Energy & Fuels 2008, 22, 561–563
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Circular Dichroism of Crude Oils: Influence of Micelle Formation A. V. Potapov,*,†,‡ S. F. Kolyakov,† V. N. Krasheninnikov,† and R. Z. Syunyaev‡ Institute of Spectroscopy of Russian Academy of Sciences, 142190, Troitsk, Moscow region, Russia, and Gubkin Russian State UniVersity of Oil and Gas, 119991, Leninsky prospect 65, Moscow, Russia ReceiVed April 3, 2007. ReVised Manuscript ReceiVed October 25, 2007
Circular dichroism (CD) and absorption spectra of toluene solutions of crude oils, collected from two different reservoirs, have been measured in the spectral range, where the optical activity of crude oil solutions is due to the aggregation of asphaltene molecules and for asphaltene concentrations up to values over the critical micelle concentration (cmc). Concentration dependences of CD and the absorption signal in the maximum of the registered CD band (410 nm) in the range of low asphaltene concentrations are described by linear functions. Micelle formation of asphaltenes in crude oil solutions leads to a sharp increase of the CD signal and a violation of the linear concentration dependence of the absorption signal. A further increase of the asphaltene concentration is accompanied by saturation of the CD signal, which is possibly due to the formation of a stable phase by micelles in solution.
Introduction Depletion of easily accessible stocks of hydrocarbons has resulted in the attraction of attention to extraction and processing of heavy high viscous crude oils and natural bitumens. These are high-concentrated dispersed systems with the significant contents of high-molecular asphaltene substances. Crude oil systems are objects of soft matter physics, for which significant correlation between their properties on micro- and macroscopic levels of organization is typical.1 The possibility of regulation of microstructure parameters at oil production, transportation, and refining is a basis for new effective physical and chemical technologies. Phase changes taking place in technological processes are accompanied by the appearance, formation and dispersion of microheterogeneous systems. The degree of dispersity is considered as an additional thermodynamic parameter, and its analysis is a source of the initial information for the choice of strategy of new technologies. Asphaltenes contain the molecular fragments with different electronic heterogeneity. Favorable conditions for the formation of change-transfer complexes with partial or full charge transfer from donor to acceptor exist in such systems. The most wellknown structural model of asphaltene molecules is the compact polycondensed structure with aromatic fragments settled down on the periphery of a molecule (“continental”). In other cases, the different fragments interconnect themselves by methylene chains (“archipelago”).2 The total action of intermolecular forces leads to the stacking of flat aromatic fragments.3 The value of the stacking energy is determined by a degree of overlapping of the bases.4 Calculations show3 that the dependence of the stacking interaction energy has a minimum relative to the rotation angle around a normal axis. Asphaltenes under such * To whom correspondence should be addressed. Fax: 0074953340886. E-mail:
[email protected]. † Institute of Spectroscopy of Russian Academy of Sciences. ‡ Gubkin Russian State University of Oil and Gas. (1) Syunyaev, R.; Safieva, R.; Safin, R. J. Pet. Sci. Eng. 2000, 26, 31. (2) Murgich, J. Pet. Sci. Technol. 2002, 20, 1029. (3) Molecular Interactions; Ratajczak, H., Orville-Thomas, W. J., Eds.; John Wiley and Sons: Chichester, U.K., 1981. (4) Murgich, J.; Rodriguez, J.; Aray, Y. Energy Fuels 1996, 10, 68.
conditions spontaneously turn relative to each other depending upon the composition of fragments and heteroatoms. Thus, a molecular level of asphaltene organization in oil systems is not the main level. Macroscopic properties of oils are determined by the permolecular level of structure organization. Aggregation of asphaltene molecules with the increase of their concentration in liquid medium occurs by a step mechanism. In the initial stages of aggregation, asphaltenes are bonded according to the stacking model.5–7 In the final stage of aggregation, asphaltenes form the micelles, spherical aggregates, capable of retaining the water molecules in the interior pools. The critical micelle concentration (cmc) of asphaltenes in crude oils is in the range of 2–12 g/L according to different experiments.8–10 Asphaltene molecules have an irregular structure, unsaturated bonds, and aromatic and naphthenic fragments. Electrons of chromophores move in asymmetric electric fields. These conditions are necessary to display optical chirality of molecules and their associates.11 It gives an opportunity to apply circular dichroism (CD) spectroscopy for the analysis of the formation stages of asphaltene permolecular structures in crude oil disperse systems. CD spectroscopy has already been used for the analysis of the structure of crude oil systems in our previous work.12 The significance of permolecular structures formed in the initial stages of asphaltene aggregation in solutions of crude oil in toluene for optical activity of oil was demonstrated. (5) Andreatta, G.; Bostrom, N.; Mullins, O. C. Langmuir 2005, 21, 2728. (6) Evdokimov, I. N.; Eliseev, N. Yu.; Akhmetov, B. R. J. Pet. Sci. Eng. 2003, 37, 135. (7) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (8) Syunyaev, R. Z. Conception of permolecular structures and extreme states in oil dispersed systems (in Russian), Habilitation work, Gubkin Russian State University of Oil and Gas, Moscow, Russia, 1999. (9) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6. (10) Andersen, S. I.; Christensen, S. D. Energy Fuels 2000, 14, 38. (11) Velluz, L.; Legran, M.; Grosjean, M. Optical Circular Dichroism; Academic Press: New York, 1965. (12) Potapov, A. V.; Kolyakov, S. F.; Krasheninnikov, V. N.; Dumesh, B. S. J. Colloid Interface Sci. 2006, 303, 159.
