Energy Fuels 2010, 24, 4327–4332 Published on Web 07/01/2010
: DOI:10.1021/ef100376t
Observation of Liquid Crystals in Heavy Petroleum Fractions S. Reza Bagheri, Ala Bazyleva, Murray R. Gray, William C. McCaffrey, and John M. Shaw* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada Received March 27, 2010. Revised Manuscript Received June 9, 2010
In this work, we report for the first time the observation of naturally occurring discotic liquid-crystalline domains of ca. 100 μm in diameter in unreacted heavy fractions of petroleum. Precipitated solids from several bitumen and heavy oil samples exhibited the characteristic optical patterns of liquid crystals when observed under cross-polarized light. Samples included asphaltenes precipitated from Athabasca and Cold Lake bitumen (Canada), Maya heavy crude oil (Mexico), and Safaniya crude oil (Saudi Arabia) and a maltene fraction of Athabasca bitumen. The liquid-crystal domains appeared in asphaltene solids at ∼330 K in a nitrogen atmosphere and disappeared at ∼430 K. Upon cooling and subsequent reheating, the domains did not reappear. Liquid-crystal domains also appeared and then disappeared in the presence of toluene vapor at room temperature. Because the liquid crystals exhibit both thermotropic and lyotropic behavior, they are amphotropic. While amphotropic liquid crystals are known to arise in biological systems, the specific attributes of the liquid-crystal structures observed here are the first reported occurrence of such behavior in nature. The presence of liquid crystals in petroleum solids enriches our understanding of the complex phase and interfacial behavior of these materials and may provide new opportunities for partitioning petroleum.
structure. Some of the larger molecules, with masses over 700 Da, form aggregates of ∼5 nm suspended in liquid petroleum.6,7 These components, termed asphaltenes, can be isolated by precipitation in an excess of solvents, such as n-pentane or n-heptane. The asphaltene fraction is implicated in a wide range of production and processing problems, such as plugging of production pipelines, stabilization of emulsions, and fouling of process equipment. This fraction includes some hydrocarbons, but molecules including sulfur, nitrogen, oxygen, vanadium, and nickel,8 in addition to carbon and hydrogen, are prevalent. Petroleum phase behavior is equally complex. Up to four phases in equilibrium are observed for reservoir fluids9 and bitumen fraction and light hydrocarbon mixtures.10 Recently, a combination of calorimetry and rheology was applied to demonstrate that heavy oil and bitumen fractions undergo complex transitions from solid-like behavior to liquid behavior over the temperature interval of 150-520 K.11 These fractions exhibited a minimum of three phases (solid maltenes, solid
Introduction Liquid crystals are a state of matter that can exhibit the physical properties of both crystals and isotropic liquids.1 Naturally occurring liquid crystals have been observed in biological membranes and vesicles. The long-range order of the molecules in these fluids can give rise to unique optical and electronic properties. In normal liquids, molecules are disordered or isotropic, while in crystalline solids, the molecules are ordered in three dimensions. Liquid crystals exhibit ordering or anisotropy, in one or two dimensions.2 Liquid crystals are usually divided into two categories: thermotropic liquid crystals are formed by a change of the temperature, while lyotropic liquid crystals are observed with the addition of a suitable solvent.3 Some liquid crystals can show both lyotropic and thermotropic phases and are referred to as amphotropic. Amphotropic materials become anisotropic with changes in the temperature or presence of a solvent.4 With the exception of biological examples, most observations of liquid crystals have been for pure synthetic components or simple mixtures. Petroleum comprises tens of thousands of components5 with broad ranges of molar mass, elemental composition, and
(6) Chianelli, R. R.; Siadati, M.; Mehta, A.; Pople, J.; Ortega, L.; Chiang,, L. Y. Self-assembly of asphaltene aggregates: Synchrotron, simulation and chemical modelling techniques applied to problems in the structure and reactivity of asphaltenes. In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; pp 375-400. (7) Betancourt, S. S.; Ventura, G. T.; Pomerantz, A. E.; Viloria, O.; Dubost, F. X.; Zuo, J. L.; Monson, G.; Bustamante, D.; Purcell, J. M.; Nelson, R. K.; Rodgers, R. P.; Reddy, C. M.; Marshall, A. G.; Mullins, O. C. Nanoaggregates of asphaltenes in a reservoir crude oil and reservoir connectivity. Energy Fuels 2008, 23, 1178–1188. (8) Boduszynski, M. M. Composition of heavy petroleums. 2. Molecular characterization. Energy Fuels 1988, 2 (5), 597–613. (9) Shaw, J. M.; Zou, X. Y. Challenges inherent in the development of predictive deposition tools for asphaltene containing hydrocarbon fluids. Pet. Sci. Technol. 2004, 22 (7-8), 773–786. (10) Zou, X. Y.; Zhang, X. H.; Shaw, J. M. Phase behavior of Athabasca vacuum bottoms plus n-alkane mixtures. SPE Prod. Oper. 2007, 22 (2), 265–272. (11) Fulem, M.; Becerra, M.; Hasan, MD. A.; Zhao, B.; Shaw, J. M. Phase behaviour of Maya crude oil based on calorimetry and rheometry. Fluid Phase Equilib. 2008, 272 (1-2), 32–41.
