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Langmuir Films of Asphaltene Model Compounds and Their Fluorescent Properties Erland L. Nordgård,*,† Eva Landsem,‡ and Johan Sjo¨blom† Ugelstad Laboratory, Department of Chemical Engineering, and Department of Chemistry, Norwegian UniVersity of Science and Technology (NTNU), Trondheim, Norway ReceiVed March 26, 2008. ReVised Manuscript ReceiVed May 18, 2008 The relationship between the physicochemical properties of asphaltenes and asphaltene structure is an issue of increasing focus. Surface pressure-area isotherms of asphaltene model compounds have been investigated to gain more knowledge of their arrangement at an aqueous surface. Variations in interfacial activity have been correlated to proposed arrangements. The presence of a carboxylic acid has shown to be crucial for their interfacial activity and film properties. The acid group directs the molecules normal to the surface, forming a stable monolayer film. The high stability was absent when no acidic groups were present. Fluorescence spectra of deposited Langmuir-Blodgett films showed only the presence of the excimer emission for thin films of acidic model compounds, indicating a close face-to-face arrangement of the molecules. Time-correlated single photon counting (TCSPC) of the model compounds in toluene indicated the presence of aggregates for two of four compounds at low concentrations. However, a sudden drop of interfacial tension observed could not be correlated to the aggregation. Instead, aggregation induced by addition of a “poor” solvent showed decreased interfacial activity when aggregated due to decrease of monomers in bulk. The findings regarding these asphaltene model compounds and their structural differences show the great effect an acidic group has on their physicochemical properties.
Introduction “To understand function, study structure”. This is a famous phrase made by Francis Crick,1 the discoverer of the double helix of DNA, in 1953.2 The asphaltenes are one class of compounds present in crude oil. However, the asphaltenes are not defined by their structure, but as a solubility class. They are commonly defined as soluble in toluene, but insoluble in shortchained hydrocarbons such as n-heptane to n-pentane.3–6 The knowledge of asphaltene composition has increased a great deal, and analysis has shown the asphaltenes to be sheet-like polyaromatic hydrocarbons built up from carbon and hydrogen, together with varying amounts of heteroatoms such as nitrogen, oxygen, and sulfur, in addition to traces of metals like nickel and vanadium.7 However, these results give an average over the whole distribution, and the exact characterization of chemical structures and functional groups is still unattainable. In addition, the amount and structure of the asphaltenes may vary from well to well. Chemical composition is not the only unanswered controversy regarding asphaltenes. The molecular weight of asphaltenes has been an issue of discussion for several decades.6,8 Going from tens of thousands in value years ago,9 the currently accepted value of asphaltene molecular weights now seems to range from * Corresponding author. Phone: +47 73550325, Fax: +47 73594080, Sem Sælandsvei 4, N-7491 Trondheim, Norway. Author E-mail address:
[email protected]. † Department of Chemical Engineering. ‡ Department of Chemistry.
(1) Petsko, G. A. Genome Biol. 2000, 1, 1–2. (2) Crick, F. Gene 1993, 135, 15–18. (3) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972–978. (4) Poteau, S.; Argillier, J. F.; Langevin, D.; Pincet, F.; Perez, E. Energy Fuels 2005, 19, 1337–1341. (5) Zhang, L. Y.; Breen, P.; Xu, Z.; Masliyah, J. H. Energy Fuels 2007, 21, 274–285. (6) Schneider, M. H.; Andrews, A. B.; Mitra-Kirtley, S.; Mullins, O. C. Energy Fuels 2007, 21, 2875–2882. (7) Speight, J. G. In The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1998; pp 419-436. (8) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677–684.
500 to 1000 amu, with an average of 750 amu.8 The most promising methods today are the laser desorption ionizationmass spectrometry (LDI-MS)10 and fluorescence correlation6 and depolarization spectroscopy.8 The problem of solving this controversy arises from the self-aggregation tendencies of asphaltenes. Strong interactions through π-electrons arising from the aromatic cores dominating the average asphaltene structure3 is one reason for self-aggregation. These interactions cause instability when exposing the crude to a solvent of poor polar and hydrogen bonding character like n-heptane, together with polar interactions through the heteroatoms present. These two types of intermolecular interactions are also assumed to be the reason for the asphaltenes ability to stabilize emulsions via strong viscoelastic films,11 and the strong adsorption to surfaces.12 There seems to be two approaches to knowledge of asphaltene properties with respect to structure and functional groups. One is the continuing practice of dividing the whole asphaltene solubility fraction into numerous subfractions and studying the subfractions separately to find which subfraction is mainly responsible for undesired properties.9,13–16 The other is the use of model compounds with known chemical structures and similar physicochemical properties to those of asphaltenes. Asphaltene model molecules have been synthesized and investigated in (9) Zhang, L. Y.; Lawrence, S.; Xu, Z.; Masliyah, J. H. J. Colloid Interface Sci. 2003, 264, 128–140. (10) Martı´nez-Haya, B.; Hortal, A. R.; Hurtado, P.; Lobato, M. D.; Pedrosa, J. M. J. Mass Spectrom. 2007, 42, 701–713. (11) Singh, S.; McLean, J. D.; Kilpatrick, P. K. J. Dispersion Sci. Technol. 1999, 20, 279–293. (12) Menon, V. B.; Wasan, D. T. Colloids Surf. 1986, 19, 89–105. (13) Fossen, M.; Kallevik, H.; Knudsen, K. D.; Sjo¨blom, J. Energy Fuels 2007, 21, 1030–1037. (14) Fossen, M.; Sjo¨blom, J.; Kallevik, H.; Jakobsson, J. J. Dispersion Sci. Technol. 2007, 28, 193–197. (15) Wattana, P.; Fogler, H. S.; Yen, A.; Carmen Garcia, M. D.; Carbognani, L. Energy Fuels 2005, 19, 101–110. (16) Horvath-Szabo, G.; Masliyah, J. H.; Elliott, J. A. W.; Yarranton, H. W.; Czarnecki, J. J. Colloid Interface Sci. 2005, 283, 5–17. (17) Akbarzadeh, K.; Bressler, D. C.; Wang, J.; Gawrys, K. L.; Gray, M. R.; Kilpatrick, P. K.; Yarranton, H. W. Energy Fuels 2005, 19, 1268–1271.
