Synthesis and characterization of eight hydrophilic imidazolium-based ionic liquids and their application on enhanced oil recovery

Article abstract of DOI:10.1016/j.molliq.2017.10.069

In order to difficulties in producing large amount of crude oil and remaining oil by primary and secondary oil recovery processes, enhanced oil recovery (EOR) techniques have been developed. Water injection is one of the EOR methods which has shown great potential in recent years. The water flooding process is more effective when the injected water is enriched by chemicals which improve the oil recovery by reducing interfacial tension (IFT) and alternating wettability. In this work eight long alkyl chain imidazolium based ionic liquids (ILs) including Octyl, Decyl, Dodecyl and Tetradecyl methylimidazolium and two different anions namely Chlorid and trihydrogen diphosphate (THDP), were synthesized and characterized by ¹HNMR and elemental analysis. As a nobility it should be noted that ILs containing THDP anion have not been synthesized yet and they were used in upstream oil industry for the first time. Furthermore, some physicochemical properties were investigated for studied ILs as a function of temperature. The synthesized ILs were examined as additives in injected water to reduce the IFT in water flooding process. The critical micelle concentration (CMC) point and IFT of enriched sea water by ILs/crude oil, were measured as a function of ILs concentration. The results showed that ILs can be good candidates for EOR technology due to their significant behavior in IFT reduction and their low consumption. The consumed concentrations for ILs were observed at ppm levels, so they are favorable choices when economic concerns are considered. According to obtained results, as the alkyl chain was longer, the CMC point and IFT values were lower. Moreover the investigation of IFT values, revealed that ILs containing THDP anion were more efficient in IFT reduction compared with ILs including chloride anion. [C₁₄mim][Cl] and [C₁₄mim][THDP] were the most effective ILs which 50 and 25 ppm of these ILs, reduced the IFT values to 0.65 ± 0.04 and 0.5 ± 0.02, respectively.

Full text of DOI:10.1016/j.molliq.2017.10.069

Accepted Manuscript
Synthesis and characterization of eight hydrophilic imidazolium-
based ionic liquids and their application on enhanced oil recovery
M. Yousefi, A. Naseri, M. Abdouss, A.A. Miran Beigi
PII:
S0167-7322(17)32678-8
doi:10.1016/j.molliq.2017.10.069
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Reference:
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10 October 2017
14 October 2017
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Synthesis and characterization of eight hydrophilic imidazolium-based ionic liquids and
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ACCEPTED MANUSCRIPT
Synthesis and characterization of eight hydrophilic imidazolium-based ionic
liquids and their application on enhanced oil recovery
a
b
a, 1
b
M. Yousefi , A. Naseri , M. Abdouss * and A. A. Miran Beigi
a: Department of chemistry, Amirkabir University of Technology, Tehran, Iran
b: Research Institute of Petroleum Industry, Tehran, Iran
Abstract
In order to difficulties in producing large amount of crude oil and remaining oil by
primary and secondary oil recovery processes, enhanced oil recovery (EOR) techniques
have been developed. Water injection is one of the EOR methods which has shown great
potential in recent years. The water flooding process is more effective when the injected
water is enriched by chemicals which improves the oil recovery by reducing interfacial
tension (IFT) and alternating wettability. In this work eight long alkyl chain imidazolium
based ionic liquids (ILs) including Octyl, Decyl, Dodecyl and Tetradecyl
methylimidazolium and two different anions namely Chlorid and trihydrogen diphosphate
1
(
THDP), were synthesized and characterized by HNMR and elemental analysis. As a
nobility it should be noted that ILs containing THDP anion have not been synthesized yet
and they were used in upstream oil industry for the first time. Furthermore, some
physicochemical properties were investigated for studied ILs as a function of temperature.
The synthesized ILs were examined as additives in injected water to reduce the IFT in
water flooding process. The critical micelle concentration (CMC) point and IFT of
enriched sea water by ILs/crude oil, were measured as a function of ILs concentration. The
results showed that ILs can be good candidates for EOR technology due to their
significant behavior in IFT reduction and their low consumption. The consumed
concentrations for ILs were observed at ppm levels, so they are favorable choices when
economic concerns are considered. According to obtained results, as the alkyl chain was
longer, the CMC point and IFT values were lower. Moreover the investigation of IFT
values, revealed that ILs containing THDP anion were more efficient in IFT reduction
compared with ILs including chloride anion. [C mim][Cl] and [C mim][THDP] were the
14
14
most effective ILs which 50 and 25 ppm of these ILs, reduced the IFT values to 0.65 ±
.04 and 0.5 ± 0.02, respectively.
0
Keywords: Hydrophilic ILs, Synthesis, IFT, CMC point, EOR.
1
*
Corresponding Author (M. Abdouss), Tel.: +98-021-64545804; Fax: +98-021-66417430. Email:
majidabdouss@yahoo.com

