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Langmuir Films of Bitumen and Its Fractions Extracted from Oil Shales (Puertollano, Spain) M. Elena Díaz, Francisco J. Montes,* and Miguel A. Galán Departamento de Ingeniería Química y Textil, UniVersity of Salamanca, Plaza de los Caídos 1-5, 37008 Salamanca, Spain ReceiVed July 16, 2007. ReVised Manuscript ReceiVed September 4, 2007
Interfacial properties of bitumen (extracted from Spanish oil shales), asphaltene, and maltene monolayers at the air–water interface are studied by means of the Langmuir technique. The results obtained in this work reveal the formation of aggregation structures of asphaltenes and maltenes at the air–water interface previous to compression. Upon compression, asphaltenes form condensed monolayers while maltenes show an expanded state at low specific areas due to the formation of an overfilm. The presence of maltenes in bitumen hampers the formation of large aggregates of asphaltenes, allowing for the existence of a liquid expanded phase in bitumen films. The maltenes present the highest compressibility of the three compounds favoring the coalescence of the emulsion droplets, while asphaltene films, displaying the lowest compressibility, form rigid films that avoid coalescence. Bitumen, asphaltene, and maltene films are lost when compressed at 20 mN/m initially by rearrangement and/or formation of association structures and later by dissolution into the subphase.
1. Introduction Crude oil or bitumen consists of a very complex mixture of molecules that are usually separated into the well-known SARA (saturates, aromatics, resins, and asphaltenes) fractions. The asphaltene fraction can be separated from the rest making use of its insolubility in light alkanes, typically n-heptane or n-pentane. The deasphalted bitumen, which contains the rest of the fractions, is known as maltenes. The presence of asphaltenes and resins in crude oils has supposed an important problem in the petrochemical and oil sands industry since they are considered to be responsible for the stabilization of waterin-oil emulsions.1–3 The reason is that both asphaltenes and resins are surface active components, with a hydrophobic part, formed by an alkyl chain, and a hydrophilic part, consisting of condensed aromatic rings.4–7 Therefore, the formation of films * Corresponding author. E-mail:
[email protected]. (1) Yarranton, H. W.; Hussein, H.; Masliyah, J. H. Water-in-Hydrocarbon Emulsions Stabilized by Asphaltenes at Low Concentrations. J. Colloid Interface Sci. 2000, 228, 52–63. (2) Ese, M. H.; Yang, X.; Sjoblom, J. Film forming properties of asphaltenes and resins. A comparative Langmuir-Blodgett study of crude oils from North Sea, European continent and Venezuela. Colloid Polym. Sci. 1998, 276, 800–809. (3) Sjoblom, J.; Aske, N.; Harald Auflem, I.; Brandal, O.; Erik Havre, T.; Saether, O.; Westvik, A.; Eng Johnsen, E.; Kallevik, H. Our current understanding of water-in-crude oil emulsions.: Recent characterization techniques and high pressure performance. AdV. Colloid Interface Sci. 2003, 100–102, 399–473. (4) Acevedo, S.; Escobar, G.; Gutierrez, L.; Rivas, H. Isolation and Characterization of Natural Surfactants from Extra Heavy Crude Oils, Asphaltenes and Maltenes - Interpretation of Their Interfacial-Tension Ph Behavior in Terms of Ion-Pair Formation. Fuel 1992, 71, 619–623. (5) McLean, J. D.; Kilpatrick, P. K. Effects of asphaltene aggregation in model heptane-toluene mixtures on stability of water-in-oil emulsions. J. Colloid Interface Sci. 1997, 196, 23–34. (6) McLean, J. D.; Kilpatrick, P. K. Effects of asphaltene solvency on stability of water-in-crude-oil emulsions. J. Colloid Interface Sci. 1997, 189, 242–253. (7) Gafonova, O. V.; Yarranton, H. W. The Stabilization of Water-inHydrocarbon Emulsions by Asphaltenes and Resins. J. Colloid Interface Sci. 2001, 241, 469–478.
