1758
Langmuir 1994,10, 1758-1766
Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 2. Study of the Asphaltene-Amphiphile Interactions and Structures Using Fourier Transform Infrared Spectroscopy and Small-Angle X-ray Scattering Techniques Chia-Lu Chang and H. Scott Fogler' Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136 Received November 22, 1993. I n Final Form: March 17,1994" In the preceding paper in this issue, the influence of the chemical structure of a series of alkylbenzenederived amphiphiles on the stabilization of asphaltenes was described. In this paper, we present the results of using Fourier transform infrared (FTIR) spectroscopy and small-angleX-ray scattering (SAXS) techniques to study the interaction between asphaltenes and two alkylbenzene-derived amphiphiles, p-alkylphenoland p-alkylbenzenesulfonic acid. FTIR spectroscopy was used to characterizeand quantify the acid-base interactions between asphaltenes and amphiphiles. It was found that asphaltenes could hydrogen-bond to p-dodecylphenol amphiphiles. The hydrogen-bonding capacity of asphaltenes was estimated to be 1.6-2.0 mmol/g of asphaltene. On the other hand, the FTIR spectroscopic study indicated that asphaltenes had a complicated acid-base interaction with p-dodecylbenzenesulfonic acid (DBSA) amphiphiles with a stoichiometryof about 1.8 mmol of DBSA/g of asphaltene. The UV/vis spectroscopic study suggested that asphaltenes and DBSA could associate into large electronic conjugated complexes. Physical evidence of the association between asphaltenes and amphiphiles was obtained by SAXS measurements. It was found that asphaltenes in an alkane solutionwith p-nonylphenol (NP) amphiphiles could be dispersed as well as they were dispersed in toluene. Due to the weak X-ray scattering of NP and weak asphaltene-NP association, the measured radius of gyration of NP-asphaltene colloids was only slightly larger than that of asphaltenes. The significant change of the asphaltene's SAXS profile in the presence of DBSA clearly verified the association between asphaltenes and DBSA in apolar media. The finding that DBSA could associate itself into reverse micelles suggested asphaltenes might be surrounded and stabilized by multilayers of DBSA molecules in apolar media. However, under a higher weight ratio of asphaltene to DBSA, the shortage of DBSA to form a stable steric layer for asphaltenes could cause asphaltenes to aggregate into large colloids.
Introduction In the first paper, it was shown that the asphaltene fraction of crude oil could be stabilized in apolar media through attractive interactions with oil-soluble amphiphilic molecules.' In this paper, we further unveil the mechanism of asphaltene-amphiphile interactions and the structure of asphaltene-amphiphile associated colloidsusing Fourier transform infrared (FTIR) spectroscopy and small-angle X-ray scattering (SAXS) techniques. Infrared spectroscopy (IR) is one of the most frequently used vibrational spectroscopic techniques to study the chemical structure of crude oil asphaltenes. Various chemical groups, such as aromatics, amine, aldehyde, carbonyl, carboxylic acid, etc., have been characterized from the IR absorption spectra of asphaltene materials.24 However, the extremely complex structure and the severe intermolecular associationsof asphaltenes made it difficult to use IR spectroscopy to resolve and identify the individual bands for the specific types of chemical groups. Consequently, other analytical techniques had to be used
* To whom correspondence should be addressed. *Abstract published in Advance ACS Abstracts, May 1, 1994. (1)Chang, C. L.; Fogler, H. 5.Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 1. Effect of the Chemical Structure of Amphiphiles on Asphaltene Stabilization. Langmuir, preceding paper in this issue. (2) Yen, T. F. In Chemistry of Asphaltenes; Burger, J. W., Li, N. C., Advances in Chemistry Series 195;American Chemical Society: Washington, DC, 1981;p 39. (3) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.; Marcel Dekker: New York, 1991; Chapter 11. (4)Christy, A. A.; Dahl, B.; Kvalheim, 0. M. Fuel 1989, 68, 430.
along with IR spectroscopy to identify the chemical structure of asphaltenes. For example, the combination of base hydrolysis, silylation reactions, and differential infrared spectroscopy was applied to quantitatively determine the amounts of ketone, dicarboxylic anhydride, carboxylic acid, and 2-quinolone in asphalts.5 Infrared spectroscopic examination of the asphaltene products of an acetylation reaction revealed that 40-60% of the oxygen of asphaltenes belonged to phenolic hydroxyl groups.6 Infrared spectroscopy was also used to characterize the acid-base interaction (including hydrogen-bonding) of asphaltene material~.~-'OIt was observed that when the asphaltic concentration was diluted by CCl4 solvent, the JR band corresponding to the hydrogen-bonding stretching (3300 cm-l) shifted toward those of free O-H (3620 cm-') and free N-H (3480 cm-') stretching, indicating the existence of reversible hydrogen-bonding among asphaltic molecule^.^ It was also found that the IR intensity of the free O-H stretching band of a proton donor compound, phenol, decreased with increasing amount of asphalts in a phenol/CC& solution. From this measurement, the hydrogen-bonding capacity of the asphaltic material was quantified to be about 2 mmoVg of asphalts.8 On the basis of the infrared spectroscopic study of the interaction of phenol with asphaltenes (and resins) in CCld, it was (5)Petersen, J. C. Anal. Chem. 1975,47 (l),112. (6)Moschopedis, S.E.;Speight, J. G. Fuel 1976,55,334. (7)Petersen, J. C. Fuel 1967, 39, 295. (8) Barbour, R. V.; Petersen, J. C. Anal. Chem. 1974, 46 (2), 273. (9)Moschopedis, S. E.;Speight, J. G. Fuel 1976,55, 187. (10)Suryanarayana, I.; et el. Fuel 1990, 69, 1645.
