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Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

j ourna l h om epa ge: www.elsev ier .com/ locate /co lsur fa

dsorption of aliphatic ionic liquids at low waxy crude oil–waternterfaces and the effect of brine

ivabalan Sakthivela, Sugirtha Velusamya, Ramesh L. Gardasb, Jitendra S. Sangwaia,∗

Gas Hydrate and Flow Assurance Laboratory, Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras,hennai 600 036, IndiaIonic Liquid Laboratory, Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India

i g h l i g h t s

Aliphatic ionic liquids reduce theinterfacial tension of crude oil–watersystem.Ionic liquid + NaCl for crude oil–watersystem shows synergism in surfacephenomena.Ionic liquid with [HSO4]− or [H2PO4]−

anion shows better efficiency.Greater the chain length of ionic liq-uid, higher is the tendency to lowersurface/interfacial tension.Applicable for the recovery of resid-ual oil from matured reservoirs.

g r a p h i c a l a b s t r a c t

Proposed mechanism showing the effect of aliphatic ILs in decreasing the IFT of crude oil–water system.

r t i c l e i n f o

rticle history:eceived 8 October 2014eceived in revised form3 November 2014ccepted 6 December 2014vailable online 15 December 2014

eywords:lectrical conductivitynterfacial tensiononic liquidow waxy crude oilaClurface tension

a b s t r a c t

The need for development of better surface active agents for upstream oil and gas industry whichcan survive harsh condition of salinity are in great demand, particularly for the applications related toimproved/enhanced oil recovery, flow assurance and oil and gas production operations keeping in mindthe environmental constraints. The technical difficulties which need to be considered are those involvingthe surface forces such as surface tension (SFT) and interfacial tension (IFT) acting between the formationwater and the low waxy crude oil. In this study, we have employed the use of eight aliphatic ionic liquids(ILs), based on di- and tri-alkyl ammonium as cations and with various anions such as [CH3COO]−, [BF4]−,[H2PO4]− and [HSO4]− for the investigation of the surface phenomenon of crude oil–water system. Thesynergistic effect of NaCl along with the ILs is investigated in detail. It is observed that there is a signifi-cant reduction in the surface tension of water and the interfacial tension of crude oil–water system in thepresence of salt, particularly at higher concentration of NaCl (200,000 ppm). Effect of temperature, time,alkyl chain length of the cationic part of the ILs, nature of anions of ILs and the concentration of ILs is also

discussed. The trend in the electrical conductivity of aqueous IL solutions with various concentrationsat three different temperatures 298.15–318.15 K is also presented along with critical aggregation con-centration. The study on the effect of ILs on the SFT/IFT of water and low waxy crude oil–water systemreveal that the ILs are successful in minimizing the effect of the surface forces in the presence of salt andthereby, could pave the way for efficient enhanced oil recovery operations.

© 2014 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +91 44 2257 4825; fax: +91 44 2257 4802.E-mail address: jitendrasangwai@iitm.ac.in (J.S. Sangwai).

ttp://dx.doi.org/10.1016/j.colsurfa.2014.12.010927-7757/© 2014 Elsevier B.V. All rights reserved.

: Phys

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S. Sakthivel et al. / Colloids and Surfaces A

. Introduction

Interfacial science has a dynamic importance in the field ofetroleum engineering, especially, when it comes to the recov-ry of unconventional crude oil and the residual oil trapped ineservoirs. Recovery of residual oil trapped inside the reservoir andnhanced oil recovery are mostly influenced by key factors namely,apillary forces, viscous forces, contact angle, wettability, surfaceension (SFT) and interfacial tension (IFT). Reduction in the capillaryorces and interfacial forces helps in smoothening the mobilizationf residual oil followed by its recovery. Mobilization facilitation ofesidual oil requires a reduction in the capillary forces within theil reservoir which is to be achieved by lowering the IFT duringooding processes [1]. Furthermore, the high salinity conditions ofhe reservoir make it difficult to reduce the interfacial tension inhe typical way. Classical methods such as, polymer flooding andurfactant flooding lose their efficiency at high salinity and highemperature conditions.

Throughout our biosphere, trapped oil in depleted reservoirsount for around two-thirds of the total crude oil reserves,emaining unrecovered in the reservoirs, even after primary andecondary recovery stages [2]. Several studies were carried out inhe past, so as to improvise oil recovery techniques and, thereby,nhance oil production [3,4]. Oil and gas industries, throughout thelobe are in the quest of acquiring well developed, efficient andconomically friendly techniques for forcing out the oil which israpped in the oil reservoirs [5–8]. It is at this stage, researchersried to develop new strategies for employing ionic liquids formproved enhanced oil recovery (EOR) techniques. Though muchttention on this aspect prevails, at present there is only inadequatenformation on physical properties known for these systems withespect to crude oil for successful engineering applications [9].

Ionic liquids (ILs), widely known as green solvents [10–13],re organic salts possessing very low melting points [14]. ILsossess good catalytic properties, negligible vapour pressure, non-ammability, high chemical and thermal stability, etc. Over the lastecade, ILs have been broadly used for a wide range of applications

n science and engineering, such as organic and inorganic synthe-is, extraction and separation, catalysis, electrochemistry, biomassonversion, etc. [15–18]. The main reason behind their extensivelasses of application is due to their flexible behaviour, which cane tuned systematically by varying the cationic and anionic compo-ents. The ionic environment created by ILs enables various types of

nteractions to be possible. Details regarding the physicochemicalroperties of ILs become mandatory not only for process design butlso for the development of comparisons and estimation of theseroperties [19–21].

Many researchers [22–24] have carried out investigations onhe IFT between two immiscible liquids using various methods,uch as the Wilhelmy plate, Du Noüy ring, drop volume, sessilerop, spinning drop, pendant drop, Neumann triangle, imbed-ed fibre retraction, breaking thread method, etc. The selectionf the method depends on various factors such as, properties ofhe liquid under study, the circumstances in which the exper-ment is carried out and the stability of the liquid. The authorezave et al. [6–8] observed significant reduction in the inter-

acial tension of crude oil/water system using the ionic liquids,uch as 1-dodecyl-3-methylimidazolium chloride ([C2-mim][Cl]),-octyl-3-methylimidazolium chloride ([C8-mim][Cl]), 1-dodecylyridinium chloride ([C12-Py][Cl]) and 1-octyl pyridinium chloride[C8-Py][Cl]). The same ILs were tested at high saline conditionss well, and inferred that, these ionic liquids efficiently helped in

ecreasing the interfacial tension. Ultimately, it was found that the

FT measurements had a correlation with salinity and temperature.-Dodecyl-3-methylimidazolium chloride ([C2-mim][Cl]) was pro-

ected as a better agent in lowering the IFT and the critical micelle

icochem. Eng. Aspects 468 (2015) 62–75 63

concentration (CMC) between crude oil and brine (NaCl). The CMCwas obtained at a lower concentration of 100 ppm [6].