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1
. Introduction
In order to reducing easy to access oil, increasing global oil demand and crude oil high
price, it is important to extract more oil from the existed oil reservoirs by introducing new
technologies [1-3]. Only 20-35% of oil in place can be produced by naturally depletion [4,
5
]. Enhanced oil recovery (EOR) methods have been developed to recover large amount of
crude oil are trapped in pore structure which cannot be extracted by traditional methods
6-8]. The most commonly used secondary EOR method to improve the productivity of
the reservoirs, are composed of two main categories: miscible and immiscible injection [9-
1]. Among these technologies, water injection which is part of immiscible injection
[
1
methods is more effective when one or several chemicals are added to injected water to
improve the sweep efficiency and mobilize trapped oil in the reservoirs [12-15]. Interfacial
tension (IFT) reduction and wettability alteration to modify the capillary number in the oil
reservoirs, are two main mechanism in the enriched water injection by chemicals which
causes increase the oil recovery [16, 17].
Some additives in water injection process such as conventional surfactants are unable to
tolerate the harsh reservoir conditions such as high salinity and high temperature [18].
Furthermore, due to their high cost, high toxicity, high consumption and many other side
effect, ionic liquids (ILs) have used as a type of more cost-effective additives in water
injection [8]. Ionic liquids are molten salts which have attracted much attention as
potential alternative to traditional organic solvents [19]. Their interesting properties
includes high thermal and chemical stability, non-flammability, low vapor pressure, low
melting point and high ionic conductivity [20-23].
The application of ILs in the oil industry is very extensive. ILs have been used in refining
stage of crude oil, deposition of asphaltenes and parafines in crude oil and manufacturing
process of valuable petrochemical products [24]. In terms of molecular structure, ionic
liquids exhibit characteristics of cationic surfactants (long carbon chain substituents
appended to a charged cationic head group) [25]. Over the past few years, many
researchers applied ILs as surfactant in EOR. Benzagouta et al. (2014) [25] introduced a
special type of ionic liquid namely Ammoeng which were used for IFT reduction in
different salinity and temperature. Bin-Dahbagh et al. (2015) [2] investigated several

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ammonium and imidazolium based ILs as surfactant. Zeinolabedini Hezave et al. (2012)
[
16] studied the effect of different families (imidazolium and pyridinium) of ionic liquids-
based surfactants on interfacial tension of water/crude oil system. Lago et al. (2012) [9]
used Phosphonium IL as novel surfactant, an alternative to conventional surfactants.
In this work eight hydrophilic imidazolium based ionic liquids were synthesized and
1
characterized by HNMR and CHNO elemental analysis. The chemical structures of
synthesized are shown in Table 1. ILs containing trihydrogen diphosphate [THDP] anion
were synthesized and used in upstream oil industry for the first time. Since the
[
C mim][Cl], [C mim][Cl], [C mim][THDP] and [C mim][THDP] were liquids, their
8 10 8 10
physicochemical properties such as density, viscosity and refractive index were measured
in temperature range 10 to 90 C and atmospheric pressure. Also the effects of
temperature, alkyl chain, and anion type on physicochemical properties were investigated.
Table 1
A comprehensive literature survey showed that the effects of ILs on IFT of formation
brine/crude were discussed in previous studies, but in this work the IFT between injected
water and crude oil was studied as a known scenario in water injection process.
The synthesized ILs were used to reduce the IFT between oil and injected water. Usually
water injection process, is faced with many problems such as formations damage which is
caused by the incompatibility between injected water and formation water in reservoir. In
general, phosphate containing species such as diphosphoric acid and polyphosphoric acid
have been used as anti-scale. Hence in this research, it has been tried to synthesize the task
specific ILs that are used as water injection additives and anti-scale. Besides the effects of
anion, cation and ILs concentration on IFT reduction were investigated. The CMC point
was found for all ILs as a function of efficiency of surface active ILs.
2
2
1
. Experimental
.1. Materials
-methylimidazole, 1-chlorooctane, 1-chlorodecane, 1-chlorododecane and 1-
chlorotetradecane were pursued from Merck and were distilled before use. All other
starting materials and solvents were used as received without any further treatment. These
materials included pyrophosphoric acid (Aldrich, technical grade), diethyl ether (Merck, ≥

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9
9%), silver nitrate (Merck, ≥ 99%), ethanol (Merck, ≥ 96%), potassium hydroxide
(
Merck, ≥ 99%). High purity of N gas was prepared from Roham Gas Co. (Tehran, Iran).
2
Water used was freshly deionized and distilled before use. Physical properties of Crude oil
which was obtained from one of the Iranian oilfield are shown in Table 2. The chemical
analysis of sea water is given in Table 3.
Table 2 and 3
2
.2. Apparatus and Procedure
1
The purity of synthesized ILs was proven by HNMR (Bruker Avance 500 MHz
spectrometer) and elemental analysis CHNO (vario max, elementar, Germany). The
interfacial tension between aqueous and oleic phases, IL solution and crude oil
respectively, was measured using pendant drop method (KRUSS, SITE-100; KRUSS,
Germany). The reported accuracy of the measured IFT is ±0.01 mN/m. Water content was
measured by 851 Titrando, coulometric Karl Fischer apparatus supplied by Metrohm
(Switzerland). The densities and viscosities of ILs were measured using an automated
Anton Paar SVM-3000 digital double-tube visco-densimeter at atmospheric pressure and
temperature range 10 to 90C. The precision in experimental measurements has been
−
2
−3
found to be better than ±1×10 g.cm for the density and ±1 mPa.s for the viscosity. The
calibration was checked periodically with pure liquids (supplied by Cannon Co.) with
known density and viscosity at several temperatures.
Refractive indices of synthesized samples were measured by using a refractometer Model
J357 supplied by Rudolph Company. The apparatus was calibrated with isopropyl alcohol
and water at different temperatures from 10 to 90C.
2
.2.1. IFT measurement
Pendant drop and rising bubble are two mechanism which are considered based on density
of balk and drop in IFT measurement of liquid-liquid system. In this research, the density
of crude oil was less than sea water density, thus rising bubble mechanism was performed.
The IFT measurements were performed inside an optical glass cell filled with sea water
enriched with ILs and was placed in a jacket holder at 25±0.1C. A sufficient drop size of
oil was injected upward to the tip of needle as rising bubble which was immersed in the