0743-7463/94/2410-1758$04.50/00 1994 American Chemical Society
Asphaltene-Amphiphile Interactions and Structures
Langmuir, Vol. 10,No. 6,1994 1759
proposed that asphaltenes might be peptized as single knowing the shape of asphaltene particles. By assuming entities in crude oil by one or more resin moleculesthrough asphaltenes to be spherical, the scattering profiles were the acid-base interaction^.^ However, there is still a lack either fitted by a specific distribution function, such as the Schultz size distribution,16or transformed directly into of direct evidence that asphaltenes can be stabilized in a size distribution by a specific optimization scheme, such apolar alkane media through hydrogen-bonding with amphiphilic stabilizers. Infrared spectroscopy has also as those proposed by Letcher and Schemel6 or by Vonk.17 Other geometries such as a disk with a thickness of 0.3-1 been applied to qualitatively investigate the mutual interactions between asphaltenes and resins.1° However, nm have also been used to interpret the small-angle scattering data of asphaltenes.18J9 However, it should be the complexity and similarity of the chemical structures noted that the small-angle scattering curves can be of asphaltenes and resins make it extremely difficult to approximated by a variety of geometries or size distribustudy the asphaltene-resin interactions quantitatively. tions. Any analysis of the scattering data of asphaltenes Infrared spectroscopy has also been used to study the based on the specific geometry and/or size distribution electron donor-acceptor interaction (i.e., change transfer can only be treated as a simplified approximation. behavior) of asphaltenes.11-13 Asphaltene’s aromatic C-H The dimension of asphaltene colloids was found to out-of-planebending vibration bands located between 865 change under different conditions and treatments.l”l8 A and 760 cm-l were found to shift toward higher frequencies study using small-angle neutron scattering (SANS) tech(blue shift) as the electron acceptors (e.g., nitrobenzene) niques showed that, by raising the temperature, larger were added to the asphaltene solution. On the basis of asphaltene colloidsin toluene could dissociateinto smaller these results, it was proposed that the charge transfer asphaltene colloids with less polydispersity.16 Moreover, process, which was associated with a partial removal of an increasing the pressure of the pentane-crude oil mixture electron from a bonding orbital (a) of the donor to an can alsoslightlyreduce the size of the asphaltene colloids.16 antibonding orbital (a*) of the acceptor, might exist in The association between asphaltenes and natural resin asphaltenes.ll In addition to r r * electronic transition, molecules has been deduced from the fact that the SANS the lone electron pair (n) of various heteroatoms (e.g., intensity of asphaltene-natural resin mixtures was higher sulfur, nitrogen, oxygen, etc.) of asphaltenes could also than that of the summation of the scattering contributed induce the na* electronic transition.12 Moreover, the by each species.lg The above studies suggest that ashighly electronic conjugated structure of alternated single phaltene-resin colloids in crude oil behave as micelles and double bonds, such as large fused aromatics and which are either associated or dissociated depending upon polyenes with or without variousheteroatoms,always exists the thermodynamic state of the crude. In addition, in asphaltenes. This conjugated structure can even reduce chemical treatments such as hydrotreating may also affect the energy gap for the electron transition from the ground the aggregation of asphaltene colloids.l7 states (e.g., n, ?r) to the excited states (e.g., a*). Therefore, The above studies discussed the characterization of the asphaltenes are capable of absorbing light ranging from structure and functional groups of the asphaltene fraction the UV to near-IR range.3J2 Indeed, the asphaltene itself; Relatively few studies have been carried out on the fraction with greater polarities and larger molecular interactions between asphaltenes and amphiphilic molweights showed stronger electron absorption at the longer ecules which have well-defined chemical structures. In wavelength of light.3113 However, the broad absorption part 1,it was found that two types of alkylbenzene-derived spectrum of petroleum asphaltenes in the range between amphiphiles, namely, p-alkylphenol and p-alkylbenzeneUV and near-IR showed very little fine structure and could sulfonic acid, were effective in stabilizing asphaltenes yet only be interpreted in general terms.13 behaved quite differently in alkane solventswith different Small-angle scattering of X-rays and neutrons is a molecular weights. Therefore, in this paper, we describe powerful technique for the characterization of the physical the results of a mechanistic study on the interaction structure of solutionscontainingcolloids with length scales between asphaltenes and these two types of amphiphiles. from 10 to 1000 A (e.g., microemulsions, surfactant Fourier infrared transform spectroscopy was used primicelles, macromolecules, polymer lattices). This techmarily to characterize and quantify the acid-base interacnique has been applied to determine the size distribution and association behavior of asphaltene ~ o l l o i d s . ~ J ~ tions ~ ~ between asphaltenes and amphiphiles. Small-angle X-ray scattering techniques were used to study the physical Using a Bonse-Hart small-angleX-ray diffractometer with structure of asphaltene colloids in these amphiphile a minimum magnitude of the scattering vector of 2.15 X solutions and the behavior of the association between 10-3 A-l, Dwiggins found that the stabilized asphaltic asphaltenes and amphiphiles. colloids in crude oil had a radius of gyration in the range of 20-50 A.14 Because asphaltenes are a complex mixture Experimental Section of natural molecules with different polarities and geomMaterials. Asphaltenes used in this study were either etries, it is not possible to derive the actual size distribution obtained from Mobil Research & Development or prepared by of asphaltenes from the small-anglescattering data without precipitating them from the Mobil crude oil according to the standard procedure in ASTM 2007D. The properties of as(11) Yen, T. F. Fuel 1973,52,93. (12) MuUins, 0. C. Anal. Chem. 1990,62,506. (13) Jennings, P. W.; Pribanic, J. A. S.; Mendes; T. M., Smith, J. A. In Chemistry and Characterization of Asphalts, preprints; Youtcheff, J., Mitt, T., Eds.; American Chemical Society: Washington, DC, 1990; Vol. 2.5 (2). ~ _ ,n, 2R2 r
(14) Dwiggins, C. W. J . Appl. Crystallogr. 1978, 11, 615. (15) Sheu, E. Y.; Liang, K. 5.;Sinha, H. A.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153(2), 399. (16) Norman, F. C.; Quintero, L.; Pfund, D. M.; Fulton, J. L.; Smith, R. D.; Capel, M.; Leontaritis, K. Langmuir 1993, 9, 2035. (17) Senglet, N.; Williams, C.; Faure, D.; Des Courieres, T.; Guilard, R. Fuel 1990, 69, 73. (18) Herzog, P.; Tchoubar, D.; and Espinat, D. Fuel 1988,67,245. (19) Espinat, D. Presented at the SPE International Symposium on Oil Field Chemistry, New Orleans, LA, 1993; SPE Paper No. 25187.