Wang et al. [25] investigated the IFT of a few [Cn-mim][BF4]/alkane systems and [Cn-mim][PF6]/alkane systems atdifferent temperature range (288.15–328.15 K) at atmosphericpressure. It was detected that on increasing the temperature of thesystem, IFT decreased linearly. Similar results were observed whenincreasing the alkyl chain length of head group of ionic liquids.The same trend was observed for [Cn-mim][PF6]/alkane system.The information on the IFT of a few [Cn-mim][BF4]/alkane systemsand [Cn-mim][PF6]/alkane systems were measured by Wang andhis co-workers. The interfacial tensions of the four imidazoliumbased ILs ([Cn-mim][BF4]; where n = 3–6) with hexane, heptane,and cyclohexane were taken at different experimental tempera-tures in the range of 288.15–328.15 K at atmospheric pressure. Itwas identified that, with increase in temperature, the interfacialtension decreased. The interfacial tension values were also foundto decrease with increase in alkyl chain length for all the ILs/alkanesystem. A linear decline of IFT with rise in temperature was alsoobserved. Similar results were observed for the interfacial tensionsof four other imidazolium based ILs ([Cn-mim][PF6]; where n = 5–8)with hexane and heptane [25,26].

Gardas et al. [27] measured the interfacial tension of water andseveral n-alkane system (n-hexane/n-octane/n-decane) in the pres-ence of various imidazolium ionic liquids in the temperature rangeof 293–323 K with an interval of 10 K at atmospheric pressure. Theyfound that the ionic liquid containing longer chain length on theheader part showed a better performance on the reduction of inter-facial tension of the system. On the other hand, they also observed adecreasing trend on the IFT on increasing the temperature. In addi-tion, they also noted that the IFT of any ILs/n-alkane system wasmuch lower than the individual IFT of ILs in air, and IFT of n-alkanein air.

The classical surfactants were not found to be effective ascompared to ILs in reducing the IFT at high temperature condi-tions. Ye et al. [14] performed the IFT analysis of crude oil/watersystem by using gemini surfactants, such as butyl-�, �-bis(tetradecyldimethylammonium bromide) and the butyl-�, �-bis(hexadecyldimethylammonium bromide). In this case, the authornoticed that an increase in the temperature of the system in thepresence of surfactants did not improve the reduction in IFT asexpected. However, for the study of the effect of temperature, com-bination of surfactant system was not found to exhibit significanteffect on the IFT due to synergism [14]. It is, thus, inferred that theILs are capable to decrease the IFT considerably, that is otherwise,quite impossible for the conventional surfactant especially at highsalinity and high temperature conditions. It was also found that theeffect of ionic liquid on IFT reduction was encouraging and in addi-tion, higher concentration of salt did not affect the stability of theIL solution [6–8].

Effectual separation of any liquid from a mixture of immiscibleliquids primarily involves the phenomenon of SFT and IFT beingalleged as the dependent factors. The interfacial tension betweenbitumen/silica systems was found to be decreased in the additionof ionic liquids, facilitating the easier recovery of bitumen [28]. Inthe case of bitumen and silica, the adhesion force is decreased in thepresence ionic liquids than in the normal aqueous solution. Simi-larly, the contact angle of bitumen/water droplet was about ∼90◦,but in the case of bitumen/ionic liquids system it decreased to anangle of ∼73◦ [29]. These results illustrate that the separation iseasier by using aqueous solution of ionic liquids rather than onlywith the water.

Vast unmapped space in the field of crude oil recovery demandsfor crafting and employing suitable ionic liquids. Even thoughnumerous investigations [6–8,10,11,30] have been performed inthis respect, it is found that the majority of the conventional

6 : Phys

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4 S. Sakthivel et al. / Colloids and Surfaces A

urfactants are liable to lose their power over time at reservoirondition [7]. Consequently, the pursuit for more advanced andfficient alternatives for critical reservoir conditions of high salin-ty and high temperature is quite high. Ionic liquids are, therefore,onsidered as green solvents, which are environmental friendly andre much reliable, as compared to surfactants.

The main focus of this work is to employ task-specific ILs toecrease the IFT of crude oil–water system and in the extreme salin-

ty conditions similar to the formation brine. For this purpose, theilhelmy plate method is used to study the surface tension and

nterfacial tension on the aqueous solutions of eight aliphatic ioniciquids and low waxy crude oil system, both with and without brineNaCl) effect as a function of temperature and concentration of ILs.he experimental measurements of the temperature (288.15 > TK) < 318.15) dependence on the surface tension of water and inter-acial tension of low waxy crude oil–water system using severalmmonium ILs are presented here. This work specially aims inevealing the synergistic effect of ILs and NaCl on the reductionf SFT and IFT. Additionally, the effect of cation and anion of theonic liquids and its alkyl chain length on the cationic head of theLs on SFT and IFT is examined. Electrical conductivity is one of theasic property of any pure substance, and the determination of thelectrical conductivity is necessary for several engineering applica-ions and hence, the property of electrical conductivity of aqueousolutions of ILs has also been reported. Critical aggregation concen-ration (cac) values are important in all of the petroleum industry,pecifically for enhanced oil recovery. Critical aggregation concen-ration is the concentration above which micelles or aggregates areormed in the solution and which in turn helps in enhancing oilecovery by reducing IFT. During EOR process, to inhibit the adsorp-ion of ionic liquids onto the reservoir rock surfaces, the excessoncentration of ILs (above cac) solutions is recommended for theetter recovery of crude oil. In this study, the SFT, IFT and electricalonductivity experiments have been used to compare the criticalggregation concentration (cac).

. Experimental

.1. Materials

.1.1. Low-waxy crude oil (LWC)The present investigation uses a low-waxy crude oil (referred as

LWC” henceforth unless specified) provided by Oil India LimitedOIL), Assam, India. The viscosity of the crude oil and its SARA prop-rties are summarized in Table 1.

.1.2. Ionic liquidsIn the present investigation, eight ammonium based aliphatic

Ls, namely, diethylammonium phosphate [Et2NH2]+[H2PO4]−,iethylammonium sulfate [Et2NH2]+[HSO4]−, triethylammoniumcetate [Et3NH]+[CH3COO]−, triethylammonium tetrafluoroborate

Et3NH]+[BF4]−, triethylammonium phosphate [Et3NH]+[H2PO4]−,riethylammonium sulfate [Et3NH]+[HSO4]−, tripropylammo-ium sulfate [Pr3NH]+[HSO4]− and tributylammonium sulfateBu3NH]+[HSO4]− (see Table 2) were synthesized and purified

able 1ARA analysis, API gravity and viscosity report of crude oil.

No. of trials Composition

S (%) Ar (%) Re (%) As (%)

1 20.2 76.2 2.8 0.6

2 20.4 76.3 3.1 0.3

3 20.6 76.0 3.1 0.3

Average 20.4 76.2 3.0 0.4

: Saturates; Ar: Aromatics; Re: Resins; As: Asphaltenes.

icochem. Eng. Aspects 468 (2015) 62–75

according to the previous available literature [31]. All the eightionic liquids were synthesized by neutralization of equimolarbase with the corresponding acid. In a 500 mL round-bottomedflask one equivalent of base was taken and placed in an oil-bathat 60 ◦C, fitted with a reflux condenser. To the above base, thecorresponding acid solution (one equivalent) was slowly addeddrop-wise with vigorous stirring for about 1 h. Then the reactionmixture was stirred for 2 h at 70◦ C to ensure completion ofthe reaction. Water was removed from the reaction mixture byheating the residue at 80◦ C in high vacuum till the weight of theresidue remained constant. These ILs have been confirmed using1H nuclear magnetic resonance (1H NMR) spectroscopy. Prior touse, all the ILs were dried under vacuum at 353 K for about 48 h toremove the moisture. Table S1 provides the detailed list of startingmaterials used for the synthesis of ionic liquids, their CAS number,source and purity.