*To whom correspondence should be addressed. Telephone: 780-4928236. E-mail:
[email protected]. (1) Khoo, I.-C. Liquid Crystals, 2nd ed.; Wiley-Interscience: Hoboken, NJ, 2007. (2) de Gennes, P.-G.; Prost, J. The Physics of Liquid Crystals, 2nd ed.; Oxford University Press: New York, 1995. (3) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim, Germany, 2003. (4) Corcoran, J.; Fuller, S.; Rahman, A.; Shinde, N.; Tiddy, G. J. T.; Attard, G. S. Amphitropic liquid crystals. Part 1.;Effect of a thermotropic mesogen on lyotropic mesomorphism, and of a surfactant on thermotropic mesomorphism. The C16EO8-5-CB-water system. J. Mater. Chem. 1992, 2 (7), 695–702. (5) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Resolution of 11 000 compositionally distinct components in a single electrospray ionization Fourier transform ion cyclotron resonance mass spectrum of crude oil. Anal. Chem. 2002, 74, 4145–4149. r 2010 American Chemical Society
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asphaltenes, and liquid maltenes). Precipitated asphaltenes undergo a multi-stage transition from solid to liquid that commences at ∼340 K and terminates at ∼520 K,12 comprising an exothermic transition at 400-425 K superimposed on a broad endothermic transition spanning the interval from 340 to 520 K. The endothermic transition is reversible, while the exothermic phase transition observed over the 400-425 K temperature interval was tentatively attributed to an irreversible or slowly reversing dissolution of part of the asphaltenes in the growing liquid phase.11 Phase angles measured during oscillatory rheometry showed that the broad endothermic transition between 340 and 520 K is a solid-liquid transition.11 Consequently, samples of asphaltenes precipitated from crude oil comprise a minimum of three phases at room temperature. Thermal and catalytic reactions of heavy petroleum fractions can lead to the formation of a carbonaceous mesophase, an intermediate phase that is characterized as a discotic nematic liquid crystal.13-17 Reactively modified materials, such as electrode binder pitches and mesophase pitch, which are derived from the residue fraction of thermally or catalytically cracked products, were shown to contain small crystallites by X-ray analysis and optical microscopy.15,18 Unreacted petroleum fractions modified with the addition of surfactants have also been observed to display liquid-crystal domains.19,20 Given the diverse chemical components in petroleum, the presence of components capable of forming liquid crystals in the unreacted or unmodified material is likely. For example, large alkyl aromatics can form columnar liquid crystals.21 The presence of thousands of different components, however, might be expected to interfere with the formation of liquid-crystal phases. In particular, the smaller components would act as solvents to disrupt the ordering of liquid-crystal-forming components. Precipitated asphaltenes are powders at room temperature, eliminating this solvent action. Our hypothesis was that solids that had been precipitated from petroleum materials would exhibit liquid-crystalline domains.
