Langmuir 2001, 17, 307-313
307
Interaction and Solubilization of Water by Petroleum Asphaltenes in Organic Solution Simon Ivar Andersen,*,‡,§ Jose Manuel del Rio,*,†,| Daria Khvostitchenko,‡ Sarmad Shakir,‡ and Carlos Lira-Galeana† Thermodynamic Research Laboratory, Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, Col. Bartolo Atepehuacan, Del. Gustavo Madero CP 07730, Mexico DF, Mexico, and Department of Chemical Engineering, Bldg. 229, Technical University of Denmark, DK-2800 Lyngby, Denmark Received June 22, 2000. In Final Form: October 5, 2000 The relation between the often reported micellization of petroleum asphaltenes in organic solvents and the content of trace water in the solvent was investigated using calorimetric titration. The content of trace water in toluene (ranging from 0 to 0.047%) was found to be the driving force in establishing a measurable critical micelle concentration in the concentration range between 0.2 and 8 g/L solvent. It was determined that, for the three different asphaltenes analyzed, the calorimetric trace in the presence of 0.01% water more likely indicates a stepwise association rather than a classical micelle formation mechanism. On the basis of this result, thesolubilization of water into solutions of asphaltenes in toluene was studied, and a quasi-linear relation between asphaltene concentration and water uptake was observed. From this, the Gibbs free energy of the transference of water into the solution or asphaltene micelle was determined to be around -13.4 kJ/mol at 293 K, which is in the range of hydrogen bonding formation. The solubility of asphaltene in toluene was observed to decrease significantly in water-saturated toluene at room temperature. Finally, the addition of resins (petroleum polar components) to the asphaltene system was found to eliminate the break in the curve previously assigned to CMC. These findings are important for the further understanding of asphaltene self-association, flocculation, and deposition in the oil industry.
Introduction Petroleum consists of an array of components ranging from simple n-alkanes to complex polar aromatics of high molecular weight. It is well described in the literature that petroleum contains surface-active components that adsorb to the water-oil interface, leading to stabilization of water-in-oil emulsions.1 The heaviest and most complex part of the crude oil is called the asphaltenes, which comprise a group of molecules with very high boiling points and complex structure that is only defined as a group by their solubility behavior (see below). Due to many problems such as solid-phase separation, precipitation, and deposition during both recovery and refinery operations related to the asphaltenes, much work has been directed toward this fraction. Already in the early 1920s, Nellensteyn2 showed that asphaltenic solutions showed colloidal behavior, and “self-association” of the components is obvious in the concentration dependence of many experimental results such as viscosity, molecular weight determinations, and so forth.3 Much work has been dedicated to the association of asphaltene components, and, probably due to the lack of solubility in water, the actual interaction with trace amounts of water in organic solvents has received little or no attention, although asphaltenes fractionated from oil and added to a solvent such as toluene * Corresponding authors. E-mail:
[email protected] or (JMR) e-mail:
[email protected]. † Instituto Mexicano del Petroleo. ‡ Technical University of Denmark. § E-mail:
[email protected]. | E-mail:
[email protected]. (1) Fordedal, H.; Schildberg, Y.; Sjo¨blom, J.; Volle, J.-L. Colloids Surf. A: Phys. Chem. Eng. Aspects 1996, 106, 33. (2) Taxler, R. H. AsphaltsIts Composition, Properties and Uses; Reinhold Publishing Corp.: NY, 1961. (3) Speight, J. G. The Chemistry and Technology of Petroleum; 3rd ed.; Marcel Dekker Inc.: New York, 1998.
