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45The characterization of asphaltenes by 1H NMR relaxation method: microsecond range of spin-spin relaxation times
N.V. Shkalikov1,*, YuM. Ganeeva2, T.N. Yusupova2, V.D. Skirda1
1 Kazan State University, Kazan 420008, Russia 2 Institute of Organic and Physical Chemistry, Kazan 420088, Russia * E-mail: [email protected]
Received January 11, 2008 Revised February 20, 2008 Accepted February 21, 2008
agnefic Resonance in Solids
□ectronic Journal
Volume 10, No. 1, pages 11-19, 2008
http://mrsei.ksu.ru
The characterization of asphaltenes by *H NMR relaxation method: microsecond range of spin-spin relaxation times
N.V. Shkalikov1*, Yu.M. Ganeeva2, T.N. Yusupova2, V.D. Skirda1
1 Kazan State University, Kremlevskaya, 18, Kazan 420008, Russian Federation 2 Institute of Organic and Physical Chemistry, Russian Academy of Sciences, Arbuzov str., 8,
Kazan 420088, Russian Federation *E-mail: Nikolay. [email protected]
The asphaltene powders have been studied by *H NMR relaxation method, differential scanning calorimetry (DSC) and X-ray structure analysis. The asphaltene samples were extracted from three oils with different component and hydrocarbon-group compositions. It was found that free induction decays (FID) of various asphaltene samples contain Gaussian component with spin-spin relaxation times T2 about 10^20 ¡is, which was named a solid component. The detecting possibilities of equilibrium first-order phase transitions and non-equilibrium glass-transitions on temperature dependence of part Ps of solid component in the *H NMR signal are presented. The *H NMR relaxation and DSC methods to study temperature properties of asphaltenes are compared. The glass-transition temperatures of side aliphatic chains of asphaltene molecules are measured. We set up that crystallization of high-molecular side aliphatic chains of asphaltene molecules is possible.
PACS: 76.60.Es, 61.18.Fs, 82.56.Na, 82.56.Ub
Keywords: Nuclear magnetic resonance, solid-echo, asphaltenes, solid component
1. Introduction
The asphaltene is a general name of a great variety of high-molecular polycyclic compounds, containing heteroatoms N,
S, О and impurities of metal atoms Ni, V and others. Practically, asphaltene is a substance, which precipitates out of oil when low-molecular w-alkanes are added in oil. The quantity of precipitated asphaltenes decreases and their chain-length distribution changes from bimodal to unimodal under increasing number of carbon atoms in the molecule of w-alkane solvent [1]. In most models the asphaltenes are defined as firm quasispherical colloidal particles consisting of condensed aromatic rings grouped one above another in a stack with approximately parallel planes and surrounded by chains of aliphatic (with number of carbon atoms 5-6 and more) and/or naphthene-aromatic systems on periphery [2-4]. The size and form of colloidal asphaltene particles in solutions depend on solvent and temperature. The sizes of colloidal particles can vary in the range from 14 to 30-50 Â. Initial colloidal particles can form more and more large aggregates as a result of coagulation/flocculation, up to sedimentary stability loss and settling-out.
Asphaltenes play an important part in structure and properties formation of oil dispersed system, though, as a rule, the content of asphaltenes in the oils is insignificant (about 5-7%). It is established that the tendency of oil to settling-out of asphaltene-resin-paraffin substances is defined basically by asphaltene characteristics, namely, high aromaticity, low content of hydrogen, high condensity of aromatic rings, high density [6, 7] and the absence of structural and chemical compatibility with corresponding resins. The authors [5] name similar asphaltenes as the most polar fractions, they precipitate first during asphaltene precipitation out of oil by a standard technique. The natural oil contains insignificant quantities of the most polar fractions of asphaltenes, which have smaller molecular weights, do not contain long side aliphatic chains and can have significant amounts of sulfoxide and carbonyl groups in own structure [1]. Powder particles of these asphaltene fractions are dense, glossy particles of black color and are characterized by some ordering of the structure. The least polar fractions of asphaltenes represent a brown powder without gloss and have an amorphous structure [5].