in n-pentane or n-heptane. A fraction of Athabasca bitumen extracted with supercritical n-pentane (solubility parameter of 11 MPa1/2) was also examined, which has been discussed elsewhere.22 Athabasca and Maya asphaltenes were precipitated from the oil by the addition of 40 mL of n-pentane/g of oil. The mixture was agitated overnight at 400 rpm at room temperature and atmospheric pressure. After that, the mixture was filtered in two steps using vacuum filtration. First, it was filtered through a Fisher brand filter paper Q2, with a pore size between 1 and 5 mm. The permeate was filtered again using a 0.22 μm Millipore mixed cellulose ether membrane. To eliminate any residual oil, the filtration membranes and the flask were washed with small volumes of n-pentane until the filtrate was colorless. The membranes with the precipitated material were placed overnight in a vacuum oven at 9 kPa and 60 °C. In the case of the asphaltenes precipitated from Athabasca bitumen, an extra step was followed to reduce, as much as possible, the presence of non-asphaltenic solids (clay, sand, and some adsorbed hydrocarbons) that precipitate with the asphaltenes. For this case, the precipitate was mixed with toluene at a concentration of 10 g/L and agitated at 400 rpm for 1 h. Then, the mixture was centrifuged at 3500 rpm for 5 min at 20 °C. Afterward, the mixture was decanted. The supernatant was removed, and most of the toluene was evaporated in a rotovap. The balance of the mixture was transferred to a beaker and placed in the oven at 100 °C until all of the toluene was evaporated. Finally, the asphaltenes were placed in a vacuum oven at 9 kPa for 24 h. Cold Lake pentane-insoluble asphaltenes were provided by Imperial Oil, and Safaniya asphaltenes were produced using the American Society for Testing and Materials (ASTM) 6550 method, where asphaltenes are separated using heptane. Heptane was added at a ratio of 30 mL/g of sample if the asphaltene content was below 25% (m/m) or 25 mL/g of sample if the asphaltene content was above 25% (m/m). Then, the mixture was boiled under reflux for 60 ( 5 min, cooled, and stored for 90-120 min. The asphaltene precipitate was recovered by filtration on Whatman grade 42, 110 or 125 mm diameter, filter paper. Hot-Stage Microscopy (HSM). Powdered samples were placed on an yttrium aluminum garnet (YAG) window and inserted at the bottom of a temperature- and atmosphere-controlled cell, and a silver-plated O-ring was used to seal the YAG window. The amount of material used for HSM experiments varied between 5 and 12 mg. A Zeiss Axio-Observer inverted reflective microscope was used to observe the samples. The combined magnification of the system was 200 and 500. The powders were observed under crosspolarized light, as they were heated at 5-10 K/min under nitrogen. The temperature was measured using a thermocouple in contact with the YAG window. The inverted design permitted the examination of small particles with good contrast and resolution, unlike prior hot-stage designs, where solids sit on a heated metal substrate and are viewed from above.23 To investigate possible lyotropic behavior, the powder samples were placed on a glass slide and observed under cross-polarized light. Then, the slide was placed over a toluene bath in a sealed container at room temperature. There was no direct contact between the toluene and the sample. The slide was removed from the container and observed again under the microscope at specific time intervals. Differential Scanning Calorimetry (DSC). The heat capacity of Athabasca asphaltenes was measured in a differential scanning calorimeter TG-DSC 111 (Setaram, France) in the temperature range from 300 to 570 K. Temperature calibration to ITS 90 was performed using indium, tin, lead, and zinc (mass fractions of 0.999 99, Sigma-Aldrich Co.), as recommended by