will stabilize emulsion formation rapidly when water is added to this system. The composition of the asphaltene will affect this stability, and the oxidation of an asphaltene will enhance the stability of an emulsion made up from thism system.4 Hence, in the literature indirect evidence is given for the possible interaction between water and oxygen-containing functional groups. Asphaltenes are a generic fraction of petroleum defined operationally by the insolubility in an n-alkane (in general n-heptane) and solubility in toluene. The precipitation and separation is performed by addition of excess heptane to a petroleum sample; asphaltenes will settle as a solid dense fraction.3 As petroleum is a very complex mixture having a boiling point range from below 0 °C to above 1200 °C, the asphaltene fraction as well is a complex mixture. This material belongs to the highest boiling material in the oil and is always found in the distillation, remaining even during vacuum distillation. Much work has been dedicated to the characterization of asphaltenes from different sources, but no single asphaltene compound has been isolated; hence, the knowledge of the composition is that of an average of properties of thousands of different molecules varying in both molecular weight and polarity. The aromatic carbon content is about 50% whereas the molecular weight mentioned in the literature goes from about 700 to 10.000 g/mol, depending on method and conditions. The content of the prevailing heteroatoms N, S, and O is between 5 and 10% w/w, and density is in the range of 1.1 to 1.3 g/cm3. Despite the complexity, several similarities exist within asphaltenes from different sources when precipitated by the same procedure.3 The existence of a critical micelle concentration (CMC) in aqueous solution and the micellization at this point of surfactants is an accepted fact. In organic media, similar (4) Tort, F.; Andersen, S. I. Proceedings, First World Congress on Emulsion, Paris, 1993.
10.1021/la000871m CCC: $20.00 © 2001 American Chemical Society Published on Web 12/29/2000
308
Langmuir, Vol. 17, No. 2, 2001
effects have been described, although others have described the surfactant association as a stepwise aggregation process, hence ruling out the CMC concept in organic media. Often the existence of a change in ideal linear behavior in physical properties, such as interfacial tension as a function of concentration, has been taken as an indication of the formation of micelles and hence the existence of a micellar-monomeric equilibrium. It is obvious from data in the literature that even for simple surfactants, both in water and in nonpolar solvents, the changes observed takes place over a broad concentration range in which the micelles may or may not exist.5 The term “reversed micelle” has been used to describe self-assembly of surfactants in nonaqueous or apolar media in which aggregates are formed. These reversed micelles are thermodynamically stable associated nanostructures of amphiphatic molecules in which polar headgroups reside in the interior of the aggregate and the hydrocarbon, often tail-like, part extends into the bulk of the solvent. If water molecules are present, they play an important role in the structuring and formation of these aggregates, and extensive discussions exist on the actual role of water, even at trace levels, in the formation of the reversed micelle.6 It is obvious that the aggregates are formed more strongly and with higher aggregation number in the presence of water, but the presence of water may even be a necessity in the reversed micelle formation for some surfactant types. Hence, it is generally accepted that water will be solubilized in the interior of the reversed micelle and that the size of the micelle will grow as more water is added, (which enters the micelle core) corresponding to the formation of a water-in-oil microemulsion.6,7,8 Lagrege et al.9 showed that for the surfactant Triton X-35, micelles or aggregates were formed even in dry n-heptane; however, the surfactant itself contained traces (0.25%) of water that were not removed. The water inside the reversed micelle may either exhibit a state similar to bulk water if interacting only with other water molecules, or hydrate and interact with polar headgroups. The state of this water is a key parameter in understading how the reversed micelle is formed and the interactions keeping the micelles together. The formation of micelles is highly related to the structure both of headgroups and hydrocarbon “tails” and hence, very complex molecular mixtures may show significant effects from these structures. In comparison with asphaltenic structures, the actual reason for the hydrophilic/lipophilic behavior has not been resolved due to the mixed nature of this material. The present paper reports findings on the interaction between water at a molecular level with petroleum asphaltenes in toluene solution at concentrations in the range from 0 to 0.1% w/w. The asphaltenes have in several works been shown to exhibit behavior similar to surfactants in solution, for example, by the existense of apparent critical micelle concentrations10-13and the stabilization (5) Paula, S.; Su¨s, W.; Tuchtenhagen, J.; Blume, A. J. Phys. Chem. 1995, 99, 11742. (6) Moulik, S. P.; Paul, B. K. Adv. Colloid Interface Sci. 1998, 78, 99. (7) Bey Temsamani, M.; Maeck, M.; El Hassani, I.; Hurwitz, H. D. J. Phys. Chem. B 1998, 102, 3335. (8) Li, Q.; Weng, S.-F.; Wu, J.-G.; Zhou, N.-F. J. Phys. Chem. B 1998, 192, 3168. (9) Lagerge, S.; Grimberg-Michaud, E.; Guerfi, K.; Partyka, S. J. Colloid Interface Sci. 1999, 209, 271. (10) Rogacheva, O. V.; Rimaev, R. N.; Gubaidullin, V. Z.; Khakimov, D. K. Colloid J. USSR (Translated version) 1980, 412. (11) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497. (12) Sheu, E. Y.; De Tar, M. M.; Strom, D. A.; DeCanio S. J. Fuel 1992, 41, 299. (13) Loh, W.; Mohamand, R. S.; Ramos, A. C. S. Pet. Sci. Technol. 1999, 17, 147.