10.1021/ef070166m CCC: $40.75 2008 American Chemical Society Published on Web 11/27/2007
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Figure 1. CD spectra of R crude oil solutions in toluene for different asphaltene concentrations: 1–0.3, 2–0.6, 3–3.7, 4–5.6, 5–11.2, 6–28, and 7–56 mg/L. The optical wavelength is equal to 1 cm.
Here, we present the investigations of CD and absorption spectra of solutions of two different crude oils in toluene, with asphaltene concentrations up to values over the cmc. Experimental Section The unprocessed crude oils were directly collected from a production wells at Romashkinskoye and Volgogradskoye reservoirs (R and V crude oils) and contained 3.3 and 1.6 wt % asphaltenes, 2.2 and 0.6 wt % sulfurs, 24.8 and 9 wt % resins, and 2.5 and 2.6 wt % waxes, correspondingly. As a solvent for crude oils, we used “chemically pure” grade toluene. The concentration of oil in toluene varied from 8 mg/L to 350 g/L. The CD method is one of the optical methods that are widely used in organic and physical chemistry, as well as in the chemistry of natural and physiologically active compounds. CD is typical for optically active matters and permolecular structures possessing optical anisotropy. It appears as a consequence of different interactions of left circularly polarized and right circularly polarized light with the optically active sample. In this case, we have the difference between absorbance for vectors with right (AR) and left (AL) circular polarizations. This difference is called the CD. The CD was measured with the help of the CD spectrometer created in the Institute of Spectroscopy of the Russian Academy of Sciences. Details of the setup and an example of its using in biology were given elsewhere.13 The measurements were made in the scanning regime in a spectral range of 380–500 nm, with the subtraction of the baseline, and in the regime of signal accumulation at two wavelengths (410 nm, maximum of the spectral band, and 700 nm, zero point), with the determination of the differential signal. The absorption spectra were measured on the double-beam automatic scanning spectrophotometer “Specord M-40” in the range of 380–500 nm. For CD and absorption measurements, investigated samples were placed on quartz cavities with optical wavelengths from 1 cm to 50 µm.
Results and Discussion Figure 1 shows the CD spectra of toluene solutions of R crude oil measured in our previous work.12 One can see two spectral bands in the CD spectrum: the short-wave band with a maximum near 300 nm and the long-wave band with a maximum near 410 nm. As is well-known, the maximum in intensity of the CD spectrum of optically active objects corresponds to their resonance absorption.11 In our case, there is a correlation between the CD band position of crude oil toluene solutions (13) Kompanets, O. N. Phys.-Usp. 2004, 47, 630.
PotapoV et al.
Figure 2. CD spectra of crude oils solutions in toluene and Gaussian approximations of these spectra. (1) R crude oil (optical wavelength is equal to 50 µm). (2) V crude oil (optical wavelength is equal to 100 µm). The asphaltene concentration is equal to 5.6 g/L.
and resonance absorption of pure asphaltene toluene solutions measured by a number of authors.10,14,15 In our previous work,12 we have demonstrated that the existence of the long-wave CD band is due to the aggregation of asphaltene molecules in R crude oil solutions. Another result of that work was that the wavelength of the maximum of this band, obtained by the Gaussian approximation of the spectrum, does not depend upon the asphaltene concentration in solution and corresponds to 410 nm. The CD measurements for toluene solutions of V crude oil, performed in the present work, show a spectral pattern similar to R crude oil solutions, i.e., existence of two bands in the spectrum and invariability of the position of the long-wave band maximum at 410 nm. As an example, the long-wave CD bands of toluene solutions of R and V crude oils and Gaussian approximations of these bands are presented in Figure 2. Conditionality of the appearance of the long-wave band in the CD spectra because of the presence of aggregates of asphaltene molecules and invariability of the wavelength of its maximum motivated our further measurements. We have measured CD and absorption signals at 410 nm near the maximum of the long-wave CD band for toluene solutions of crude oils, collected from two different reservoirs. We increased the asphaltene concentration in investigated solutions up to values over the cmc. Figures 3 and 4 show the concentration dependencies of the absorption and CD signals at 410 nm. We took CD signal module for convenient joint analysis of both curves. The first result that we want to draw attention to is that CD and absorption dependencies in the range of low asphaltene concentrations (C < 1 g/L) are described by linear functions for solutions of both crude oils. However, as is well-known, asphaltenes aggregate according to the stacking model in crude oil as well as in toluene at concentrations much lesser than 1 g/L.5–7 Thus, we ascertained that asphaltene aggregation in the initial stages of aggregation (before micelle formation) does not lead to the deviation of CD and absorption concentration dependence of the toluene solution of crude oil from a linear function. (14) Evdokimov, I. N.; Eliseev, N. Yu.; Iktisanov, V. A. J. Colloid Interface Sci. 2005, 285, 795. (15) Potapov, A. V. Oil Gas Sci. Technol. 2007, in press.