ACCEPTED MANUSCRIPT
balk of sea water. The final stabilized drop was imaged and processed by the software for
measuring the IFT.
Generally, a pendant drop apparatus consists of three parts: an experimental cell, an
illuminating/viewing system and a data acquisition system to calculate the IFT value. In
this method, the profile of an oil rising bubble which is suspended in IL solution, is
determined at mechanical by the balance between gravity and surface forces. Andreas
proposed the following equations to calculate the IFT.
ꢄ
ꢁ
ꢂ ꢅꢆ
ꢃ
ꢀ
ꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢉꢊꢋꢈꢈꢈ
ꢇ
Where γ is the interfacial tension, ∆ρ is the density difference between sea water and crude
oil, D is the equatorial diameter of the drop, H is a correction factor which is related to the
e
shape factor of the pendant drop, S, defined as:
ꢂꢍ
ꢌꢀ ꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢉꢎꢋ
ꢂꢃ
Where D is the drop diameter measured horizontally at a distance D away from the apex
s
e
of the drop [26, 27].
2
.3. Synthesis of ILs
The synthesis procedure of [C mim][Cl], [C mim][Cl], [C mim][Cl] and
8
10
12
[
C mim][Cl]are given as below:
14
ILs containing chloride anion were synthesized by reacting 1-methylimidazole (0.1mol)
with excess amount of 1-cloroalkane (1-chlorooctane, 1-chlorodecane, 1-chlorododecane
or 1-chlorotetradecane) (0.11mol) without any additional solvent. At first the reaction
mixture was heated and continually stirred at 80 C for 72-120 h in a round bottomed
flask, fitted with a reflux condenser, under N atmosphere. Then it was cooled to room
2
temperature and washed thrice with 50 ml diethyl ether to remove excess starting material.
Finally the resulting products were dried under vacuum at 60 C for 48 h to remove all
solvents [28]. To prove the synthesis verification, ILs were tested with AgNO and white
3
AgCl precipitation was seen, so no further purification was required.
Synthesis of ILs containing [C mim][THDP], [C mim][THDP], [C mim][THDP] and
8
10
12
[
C mim][THDP] are given as below
14

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0
4
.1 mol of [C mim][Cl] (n = 8, 10, 12, 14) and 0.15 mol of KOH were dissolve in 30 and
n
5 ml ethanol respectively. Then KOH solution was added dropwise to the [C mim][Cl]
n
solution in a round bottomed flask equipped by magnet stirrer, and KCl white precipitate
was immediately observed. The resulting mixture was stirred for another 5 min to ensure
that the reaction was complete. [C mim][OH] solution in ethanol was obtained by filtering
n
the reaction mixture to remove all KCl participation [29]. 0.25 mol pyrophosphoric acid
per mol KOH was dissolved in water and added to [C mim][OH] solution. All the active
n
protons of H P O were neutralized to generate [C mim] P O and K P O . Then, another
4
2
7
n
4
2
7
4
2
7
0
.75 mol H P O per mol of [C mim][OH] was added in three steps to obtain
4 2 7 n
[
C mim][THDP]. Finally the mixture was placed in a rotary evaporator to remove all
n
solvents and then was dried in a vacuum oven.
.4. Characterization of synthesized ILs
2
1
The synthesized ILs were characterized by HNMR and CHNO elemental analysis, to
prove the absence of impurities. The elemental analysis and impurity contents of studied
ILs are reported in Table 4 and 5.
Table 4 and 5
1
[
C mim][Cl]: HNMR (500 MHz, D O): δH= 8.61 (1H, s), 7.36 (1H, s), 7.32 (1H, s),
8
2
4
.05 (2H, t), 3.77 (3H, s), 1.72 (2H, quintet), 1.24-1.1 (10H, m), 0.84 (3H, t).
1
[
C mim][Cl]: HNMR (500 MHz, D O): δH= 9.29 (1H, s), 7.76 (1H, s), 7.72 (1H, s),
1
0
2
4
.15 (2H, t), 3.8 (3H, s), 1.72 (2H, quintet), 1.23-1.15 (14H, m), 0.82 (3H, t).
1
[
C mim][Cl]: HNMR (500 MHz, D2O): δH= 9.12 (1H, s), 7.85 (1H, s), 7.77 (1H, s), 4.1
1
2
(2H, t), 3.83 (3H, s), 1.78 (2H, quintet), 1.30-1.21 (18H, m), 0.86 (3H, t).
1
[
C mim][Cl]: HNMR (500 MHz, D2O): δH= 8.97 (1H, s), 7.79 (1H, s), 7.75 (1H, s),
1
4
4
.17 (2H, t), 3.65 (3H, s), 1.83 (2H, quintet), 1.34-1.2 (22H, m), 0.79 (3H, t).
1
[
C mim][H P O ]: HNMR (500 MHz, D2O): δH= 9.04 (1H, s), 7.78 (1H, s), 7.82 (1H,
8
3
2
7
s), 4.12 (2H, t), 3.88 (3H, s), 1.70 (2H, quintet), 1.16-1.1 (10H, m), 0.79 (3H, s).
1
[
C mim][H P O ]: HNMR (500 MHz, D2O): δH= 9.48 (1H, s), 7.91 (1H, s), 7.85 (1H,
1
0
3
2
7
s), 4.33 (2H, t), 3.89 (3H, s), 1.75 (2H, quintet), 1.27-1.09 (14H, m), 0.82 (3H, t).
1
[
C mim][H P O ]: HNMR (500 MHz, D2O): δH= 9.21 (1H, s), 7.63 (1H, s), 7.69 (1H,
1
2
3
2
7
s), 4.21 (2H, t), 3.73 (3H, s), 1.82 (2H, quintet), 1.28-1.13 (18H, m), 0.89 (3H, t).