2.2. Methods and procedure

1H-NMR 500 MHz spectrometer (Brukar Avance, Switzerland)was used for recording 1H-NMR spectra of all the synthesized ionicliquids by using either deuterated DMSO or CDCl3 as solvents.

Water content of all the ionic liquids was determined with the helpof Karl Fischer Titrator (Analab Micro Aqua Cal100, India) and byusing the Karl Fischer reagent obtained from Merck. Before begin-ning the analysis, the instrument was calibrated using deionisedMillipore quality water. The instrument used can detect the watercontent through conductometric titration with dual platinum elec-trode, in the range 10 ppm to 100%. The ILs were observed topossess water content less than 1000 ppm. The aqueous solu-tions of the various concentration of the ILs namely, 50, 100, 250,500, 750, 1000–10,000 ppm of the solutions of the eight ammo-nium based ionic liquids were prepared. All the above solutionswere freshly made for each and every measurements such as,for the determination of surface tension, interfacial tension, salteffect and electrical conductivity. In the same way, different con-centrations of aqueous solutions of NaCl were made. RADWAGelectronic balance (RADWAG Wagi Elektroniczne, Poland; repeat-ability ± 0.1 mg; readability 0.1 mg) was used for all the weighingmeasurements throughout this experimental investigation.

Dynamic contact angle tensiometer (Dataphysics DCAT 11EC,Germany) was engaged to perform the surface tension and interfa-cial tension measurements. The dynamic contact angle tensiometerwas controlled by the SCAT software and the measurements weredone with the use of Wilhelmy platinum–iridium plate, type PT-11, of thickness 0.2 mm and area 3.98 mm2 with an accuracy ±1.5%at 298.15 ± 0.1 K as the probe provided by the manufacturer. Thegeneral formulae for the calculation of surface tension/interfacialtension (�) is given below,

� = F(1)

l · Cos�

where, l is the wetted perimeter (2w + 2d; w is the plate’s widthand d is the plate’s thickness) of the Wilhelmy plate and � is thecontact angle between the liquid phase and the plate. The contact

Density At 25 ◦C API gravity Viscosity at 25 ◦C

0.9000 25.71 22.70.9001 25.57 22.90.9000 25.72 22.70.9001 25.57 22.8

S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75 65

Table 2List of synthesized ionic liquids.

Cation Anion Name Abbreviation

NHH

H2PO4 Diethylammonium phosphate [Et2NH2]+[H2PO4]−

HSO4 Diethylammonium sulfate [Et2NH2]+[HSO4]−

N

H[CH3COO]

-Triethylammonium acetate [Et3NH]+[CH3COO]−

BF 4 Triethylammonium tetrafluoroborate [Et3NH]+[BF4] −

H2PO4 Triethylammonium phosphate [Et3NH]+[H2PO4]−

HSO 4 Triethylammonium sulfate [Et3NH]+[HSO4]−

N

H

HSO4 Tripropylammonium sulfate [Pr3NH]+[HSO4]−

N

H

HSO4 Tributylammonium sulfate [Bu3NH]+HSO4]−

aatpmd(

tttpppnEmatss2lcbt

wr

ngle is seldom measured in practice and instead literature valuesre used, or complete wetting (� = 0◦) is assumed. Fig. 1(a) depictshe wetting of the Wilhelmy plate at the interface of liquid and gashase. In this method, the necessary force to break the liquid film iseasured. Each measurement was performed so as to obtain 15–30

ata points and until the standard deviation reached a value of 0.03average last ‘n’ measurements; here n = 50).

The temperature was maintained using the temperature con-roller ESCY IC201 (ESCY Enterprises, India) possessing an in-builtemperature sensor with an accuracy of ±0.1 K and connected tohe tensiometer. The measurements were carried out in the tem-erature range of (288.15–318.15 K) with 10 K interval and at aressure of 0.1 MPa. Before beginning series of measurements, therobe (Wilhelmy plate) was cleaned thoroughly using concentrateditric acid, followed by distilled water wash and flame drying.ach experimental value was confirmed by triplicate measure-ent of the same at the same experimental conditions and with

n uncertainty of ≤0.03 mN/m. Before beginning the experiments,he performance of the instruments was verified by measuring theurface tension of double distilled water. The measured surface ten-ion of the double distilled water was found to be 71.96 mN/m at98.15 K, which is very much consistent with the reported standard

iterature [32] value 71.99 mN/m. Similarly, the measured interfa-ial tension of the immiscible water–hexane system was found toe 50.35 mN/m at 298.15 K, which is also in good agreement with

he literature [33] value of 50.38 mN/m.

The mixture of aqueous solutions of IL and low waxy crude oilas stirred vigorously using a magnetic stirrer for about 12 h at the

equired experimental temperature. This mixture was then allowed

to stand for few hours to separate the mixture into two clear layers.Required amount of oil phase solution and aqueous phase solutionwas separated and used for the measurement of both SFT and IFT.Fig. 1(b) gives the pictorial representation of the overall experi-mental set-up of tensiometer coupled with temperature controlleralong with computer.

3. Results and discussion

In the present investigation, eight aliphatic based ammo-nium ILs namely, [Et2NH2]+[H2PO4]−, [Et2NH2]+[HSO4]−,[Et3NH]+[CH3COO]−, [Et3NH]+[BF4]−, [Et3NH]+[H2PO4]−,[Et3NH]+[HSO4]−, [Pr3NH]+[HSO4]− and [Bu3NH]+[HSO4]− arestudied for their effect on the SFT of water and for the influenceon the IFT of low waxy crude oil–water system. Effect of brine andconcentration effect of different ILs as a function of temperatureare also discussed in detail in the following sections.

3.1. Effect of ILs on SFT of distilled water

Surface tension is one of the basic data for surface chem-istry, with the value depending on temperature and the nature ofcation and anion for the systems containing ILs. Surface tensionis too important for the field of enhanced oil recovery tech-

nique. Addressing the SFT of water in the presence of ionic liquidsalong with brine really helps to understand the reduction of sur-face phenomena of crude oil–water system. ILs reduces the SFTof water which helps the latter in contacting more of the pore

66 S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75

Ffo

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71.9

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558

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52.7

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ig. 1. (a) Schematic diagram of the Wilhelmy plate at the liquid–vapour (gas) inter-ace. (b) Schematic representation of the experimental set-up for the determinationf crude oil–water interfacial tension.

paces in the reservoir, thereby, reducing the inaccessible pore vol-me. This could help in the recovery of trapped oil from the porepaces achieved through IL supplemented water flooding. More-ver, water helps in the capillary action when its surface tension iseduced. Reduction in SFT causes a reduction in the surface areaontacting and, thus, favours faster contact of water with porepaces. In general, water can be drawn very effectively into narrowpenings in porous materials, such as, soils.