of these two fractions at the air/water and oil/water interfaces is possible. Furthermore, rigid films separating water droplets in water-in-oil emulsions preventing coalescence are considered to be formed mainly by asphaltenes and resins.2,8 Water-in-oil emulsions can be formed in oil production plants in reservoirs and pipes, where high turbulence and pressure drops take place. It is also possible that the emulsions are deliberately formed with purification purposes.7 In the oil sand industry, the bitumen is extracted by means of hot water, a process that results in a bitumen solution that, in addition to the bitumen and the solvent, contains 2–3% water in the form of water-in-bitumen emulsion.9 Also, the work of recovery of oil in marine spills is hampered by the formation of these emulsions.10 Because of the higher viscosity and volume of the water-in-oil emulsions in comparison with the crude oil alone, transport costs, among others, are increased. Additionally, corrosion problems and catalyst poisoning may arise as a consequence of the presence of contaminants in the emulsified water.11 Moreover, the presence of water in the final product may be the cause of not complying with some particular product specifications. Therefore, breaking water-in-oil emulsions becomes essential in these types of industries. Chemical and thermal processes can be used with this purpose. However, these treatments often fail, and more efficient water separation processes are needed. But first, the stabilization of these waterin-oil emulsions has to be completely understood. (8) Mohammed, R. A.; Bailey, A. I.; Luckham, P. F.; Taylor, S. E. Dewatering of Crude-Oil Emulsions.2. Interfacial Properties of the Asphaltic Constituents of Crude-Oil. Colloids Surf., A 1993, 80, 237–242. (9) Zhang, L. Y.; Xu, Z. H.; Masliyah, J. H. Characterization of adsorbed athabasca asphaltene films at solvent-water interfaces using a Langmuir interfacial trough. Ind. Eng. Chem. Res. 2005, 44, 1160–1174. (10) Lobato, M. D.; Pedrosa, J. M.; Hortal, A. R.; Martinez-Haya, B.; Lebron-Aguilar, R.; LagoS. Characterization and Langmuir film properties of asphaltenes extracted from Arabian light crude oil. Colloids Surf., A 2007, 298, 72–79. (11) Solovyev, A.; Zhang, L. Y.; Xu, Z. H.; Masliyah, J. H. Langmuir films of bitumen at oil/water interfaces. Energy Fuels 2006, 20, 1572–1578.
10.1021/ef7004095 CCC: $37.00 2007 American Chemical Society Published on Web 10/06/2007
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Considering that the formation of bitumen films at the water/ oil interface is essential to the stabilization of emulsions, the knowledge of the interfacial properties of bitumen, as well as its fractions, asphaltenes, and maltenes, can be considered very valuable. Film formation of surface active materials can be studied by means of the Langmuir technique. The first research group that used the Langmuir balance technique to investigate the interfacial properties of crude oil was that of Kimbler et al.12 They reported the 2D phases showed by crude oil films when compressed alone or after the addition of a nonionic surfactant. According to their results, the crude oil shows the gaseous, liquid expanded, and condensed phases when compressed alone while only expanded films are formed when mixed with the nonionic surfactant. After the investigation presented by Kimbler and co-workers,12 several other studies have been done on the interfacial properties of bitumen and fractions by means of the Langmuir technique.2,8–11,13–16 Jones et al.13 used the mentioned technique to distinguish different types of crude film attending to the dynamics of relaxation that, according to these authors, determine the emulsion stability. Leblanc et al.14 studied the interfacial film behavior of maltenes and different fractions of asphaltenes at the air–water interface concluding that, while asphaltenes form true monolayers, maltenes form multilayers as they are under increasing surface pressures. Mohamed et al.8 studied asphaltenes and resins, showing that asphaltenes and asphaltene–resin mixtures are responsible for stabilizing water-in-oil emulsions while resins by themselves cannot be the cause of the stabilization. Ese et al.2 also studied asphaltenes, resins, and their mixtures. According to the results reported by these authors,2 the compressibility of the maltene films is much higher than that of asphaltenes while the degree of aggregation of these is higher than that of maltenes. Zhang et al.16 studied the interfacial properties of different fractions of Athabasca asphaltenes and concluded that, even at low surface pressures, the different asphaltene fractions form monolayers, which allows the stabilization of emulsions. Zhang et al.9 used an oil–water interface to study the interfacial behavior of Athabasca asphaltenes. Their results indicate the importance of the aromacity of the crude medium in the compressibility of the asphaltene monolayers. Solovyev et al.11 studied the interfacial behavior of bitumen, asphaltene, and maltene films at the toluene–water interface after washing the films with toluene and/or heptane. According to the reported results, asphaltene films are irreversibly adsorbed at the toluene–water interface while maltene molecules are lost from the interface into the toluene. Lobato et al.10 studied asphaltenes extracted from Arabian light crude oil. They showed the importance of the aggregation phenomenon in asphaltenes by means of mass weight distribution as well as interfacial studies. In this work, the interfacial behavior at the air–water interface of bitumen extracted from oil shales (Puertollano, Spain) as well as asphaltenes and maltenes extracted from the original bitumen are studied. Isotherm studies, monolayer hysteresis, and stability as well as the influence of the initial amount of monolayer (12) Kimbler, O. K.; Reed, R. L.; Silberberg, I. H Soc. Pet. Eng. J. 1966, 6, 153. (13) Jones, T. J.; Neustadter, E. L.; Whittingham, K. P. Water-in-Crude Oil Emulsion Stability and Emulsion Destabilization by Chemical Demulsifiers. J. Can. Pet. Technol. 