2233r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels 2010, 24, 2233–2239 : DOI:10.1021/ef900819pPublished on Web 01/20/2010

Effects of Temperature and Pressure on the Phase State ofOils andAsphaltene Solutions

Observed Using Dielectric Spectroscopy†

Rustem Z. Syunyaev* and Victor V. Likhatsky

Department of Physics, Gubkin Russian State University of Oil and Gas, Moscow, Russia

Received July 30, 2009. Revised Manuscript Received December 24, 2009

The dielectric spectra ofCaspianOil weremeasured at atmospheric pressure in temperature range from123to 203K.Also dielectric responses ofKumkolskayaOil and asphaltene solutions were obtained in pressurerange up to 1 GPa and in temperature interval 250-320 K. The dependences of glass transitiontemperature (Tg) on pressure for oil samples were obtained for the first time by dielectric spectroscopy,they increase approximately from 125 at atmospheric pressure to over 280K at 1GPa. The phase diagramsof oil samples were constructed using Williams-Landell-Ferry equation. For asphaltene solutions, therelaxation times dependon viscosity only prior to the beginning of a crossover region nearby crystallizationpressure of solvents. It is shown that the dielectric strength is not proportional to total concentration ofasphaltenes in polydisperse solutions. The values of the thermal expansion coefficient of the free volume(Rf), the isothermal compressibility of the free volume (βf), and the relative free volume at Tg (fg) wereobtained for the examined oils. The received data are in agreement with the data obtained by independentthermophysical methods.

I. Introduction

Any oil is a complex system containing many hydrocar-bons, which differ significantly both in structure and physi-cochemical properties.When changing parameters of externalinfluences (pressure, temperature, frequency of the electro-magnetic field, etc.), such a system is continuously evolving.Differences in the degree of intermolecular interactions ofcomponents lead to the transformation of its phase state. Thephase behavior of oil system is inmany respects determinedbythe content of resin-asphaltene substances. The aggregat-ion processes depend on the thermodynamic parameters.1

Resin-asphaltene substances determine the ability of oilsystems to aggregate, as well as their surface-active andviscoelastic properties.2 The polarity of asphaltenes and resinsmakes it possible to register the dielectric response. Oil is asystem of components with different solubility parameters: itcan be presented as a solution of polar high-molecular resin-asphaltene components in nonpolar or low-polar solvents.3

The structural organization of such systems is determined bysupramolecular scale. In contrast to usual polymer solutionsin which there is regularity at least along the molecular chainof the polymer, the degree of disorder in the oil system ismuch

higher. Suchmolecular-disorderedorganic systems are amonga class of substances called organic glass.4

Pressure and temperature determine the distribution func-tion of relaxation times (RT). For most systems, the principleof thermobaric equivalence holds. This principle implies thatRT are reduced with increasing temperature or with decreas-ing pressure. In relaxation spectrometry the principle is alsosupplemented by the frequency equivalence.5 Generally, themeasured value is a function of temperature (T), pressure (P),frequency (ν).

According to conventional classificationR-process (dipole-segmental losses) is associated with quasi independent fluc-tuations of largemacromolecular segments,β-process (dipole-grouping losses) is responsible for the small-scale movementsof chemical groups of the macromolecules. λ-Process reflectsthe slow process of relaxation of some ordered clusters. Thesecommon universal processes are observed irrespective of thechemical nature of substances (Figure 1). Generally, theremay be several relaxation processes (RP) in a complex multi-component system, such as an oil system. In a multicompo-nent oil system, the RT spectrum is observed. This leads to acomplication of a common interpretation of the dielectricresponse.

The conductivity ofmaterials is determined by the competi-tion of several mechanisms of charge transport: phoretic,ionic, and hopping electron-hole. The presence of all typesof conductivity is typical for oil; however, their contributionsare not identical under different conditions. The influence ofthermodynamic parameters on conductivity mechanisms isdifferent. Oil can be attributed to a class of organic semicon-ductors in magnitude of conductivity.

The dielectric spectroscopy (DS) method is frequently usedfor the analysis of oil systems at atmospheric pressure. The

† Presented at the 10th International Conference on Petroleum PhaseBehavior and Fouling.