Andersen et al.
of water-in-oil emulsions.1 This paper discusses the basis of the reverse micelle of pure surfactants in apolar media and the effects of water as reported in the literature in relation to the complex asphaltene chemistry. This is used in the analysis of new experimental data on petroleum asphaltenes in solutions with and without trace water present, using calorimetric titration as well as static solubilization experiments, as a function of asphaltene concentration. Finally, the presence of other polar petroleum constituents (so-called resins) are shown to interact with either water or asphaltenes such that no CMC is observed. These findings are important for the further understanding of asphaltene self-association, flocculation, and deposition in the oil industry. Experimental Section Calorimetric titration was performed on a LKB 2277 bio activity monitor microcalorimeter equipped with a perfusion cell at 35.05 °C by titration of 1.50 mL of pure solution with aliquots of 0.020 ( 0.0002 mL of asphaltene solutions of concentration Ca (ca 30 g/L) well beyond the assumed CMC. Injection was performed with a Hamilton automatic syringe coupled to a PC. The heat signal was calibrated with the internal electronic calibration device. CMC was determined from the change in heat of dissociation, mixing, and dissolution as a function of total concentration. The heat evolved upon dilution of a micelle solution is dependent on the total surfactant or, in this case, the asphaltene concentration of the resulting solution:
Below CMC: ∆hdilution, total ) ∆hdilution, micelle + ∆hdemicellization + ∆hdilution, monomer + Q Beyond CMC: ∆hdilution, total ) ∆hdilution, micelle + Q The various contributions come from initial dilution of the micelles (∆hdilution, micelle), then a demicellization contribution when micelles dissociate (∆hdemicellization), and finally a contribution from the dilution of resulting monomers (∆hdilution, monomer). Q is heat caused by external influences, such as heat from stirring, frictional effects during liquid injection, and minor differences in solvent composition caused by evaporation from the syringe tip. The latter can be minimized but has been found to give a minor contribution in solvent-into-solvent experiments. However, in this analysis the contribution from possible rearrangements of different molecules into new types of aggregates or micelles was not included. Scattering was seen in the first few injections, as is often observed in the literature, probably due to gradients in the syringe tip. The effect of different water concentrations was investigated by mixing water-saturated toluene (TW) and dry toluene (T) in different volumetric proportions (T/TW: 1/3, 1/1, and 3/1). Asphaltenes were difficult to dissolve in water-saturated toluene, and plugging of the syringe was experienced. Hence, the pure TW experiment could not be performed properly but was done by injection of a solution with less water into a water-saturated solution, such that the resulting solution was almost saturated with water. Solubilization. A solution of asphaltenes in toluene with a fixed concentration was slowly poured upon a water phase (deionized), avoiding mixing of the organic and the aqueous phase. The two-phase system was left to equilibrate in the dark, and samples of the organic phase were taken at time intervals and analyzed for water content. Water was determined by Karl Fischer titration on a 652 KF-Coulometer using Hyrdanal coulomat AG(RH 34836) with 30% toluene as reagent. The KF system was calibrated with solutions of known water content in the range of interest. Effects of asphaltene concentration were examined as well. Asphaltenes were obtained by heptane precipitation following modified IP 143 standards by addition of 30 vol heptane/g oil.