phaltene samples and the information on the amphiphiles for this study, including p-nonylphenol, p-dodecylphenol, and p-dodecylbenzenesulfonicacid, were described previously.1 Fourier Transform Infrared (FTIR) Spectroscopic Measurement. Fourier transform infrared spectroscopy was used to quantify the acid-base interactions between asphaltenes and amphiphiles and to further unveil the mechanism of stabilizing asphaltenes by amphiphiles in alkane solutions. T w o amphiphiles,p-dodecylphenol(DP) and p-dodecylbenzenesulfonic acid (DBSA), were used in this study. First, the IR absorption bands responsible for the acid-base interaction between asphaltenes and these amphiphiles were identified. These bands were, namely,the free O-Hstretching band peak located at 3620 cm-1 for DP amphiphilesand the broad SO-H stretching bands
Chang and Fogler
1760 Langmuir, Vol. 10, No. 6, 1994 ranging from 2000 to 3000 cm-1 for DBSA amphiphiles.20 In the experiments studying the asphaltene-DP interaction, the decane solutions contained sufficient concentrations of DP amphiphiles (6.25 and 8w t % ) to completelydissolve up to 6 wt % asphaltenes. In the study on the asphaltene-DBSA interactions, asphaltenes were completely dissolved in two solvents, decane and CCL, containing 4 and 2 wt 9%DBSA,respectively. Teflon tubes (Nalge Co.) were used to accommodate the solution in order to prevent the adsorption of asphaltenes and amphiphiles to the sample vial. After asphaltenes were completelydissolvedin the solution, samples were agitated for at least 6-24 h and then measured using an FTIR spectrophotometer. The FTIR spectrophotometer and the operating procedure were the same as previously described.' Small-Angle X-ray Scattering (SAXS) Measurement. Small-angleX-ray scattering was used to characterizethe physical structure of asphaltenes in amphiphile solutions in order to understand the association between asphaltenes and amphiphiles. Asphaltene molecules can scatter X-rays because they have a higher electron density than that of the most aromatic and aliphatic solvents. This higher electron density is due to asphaltene's higher carbon-to-hydrogen molar ratio and various heteroatoms including oxygen, nitrogen, sulfur, and metal compounds. Before the experimental procedure is described, the relevant X-ray scattering theory is olitlined as follows:*'Pa For a molecular system, the scattered intensity Z(q) of X-rays can be expressed by
Equation 1 can be rewritten as
where q = the scattering vector which has the magnitude q = (47r/X) sin(8/2), with X and 0 being the X-ray wavelength and the scattering angle, respectively,Ap(r) = p(r)- (p(r)) = theelectron density fluctuation from the mean value (p(r)) of the system, and y ( r ) = J Ap(r1) Ap(r3) d3rl,with r = Irl - r21, is the correlation function. For a liquid solution, eq 1can be simplifiedby averaging all possible orientations of q with respect to r. Therefore, (3) where y ( r ) in eq 3 can be determined by the inverse Fourier transform: (4)
Usually the distance distribution function P(r) defined as the product of the radial dimension and the correlation function, y ( r ) , Le.
P(r) = r2y(r)
(5)
was used to express the heterogeneity of the system.22 Ifthe concentration of particles is so dilute that the interparticle scattering can be neglected, the correlation function y ( r ) for a solution containingNdispersed particles which have a significant electron density difference (Ap) from the solvent can be approximated by
d r ) =N(Apho(r)
(6)
yo(r)is the relative correlation function which relates only to the geometry of the dispersed particle. Note that yo(0)is the volume (20) Colthup, N. B.; Wiberley, S. E. Introduction to Infrared and Ranan Spectroscopy, 3rd ed.; Academic Press: New York, 1990. (21) Guinier, A.; Fournet, G. Small-Angle Scattering of X-Rays; Wiley: New York, 1955. (22) Clatter, 0.; Kratky, 0.Small-Angle X-ray Scattering; Academic Press: New York, 1982.