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1
[
C mim][H P O ]: HNMR (500 MHz, D2O): δH= 8.89 (1H, s), 7.85 (1H, s), 7.77 (1H,
1
4
3
2
7
s), 4.12 (2H, t), 3.71 (3H, s), 1.88 (2H, quintet), 1.32-1.15 (22H, m), 0.85 (3H, t).
3
3
. Result and discussion
.1. Physicochemical properties of ILs
The physicochemical properties of [C mim][Cl], [C mim][Cl], [C mim][THDP] and
8
10
8
[
C mim][THDP] are given in Table 6, over a wide temperature range of 10 to 90 C. The
8
density (ρ) and refractive index (n ) were fitted by least squares using the polynomial of
D
second order expression given by equation (4).
2
Z = A + A ×T+A ×T
(4)
(5)
o
1
2
The viscosity values can be represented using the following Ln type equation (5):
ln η (mPa s) = A /T- A
o
1
-3
Where z is  (g.cm ) and n , T is the temperature (C) and A , A and A are the
D
o
1
2
adjustable parameters [30].
The best fit parameters for density, viscosity and refractive index are listed in Table 7.
Table 6 and 7
3
.2. Density and viscosity
The effects of temperature, cation and anion type on the density and viscosity of
synthesized ILs are shown in Table 6 and Figures 1 and 2. As expected, the measured
densities and viscosities were decreased as a function of temperature. According to Fig. 1,
the density of ILs was decrease with increasing the alkyl chain length at each temperature.
The data clearly indicated that the density was increased in order [C mim][THDP] >
8
[
[
C mim][DHP] > [C mim][Cl] > [C mim][Cl]. The higher density values for
10 8 10
C mim][THDP] and [C mim][THDP] was due to presence of more hetero atoms in
8
10
comparison with other anions. In general, the density of ILs was increased when anion
molecular weight was increased [31]. As shown in Fig. 2, the investigation on viscosity
values of ILs exhibited that as the alkyl chain was increased from C to C , the viscosity
8
10
was increased which was caused by increasing the van der Waals interaction between the
aliphatic alkyl chains [32, 33]. The main reason for high viscosity of [C mim][THDP] and
8
[
C mim][THDP] is presence of hydrogen bonding in anions structure and also high
10
molecular weight of these anions compared with ILs containing chloride anion.

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Fig. 1 and 2
3
.3. Refractive Index
The refractive index is a key parameter to determine the purity and electronic
polarizability of ILs. The temperature dependence of refractive indices for synthesized ILs
are presented in Table 6 and Fig. 3. The n values for the synthesized ILs were decreases
D
in a linear manner with increasing temperature. Figure 4 showed the effects of
temperature, anion and cation type of ILs on the refractive indices values. The results
showed that the shorter alkyl chain length led to the higher refractive index. It can be
explained that, ILs with longer alkyl chain, cannot be close packed in microscopic
structure [34]. Increasing trend of refractive index for studied ILs, was as follows:
[
C mim][Cl] > [C mim][Cl] > [C mim][THDP] > [C mim][THDP]. For synthesized ILs,
8 10 8 10
the refractive indices values were higher than the range of most organic materials.
Fig. 3
3
.4. Surface activity of ILs
The critical micelle concentration (CMC) is an important characteristic of a surfactant. At
this concentration, surfactant molecules which were placed at surface of solution, associate
to form micelles and any further addition of surfactant arrange as micelles, as a result the
IFT is not reduced anymore as the surfactant concentration was increased. The CMC point
determines surfactant efficiency as the CMC is lower, the surfactant have better efficiency.
A lower CMC indicates that less concentration of surfactant is require to obtain the
minimum IFT value [6, 25 and 35]. In order to find the CMC of investigated ILs, several
experimental IFT tests were performed at different concentrations of ILs in sea water. The
salinity was considered constant concentration in this work, because the effect of salinity
on TFT have been done in pervious works [6, 16]. However the harsh salinity condition
was applied to evaluate the stability of ILs in sea water salinity. All synthesized ILs were
stable and efficient in harsh salinity of sea water. The CMC values are listed in Table 8.
Table 8
Table 8 revealed that as the alkyl chain was longer, the CMC value was lower. In order to
low CMC value for many synthesized ILs, they can be good candidate when economical
concern is a main factor.
3
.5. Effect of ILs concentration on IFT between sea water and crude oil