Experimental Section Materials. In this work, asphaltenes were precipitated from Athabasca and Cold Lake bitumen (Canada), Maya heavy crude oil (Mexico), and Safaniya crude oil (Saudi Arabia) by dilution (12) Lastovka, V.; Fulem, M.; Becerra, M.; Shaw, J. M. A similarity variable for estimating the heat capacity of solid organic compounds; Part II. Application: Heat capacity calculation for ill-defined organic solids. Fluid Phase Equilib. 2008, 268 (1-2), 134–141. (13) Brooks, J. D.; Taylor, G. H. Formation of graphitizing carbons from liquid phase. Nature 1965, 206 (4985), 697–699. (14) Honda, H. Carbonaceous mesophase;History and prospects. Carbon 1988, 26 (2), 139–156. (15) Park, Y. D.; Mochida, I. A 2-stage preparation of mesophase pitch from the vacuum residue of FCC decant oil. Carbon 1989, 27 (6), 925–929. (16) Yen, T. F. The colloidal aspect of a macrostructure of petroleum asphalt. Fuel Sci. Technol. Int. 1992, 10 (4-6), 723–733. (17) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Asphaltene molecular structure and chemical influences on the morphology of coke produced in delayed coking. Energy Fuels 2006, 20 (3), 1227–1234. (18) Pollack, S. S.; Alexander, L. E. X-ray analysis of electrode binder pitches and their cokes. J. Chem. Eng. Data 1960, 5 (1), 88–93. (19) Sadeghi, K. M.; Sadeghi, M. A.; Jang, L. K.; Chilingarian, G. V.; Yen, T. F. A new bitumen recovery technology and its potential application to remediation of oil spills. J. Pet. Sci. Eng. 1992, 8, 105–117. (20) Lian, H. J.; Lin, J. R.; Yen, T. F. Peptization studies of asphaltene and solubility parameter spectra. Fuel 1994, 73 (3), 423–428. (21) Wu, J.; Grimsdale, A. C.; Mullen, K. Combining one-, two- and three-dimensional polyphenylene nanostructures. J. Mater. Chem. 2005, 15, 41–52.
(22) Chung, K. H.; Xu, C.; Gray, M.; Zhao, Y.; Kotlyar, L.; Sparks, B. The chemistry, reactivity, and processability of Athabasca bitumen pitch. Rev. Process Chem. Eng. 1998, 1, 41–79. (23) Perrotta, A. J.; McCullough, J. P.; Beuther, H. Pressuretemperature microscopy of petroleum-derived hydrocarbons. Prepr.Am. Chem. Soc., Div. Pet. Chem. 1983, 28 (3), 633–639.
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Figure 1. Athabasca asphaltenes (C5) under cross-polarized light at room temperature.
Figure 2. First liquid-crystal particles (marked by the circle) appeared in Athabasca asphaltenes (C5) at 342 K.
the Gesellschaft f€ ur Thermische Analyse e.V. (GEFTA).24-28 Energy calibration was performed using the Joule effect method in the factory and checked by measuring the heat of fusion, ΔfusHm, of naphthalene, indium, and tin. The agreement with recommended literature values26-29 was within 2%. Heat-capacity, cp (heat flow rate), calibration was performed using synthetic sapphire, a primary reference material according to the National Institute of Standards and Technology (NIST, SRM 720) and the International Confederation for Thermal Analysis and Calorimetry (ICTAC).29 The uncertainty of the cp measurements was estimated to be less than 2% (0.02 J K-1 g-1) in the studied temperature range. All heat-capacity data were obtained using a continuous three-step method:30 “(1) empty - (2) reference material (sapphire) - (3) sample under study” for a measuring cell. An empty reference cell was present for all three runs. The measurements were carried out with a heating rate of 2 K min-1 with isothermal periods of 3600 s at the beginning and end of each trial. Hermetically sealed stainless-steel cells, with a maximum pressure of 10 MPa at 573 K, were applied in experiments. Figure 3. Liquid-crystal spheres with Maltese crosses in Athabasca asphaltenes (C5) at 358 K.