10.1021/la800945m CCC: $40.75 2008 American Chemical Society Published on Web 07/25/2008
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several studies using modified polyaromatic compounds.6,8,17–19 These studies have mainly focused on the issue of aggregation or the molecular weight, and there has been suggested aggregation due to π-stacking as a result of aromatic interactions as well as hydrogen bonding interactions. Little attention has been focused on the use of model compounds in studies of interfacial tension, film properties, and emulsion stability. We have recently synthesized and investigated three model compounds for asphaltenes based on a perylene bisimide (PBI) core with different substitutions.20 In contrast to other studies using the PBI core,6,8 we utilized different substituents on only one side, giving asymmetric model molecules with one polar and one nonpolar part, which can be considered polyaromatic surfactants. These molecules were highly interfacially active. Fossen et al. have examined the dynamic interfacial tension profiles of asphaltenes divided into both two and four subfractions, varying the heptaneto-toluene ratio as model oil.13,14 Even though most asphaltene samples are only moderately interfacially active with γo/w in the region 25-30 mN/m at 0.1 g/L in toluene, a subfraction obtained at 18:1 heptane-to-toluene ratio has been shown to be distinctly more interfacially active, lowering the value to 15-20 mN/m.13 The characterization of this subfraction showed that this group contained more branched aliphatic chains, and perhaps most important, contained larger amounts of hydroxylic and carboxylic groups.21 These highly polar and hydrogen bonding groups can greatly enhance the interfacial activity by directing these groups into the aqueous phase at an interface. The presence of an acidic function would also lead to a pH dependence of the interfacial tension due to higher interfacial activity of charged species when the molecules come in contact with an alkaline phase. This pH dependence has in fact been shown for asphaltenes. The interfacial activity and emulsion stability was enhanced at both acidic (pH < 4) and alkaline (pH > 8) conditions indicating that the asphaltenes contained both acidic and basic functions and the presence of these was responsible for the properties of the whole fraction.4 The asphaltenes stabilize emulsions through elastic interfacial films which prevent the coalescence of emulsion droplets. Irreversible adsorption to the interface is one factor which increases the stability, and the strong viscoelastic properties are a result of strong interactions between the molecules at interfaces.11 It has been observed that the interfacial film formed by asphaltene adsorption can be flexible or rigid, dependent on the state of the asphaltene and the amount of resins in the film.22 Adsorption onto oil/water and oil/gold interfaces was studied by quartz crystal microbalance and a non-steady-state region was observed at low concentrations, but the adsorption reached steady state at high concentrations of asphaltenes. The threshold concentration was dependent on the resin-to-asphaltene ratio and related to the rigid-to-flexible transition of the film. A model was proposed where the rigid film with small amounts of resins was not sterically stabilized toward flocculation and bridging between emulsion droplets could occur. This model supports the observation that water-in-oil emulsions are stable toward flocculation at higher concentrations of bitumen. The rigidity at low concentrations stems from the strong bonds between nonstabilized asphaltenes while the flexibility is due to steric (18) Rakotondradany, F.; Fenniri, H.; Rahimi, P.; Gawrys, K. L.; Kilpatrick, P. K.; Gray, M. R. Energy Fuels 2006, 20, 2439–2447. (19) Lopez-Linares, F.; Carbognani, L.; Gonzalez, M. F.; Sosa-Stull, C.; Figueras, M.; Pereira-Almao, P. Energy Fuels 2006, 20, 2748–2750. (20) Nordgård, E.; Sjo¨blom, J. J. Dispersion Sci. Technol. 2008, in press. (21) Fossen, M.; Kallevik, H.; Lobato, M. D.; Knudsen, K. D.; Sjo¨blom, J. To be submitted to Energy Fuels. (22) Goual, L.; Horvath-Szabo, G.; Masliyah, J. H.; Xu, Z. Langmuir 2005, 21, 8278–8289.
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repulsion among resin-stabilized asphaltenes. To understand what causes the strong interactions of the rigid film, a better understanding of the asphaltenes at a molecular level is needed. The geometry and arrangement of the asphaltene molecules at interfaces is still a problem to be addressed. For a long time, an arrangement with the polycyclic ring lying flat on the surface, or interface, and the alkyl chains pointing away has been accepted.9,11,23 However, a recent study has questioned this assumption.24 Lobato et al. showed on an Arabic asphaltene sample that the increase in surface pressure on a Langmuir film occurred at a mean molecular area of much lower size (>100 Å2) than what is possible for a flat-on arrangement. The authors proposed that the molecules were either tilted or associated in stacked arrays. A tilted or head-on arrangement will give a film where the aromatic sheets are arranged face-to-face normal to the surface, and this gives a reasonable explanation for the irreversibility and the viscoelastic properties of asphaltene films. The formation of aromatic sheets normal to the aqueous phase will give a strong stabilization through intermolecular π-electrons. It is essential to know the arrangement of asphaltenes at a surface or an interface to be able to avoid, or inhibit, the formation of such stable films. In this paper, we use polyaromatic surfactants as model molecules for asphaltenes to investigate the reason for high interfacial activity, and correlate formation of highly stable Langmuir films to a head-on arrangement of the molecules via fluorescence spectroscopy of deposited Langmuir-Blodgett films. The morphology of the films has been visualized by Brewster angle microscopy (BAM). Results of the model compounds are compared to results from real asphaltene samples, and questions regarding importance of acidic groups and arrangement of molecules at the surface are discussed.