The procedure followed for the SFT experiments is as follows.fter accomplishing the standard calibration procedure of theynamic contact angle tensiometer as per Section 2.2, the surfaceension measurements of the water + ionic liquid system are mea-ured using the tensiometer for the eight aliphatic ionic liquids atifferent temperatures in the range of 288.15–318.15 K with an

nterval of 10 K and at 0.1 MPa with various concentrations. Eachxperimental measurement is repeated minimum three time toeduce the uncertainty to a value ≤0.03 mN/m of the measurement.he data for SFT of ILs + water system is reported in Table S2–S9 inupplementary material. It is observed that the surface tension ofhe aqueous solutions of ionic liquids keeps on diminishing withncrease in the concentration of ionic liquids at the same temper-ture and decreasing linearly on increasing the temperature from88.15 K to 318.15 K. The effect of chain length of the ionic liquidsn the property of surface tension is also investigated. It is found

hat the ionic liquid possessing longer chain length shows greaterffect on the reduction of surface tension at a particular experi-ental condition. This can be evidently seen from Table 3 that, the

urface tension of two carbon system ionic liquid, [Et3NH]+[HSO4]− Tab

le

3Su

rfac

e

ten

ILs

con

c.

0 50

100

250

500

750

1000

2000

3000

4000

5000

6000

7000

8000

9000

10,0

00

The

stan

da

S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75 67

(a) (b)

(c) (d)

100007500500025000

50

55

60

65

70

75

Su

rfac

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(m

N/m

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IL Concentration (ppm)

[Et2NH

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+[H

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4]

-

10000750050002500045

50

55

60

65

70

75

Su

rfac

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(m

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IL Concentration (ppm)

[Et2NH

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-

10000750050002500045

50

55

60

65

70

75

Surf

ace

tensi

on (

mN

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IL Concentration (ppm)

[Et3NH]

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-

10000750050002500045

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55

60

65

70

75

Su

rfac

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(m

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[Et3NH]

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10000750050002500045

50

55

60

65

70

75

Su

rfac

e te

nsi

on

(m

N/m

)

IL Concentration (ppm)

[Et3NH]

+[H

2PO 4

]-

10000750050002500045

50

55

60

65

70

75

Su

rfac

e te

nsi

on

(m

N/m

)

IL Concentration (ppm)

[Et3NH]

+[HSO

4]

-

(f)(e)

(g) (h)

100007500500025000

45

50

55

60

65

70

75

Su

rfac

e te

nsi

on

(m

N/m

)

IL Concentration (ppm)

[Pr3NH]

+[HSO

4]

-

100007500500025000

45

50

55

60

65

70

75

Su

rfac

e te

nsi

on

(m

N/m

)

IL Concentration (ppm)

[Bu3NH]

+[HSO

4]

-

Fig. 2. Effect of ILs at varying concentration on the surface tension of aqueous solution at four different temperatures of �288.15, 298.15, 308.15 and 318.15 K.( H]+[CH[

dctoOu5

a) Effect of [Et2NH2]+[H2PO4]−; (b) effect of [Et2NH2]+[HSO4]−; (c) effect of [Et3NEt3NH]+[HSO4]−; (g) effect of [Pr3NH]+[HSO4]−; (h) effect of [Bu3NH]+[HSO4]− .

ecreases from 71.95 mN/m to 53.73 mN/m on increasing the con-entration from 0 ppm to 50 ppm at 298.15 K. Likewise in the case ofhree carbon system ionic liquid, [Pr3NH]+[HSO4]−, the reduction

f surface tension is about 52.99 mN/m at 50 ppm concentration.n the other hand, for the case of four carbon system ionic liq-ids, [Bu3NH]+[HSO4]−, the surface tension is reduced to a value of2.33 mN/m. It is, therefore, clear that the ionic liquids with longer

3COO]−; (d) effect of [Et3NH]+[BF4]−; (e) effect of [Et3NH]+[H2PO4]−; (f) effect of

alkyl chain perform effectively in reducing the SFT of ILs + water sys-tem, whereas, the performance of the ILs with lesser chain lengthis not much significant.

In an identical way, ionic liquids having the same cation([Et3NH]+) and different anion ([H2PO4]−; [HSO4]−; [CH3COO]−;[BF4]−) are also explored in detail for SFT reduction phe-nomenon. For example, at 50 ppm concentration, for the IL

68 S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75

of ILs

[7tftct[tvbevItttm(atsardotttiatwpc

ooc

Fig. 3. Proposed mechanism showing the effect

Et3NH]+[H2PO4]−, the surface tension value diminishes from1.95 mN/m to 56.90 mN/m; for [Et3NH]+[CH3COO]−, the surfaceension value decreases from 71.95 mN/m to 54.86 mN/m; andor [Et3NH]+[BF4]−, the surface tension drops from 71.95 mN/mo 58.64 mN/m. This means that the ILs with the commonation [Et3NH]+, and with different anion shows efficiency inhe reduction of surface tension in the following trend as,HSO4]− > [CH3COO]− > [H2PO4]− > [BF4]− at 298.15 K. However,his trend may have some minor deviations depending upon theariations in the concentration of ILs. This trend would be mainlyecause of the different individual physicochemical properties ofvery anion of ionic liquids with water which further leads to theirarying behaviour upon interaction with water. In this trend, theL containing HSO4

− as anion constantly shows a better efficiencyo reduce the SFT than the other ILs. However, other anions withhe same cation (Et3NH), may show minor deviations in the trendo a slight extent. The lower concentrations (<5000 ppm) show a

inor deviation in the same series, but the higher concentrations6000–10,000 ppm) reveal a consistency in the trend in the orders, [HSO4]− > [CH3COO]− > [H2PO4]− > [BF4]−. This could be due tohe very lower concentration of the IL solution prepared. Fig. 2(a–h)hows the graphical representation of the effect of eight aliphaticmmonium based ionic liquids on the surface tension of theirespective aqueous solutions at various concentration and at fourifferent temperatures 288.15–318.15 K at 10 K interval. The SFT isbserved to decrease with increase in the concentration of ILs andemperature. The reason for the reduction of the surface tension ofhe corresponding water–ionic liquid system could be accredited tohe capability of the ionic liquid to interrupt the hydrogen bondingnteractions at the surface of water or its preferential likeness to getdsorbed at the surface rather than the bulk, thereby, decreasinghe surface tension of the system [34]. In such a case, the surfaceould be occupied by the ionic liquids and may be sustained at thatosition by the polar interactions between the ILs and the low waxyrude oil and this hypothetical mechanism is displayed in Fig. 3.

The percentage reduction of the surface tension of the aque-us solution of the IL, [Bu3NH]+[HSO4]− at various concentrationf ILs at four different temperatures is shown in Fig. S1(a). Effi-iency of the percentage reduction in surface tension of aqueous IL

in decreasing the IFT of crude oil–water system.

[Bu3NH]+[HSO4]− system is in the range of 25–35%, and this trendkeeps on increasing with increase in the concentration of the IL.The corresponding graph indicating the surface tension values atfour different temperatures and with various concentration of theIL is shown in Fig. S1(b). It is evident that, the decrease in the SFTwhich shows the linear decline with increase in temperature foreach concentration of IL solution.

3.2. Effect of ILs on the IFT of low waxy crude oil–water systems

Experiments for the determination of the interfacial tension ofthe low waxy crude oil–aqueous ionic liquid solutions are per-formed at four different experimental temperatures from 288.15 Kto 318.15 K at an interval of 10 K and at the atmospheric pres-sure (0.1 MPa). Analogous to the above Section 3.1, low waxy crudeoil–aqueous ionic liquid solution system shows a linear decrease inthe IFT with an increase in the temperature at the same experimen-tal conditions. The data corresponding to one particular low waxycrude oil–aqueous ionic liquid ([Bu3NH]+[HSO4]−) system is givenin Table 4 and for the rest of the ILs, data are given in supplementarymaterial (Tables S10–S16).