Petroleum Asphaltenes in Organic Solution Modifications were in the handling and precipitation, with all steps at room temperature. The samples were washed after separation in heptane using ultrasonication and centrifugation as intermediate steps. The asphaltenes were assumed to be “pure” when the heptane was either colorless or slightly yellow. One system was a purified asphaltic reservoir deposit. Resins (see discussion below for definition and details) were obtained from deasphaltened oil by sequential elution from activated silica columns.3 The n-pentane asphaltenes were obtained using a similar procedure to the n-heptane asphaltenes. Asphaltenes of four different origins were examined. Codes given for these are KU, KVB, Y, and OMV, referring to internal identification. Toluene was HPLC grade and was dried by distillation in a glass still and stored over molecular sieves or by the addition of sodium sulfate. Water was deionized. Heptane was HPLC grade and was used as received. Toluene was saturated with water by mixing toluene with excess water and allowing the mixture to stand for at least 3 days.
Langmuir, Vol. 17, No. 2, 2001 309
Figure 1. Heat/injection vs asphaltene concentration in toluene in the absence and presence of water (0.013%) for two different asphaltenes: triangles, OMV heptane asphaltenes; circles, KVB heptane asphaltenes; solid symbols, with 0.013% water in toluene; open symbols, dry toluene.
Results and Discussion Microcalorimetry has been used in the determination of critical micelle concentrations and determinations of heat of micellization of well-defined system with increasing frequency over the past decade.5,14-16 The titration procedure obviously provides some advantages but may also for complex systems yield very complex responses that may give rise to doubts concerning the results, especially when working with ill-defined mixtures with apparent surface-active properties such as the petroleum asphaltenes. In 1991, Andersen and Birdi11 published data using calorimetric titration on the association of petroleum asphaltenes in various organic solvents and mixtures, which indicated that a critical micellar concentration region could be present. This CMC was found to be related to the solvent composition and could be related to the solubility paramter for the nonpolar mixtures of various n-alkanes and toluene.11,17 These measurements confirm experiments performed with various asphaltenes using interfacial tension methods.10,12,13 Further work on this subject was presented recently using microcalorimetric titration.18,19 However, these new mesaurements show that a thorough analysis of the data indicated not a CMC type of transition region, as observed for surfactants, but more likely a transition from one association type to another, such that an operational CMC is more the correct term. Normally toluene, when received, contains water in the range of 0.01% w/w. To examine if this has any relation with the occurrence of the break in the heat/injection versus concentration, as is observed for many surfactants in nonaqueous solvents, experiments were carried out using dry toluene. In Figures 1 and 2, the curves are reported for two different asphaltenes in the presence (0.013% w/w by Karl Fischer titration) and absence of trace water20 by plotting heat per injection or cumulated heat versus asphaltene concentration. As observed, the removal of water also removes the observed break in the curves. These findings indicate that there is a specific interaction between asphaltene molecular types and the water in solution that apparently leads to the apparent CMC. The interaction of asphaltene and water is not (14) Majhi, P. R.; Moulik, S. P. Langmuir 1998, 14, 3986. (15) Majhi, P. R.; Moulik, S. P. J. Phys. Chem. B 1999, 103, 5977. (16) Kresheck, G. J. Am. Chem. Soc. 1998, 129, 10694. (17) Andersen, S. I. Ph.D. Thesis, Department Physical Chemistry, Technical University of Denmark. 1990. (18) Andersen, S. I.; Christensen, S. D. Energy Fuels 2000, 14, 38. (19) Andersen, S. I. In Asphaltenes and Waxes; Lira-Galeana, C., Ed.; In Press. (20) Shakir, S. A. Characterization of Asphaltenes by Calorimetry. Minor M. Sc. thesis work, Department of Chemical Engineering, Technical Unviersity of Denmark, 1999.