( V) of a single dispersed particle. Instead of evaluating y ( r )using eq 4, a Taylor's series expansion of eq 1at small values of q leads to the following equations for a solution containing N dilute dispersed particles:
= Nv(Ap)' exp(-q2Ri/3)
(7)
Equation 7, called the Guinier approximation, can be used to estimate the radius of gyration (R,)of the dispersed particle from the slope of a straight line which is fitted to the log(Z(q)) versus q 2 data points in the small q region. However, this approximation is valid only if the q range for the calculation and the resultant R, value satisfy the constraint that qRg < 1. In this study, the SAXS instrument used for most of the measurements was equipped with a traditionally sealed-off Fe tube (Phillips Co.) which generated X-rays with a wavelength of 1.94A,a Kratky camera (Anton Paar Co.), and a one-dimensional position-sensitive proportional counter (M.Braun Co.). The proportional counter had a resolution of 8 = 0.003 deg/channel, and the Kratky camera provided the q values ranging from 0.008 to 0.2 A-1. The X-ray generator was operated at 40 kV and 10 mA. Three kinds of solutions containing completely dissolved asphaltenes were used in this study. They were (1)pure toluene, (2) 10 wt % p-nonylphenol in dodecane (or heptane), and (3) 5 wt % p-dodecylbenzenesulfonic acid in heptane. In order to avoid interparticle scattering, the maximum concentration of asphaltenes waa limited to 4 wt %. The liquid cell used to hold asphaltene solutions for SAXS measurements had a pair of very thin parallel mica windows and a fixed path length of 1/8in. In data analysis, the measured scattering intensity I ( q ) was plotted in the logarithmic scale in order to separate the scattering effects contributed by the geometry of the dispersed phase (i.e., yo (r)) from other factors such as N, V, and Ap (see eq 6). The radii of gyration of asphaltenes and asphaltene-amphiphile associated colloids were first estimated by the Guinier approximation using eq 7. However, because asphaltene molecules are polydisperse and structurally complex, it is difficult to define their actual size distribution. Without losing any generality, the distance distribution function P(r) in eq 5 was further used to represent detailed information on the asphaltene structure in the solution. Because the X-ray scattering I ( q ) profiles in this study were smeared by the slit collimation, these I ( q ) profiles were first extrapolated to small q values (q < 0.008 A-9 using the Guinier law, and then desmeared by the procedure of Schmidt and Hight.a Afterward,the desmeared SAXS profie was Fourier transformed to P(r) using eqs 4 and 5.
Results and Discussion Acid-Base Interaction between Asphaltenes and Amphiphiles (FTIR Study). Interaction between Asphaltenes and pAlkylpheno1. The acid-base (i.e., hydrogen-bonding) interaction between asphaltenes and the hydroxyl (OH) group of p-dodecylphenol (DP) amphiphiles in decane solution was investigated by monitoring the absorption intensity of DP's free OH stretching band peak located at 3620cm-l. It was found that, at high DP concentration,DP molecules tended to hydrogen-bond among themselves. In order to use this 3620-cm-1 peak to study the DP-asphaltene interactions, the hydrogenbonding behavior of DP molecules themselves was investigated first. In Figure la, the FTIR spectra show that the bonded OH stretching band in the range of 3200-3550 cm-1 does not appear below 1wt % DP but does appear at 2 wt % DP in decane, suggesting that the hydroxyl (OH)groups of different DP amphiphiles start hydrogenbonding together at between 1 and 2 wt % DP. The integrated absorbanceof the DP's free OH stretching band peak was calculated from the area of this 3620-cm-l peak. (23) Schmidt, P. W.; Hight, R. Acta Crystallogr. 1960, 13, 480.
Asphaltene-Amphiphile Interactions and Structures
Langmuir, Vol. 10, No. 6,1994 1761 0.3
Free OH
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Figure l b further shows that the extent of increment of the integrated absorbance gradually decreases as the DP concentration increases from 1to 8 wt % , indicating the percentage of hydrogen-bonded DP molecules increases with increasing DP concentration. The hydrogen-bonding band in the range of 3200-3500 cm-l was not investigated in this study, because of the difficulty in differentiating the contribution of the asphaltene-DP interactions to this band from that of ihe DP-DP interactions. Figure 2a shows the FTIR spectra of the DP's free OH stretching band peak of the decane solutions in the presence of 6.25 wt % DP at different concentrations of stabilized asphaltenes. The absorbance of this 3620-cm-l peak decreases successivelywith increasing concentration of asphaltenes from 2 to 6 wt %. The integrated absorbance of this 3620-cm-l peak was plotted in Figure 2b. It shows that this DP's free OH stretching peak decreases up to 10-15% at 5-6 wt % asphaltenes, indicating that part of the DP molecules are hydrogenbonding with asphaltenes. In order to quantify the asphaltene-DP interaction, we assumed that the equilibrium constant between the free DP molecules and the DP molecules with hydrogen-bonding among themselves was not influenced by the presence of asphaltenes. Consequently, the quantity of the free DP and selfassociated DP was deduced using the calibration curve in Figure lb. Next, this value was subtracted from the total amount of added DP to obtain the quantity of DP interacting with asphaltenes. Figure 3 shows that the amount of p-dodecylphenol hydrogen-bonding with asphaltenes from the above calculation increases steadily with increasing asphaltene concentration. From the initial slope of the curves in Figure 3, it was found that the hydrogen-bonding capacity of asphaltenes increases from approximately 1.6 mmol/g of asphaltenes a t 6.25 wt 5% DP to 2 mmol/g of asphaltenes a t 8 w t % DP. The
1 3 4 5 Wt% of total asphaltenes
6
7
(b)
(b)
Figure 1. (a) FTIR spectra of different amountsof DP dissolved in decane. (b) Integrated absorbance of the free OH stretching band (3620cm-1) for different amountsof DP dissolved in decane.
1
8wt%DP
Figure 2. (a) FTIR spectra of the free OH stretching band of DP for different amounts of asphaltenes dissolved in 6.25 w t % DP/decane solutions. (b) Integrated absorbance of the free OH stretching band of DP for different amounts of asphaltenes
dissolved in DP/decane solutions. 2.5
Wt% of total asphaltenes
Figure 3. Weight percentage of DP hydrogen-bonding with
asphaltenes in DP/decane solutions.
hydrogen-bonding capacity of this asphaltene sample is very similar to that obtained by Barbour and Petersen using phenol/CCl, solution (see Introduction). However, as opposed to the alkane solvent used in our study, CCl, solvent used by Barbour and Petersen is a fairly good solvent for asphaltenes. As previously stated, their study did not provide direct evidence that asphaltenes can be stabilized in apolar media through hydrogen-bonding interaction with amphiphilic stabilizers. Interaction between Asphaltenes and pAlkylbenzenesulfonic Acid. The acid-base interaction between asphaltenes and p-dodecylbenzenesulfonicacid (DBSA, n-C12HghSOSH) was characterized using FTIR and UV/ vis spectroscopies. Figure 4a shows that, in the range of 2000-2800 cm-', the absorbance of the CCl, solution containing 2 wt % asphaltenes and 2 wt % DBSA decreases by 30-50 9% from that of pure 2 wt % DBSA in CC4.It is known that DBSA's SO-H stretching band has a significant absorbance in this wavelength range. Therefore, decreas-
Chang and Fogler 1,
-2%DBSA/CC14
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Wavenumber (cm")
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I
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Figure 6. Comparison of the UV/visible spectra of asphaltenes in toluene and in the 5 w t % DBSA/heptane solution.