Experimental Section The synthetic routes toward asphaltene model compounds have been published elsewhere.20,25 Contrary to our previous experiments, the molecules have now passed an extra purification step, where they have been purified with flash chromatography using a 95/5 mixture of chloroform and methanol as the eluent. Highly pure compounds are necessary in Langmuir experiments for true surface concentrations, which is essential for the calculation of the mean molecular area. The outcome of reactions and purity of the products has been confirmed by 1H NMR and TLC analysis, and the purities are estimated to be >98%. Four model molecules were investigated in this study, and their structures, molecular weights, and abbreviations are given in Figure 1. Interfacial tensions (IFT) of the model compounds were measured with a CAM 200 pendant drop equipment (KSV Instruments) which measures the dynamic interfacial tension between an oil phase and an aqueous phase as a function of time. The instrument is equipped with a CCD video camera with telecentric optics, a frame grabber, and an LED based background light source. Images of the oil droplet were taken every 2 s up to 500 s and the diameter of the syringe was 0.7 mm. The model compound was dissolved in a 0.5 mM stock solution in toluene, and diluted to the desired concentration with the appropriate solvent. In all cases the aqueous phase consisted of a pH 9 buffer obtained from Merck. The experiments were conducted forming an oil droplet (∼3-7 µL) from a syringe upward in the buffer solution. Surface pressure-area isotherms were recorded using a KSV Langmuir trough of effective film area 364 × 75 mm (Finland) with a double barrier system and a Wilhelmy plate of platinum. All experiments were performed at room temperature. The trough was (23) Leblanc, R. M.; Thyrion, F. C. Fuel 1989, 68, 260–262. (24) Lobato, M. D.; Pedrosa, J. M.; Hortal, A. R.; Martinez-Haya, B.; LebronAguilar, R.; Lago, S. Colloids Surf., A 2007, 298, 72–79. (25) Holman, M. W.; Liu, R.; Adams, D. M. J. Am. Chem. Soc. 2003, 125, 12649–12654.
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Nordgård et al. emission wavelength of 640 nm, which covers both photons from monomers and the aggregated state. For C5 Pe and BisA, the lifetimes could be fitted to monoexponential decays, while PAP and TP could be fitted well with biexponential decays. χ2 values were close to unity. (See Supporting Information).
Results and Discussion
Figure 1. Structures, abbreviations, and molecular weights of four molecules synthesized as asphaltene model compounds to be studied. Correct nomenclature is, using C5 Pe as an example, N-(1-hexylheptyl)N′(5-carboxylicpentyl)-perylene-3,4,9,10-tetracarboxylicbisimide.
made of Teflon and the barriers of Delrin. The trough and barriers were thoroughly cleaned with acetone, ethanol, tap water, and ultrapure water before use, and by aspirating the surface with a Pasteur pipet connected to a vacuum-aspirator. The surface was assumed pure when the surface pressure of subphase did not exceed 0.20 mN/m upon full compression. Isotherms were recorded at different pH values of the aqueous subphase: pH 6 using ultrapure water with a resistivity of 18.2 Ω, pH 9 buffer from Merck, and pH 7 from Fluka. pH 11.1 and pH 3 buffers were prepared using Na2HPO4/NaOH and potassium hydrogen phthalate/HCl, respectively. Asphaltene model compounds were dissolved in spectrophotometric grade chloroform at a concentration of 1.0 mM and an amount of 40-50 µL was spread on the subphase with a 50 µL Hamilton syringe, depending on the compound or experiment. The solvent was allowed to evaporate over 5 min before compression. The compression of the films was carried out at a barrier speed of 5 mm/min and the target pressure chosen to be 100 mN/m. Langmuir-Blodgett depositions of the films formed were carried out using a KSV dipping arm onto thin glass plates with thickness of 0.14 mm. The film was deposited in the upward direction with the glass plate in the subphase before spreading. The films were deposited at a constant surface pressure of 20 mN/m with a dipping speed of 1 mm/min and transfer ratios near unity, except for the BisA derivate. A KSV Brewster angle microscope (BAM) was utilized for visualization of the film morphology. This instrument is placed above the trough with a laser directed normal to the trough direction and at the Brewster angle in the z-direction. A standard 10 mW He-Ne laser with a Glan-Thomson polarizer emits p-polarized light. To avoid stray light from the surface, a black wedge-shaped plate was placed at the bottom where the laser was pointed. Digital photos were taken with a 768 × 494 pixels CCD camera through a 10× magnification and a spatial resolution of ∼2 µm. Steady-state (SS) fluorescence spectra were obtained with a Jobin Yvon Fluorolog 3 spectrophotometer with Fluoresscence software. The light source was a 450 W Xe-lamp filtered through singlegrating monochromators. Samples were excited at 495 nm and frontface illumination geometry was utilized due to the high optical density of the samples. For solutions, slits of 0.5 mm were used, while the slit widths were 5 mm in the case of LB films. SS spectra of the LB films were carried out by carefully placing the glass slide in between the cuvette holder and an empty cuvette. Fluorescence lifetime of the same samples used in the IFT measurements was recorded with the Datastation software and fitted using the DAS6 software program. A 495 nm nano-LED diode operating at 1 MHz repetition rate was the excitation source and the photons were detected at an