1978, 17, 100–108. (14) Leblanc, R. M.; Thyrion, F. C. A study of monolayers of asphalts at the air/water interface. Fuel 1989, 68, 260–262. (15) Zhang, L. Y.; Xu, Z. H.; Mashyah, J. H. Langmuir and LangmuirBlodgett films of mixed asphaltene and a demulsifier. Langmuir 2003, 19, 9730–9741. (16) Zhang, L. Y.; Lawrence, S.; Xu, Z. H.; Masliyah, J. H. Studies of Athabasca asphaltene Langmuir films at air-water interface. J. Colloid Interface Sci. 2003, 264, 128–140.
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forming material and the subphase temperature and pH in the resulting isotherms are investigated. The results obtained in this work provide new information regarding the aggregation phenomenon, compressibility, and the mechanisms of destabilization of bitumen, asphaltene, and maltene monolayers. 2. Experimental Setup and Procedure 2.1. Bitumen Samples and Chemicals. ACS-grade toluene (99.7%) and puriss. p.a.-grade n-heptane (99.5%) were purchased from Sigma-Aldrich Chemical Co. ACS-grade 2-propanol (99.8%), ACS-grade hydrochloric acid (37%), and ACS-grade sodium hydroxide (99%) were provided by Scharlau Chemie Co. All of these products were used as received without further purification. Ultrapure water (resistivity of 18.2 MΩ cm) obtained from an Elgastat UHQ II system was used as the subphase in all the experiments. The pH of the pure water was changed by using dilute solutions of the hydrochloric acid and sodium hydroxide. Bitumen-in-toluene solutions were obtained in a previous set of experiments by means of supercritical extraction from oil shales coming from Puertollano (Spain) using toluene as solvent.17 In order to completely eliminate any solids, the solution was filtered with a 0.2 µm nylon-66 membrane. After filtration, toluene was removed by natural evaporation in a hood. The final bitumen-in-toluene solution used in this work was obtained by dissolving the solidfree bitumen in toluene to a concentration of 1.19 mg/mL. The asphaltene fraction was extracted from the solid-free bitumen by precipitation with n-heptane (volume ratio 1:40). After stirring the mixture for 2 h, the supernatant was separated from the precipitate and an excess of n-heptane was again added. This washing process was repeated until the supernatant became close to colorless. The precipitate, which constitutes the asphaltenes, is dissolved in toluene to a concentration of 1.17 mg/mL. The maltene fraction was obtained after natural evaporation of the n-heptane from the solution obtained as supernatant from the successive washings of the asphaltenes. Finally, the maltene fraction was dissolved in toluene to a concentration of 1.38 mg/mL. 2.2. Langmuir Trough. Interfacial studies were performed using a 612D Nima Technology trough equipped with two PTFE (polytetrafluoroethylene) barriers that move symmetrically toward the center of the trough compressing the monolayer of molecules on the water surface. The symmetric compression of the monolayer allows the use of lower barrier speeds, reducing the resulting shear resistance. The trough has an area of 600 cm2, and it is made of virgin PTFE supported by an anodized alloy support plate. A Wilhelmy plate attached to a force balance is used to measure the interfacial pressure (π) and the signal from the balance controls the moving barriers. The plate is a strip of chromatography paper (Whatman’s Chr 1). The area between the barriers (A), the surface pressure, and the time are monitored during each experiment. In order to assure the cleanliness of the trough working environment, the trough is kept inside a horizontal laminar flow cabinet model IDL-78H, made by Indelab. The temperature of the subphase was maintained constant by means of an external temperature controller unit Lauda RE 104. 2.3. Experimental Procedure. Experimental runs were started by wiping the trough over with a Kimtech Science paper tissue moistened with 2-propanol. Ultrapure water was then poured until filling up the trough. Cleanliness of the water was checked simply by producing an isotherm at a high compression rate. If the isotherm is not completely flat, an aspirator pump is used to suck off any dust or contaminant at the water surface. Once the subphase is completely clean, a given volume of the particular surfactant-intoluene solution is spread drop by drop over the water surface. The solvent is allowed to evaporate for 10 min. Unless otherwise (17) García-Hourcade, L.; Torrente, C.; Galan, M. A. Study of the solubility of kerogen from oil shales (Puertollano, Spain) in supercritical toluene and methanol. Fuel 2007, 86, 698–705.
