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Journal of Dispersion Science and Technology
ISSN: 0193-2691 (Print) 1532-2351 (Online) Journal homepage: http://www.tandfonline.com/loi/ldis20
Effect of Gemini surfactant additives on pour point
depressant of crude oil
Sahar M. Ahmed, Taisir. T. Khidr & Dina A. Ismail
To cite this article: Sahar M. Ahmed, Taisir. T. Khidr & Dina A. Ismail (2017): Effect of Gemini
surfactant additives on pour point depressant of crude oil, Journal of Dispersion Science and
Technology, DOI: 10.1080/01932691.2017.1385483
To link to this article: http://dx.doi.org/10.1080/01932691.2017.1385483
Accepted author version posted online: 09
Oct 2017.
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Download by: [University of Florida]
Date: 28 October 2017, At: 12:50
Effect of Gemini surfactant additives on pour point
depressant of crude oil
Sahar M. Ahmed
Downloaded by [University of Florida] at 12:50 28 October 2017
Egyptian Petroleum Research institute (EPRI), Nasr City, Cairo, Egypt
Taisir. T. Khidr
Egyptian Petroleum Research institute (EPRI), Nasr City, Cairo, Egypt
Dina A. Ismail
Egyptian Petroleum Research institute (EPRI), Nasr City, Cairo, Egypt
Address correspondence to Dina A. Ismail, Egyptian Petroleum Research institute (EPRI), 1
Ahmed El- Zomor St., Nasr City, 11727 Cairo, Egypt. E-mail: dinaabdlkader@hotmail.com
ABSTRACT
Efficiencies of cationic gemini surfactant additives in improving the pour point depressant
of crude oil were investigated. The length of alkyl chain is a major factor affecting the
improvement of the pour point depression. The adsorption behavior of these gemini surfactants
at air/solution and oil/solution interfaces were investigated by measuring the surface tension and
interfacial tension as functions of concentration. It is found that there is a good relation between
surface properties especially interfacial tension of the gemini surfactants and their efficiency in
depressing the pour point. Also, the surface parameters and free energies of micellization and
adsorption confirm the decreasing and improving of pour point depression. Crystallization study
1
in curde oil revealed the relationship between the structure and activity of gemini surfactant
additives. It is found that the X-ray diffraction patterns of waxes with additives are remarkably
different from those without additives. The mechanism of the depressants action has been
suggested according the adsorption of each additive. Adsorption of the additive on the surface of
the wax particles inhibits their growth and alters the crystal habits through micelle core.
Pretreatment of the crude oil with pour point depressants, has received the greatest acceptance due
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to its simplicity and economy.
GRAPHICAL ABSTRACT
KEYWORDS: crude oil, Gemini surfactants, pour point depressant, surface properties, X-ray
diffraction
Introduction
Wax deposition is a serious and long-standing problem in waxy crude oil storage and
transportation. With decreasing temperature, the paraffin precipitates from crude oil and forms
2
wax crystals. The pour point is the lowest temperature at which the oil will just flow, under
standard test conditions.