Figure 2. Cumulated heat of titration vs asphaltene concentration in toluene in the absence and presence of water (0.013%) for two different asphaltenes: triangles, OMV heptane asphaltenes; circles, KVB heptane asphaltenes; solid symbols, with 0.013% water in toluene; open symbols, dry toluene.
surprising in that asphaltenes added to solvents lead to enhanced water-solvent emulsion stability by adsorption of asphaltene molecules at the solvent-water interface.21 Possible candidates for this interaction are polar functional groups such as carboxylic acid and phenol; also, sulfoxide is known to be polar and may enhance the stability of water-in-oil emulsions.22 On the basis of these observations, it can be assumed that the water in the organic solvent acts as a bridge between asphaltene molecules that may, due to the large size, be sterically hindered in the association. Water may also form a microdroplet that makes up the center of the aggregate. To further investigate this behavior, the effect of changing the solvent water content was examined by mixing dry and water-saturated toluene before dissolution of the asphaltenes and titrating into the same solventwater mixture. It was observed that KU asphaltenes had a lower solubility in water-saturated toluene, although this water concentration is only approximately 0.047% w/w at room temperature. The solubility dropped from about 80 g/L in dry toluene to about 30 g/L in watersaturated toluene. Hence, already this indicates a large influence and implies interaction between water and asphaltene. Calorimetric titrations were performed at 35.05 °C. For this particular asphaltene (KU), the drying of the toluene did not lead to a constant value of heat per injection but rather a constant decline with increasing addition of asphaltenes, yielding a curve where no break could be observed. The reproducibility of two titrations at a water concentration of 0.024 w/w is shown in Figure 3. (21) McLean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 196, 23. (22) Seifert, W. K.; Howells, W. G. Anal. Chem. 1969, 41, 553.
310
Langmuir, Vol. 17, No. 2, 2001
Andersen et al.
Figure 3. Heat/injection versus asphaltene concentration in toluene; repetition of the same experiment with 0.024% water (asphaltene KU). Figure 6. Apparent CMC of asphaltene KU versus water content in toluene at 35 °C. Table 1. Apparent Critical Micelle Concentrations of Different Asphaltenes and at Different Water Concentrations by Calorimetry at 35.0 °C
Figure 4. Heat/injection vs asphaltene concentration in toluene in the absence and presence of water at concentrations between dry and water-saturated toluene; asphaltene KU (toluene dry, open circle; 0.0118% water, solid triangle; 0.023% water, solid diamond; 0.0353% water, solid square; 0.047% water solid circle).
Figure 5. Cumulated heat of titration vs asphaltene concentration in toluene in the absence and presence of water at concentrations between dry and water-saturated toluene; asphaltene KU (toluene dry, open circle; 0.0118% water, solid triangle; 0.023% water, solid diamond; 0.0353% water, solid square; 0.047% water, solid circle).
In Figures 4 and 5, the plots of either heat per injection or cumulative heat versus asphaltene concentration are given for four different water concentrations. In all cases, it has been found that ∆Hp (heat per injection) is positive. It was also found that ∆Hp in the absence of water decreases with a constant slope in the two series of experiments when heat/injection is plotted versus asphaltene concentration (Figures 1 and 4). In the presence of water, three regions of concentrations were found in the two series. From 0 to around 3 g/L, ∆Hp is bigger than the value in the absence of water. Between 3 and 7 g/L there is a transition region. Above 7 g/L ∆Hp exhibits a linear decrease similar to that when no water is present. In Figures 2 and 5, the cumulative heat ∆Hc as a function of asphaltene concentration is given. In the absence of water, no change in the slope of ∆Hc as a function of the
water concentration (%)
CMC (g/L)
asphaltene sample
0.013 0.013 0 0.012 0.023 0.035 0.047
3.4 6.4 0 5.7 6.4 3.7 3.2
KVB OMV KU KU KU KU KU
concentration was observed. In all cases in which water is present, a change is found in the slope. Recently, experimental results from surface tension and calorimetry23 for the surfactant CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) in aqueous solution have been reported, in which changes of slope in the surface tension and in ∆Hc in the same regions of concentration in both techniques were found. These authors reported the changes as evidence of the micellar behavior of CHAPS. Our plots of ∆Hc as a function of concentration show a similar behavior to CHAPS. Above and below the transition region the experimental values given in Figures 2 and 5 were linear-fitted. The intercept between these two fits was interpreted as a possible CMC. For the experimental data of Figures 5 and 2, before the