*To whom correspondence should be addressed. E-mail: syunyaev@gubkin.ru.(1) Akbarzadeh,K.; Hammami, A.; Kharrat, A.; Zhang, D.; Allenson,

S.; Creek, J. L.; Kabir, S.; Jamaluddin, A.; Marshall, A. G.; Rodgers,R. P.; Mullins, O. C.; Solbakken, T. Oilfield Rev. 2007, 19, 22–43.(2) Structures and Dynamics of Asphaltenes, 1st ed.; Mullins, O. C.,

Sheu, E. Y., Eds.; Plenum Press: New York, 1998.(3) Wiehe, I. A. Process Chemistry of Petroleum Macromolecules, 1st

ed.; CRC Press: Boca Raton, FL, 2008.(4) Kob, W. In Soft and Fragile Matter: Nonequilibrium Dynamics,

Metastability and Flow; Cates, M. E., Evans, M. R., Eds.; Institute ofPhysics Publishing, Copublished by Scottish Universities Summer School inPhysics: Bristol, U.K., 2000.

(5) Bartenev, G. M.; Barteneva, A. G. Relaxation Properties ofPolymers; Khimiya: Moscow, Russia, 1992.

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Energy Fuels 2010, 24, 2233–2239 : DOI:10.1021/ef900819p Syunyaev and Likhatsky

substantiation of its applicability was given earlier.2,6,7 TheDS method is used for the description of aggregation pro-cesses of asphaltenes,6,8-10 the dipole moments of asphal-tenes.11,12 Correlation of structural and dynamic parametersof oil with the parameters of the dielectric responsewas shownin the literature.13-16 Various frequency ranges were investi-gated.17,18 Time-domain spectroscopy was used by severalgroups.13,19 The influence of the pressure on the dielectricresponse is less known.10,18

The aim of this work was to study the behavior of dielectricparameters, the nature of the conductivity of oil and asphal-tene solutions in the range of pressures and temperatures nearglass transition (GT) and crystallization. The dielectric re-sponse is a well-known tool for the analysis of such processesin polymers.5,20-23

II. Materials and Methods

II-1. Oil Samples. Caspian oil (oil 1)24 and Kumkolskayaoil (oil 2) were used as samples. Physicochemical para-meters of dead Caspian and Kumkolskaya oils are pre-

sented in Table 1 (the term “dead oil” means that the oilsamples do not contain dissolved gas). Viscosity and densityof oil 2 have been measured at atmospheric pressure(ASTM D 7042 and ASTM D 5002, respectively).25,26

Values of viscosity and density of oil 2 are presented inTable 2.

II-2. Asphaltene Solutions. The asphaltenes were extractedfrom oil 2 according to the method described in ASTMD 6560.27 Toluene and benzene were used as a solvent. Thereasons for choosing the solvents are the following:

(1) Nonpolarity or low polarity of their molecules (inorder to determine the contribution of polar compo-nents to spectra).

(2) Asphaltene aggregation processes are going activelyin the critical region near the phase transition. Thecapabilities of the experimental setup allowed to studythese areas for pure solvents.

(3) Good solubility of asphaltenes (to obtain stablecolloidal solutions).

Parameters of solvents: benzene and toluene (Merck “proanalysi”, Germany) are listed in Table 3.28 Toluene (A/T)and benzene (A/B) solutions of asphaltenes were studied atconcentration of 3 wt % (27 g/L). A/T solutions were alsostudied at 0.3 wt % (2.7 g/L). Solutions were prepared bystandard procedure.

Benzene and toluene vary significantly in physical proper-ties. The melting temperature of toluene is much lower thanthat of benzene at atmospheric pressure (Table 3). Thecrystallization pressures of benzene29-31 and toluene32 areequal to 0.058 and 0.843 GPa at 298 K, respectively. The

Figure 1. General view of relaxation processes (RP) in disorderedmaterials.