The efficient solution to these problems is deployment of chemical additives such as pour
point depressants (PPDs) and flow improvers (FIs). PPDs/FIs doped in waxy crude oils could
modify crystal growth habits by nucleation, adsorption and co-crystallization effects, effectively
inhibiting the tendency of wax crystals to interconnect into fixed three-dimensional networks.[1–3]
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Consequently, pour point, viscosity and yield stress values of waxy crude doped with PPDs/FIs
are substantially reduced. PPDs/FIs have been contained oil soluble long chain alkyl group and a
polar moiety in the molecular structure. The long chain alkyl group insert into wax crystal and
polar moiety exist on the wax surface and reduces wax crystal size.[4]
S. I Inyckyj and C. O. Cole[5] thought that the response of fuels to flow improvers could be
substantially improved by utilizing a dual functional flow improver's composition, which is a
combination of two different waxes modifying compounds. One of these functions act as a wax
growth arrester while another act as nucleating agents. These additives were a combination of the
conventional flow improvers and wax dispersants. The structure and composition of wax
dispersants are similar to conventional flow improver in some feature, but different in others.[6,7]
They often possess highly polar functional groups. This polarity may reach a surfactant character,
which is considered as the basic prerequisite for the dispersion potential. Polar nitrogen containing
polymers can function as wax dispersants and flow improvers simultaneously in one component
additive.[8]
Geminis are a special class of surfactants where two monomeric surfactants (two hydrophilic and
two hydrophobic groups) are coupled together via a spacer. Because of the unique structure,
gemini surfactants have some superior properties, such as higher surface activity, lower critical
3
micelle concentration (CMC)[9,10] a higher efficiency in reducing the oil/water interfacial
tension.[11] Moreover, cationic surfactants have been proposed as additive reagents in the
mitigation and remediation of organic contaminated soils.[12] In petrochemical applications,
geminis have been used in emulsified fuels for diesel engines. Improved engine performance,
reduced emissions, and reduction in torque are some of the improved qualities observed with the
emulsified fuels containing gemini surfactants compared to neat diesel as well as conventional
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surfactant.[13]
Maithufi et al.[12] had synthesized Gemini surfactants and evaluated as wax dispersants
(WDs) or wax anti settling additives (WASAs) in crude oil derived diesel. Bi-sulfonate Gemini
Surfactant had been investigated as Viscous Oil Emulsifier/Viscosity Reducer.[14]
The present work deals with the synthesis and evaluation of some cationic gemini
surfactant additives as pour point depressant for waxy crude oil. Also we intend to study the
interaction between cationic gemini surfactant additives and the wax constituent in waxy crude oil
through wide angle x-ray diffraction in order to elucidate this phenomenon at typical field
conditions. Study the effect of these surfactants on surface properties.
2. Experimental
2.1 Material
Dibromohexane, quinoline, fatty acids and acetone were supplied from Aldrich. 2.2. Crude
oil used: Egyptian waxy crude oil was submitted from Qarun Petroleum Company. Their
physicochemical properties are listed in Table 1. The n-paraffin distribution of the isolated waxes
was determined by gas chromatography analysis according to ASTM D 2887 standards.
4
2.3. Synthesized cationic Gemini surfactants
A typical experimental procedure was as previously explained according to Ahmed, et al.
method.[11] The esters were quaternized by di bromohexane in acetone the product samples were
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expressed as G12, G14 and G18 according to the used ester.
2.4. Surface tension measurements
Surface tension measurements were performed for freshly prepared solutions of surfactants
derivatives in the concentration ranges of (0.1-0.00001mol /L) using a Kruss du Nouy tensiometer,
Type K6 (Kruss GmbH, Hamburg, Germany) at 25oC.
2.5. Evaluation test
2.5.1. Pour point measurement
The tested crude oils were heated up to 60oC and the Gemini surfactant additives were
added at different concentration 300, 500, 700, 1000 and 2000ppm . The pour point temperature
was measured at temperature started from 45oC to solidification according ASTM D 97–93.
2.5.2. X-ray diffraction spectroscopy
5
The structure of the asphaltene isolates from crude oil and asphaltene + 1000ppm (G18)
and asphaltene + 1000ppm (G14) were characterized using X-ray powder diffractometer, PA
Nalytical X'Pert PRO MPD (Netherland). Cu Kα radiation with wave length = 1.5418Å was used
at a rating of 40kV, 40mA . The diffraction patterns were recorded at room temperature in the
angular range of 4-70º (2θ) with step size 0.02º (2 θ) and scan step time 0.4(s). The crystalline
phases were identified using the ICDD-PDF database.
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3. Result and Discussion
3.1. Surface properties
Geminis have remarkably low CMC values compared to the corresponding conventional
surfactants of equivalent chain length. The CMC values of the synthesized Gemini surfactants
were determined from the break point of the surface tension (m N/m) versus concentration (on log
scale) curve. Table 2, showed increasing in the surface tension of the Gemini surfactants by
increasing the hydrocarbon length of Gemini this can be attributed to the hydrophobic effect of the
alkyl chains. While the CMC value decreases with an increase in the hydrophobic chain length of
the molecule, which results in increasing the repulsion between the polar medium (H2O) and the
non-polar chains so that the molecules tend to aggregate at lower concentration.[15]
The effectiveness (surface pressure) of gemini surfactants shown in Table 2. The most
efficient surfactant is one that gives the greatest lowering in surface tension for a given CMC. The
Gemini surfactant G18 is the most efficient in achieving the maximum reduction (26 mN/m) of
the surface tension at its CMC.