Results and Discussion Microscope Observations. Because liquid crystals may be thermotropic, lyotropic, or amphotropic, the sample thermal history and solvent environment were key variables in this study. At room temperature, asphaltene powders include dark domains and domains that appear as bright points under crosspolarized light (Figure 1). For asphaltenes at approximately 330 K, a phase transition occurs and the first liquid-crystal
domains appear as droplets with Maltese crosses (Figure 2). Dependent upon their size, their color ranges from yellow and orange for small spheres to red for large spheres (Figure 3). The Maltese crosses (or disclinations with four dark brushes) rotate in the direction of the polarizer and analyzer rotation, as observed in concentric lamellar liquid crystals comprising the surfactant, co-surfactant, and solvent.31-33 Lamellar texture is considered a feature of lyotropic liquid crystals, but it has not been observed previously for theromotropic liquid crystals. As temperature is increased to ∼350 K, the number of liquidcrystal domains increases and then decreases upon further heating. At a temperature between 350 and 430 K, three different phases can be detected in asphaltene samples: solid, liquid crystal, and isotropic liquid, which is dark under cross-polarized light (Figure 4). The liquid becomes isotropic at ∼430 K. For the fraction of Athabasca bitumen extracted with supercritical
(24) Hohne, G. W. H.; Cammenga, H. K.; Eysel, W.; Gmelin, E.; Hemminger, W. The temperature calibration of scanning calorimeters. Thermochim. Acta 1990, 160 (1), 1–12. (25) Cammenga, H. K.; Eysel, W.; Gmelin, E.; Hemminger, W.; Hohne, G. W. H.; Sarge, S. M. The temperature calibration of scanning calorimeters. 2. Calibration substances. Thermochim. Acta 1993, 219, 333–342. (26) Sarge, S. M.; Gmelin, E.; Hohne, G. W. H.; Cammenga, H. K.; Hemminger, W.; Eysel, W. The caloric calibration of scanning calorimeters. Thermochim. Acta 1994, 247 (2), 129–168. (27) Gmelin, E.; Sarge, S. M. Calibration of differential scanning calorimeters. Pure Appl. Chem. 1995, 67 (11), 1789–1800. (28) Sarge, S. M.; Hemminger, W.; Gmelin, E.; Hohne, G. W. H.; Cammenga, H. K.; Eysel, W. Metrologically based procedures for the temperature, heat and heat flow rate calibration of DSC. J. Therm. Anal. 1997, 49 (2), 1125–1134. (29) Sabbah, R.; An, X. W.; Chickos, J. S.; Leitao, M. L. P.; Roux, M. V.; Torres, L. A. Reference materials for calorimetry and differential thermal analysis. Thermochim. Acta 1999, 331 (2), 93–204. (30) H€ ohne, G. W. H.; Hemminger, W. F.; Flammersheim, H.-J. Differential Scanning Calorimetry, 2nd ed.; Springer Verlag: Berlin, Germany, 2003.
(31) Figueiredo Neto, A. M.; Salinas, S. R. The Physics of Lyotropic Liquid Crystals: Phase Transitions and Structural Properties; Oxford University Press: New York, 2005. (32) Ge, L. L.; Chen, L. P.; Guo, R. Microstructure and lubrication properties of lamellar liquid crystal in Brij30/[Bmim]PF6/H2O system. Tribol. Lett. 2007, 28 (2), 123–130. (33) Friberg, S. E. Emulsion stability. In Emulsions;A Fundamental and Practical Approach; Sj€oblom, J., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; pp 17-18.
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Figure 5. Supercritical pentane extract from Athabasca bitumen at 356 K. Table 1. Phase Transition Temperatures for Different Samples Used in This Study
sample Athabasca asphaltenes (C5) Maya asphaltenes (C5) Cold Lake asphaltenes (C5) Safanya asphaltenes (C7) fraction of Athabasca bitumen extracted with supercritical n-pentane
temperature at temperature at which which liquid crystals liquid crystals appear (K) disappear (K) 338 340 341 371 316
423 435 431 433 373
treated sample of asphaltenes that no longer possessed liquidcrystalline domains was washed with n-pentane and then vacuum-dried at room temperature, liquid-crystal domains reappeared upon heating to ∼330 K and then disappeared again when the sample was reheated above 430 K. Exposure of precipitated asphaltene powder to toluene vapor at room temperature also gave transient formation of a liquid-crystal phase, which disappeared over time as more toluene was absorbed. Liquid crystals appeared after ca. 10 min of exposure to toluene, but the liquid-crystal domains became isotropic after 30 min of exposure, as shown in Figure 7. Finally, a film of asphaltenes that was cast from a solution of methylene chloride, to minimize segregation of molecules according to structure, did not show liquid-crystalline behavior upon heating from room temperature to 463 K. These microscopy observations clearly illustrate that multiple thermal and solvation pathways exist for the formation of liquid crystals in petroleum fractions and that dissolution in strong solvents or petroleum liquids at higher temperatures can disrupt their order or prevent their formation. Thus, the observed liquid crystals are amphotropic in nature.34 Calorimetry. A phase behavior study for one of the asphaltenes, Maya pentane asphaltanes, based on calorimetric and rheological data was reported previously.11 Here, additional illustrative calorimetric data are reported for Athabasca asphaltenes. Three sets of values are reported in Figure 8. The first scan is for freshly precipitated asphaltenes. The second scan is for a second heating cycle. Data are reported as apparent heat-capacity values. A correlation for the heat capacity of
Figure 4. Athabasca bitumen asphaltenes at 346 K (a) under crosspolarized light and (b) under normal light. The comparison to the photo under normal light shows the co-existence of solid or glass, liquid crystal, and an isotropic phase.