Table 1. Physicochemical Parameters of Dead Caspian and

Kumkolskaya Oils

Caspian Oil(oil 1)

Kumkolskaya Oil(oil 2)

density (kg/m3) 801 844pour point (K) 235 284paraffin (wt %) 4.5 12asphaltene and resins (wt %) 2.7 7.7fractional composition %, up to623 K

76.4 55.3

Table 2. Viscosity and Density of Oil 2 (Measured at

Atmospheric Pressure)

temperature (K)

292 310 320

viscosity (mPa 3 s), η 14.8 8.0 5.3density (kg/m3), F 844 826 815

(6) Sheu, E. Y.; De Tar, M. M.; Storm, D. A. Fuel 1994, 73, 45–50.(7) Sheu, E. Y. J. Phys.: Condens. Matter 1996, 8, A125–A141.(8) Maruska, H. P.; Rao, B. Fuel Sci. Technol. Int. 1987, 5, 119–168.(9) Sheu, E. Y.; Storm, D. A.; Shields, M. B. Energy Fuels 1994, 8,

552–556.(10) Syunyaev, R. Z.; Abid, R. S. Colloid J. 1994, 56, 180–184.(11) Goual, L.; Firoozabadi, A. AIChE J. 2002, 48, 2646–2663.(12) Wattana, P.; Fogler, H. S.; Yen, A.; Carmen Garc�ia, M. D.;

Carbognani, L. Energy Fuels 2005, 19, 101–110.(13) Saraev,D. V.; Lunev, I. V.; Yusupova, T.N.; Tagirzyanov,M. I.;

Yakubov, M. R.; Gusev, Y. A.; Romanov, G. V. Oil and Gas Business[Online] 2005, http://www.ogbus.ru/eng/authors/Saraev/Saraev_1e.pdf(accessed June 3, 2005).(14) Sheu, E. Y.; Acevedo, S. Fuel 2006, 85, 1953–1959.(15) Syunyaev, R. Z.; Safieva, R. Z.; Safin, R. R. J. Petrol. Sci. Eng.

2000, 26, 31–39.(16) Rejon, L.; Manero, O.; Lira-Galeana, C. Fuel 2004, 83, 471–476.(17) Goual, L. Energy Fuels 2009, 23, 2090–2094.(18) Syunyaev, R. Z.; Balabin, R.M. J. Dispersion Sci. Technol. 2007,

28, 419–424.(19) Syunyaev, R. Z.; Karpov, S. A.; Karpova, V. V. Chem. Technol.

Fuels Oils 2001, 37, 131–133.(20) Sperling, L. H. Introduction to Physical Polymer Science, 3rd ed.;

Wiley-Interscience: New York, 2001.(21) Mierzwa, M.; Floudas, G.; �Step�anek, P.; Wegner, G. Phys. Rev.

B 2000, 62, 14012–14019.(22) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed.; Wiley:

New York, 1980.(23) Floudas, G. In Broadband Dielectric Spectroscopy; Kremer, F.,

Sch€onhals, A., Eds.; Springer: Berlin, 2003.(24) Nadirov, N. K. Tengiz;Sea of Oil, Sea of Challenges; Fylym:

Almaty, Kazakhstan, 2003.

(25) Standard Test Method for Dynamic Viscosity and Density ofLiquids by Stabinger Viscometer (and the Calculation of KinematicViscosity); ASTM D 7042-04; American Society for Testing and Materials(ASTM): Philadelphia, PA, 2004.

(26) Standard Test Method for Density and Relative Density of CrudeOils by Digital Density Analyzer; ASTM D 5002-99(2005); AmericanSociety for Testing and Materials (ASTM): Philadelphia, PA, 2005.

(27) Standard TestMethod forDetermination of Asphaltenes (HeptaneInsolubles) in Crude Petroleum and Petroleum Products; ASTM D 6560-00(2005); American Society for Testing and Materials (ASTM): Philadel-phia, PA, 2005.

(28) CRCHandbook Chemistry and Physics, 85th ed.; Lide, D. R., Ed.;CRC Press: Boca Raton, FL, 2004.

(29) Bridgman, P. W. Phys. Rev. 1914, 3, 153–203.(30) Easteal, A. J.; Woolf, L. A.; Wilson, F. L. Int. J. Thermophys.

1985, 6, 275–284.(31) Xu, W.; Zhu, R.; Tian, Y.; Li, H. J. Chem. Eng. Data 2007, 52,

1975–1978.(32) Osugi, J.; Shimizu,K.;Yasunami,K.;Moritoki,M.;Onodera,A.

Rev. Phys. Chem. Jpn. 1968, 38, 90–95.