Circular Dichroism of Crude Oils
Figure 3. Dependencies of absorption and CD signals of solutions of R crude oil in toluene at λ ) 410 nm on the asphaltene concentration. We took a module of the CD signal for convenient joint examination of CD and absorption signal dependencies. All values are brought in correspondence with values for the optical wavelength of 1 cm.
Figure 4. Dependencies of absorption and CD signals of solutions of V crude oil in toluene at λ ) 410 nm on the asphaltene concentration. We took a module of the CD signal for convenient joint examination of CD and absorption signal dependencies. All values are brought in correspondence with values for the optical wavelength of 1 cm.
However, when we increase the asphaltene concentration over 1 g/L and thus work in the range of micelle formation, introduced curves become more complicated. We registered the violation of linear concentration dependences of the absorption signal in the range of asphaltene concentrations of 1–2 g/L for R and V crude oils (see Figures 3 and 4, correspondingly). This result can be explained by reorganization of the solution structure (micelle formation of asphaltene molecules) as well as possible adsorption of asphaltenes on the walls of thin cavities used in the measurements. At the same concentrations, we did not observe the violation of linear concentration dependences of the CD signal in the limits of measurement error. A possible explanation of a remarkable change of the absorption signal and a lack of such a change of the CD signal is adsorption of asphaltene monomers on the walls of thin cavities, which does not influence the intensity of the long-wave CD band, caused by the existence of asphaltene aggregates. A further increase of the asphaltene concentration leads to a sharp increase of the registered CD signal (see Figures 3 and 4). For convenient joint analysis of both concentration dependencies of the CD signal, obtained for solutions of two crude oils, we show these curves once more in Figure 5.
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Figure 5. Dependencies of the CD signal of solutions of crude oils in toluene at a maximum of the registered CD band (λ ) 410 nm) on the asphaltene concentration. (1) R crude oil, (2) V crude oil, and (3) trend line. We took a module of the CD signal as in previous figures. All values are brought in correspondence with values for the optical wavelength of 1 cm.
As one can see from Figure 5, a sharp increase of the CD signal occurs in the same concentration range of 3–4 g/L for solutions of both crude oils. An increase of the CD signal could be explained by the considerable reorganization of the structure of the crude oil solution. At given concentrations, this reorganization should be bound with the micelle formation of asphaltenes. The violation of linear concentration dependences of the absorption signal in this concentration range also becomes considerable (see Figures 3 and 4). Thus, cmc values of asphaltenes in toluene solutions of R and V crude oils collected from different reservoirs and having different compositions are in the concentration range of 3–4 g/L. A further increase of the asphaltene concentration (over cmc) is accompanied by a nonlinear increase of CD and absorption signals. However, both curves describing the absorption signal variation are monotone, whereas both curves describing CD signal variation have a break point at an asphaltene concentration of 5.5 g/L. As one can see in Figure 5, at concentrations over the break point, saturation of the CD signal takes place for both oil solutions. We can only assume that the observed saturation is due to the formation of the stable phase by micelles in solution, possibly similar to a liquid crystal. Summary. CD and absorption spectra of toluene solutions of two different crude oils (collected from different reservoirs and having different compositions) have been measured in the frequency range where the optical activity of crude oil is due to the aggregation of asphaltene molecules. The micelle formation of asphaltenes is accompanied by a sharp increase of the CD signal and violation of linear concentration dependences of the absorption signal. The cmc of asphaltenes in investigated crude oil solutions is in the concentration range of 3–4 g/L. Thus, CD spectroscopy is a new and convenient method for the determination of the cmc of asphaltenes in crude oils. Saturation of the CD signal at asphaltene concentrations over the cmc is possibly due to the formation of a stable phase by micelles in solution. Acknowledgment. The support of the Russian Science Support Foundation is gratefully acknowledged by A.V.P. EF070166M