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Figure 1. π vs a isotherms of bitumen, asphaltenes, and maltenes on pure water at 20 °C compressed at 20 mm/min.
mentioned, the barrier speed is set at 20 mm/min and the temperature at 293 K.
3. Experimental Results and Discussion 3.1. Surface Pressure vs Area (π–a) Isotherms. Typical isotherms of the bitumen and its fractions, maltenes and asphaltenes, are shown in Figure 1. The added volume of each surfactantin-bitumen solution was selected in order to add approximately the same amount (0.11 ( 0.01 mg) of the compound in each case. The area is expressed per milligram of spread material (denoted as a). Considering that the asphaltenes form aggregates,2,10 expressing the area as a specific quantity is preferable since it does not lead to incorrect values of the sizes of molecules. The differences between the obtained isotherms shown in Figure 1 are obvious. These differences are described below regarding the shape, characteristic areas, and compressibility. The analysis of the shape of the surface pressure–area isotherms of monolayers of particular surface active materials at the air/water interface is usually based on slope changes. In this way, two-dimensional gas, liquid, and solid phases have been recognized.18 In the cases presented in Figure 1, different shapes of the resulting isotherms can be readily observed. According to the results obtained for asphaltenes, at large specific areas, the molecules are far enough apart and present negligible interaction with one another. At this stage, the surface pressure remains unchanged and the monolayer behaves as a 2D gas, being the molecules randomly oriented at the air/water interface. As the gaseous monolayer is further compressed, the isotherm takes off, initiating a steep increase that continues up to values of the surface pressure of 40 mN/m. At this point, a change in the slope of the curve is observed which has been associated with the folding and buckling of the monolayer.16 Further compression up to the limiting barriers position does not lead to a catastrophic collapse of the monolayer. Instead, the surface pressure increases slowly which indicates that the buckled monolayer at this state is still somehow elastic. (18) Gaines, G. L., Jr. Insoluble Monolayers at the Gas-Liquid Interfaces; John Wiley & Sons: New York, 1966.
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The inexistence of slope changes in asphaltene isotherms has been previously reported by other authors with different asphaltene origin.2,8,10,16 Zhang et al.16 attributed the nonexistence of phase transitions to the mixture of molecules with different transition points that constitute the asphaltene fraction. However, if this were the reason, bitumen isotherms would have the same kind of shape. This, as will be shown later, is not the case, and bitumen isotherms show clear transitions between gas, liquid expanded, and liquid condensed phases. On the contrary, the steep curve obtained with asphaltenes may be indicating a transition directly from the gas phase to a condensed phase (probably liquid condensed). Equivalent types of isotherms are obtained with typical monolayer forming materials (such as carboxylic acids) at high pH in the presence of divalent (even trivalent) counterions.19,20 The liquid phase present in isotherms at sufficiently low pH values in the absence of counterions starts shrinking at very low concentrations of cations in the subphase at higher pH values and eventually disappears as the concentration increases. This is basically due to the condensing effect of the counterion caused by the formation of salts formed by two or three ionized amphiphilic molecules and the cation. Equivalent behavior of the asphaltenes is likely to be the reason for the nonexistence of the liquid phase. The formation of association structures analogous to the salts mentioned above impedes the existence of the liquid expanded phase, and the monolayer evolves as the surface pressure is increased from an expanded state to a condensed state. The isotherm obtained for bitumen is equivalent to the characteristic isotherm of a carboxylic acid with three very well differentiated regions:18 the gas phase, at low surface pressures, the liquid expanded phase that appears as the compression persists up to a surface pressure of around 17 mN/m, and the liquid condensed phase that is present from π ≈ 17–33 mN/m. There is a last region, observed at higher surface pressures, that corresponds to the folding of the bitumen monolayer at the air–water interface, equivalent to that observed in the asphaltene isotherm. The maltene isotherm is equivalent to that of bitumen with the gas and the liquid regions well differentiated. However, the maltene films can be compressed to much smaller areas, and the condensed and buckling regions (if they exist) cannot be detected clearly even at the maximum