ACCEPTED MANUSCRIPT
The imidazolium based synthesized ILs in this research were composed of four different
alkyl chain length (Octyl, Decyl, Dodecyl and Tetradecyl) and two anion (chloride and
trihydrogen phosphate). The IFT of sea water which was enriched by ILs and crude oil
was measured at several concentration. Besides the effects of cation and anion type and
concentration of ILs were investigated on IFT reduction. According to obtained results
listed in Table 9, all eight ILs showed very interesting behavior in IFT reduction. As a
result, it can be concluded that the synthesized ILs were able to tolerate the harsh
conditions of sea water concentration and they were favorable choices for EOR process,
due to IFT reduction.
Table 9
Commonly, long alkyl chain ILs, like surfactant, are consisted of hydrophilic imidazolium
head group and hydrophobic alkyl chain tail in cation part. The hydrophobic part extend
into the oily phase and the hydrophilic part likes to stay in water phase. As the alkyl chain
is longer, the capability of the ILs to reduce the IFT is increased which could be due to the
effect of hydrophobic long alkyl chain tail [35]. As clearly can be found in Table 8 and 9,
the CMC point and IFT values is decreased as the alkyl chain length is increased.
According to obtained results from Figures 4, 5 and Table 9, as the ILs concentration were
increased from 100 to 3000 ppm for [C mim][x], (n: 8, 10, 12 and x:Cloride and THDP),
n
the IFT of sea water/crude oil were deceased. Due to very low IFT values for
[
C mim][Cl] and [C mim][THDP], ILs concentration was increased from 10 ppm and
14 14
the IFT was decreased to very low values as shown in Table 9.
Fig. 4 and 5
Furthermore, a detailed investigation in Table 9 and Fig. 4, 5, revealed that a sharp
reduction in the IFT was observed when the IL concentration was changed before and
after the CMC point of corresponding IL. Comparing the IFT values of ILs containing
chloride anion and those who were included THDP anion, it can be concluded that the
anion types of ionic liquids is affected on IFT of sea water/ crude oil system. There were a
few reports concerning the effects of the anion on IFT between sea water and crude oil.
The results demonstrated that the efficiency of ILs containing THDP anion in IFT
reduction, was better than ILs with chloride anion. The investigation revealed that the
system containing ILs with bigger size and higher viscosity of anion, exhibited higher IFT
reduction.

ACCEPTED MANUSCRIPT
Commonly increasing the viscosity of injected fluid is improved the sweep efficiency, so
ILs which have higher viscosity, is more effective to reduce IFT [36].
Conclusion
The eight hydrophilic imidazolium based ionic liquids were synthesized and characterized
1
by HNMR and CHNO elemental analysis. ILs including [C mim][Cl], [C mim][Cl],
8
10
[
[
C mim][Cl], [C mim][Cl], [C mim][THDP], [C mim][THDP], [C mim][THDP] and
12 14 8 10 12
C mim][THDP] were used as surfactants to reduce the IFT between enriched sea water
1
4
by ILs and crude oil in water injection. As a nobility it should be noted that the effect of
synthesized ILs on water injection process as known scenario in secondary EOR, has been
proposed for the first time. Moreover, as another nobility ILs containing THDP anion,
have not been synthesized yet. The physicochemical properties such as density, viscosity
and refractive index were measured for [C mim][Cl], [C mim][Cl], [C mim][THDP] and
8
10
8
[
C mim][THDP] at temperature range of 10 to 90 C. According to the obtained results
1
0
unlike viscosity, other properties such as density and refractive index were decreased
while alkyl chain length was increased.
The CMC point as a function of optimized economical and efficient concentration for all
ILs was determined and the IFT as a function of sweep efficiency (capillary number) in
water injection process, was measured at different ILs concentration by pendant drop
method. The results revealed the high efficiency of synthesized ILs in IFT reduction of sea
water/crude oil. The used sea water salinity was about 43000 ppm. According to the
obtained results, as the alkyl chain length was increased, the CMC point and IFT values
were decreased, it means that less IL is required to obtain the minimum IFT, so these ILs
are favorable choices in economic circumstances. The best IFT values (0.65 ± 0.04 and 0.5
-1
±
0.02 mN.m ) were obtained by [C mim][Cl] and [C mim][THDP] while, only 50 and
14 14
2
5 ppm of these ILs was added to sea water. Furthermore the experimental IFT data
showed that ILs including THDP anion had the better performance as surfactant to
reducing the IFT which is related to the higher viscosity of these ILs compared with ILs
having chloride anion.

ACCEPTED MANUSCRIPT
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[
9
[
1
[
[

ACCEPTED MANUSCRIPT
Table. 1. Chemical structures of the synthesized ionic liquids
Ionic Liquid Cation part Anion part Abbreviation
-Octyl-3-
M.W
g/mol)
Purity
(%)
Formula
(
1
methylimidazolium
chloride
C H ClN2
[
C mim][Cl]
12 23
230.78
258.2
99
99
8
1
-Decyl-3-
14 27
C H ClN2
methylimidazolium
chloride
[C mim][Cl]
10
1
-Dodecyl-3-
16 31
C H ClN2
methylimidazolium
chloride
[C mim][Cl]
286.88
314.94
99
99
1
2
1
-Tetradecyl-3-
methylimidazolium
chloride
C H ClN2
18 35
[
C mim][Cl]
1
4
1
-Octyl-3-
methylimidazolium
threehydrogen
diphosphate
[
C mim][
THDP]
8
C H N O P
12 26 2 7 2
372.29
400.34
98
98
1
- Decyl-3-
methylimidazolium
threehydrogen
diphosphate
[
C mim][
10
C H N O P
14 30 2 7 2
THDP]
1
- Dodecyl-3-
methylimidazolium
threehydrogen
diphosphate
[
[
C mim][
12
THDP]
C H N O P
16 34 2 7 2
428.40
456.45
98
98
1
-Tetradecyl-3-
methylimidazolium
threehydrogen
diphosphate
C mim][
14
C H N O P
18 38 2 7 2
THDP]

