric ratio of water to oil in the spinning droptensiometer is about 200. Samples were assumed to be equi-librated when measured values of IFT remained unchangedfor half an hour. All experiments were performed at30.0� 0.5�C.

The aqueous phase in all studies reported here was 0.1mass% 366 (494) and 1.0 mass% NaCl.

3. RESULTS AND DISCUSSION

Measuring the IFTs of a standard surfactant solutionagainst a homologous series of n-alkane, each surfactantor surfactant mixture in a reference series will produce aminimum IFT. The alkane carbon number (ACN) for theminimum IFT calls the nmin of this surfactant sol-ution.[26,27] For a surfactant or surfactant mixture, the nmin

value can represent its hydrophilic-lipophilic balance(HLB) ability; the higher its nmin value, the lower its HLB.

Similar to the concept of nmin, an EACN for any hydro-carbon, hydrocarbon mixture, or crude oil can be definedusing the following procedure[26,27]: Using surfactants orsurfactant mixtures of the same structural type to makeup a series of formulations with different nmin values, whichhas been carried out by adjusting the structure and mixedratio of the surfactants, measure the IFT of each formu-lation against the oil whose EACN is desired, and thenmake a plot of IFT versus the nmin value. If the curvehas a minimum, the nmin value of surfactant formulationfor that point defines the EACN of the oil, which is aninvariant oil property and, in particular, does not changealong with the surfactant structure. The EACN value of

an oil phase can represent its influence on the partition ofsurfactant in the oil phase. It is more difficult for a surfac-tant to partition into an oil phase having a higher EACNvalue than one having a lower EACN value.

In this study, the nmin values of the two alkylbenzenesulfonates 366 and 494 (see Figure 1) are the same valueof 925 and the EACN value of n-Nonane is 9.

3.1. Effect of Organic Additives on Dynamic IFT ofModel Oil

3.1.1. Effect of Alkyl Chain Lengths of Carboxylic Acidsand Alcohols on Dynamic IFTs of Model Oil

The dynamic IFTs between two alkylbenzene sulfonates366=494 and n-Nonane containing 10mmol=L carboxylicacids with different chain lengths were investigated andshown in Figure 2. We can see that the dynamic IFTs dropgradually to the stable values for 366=494 against modeloils. It also shows that both the two surfactants show highinterfacial activity and can produce ultra-low IFT againstn-Nonane without carboxylic acid added under our experi-mental conditions. However, it is obvious that the IFTs of366 and 494 solutions become higher after the addition of10mmol=L carboxylic acids with different chain lengths.

Figure 3 shows the stable values of dynamic IFTs as afunction of carboxylic acid chain length for 366=494against n-Nonane model oils. The carboxylic acids at10mmol=L concentration in n-Nonane model oils havethe ability to increase IFT. The trend of increasing IFTbecomes stronger with the increasing of carboxylic acidchain length and the stable values increased gradually.

Once the aqueous and oil phases contact, surfactantmolecules in the aqueous phase begin to assemble ontothe interface, resulting that interfacial concentrationincreases and the IFT values descend along with time. Atthe same time, interfacial surfactant molecules will par-tition into the oil phase because of the concentration gradi-ent. Therefore, the interfacial surfactant concentration willincrease monotonously and arrive at the plateau value untilpartition equilibrium among the interface and the bulkphases appears. As a result, dynamic IFTs drop graduallyand reach the stable values as function of the time.

Organic additives in oil phase can influence the partitionof added surfactants in oil phase, aqueous phase and inter-face by changing the EACN value of model oil, and further

FIG. 1. Structures of 366 and 494.