0.15
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1500
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Figure 4. (a) FTIR spectra of DBSA in solution before and after the addition of 2 w t % asphaltenes. (b)Differential FTIR spectra of asphaltenes after subtracting FTIR spectra of asphaltene/2wt % DBSA/CC&solutionsfrom that of 2 wt % DBSA/ CC4 solution. ing DBSA's SO-H stretching band in the presence of asphaltenes can be due to the breakage of DBSA's SO-H bond by irreversiblytransferring the proton of its sulfonic acid (S03H) group to the corresponding conjugate basic groups on asphaltenes. In order to further study this interaction, differential FTIR spectra of asphaltenes were obtained by subtracting the FTIR spectra of asphaltenel DBSA solutions from that of the pure DBSA solution. Figure 4b shows that the differential FTIR spectra of asphaltenes are significantly different from that of asphaltenes being dissolved in the pure CC4 (CS2) solvent (Figure 2, part 1). First, the appearance of a significant broad band located at 3300 cm-1 suggests a hydrogenbonding interaction occurs in the asphaltene/DBSA solution. The hydrogen-bonding between the DBSA's S=O group and the asphaltene's acidic groups may account for the appearance of this band. It is also known that the commercial DBSA is usually hydrated into n-CnH25PhS03-H30+ to a certain extent. From the purity of our DBSA sample,97 w t % ,the maximum percentage of DBSA that is hydrated would be 56%. Therefore, the hydronium (H30+)group of hydrated DBSA may also hydrogen-bond with asphaltenes and contribute to the absorption at 3300 cm-1. In addition, the absorption in the range of 10001400 cm-', which corresponds to the SO2 (SO3-) groups of DBSA and polar groups of asphaltenes, also differs from that of pure asphaltenes significantly. The differential FTIR spectra of asphaltenes also show that, in the range above 3500 cm-1, avery broad absorption appears. This absorption band increases continually with increasing FTIR wavenumber until a value of 4000 cm-l is reached. This trend suggests that the absorption may further increase in the near-IR range (10000-4000 cm-l). To further unveil this phenomena, the electronic absorption of asphaltenes in toluene and in DBSNdecane solution in a UV/vis range was studied. Figure 5 shows the UV/vis absorption of asphaltenes in the presence of DBSA is significantly stronger than that without DBSA. This
observation provides supporting evidence that the broad band appearing from 3500 to 4000 cm-l in Figure 4b is an extension of the extra electronic absorption in the UV/vis range. As discussed in the Introduction, asphaltenes have a broad absorption profile ranging from UV to near-IR due to the m* and nr* electronic transitions of the asphaltene's highly conjugated electronic structure with the alternation of single and double bonds, such as polyaromatic rings and polyenes. For a DBSA molecule, the connection of its aromatic benzene group to its sulfonic group also results in an electronic conjugated structure. Therefore, the increase of the UV/vis absorption of asphaltenes in the presence of DBSA suggests that asphaltenes and DBSA molecules may associate into an even larger electronic complex structure than that of asphaltene itself. The spectroscopicstudies described above suggest that the asphalteneDBSA interaction may be initiated by the breakage of the SO-H bond of DBSA and followed by the association of asphaltenes and the p-dodecylbenzenesulfonate group into a larger conjugated structure. Due the chemical structural complexity of asphaltenes, the actual mechanisms of this asphaltene-DBSA interaction are very difficult to identify. One of the most probable mechanisms is the electrophilicaddition reaction between DBSA and the *-electron of asphaltene~.~~ Asphaltenes contain a significant portion of *-electrons in their highly electronic conjugated structure. These asphaltene ?r-electrons can serve as a source of electrons (i.e., Lewis base) for the compounds which are deficient of electrons such as DBSA (i.e., Lewis acid). Therefore, DBSA molecules can react with the *-electrons of asphaltenes by initial breaking ita SOgH bond and subsequently bonding its p-dodecylbenzenesulfonate group to asphaltenes. This electrophilic addition reaction is illustrated as follows: SOB-H
I I
+ -C=C-
asphahenes
-
I t
-C-C-
(6)
Another possible mechanism which could cause the decrease of SO-H vibrational absorption and the increase of electronic absorption is the acid-base exchange interaction between DBSA and the specific acid-base complex groups of asphaltenes as illustrated in the following expression:
(9)
where A, and B, represent the acidic and basic groups of asphaltenes, respectively. The products, i.e., -PhSO3(24) Morrison, R. T. Organic Chemistry, 6th ed.; Prentice-Hall: Englewood Cliffs, NJ, 1992.
Langmuir, Vol. 10, No. 6,1994 1763
Asphaltene-Amphiphile Interactions and Structures I
100,
\ ;
l o l #,";,'
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Figure 6. Amount of DBSA undergoing the irreversible acidbase reaction with asphaltenes in 4 wt % DBSA/decane solution and in 2 wt % DBSA/CC&solution.