compression attainable with the barriers, although a change in the slope of the isotherm is obtained at a surface pressure of about 14 mN/m. The ability of the maltenes of packing at very small areas without reaching a condensed state is due to the different surface active materials that are composed of. Among them, the resins have been shown2 to display an analogous behavior of that of maltenes, something that has been interpreted as the formation of an overfilm at the same time that the molecules are forced together by the increase in the surface pressure. The differences between the isotherms obtained for the bitumen and the two fractions, asphaltenes and maltenes, are based on their composition. The asphaltenes form aggregates and show a condensed state at higher specific areas than the other two components probably due to the formation of particles. On the contrary, the maltenes show an expanded state at low specific areas probably due, in this case, to the formation of an overfilm. The case of bitumen is a mixture of both cases mentioned above. The presence of maltenes is likely to hamper the formation of large aggregates of asphaltenes, allowing for (19) Langmuir, I. Mechanical properties of monomolecular films. J. Franklin Inst. 1934, 218, 143–171. (20) Myers, R. J.; Harkins, W. D. Effect of traces of metallic ions on films at interfaces and on the surface of water. Nature (London) 1937, 139, 368.
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Table 1. Characteristic Area Values of Bitumen, Asphaltene, and Maltene Monolayers a (m2/mg) bitumen asphaltene maltene
a0
aT
aC
0.37 0.42 0.34
0.126
0.095 0.264
0.128
the existence of a liquid expanded phase in the bitumen. Additionally, the presence of asphaltenes allows for the existence of a condensed state. For comparative purposes only, the extrapolated specific areas (a0), the critical areas (aC), and the transition areas (aT) obtained from the isotherms shown in Figure 1 are shown in Table 1. The differences between the three fractions are evident and show, qualitatively, the different monolayer behavior at the air–water interface of bitumen, asphaltenes, and maltenes. In order to quantify the rigidity of the films, the compressibility (C5) is defined18 as Cs ) -
1 dA A dπ
(1)
This parameter is very important to quantify the stability of emulsions.2 The higher the compressibility, the easier the emulsion droplets coalesce while a rigid film avoids coalescence phenomena. According to the results obtained for the compressibility, which confirm the qualitative explanations given above, the maltene film displays the highest compressibility (least rigidity) with C5 ≈ 0.075 m/mN while the asphaltene film the lowest with C5 ≈ 0.012 m/mN. Bitumen films show a variation of compressibility depending on the surface pressure. In this way, when the liquid expanded state prevails, C5 ≈ 0.05 m/mN. As the surface pressure is increased, the transition from liquid expanded to liquid condensed takes place with the consequent decrease in C5 until reaching a value of around 0.025 m/mN when the transition is completed. 3.2. Monolayer Hysteresis. The results of three consecutive compression–expansion cycles of bitumen, asphaltene, and maltene monolayers at 20 °C and 20 mm/min are shown in Figure 2. The maximum surface pressure at which the monolayers were compressed was set to 20 mN/m. This value is safely below the value of pressure at which collapse takes place for the three components. A period of 15 min between compression–expansion cycles was allowed for the relaxation of the film. Comparing the three compression–expansion cycles obtained for each of the fractions (Figure 2), three characteristics are readily observed: • First, according to the results obtained in this work, the compression and expansion curves for the successive cycles are not superimposable for any of the three substances, evidencing the existence of a hysteresis phenomenon. The existence of hysteresis, which is not observed with fatty acid monolayers, indicates that the molecules in the monolayer do not relax back to the initial state. According to Figure 2, it is clear that large hysteresis is displayed by the asphaltenes and the bitumen while the maltenes show small hysteresis. A possible explanation is that the maltenes present the highest flexibility and are able to almost recover their initial state even after three compression– expansions cycles. Resins, a fraction of the maltenes, were studied by Esse et al.,2 who reported that due to their polar nature they do not associate in particles to the same degree as the asphaltenes. If, as resins, maltenes are assumed to slightly associate, this would explain the lower degree of hysteresis. Additionally, Lobato et al.10 reported that the hysteresis observed
Figure 2. Compression–expansion cycles of bitumen, asphaltene, and maltene films on pure water at 20 °C. Barriers velocity: 20 mm/min.