DYNAMIC IFT OF ALKYLBENZENE SULFONATES 625

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affect the IFT in surfactant=brine=model oil systems. Forthe two low IFT systems, the addition of carboxylic acidschanges IFT mainly by altering the EACN=nmin values.[20]

The nmin values of 366 and 494 are both 9, which means the366 (494) molecules have almost the same trend to partitioninto aqueous phase and oil phase when the EACN=nmin

value is unity, and then the interfacial surfactant concen-tration reaches maximum. However, the EACN value ofn-Nonane with carboxylic acids decreases. With the chainlength of carboxylic acids becoming longer, the trend ofdecreasing EACN values becomes stronger, leading thatthe EACN=nmin value deviates from unity.[20,25] As a result,the stable value of IFT increases and antagonism has beenobserved. This is due to the relatively stronger tendency of366 (494) molecules partitioning into n-Nonane model oilscontaining long-chain carboxylic acid, which results in thedecreasing of interfacial surfactant concentration and thenthe increasing of IFT.

FIG. 4. Effect of carboxylic alcohol chain length on dynamic IFT

between 366 (A)=494 (B) and n-Nonane model oils (alcohol concentration

10mmol=L).

FIG. 2. Effect of carboxylic acid chain length on dynamic IFT

between 366 (A)=494 (B) and n-Nonane model oils (acid concentration

10mmol=L).

FIG. 3. Effect of carboxylic acid chain length on the stable value of

dynamic IFT of n-Nonane model oils (acid concentration 10mmol=L).

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Figure 4 shows the dynamic IFTs of 366=494 againstn-Nonane model oils as a function of carboxylic alcoholchain lengths. Figure 5 shows the effect of carboxylicalcohol chain length on stable values of dynamic IFTs.We can see from Figures 4 and 5 that the dynamic IFTsdrop gradually to the stable values for 366 (494) againstn-Nonane model oils. The carboxylic alcohols at10mmol=L concentration in model oils also have the abil-ity to increase IFT. These experimental results are similarto those of carboxylic acid model oils.

3.1.2. Effect of N-Octadecanoic Acid and N-OctadecanolConcentration on Dynamic IFT of Model Oil

Figure 6 shows the dynamic IFTs as a function ofn-Octadecanoic acid concentration for 366=494 againstn-Nonane model oils. We can see from Figure 6 that thedynamic IFTs of n-Nonane added n-Octadecanoic aciddrop to the stable values quickly.

Figure 7 shows the stable values of dynamic IFTs as afunction of n-Octadecanoic acid concentration for366=494 against n-Nonane model oils. The n-Octadecanoicacid in model oils has the ability to increase IFT. More-over, the IFT values continue to increase with the increas-ing of n-Octadecanoic acid concentration. According toour earlier studies, the magnitude of decreasing EACNvalue becomes larger with the increasing of oleic acid con-centration.[20] Therefore, the EACN=nmin value tends to befurther from unity by increasing n-Octadecanoic acid con-centration in oil phase and the stable value of IFT increasescontinuously.

Figure 8 shows the dynamic IFTs as a functionof n-Octadecanol concentration for 366=494 againstn-Nonane model oils. Figure 9 shows the stable values ofdynamic IFTs as a function of n-Octadecanol concen-tration. Similarly behaviors to results in Figure 6 and 7

FIG. 6. Effect of n-Octadecanoic acid concentration on dynamic IFT

between 366 (A)=494 (B) and n-Nonane model oils.

FIG. 7. Effect of n-Octadecanoic acid concentration on the stable

value of dynamic IFT between 366=494 and n-Nonane model oils.

FIG. 5. Effect of carboxylic alcohol chain length on the stable value of

dynamic IFT of n-Nonane model oils (alcohol concentration 10mmol=L).

DYNAMIC IFT OF ALKYLBENZENE SULFONATES 627

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can be observed, which means that the mechanisms respon-sible for affecting IFT for n-Octadecanol oil andn-Octadecanoic acid oil are the same.