It is noticed that paraffin hydrocarbons coprecipitate together with asphaltenes during the standard procedure of asphaltene precipitation out of oil by low-molecular w-alkanes. There are some assumptions explaining this phenomenon. The authors [1] consider that complexity of structure and molecular weight of asphaltene aggregates increase approaching the boundary of flocculation during addition of low-molecular w-alkanes into the oil. Asphaltenes possess superfluous ability to association through hydrogen bonds near the flocculation point that leads to coprecipitation of other oil components, in particular, high-molecular paraffin hydrocarbons. Philp and coworkers [8] believe that coprecipitation of high-molecular hydrocarbons, such as paraffins and asphaltenes, occurs because of equally low solubility in low-molecular w-alkanes (solubility in w-pentane is equal 1.3 mg / 100 ml approximately). Recently, a number of publications [9, 10] appeared, whose authors claim that Van der Waals interactions between lateral side aliphatic chains of asphaltene molecules are responsible for the least thermally steady asphaltene aggregates formation [11]. Further, the same authors assume that asphaltenes interact with w-alkanes (w-alkanes are dissolved in the solvation sphere of colloidal asphaltene aggregates), and this interaction causes flocculation of asphaltene molecules and its precipitation out of oil [12]. It is found that interaction between asphaltenes and paraffins depends on the structure of asphaltenes [13], concentration and dispersion degree of asphaltenes [14].
The paper [15] claims that asphaltene aggregates can be in the three different phases. The heat capacity of asphaltenes is similar to the heat capacity of solid crystal at the temperatures essentially below -30°C, until they do not undergo phase transition to the amorphous state at the heating above -30°C. The amorphous phase of asphaltenes arises due to interactions polar side aliphatic chains and keeps up to temperatures 25-30°C. During further heating, asphaltenes form more dense structures (stable up to temperatures of the order 100°C) due to bonding between peri-condensed aromatic surfaces, and then asphaltenes suffer phase transition to the ordering crystal phase in the temperature range 100-180°C. At more high temperatures, the amorphous phase of asphaltenes is softened and the crystal phase of asphaltenes begins to melt only at temperatures 220-240°C. Finally, at the temperature 350°C, asphaltenes are destroyed forming mesomorphic anisotropic melt, which foregoes coke formation.
The authors [16] studied thermal properties of asphaltenes in the stream of nitrogen by means of differential scanning calorimetry (DSC) and discovered two typical phase transitions for asphaltenes. So their asphaltene samples suffer glass-transition in the range of temperatures 120-130°C, which relates to non-polar amorphous phase of asphaltenes. The endothermic effect observed by authors at temperature 180°C under heating of asphaltenes is explained as melting of crystal phase, which is characteristic for a polar asphaltene fraction.
Thus, from the above mentioned data follows that the composition and structure of asphaltenes play an important part in structure and properties formation of oil disperse system. Studying asphaltene phase transitions mechanisms is necessary for understanding the internal structural organization of natural oil and its changes with temperature. This information is important for oilfield exploitation technologies optimization and also for oil refining and transportation.
The purpose of our work is to find structural features of different oil asphaltenes using 'H nuclear magnetic resonance (NMR) relaxation method. It is also supposed to give the analysis of *H NMR relaxation method opportunities in comparison with DSC method on the example of asphaltenes.
2. Samples
The asphaltene samples (powders) extracted from three different oils, essentially differing in component and hydrocarbon-group compositions (Table I) have been chosen as objects of research. It is seen from Table I that the
Dachnaya oil is characterized by high content of asphaltenes, Mordovo-Karmal bitumen is characterized by high content of resins and Mamurinsk oil is characterized by high content of paraffins (wax contains paraffins).
Table I. Physical-chemical research data of the Mordovo-Karmal bitumen (Tatarstan; extracted by in-situ combustion method), Dachnaya oil well №3577 (Tatarstan) and Mamurinsk oil well №2I (Samara region)
Oil samples Gasoline fraction, wt.% Wax, wt. % Benzene resins, wt. % Alcohol-benzene resins, wt. % Asphaltenes, wt. %
Dachnaya oil well №3577 1S.S 55.7 16.4 2.0 7.1
Mordovo-Karmal bitumen 10.1 52.1 25.0 S.0 4.S
Mamurinsk oil well №21 21.2 60.2 13.7 3.6 1.4
The asphaltene precipitation was carried out by petroleum-ether (boiling temperature 40-70°C) with volume proportion: oil/solvent = 1/40. The precipitated asphaltenes were washed on the filter in the Soxhlet's apparatus by hot petroleum-ether until the solvent becomes colorless. Then obtained asphaltenes were dissolved in benzene to eliminate non-hydrocarbon impurity, after that benzene was removed. Finally, asphaltenes were dried up to constant weight in vacuum.