transition region, the fitting function was of the type y ) bx because a statistical study showed that an independet term could not be justified. Above the apparent transition, a type y ) a + bx could obviously apply. In all cases, above and below the transition region the squared correlation coefficient was above 0.99. The calculated values of CMC are shown in Table 1. The dependence of CMC on water content of the toluene solvent at 35 °C is given in Figure 6. A bell-shaped relation is observed, which could indicate that different types of aggregates are formed at low and high water contents; also, the apparent lowering of solubility of the asphaltenes in the water saturated toluene may affect the formation of aggregates. Nevertheless, it is obvious that asphaltene association or micellization, as it has been called, is related to the content of water present. In experiments such as surface tension experiments, it can be difficult to control these very low concentrations of water, as most experimental setups are of the open-cup wilhelmy type; hence, humidity will enter the solvent. Figure 7 reports the static solubilization experiment as a function of time. It is observed that the uptake of water (23) Giacomelli C. E.; Vermeer A. W. P.; Norde W. Langmuir 2000, 16, 4853.
Petroleum Asphaltenes in Organic Solution
Figure 7. Solubilization of water in asphaltene KU-toluene solution (30 g asphaltene/L) and in pure toluene as a function of time. Also shown is the blank test of uptake in asphaltene/ toluene solutions from humidity in the air.
Langmuir, Vol. 17, No. 2, 2001 311
w/w is well in agreement with scarce literature data on the solubility of water in hydrocarbons. For benzene, the solubility is approximately 0.051% w/w at room temperature, whereas when no interaction between the π-orbitals and water is present, as in heptane, the solubility is 0.014% w/w.26 Similarly for cyclohexane the water solubility is much lower. Wishnia27 found an increase of the solubility of small alkanes in aqueous micellar solution with respect to their solubilities in pure water. It is accepted28 that this fact is due to the transfer process of the small alkanes from the water to the interior of the micelle. In our case, the increase of the water solubility in the asphaltene solution in organic media can be interpreted in terms of the water-asphaltene interaction. In this way, the solubility of water (in grams per kilogram) is expressed as
S ) kfK + sT fT
Figure 8. Solubilization of water in asphaltene solutions in toluene as a function of asphaltene concentration.
at equilibrium is almost three times as high in the presence of asphaltenes than when saturating the pure toluene. A slow uptake is observed for the asphaltene solution. This could be caused by formation of a microemulsion zone at the oil-water interface due to the shear caused by the flux of the water into the organic phase. This is observed as the presence of a thin light brown layer, which is typical for water-in-oil emulsions.24 Similar experiments have been performed in reversed micelle studies with respect to the effect of surfactant concentration as an indication of the role of water in the micellization.25 Figure 8 shows the saturation concentration of water as a function of asphaltene KU concentration from 0 to 20 g/L. The first approximation is that a linear relation indicates a one-to-one relation between the uptake of water and the asphaltene concentration: assuming a molecular weight of 1000 g/mol for the asphaltenes, it is calculated that approximately 1.2 molecules of water associate with one molecule of asphaltene. A closer examination of the solubilization data in the low-concentration region (0-2 g/L) and in the high asphaltene concentration region (8-20 g/L) reveals that the local linear dependence slope is almost doubled at high concentration, indicating that between the two ranges, a restructuring of the asphaltene-water aggregates may take place leading to a better uptake of water; this, indeed, could be the formation of water pool-like (microdroplet) centers in the asphaltene “micelles” or a transition from monomeric hydration to swelling of micelles. The concentration range where the change takes place is in agreement with the calorimetric work. However, at present, despite the good reproducibility of single points, the deviations observed in the timedependent study (Figure 7) do indicate that one should hesitate in drawing too detailed conclusions on these experiments, as well as on one single type of asphaltene. Also, the data of the equilibrium water uptake for 30 g/L is seen to be almost similar to the 20 g/L, which is in agreement with the low solubility of the asphaltenes in the presence of water. The y-axis intercepts of 0.047 % (24) Schramm, L. L. Emulsions: Fundamentals and Applications in the Petroleum Industry; Advances in Chemistry Series 213; American Chemical Society: Washington, DC, 1992. (25) Abou-Nemeh, I.; Bart. H. J. Langmuir 1998, 14, 4451.