At first, eight different ammonium-based ionic liquids contain-ing three different chain lengths are utilized to study the reductionof interfacial tension measurements of low waxy crude oil–watersystem in the presence of ionic liquids without NaCl salt and thesame set of experiments are carried out in the presence of NaCl salt(with varying concentrations) along with ionic liquid solution. Thereduction in the interfacial tension of low waxy crude oil–aqueousionic liquid solution system primarily depends on the adsorptionability of ionic liquid at the interface between crude oil–water sys-tems. The impact of any change in the concentration of the ILs,temperature of the system and the nature of ionic liquids are alsoexpected to have a great impact on the IFT of crude oil–aqueousionic liquid solution.

Fig. 4(a–h) illustrates the influence of the eight different ammo-

nium based ionic liquids on the interfacial tension of low waxycrude oil–aqueous ionic liquid system at different temperaturesfrom 288.15 K to 318.15 K at an interval of 10 K and at atmosphericpressure. In this observation, the concentration of ILs plays a major

S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75 69

(a) (b)

(c) (d)

100007500500025000

20

25

30

35

40

Inte

rfac

ial

Ten

sio

n (

mN

/m)

IL Concentration (ppm)

[Et2NH]

+[H

2PO 4

]-

1000080006000400020000

20

25

30

35

40

Inte

rfac

ial

Ten

sio

n (

mN

/m)

IL Concentration (ppm)

[Et2NH]

+[HSO

4]

-

1000080006000400020000

20

25

30

35

40

Inte

rfac

ial

Ten

sio

n (

mN

/m)

IL Concentration (mN/m)

[Et3NH]

+[CH

3COO]

-

1000080006000400020000

20

25

30

35

40

Inte

rfac

ial

Ten

sio

n (

mN

/m)

IL Concentration (ppm)

[Et3NH]

+[BF

4]

-

100008000600040002000015

20

25

30

35

40

Inte

rfac

ial

Ten

sio

n (

mN

/m)

IL Concentration (ppm)

[Et3NH]

+[H

2PO 4

]-

100008000600040002000015

20

25

30

35

40

Inte

rfac

ial

Ten

sio

n (

mN

/m)

IL Concentration (ppm)

[Et3NH]

+[HSO

4]

-

(f)(e)

(g) (h)

100008000600040002000015

20

25

30

35

40

Inte

rfac

ial

Ten

sio

n (

mN

/m)

IL Concentration (ppm)

[Pr3NH]

+[HSO

4]

-

1000080006000400020000

15

20

25

30

35

40

Inte

rfac

ial

Ten

sio

n (

mN

/m)

IL Concentration (ppm)

[Bu3NH]

+[HSO

4]

-

Fig. 4. Effect of ILs at varying concentration on the interfacial tension of oil/water system at four different temperatures of � 288.15, 298.15, 308.15 and 18.15 K.( H]+[CH[

r[ttt

a) Effect of [Et2NH2]+[H2PO4]−; (b) effect of [Et2NH2]+[HSO4]−; (c) effect of [Et3NEt3NH]+[HSO4]−; (g) effect of [Pr3NH]+[HSO4]−; (h) effect of [Bu3NH]+[HSO4]− .

ole in the IFT study. Unlike the conventionally used surfactants

14,35], these ILs are observed to work effectively even at highemperature situations. In the present investigation, the interfacialension between the low waxy crude oil–aqueous ionic liquid solu-ions is found to be 36.26 ± 0.01 mN/m at 288.15 K. The results of

3COO]−; (d) effect of [Et3NH]+[BF4]−; (e) Effect of [Et3NH]+[H2PO4]−; (f) Effect of

the witnessed systems in the presence of different di- and tri-alkyl

ionic liquids show a remarkable performance.

As per Table 5, on comparing the interfacial tension of the ionicliquids containing two carbon, e.g., [Et3NH]+[HSO4]−, three carbon,e.g., [Pr3NH]+[HSO4]− and four carbon, e.g., [Bu3NH]+[HSO4]−,

70 S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75

Table 4Interfacial tension (IFT) of low waxy crude oil–water system in the presence of IL,[Bu3NH]+[HSO4]− at various concentrations and at four different temperatures.

Temperature 288.15 K 298.15 K 308.15 K 318.15 K

[Bu3NH]+[HSO4]− conc. (ppm) Interfacial tension (mN/m)

0 36.26 34.26 31.26 28.9750 23.44 22.42 20.48 19.25100 23.07 22.05 20.10 18.77250 22.68 21.85 19.77 18.47500 22.41 21.25 19.16 18.17750 22.05 21.01 18.65 17.651000 21.76 20.52 18.24 17.192000 21.36 19.62 18.07 16.893000 20.86 18.93 17.86 16.464000 20.34 18.34 17.49 16.225000 19.83 17.75 17.14 15.886000 19.37 17.43 16.89 15.447000 18.66 17.14 16.47 15.058000 18.22 16.86 16.14 14.529000 17.54 16.69 15.97 14.20

T

teitsloecvow

budt[t[chthHl5brstc3otlilo1i2

5

ten

sion

(IFT

)

of

low

wax

y

cru

de

oil–

wat

er

syst

em

in

the

pre

sen

ce

of

ILs,

at

vari

ous

con

cen

trat

ion

s

and

at

298.

15

K.

(pp

m)

Ion

ic

liqu

ids

(ILs

)

[Et 2

NH

2]+

[H2PO

4]−

[Et 2

NH

2]+

[HSO

4]−

[Et 3

NH

]+[C

H3C

OO

]−[E

t 3N

H]+

[BF 4

]−

[Et 3

NH

]+[H

2PO

4]−

[Et 3

NH

]+[H

SO4]−

[Pr 3

NH

]+[H

SO4]−

[Bu

3N

H]+

[HSO

4]−

34.2

6

34.2

6

34.2

6

34.2

6

34.2

6

34.2

6

34.2

6

34.2

625

.72

24.5

6

24.5

6

24.3

0

23.8

6

23.5

7

23.2

8

22.4

225

.22

24.0

6

24.0

6

23.7

7 23

.25

23.1

3

22.9

6

22.0

524

.52

23.8

0

23.4

9

23.1

5 22

.88

22.7

9

22.4

6

21.8

524

.02

23.2

7

22.6

0

22.4

9

22.3

2

22.4

7

22.0

5

21.2

523

.41

22.6

1

22.1

5

21.9

0

21.7

6

21.8

3

21.7

5

21.0

123

.10

22.1

3

21.8

7

21.0

7

21.3

5

21.3

5

21.1

7

20.5

222

.55

21.8

2

21.3

1

20.8

8

20.8

6

20.8

2

20.8

2

19.6

221

.75

21.1

9

21.0

1

20.4

5

20.5

2

20.1

4

20.3

3

18.9

321

.15

20.7

6

20.7

3

20.0

4

20.1

3

19.8

7

20.0

4

18.3

420

.57

20.1

5

20.3

6

19.8

8

19.8

6

19.5

7

19.5

6

17.7

520

.05

19.8

7

19.7

0

19.5

3

19.5

2

19.2

4

18.9

5

17.4

319

.63

19.5

3

19.3

3 19

.16

19.2

1

18.9

5

18.3

8

17.1

419

.24

19.2

3

18.9

0 18

.86

18.7

5

18.4

6

17.8

6

16.8

619

.02

18.4

9

18.7

8

18.7

5

18.5

5

18.0

4

17.3

9

16.6

918

.89

18.2

2

18.6

3

18.5

7

18.4

1

17.8

5

17.1

8

16.5

5

rd

un

cert

ain

ties

are

u(T)

=

0.1

K, u

(IFT

) =

0.03

mN

/m, u

(IL

con

c.) =

1

pp

m.