3. Experimental methods
Nuclear magnetic resonance (IH NMR) relaxation
Free induction decays (FID) of various asphaltenes contain Gaussian component with spin-spin relaxation times T2 about 10^20 (is (see Fig.1). We are name the component with Gaussian form of FID a solid component, because immobile protons with high arrangement density can possess such a FID form. We believe that crystalline and amorphous solids, containing hydrogen, can possess such short spin-spin relaxation times T2. We choose part Ps of solid
component in the 1H NMR signal of asphaltenes as a parameter, which describes the content of solid-state formations
[17, 18]. Its measurement is difficult to perform applying direct measurement from FID, because NMR equipment has
dead time ^ (in our case ^ = 13 is). Therefore it is necessary to use Solid-Echo pulse sequence (90°x - т - 90°y - т) [19-21] for correct measurement of parameter Ps. Transverse magnetization decay of asphaltenes in Solid-Echo pulse sequence is similar to FID and can be described by equation:
M (t, t) = Ml(t ) + M s (t, t),
Ml(t ) = X Mli (0) • exp
Ms (t, t) = Ms (0, t) • exp
t
T
(1)
2li J f t
2
V
T
In
where Mb(0) is magnetization of liquid component (Lorentzian component) at the maximum of solid-echo signal; Ms(0,r) is magnetization of solid component (Gaussian component) at the maximum of solid-echo signal, which depends on time interval т between RF-pulses; T2u is spin-spin relaxation time of liquid component; T2s is spin-spin relaxation time of solid component.
Solid component magnetization dependence on time т (see Fig.2) can be presented in the form:
Fig.1. Transverse magnetization decay of solid component in the 1H NMR signal for different oil asphaltenes at room temperature 24°C (solid-echo pulse sequence, t = 13 is). Fitting curves computed on the basis of the equations (1) for experimental points are shown by solid curves.
Ms (0, t) = M s(0,0) • exp
>-р\\
in
(2)
where Т2Г is relaxation time of solid component magnetization caused by irreversible processes (flip-flop transitions, multi-quantum transitions); n is power parameter, for which the dependence of іп^^т)) from т" is a linear function. The value of parameter n is defined by the properties of dipole-dipole interactions and the nature of asphaltene solid-state formations.
Fig.2.
2*t,
Solid component magnetization dependence on time interval t in the pulse sequence Solid-Echo for different oil asphaltenes. Fitting curves computed on the basis of the formula (2) for experimental points are shown by solid curves.
Thus, expressions to define part Ps of solid component in the 'H NMR signal of oil can be written down as follows:
P =
M s(t = 0,t = 0) M(t = 0,t = 0) ’ Mb. (t = 0)
M (t = 0,t = 0)
E p+p = i,
(3)
where Pi,is part of liquid component in the 'H NMR signal of oil.
Temperature dependences of part Ps of solid component in the 'H NMR signal of different oil asphaltenes are well described by the equation:
N \P amorph/cryst
Ps(T) = 100-E-----------„ x-------------, (4)
exp
w.
+1
where APsiamorph/cryst is increment of Ps(T) due to mixture component vitrifying (devitrifying) or crystallizing (melting) at cooling (heating); T0i is mean temperature of the phase transition; w{ is parameter, which characterizes the phase transition temperature interval; N is number of phase transitions.
In the equation (4) summation is made on a set of Fermi’s functions, each of which relates to first order phase transition or to glass-transition. The opportunity of Fermi’s function application for computation of observable phase transitions on Ps(T) dependence can be explained by comparing derivative of Fermi’s function with Gaussian distribution function (see Table II), because crystallizing and vitrifying processes are described by Gaussian distribution.
Table II. Ratio of the 4-th moment to the 2-nd moment squared for different distribution functions
i =1
Distribution function Expression for function M4/(M2)2
Gaussian 1 2 (X-*o)2 e w2 w 3
Lorentzian 2 w n 4 - (x - x0)2 + w2 6.988 (for truncated function on the level 1%)
derivative of Fermi’s function 1 1 w - e w + e w + 2 4.2
0,1
0,01-
1E-3-
Dachnaya oil asphaltenes Mordovo-Karmal bitumen asphaltenes Mamurinsk oil asphaltenes
0 10 20 30 40
50 t, ^s
70 80 90 100
Fig.3. Transverse magnetization decay of 1H NMR signal for different oil asphaltenes at room temperature 24°C (solid-echo pulse sequence, t = 11 p.s)
It’s possible to claim that derivative of Fermi's function is suitable for approximate computation of phase transition processes, because ratios of the 4-th moment to the 2-nd moment squared for derivative of Fermi's function and Gaussian distribution function are insignificantly different.