A,, and H-B,,, may still associate together in apolar media through acid-base attractions after this reaction. Our preliminary FTIR measurements show that the addition of ethanol, p-dodecylphenol (containing the hydroxyl (OH) group), and acetone (with the carbonyl (C=O)group) to the DBSA solution does not decrease the DBSA's SO-H stretching band, which suggests that perhaps only a few specificfunctionalgroups of asphaltenes such as metal ions (e.g., vanadium and nickel) may be involved in this type of reaction. In addition to the above two mechanisms, the physical association between DBSA and the polar groups of asphaltenes may also generate the electron donor-acceptor complex structure to bring about the red shift of the electronic absorption. At last, HzO in the commercial DBSA may also undergo an electrophilic addition to the asphaltene's ?r-electronby the catalysis of acids (e.g., DBSAIz4and cause the absorption band in the range of 2000-3000 cm-l to decrease. From the above description, one can see that the acidbase interactions between asphaltenes and DBSA involve quite a few mechanisms. This asphaltene-DBSA attractive interaction appears so strong that the asphalteneDBSA precipitates are much less soluble than asphaltenes themselves. We found that the precipitates generated from the heteroaggregation between asphaltenes and DBSA cannot even be dissolved by an aromatic solvent such as toluene. The overall irreversible asphaltene-DBSA interaction was quantified by the decrease of the FTIR absorption in the range of 2000-3000 cm-'. The extent of this decrease, as shown in Figure 4b, is quite similar at different wavenumbers ranging from 1800 to 2500 cm-l. Therefore, the absorption intensity at 2200 cm-l was chosen to estimate the percentage of the DBSA having an acidbase interaction with asphaltenes. Figure 6 shows that the percentage of DBSA interacting with asphaltenes increases with increasing weight ratio of asphaltenes to DBSA. When the value of the asphaltene-to-DBSA weight ratio is below 0.5, the percentage of DBSA interacting with asphaltenes increases linearly with increasing weight ratio, suggesting that there is sufficient DBSA to interact completely with all available active groups of asphaltenes. The stoichiometry of the acid-base interaction between asphaltenes and DBSA is estimated to be about 1.8 mmol of DBSAIg of asphaltene. When the weight ratio is above 0.5, the percentage of DBSA interacting gradually deviates negatively from the extrapolation of the linear fitting of data below 0.5 weight ratio, indicating that DBSA cannot interact with all of the asphaltene's active groups. Structure of Asphaltenes in Amphiphile Solutions (SAXS Study). The structure of stabilized asphaltenes in three solutions [(l)pure toluene, (2) 10 wt % p nonylphenol (NP)/dodecane solution, and (3) 5 wt %
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Figure 7. SAXS intensity I ( q ) versus q curves for asphaltenes (a) in toluene solutions, (b) in 10 wt % NP/dodecane solutions, and (c) in 5 w t % DBSA/heptane solutions. p-dodecylbenzenesulfonicacid (DBSA)/heptanesolution1 was studied using SAXSmeasurements. After subtracting the background and smoothing the data, the scattering profiles were plotted in Figure 7. Figure 7a shows the scattering profiles of asphaltenes in the pure toluene solvent. When amphiphiles are not present in the toluene solution, the scattered X-rays are attributed only to the asphaltenes. These scattering profiles do not have any noticeable upturn in the small q range, suggesting the absence of large asphaltene aggregates. The slope of the scattering curves also remains virtually unchanged as the concentration of asphaltenes increases from 1to 4 w t 7%, indicating that asphaltene colloids are well dispersed in toluene without any strong association among themselves even if the asphaltene concentrationreaches 4 wt 7%.Figure 7b shows that the SAXS scattering profiles of asphaltenes in a 10 wt 5% NP/dodecane solution are very similar to those of asphaltenes in toluene, and that their slopes also remain unchanged as the asphaltene concentration increases from 1to 4 wt %. Therefore, asphaltene colloids in the p-nonylphenol/dodecane solution are stabilized as well as in the pure toluene. Figure 7c shows the scattering profiles of asphaltenes in the 5 wt % DBSA/heptane solution. Because the scattering profile of the 5 wt % DBSA/heptane solution (without asphaltenes) decreases slightly with increasing q value, DBSA molecules themselves can apparently induce the scattering of X-rays. The slope of scattering profiles increases gradually from 0.3 to 3 wt 5% asphaltenes because more and more asphaltenes contribute to the X-ray scattering. The scattering profile for 3 wt % asphaltenes shows a noticeable upturn a t q < 0.04 A-1. This upturn in the small q range indicates that asphaltene particles may start to aggregate into large associated colloids.