ACCEPTED MANUSCRIPT
Table 2. Physical properties of used crude oil
Property
Crude Oil
Density
of
Residual
Oil
(ambient
0.9135
condition)(g/lit)
Molecular Weight of Stock Tank Oil (g/mol)
Viscosity (ambient condition) (cP)
298
4.2325
23.25
API Gravity of Residual Oil (API)

ACCEPTED MANUSCRIPT
Table 3. Chemical analysis of sea water.
Component
Calcium
Composition / ppm
500
1642
12650
460
0.1
Magnesium
Sodium
Potassium
Lithium
Barium
0.6
Strontium
Fe
4.7
<0.05
23042
4
Chloride
Carbonate
Bicarbonate
Sulfate
92
3070
2
Sulfide
Total dissolved solids (TDS)
43788

ACCEPTED MANUSCRIPT
Table 4. The CHNO elemental analysis for synthesized ILs
Chemical
Formula
MW
(g/mol)
% C
% H
% N
% O
IL
cal
exp
cal
exp
cal
exp
cal
-
exp
-
C mim cl
8
C H ClN
62.45 62.33 10.04 10.11 12.14 11.98
1
2
23
2
2
2
2
230.78
258.2
C mim cl
1
0
C H ClN
64.96
64.8
10.51
10.7
10.82 10.72
-
-
-
-
-
-
1
4
27
C mim cl
1
2
C H ClN
66.98 67.06 10.89 10.77
9.76
8.89
7.52
6.99
6.54
6.14
9.82
8.77
7.74
7.07
6.42
6.28
1
6
31
286.88
314.94
372.29
400.34
428.40
456.45
C mim cl
1
4
C H ClN
68.65 69.01
38.71 38.56
42.00 41.92
44.86 45.04
11.2
7.04
7.55
7.99
8.39
11.06
6.88
7.61
8.11
7.96
1
8
35
C mim
8
C H N O P
30.08 29.89
27.97 28.01
26.14 26.07
24.54 24.73
1
2
26
2
7
2
2
2
2
THDP
C mim
1
0
C H N O P
1
4
30
2
7
THDP
C mim
1
2
C H N O P
1
6
34
2
7
THDP
C mim
1
4
C H N O P
47.36
47.8
1
8
38
2
7
THDP

ACCEPTED MANUSCRIPT
Table 5. Impurities of the synthesized ionic liquids.
[
C mim] [C mim] [C mim] [C mim] [C mim] [C mim] [C mim] [C mim]
8
10
12
14
8
10
12
14
Species
[Cl]
[Cl]
[Cl]
[Cl]
[THDP]
[THDP]
[ THDP] [ THDP]
Results
a
Water content ,
ppm
6
40
480
340
310
420
510
280
350
b
Chloride , ppm
152914
<10
<5
137183
<10
<5
123628
<10
<5
108963
<10
<5
<10
<10
<5
<10
<10
<5
<10
<10
<5
<10
<10
<5
b
Bromide , ppm
c
Sulfate , ppm
c
Heavy metals
<
0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
(
as Pb), ppmd
Ash content ,
mass%
<
0.001
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
Test methods are a) Karl Fischer, b) potentiometry, c) turbidimetry, d) Electric furnace and gravimetry
The uncertainty in temperature and pressure were ±0.01 ºC and ±0.01 bar respectively.