Interfacial Properties of Asphaltene Model Compounds. The interfacial tensions of the model compounds with acidic groups between toluene and pH 9 in the region 1-20 µM are shown in Figure 2a. This concentration region was chosen because in this region a sudden drop in the IFT was observed in previous studies of such compounds.20 All compounds show the same high interfacial activity and the shapes of the curves are very similar. The previous study showed a more gradual drop in the IFT for PAP, but this has later been assigned to more contamination of the BisA derivate, which is one intermediate toward the asymmetric compounds, resulting in inaccurate concentration of the active molecule, PAP. The sudden drop in IFT is distinct for all acidic compounds. The value of γo/w is reduced to half by increasing the concentrations from 5 to 10 µM. Comparing the values of γo/w of all four model compounds at 50 µM, given in Table 1, indicates what is necessary for high interfacial activity. The three acidic molecules are very interfacially active, while the nonacidic BisA can hardly be considered interfacially active, with an interfacial tension as high as 32 mN/m. Even though the aromatic core is equal and there are several polar CdO bonds in the compound, the absence of the acidic group changes the behavior of BisA completely compared to the three other model compounds. The time dependence in Figure 2b also indicates that the surface active compounds adsorb quickly to the interface and reach equilibrium within 100 s. The latter also supports that the synthesized compounds have high purity because impurities will increase the time needed to reach equilibrium. A difference in the interfacial tension is also observed between C5 Pe, with an aliphatic acid as the headgroup, and the two compounds with aromatic ring(s) present in the headgroup. Another interesting observation is that TP and PAP gives the same values, even for a larger headgroup in TP. To explain this, a hypothesis of how these molecules arrange themselves at an aqueous surface must be established. Langmuir Isotherms of Model Compounds. The Langmuir technique is very suitable for identifying molecular arrangements, phases, and phase transitions in films of monomolecular thickness. The value of the mean molecular area when the surface pressure changes gives information of how much space a molecule requires in a phase. Langmuir isotherms of all model compounds at pH 6 are shown in Figure 3a. The acidic compounds collapse at a high surface pressure, indicating formation of stable films. A collapse of a film can be found as a rapid decrease in pressure from a solid state, or as a horizontal break from a liquid state. The compounds under investigation show smooth leveling off of the surface pressure, likely to be associated to a three-dimensional collapse from the liquid state, and from the steepness of the slope most likely from the liquid-condensed state (L2). Another important observation is the difference in molecular size between C5 Pe and the two aromatic-acidic PAP and TP. A configuration where all aromatic parts are aligned with their aromatic cores flat on the surface, a flat-on configuration, would also yield differences between PAP and TP because the aromatic group in the head of TP is larger. A measurement of the space each molecule requires in the film can be considered to be the limiting area (A0) which is obtained by extrapolation at the intersection of the abscissa axis
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Figure 2. (a) Interfacial tension of three acidic asphaltene model compounds between toluene and pH 9. The values at each concentration have been taken as the average of the 10 last seconds of a 500 s measurement. (b) Time-dependent interfacial tension curves at 50 µM for all model compounds. Table 1. Interfacial Tension of Asphaltene Model Compounds at 50 µM after 500 s between Toluene and pH 9 compound C5 Pe PAP TP BisA
IFT [mN/m] 1 4 4 32
with the tangent at the Π-A isotherm. For PAP and TP, A0 is 60 and 61 Å2 respectively, while C5 Pe has an A0 around 46 Å2. All values are inconsistent with a flat-on arrangement. Modeling of the bisimide core in other studies demonstrated the core to have an area of 14.2 × 9.2 Å,26 giving a size of the core from nitrogen to nitrogen to be 130.6 Å2. Taking into account that the alkyl chains also need some space in the core plane, the value for a flat-on arrangement is too high. The edge-on arrangement, where the longest side of the aromatic core touches the surface, is another possibility. With an intermolecular distance of 3.4 Å as a minimum27,28 the molecular area for C5 Pe without the alkyl chains is 48.3 Å2. The limiting area found was 46 Å2, so the edge-on arrangement might be possible taking some deviations into account. Again however, the branched and linear alkyl chains will demand extra space, so the theoretical area with the edge-on arrangement will be even higher. In addition, assuming the edgeon arrangement, TP should have a limiting area difference of more than 1 Å2 compared to PAP due to the extra pyrrole ring. The last possibility is the head-on configuration, where the shortest side of the aromatic core touches the surface, giving a minimum molecular area of 31.2 Å2. This arrangement will direct the acidic groups into the aqueous phase, the branched hydrocarbon chain out from the surface, and all the aromatic cores face-to-face. Taking into account that the branching of the hydrocarbon side of the core will force the molecules apart, no large differences either in A0 or AC of PAP and TP indicates that the aromatic ring(s) in the polar head groups must be aligned in the same direction as the molecular plane. PAP and TP has branching in the polar head as well, which explains why these two compounds have a larger molecular area than C5 Pe. The last compound under consideration, BisA, surprisingly shows a lower value for molecular area. The polar groups of BisA are in the core, so a (26) Antunes, P. A.; Constantino, C. J. L.; Aroca, R. F.; Duff, J. Langmuir 2001, 17, 2958–2964. (27) Liu, S. G.; Sui, G.; Cormier, R. A.; Leblanc, R. M.; Gregg, B. A. J. Phys. Chem. B 2002, 106, 1307–1315. (28) Chen, Z.; Stepanenko, V.; Dehm, V.; Prins, P.; Siebbeles; Laurens, D. A.; Seibt, J.; Marquetand, P.; Engel, V.; Wu¨rthner, F. Chem. Eur. J. 2007, 13, 436– 449.