Chang and Fogler
1764 Langmuir, Vol. 10,No. 6,1994 1000
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The verification of asphaltene-amphiphile interactions was carried out by carefully comparing the small-angle scattering profiles of asphaltenes in different solutions. Figure 8a shows SAXS profiles of 4 wt 7% asphaltenes in toluene, in 10 wt % NP/dodecane solution, and in 10 wt 7% NP/heptane solution. It is clear that the scattered intensity of asphaltenes decreases successively from heptane (10 wt % NP) to dodecane (10 wt % NP) to toluene due to the successive decrease in the electron density difference (Ap) between asphaltenes and solvents. The magnitude of the electron density difference between asphaltenes and solvents can affect the shape of the scattering profile of asphaltenes. However, Figure 8a shows that even though the scattered intensities of asphaltenes in the NP/heptane solution and in the NP/ dodecane solution are different, they still have the same slope which is slightly larger than the slope of the asphaltene/toluene scattering curve. This slightly steeper slope in the scattered intensity of asphaltenes in NP/alkane solutions may be attributed to the asphaltene-NP association rather than the electron density difference between asphaltenes and solvents. Evidence for the association between asphaltenes and DBSA in heptane solutions is shown in Figure 8b. Note first that the electron density differences between the dispersed phase and the solvent medium were unimportant because the same solvent, heptane, was used for all measurements. If asphaltenes and DBSA do not associate together, the scattered intensity of the solution containing both DBSA and asphaltenes should be equal to the summation of the scattered intensity of the asphaltene solution and that of the DBSA solution. However, Figure 8b shows that the scattering profile for the heptane solution
containing 2 wt % asphaltenes and 5 wt % DBSA has a significantly stronger intensity and a steeper slope than the summation of the scattering of 5 wt 5% DBSA/heptane solution and that of 2 wt 7% asphaltenedheptane (with 10 wt % NP) solution. This difference clearly demonstrates that DBSA can associate with asphaltenes. Similar results were also found even in the cases where the concentration of asphaltenes was as low as 0.3 wt %. Equation 1shows that the intensity of scattered X-rays is proportional to the square of Ap, the electron density fluctuation from its mean value. It is apparent that asphaltenes have a significantly higher electron density than both aromatic and aliphatic solvents used in this study. However, p-nonylphenol (NP) does not have a significantly higher electron density than the aliphatic solvents. p-Nonylphenol's head group, with only one oxygen atom, is a weak scattering entity, and its aliphatic tail has virtually the same electron density as alkane solvent. Therefore, the actual dimension of asphalteneamphiphile aggregates could be significantly underestimated by not taking into account the aliphatic stabilization layer built around asphaltenes by NP amphiphiles. Moreover, because of the weak asphaltene-NP interaction, the concentration of NP attached to asphaltene surfaces is not significantly higher than the bulk average NP concentration, making the electron density difference between the NP layer attached to asphaltenes and the mean value even smaller. This result explains why the SAXS profile for asphaltenes in 10 wt % NP/alkane solution, as shown in Figure 8a, is very similar to that in toluene, leading to difficulty in identifying the association between asphaltenes and p-nonylphenol amphiphiles. Although DBSA also has an aliphatic tail which does not cause the scattering of X-rays, ita head group, the sulfonic acid (SOsH), is a very strong X-ray scattering entity.26 The scattering amplitudes of X-rays for sulfur, oxygen, carbon, and hydrogen are 4.5 X 10-12, 2.25 X 10-12,1.69 X 10-12, and 0.28 X 1&12 cm, r e s p e c t i ~ e l y . ~The 6 ~ ~average ~ scattering amplitude of a sulfonic group (SO3-) is 10times (((4.5 + 3 X 2.25)/4) X 10-12 = 2.8 X 1&12cm) that of the hydrogen atom. Therefore, the scattering profiles of DBSA/heptane solutions in Figure 8b are primarily attributed to the sulfonic acid group even though the DBSA's benzene group may also induce a portion of X-ray scattering. Figure 9 shows the Guinier plots (log(&)) vs q2)of some of the scattering profiles in Figure 7. Curves 1 and 2 correspond to 4 wt 7% asphaltenes in toluene and in 10 wt % NP/dodecane solution, respectively. It was found in our size exclusion chromatographic experiments that asphaltenes are quite polydisperse.' The slight nonlinearity of curves 1 and 2 also supports this polydisperse result. However, in the range where q 2 is less than O.OOO8 A-2, the log(l(q))vs q2 relationship still appears reasonably linear. Therefore, the Guinier approximation described in eq 7 was used to estimate the radius of gyration (R,) of asphaltenes. The apparent Rg of asphaltenes is calculated to be 37 and 42 A in toluene and in 5 wt 7% NP/dodecane solution, respectively. The extremely weak X-ray scattering strength of NP amphiphiles makes the size of asphaltene-NP associated colloids (42 A) only slightly larger than the asphaltene species itself (37 A). As shown by curve 3, the Guinier plot of the scattering profile of 5 wt 5% DBSA in heptane solution has a linear trend up to q2 = 0.015 A-2, suggesting that DBSA should be quite (25) Wu, D. Q.; Chu, B.; Lundberg, R.; Macknight, W. J. Macromolecules 1993,266,1000. (26) Chen, S. H. Annu. Reo. Phys. Chem. 1986,37,351.
Asphaltene-Amphiphile Interactions and Structures I
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)'"(s Figure 9. Guinier plots to estimate the radii of gyration (R,) of asphaltenes and DBSA in different solutions: curve 1,4wt % asphaltenes (in toluene),R, = 37 A;curve 2; 4 wt % asphaltenes (in 10 wt % NP/dodecane solution), R, = 42 A; curve 3,5wt % DBSA (in heptane), R, = 15 A; curve 4 , 3 wt % asphaltenes and 5wt 7% DBSA (inheptane),R, = 75A. All Guinier approximations satisfy qR, < 1. monodispersed. The estimated R, of DBSA is 15 A. Because this value is significantly larger than the major scattering entity of DBSA, the sulfonic acid group, DBSA molecules should associate themselves into reversed micelles in apolar media. This DBSA micellar formation may presumably involve the acid-base interaction between the SO-H group and the S=O group of different DBSA molecules. When the concentration ratio of DBSA to asphaltene is high, excessive DBSA molecules may attach the inner DBSA molecules which bind directly to the asphaltene core to form multiple layers of DBSA molecules around asphaltenes. However, in heavier alkane solutions, the stronger association between DBSA