6
Maximum Surface Excess (max) values of Gemini surfactants are represented in Table 2.
(max) increases by increasing the number of hydrophobic chains on the head group due to the
increase of the interaction with the water phase so the surfactant molecules are directed to the
interface, which decreases the surface energy of the solution.
The minimum area per molecule at the aqueous solution/air interface for the Gemini
surfactants is listed in Table 2. The minimum surface area (Amin) decreases with an increase in the
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number of hydrophobic chains in the surfactant molecules due to the higher accumulation of these
molecules at the interface and a smaller available area per molecule.[16,17]
Efficiency (PC20) values of the prepared Gemini surfactants are given in Table 2. From
these data, it was observed that increasing the number of alkyl chains produce a decrease in
efficiency. This is due to the fact that the efficiency of adsorption at interfaces increases linearly
with an increase in the number of carbon atoms in the hydrophobic group.[18]
From Table 2 values of the standard free energies of micellization and adsorption (Gomic
and Goads) are always negative, indicating that these two processes are spontaneous. The change
in values of both -Gomic and-Goads suggesting that the driving force of micellization or adsorption
is derived from the hydrophobic moieties due to the interaction between hydrocarbon chains.<link
rid="bib19">[19]
The interfacial tension of the oil 0.1% surfactant systems at room temperature was
measured (Table 2). It is clear from the data that the gemini surfactants have good interfacial
tension values against paraffin oil. Also, the lower values of the interfacial tension indicate the
potential to use these surfactants in several applications.
3.2. Effect of the Gemini surfactants on pour point of crude oil
7
The activity of three samples (G12, G14, and G18) Gemini surfactants as pour point
depressant / wax dispersants was evaluated on the basis of their chemical structure. The three
samples were tested to improve the flow properties of the investigated crude oil in term of pour
point depression at concentration of 300, 500, 700, 1000 and 2000ppm . The pour point values
were measured and listed in Figure 1. In general, it was observed that increasing concentration of
additives up to 2000ppm decreasing pour point values. The effectiveness of pour point depression
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increases in order of G18 > G14 > G12. The relation between side chain length content and the
depression of pour point against different concentration in ppm was represented in Figure 1.
Additive G18 adduct shown a maximum effectiveness of pour point depression at 1000ppm (∆pp
= 21oC). The reason was that the G18 long-chain ester functioned by adsorption on to the crystals
being formed to redirect their crystal formation and by co-crystallization to form much smaller
isotropic crystals and higher solubility wax in the mixed crystals serving as the flow improver.
Under the same conditions, the long alkyl chain could improve the compatibility of the G18 longchain esters in the crude oil, and could destroy the formed interlocking network of waxes. It is
interest to mention that, the gemini compounds (G12, G14, G18) are soluble in water. According,
the surface-active properties for the tested compounds can be correlated with their effectiveness to
depression the pour point. The surface active properties critical micelle concentration (cmc),
(πCMC), free energy of micellization (Gomic) and free energy adsorption (Goads), the most negative
Goads value for G18 means that the maximum adsorption on the growing wax crystals was
obtained to prevent formation of wax crystal networks which enhances the pour point depression
for crude oil it can be seen that, the gemini compound G18 (for which the Goads equals -22.848
kJ/mol) exhibits the maximum effectiveness of pour point depression (∆pp = 21oC). This can be
8
attributed to the adsorption of the additive on the surface of the wax crystals keeping then separated
from each other.