Temperature dependences of 1H NMR signal parameters of asphaltene powders were measured in the temperature range -70^150oC using pulse NMR apparatus with resonance frequency 19.08 MHz on nuclei 1H, dead time tp = 13 ^s of receiver and magnetic field homogeneity 0.01 gauss/cm of electromagnet. This pulse NMR apparatus is manufactured by scientists of molecular physics department of Kazan state university.
Differential scanning calorimetry (DSC)
Asphaltene phase transitions were additionally checked in the range of temperatures 20^150°C by differential scanning calorimeter C80 manufactured by
company Setaram (France). The speed of scanning equals 1 K/minute, hanging weight equals 20 mg.
t, |as
Fig.4. Transverse magnetization decay of 'H NMR signal for different oil asphaltenes at high temperature 145°C (solid-echo pulse sequence, t = 11 p.s)
X-ray structure analysis
X-ray diffractograms of asphaltene powders at room temperature were measured by diffractometer DRON-3M (Russia).
4. Results and discussion
Transverse magnetization decays of 'H NMR signal of different oil asphaltenes at temperatures 24 and 145°C are presented in figures 3 and 4, respectively. It’s possible to see that decays of all asphaltene samples at these temperatures contain Gaussian component (solid component) and at least one Lorentzian component (liquid component).
Results of exponential decomposition of 'H NMR signals of asphaltenes at temperatures 24 and 145°C are presented in the tables III and IV, respectively. Notations in the tables are presented as follows: Pi -total part of liquid components in the 1H NMR signal of asphaltenes with average spin-spin relaxation time T2l; Ps - part of solid component in the 'H NMR signal of asphaltenes with spin-spin relaxation time T2s.
Table ID. Decomposition of 1H NMR signals of asphaltenes at room temperature (24 °C) on exponential components with Lorentzian and Gaussian forms
Samples of oil asphaltene Pl,% T2l, ^s Ps,% T2s, ^s
Dachnaya oil, well №3577 34.7 '6.5 65.3 '4.9
Mordovo-Karmal bitumen 3.2 53.0 96.8 '4.5
Mamurinsk oil, well №2' '6.8 24.4 83.2 '0.6
Table IV. Decomposition of H NMR signals of asphaltenes at high temperature (145 °C) on exponential components with Lorentzian and Gaussian forms
Samples of oil asphaltene Pl,% T2l, ^s Ps,% T2s, ^s
Dachnaya oil, well №3577 50.7 52.5 49.3 '7.9
Mordovo-Karmal bitumen 57.' 33.4 42.9 '6.8
Mamurinsk oil, well №2! 88.7 455.8 U.3 '8.2
Thus, the 'H NMR signals of all asphaltene samples contain a solid component at room temperature (24°C) as well as at high temperature ('45°C). It’s possible to assume that components with Lorentzian decay form relate to mobile side aliphatic chains of asphaltene molecules.
Temperature, deg. Celsius
Fig.5. Temperature dependences of part Ps of solid component measured during heating for different oil asphaltenes (solid-echo pulse sequence, t = '3 p.s). Fitting curves computed on the basis of the formula (4) for experimental points are shown by solid curves.
Temperature dependences of part Ps (Fig.5, 6) and spin-spin relaxation time T2s (Fig.7) of solid component in the 'H NMR signal were measured for investigated asphaltene samples. The part Ps of solid component for all asphaltene samples decreases during heating in the wide temperature range from -70 to '50°C. Ps(T) dependence for Mamurinsk oil asphaltenes only shows temperature hysteresis effect (Fig.6) during heating and cooling in the range of temperatures 90-'05°C, that indicates the first order phase transition. This phase transition is characterized by the change of solid component magnetization APscryst = 52.5 ± 3.5% and the average temperature 98.8 ± 2°C. Part Ps of solid component decreasing during heating with missing temperature hysteresis effect in this temperature range can be explained by devitrifying side aliphatic chains of asphaltene molecules and showing up their mobility. The glass-transition of Mamurinsk oil asphaltenes is characterized by the change of solid component magnetization APsamorph = 33.6 ± 2.5% with the average temperature 27.'°C. The glass-transitions of
80 85 90 95 100 105 110
Temperature, deg. Celsius
115 120
Fig.6.