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Energy Fuels 2010, 24, 2233–2239 : DOI:10.1021/ef900819p Syunyaev and Likhatsky

dependences of viscosity of benzene and toluene on pressureare known.33

II-3. Experimental Setup. Dielectric spectra of Oil 1 wereobtained on broadband dielectric spectrometer NOVO-CONTROL34 with a temperature control system QUATROCryosystem. Spectra were measured in the frequency rangefrom 10-2 to 105 Hz at temperatures from 123 to 203 K atatmospheric pressure.

Dielectric parameters of Oil 2 were measured by thefrequency response analyzer in the frequency range from10-3 to 106 Hz. The analyzer was developed at PhysicalFaculty of Umea University (Sweden).35,36 The standardimpedance analyzer HP 4192 A, which measure the capaci-tance of the condenser with sample directly, was used in ahigh-frequency range (102-106 Hz). In a low-frequencyrange (10-3-102 Hz) the special comparison circuit wasdeveloped. The control was carried out by computer “MicroVAX II” which set a necessary frequency step (in a logarith-mic scale), supervised pressure and temperature of a sample.

Dielectric spectra of capacitance (proportionally to di-electric permittivity) of three types were used for analysis ofRP: (a) oil 1 at atmospheric pressure in the temperatureinterval from 123 to 203 K, (b) oil 2 in the range of pressureup to 1 GPa at temperatures from 250 to 320 K, and (c) twoconcentrations of asphaltenes in two solvents at tempera-tures 294 and 312 K.

III. Results

The dielectric spectra of both oil samples are similar. Anexampleof thedielectric spectraofOil 1 is presented inFigure2.The loss peak is visible at temperatures from 123 K. Theprocess caused by conductivity begins to appear only from

173 K (in the frequency range from 10-2 to 101 Hz). Both RPshift to higher frequencies with increasing temperature. Treat-ment shows that the loss peak can be interpreted as acombined R and β-process. The rise of temperature, inaccordance with the general principle of relaxation thermo-baric-frequency equivalence, leads to a shift of the maximumloss in a range of high-frequencies. The increase of pressure,on the contrary, leads to a shift of themaximum loss in a rangeof low-frequencies.

The dielectric response of the conduction current is ob-served in a low-frequency range (10-2-101 Hz). It covers thepeak, which characterizes the dipole-cluster loss (λ-process)(Figure 1). Peak, characterized by R-process, is clearly visiblein the frequency range from 103 to 105 Hz. It is this area thatwas allocated for the analysis of oils.14

“Freezing” of any process (λ, R or β) means a change ofmobility of kinetic units and therefore change of entropyin configuration space. Such structural reorganizationreflects the phase transitions in the system and allows torelate these phase transitions to the so-called entropy phasetransitions.37,38

Typical spectra of asphaltene solutions are presented inFigure 3. The conductivity of asphaltene solutions is quitehigh and λ-process is practically completely covered by it.However it is possible to separate them in the phase transitionregion.

IV. Data Treatment and Discussion

IV-1. Oil 1 (Caspian Oil). Glass Transition through Tem-

perature.Dielectric permittivity is a complex function of ν,P,and T

ε�ðν,P,TÞ ¼ ε0ðν,P,TÞ- iε00ðν,P,TÞ ð1Þwhere ε0 and ε00 are real and imaginary parts of the complexdielectric function ε*, respectively. The real and imaginary

Table 3. Parameters of Solvents

solvent

molar

mass

(g/mol)

density

(g/cm3)

melting point at

atmospheric

pressure (K)

dipole

moment

(D)

isothermal

compressibility

(Pa-1) at 293 K

benzene 78.11 0.88 278.4 0 0.966� 10-9

toluene 92.14 0.87 178 0.37 0.896�10-9

Figure 2. Dielectric spectra of oil (sample oil 1: low temperatures,atmospheric pressure). Relaxation times (RT) of R-process increasewith decreasing temperature. (tan δ = ε0 0/ε0 is the tangents of thephase angle between ε0 and ε0 0).

Figure 3. Typical dielectric spectrum of asphaltene solution (A/T,3 wt%, 312 K, 0.1 GPa). Open and filled squares show the real (C0)and imaginary (C0 0) parts of complex capacitance, respectively.

(33) Bridgman, P. W. Proc. Am. Acad. Arts Sci. 1926, 61, 57–99.(34) Novocontrol Technologies GmbH & Co. KG. http://novocon-

trol.de/.(35) Forsman, H.; Andersson, P.; B€ackstr€om, G. J. Chem. Soc.,

Faraday Trans. 2 1986, 82, 857–868.(36) Forsman, H. Mol. Phys. 1988, 63, 65–75.