10,000 17.21 16.55 15.84 14.02

he standard uncertainties are u(T) = 0.1 K, u(IFT) = 0.03 mN/m, u(IL conc.) = 1 ppm.

he latter IL possessing the longest alkyl chain shows enhancedfficiency in the reduction of interfacial tension at the same exper-mental condition. Though several reasons could be endorsed forhe above statement based on various strategies, akin to the trendeen in this study, the same was observed in one of the previousiterature by Smit et al. [36] where greater reduction in IFT wasbserved in the presence of longer hydrophobic tail. In addition,ven at lower concentration, the ILs with longer hydrophobic alkylhain exhibit enhanced reduction in IFT. This may be due to thean der Walls force of attraction between the hydrophobic partf ionic liquids (alkyl chain) and the hydrophobic part of the lowaxy crude oil system [27].

Besides the above factor, the anionic species themselves areelieved to be responsible for fine tuning the action of ionic liq-ids on the crude oil–water system. With the intention to gatherata to support this concept, a set of experiments with ILs con-aining the common cation, [Et3NH]+ and anions such as, [HSO4]−,H2PO4]−, [BF4]− and [CH3COO]− are performed. The results revealhat the effect of the various anions on decreasing the IFT is as,HSO4]− > [H2PO4]− > [BF4]− > [CH3COO]− at 298.15 K. The reasonould be attributed mainly to the physicochemical properties of theigher molecular weight anions which steered the better interac-ion with crude oil moiety. In this particular case, the IL containingigher molecular weight is showing better reduction in the IFT.owever, slight changes in the above order may be observed at

ower concentration of the sample solution and about additional–13% reduction in IFT is observed on increasing the temperaturey 10 K. Moreover, for 50 ppm concentration of any IL, significanteduction in IFT is observed. For instance, the IL, [Et3NH]+[HSO4]−

hows the IFT value as 24.78 mN/m. Likewise, for [Pr3NH]+[HSO4]−,he IFT drops from 36.26 mN/m (0 ppm) to 24.26 mN/m. In thease of [Bu3NH]+[HSO4]− the drop in the IFT is 23.44 mN/m from6.26 mN/m at 288.15 K. Table 6 shows the comparison of thebserved IFT results of this present investigation (eight ILs) withhe existing reported [6–8] IFT values of crude oil–aqueous ioniciquid system. The observed IFT of low waxy crude oil–aqueousonic liquid solution is much lesser than the individual SFT ofow waxy crude oil and ionic liquid alone. For example, the IFTf low waxy crude oil with the IL, [Et3NH]+[CH3COO]− is about0.76 mN/m at 298.15 K, whereas the SFT of low waxy crude oil

s about 38.14 mN/m and for the IL, [Et NH]+[CH COO]− is around

3 38.18 mN/m.

Fig. 5 gives the efficiency of the ammonium based ILs at 0,0, 1000 ppm concentration in IFT reduction. Comparing all the Ta

ble

5In

terf

acia

l

ILs

con

c.

0 50

100

250

500

750

1000

2000

3000

4000

5000

6000

7000

8000

9000

10,0

00

The

stan

da

S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75 71

Table 6Comparing the observed IFT results of this present investigation (eight ILs) with the existing reported IFT values of crude oil–aqueous ionic liquid system.

IFT (mN/m) ILs concentration (ppm) IFT reduction (%) Reference

0 100 250 500 750 1000

[C12mim]+[Cl]− 39.98 37.59 – – – 28.44 28.86 [6][C8Py]+[Cl]− 39.98 38.49 35.82 31.68 30.21 29.53 26.14

[7][C12Py]+[Cl]− 39.98 37.12 33.50 8.72 7.42 7.38 81.54[C8mim]+[Cl]− 39.98 38.77 35.77 34.55 29.87 16.77 58.05[C12mim]+[Cl]− 39.98 37.59 34.21 32.36 29.12 28.44 28.86[C12mim]+[Cl]− 47.3 45.2 – – – 43.3 8.46 [8][Et2NH2]+[H2PO4]− 34.26 25.22 24.52 24.02 23.41 23.10 32.57

This work

[Et2NH2]+[HSO4]− 34.26 24.06 23.80 23.27 22.61 22.13 35.41[Et3NH]+[CH3COO]− 34.26 24.06 23.49 22.60 22.15 21.87 36.16[Et3NH]+[BF4] − 34.26 23.77 23.15 22.49 21.90 21.07 38.50[Et3NH]+[H2PO4]− 34.26 23.25 22.88 22.32 21.76 21.35 37.68[Et3NH]+[HSO4]− 34.26 23.13 22.79 22.47 21.83 21.35 37.68[Pr3NH]+[HSO4]− 34.26 22.96 22.46 22.05 21.75 21.17 38.21[Bu3NH]+[HSO4]− 34.26 22.05 21.85 21.25 21.01 20.52 40.11

Fig. 5. Reduction of IFT of low waxy crude oil–water in the presence of various ILsa

Ihtreosacfits

both SFT and IFT with respect to the various alkyl chain lengthsconsidered.

TSH

T

t 0, 50, 1000 ppm concentration and at 298.15 K temperatures.

Ls under this category, it is observed that the ILs having longerydrophobic alkyl chain show greater percentage of IFT reduc-ion. Fig. S2(a) (supplementary material) displays the percentageeduction of interfacial tension upon addition of ionic liquids. Forxample, the IL [Bu3NH]+[HSO4]− shows 34–53% reduction in IFTf crude oil–water at different concentration of ILs. As usual theystem containing higher concentration of IL shows higher percent-ge of IFT reduction than the system with lower concentration. Theorresponding diagram for the interfacial tension values at four dif-erent temperatures and with various concentrations is presentedn Fig. S2(b). It clearly shows the linear decline with increase in

emperature for each concentration of the IL solution–crude oilystem.

able 7urface tension (SFT) and interfacial tension (IFT) for water + ILs and water + ILs + crude oere three ILs are chosen from among the others possessing 2, 3 and 4 carbon containing

NaCl conc. [Et3NH]+[HSO4]− [Pr3

SFT IFT SFT

0 52.25 21.35 52.050 51.24 20.74 50.8100 50.58 19.52 48.9500 50.17 18.29 47.11000 49.26 17.44 45.25000 47.25 16.20 43.810,000 45.21 14.22 42.150,000 40.12 12.56 39.5100,000 38.54 10.23 35.6200,000 34.51 09.36 32.1

he standard uncertainties are u(T) = 0.1 K, u(SFT/IFT) = 0.03 mN/m, u(IL conc./NaCl conc.)

Fig. 6. Comparison of SFT and IFT of water, water–low waxy crude oil systemsrespectively at various concentration of ILs, at 10,000 ppm of NaCl and at the tem-perature of 298.15 K.