We have proposed a possible mechanism to explain thisphenomenon as below.[20–22] The reduction of IFT dependsdirectly on molecules of solvent at the interface replaced bymolecules of surfactant. The higher the interfacial surfac-tant concentration is, the lower the IFT. At the same time,the interfacial surfactant concentration is controlled bythe desorption barrier of the surfactant. When the surfac-tant has a tendency to partition into the oil phase, it has asmall desorption barrier from interface to oil phase and alarge desorption barrier from interface to aqueous phase.On the contrary, when the surfactant has a tendency to par-tition into the aqueous phase, it has a small desorption bar-rier from interface to aqueous phase and a large desorptionbarrier from interface to oil phase. Both of the two types ofsurfactants mentioned above have low interfacial concen-trations and comparatively high IFTs. Only when desorp-tion barriers from interface to oil phase and aqueous areboth large does the IFT become low. Moreover, the desorp-tion barrier from interface to oil phase increases withincreasing alkane model oil chain length; consequently,the IFT will decrease for an oil-soluble surfactant andincrease for a water-soluble one. When desorption barriersfrom interface to oil phase and aqueous are equal, the IFTminimum occurs. The synergism and antagonism mech-anism between surfactants and surface-active organic com-ponents in oil phase can be conclude that the factors alterthe desorption barrier of surfactants.

When the surfactant can partition into both aqueousand oil phase, the interfacial surfactant concentration ishigh and the average area of surfactant molecules is closeto its limiting area. Thus, the adsorption of surface-activecomponent molecules onto interface is not independent.It is reasonable that interfacial concentration ofsurface-active component is much lower than that of sur-factant due to its low surface activity. The organic addi-tives can influence the polarity of the oil phase and leadto the change of the partitioning ability from the interfaceto the oil phase for surfactant molecules. For a low IFTsystem, the addition of organic additives changes IFTmainly by altering the EACN=nmin values. When theEACN=nmin value is unity, the surfactant molecules havealmost the same trend to partition into aqueous phase oroil phase, and the interfacial surfactant concentrationreaches maximum. Antagonism has been observed whenthe EACN=nmin value deviates from unity by addingorganic additives. This is due to the stronger trend of par-titioning into aqueous or oil phase for surfactant mole-cules, which results in the decreasing of interfacialsurfactant concentration and the increasing of IFT. Syn-ergism has been observed when the EACN=nmin value isclose to unity for the same reason.

FIG. 8. Effect of n-Octadecanol concentration on dynamic IFT

between 366 (A)=494 (B) and n-Nonane model oils.

FIG. 9. Effect of n-Octadecanol concentration on the stable value of

dynamic IFT between 366=494 and n-Nonane model oils.

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3.1.3. Effect of Alkane Carbon Number on IFTs ofN-Octadecanol Model Oil

In order to prove above mechanism, we have measuredIFTs of 366 and 494 against 10mmol=L n-Octadecanolmodel oil systems, as seen in Figure 10 and 11, respectively.We can see clearly from these Figures that the IFT mini-mum appears with increasing of alkyl carbon number ofmodel oils. For 366 and 494, the alkyl carbon numbers cor-responding to minima shift from 9–12 and 9–11, respect-ively. In the case of model oil above this breaking point,the addition of 10mmol=L n-Octadecanol will reduceIFT, and the opposite behavior will be observed below thispoint. The experimental results in Figures 10 and 11 can beexplained well by our provided mechanism above.

3.2. Effect of Crude Oil Fractions on IFT ofAlkylbenzene Sulfonate Solutions

Figure 12 shows the effect of different oil fraction onstable values of dynamic IFTs for model oils against 0.1mass% 366 and 1.0 mass% NaCl. We can see from theresults that acidic fractions have the remarkable effect ofincreasing the stable value of IFT than the other oil frac-tions at the same concentration. The effects of acidic frac-tions separated from four types of crude oils have the sametendency, and that the stable values are basically increasingto the 1 mN.m�1 order of magnitude.

In addition, we can also find that the resins in jet fuel,similar to the acidic components, cause the stable valuesincreasing by reason of surface-active acidic fractionsmostly distributing in resins; the extent of increasing stablevalues by asphaltenes referring to jet fuel was smaller thanacidic components and resins; the effect of de-acidic oil onthe stable values was the weakest as a whole. Furthermore,the stable values of crude oils were higher than that of jetfuel and de-acidic oil. In other words, under our experi-

mental conditions, crude oil fractions had the effect ofincreasing the stable values with the order of acidic compo-nents> resins> asphaltenes and crude oil> de-acidicoil> jet fuel. So from the results we can conclude thatacidic components in crude oil play an important role ininfluencing IFT.