ACCEPTED MANUSCRIPT
Table 6. Density (ρ), viscosity (η) and refractive index (n ) of synthesized ILs
D
−3
T/C
ρ/(g.cm )
η/(mPa.s)
n
D
[
C mim][Cl]
8
1
2
2
3
4
5
6
7
0
0
5
0
0
0
0
0
1.0189 ± 0.0174
1.0137 ± 0.0117
1.0119 ± 0.0183
1.0081 ± 0.014
1.0022 ± 0.0088
0.9970 ± 0.0077
0.9903 ± 0.0123
0.9852 ± 0.0148
-
1.51162 ± 0.0018
1.51011 ± 0.0026
1.50867 ± 0.0030
1.50721 ± 0.001
1.5043 ± 0.0024
1.50152 ± 0.0016
1.49871 ± 0.0022
1.49587 ± 0.0031
34100 ± 1770
20900 ± 1049
11040 ± 2016
4130 ± 333
1980 ± 329
940 ± 149
338 ± 78
8
9
0
0
0.9791 ± 0.0108
0.9736 ± 0.0109
137 ± 34
56 ± 15
1.49343 ± 0.0033
1.49079 ± 0.0041
[
C mim][Cl]
10
1
2
2
3
4
5
6
7
8
9
0
0
5
0
0
0
0
0
0
0
-
-
-
-
-
-
-
1.50018 ± 0.0054
1.4988 ± 0.0026
1.4971± 0.002
1.49362 ± 0.0058
1.48962 ± 0.008
1.48646 ± 0.008
1.48334 ± 0.0053
1.48059 ± 0.0063
1.47869 ± 0.0027
0.9834 ± 0.0094
0.9788 ± 0.0042
0.9739 ± 0.0064
0.968 ± 0.0029
0.9619 ± 0.0062
0.9558 ± 0.0086
0.9501 ± 0.0103
0.9433 ± 0.0091
5721 ± 370
2648 ± 304
1292 ± 183
489.3 ± 91
248.15 ± 58
152.46 ± 64
[
C mim][THDP]
8
1
2
2
3
4
5
6
7
8
9
0
0
5
0
0
0
0
0
0
0
-
-
-
-
1.47193 ± 0.0053
1.4707 ± 0.0049
1.46949 ± 0.0072
1.46851 ± 0.0054
1.46654 ± 0.005
1.46466 ± 0.0087
1.46264 ± 0.0092
1.46073 ± 0.0048
1.4588 ± 0.004
1.4255 ± 0.0121
1.4219 ± 0.0143
1.4163 ± 0.018
1.4090 ± 0.0109
1.4030 ± 0.0066
1.3946 ± 0.0038
1.3895 ± 0.0112
1.3812 ± 0.0101
23469 ± 1783
14620 ± 1288
7114 ± 676
2983 ± 594
1313 ± 329
683 ± 237
369.45 ± 79
210.35 ± 56
1.45692 ± 0.007
[
C mim][THDP]
10
1
2
2
3
4
5
6
7
8
9
0
0
5
0
0
0
0
0
0
0
-
-
-
-
-
-
-
-
1.3587 ± 0.0135
1.3536 ± 0.0133
1.3483 ± 0.0096
1.3438 ± 0.0105
1.3384 ± 0.0022
1.3336 ± 0.0066
8048 ± 556
3521 ± 680
1802 ± 318
893.2 ± 173
421 ± 54
253.1 ± 31
1.44929 ± 0.0040
1.44718 ± 0.0063
1.44497 ± 0.0032
1.44308 ± 0.0071
1.44123 ± 0.0042
1.43902 ± 0.0067
The uncertainty in temperature and pressure were ±0.01 ºC and ±0.01 bar respectively.

ACCEPTED MANUSCRIPT
Table 7. Fitting parameters of Eqs. (1), (2) used for correlation of the physical properties of ILs studied.
2
Physical property
A0
A1
A2
R
[
C mim][Cl]
8
−
3
-7
ρ/(g.cm )
nD
1.0249
1.5148
-21.999
-0.0006
-0.0003
9521.8
-2×10
-2×10
-
0.9992
0.9980
0.9963
-
7
η/(mPa.s)
[
C mim][Cl]
10
−
3
-7
ρ/(g.cm )
nD
0.9971
1.509
-18.536
-0.0006
-0.0004
8524.7
-3×10
1×10
-
0.9988
0.9984
0.9956
-
6
η/(mPa.s)
[
C mim][THDP]
8
−
3
-7
ρ/(g.cm )
nD
1.4406
1.474
-16.563
-0.0006
-0.0002
7937
-8×10
-1×10
-
0.9985
0.9991
0.9991
-
7
η/(mPa.s)
[
C mim][THDP]
10
−
3
-7
ρ/(g.cm )
nD
1.3795
1.458
-16.334
-0.0005
-0.0002
7927.9
2×10
2×10
-
0.9997
0.9975
0.9993
-
7
η/(mPa.s)

ACCEPTED MANUSCRIPT
Table 8. The obtained CMC point of synthesized ILs
-
1
IL
CMC point (ppm)
IFT (mN.m )
[
[
[
[
[
[
[
[
C mim][Cl]
500
350
150
50
350
250
100
25
1.9 ± 0.39
1.67 ± 0.23
1.43 ± 0.15
0.65 ± 0.04
1.52 ± 0.19
1.21 ± 0.03
1.1 ± 0.09
0.5 ± 0.02
8
C mim][Cl]
10
C mim][Cl]
12
C mim][Cl]
14
C mim][THDP]
8
C mim][THDP]
10
C mim][THDP]
12
C mim][THDP]
14
Table 9. Effect of synthesized ILs on IFT of crude oil/enriched sea water
-
1
IFT (mN.m ) ± a
[
IL]
(
ppm)
[C
[
8
mim]
Cl]
[C
[THDP]
8
mim]
[C10mim]
[THDP]
[C12mim]
[THDP]
[C14mim]
[THDP]
[
C mim][Cl]
[C mim][Cl]
12
[C mim] [Cl]
14
1
0
0
11.66 ± 0.71
11.78 ± 0.69
12.03 ± 0.65
11.97 ± 0.67
11.58 ± 0.72 11.62 ± 0.65 12.11 ± 0.58 11.82 ± 0.61
1
2
5
7
0
5
0
5
-
-
-
-
-
-
-
-
-
-
-
-
3.49 ± 0.23
2.85 ± 0.37
0.65 ± 0.04
-
-
-
-
-
-
-
-
-
-
4.82 ± 0.21
0.5 ± 0.02
0.33 ± 0.06
0.31 ± 0.09
3.89 ± 0.09
2.43 ± 0.09
1.1 ± 0.09
-
-
1
1
2
2
3
5
7
00
50
00
50
50
00
50
8.23 ± 0.62
-
6.46 ± 0.71
-
5.23 ± 0.16
1.43 ± 0.15
-
1.22 ± 0.09
-
0.98 ± 0.1
0.82 ± 0.07
0.78 ± 0.09
0.82 ± 0.06
0.86 ± 0.1
0.85 ± 0.09
0.5 ± 0.08
7.12 ± 0.43
-
5.21 ± 0.36
-
0.26 ± 0.08
-
-
-
-
-
4.83 ± 0.43
3.92 ± 0.61
1.67 ± 0.23
1.35 ± 0.1
1.16 ± 0.09
1.08 ± 0.12
1.04 ± 0.14
1.08 ± 0.09
1.04 ± 0.12
-
-
-
5.23 ± 0.53
3.56 ± 0.46
1.9 ± 0.39
1.63 ± 0.28
1.59 ± 0.35
-
0.53 ± 0.04
3.62 ± 0.62
1.52 ± 0.26
1.35 ± 0.13
1.39 ± 0.1
1.3 ± 0.09
1.22 ± 0.1
1.2 ± 0.08
1.23 ± 0.13
1.21 ± 0.19
1 ± 0.08
0.95 ± 0.11
0.95 ± 0.09
0.87 ± 0.13
0.82 ± 0.06
0.9 ± 0.09
0.83 ± 0.14
0.96 ± 0.13
-
0.81 ± 0.09
-
0.78 ± 0.09
0.62 ± 0.1
0.61 ± 0.03
0.43 ± 0.07
0.27 ± 0.05
-
-
0.52 ± 0.07
0.2 ± 0.09
-
-
1
000
500
000
000
0.5 ± 0.03
0.2 ± 0.08
1
2
3
-
-
-
-
-
-
1.52 ± 0.23
1.5 ± 0.18
⁕
a is standard deviation obtained using minimum three dependent measurements for each data point.