molecules could produce larger asphaltene-DBSA associated colloids which may become sufficiently heavy to precipitate out of solution.' Curve 4 shows the Guinier plot of 3 wt % asphaltenes in 5 wt % DBSA/heptane solution. The significant upturn in the small q2 range indicates that asphaltenes aggregate into much larger colloids. The R, of these aggregates estimated from the slope of the curve in the smallest q range (q2 < 0.0002A-2) is at least 75 A. Indeed, the finding that 4 wt 5% asphaltenes cannot be completely dissolved in the 5 wt $' 6 DBSA/heptane solution suggests 3 wt 5% asphaltenes may associate with almost all of the DBSA and are close to their solubility limit. The distance distribution functions,P(r),for asphaltenes in different solutions were calculated and plotted in Figure 10 using the scattering data in Figure 7. P ( r ) is the probability of finding two scatteringpoints that are spaced by a distance of r. Figure 10a shows that P ( r ) curves of asphaltenes in toluene and in the p-nonylphenolldodecane
r (b)
Figure 10. Distance distribution function P(r) of (a) 4 wt % asphaltenes in toluene solution and in 10 wt % NP/dodecane solution and of (b) different amounts of asphaltenes in 5 wt % DBWheptane solution. solution have similar sizes ranging up to about 130-140 A, and the positions of the P(r) peak are located between 35 and 45 A. The intensity difference of P ( r ) between these two curves is mainly due to the difference of the electron densities between asphaltenes and these two media. Figure 10b shows that the range and the intensity of the distance distribution function of asphaltene-DBSA associated colloids increase with increasing concentration of asphaltenes. For the 5 wt % DBSNheptane solution without any asphaltenes, P(r)ranges up to 50 A and the P ( r ) peak is located at 15 A. As the asphaltene concentration is increased from 0.3to 1 to 3 wt % ,the maximum dimension of P ( r ) shifts successively upward from about 120 to 140 to 260 A, and the location of the P ( r ) peak also moves successively from 20 to 30 to 50 A. At 0.3 and 1 wt % asphaltenes, theP(r) profiles of the 5 wt 7% DBSNheptane solutions are still distributed over smaller r ranges more than those in Figure loa, indicating there still exist free DBSA molecules without associating with asphaltenes. At 3 wt 9% asphaltenes, the shift of the entire P ( r ) profile significantly toward larger r indicates asphaltenes may start aggregating. The maximum r value, i.e., 260 A, becomes approximately twice as large as the maximum r value of asphaltene particles, i.e., 130 A, suggesting that asphaltene particles may have formed the binary aggregates. From the experimental results of the above FTIR (and UV/vis) spectroscopic measurement and SAXS study, one can clearly see that asphaltenes from natural crude oil are capable of associating with amphiphilic molecules into micelle-like colloids in apolar media through acid-base interactions between asphaltenes and amphiphiles. Especially, the spectroscopicstudy shows that the acid-base interaction between asphaltenes and amphiphiles appears fairly complex and involves different mechanisms, including reversible hydrogen-bonding, electron donor-acceptor transfer interaction, and the irreversible electrophilic
1766 Langmuir, Vol. 10, No. 6, 1994
addition reaction. These observations demonstrate the complexity of the acid-base behaviors of asphaltenes. Asphaltene colloids in crude oil can be peptized by the natural amphiphilic stabilizers, i.e., natural resins, by reversible hydrogen-bonding and electron donor-acceptor transfer interaction. On the other hand, the irreversible electrophilic addition reaction does not likely take place between asphaltenes and natural resins. Nevertheless, the electrophilic addition reaction involved between asphaltenes and DBSA may account for the cause of asphaltene deposition during the acidization of reservoir formations using aqueous fluids containing strong HC1 and HF acids. Upon contacting reservoir crude oil, these strong acids can carry out electrophilic addition reactions with asphaltenes similar to that shown in eq 8. This reaction may further induce the polymerization of asphaltenes and cause asphaltene deposition in the reservoir.27 This study also shows that p-alkylphenol and p-alkylbenzenesulfonic acid can be used to characterize and quantify the acid-base reactivity of asphaltenes and, therefore, should be applicable to the characterization of asphaltic material from other crude oils.
Conclusions In this study, the mechanism of stabilizing asphaltenes in alkane solutions by two types of alkylbenzene-derived amphiphiles, p-alkylphenol and p-alkylbenzenesulfonic acid, was investigated by FTIR spectroscopy and SAXS. The FTIR spectroscopic study verified that asphaltenes could form acid-base interactions with both p-alkylphenol amphiphiles and p-alkylbenzenesulfonicacid amphiphiles. The capacity of asphaltenes for hydrogen-bonding with p-dodecylphenol was estimated to be 1.6-2.0 mmol/g of (27) Leontaritia, K. J. Private communication.
Chang and Fogler
asphaltene. The interaction between asphaltenes and p-dodecylbenzenesulfonic acid (DBSA) appears quite complex. The decrease of the absorption of DBSA's SO-H stretching band and the increase of the electronic absorption of asphaltenes suggest one of the asphaltene-DBSA interactions may involve an electrophilicaddition reaction of DBSA (Lewis acid) to the ?r-electron of asphaltenes (Lewisbase). The stoichiometry of the overallasphalteneDBSA interaction was approximated to be 1.8 mmol of DBSA/g of asphaltene. SAXS studies further demonstrated the association between asphaltene particles and p-nonylphenol and DBSA. It was found that the dimension of asphaltene particles was quite similar to that of asphaltene-pnonylphenol associated colloids. The significant change of the SAXS profiles by the addition of DBSA to asphaltenes reflects the strong association between asphaltenes and DBSA. The observation that DBSA molecules can associate with themselves suggests asphaltenes might even be surrounded by multilayers of DBSA molecules in apolar media. A shortage of DBSA amphiphilic stabilizers can cause asphaltenes and DBSA to flocculate into large aggregates which can also be characterized by the SAXS measurement.
Acknowledgment. The authors thank the following sponsors of the industrial affiliate program of the University of Michigan: Chevron Oil Field Research Co., Conoco., Halliburton Services, Marathon Oil Co., Mobil Research & Development,Phillips Petroleum, Texaco,and Unical. We also thank the Department of Energy for financial support. The permission of Professor E. Gulari to use his FTIR spectrophotometer and the assistance of Dr. R. Hristov in the SAXS experiments are also greatly appreciated.