Figure 13 shows the effect of different oil fraction onstable values of dynamic IFTs for model oils against 0.1mass% 494 and 1.0 mass% NaCl. We can also see fromthe results that the acidic fractions are the most dominatingfactor affecting IFT. And the resins can increase the stablevalues as well as the acidic fractions. Besides, the asphal-tenes have less ability of increasing the stable values thanacidic fractions and resins and the de-acidic oil was theweakest. The stable values of crude oils are obviouslyhigher than that of jet oil and de-acidic oil. In general,

FIG. 10. Effect of alkane carbon number on the stable value of

dynamic IFT for 366 solutions and n-alkane model oils containing

10mmol=L n-Octadecanol.

FIG. 11. Effect of alkane carbon number on the stable value of

dynamic IFT for 494 solutions and n-alkane model oils containing

10mmol=L n-Octadecanol.

FIG. 12. Effect of different crude oil fractions on stable value of IFT

for 366 solution.

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the tendency of different crude oil fractions influencingthe stable values of IFTs of 494 is similar to that of 366.Therefore, the discussions for 366 above can also beapplied to 494.

However, property of different kinds of crude oils isdissimilar, and the amount of acidic components is alsodistinct in different crude oil. Although not all carboxylicacids present in crude oil have been fully identified, it isknown that their number of carbon atoms falls in the rangefrom C8 to C26 and their molecular weights are in therange of 200 to 700.[28–30] Therefore, the effect of 5% massacidic fractions on stable values was approximately equalto 70–100mmol=L n-Octadecanoic acid. We can see fromFigure 7 that 70–100mmol=L n-Octadecanoic acid addi-tives can increase the stable values approximately to 1mN.m�1. So our experimental results for model oils canuseful for understanding the IFT character of crude oil.

4. CONCLUSIONS

On the basis of the work above, the following conclu-sions can be obtained:

1. The addition of carboxylic acids and alcohols candecrease the EACN values, which can quantitativelyrepresent the influence of oil phase on the partition ofsurfactant in the oil phase. The trend of decreasingEACN values becomes stronger with increasing car-boxylic acid (alcohol) chain length and acid (alcohol)concentration.

2. The interfacial activity of surfactants and the ability ofadsorption onto the interface control the dynamic IFTs.Organic additives in oil phase have the ability toincrease IFT, and the stable values increase with theincreasing chain length and concentration under ourexperimental conditions due to deviation of EACN=nmin values from unity.

3. Crude oil fractions, such as acidic fractions, resins andasphaltenes, have the same trend on IFT as organicadditives. Acidic fractions are the dominant factorsaffecting the stable values of model oils. In the case ofsurfactant solution and model oil systems withEACN=nmin value of unity, the increasing order ofIFT are acidic components> resins> asphaltenes andcrude oil> de-acidic oil> jet fuel.

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(1978) J. Am. Oil Chemists Soc., 55: 513–520.[20] Zhang, L., Luo, L., Zhao, S., and Yu, J.Y. (2002) J. Colloid

Interface Sci., 249: 187–193.[21] Zhang, L., Luo, L., Zhao, S., and Yu, J.Y. (2002) J. Colloid

Interface Sci., 251: 166–171.

FIG. 13. Effect of different crude oil fractions on stable value of IFT

for 494 solution.

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[25] Zhao, R.H., Zhang, L., Zhang, L., Zhao, S., Yu, J.Y. (2010)Energy Fuel, 24: 5048–5052.

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[27] Cash, L., Cayias, J.L., Fournier, G., Macallister, D., Schares,T., Schechter, R.S., and Wade, W.H. (1977) J. ColloidInterface Sci., 59: 39–44.

[28] Scheele, G.F. and Meister, B.J. (1968) AIChE J., 14: 9–15.[29] Seifert,W.K.andTeeter,R.M. (1970)Anal.Chem., 42: 180–189.[30] Zhang, L., Luo, L., Zhao, S., Xu, Z.C., An, J.Y., and Yu,

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