3.3. Synergistic effect of IL and salt mixture

Surface tension of normal aqueous ionic liquid solution(water + IL) is compared with the system containing NaCl salt atvarying concentrations and is shown in Table 7. Table 7 also pro-vides the IFT between low waxy crude oil–aqueous ionic liquidsolutions containing 1000 ppm of IL and the effect of various con-centration of NaCl. The ILs are chosen from those with same anionbut with different alkyl chain length, i.e., two, three and four car-bon tails on their cationic ammonium moiety. The data shows thesame trend as discussed in the previous Sections 3.1 and 3.2 for

Consequently, the system comprising IL + NaCl + water with lowwaxy crude oil appears to exhibit synergism. Fig. 6 shows the

il system, respectively, with the effect of salt on ILs (at 1000 ppm) and at 298.15 K. alkyl chain.

NH]+[HSO4]− [Bu3NH]+[HSO4]−

IFT SFT IFT

3 21.17 50.64 20.524 20.49 48.63 19.635 19.52 47.22 18.282 18.23 45.81 17.426 16.75 43.69 16.228 15.12 41.33 14.852 13.79 39.69 13.146 11.26 35.29 11.039 09.58 33.65 09.065 08.86 29.21 08.56

= 1 ppm.

72 S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75

(b)(a)

(c)

200000150000100000500000

10

20

30

40

50

SF

T/I

FT

(m

N/m

)

NaCl Concentration (ppm)

SFT of [Et3NH]

+[HSO

4] at 1000 ppm

-

IFT of [Et3NH]

+[HSO

4] at 1000 ppm

-

200000150000100000500000

10

20

30

40

50

SF

T/I

FT

(m

N/m

)

NaCl Concentration (ppm)

SFT of [Pr3NH]

+[HSO

4] at 1000 ppm

-

IFT of [Pr3NH]

+[HSO

4] at 1000 ppm

-

200000150000100000500000

10

20

30

40

50

SF

T/I

FT

(m

N/m

)

NaCl Concentration (ppm)

SFT of [Bu3NH]

+[HSO

4] at 1000 ppm

-

IFT of [Bu3NH]

+[HSO

4] at 1000 ppm

-

F oil–wa

ctswwoobagtigFsobp

ti1N2ttett2foasbi

c(

ig. 7. Surface tension (SFT) of water and interfacial tension (IFT) of low waxy crudet 298.15 K.

omparison of surface tension of water + IL ([Bu3NH]+[HSO4]−) sys-em with the system water + IL ([Bu3NH]+[HSO4]−) + NaCl. In theame way the comparison of interfacial tension of water + lowaxy crude oil + IL ([Bu3NH]+[HSO4]−) systems with the systemater + low waxy crude oil + IL ([Bu3NH]+[HSO4]−) + NaCl at vari-

us concentration of ILs and at 10,000 ppm of NaCl and 298.15 K. It isbserved that the system containing NaCl along with ILs shows theetter efficiency in the reduction of SFT and IFT for the water + ILsnd water + crude oil + ILs system. This is indeed due to the syner-istic combination of NaCl and ILs to reduce the SFT or IFT. In fact,his promising indication would really help in specifically employ-ng the use of ILs in the presence of salts for the upstream oil andas applications such as enhanced oil recovery, or flow assurance.ig. 7 shows the SFT of water and IFT of low waxy crude oil–waterystem in the presence of 1000 ppm ILs, at various concentrationsf NaCl and at 298.15 K. The study indicate that these ILs can act asetter alternatives to the existing conventional surfactants in theresence of high salinity and high temperature.

Fig. 8(a–c) shows the effect of time on the reduction of surfaceension of various aqueous systems of the IL, [Bu3NH]+[HSO4]−

n three different concentrations namely, 1000 ppm, 5000 ppm,0,000 ppm and in the presence of different concentrations ofaCl (brine salt) namely, 10,000 ppm, 50,000 ppm, 100,000 ppm,00,000 ppm and at 298.15 K. During the above measurements, ashe time increases, there seems to be a minimal reduction in surfaceension. The synergistic effect of NaCl with ionic liquids is appar-nt from the system containing the IL, [Bu3NH]+[HSO4]−, wherehe surface tension of its aqueous solution is reduced from 71.96o 50.64 mN/m at 1000 ppm of the corresponding ionic liquid at98.15 K, whilst for the same system, with NaCl salt (200,000 ppm),urther reduction in the surface tension to about 29.21 mN/m isbserved at the same temperature. This is indeed very remarkables the percentage of surface tension reduction is observed to beignificantly greater in the presence of NaCl. Similar studies haseen extended for ILs [Et3NH]+[HSO4]−, [Pr3NH]+[HSO4]− and this

s given in supplementary material (Figs. S3 and S4).Fig. 9(a–c) shows the effect of time on the IFT on the low waxy

rude oil–aqueous solution system with NaCl and the ionic liquid[Bu3NH]+[HSO4]−) at different concentrations and at 298.15 K. As

ater system in the presence of 1000 ppm ILs, at various concentrations of NaCl and

time increases, only a very small reduction in IFT of the system isobserved. The corresponding effect has been investigated for othertwo ILs [Et3NH]+[HSO4]−, [Pr3NH]+[HSO4]− also and it is providedin the supplementary material (Figs. S5 and S6). As observed inSection 3.2, that in the absence of salt, the interfacial tension ofany aqueous ionic liquid solution–low waxy crude oil system isobserved to minimum at 20.52 mN/m at 298.15 K for 1000 ppm ILs.However, when the same set of experiments is performed in thepresence of NaCl salt, the minimum IFT is observed to be around8.56 mN/m.

As the effect of different alkyl chain lengths is concerned, thesame trend prevails in the study of synergism of salt along withILs as discussed in the earlier section. The presence of NaCl saltalso helps to decrease the critical aggregation concentration (cac)of the various systems containing the different ILs. This is observedduring both SFT and IFT measurements, which could be due tothe synergistic effect of NaCl and IL mixture on SFT/IFT mea-surements [37–39]. Solubility limit of ionic liquid also decreasesupon adding NaCl in the system and, hence, makes the IL lessionized. The ILs get strongly adsorbed at the interface, and fur-ther helps in reducing the SFT/IFT. Hence, the addition of NaClalong with IL certainly helps in minimizing the concentrationof ILs being used to achieve lower SFT/IFT. As per the availableprevious literature [2,40], it was observed that the addition of sur-factant reduced the IFT of the low waxy crude oil–water system byadsorbing the surfactant at the interface between low waxy crudeoil–water. Perhaps, the analogous consequence could be the rea-son for the reduction of SFT/IFT in the systems considered in thispresent study, the only difference being the ILs used instead of thesurfactants.