(1)
where fK and fT are the weigh fractions of asphaltene and toluene, sT (in grams per kilogram) is the solubility of water in toluene, and k is a measure of the saturation of the water-asphaltene interaction (in grams per kilogram). The parameter k is calculated by the slope of a plot of S against fK:
S ) (k - sT)fK + sT
(2)
Using the data of Figure 8 and neglecting the asphaltene contribution to the total solution volume the results of a linear fit are sT ) 0.471 ( 0.013 g/kg and k ) 23.8 ( 1.3 g/kg. Estimating a molecular weight of 1000 for the asphaltenes and expressing k in units of mol/mol, the result is 1.3, in agreement with our above estimation. The standard free energy of Gibbs for the transfer process of a mole of free water molecules in toluene solution to a state interacting with the asphaltene molecules is calculated by:
∆G°trans ) RT ln
xwT xwA
(3)
where xwT and xwA are the values of sT and k expressed in molar fractions. It is possible to get a reasonable estimation of ∆G°trans observing the following fact: for a molecular weight of the asphaltene between 500 g/mol and 1000 g/mol, the standard free energy of transfer goes from -12.5 kJ/mol to -13.4 kJ/mol. For the interval between 5 × 103 g/mol and 20 × 103 g/mol, ∆G°trans goes from -14.6 kJ/mol to -14.9 kJ/mol (and taking MWa ) 100 × 103, ∆G°trans is -14.9 kJ/mol). Then, and for this sample only, a reasonable estimation of the standard free energy of Gibbs for the transfer process of a mole of free water molecules in toluene solution to a state interacting with the asphaltene molecules is:
∆G°trans ) -13.4 ( 2.5 (kJ/mol)
(4)
This magnitude represents an average over the molecular weight span reported for asphaltenes in the literature and is found to be in the range of the hydrogen bonding energy, which for various processes is in the range of -10 to -40 kJ/mol.29 Hence, it is obvious that hydrogen (26) McCain, W. D., Jr. Properties of Petroleum Fluids, 2nd ed.; PennWell Publishing Co.: OK, 1990; p 463. (27) Wishnia, A. J. Phys. Chem. 1963, 67, 2079. (28) Tanford C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1973.
312
Langmuir, Vol. 17, No. 2, 2001
Andersen et al.
Figure 9. Cumulated heat vs asphaltene Y concentration in toluene in the absence and presence of resins. See text for details.