(37) Frenkel, D. In Statistical Physics: Invited Papers from STAT-PHYS 20; Gervois, A.; Iagolnitzer, D.; Moreau, M.; Pormeau, Y.; North-Holland: Amsterdam, 1999; pp. 26-38.

(38) Frenkel, D. In Soft and Fragile Matter: Nonequilibrium Dy-namics, Metastability and Flow; Cates, M. E., Evans, M. R., Eds.; Instituteof Physics Pub., Copublished by Scottish Universities Summer School inPhysics: Bristol, U.K., 2000.

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Energy Fuels 2010, 24, 2233–2239 : DOI:10.1021/ef900819p Syunyaev and Likhatsky

parts of the dielectric response are related by the Kramers-Kronig relations,39 but in the experiment, they are measuredseparately.

RT of R-processes for Oil 1,2 are obtained using Havri-liak-Negami (HN)40 model with an additional term takinginto account the conductivity contribution. Debye equationrelates the frequency of maximal loss (νp) to RT (τ)

2πνpτ ¼ 1 ð2ÞRT is the main parameter that determines the hierarchy ofstructural levels. As GT temperature is approached, thegrowth of RT reflects the consecutive “freezing” of molecu-lar and more large-scale movements. The GT means atransition of disordered liquid phase into the amorphousphase with high value of viscosity (1010 poise).22,41 ThereforeRT of R-process increases significantly. GT is not a thermo-dynamic transition, but a kinetic. In practice, it is assumedthat logarithm of RT (R-process) in the glassy state exceedsRT in the liquid approximately by a factor of 10. Figure 4shows the temperature dependence ofRTofR-process for oil1 at atmospheric pressure.

Temperature dependence of RT is defined by well-knownWilliams-Landel-Ferry (WLF) equation22,42

logτgτ¼ ðB=2:303fgÞðT - TgÞ

ð fg=Rf ÞþT - Tgð3Þ

where B is the empirical constant, Tg is the GT temperature,fg is the relative free volume atTg,Rf is the thermal expansioncoefficient of free volume.

The change of relative free volume with temperature f =fgþ Rf(T-Tg) is presented in Figure 5. The GT temperature(Tg = 114.7 K) at atmospheric pressure and the thermalexpansion coefficient (Rf= 0.00138K-1) were calculated for

oil 1. The calculated value of fg for oil 1 ( fg = 0.0261) is inagreement with reported data on fg for polymers ( fg≈ 0.025for many polymers).20,23

The temperature coefficient of activation energy (WR)20 is

estimated for oil 1. This coefficient is a function of tempera-ture and is an analogue of activation energy for cooperativesegmental movement. As GT temperature is approached, itgrows for R-processes. It is possible to receive the followingequation from general Arrhenius relation

WðTÞR ¼ 2:303 3R 3d log τ

d 1=Tð4Þ

where R is the gas constant.Temperature coefficient of activation energy is shown as

inset in Figure 4.IV-2. Oil 2 (Kumkolskaya Oil) Glass Transition through

Pressure. Spectra of oil 2 are investigated in the pressurerange from 0.1 up to 1.0 GPa. Processing of spectra wascarried out in the same manner as for oil 1. Dependence ofRT on pressure was obtained at fixed temperatures (Figure 6).The data were fitted by the modified and simplified WLFequation within the framework of the free volume theory22

logτ

τa¼ D 3P

2:303 3 ðPg - PÞ ð5Þ

whereD is a dimensionless fitting parameter; τa is the RT atatmospheric pressure;Pg is the pressure ofGT at a constanttemperature. Pg values at fixed temperatures define bor-ders of GT region. Results are presented in Figure 7.Earlier the GT region of Kumkolskaya Oil was studiedby a complex of thermophysical methods (the differentialscanning calorimetry and the transient hot-wire method).43

The results of these studies are shown in Figure 7 forcomparison. The curve of GT for oil 2 was constructedby analogy with the curves of previous work.43 The GTtemperature for oil 2 is equal to 136.8 K. As a firstapproximation, assume that the derivatives dTg/dP for oils(oil 1 and oil 2) are equal; Figure 7 shows the expected GTregion for oil 1.