Temperature dependences of part Ps of solid component measured during heating and cooling for Mamurinsk oil asphaltenes (solid-echo pulse sequence, t = '3 ¡is). Fitting curves computed on the basis of the formula (4) for experimental points are shown by solid curves.
Fig.7.
21 20 19 18 17 16 15 1 14 13 12 11 10 9
-60 -40 -20 0 20 40 60 80 100 120 140
Temperature, deg. Celsius
Temperature dependences of spin-spin relaxation time T2s of solid component measured during heating for different oil asphaltenes (solid-echo pulse sequence,
A '
, Dachnaya oil asphaltenes - Mordovo-Karmal bitumen asphaltenes A A *
‘ Mamurinsk oil asphaltenes
■ ■»! 9 ■ ■■ A i ■ ® 9 3 - ■ A A ■ n> • 9 . A •
aaaaaAa aaAaA
t = '3 is)
600 n
550-
500-
450-
400-
350-
300-
250-
200-
150-
100-
50-
0-
Dachnaya oil asphaltenes Mordovo-Karmal bitumen asphaltenes Mamurinsk oil asphaltenes__________
Mamurinsk oil asphaltene
MK bitumen & Da4naya oil asphaltenes
10 15 20 25
30 35 40
2*©, degree
45 50 55 60 65
Dachnaya oil asphaltenes are characterized by two changes of solid component magnetization APs'amorph = 2'.5 ± 2.0% and APs2amorph = 34.5 ± 2.5% with the average temperatures -46.9 and 59.6°C, respectively. The glass-transitions of Mordovo-Karmal bitumen asphaltenes are characterized by two changes of solid component magnetization
amorph 2 = •
2.0°C and 87.2°C,
APs'amorph = '2.6 ± L5% and APs2amorph = 29.8 ± 2.5%
Fig.8.
X-ray diffractograms for different oil asphaltene powders at room temperature 20°C
samples. An enormous endothermic effect in the temperature asphaltenes, which can be attributed to melting crystal phase of high-molecular paraffins. It should be noted that paraffin crystal phase presence is additionally confirmed by IR spectroscopy data, according to which the Mamurinsk
with the average temperatures respectively.
It should be noted that so wide a range of glass-transition temperatures -50^90°C of side aliphatic chains of asphaltenes can be caused by distribution of their length, variation of their isomeric structure (saturated and unsaturated aliphatic chains) and the insertion of heteroatoms (O, N and others).
Temperature dependences of spin-spin relaxation time T2s of solid component magnetization for Dachnaya oil asphaltenes and Mordovo-Karmal bitumen asphaltenes are changed in the same manner and do not show uneven changes. However, the spinspin relaxation time T2s for Mamurinsk oil asphaltenes during heating in the range of temperatures from 60 to U0°C sharply enough increases from U to '9 is, that is well explained by melting one of the components of multiphase system at these temperatures.
Thus, the observed first order phase transition at temperature 98.8 ± 2°C for Mamurinsk oil asphaltenes can be explained by the presence of high-molecular hydrocarbonic impurity. The data of temperature dependences of 'H NMR relaxation show that there are no other impurities of high-molecular hydrocarbons in the Dachnaya oil asphaltenes and Mordovo-Karmal bitumen asphaltenes.
The measurements of X-ray diffractograms (see Fig.8) of investigated asphaltenes at room temperature have been performed to confirm this hypothesis. The data of X-ray structure analysis show that Dachnaya oil asphaltenes and Mordovo-Karmal bitumen asphaltenes have amorphous structure, and that Mamurinsk oil asphaltenes contain both amorphous and crystal phases.
X-ray diffractograms (Fig.9) of spectral pure graphite and pure paraffin (mixture of paraffin hydrocarbons C^-Css) were measured to establish the type of lattice for crystal phase of Mamurinsk oil asphaltenes. Figure '0 shows that X-ray diffractograms of Mamurinsk oil asphaltenes and pure paraffin are very similar. Now, we can conclude that the crystal phase of Mamurinsk oil asphaltenes is characterized by the same lattice spacing as crystal paraffin.