flat-on arrangement would seem logical. One explanation for the low molecular area calculated might be that BisA is not arranged as a monolayer, but stacked in columns which in turn are aligned flat-on. The flat-on arrangement would also give interactions with the maximum number of polar groups of each molecule with the aqueous surface. When having more than one layer on the surface, the intention of calculating a mean molecular area is no longer applicable. Hence, a conclusive arrangement of BisA cannot be made. Zooming in on the area where BisA increase in pressure, Figure 3b shows a jagged isotherm rather than a smooth increase, supporting that this point is beyond the film collapse and multilayers have started to be stacked. The edge-on arrangement into multilayers is the other possible arrangement, where molecules “slip out” of one layer to the next upon compression. This arrangement will decrease the number of polar group-water interactions, but will arrange the aromatic core planes normal to the surface and addition of more layers due to “slip-out” can be stabilized via carbonyl-carbonyl interlayer interactions. As a middle point between the stacked flat-on and the edge-on, a tilted arrangement is also plausible. The head-on arrangement for BisA is however not likely, because this will give an unfavorable interaction between alkyl chains from the “head” side sticking into the aqueous phase. Proposed arrangements of C5 Pe, PAP, and BisA are shown in Figure 4, which can explain the results in the Langmuir isotherms. The dramatic effect of an acidic group on such a system is evident. In the figure, the stacked flat-on arrangement is shown for BisA as an example, but as mentioned, no conclusion of the exact arrangement of BisA can be made. Concerning the asphaltenes, this study supports the recent assumptions that the surface active molecules are either tilted or normal to the surface plane, instead of lying with the aromatic core flat on the surface. This study can however not exclude the edge-on arrangement for the acidic molecules, but this is just a matter of whether the aromatic cores have a circular or ellipsoid shape. The most important is that the aromatic cores are facing each other, and the hydrogen-bonding group is stabilizing the monolayer toward “slip-out”. Using Langmuir isotherms to discuss molecular arrangements at surfaces is not a direct interpretation, because the value of mean molecular area is assuming only monolayer coverage and control of the amount of molecules spread on the surface. Nevertheless, it gives a good indication when used and compared with other experimental techniques. The observed molecular areas of the acidic molecules alone so far cannot exclude the possibility of a stacked flat-on
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Figure 3. (a) Surface pressure-area Langmuir isotherms of all four model compounds spread onto ultrapure water of pH 6. 40 µL of a 1.0 mM chloroform solution for each compound was the spreading amount, and the barrier speed was 5 mm/min. (b) Enlargement of the isotherm of BisA.
arrangement, but the BAM images presented later contradict the presence of more than one layer for these compounds. The interfacial tension measurements were carried out at pH 9, and the reason for the alkaline conditions was to highlight the differences between the compounds. However, the Langmuir isotherms above were carried out at pH 6, and correlation of the film properties and the IFT results must be done under the same conditions. Hence, the isotherms must be carried out at pH 9 as well. Figure 5 shows the isotherms obtained at pH 9. BisA was not studied since it has no pH dependence. As expected, the A0 values for all compounds are higher due to increased electrostatic repulsion between the ionized carboxylate groups. The collapse areas are close to what is obtained at pH 6 indicating that a saturated interface is packed in the same way, irrespective of the pH. In contrast to what was observed at pH 6, the isotherms at pH 9 show a phase transition around 15 mN/m. This might be a conformational transition of the rotation of the headgroup to obtain the most efficient packing, which is not necessary at pH 6. There might be two different stabilizing mechanisms for the monolayers of the acidic model compounds, aromatic interactions, and the formation of metal naphthenates. Naphthenic acids, which are cyclic, aliphatic acids, give monolayer films which increase in surface collapse pressure as a function of pH due to formation of metal naphthenates. This has been illustrated for 5β(H)-cholanic acid and decahydro-1-naphtalenepentanoic acid,29 and acidity constants at the surface, KSa, could in fact be calculated from the surface collapse pressures. Pressure-area isotherms over the pH range for the acidic model compounds are shown in Figure 6a-c. No significant increase in the surface pressures were observed for any compounds, with intermediate pH giving highest collapse pressures. Thus, no correlation between the film stability and the formation of the metal naphthenate of the model compounds were found, and no acidity constants at the surface could be calculated for these compounds. The aromatic interactions seem in this case to be the factor responsible for the film stability. More efficient packing of the molecules of C5 Pe will enhance the aromatic π-electron interactions, making the film even more stabilized through the aromatic sheet-like arrangement. Measuring the loss of area at a constant pressure over time is one way to highlight the stability of a Langmuir film. Figure 7 shows the stability of the model compounds at 20 mN/m and pH 6. Only a loss of a few percent during the initial time span is observed (29) Havre, T. E.; Ese, M.-H.; Sjo¨blom, J.; Blokhus, A. Colloid Polym. Sci. 2002, 280, 647–652.
for the acidic models, indicating highly stable films. The results also show that the compounds will not migrate into the bulk aqueous phase, or “slip-out” of the monolayer to form a new layer. BisA, however, is not stable with a loss of 35% of total area during 500 s. This supports the assumption that BisA creates multilayers, and the loss in area can be attributed to molecules “slipping-out” from one layer to another. Such “slip-out” has also been observed in Langmuir films of similar hydrophobic tetra-(n-alkoxy-carbonyl)-perylenes.30 Another reason for the instability of the BisA film could be dissolution into the aqueous subphase. However, from the structures of the model compounds in Figure 1, BisA is the most hydrophobic compound and should be the least soluble of all in water. If dissolution into the water phase is the reason for unstability of BisA, the acidic compounds should be even more unstable. Hence, dissolution into the subphase does not seem likely. The findings bring out the question of why the asphaltenes form so strong and elastic films, and the head-on arrangement gives a good explanation. The isotherm curves of the model molecules have many similarities to films prepared with asphaltenes.31,32 That is, an A0 in the area 50-100 Å2 and a rise with a horizontal break. The rise is more gradual in contrast to the acidic model compounds, but steeper than for BisA. This gives a reason to believe that asphaltene films are a mixture of acidic (or other surfactant-like structures) and nonacidic compounds, which are aligned normal or parallel to the surface dependent on the presence of strong hydrogen-bonding functionalities, and the observed isotherm is an additive of the different arrangements. Film Properties. The perylene bisimide (PBI) as aromatic core for the model compounds was initially chosen not only for synthetic reasons, but also for its well studied fluorescence properties.33–38 Many derivates of PBIs are known to aggregate (30) Hertmanowski, R.; Chrzumnicka, E.; Martynski, T.; Bauman, D. J. Lumin. 2007, 126, 323–332. (31) Grijalva-Monteverde, H.; Arellano-Tanori, O. V.; Valdez, M. A. Energy Fuels 2005, 19, 2416–2422. (32) Cadena-Nava, R. D.; Cosultchi, A.; Ruiz-Garcia, J. Energy Fuels 2007, 21, 2129–2137. (33) Cano, T. d.; Parra, V.; Rodrı´guez-Me´ndez, M. L.; Aroca, R.; Saja, J. A. d. Org. Electron. 2004, 5, 107. (34) Vitukhnovsky, A. G.; Sluch, M. I.; Warren, J. G.; Petty, M. C. Chem. Phys. Lett. 1991, 184, 235. (35) Heinz Langhals, W. L. Eur. J. Org. Chem. 1998, 1998, 847–851. (36) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373–6380. (37) Neuteboom, E. E.; Meskers, S. C. J.; Meijer, E. W.; Janssen, R. A. J. Macromol. Chem. Phys. 2004, 205, 217–222. (38) Xie, R.; Xiao, D.; Fu, H.; Ji, X.; Yang, W.; Yao, J. New J. Chem. 2001, 25, 1362–1364.