Figure 4. RT of R-process vs temperature for oil 1 Inset: Change oftemperature coefficient of activation energy with temperature.Explanation in text.

Figure 5. Relative free volume of Oil 1 vs temperature at atmo-spheric pressure.

(39) Jonscher,A.K.Dielectric Relaxation in Solids; ChelseaDielectricsPress: London, 1983.(40) Havriliak, S.; Negami, S. Polymer 1967, 8, 161–210.(41) Sazhin, B. I.; Lobanov, A. M.; Eidelnant, M. P.; Koikov, S. N.;

Romanovskaya, O. S. Electrical Properties of Polymers; Khimiya:Leningrad, Russia, 1970.(42) Williams, M. L.; Landel, R. F.; Ferry, J. D. J. Am. Chem. Soc.

1955, 77, 3701–3707.(43) Kutcherov, V. G.; Lundin, A.; Ross, R. G.; Anisimov, M. A.;

Chernoutsan, A. I. Int. J. Thermophys. 1994, 15, 165–176.

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Energy Fuels 2010, 24, 2233–2239 : DOI:10.1021/ef900819p Syunyaev and Likhatsky

Within the framework of the thermodynamic approach,the GT is similar to the second order phase transition.20

Differences in Tg values are determined by the componentcomposition of oil at atmospheric pressure (Table 1). Thepour point of oil 1 correlated with GT temperature44 issignificantly lower than the pour point of oil 2.

The values of fg and Rf were determined earlier for Oil 1. Itmight be supposed that the value of Rf for oil 2 would beequal to the value of Rf for oil 1.

20,22 Using a value of dTg/dPat atmospheric pressure the coefficient of isothermal com-pressibility (βf)

20 for oil 2 is calculated

βf ¼ Rf 3∂T

∂P

� �τ

ð6Þ

Measured value (βf = 3.7� 10-10 Pa-1) is a little lower thanthe reported data in the literature.45 Probably the reason isthat the calculation is carried out at GT temperature. With a

knowledge of Rf and βf, it is possible to estimate the relativefree volume in oil for any pressures and temperatures by eq 7

f ðT ,PÞ ¼ f0 þRfðT -TgÞ- βfðP-PgÞ ð7Þwhere f0 is the relative free volume at Tg, Pg.

IV-3. Oil Viscosity at Low Pressures. Reach-throughconductivity current is inversely proportional to frequency(Icond � σ0/ν), where σ0 designates static electrical conduc-tivity of the DC. Measured values of an imaginary part ofcomplex capacitance on a frequency of 10-3 Hz enable us todetermine σ0.

In the general case, movement of different charge carrierssuch as colloid particles, ions, and single charges causesconductivity of materials. Contributions of different typesof oil conductivity are not identical under various condi-tions. Phoretic conductivity associatedwithmovement of thecharged particles decreases with increase of pressure. Attemperatures above pour point of oil 2 (284 K) the increaseof pressure results in reduction of conductivity. In this case,the phoretic and ionic conductivities are main contributionsto total conductivity. As a first approximation, the viscosity,and both phoretic and ionic conductivities are determined bythe kinetic mobility of charged particles at low pressures. Inthe literature it is known as Walden’s rule46,47

ση ¼ const ð8Þwhere σ is the conductivity and η is the viscosity.

With increase of viscosity, conductivity of system shoulddecrease and vice versa.Decrease of viscositywith increase oftemperature or with reduction of pressure should result ingrowth of conductivity. The data presented in Figure 8 showthat the conductivity linearly decreases with pressure over arange of pressures up to 0.3 GPa.

Conductivity can be represented as follows:

σ ¼ A exp -EðP,TÞRT

� �ð9Þ

where A is the pre-exponential factor determined by thenature of the liquid, E is the Arrhenius activation energy.

Figure 6.Relaxation times (RT) ofR-process vs pressure at differenttemperatures for oil 2.

Figure 7. Phase diagram for oil samples. The glass transition (GT)region determined by Kutcherov et al. 43 is shown for comparison.

Figure 8.Conductivity of oil 2 vs pressure at different temperatures.