ACCEPTED MANUSCRIPT
1
1
1
1
1
.5
.4
.3
.2
.1
1
0
.9
0
20
40
60
80
100
T / C
3
Fig. 1. Experimental values of density, ρ (g/cm ) against temperature (C) for (●) ([C mim][Cl]), (♦)
8
(
[C mim][Cl]), (□) ([C mim][THDP]) and (×) ([C mim][THDP])
10 8 10

ACCEPTED MANUSCRIPT
3
3
2
2
1
1
5000
0000
5000
0000
5000
0000
5
000
0
0
20
40
60
80
100
T / C
Fig. 2. Experimental values of viscosity η (mpa.s) against temperature (C) for (●) ([C mim][Cl]), (♦)
8
(
[C mim][Cl]), (□) ([C mim][THDP]) and (×) ([C mim][THDP])
10 8 10

ACCEPTED MANUSCRIPT
1
1
.52
.51
1
.5
1
1
1
1
1
1
1
.49
.48
.47
.46
.45
.44
.43
0
20
40
60
80
100
T / C
Fig. 3. Experimental values of refractive index n , against temperature (C) for (●) ([C mim][Cl]), (♦)
D
8
(
[C mim][Cl]), (□) ([C mim][THDP]) and (×) ([C mim][THDP])
1
0
8
10

ACCEPTED MANUSCRIPT
Fig. 4. Effect of ILs (containing chloride anion) concentration on the IFT of sea water and crude oil, (●)
[
C mim][Cl], (■)[C mim][Cl], (∆) [C mim][Cl] and (×) [C mim][Cl]
8 10 12 14

ACCEPTED MANUSCRIPT
Fig. 5. Effect of ILs (containing chloride anion) concentration on the IFT of sea water and crude oil, (○)
[
C mim][THDP], (□) [C mim][THDP], (▲) [C mim][THDP] and (♦) [C mim][THDP]
8 10 12 14

ACCEPTED MANUSCRIPT
3
Fig. 1. Experimental values of density, ρ (g/cm ) against temperature (C) for (●) ([C mim][Cl]), (♦)
8
(
[C mim][Cl]), (□) ([C mim][THDP]) and (×) ([C mim][THDP])
10 8 10
Fig. 2. Experimental values of viscosity η (mpa.s) against temperature (C) for (●) ([C mim][Cl]), (♦)
8
(
[C mim][Cl]), (□) ([C mim][THDP]) and (×) ([C mim][THDP])
10 8 10
Fig. 3. Experimental values of refractive index n , against temperature (C) for (●) ([C mim][Cl]), (♦)
D
8
(
[C mim][Cl]), (□) ([C mim][THDP]) and (×) ([C mim][THDP])
10
8
10
Fig. 4. Effect of ILs (containing chloride anion) concentration on the IFT of sea water and crude oil, (●)
C mim][Cl], (■)[C mim][Cl], (∆) [C mim][Cl] and (×) [C mim][Cl]
[
8
10
12
14
Fig. 5. Effect of ILs (containing chloride anion) concentration on the IFT of sea water and crude oil, (○)
C mim][THDP], (□) [C mim][THDP], (▲) [C mim][THDP] and (♦) [C mim][THDP]
[
8
10
12
14

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Graphical Abstract
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Highlights
1
- Four new ILs containing trihydrogen diphosphate anion, were synthesized and
characterized for the first time.
2
- The used ILs had significant efficiency in reducing interfacial tension in water
injection process.
3
4
- As the alkyl chain was longer, the CMC point and IFT value were more reduced.
- These ILs are favorable choices in economic circumstances due to their very low
consumption.