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Figure 4. Suggested arrangement of C5 Pe, PAP, and BisA at the aqueous surface in low-compressible phases. The arrangement of TP is assumed to be the same as PAP, and the arrangement shown of BisA is one of two plausible arrangements.
in solution due to face-to-face stacking, and this stacking will yield the presence of the excimer (“excited dimer”) emission in fluorescence.28,36,37 In contrast to the typical 3-peak pattern of the monomer from 530 to 650 nm, the excimer, or aggregate band, is a broad, unstructured band from 600 to 800 nm.25 The monomer band is nearly unaffected by the substitution onto the nitrogens,27 and many PBIs show fluorescence quantum yields close to unity.35 In addition, fluorescence lifetimes of PBI monomers are quite constant at 4 ns and shown to be stable toward oxygen quenching and not significantly affected by the substitution or environment polarity.39 The aggregate lifetime, however, varies from 8 to 33 ns,28,34,37 but is in all cases significantly larger than the monomer lifetime. To give further
evidence of the face-to-face arrangement of the molecules in the Langmuir films, a Langmuir-Blodgett deposition onto a hydrophilic glass substrate was carried out from the liquid condensed state at 20 mN/m. The intention was that face-to-face arrangement would yield the aggregate band in fluorescence, and the absence of this arrangement would yield the monomer band. Figure 8 shows the steady-state spectra of deposited films of the model compounds. Except a small shoulder for PAP and BisA at 535 nm, which is maximum emission peak for the monomer, the films show only the aggregate band and only minor differences (39) Langhals, H.; Karolin, J.; Johansson, L. B.-Å. J. Chem. Soc., Faraday Trans. 1998, 94, 2919–2922.
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Figure 5. Surface pressure-area Langmuir isotherms of acidic model compounds spread onto an aqueous buffer of pH 9. 40 µL of a 1.0 mM chloroform solution for C5 Pe and 50 µL of PAP and TP was the spreading amount. The barrier speed was 5 mm/min.
between the three acidic compounds. The spectra are close to those obtained with monolayer films of similar compounds with the PBI core and different substituents.27,33 The results indicate that the films in this study are arranged with the aromatic cores face-to-face. Even traces of the monomer peak can be seen for PAP and BisA; this is a very small amount taking into account the large decrease in the fluorescence quantum yield going from a monomer to an aggregate. BisA displays a slightly different spectrum, with higher monomer peak intensity and an aggregate band consisting of contributions from two transitions. These two transitions might be Y- and E-type excimer emission, depending on how ideal the dimer packing is.38 The presence of a second packing arrangement again indicates that BisA is not organized in the same way as the acidic model compounds, and the transfer ratio (TR) of 1.5 supports the fact that more than one layer of BisA was transferred. The TR is the relationship between the loss of trough area and the area of substance to be deposited. In some cases, the substrate thickness can cause deviations, but in our case the glass substrates had a thickness of 0.14 mm and should not be significant. The roughness of a glass surface, however, might cause the molecules to crumple up in some areas and can for example be the reason the band of PAP slightly differs from TP and C5 Pe. In the future, deposition onto a smoother surface, like mica, could be a better approach to verify small band differences. However, in this study we used fluorescence to characterize the LB films to exclude the possibility of a monolayer with a flat-on arrangement. This arrangement should not give the excimer emission which is very dominant in the observed spectra. The stability of the films and the arrangement of the acidic model compounds are plausible explanations for the high interfacial activity of these. The sudden drop in γo/w is however still an observation to be highlighted. BAM was utilized in order to view this question. The brightness of areas in the images is equal to the relative density and thickness of the surface. Figure 9a-c shows images taken upon compression of C5 Pe at pH 6 both in a highly compressible area (A > A0) and in the liquid condensed phase (AC < A < A0) and after film collapse (A < AC). To compare, images taken of BisA are shown in Figure 9c,d. The images correspond to the points in the isotherms marked a-e in Figure 9f. Several islands of higher density are observed together with large areas of low density (black areas). A highly compressible area is actually a phase transition between islands of a liquid condensed phase (bright areas) and the gaseous phase (dark areas).