n-pentane, liquid crystals appear at 316 K and disappear at 373 K (Figure 5). The liquid-crystal appearance and disappearance temperatures for all samples are shown in Table 1. The images in Figures 1-3 clearly show that the liquid crystals make up only a fraction of each sample. Only some of the particles in the images show liquid-crystalline domains. The liquid-crystal domains consist of a thin film covering a core of solid, as illustrated in Figure 6, for the supercritical pentane extract from the Athabasca bitumen sample at 358 K. All of the liquid-crystalline domains examined for all samples consisted of a “ring” of liquid crystal around a core of solid. This structure may be due to the precipitation history of the particle, with the core forming first, followed by the outer layer that is capable of liquid-crystal formation. Irrespective of their origin, these micrographs provide clear visual evidence of heterogeneity in precipitated asphaltenes. Once liquid crystals appeared in a sample upon heating, they remained stable upon cooling, at least for a period of days. When samples were heated above the upper temperature boundary for liquid crystals, to give an isotropic melt, and then cooled to room temperature, the liquid-crystal domains did not reappear over a period of days. A further study is required to determine whether these transitions are irreversible or very slowly reversible. The solvent environment was also observed to play a key role in liquid-crystal formation. For example, when a heat-
(34) Tschierske, C. Amphotropic liquid crystals. Curr. Opin. Colloid Interface Sci. 2002, 7 (5-6), 355–370.
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Figure 6. Close-up of the liquid-crystal domain from the supercritical pentane extract from the Athabasca bitumen sample at 358 K (a) under cross-polarized light and (b) under normal light. The particle is 82 μm in diameter.
solid Athabasca pentane asphaltenes based on elemental analysis is included as a trend line for single-phase calorimetric behavior.12 As in the case of Maya asphaltenes,11 heat-capacity measurements for freshly precipitated Athabasca asphaltenes (first scan of Figure 8) revealed a large endothermic phase transition starting at ≈320-330 K, where the apparent heat-capacity data diverge from solid behavior, and ending at ≈520 K, and a smaller overlapping exothermic peak between 400 and 425 K. The temperature of the disappearance of the liquid-crystalline phase, as observed by microscopy, coincides with the exothermic peak from the DSC measurements. The exothermic transition observed for asphaltene samples is consistent with dissolution of the liquid crystals into the surrounding liquid phase. Dissolution enthalpies include two contributions: fusion (endothermic) and solvation (exothermic). Because the enthalpy for the liquid crystal to liquid transition is small,35-38 solvation dominates, leading
Figure 7. Interaction of Athabasca asphaltenes (C5) with toluene vapor: (a) initial sample, (b) sample after 10 min, and (c) sample after 30 min.