for asphaltenes was likely to be due to the formation of association structures. However, Zhang et al.,16 who studied the hysteresis of asphaltenes, discarded the possibility of a definitive role of relaxation dynamics in the observed hysteresis. In this way, they proposed that the hysteresis of the asphaltenes was due to the formation of multilayers at the air/water interface or to the dissolution into the water subphase of the smaller molecules present in the mixture of substances that constitute the asphaltenes. However, if this were the case, the lost of material would displace the compression–expansion cycles to the left (lower specific areas).9 However, as will be explained later, in the cases presented here the exact opposite occurs. The behavior displayed by the bitumen corresponds to the mixture of asphaltenes and maltenes. After the first compression, the asphaltenes and maltenes have formed particulate structures that do not relax back to the original structure, according to the expansion path. • Second, successive hysteresis loops are repeatable; that is, the shape of the loop after the first cycle is roughly the same as after successive cycles for the three compounds. These results differ from those obtained with asphaltenes by other authors10,16 who reported a significant reduced hysteresis as the number of compression–expansion cycles was increased. Those results were interpreted as irreversible changes suffered by the asphaltene monolayer after the first compression. Lobato et al.10 reported that the reduction of the hysteresis observed for asphaltenes was likely to be due to the persistence of structures after the expansion of the monolayer. Something similar is likely to be
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Figure 3. Stability of bitumen, asphaltene, and maltene monolayers on pure water at 20 °C. (a) Comparison between the three components. (b, c, d) Comparison for a single component after one compression at 20 mm/min, 10.5 mm/min, and a compression–expansion cycle.
taking place in the cases presented here for asphaltenes and to a much lower extend for the maltenes. However, the unchanged hysteresis loops obtained here indicate either that (1) the formation of new association structures is only partial at 20 mN/ m, and several cycles (more than three) are needed to complete the total number of aggregates possible, or that (2) after the first cycle, some of them relax back to the original state while the rest stays as aggregates. If all of the structures could relax back, no hysteresis would be observed. On the other hand, if all of the structures continued, a decrease on the compressibility would be expected. • Finally, the compression–expansion cycles do not cross for either bitumen or asphaltenes and maltenes. As the number of cycles increases, the isotherms both during the compression and expansion sections are displaced toward higher specific areas. For the case of maltenes, this displacement, although the hysteresis is not remarkable, is reduced as the number of cycles increases. On the contrary, the asphaltenes show a remarkable displacement that increases as the number of cycles is increased. Bitumen shows an intermediate behavior being the mixture of asphaltenes and maltenes. The displacement is pronounced after the first cycle of compression being reduced after the second one. Again, the formation of aggregates explains the increase in the specific area that corresponds to a fixed value of the surface pressure as a consequence of successive cycles. The formation of association structures leads to a transition from the gas phase to a condensed phase as the surface pressure is increased at progressively higher specific areas.
3.3. Monolayer Stability. Figure 3 shows the loss on the area occupied by monolayers of bitumen as well as asphaltenes and maltenes when held at a constant surface pressure of 20 mN/m. Stability is expressed in terms of the ratio, at constant π, of the film area A, at time t, to the area at zero time, A0, which corresponds to the area measured when the target surface pressure (20 mN/m) is reached. The variation of A/A0 with time is monitored for 150 min in order to study the stability of the films. Figure 3a shows the comparison between the stability of bitumen, asphaltene, and maltene films when the compression was performed at 20 mm/min. The shape of the three curves is equivalent: an initial sharp decrease on the relative area followed by a slower but continuous decrease. The maltene films also show a plateau after ∼2 h of compression. In this way, during the first hour, 75–80% of the observed area loss takes place. This may be indicating the existence of at least two different mechanisms accounting for the decrease of the area. Possible mechanisms are reorganization, dissolution, or collapse.18 The shape of the curves can give information on the particular cause of instability. In this way, if the curve is convex, the area loss occurs by collapse, while if it is concave, it occurs by dissolution. The kinetics of the collapse and dissolution processes can be very complicated even though first-order processes have been proposed.21 According to the results obtained for the three compounds, the variation of the ratio A/A0 (21) Binks, B. P. Insoluble Monolayers of Weakly Ionizing Low Molar Mass Materials and Their Deposition to Form Langmuir-Blodgett Multilayers. AdV. Colloid Interface Sci. 1991, 34, 343–432.