3.3. X-Ray diffraction measurements
X-Ray diffraction investigation of asphaltene in absence and presence of surfactants
(1000ppm ). The particle size of asphaltene in presence of G14 and G18 additive were determined
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by using the Scherer equation.<link rid="bib"><link rid="bib"><link rid="bib20">[20]
D  B  /  B Cos  
Where D is the mean particle diameter, B is Scherer constant (0.89), λ is wave length of
the x- ray beam = 1.5405Å, β is full – width half maximum FWHM of diffraction lines, and θ is
the diffraction angle. The computed values of particle size Table 3. showed increasing the order
of asphaltene with 1000ppm of G18 additive = 1.05 < asphaltene with 1000ppm of G14 additive
= 6.01 < Asphaltene = 8.88. The Gemini surfactant additives are considered to be well in the
primitive molecular orientating and nucleating process. G18 additive is considered more efficient
than G14 additive. Figure 2 and Table 3. represent the XRD spectrogram for G18 additive and
G14 additive. The PPD molecules will get into the lattice part of wax crystal cells causing change
in the surface properties of the wax crystals. It will also bring the n-alkane molecules to deposit on
the surface of the wax crystals. Adsorption can also decrease the plane density, but the changes of
the surface properties of the wax crystal are different than those with co- crystallization.
Conclusions
Three Gemini surfactants were investigated as pour point depressants for crude oil. The
maximum pour point reduction was obtained when the crude oil was treated by the 1000ppm G18,
9
and the minimum pour point reduction was obtained for the crude oil treated by the 1000ppm G12.
The longer the chain of Gemini long-chain esters apparently mixed with the paraffin chains
existing in the crude oil, the better the effect on preventing their facile crystallization. Study XRay diffraction of asphaltene and asphaltene with Gemini surfactant additives.
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[8] Al-Sabagh, A. M.; Khidr, T. T.; Moustafa, H. Y.; Mishrif, M. R.; Al-Damasy, M. H.
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[11] El-Dib, F. I.; Ahmed, S. M.; Ismail D. I.; Mohamed, D. E. Synthesis and Surface Properties
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Surfact. Deterg. 2011, 14, 85–90. DOI: 10.1007/s11743-010-1207-6.
[16] El-Sukkary, M. M. A.; Shaker, N. O.; Ismail, D. A.; Ahmed, S. M.; Zaki, M. F.; Awad, I. A.
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[20] Cullity, B. D.; Stock, S. R. Element of X-Ray Diffraction, 3rd ed.; Addison-Wesley, 2001.
11
Table 1. Physicochemical properties of the investigated crude oil.
Test
Method
Result
Density@15°C Kg/L
ASTM D1298
0.8652
Pour point °C
ASTM D97
24
Kinametic viscosity at 40 °C
ASTM D445
10.440
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C.st
at 100 °C C.st
2.300
Asphaltene Content, wt.%
IP 134
0.711
Sulfur Content wt%
ASTM D4294
0.669
Wax Content, wt%
UOP 46/64
9.52
n-paraffins, wt%
ASTM D2887
84.20
Iso- paraffin, wt%
ASTM D2887
5.88
Total paraffins content, wt%
Urea adduct
90.08
12
Table 2. Surface parameters and standard free energy of micellization and adsorption of the
synthesized Gemini surfactants.
Gemin

cmc
πCMC
max.1010m
Amin
Pc20mol /
Gomic
Goads
Inter.
i Surf.
mN/
mol/
mN/
ol cm2
nm2
L
kJ/mol
kJ/mol
Tensio
m
L
m
41
0.020
31
G12
n
0.388
4.27
0.00032
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9
G14
43
0.016
29
0.440
3.77
0.00145
0
G18
46 0.010
26
0.513
19.44
9
3
20.46
20.53
7
3.24
0
13
19.36
0.0022
11
9
3
22.79
22.84
4
8
5
Table 3. X-Ray Diffraction results of the asphaltene in absence and presence of additive.
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Sample
Pos. (2Ɵo)
FWHM
d-spacing
(2Ɵo)
(Å)
Rel.
Int.
(%)
Aspphaltene
22.2754
0.1574
3.99103
3.16
Asphaltene + 1000ppm G12
30.0245
0.2298
2.9479
4.31
Asphaltene + 1000ppm G14
30.2563
0.2362
2.95402
4.34
Asphaltene + 1000ppm G18
40.8703
0.9446
2.20806
1.86
14
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Figure 1. Effect of Gemini surfactants on the pour point of crude oil.
15
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Figure 2. XRD spectra for aspheltene in presence of (a)-1000ppm of G12, (b)-1000ppm of G14
and (c)-1000ppm of G18.
16
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