(44) Kutcherov, V. G.; Chernoutsan, A. I. Int. J. Thermophys. 2006,27, 474–485.(45) Handbook of Table for Applied Engineering Science, 2nd ed.; Bolz,

R. E., Tuve, G. L., Eds.; CRC Press: Cleveland, OH, 1973.

(46) Walden, P. Z. Phys. Chem. 1906, 55, 207–249.(47) Bockris, J. O.; Reddy, A. K. Modern Electrochemistry: Ionics,

2nd ed.; Plenum: New York, 1998.

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Energy Fuels 2010, 24, 2233–2239 : DOI:10.1021/ef900819p Syunyaev and Likhatsky

At low pressures, oil is far from the glassy state. It allows usto expand the Arrhenius activation energy in a series and toobtain dependence of viscosity on pressure under isothermalconditions

log η ¼ log η0 þ 0:431

RT

∂E

∂PP ð10Þ

where η0 is the viscositymeasured at atmospheric conditions.Derivative ∂E/∂P is determined from the slope of linear partsof conductivity isotherms shown in Figure 8.

Figure 9 shows the relative oil viscosity (η/η0) dependenceon pressure. The similar diagram reported by Muskat48 ispresented for the comparison.

Pressure dependence of density of oil 2 was received ear-lier.18

IV-4. Asphaltene Solutions.As previouslymentioned, oil isa multicomponent system containing structural kinetic units(clusters). In these kinetic units, cores are formed by asphaltenenanoaggregates surrounded by solvate shells with consistentlydecreasing potentials of intermolecular interactions. Undercertain conditions, arising units interact among themselves,forming the spatial net (i.e., gel). It defines viscoelastic behaviorof oil. Asphaltene solutions form sols in individual solvents.1

The spectra of imaginary part of dielectric permittivity(Figure 3) show that conductivity covers dipole-cluster λ-relaxation. Therefore, they were described by a superposi-tion of a conductivity contribution and two HN RP. Theparameters of HN model for λ-process covered by conduc-tivity is found to be similar to the parameters of Debyemodel. Asphaltene clusters do not interact with each other.This process is basic.

As discussed above, theGTprocess is characteristic for oil.As opposed to oil, the dielectric response of asphaltenesolutions will be determined by crystallization of solvent.

IV-4.1. Asphaltenes in Benzene (Low Pressures). Compar-ison with spectra of oil shows that dielectric response iscaused by presence of asphaltene components. Solution A/Bat 3 wt% represents sol system. As the critical point (pressurecrystallization of benzene) is approached, total RT decreasessteeply (Figure 10). It occurs before the critical pressure.Withfurther increase in pressure, the rotational degrees of freedomof the clusters are “frozen” because of the crystallization ofsolvent. At pressure above crystallization pressure of benzene,rotational motion of clusters is not any longer responsible forrelaxation but β-process defines relaxation. The increase ofpressure will repress dipole-segmental movements, and in thesequel the relaxation will be determined by movement ofindividual polar groups (dipole-grouping losses).

Average radius of quasispherical asphaltene clusters wascalculated by Debye equation (eq 11) using viscosity valuesdetermined by interpolation33

τ ¼ 3Vη

kBTð11Þ

where V is the asphaltene cluster volume and kB is theBoltzmann constant. The obtained values of average radiusof quasispherical asphaltene clusterswere equal to 200( 10 nmin the range of pressure up to critical.10

IV-4.2. Asphaltenes in Toluene (A/T) (HighPressures).Thedependences of RT on pressure for A/T solutions at twoconcentrations and two temperatures are shown in Figure 11.The increase of τ with pressure occurs due to the rise in theviscosity. As the pressure crystallization point of toluene atlower temperature (294 K) is approached, a jump-like de-crease of RT for high concentrated solution (3 wt %) occursat pressure 0.7 GPa. High concentrated solution (3 wt %) atlower temperature is similar in properties to glass-formingsystems (like oil). At higher temperature (312K), this effect isnot observed. Probably, the crystallization pressure of to-luene and crossover region is shifted to higher pressures(above 1 GPa). The decrease of concentration by a factorof 10 (0.3 wt %) leads to a shift of the crossover region tohigher pressures. Asphaltene nanoaggregates do not distortthe matrix of the solvent and the crossover region does notarise at low temperature (294 K).