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follows a concave curve what would be indicating the dissolution of the monolayer into the subphase even though reorganization can be the predominant cause of the decrease on A/A0 at the initial stage. What it is clear from Figure 3a is that bitumen films are lost faster than asphaltene films and these faster than maltenes films. This means that, for maltenes, after reaching the target pressure, the formation of bi- or trilayers at the air–water interface does not persist, and after a period of reorganization, the film stays considerably stable. On the other hand, asphaltene and bitumen films remain unstable even after 150 min, being lost probably by dissolution into the subphase. Two additional studies have been performed in order to bring more information for the identification of the particular mechanism that leads to the loss of the monolayer. In this way, first, the velocity of compression to achieve the target pressure was reduced to 10.5 mm/min for each of the films, and second, the films were compressed to 20 mN/m, expanded, and compressed again. The results are shown in Figure 3b–d. Very interesting information can be obtained from the behavior of the films under these three different stability experiments. According to the results obtained, the greatest loss of monolayer takes place after the compression at 20 mm/min for the three components. However, the greatest decrease of the bitumen and maltene film losses takes place with a slower compression velocity while the asphaltene film loss is greatest reduced when using the routine of one cycle of compression–expansion. Additionally, after the first sharp decrease, the velocity of decrease of the relative area is very close for the three curves and the three compounds. These experimental facts lead to the following conclusions: • At the initial stage of the stability studies, the rearrangement and/or formation of association structures of bitumen and maltene molecules is significantly favored by slow compression rates. In contrast, asphaltene molecules show enhanced rearrangement probably forming aggregates after one cycle of expansion–compression at 20 mN/m. • After a certain period of time, the rate of bitumen and asphaltenes monolayer area decrease follows a first-order rate, probably due to dissolution, independently of the velocity of compression and the existence or not of previous cycles of compression–expansion. Additionally, the rate constant (k) is approximately constant for bitumen (k ≈ -7 × 10-4 s-1) as well as for asphaltene films (k ≈ -3 × 10-4 s-1). For the case of maltenes, instead of a first-order rate decrease, approximately constant values of the relative area are obtained after the first stage of rearrangement, meaning that maltene monolayers are not lost by dissolution. 3.4. Effect of Trough Concentration on Surface Pressure vs Area Isotherms. In this section, the influence of the amount of monolayer forming material spread over the air–water interface on the resulting isotherm is investigated. The spreading solution concentration is not changed, and by varying the volume spread, the initial total amount (in milligrams) is changed. Figure 4 shows representative results obtained for bitumen, asphaltene, and maltene films. The results are equivalent for all components: (1) As the initial total amount of monolayer material is increased, the isotherms are displaced toward decreasing specific areas, that is, toward increasing absolute areas. (2) The shape of the isotherms is not substantially affected by the total amount of surface active material initially added. From the results obtained, it seems clear that even before any compression of the monolayer, when it is still in the gas state, the molecules of these components arrange differently as the amount present in the gaseous monolayer is increased. Ese
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Figure 4. Influence of the initial amount of surface active material on the π vs a isotherms of bitumen, asphaltenes, and maltenes on pure water at 20 °C compressed at 20 mm/min.