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A recent paper presenting BAM images upon compression of an asphaltenic film showed the same occurrence of islands of high density coexisting with a gaseous phase.32 The pressure increase set in when the appearance of dark, gaseous areas was absent, and this is also the case for C5 Pe. At 4 mN/m, after reaching the L2 phase, the film is a uniform phase of high density which after collapse shows a “crystalline” pattern indicating multilayers (Figure 9c) The latter was also observed visually as appearance of purple streaks near the barriers, which has also been observed on asphaltene films.9 The pattern for BisA shows a chaotic phase behavior before pressure increase, and at only 2 mN/m is the brightness of the image equal to the brightness of the collapsed film of C5 Pe. No smooth monolayer was observed on compression of BisA. The BAM images also support that the acidic molecules are not arranged as stacks, either flat-on or edge-on. Comparison of images in Figure 9b and c shows that the image of C5 Pe (Figure 9b) is a homogeneous layer of grayish light intensity, while at the same surface pressure, BisA gives a brighter image (Figure 9c) with more intensity variations in the layer, corresponding to formation of multilayers. Hence, the stacking arrangement can be excluded for C5 Pe, and most likely for the other acidic molecules as well. The coexistence of a gaseous and liquid phase which finally emerges to a uniform liquid phase can give an idea of the mechanism behind the sudden drop in IFT of the acidic model compounds. One can imagine the surface of the oil droplet submerged in the aqueous phase to be covered with islands of surfactant, which grow in size until all surfactant molecules are packed closely enough to form the stable film. When this twodimensional network has formed, the film is stabilized through π-electrons over the whole droplet which causes a highly interfacially active system. The reason BisA is not interfacially active is that it does not form a stable 2D network in the interface plane. And of course, it does not contain an acidic or other strong hydrogen-bonding functionality which greatly enhances the affinity for the interface and the stability of a film. These results supports the findings by Fossen et al.13,14 that there was one subfraction of the asphaltenes which was more interfacially active and this contained COOH or other strong hydrogen-bonding functionalities, and the results are also consistent with the pH dependence of interfacial tension and emulsion stability of asphaltenes.4 Effect of Aggregation on Interfacial Properties. When previously discovering these highly interfacially active compounds, two possible explanations were proposed: either formation of an efficient packing of molecules forming a stable film, or aggregation in the bulk phase. The Langmuir experiments show that an efficient packing occurs and the model compounds form a 2D network and a stable film, likely to yield high interfacial activity. The aggregation mechanism, however, has so far not been discussed. In asphaltene research, the terminology “critical nanoaggregate concentration” (CNAC) has been introduced to describe the tendency of aggregation in organic solvents, but depending on the experimental method used, large variations in the CNAC values have been found. However, the currently assumed CNAC range is 50-150 mg/L depending on the origin of the asphaltene sample.40 There has been much speculation regarding whether asphaltene monomers or the aggregates are stabilizing water-in-oil emulsions, and at these low concentrations, sensitive instrumental techniques are needed to obtain reliable data. Fluorescence lifetime measurements of the IFT solutions were carried out in order to see if there was an abrupt change (40) Andreatta, G.; Bostrom, N.; Mullins, O. C. Langmuir 2005, 21, 2728– 2736.
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Figure 6. Surface pressure-area Langmuir isotherms of acidic model compounds as a function of pH for (a) C5 Pe, (b) PAP, and (c) TP. The pH was controlled using buffer solutions, and pH 5.8 consisted of ultrapure water with [Na+] ) 0.05 M to have a salinity comparable to the buffer solutions.
Figure 7. Film stability of all model compounds measured as the loss in area available in the trough over time at a constant pressure, Π ) 20 mN/m. The compounds were spread onto ultrapure water of pH 6.
during the 1-20 µM series regarding fluorescence lifetime. As mentioned, the aggregate lifetimes of PBI derivates are much longer than the monomer lifetime. Figure 10 shows the average lifetime of all four model compounds as a function of concentration. Since different exponential decays were observed, a plot of the average fluorescence lifetime, jτ, is more convenient. For a two-exponential decay the average lifetime is given by jτ ) f1τ1 + f2 τ2 where fi is fraction of lifetime τi. The results show that the two acidic-aromatic model compounds PAP and TP showed the presence of both monomer and aggregate due to higher average lifetime, but C5 Pe and BisA could be well fitted with a monoexponential decay with the expected monomer lifetime of ca. 4 ns. The monomer lifetime
Figure8. Normalizedsteady-statefluorescencespectraofLangmuir-Blodgett films of asphaltene model compounds deposited onto glass at 20 mN/m surface pressure and 1 mm/min dipper speed. The spectra were recorded with front-face illumination onto the glass placed between an empty cuvette and the cuvette holder. Films were excited at 495 nm.
was nearly the same in the fitted decays for PAP and TP, indicating that the techniques differentiate well between monomeric and aggregated molecules. Lifetimes, fractions, and χ2-values for all samples are given in the Supporting Information. Some of the samples for PAP and TP displayed shorter average lifetime. The reason for this is unknown, but small contaminations in the glass vials used for sample preparation could enhance monomer stability or decrease aggregation in those particular samples. For example, the presence of trace water in studying asphaltene aggregation
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Figure 10. Average fluorescence lifetime 1-20 µM of all model compounds in toluene. Samples were excited at 495 nm and the emission at 640 nm was recorded using a TCSPC detector. C5 Pe and BisA could be fitted with a monoexponential decay, while PAP and TP exhibit biexponential decays.
Figure 9. BAM images of Langmuir compression of C5 Pe (a) before compression, A > A0, (b) in the liquid condensed phase at 4 mN/m, (c) after film collapse, A < AC; and BAM images of BisA (d) before compression, A > A0, and (e) at 2 mN/m. (f) Langmuir isotherms of C5 Pe and BisA with markings (a-e) when the BAM images was taken.
has been shown to be crucial.41 When finding CNAC at low concentrations, it was also pointed out that the toluene used was not dried.40 Perhaps no aggregation would have been observed in this study if ultradry conditions were used, but to find a CNAC has not been the objective in this work. The goal was to find out if the interfacial activity was a result of aggregation, and the samples studied were precisely the same used in the IFT measurements and from the same vials, and there are no apparent correlations between interfacial tension drops and aggregation. In fact, studying Figure 2 closely, the IFT value of PAP at 17.5 µM is slightly lower than at 20 µM. Compared to the same sample in Figure 10, this sample indicated no aggregates in solution, indicating that it is only the monomer which is interfacially active. Since C5 Pe does not aggregate, but is more interfacially active than PAP and TP, the aggregation mechanism (41) Andersen, S. I.; del Rio, J. M.; Khvostitchenko, D.; Shakir, S.; LiraGaleana, C. Langmuir 2001, 17, 307–313.