bonding, may be involved either between oxygen or nitrogen-containing groups or between water and aromatic rings. It is, of course, necessary here to remember that petroleum asphaltene is a complex mixture of very different components and than some molecules may be more active in the interaction with water that others as well as having more sites (functional groups) per molecule than others. Effect of the Presence of Resins. In the collidal approach to asphaltene dispersion and stability in crude oils, Pfeiffer and Saal30 proposed that asphaltenes, which are inherently insoluble in oil, are dispersed in the oily medium by resins. Resins are a polar fraction that can either be obtained by solubility criteria (such as being soluble in heptane but insoluble in pentane) or by chromatographic procedures.3 In our work in 1991,11 the effect of coprecipitated nonasphaltenic material was examined in toluene with trace levels of water; this material was found to produce an increase in CMC as more material was precipitated, either by changing the precipitant or the temperature of precipitation. The changes in CMC were small from about 3 g/L to about 7 g/L, when going from a precipitation yield of 10% to 40% of the total oil, and the main effect was proposed to be a dilution effect, in the sense that the added material apparently did not participate or diminish the micellization expressed by CMC. To further investigate this effect but in a slightly different manner (with the concept of resins dispersing the asphaltene in mind, a series of experiments were performed to look at resin-asphaltene interactions in oil Y. Asphaltenes precipitated in n-pentane was assumed to contain resin type molecules as more material is precipitated than in n-heptane, and also resins (polars) were recovered by open column chromatography on activated silica. The latter resins were added to the toluene solvent both in the cell and in the asphaltene solution to be injected, such that the concentration of resins remained constant throughout the experiment and that the asphaltene aggregate in the concentrated solution was modified. Hence, only the asphaltene concentration was changed during the experiment, in which the solvent is a toluene-resin solution. The resin concentration was 27.8 g/L toluene throughout the experiment. In the other type of experiment, the pentane asphaltene aggregate in toluene was examined such that the entire concentration was changed, but in the presence of resin-type material, which was assumed to coprecipitate, as pentane is used instead of heptane. The ratio of heptane-to-pentane asphaltene was 0.74, corresponding to a dilution of “active heptane asphaltenes” by approximately 35%. As can be seen in Figure 9 analysis (29) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: NY, 1974 (30) Pfeiffer, J. Ph.; Saal, R. N. J. J. Phys. Chem. 1940, 44, 139.
Figure 10. Heat/injection vs asphaltene Y concentration in toluene in the absence and presence of resins. See text for details.
of both the n-C5 asphaltenes and the resin-doped solution data shows that the asphaltene CMC cannot be detected in the concentration range investigated. As mentioned above, the changes observed in the past experiments were small and within the concentration range examined, 0-8 g/L, such that the addition of resins or the n-heptane-npentane coprecipitated fraction should not move CMC to a very high value outside this concentration range. Analysis by the heat/injection technique indicates that no abrupt change in the curve can be observed for n-pentane or resin-doped asphaltenes (Figure 10). At the same time, the heat of mixing decreases significantly indicating that the structures present in the C7 asphaltene-toluene solution is not present in the C5 asphaltenes or in the resin-asphaltene solutionsor cannot be broken by the dilution. The present heat analysis, however, indicates that not only the dilution phenomenon mentioned by Andersen and Birdi (1991) but also the interaction between molecules are responsible for changing CMC. The date reported herein is performed with toluene containing trace amounts of water. This might also have an effect on the resin-asphaltene interactions, as the polar resins may interact with water such that this cannot react with the asphaltenes to the same extend as in the pure asphaltene-water-toluene systems investigated above. Another possible explanation is that resin-asphaltene interactions are stronger than water-asphaltnene and water-resin, leading to the exclusion of water from the system. This will be examined soon. Conclusions The interaction of asphaltenes and water at trace level concentrations was shown using both calorimetric titration and solubilization experiments. The calorimetric titration shows that the critical micelle concentration reported in the past for petroleum asphaltenes in solution is highly affected by water in such a way that it must be assumed that water plays a major role in the micellization or association process. From the solubilization experiments, the Gibbs free energy of transfer of free water to the asphaltene aggregate was found to be -13.4 ( 2.5 kJ/mol, which could point in the direction of hydrogen bond formation. The solubility of the asphaltenes was observed to decrease as more water was present in the toluene solvent. Finally, resins were observed to interact with the asphaltene-toluene-trace water system in a way in which no CMC could be detected. These findings are important for the further understanding of asphaltene self-association, flocculation, and deposition in the oil industry. Acknowledgment. The excellent technical support of Mr. Z. Tecle, Department Chemical Engineering, Technical University of Denmark, is highly appreciated both in the calorimetric and the Karl Fischer work. The
Petroleum Asphaltenes in Organic Solution
support of Teknisk Kemisk Fond, Technical University of Denmark and DONGS Jubilaeums Fond, during the sabbatical stay of S.I.A. at Instituto Meixcano del Petroleo, and Universidad Autonoma Metropolitana-Iztapalapa,
Langmuir, Vol. 17, No. 2, 2001 313
both Mexico City, Mexico is highly appreciated, as is the support of the two latter host organizations in Mexico. LA000871M