This can be now claimed that the ILs have the ability to actas better surface active agents experimentally, than the currentlyexploited surfactants, where the latter fail to perform at high tem-perature and salinity conditions. As discussed earlier, it is evidentfrom the Figs. 8(a–c) and 9(a–c), that the overall reduction of SFT/IFT

with respect to time is not much convincing. This occurrence canbe clarified on the basis of surface active agents formed from thelow waxy crude oil and their formation kinetics. Primarily, the for-mation of the surface active agents may be slow, and hence the

S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75 73

(b)(a)

(c)

80706050403020100

28

32

36

40

Su

rfac

e te

nsi

on

(m

N/m

)

Time (min)

1000 ppm of [Bu3NH]

+[HSO

4]

-

80706050403020100

28

32

36

40

Surf

ace

tensi

on (

mN

/m)

Time (min)

5000 ppm of [Bu3NH]

+[HSO

4]

-

80706050403020100

28

32

36

40

Surf

ace

tensi

on (

mN

/m)

Time (min)

10000 ppm of [Bu3NH]

+[HSO

4]

-

Fig. 8. Time-hold study – effect of NaCl on the surface tension of the aqueous solution containing the IL, [Bu3NH]+[HSO4]− at three different concentrations, namely (a) 1000,(b) 5000 and (c) 10,000 ppm and at 298.15 K. � 10,000, 50,000, 100,000; 200,000 ppm of NaCl.

(b)(a)

(c)

80706050403020100

8

10

12

14

Inte

rfac

ial

Ten

sio

n (

mN

/m)

Time (min)

1000 ppm of [Bu3NH]

+[HSO

4]

-

80706050403020100

8

10

12

14

Inte

rfac

ial

Ten

sion (

mN

/m)

Time (min)

5000 ppm of [Bu3NH]

+[HSO

4]

-

80706050403020100

8

10

12

14

Inte

rfac

ial

Ten

sio

n (

mN

/m)

Time (min)

10000 ppm of [Bu3NH]

+[HSO

4]

-

Fig. 9. Time-hold study – effect of NaCl on the interfacial tension of the aqueous solution containing the IL, [Bu3NH]+[HSO4]− at three different concentrations, namely (a)1000, (b) 5000 and (c) 10,000 ppm and at 298.15 K. � 10,000, 50,000; 100,000; 200,000 ppm of NaCl.

74 S. Sakthivel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 468 (2015) 62–75

1000075005000250000

5000

10000

15000

20000

Conduct

ivit

y (

µS

/cm

)

ILs Concentration (ppm)

45003000150000

2000

4000

6000

8000

10000

cac

Fig. 10. Electrical conductivity of aqueous solution containing the vari-o + −

[

lcp(alwmi

3

a5mtsdoeppm

ilulti1tetttatcoc3

Fig. 11. Electrical conductivity of aqueous solution containing the various ILs at

us ILs at varying concentration and at 298.15 K. � [Et2NH2] [H2PO4] ,[Et2NH2]+[HSO4]− , [Et3NH]+[CH3COO]− , [Et3NH]+[CH3COO]− ,

Et3NH]+[BF4]− , [Et3NH]+[H2PO4]− , [Pr3NH]+[HSO4]− , [Bu3NH]+[HSO4]− .

ess accumulation of ILs and surface active agents at the interfacean happen and the broadening of the double layer cannot appro-riately occur. With increase in time, the surface active moleculepolar moiety from low waxy crude oil) gather from the crude oilnd causes the thinning of this double layer at the interface, therebyowers the SFT/IFT (Fig. 2). The source of the polar moiety of low

axy crude oil is obtained from the literature [41]. Once this accu-ulation process stretches towards its end, there is no reduction

n the SFT/IFT observed.

.4. Electrical conductivity

Akin to the SFT experiments, various concentrations of the eightmmonium-based aliphatic ionic liquids solutions in the range of0–10,000 ppm are prepared and their electrical conductivity iseasured at three different temperatures of 298.15–318.15 K. All

he dilute solutions of ILs at different concentration are found tohow a direct proportionality with the observed electrical con-uctivity values at a particular temperature. Fig. 10 illustrates thebserved electrical conductivity values at different concentration ofight ammonium based aliphatic ILs at 298.15 K in the atmosphericressure. The results of the same set of experiments at the tem-eratures of 308.15 K and 318.15 K are given in the supplementaryaterial (Figs. S7 and S8).As shown in Fig. 10, the critical aggregation concentration (cac)

s interpreted from the point of intersection of the two straightines in the graph of concentration of ILs versus conductivity val-es. Though, similar approach can be extended to all other ionic

iquids solution at all temperatures, but not shown in the figure forhe sake of simplicity. With respect to all the ionic liquids under thisnvestigation, it is observed that the cac value is found to be below000 ppm concentration of ILs. This study also voices out the facthat, the cac value of any particular system is comparatively low-red on increasing the temperature of that system. The reason forhis observation could be attributed to the decreasing hydration ofhe hydrophilic moiety at higher temperatures rapidly favouringhe process of aggregation [42]. The ionic liquids with lengthierlkyl chain show greater effect on the reduction of cac value thanhe shorter alkyl chains on the head part of the ionic liquids. Fig. 11

ompares the effect of different ILs on the electrical conductivityf aqueous solution containing the various ILs at 1000 ppm con-entration and at three different temperatures 298.15 K, 308.15 K,18.15 K. The trend observed in the mentioned Fig. 11, is because of

1000 ppm concentration and at three different temperatures 298.15 K, 308.15 K,318.15 K.

the fact that, the ILs possessing the least molecular weight amongstthe ILs studied, shows greatest performance towards the electricalconductivity, since the electrical conductivity is directly propor-tional to the weight of the substance [43].

4. Conclusion

The present study investigated the effect of various ionic liq-uids on the surface tension of water and the interfacial tensionof low waxy crude oil–water systems with and without NaCl.The experiments were carried out in the temperature range of(288.15–318.15 K) with 10 K interval and at a pressure of 0.1 MPa.The results indicated that the combination of ionic liquid and brinecomparatively worked better in effectively reducing the surfacetension/interfacial tension of the various systems considered. TheSFT of aqueous solution containing IL along with NaCl is reducedfrom 38.51 mN/m to 29.21 mN/m while in the absence of NaCl, theSFT drop is from 57.11 mN/m to 50.64 mN/m at 298.15 K. In addi-tion, IL along with NaCl is observed to reduce the IFT of crudeoil–water significantly from 11.24 mN/m to 8.56 mN/m, whereasin the absence of salt, the IL (at 1000 ppm) reduces the IFT of thesame from 23.10 mN/m to 20.52 mN/m at 298.15 K. This reveals thesynergism effect between IL and NaCl in lowering the SFT/IFT of thevarious systems. The use of a bare minimum amount of IL reducedthe interfacial tension between low waxy crude oil–water, whichin-turn may be expected to increase the mobility of the trappedoil from the depleted reservoirs. The effect of increasing the alkylchain length on the ILs, the increase in concentration of ILs and tem-peratures are found to have a positive impact on the reduction ofSFT/IFT. The notion of using longer alkyl chain containing ILs couldalso be extended as a part of additional investigation for enhancedoil recovery applications so as to achieve better IFT reduction. Theelectrical conductivity also showed a linear increase with increas-ing IL concentration. The ILs can provide better alternatives to theconventional surfactant which can survive harsh condition of salin-ity for the applications related to improved/enhanced oil recovery,flow assurance and oil and gas production operations.

Acknowledgements

Financial support from Oil India Limited through grantCONT/HC/C-5/0931/2012 is highly appreciated and acknowledged.We also extend our gratitude to Dr. Srinivasan V. Raju, General Man-

ager (R&D) and Mr. Prashant Dhodapkar, Chief Research Scientist,Research and Development/Oil India Limited for technical supportrelating to the studies of crude oil.

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S. Sakthivel et al. / Colloids and Surfaces A

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/.colsurfa.2014.12.010.

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