et al.2 and Lobato et al.10 reported an equivalent behavior for asphaltenes when the spreading solution concentration was varied between 0.1–15 mg/mL and 1–8 mg/mL, respectively. They observed that, as the concentration of the spreading solution is increased, the area occupied by the monolayer at a given surface pressure increases, too. They attributed this behavior to a higher extent of formation of aggregates in the spreading solution that was kept after the spreading at the air–water interface. These larger or more numerous association structures would occupy less specific area at a constant surface pressure. Lobato et al.10 mentioned that even though the mechanism described above was probably the one explaining the variation on the area occupied by the monolayer at constant surface pressure when different spreading solution concentrations were used, it was not clear where the structural changes were taking place: at the spreading solution or at the air–water interface. Considering that in the results shown in Figure 4 the spreading solution concentration was kept constant for each of the components and that the behavior is equivalent to that reported by Ese et al.2 and Lobato et al.,10 it can be concluded that definitely the aggregation takes place at the air–water interface. This fact does not mean that it does not take place in the spreading solution also, but what is clear is that it does take place at the interface during the 10 min period that the monolayer stays before compression intended for evaporation of the solvent. Additionally, this enhancement of the formation
Langmuir Films of Bitumen and Its Fractions
Energy & Fuels, Vol. 21, No. 6, 2007 3461
the values of a0, aC, and aT increase as the temperature is decreased. The reason behind the contraction of the monolayers at higher temperatures is a consequence of the greater flexibility of the hydrophobic chains of the molecules of asphaltenes, maltenes, and bitumen. The greater flexibility is translated into a higher compressibility due to the formation of highly packed structures. Similar effects of the temperature on asphaltene films were shown by other authors8,10,16 while here we are showing approximately the same behavior on bitumen films. Maltene films, as mentioned above, are clearly affected by the reduction of the temperature while the increase of the temperature does not seem to further affect the compressibility. No effect of the subphase pH was observed on the resulting isotherms (results not shown) for pH values of 3, 5.5, and 8.75. Bitumen, asphaltene, and maltene films isotherms are basically equal at these three values of the pH. Zhang et al.16 show the same behavior for asphaltene films. The reason behind this behavior can be related to the packing of the hydrophilic head groups and the hydrophobic alkyl chains. Considering that the variation of the pH affects the ionization of the head groups which could cause the condensation of the monolayer due to the formation of salts18 and the fact that the ionization of the head groups does not affect the resulting isotherms, the alkyl chains need to be responsible for the resulting packing of the bitumen, asphaltene, and maltene films. Conclusions
Figure 5. Influence of the subphase temperature on the π vs a isotherms of bitumen, asphaltenes, and maltenes on pure water.
of aggregates caused by the increased of the amount of monolayer forming material added to the interface is observed for the three components which confirms that both asphaltenes and maltenes aggregate to some extent at the air–water interface. 3.5. Temperature and pH Effect on Surface Pressure vs Area Isotherms. The effect of the subphase temperature on the resulting isotherms of the three components is shown in Figure 5. The three temperature values were measured directly in the trough being 12, 20, and 32 °C. According to the results obtained, the shape of the isotherms does not seem to be affected by the temperature although higher surface pressures are attainable at lower temperatures. It is also clear, in view of Figure 5, that the isotherms are displaced toward lower specific areas as the temperature is increased, except for the maltene films which do not show clear differences between the temperatures of 20 and 32 °C. As a consequence of this displacement,
Interfacial properties of bitumen and its fractions, asphaltenes and maltenes, were analyzed using a Langmuir trough. According to the results obtained in this work, asphaltenes form rigid films containing aggregates that show a condensed state, even at high specific areas, and large hysteresis. Maltene films, also formed by aggregates, present the highest compressibility and form an overfilm under increasing surface pressure values. Maltene particulate structures show small hysteresis, and they are able to almost recover their initial state after expansion of the film. Bitumen films present an intermediate behavior as the presence of maltenes is likely to hamper the formation of large aggregates of asphaltenes. Monolayer stability studies reveal that the three components undergo rearrangement and/or formation of association structures first, a phenomenon that is favored by slow compression rates in the case of bitumen and maltenes and by compressing and expanding the monolayer in the case of asphaltenes, followed by dissolution in the case of bitumen and asphaltene films. The temperature of the subphase does not affect the shape of the isotherms of none of the components, although, in general, the isotherms are displaced toward lower specific areas as the temperature is increased due to the greater flexibility and compressibility of the hydrophobic chains of the molecules of asphaltenes, maltenes, and bitumen. No effect of the subphase pH was observed on the resulting isotherms being the alkyl chains responsible for the resulting packing of the films. Acknowledgment. This research was performed under grants from the Junta de Castilla y León (SA022B06) and from the Ministerio de Educación y Ciencia (Spain) (CTQ2006-01945/PPQ). EF7004095