Referring to the DSC data for investigated asphaltene samples (Fig.U), we notice two regions on the thermograms of asphaltenes in the temperature range 90^U0°C and 110^130°C. Some doubts arise with regard to thermal processes taking place in the temperature range 110^130°C for all asphaltene range 90^U0°C is observed for Mamurinsk oil
1SGG
14GG
^GG
12GG
11GG
1GGG
9GG
SGG
7GG
6GG
SGG
4GG
300
2GG
1GG
G
spectral pure graphite
pure paraffin
...-spectral pure graphite -
jl * pure paraffin "
і
oil asphaltenes are characterized by high content of unbranched aliphatic chains. The presence of the ordered structures in the Mamurinsk oil asphaltenes is confirmed by the peak on frequency 888 cm-1.
In conclusion, it’s possible to evaluate the content of crystal phase of high-molecular paraffins in the Mamurinsk oil asphaltenes using the next equation:
ciyst AP cryst CH amorph
m
asph
asph
amorph
asph
AP
amorph
C
H cryst
asph
masph _ mCIystasph + m
C Hamorph ___________c H
asph asph
amorph
asph
(5)
1G 1S 2G 2S
3G 35 4G 2*©, degree
45 5G 55 6G 65
C Hcryst ___________с
asph
Fig.9.
X-ray diffractograms for spectral pure graphite and pure paraffin at room temperature 20°C
paraff 5
where masph, mcrystasph, mamorphasph are weights of asphaltenes, their crystal and amorphous phases; ^scryst (^,5™“^) is increment of Ps(T) due to melting (devitrifying) of asphaltene components
(APsamorph _ 1GG%-APscryst; ^scryst = 52.5 і 3.5% for
Mamurinsk oil asphaltenes), CHasph and CHparaf are amounts of protons in asphaltenes (about 8 wt.%) and paraffins (about 14.8 wt.%), respectively. It’s possible to approximately estimate the weight content of crystal phase of high-molecular paraffins in the Mamurinsk oil asphaltenes by using equations (5). The value 37.5 wt.% obtained from 'H NMR data is in quite good agreement with the value 4б wt.% obtained from DSC data.
5. Summary
The comparison of 'H NMR relaxation and DSC methods for studying temperature properties of asphaltenes shows that temperature dependence of part Ps of solid component in the 'H NMR signal allows to detect equilibrium first-order phase transitions (presence of temperature hysteresis effect) and non-equilibrium glass-transitions (absence of temperature hysteresis effect). However, we can identify only first-order phase transitions from DSC curves.
The results of the complex research of asphaltenes, extracted from three different oil samples (Dachnaya oil with high content of asphaltenes; Mordovo-Karmal bitumen with high content of resins and Mamurinsk oil with high content of paraffins), by 'H NMR relaxation, DSC, X-ray structure analysis and IR spectroscopy methods show that asphaltenes are able to contain the crystal phase of high-molecular paraffins. We believe there is no answer to the question: whether these paraffin chains are bonded to asphaltenes, i.e. whether they are structural elements of asphaltene molecules, or these paraffin chains are Fig.11. DSC curves of heating for different oil asphaltenes individual paraffin molecules coprecipitated together
with asphaltenes [1, 8-14]. It is necessary to point out that high-molecular side aliphatic chain of asphaltene molecule is able to crystallize of its own at cooling. Also, the hypothesis about the opportunity of crystallization of high-molecular side aliphatic chains of asphaltene molecules is partially confirmed on the basis of the dissolution stage of investigated asphaltenes in benzene, which has to separate paraffin and asphaltene molecules.
Finally, it should be noted that low-molecular side aliphatic chain of asphaltene molecule is only able to vitrify at cooling. Such a wide range of glass-transition temperatures -50^90°С of side aliphatic chains of asphaltene molecules can be caused by distribution of their length, variation of their isomeric structure (saturated and unsaturated aliphatic
6GG
55G
5GG
45G
4GG
350
300
25G
2GG
15G
5G
G
30 З5 4G 2*©, degree
65
Fig.10. Comparison of X-ray diffractograms for Mamurinsk oil asphaltenes and pure paraffin
1.GG
G.75
G.5G
G.25
G.GG
$ -G.25
at -G.5G
-G.75
-1.GG
-1.25
9G 1GG 11G 12G ^G
Temperature, deg. Celsius
15G
chains) and the insertion of heteroatoms (O, N and others). Additional research is needed to understand in detail the influence of side aliphatic chains structure of asphaltene molecules on their glass-transition temperatures.
Acknowledgement
We thank R.A. Nazipov for his help in measurements of X-ray diffractograms. The work is supported by Schlumberger R&D and the Federal Agency of Science (under contracts N 02.451.11.7019 and N 02.445.H.7402).
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