to the appearance of an exotherm. This exothermic peak is absent from the second scan, consistent with the lack of liquidcrystal formation upon reheating. On the basis of these observations, we conclude that the exothermic peak is due to the dissolution of the liquid-crystal phase. Asphaltene Phase Behavior. Precipitated asphaltenes comprise a minimum of two solid phases at room temperature. One of the solids, solid I, undergoes a direct endothermic transition to liquid, which begins above room temperature, while the other solid, solid II, undergoes an endothermic transition to a liquid crystal followed by an exothermic dissolution as
(35) Sorai, M.; Suga, H. Studies on mesogenic disc-like molecules. 2. Heat-capacity of benzene-hexa-n-heptanoate from 13 to 393 K. Mol. Cryst. Liq. Cryst. 1981, 73 (1-2), 47–69. (36) Sorai, M.; Yoshioka, H.; Suga, H. Studies on mesogenic disc-like molecules. 3. Heat-capacity of benzene-hexa-n-octanoate from 13 to 393 K. Mol. Cryst. Liq. Cryst. 1982, 84 (1-4), 39–54. (37) Stegemeyer, H.; Baumgartel, H.; Franck, E. U.; Grunbein, W. Liquid Crystals; Springer: New York, 1994. (38) Collings, P. J.; Hird, M. Introduction to Liquid Crystals: Chemistry and Physics; Taylor and Francis, Inc.: Philadelphia, PA, 1997.
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of the phase behavior of asphaltenes and other petroleum fractions. This heterogeneous and path-dependent phase behavior likely accounts for the difficulty in predicting the precipitation of asphaltenes during production and subsequent refining of petroleum. Further work is required to determine which components of asphaltenes are responsible for the formation of liquid-crystal domains, which substrates they form, and the conditions under which liquid crystals are stable, metastable or unstable, and to quantify their contribution to molecular aggregation in crude oils. Clearly, the observation of liquid-crystal domains in precipitated samples from bitumen and heavy oils suggests that molecular self-assembly may enable new separation methods for components of petroleum. Rather than separation by the current methods of distillation based on boiling point, precipitation, or solubility, the controlled formation of liquid-crystalline phases may allow for separation of structurally similar molecules that align to form a concentric lamellar phase.
Figure 8. Temperature dependence of the heat capacity for C5 Athabasca asphaltenes: solid line, first scan; dashed line, second scan; dotted line, predicted heat capacity of solid asphaltenes.12 The vertical solid lines demark the temperature range where liquid crystals are observed for C5 Athabasca asphaltenes.
Conclusions A portion of asphaltenes precipitated from Athabasca and Cold Lake bitumen (Canada), Maya heavy crude oil (Mexico), and Safaniya crude oil (Saudi Arabia) and a maltene fraction of Athabasca bitumen exhibit liquid-crystal behavior. Because these naturally occurring hydrocarbons are drawn from different geological basins with different solubility properties and average chemical compositions, liquid-crystal formation appears to be a common phenomenon in petroleum solids. The liquid-crystal domains are shown to arise from multiple thermal and solvent addition pathways and are consequently amphotropic in nature. This is the first report of naturally occurring non-biological amphotropic liquid crystals that arise from unmodified or unreacted natural materials. The composition, equilibrium properties, formation, and dissolution kinetics of these liquid crystals, implications for hydrocarbon surface and separation science, and hydrocarbon production and refining remain to be explored.
the temperature is raised. Thus, asphaltenes comprise three phases (solid I, solid II, and liquid) from approximately room temperature, where the heat capacity begins to deviate from the heat capacity of a solid hydrocarbon12 (Figure 8), to the temperature where liquid crystals appear (see Table 1 for values). From that temperature to the temperature where liquidcrystal dissolution is complete, up to four phases are present (solid I, solid II, liquid crystal, and liquid). Once the liquid crystals dissolve, two phases (liquid and solid I) are present up to at least 500 K. Upon subsequent cooling to room temperature, asphaltene samples did not return to the same phase states as present initially, at least not over the duration of experiments performed here. Neither calorimetry nor microscopy indicated the presence of liquid crystals and, by inference, solid II, upon reheating. Consequently, the phases present in precipitated asphaltenes may not be at phase equilibrium with each other. Implications of Liquid-Crystal Observations. The observation of liquid crystals enriches our understanding of the complexity
Acknowledgment. The work was funded by UOP LLC and the Natural Sciences and Engineering Research Council of Canada.
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