IV-4.3. Dielectric Strengths and Dipole Moments. Theconcept of the dielectric strength (Δε) as the difference

Figure 9. Viscosity of oil 2 vs pressure. The viscosity is estimatedfrom conductivity data at low pressures. Inset: Comparison withknown data.48

Figure 10.Relaxation times (RT) vs pressure (A/B, 3 wt%, 294 K).

(48) Muskat, M. Physical Principles of Oil Production, 1st ed.;McGraw-Hill Book Co.: New York, 1949.

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Energy Fuels 2010, 24, 2233–2239 : DOI:10.1021/ef900819p Syunyaev and Likhatsky

between low frequency permittivity (static dielectric permit-tivity εs) and permittivity at the high frequency limit (ε¥) isintroduced in the theory of dipole relaxation.39 The dielectricstrength and values of the dipole moment (μ) of N kineticunits (N = concentration) are related by the followingrelation

Δε ¼ εs - ε¥ ≈Z

ε00ðνÞdlog ν≈ N 3μ2

Tð12Þ

Exact calculation of the coefficients of proportionality ineq 12 was not carried out. Qualitative dependences areshown in Figure 12. The values of dielectric strength arepresented in relative units. The presented dependences showthat dipole moments of asphaltene clusters are not constantand they depend on temperature, pressure, and concentration.

According to eq 12, the increase of concentration (N) by afactor of 10 (from 0.3 up to 3 wt%) should result in a rise ofthe dielectric strengths by a factor of 10. According toFigure 12, there is an increase of only two times. Relationsof temperatures also do not coincide with relations of di-electric strengths (eq 12). This can probably be explained bythe fact that the dipole moment of asphaltene cluster has apower dependence on the number of monomers forming thecluster.15 The increase of the total concentration in thesolution (Ctotal) results simultaneously in two parallel pro-cesses: (1) growth of nanoaggregates and clusters, accom-panied by a redistribution of monomers between them and(2) formation (disappearance) of new particles.

The dielectric strength will be proportional to N 3 μ(n)2/T

provided that Ctotal ≈ N 3 n, where N is the concentration ofindependent kinetic units (clusters), μ(n) is the dipole mo-ment of each of them, and n is the number of monomers inkinetic unit (cluster). This issue requires further research.

V. Summary

The dielectric relaxation spectra for two oils (CaspianOil 1)and (Kumkolskaya Oil 2) and solutions of asphaltenes ex-tracted from oil 2 are measured in a wide range of pressures(up to 1 GPa) and temperatures (123-320 K). Relaxationparameters (RT as a functions of temperature and pressure)for the investigated systems were obtained using HN model.Additional term that takes into account the conductivitycontribution was added to HN model.

The behavior of RT for sol asphaltene solutions is deter-mined near the pressure crystallization of solvents. It is shownthat the dielectric response of asphaltene solutions can bedescribed by the classical Debye model. The dipole momentsof asphaltene clusters depend nonlinearly on total concentra-tion.

The dependences of GT temperature on pressure (phasediagrams) for oil samples were obtained for the first time byDSmethod in a wide range of temperatures (123-320K) andpressures (up to 1 GPa). The set of oil parameters wasobtained from WLF equation. Within the framework of thefree volume theory developed for polymers, modified WLFequation for RT dependence on pressure has allowed tocalculate GT temperatures for Oil 2. It is shown that GTprocess is a universal phenomenon for complex oil systems.For oil 1, the temperature coefficient of activation energy wascalculated,which ranges from10 to 70 kJ/mol. The isothermalcompressibility and thermal expansion coefficient for the oilsamples were determined. The dependence of viscosity onpressure for Oil 2 was found for temperatures above the pourpoint of sample.DSmethodallows to studyphysical processesin complex oil systems, their phase state in a wide range oftemperatures andpressures.This knowledge canbeused in thedevelopment of continental and offshore oil deposits.

Acknowledgment. Prof. Backstrom G (Umea University,Sweden) and Prof. M.Anisimov (Maryland University,USA). Prof. Novikov V.F. Institute of Chemical Physics RAS(Chernogolovka, Russia). Dr. Balabin R.M. for DS experimentson Novocontrol Analyzer at low temperatures.

Figure 11.Relaxation times (RT) vs pressure at two concentrationsand two temperatures (A/T).

Figure 12. Static dielectric permittivity vs pressure at two concen-trations and two temperatures (A/T).