and the interfacial activity seem to be due to different mechanisms. PAP and TP in fact show the presence of aggregates already at 1 µM, a concentration at which they were not interfacially active. Another observation is that a solution slightly changes color when aggregates are present, which cause a red color.28 The lack of this color change was observed in the samples which were fitted to only one lifetime. Finally, it should be pointed out that the lifetime measurements and the steady-state spectra of the different LB films in Figure 8 cannot be compared with respect to whether a compound aggregates or not. The lifetimes were measured on diluted solutions, while the films were deposited from a condensed state of a compressed surface layer, representing two different cases. To contrast aggregation toward interfacial activity, PAP was dissolved at a constant concentration, 10 µM, but with varying solvent conditions. As with asphaltenes, n-hexane was expected to induce aggregation of PAP to a greater extent, but without precipitation due to the low concentration. Normalized steadystate fluorescence show how the aggregate band, with a maximum at 640 nm, increases with respect to the monomer peaks (Figure 11a). Interestingly, the aggregate band seems to be the same as the band resulting from the LB film deposited, indicating that the aggregates are also ordered face-to-face in the same way as in the film. Due to the previously mentioned much lower quantum yield of the aggregate band toward the monomer band, the peak heights cannot represent the absolute amounts of each type. However, the non-normalized spectra in Figure 11b show that the monomer concentration had a drastic reduction upon aggregation. The IFT of the same solutions were measured toward pH 9.0. Because the IFT of the pure solvents changes when going from toluene to hexane as the oil phase, the interfacial activity is shown as the reduction of the interfacial tension of the samples in comparison to the pure solvent, shown in Figure 12. What is observed is that the aggregation in bulk results in less interfacial activity. The maximum interfacial activity at 20 vol % hexane, with a reduction of nearly 25 units, can be attributed to an increased affinity toward the interface due to addition of “poor” solvent, without any start of an aggregation. However, after 20 vol % hexane, the monomers start to form larger and larger aggregates, which decrease the monomer concentration available for diffusion to the interface. In Figure 12, the IFT reduction at 40% hexane is lower than that at 60% hexane. Some deviations in the IFT value were observed for these samples so the displayed values
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Figure 11. Steady-state fluorescence spectra showing (a) intensity-normalized spectra of 10 µM PAP in 0-98 vol % n-hexane in toluene together with spectrum of a thin film of PAP deposited onto glass and (b) decrease in monomer intensity in the same solvent series. The intensity successively decreases when the vol % n-hexane increases. All samples were excited at 495 nm.
Figure 12. Reduction of the interfacial tension compared with pure solvents of 10 µM PAP in 0-98 vol % n-hexane in toluene toward an aqueous phase of buffer pH 9. The values were taken after 250 s.
have some uncertainties. Nevertheless, they had in all cases significantly less interfacial activity than at 20% hexane. The Langmuir isotherms presented in this study show that the stable films have very low compressibility from the steepness of the slope, while the compressibility is much higher for the unstable film of BisA. Although the film of C5 Pe composed of islands which finally emerged to a uniform layer, a nice isotherm was nevertheless obtained. The difference in compressibility and stability between an “active” molecule toward an inactive one shown in this study can perhaps be used as a way to indicate whether a particular asphaltene sample will cause problems regarding emulsion stability. The asphaltenes, comprising both active and inactive subfractions, might give isotherms which are the sums of contributions from the low-compressibility active fraction and the high-compressibility inactive fraction. In addition, BAM images can be used to compare the amounts of “islands” between different asphaltene samples. The asphaltene model compounds presented in this work have not been synthesized to gain a perfect match to asphaltenes and their properties. Instead, compounds with similarities to the known structural features of asphaltenes have been synthesized and by using small variations in the structures, comparable to an HLBscale, the intention is to show with these compounds what might be true for the asphaltenes when it comes to chemical structure and the influence of structure to properties. The results presented here show that the presence of a COOH group seems to be very important when it comes to interfacial activity and formation of a rigid, twodimensional film. The internal polar groups do not appear to cause
any undesired properties, but the face-to-face alignment of sheetlike aromatic cores at interfaces stabilized by terminal hydrogen bonding groups is the main problem. The results presented in this paper provide a good explanation for the observation of a more interfacially active middle fraction of asphaltenes containing more OH and COOH groups.21 In the latter study the authors also found that the fraction which precipitated first and contained a higher average molecular weight did not have the highest level of interfacial activity. BisA, having the second largest molecular weight of the asphaltene models presented here, displays no interfacial activity. Hence, a correlation between molecular dimensions and interfacial activity can not be found. Obviously acidic asphaltene fractions will be crucial for creating stable water-in-crude oil emulsions. A hypothesis that only a small fraction of the asphaltenes is responsible for the emulsion stabilization has been stated by Czarnecki et al. in different studies,42,43 but the main characteristics of this fraction have not been known. Our study indicates that the active fraction they have referred to may be the acidic asphaltene fraction, or asphaltenes containing directing polar groups.
Conclusion Comparison of acidic to nonacidic polyaromatic surfactants demonstrates that the presence of acidic groups is crucial for their film properties and interfacial activity. An efficient face-to-face packing normal to the surface in Langmuir films, as indicated by fluorescence spectroscopy, is responsible for the high interfacial activity of such compounds. The molecules, prepared as asphaltene model compounds, indicate that only small fractions of the entire asphaltene fraction may be responsible for the problems regarding stable emulsions, and these fractions contain surfactant-like structures with hydrogen bonding functionalities. The effect of aggregation in bulk showed that the interfacial activity was significantly reduced when monomers became aggregated. Acknowledgment. The authors thank The Research Council of Norway for financial support through the Petromaks program. E.L.N. also thanks Wilhelm R. Glomm for proof-reading and discussion in the preparation of the paper. Supporting Information Available: Additional experimental data for fluorescence lifetimes, fractions. and χ2-values as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org. LA800945M (42) Czarnecki, J.; Moran, K. Energy Fuels 2005, 19, 2074–2079. (43) Yang, X.; Czarnecki, J. Energy Fuels 2005, 19, 2455–2459.
