Langmuir 1994,10, 1749-1757
1749
Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 1. Effect of the Chemical Structure of Amphiphiles on Asphaltene Stabilization 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 16, 1994’ Stabilization of crude oil asphaltenes in apolar alkane solvents was investigated using a series of alkylbenzene-derivedamphiphiles as the asphaltene stabilizers. In this paper (i.e., part l),we present the study on the influences of the chemical structure of these amphiphiles on the effectiveness of asphaltene solubilization and on the strength of asphaltene-amphiphile interaction using both UV/vis and FTIR spectroscopies. The results showed that the amphiphile’s effectiveness of asphaltene stabilization was primarily controlled by the polarity of the amphiphile’s head group and the length of the amphiphile’s alkyl tail. Increasing the acidity of the amphiphile’s head group could promote the amphiphile’s ability to stabilize asphaltenes by increasing the acid-base attraction between asphaltenes and amphiphiles. On the other hand, although decreasing the amphiphile’s tail length increased the asphaltene-amphiphile attraction slightly, it still required a minimum tail length (six carbons for p-alkylphenol amphiphiles) for amphiphiles to form stable steric layers around asphaltenes. We also found additional acidic side groups of amphiphiles could further improve the amphiphile’s ability to stabilize asphaltenes. The effect of the molecular weight of alkane solvents on the amphiphile’s ability to stabilize asphaltenes was also studied. The successive decline of the effectiveness of p-alkylphenol amphiphiles on asphaltene stabilization from dodecane to pentane could be explained in part by the reduction of the asphaltene-amphiphile attractive interaction. On the contrary, the extremely strong ability of p-alkylbenzenesulfonic acid amphiphiles to associate both asphaltenes and themselves made their effectiveness on asphaltene stabilization decrease with increasing molecular weight of alkanes.
Introduction Crude oil produced from subterranean formations consists of a distribution of molecules with different chemical structures and molecular weights. The heaviest and most polar portion of crude oil, named asphaltenes, gives rise to a variety of problems during crude oil production. For example, the flocculation and precipitation of asphaltenes can plug porous formations as well as change the reservoir’s wettability from water-wet to oilwet by adsorbing to mineral surfaces.’ These asphaltene problems were recognized more than 50 years ago, and as a result much research has been conducted in order to unveil the chemical structure of asphaltenes and the mechanism causing asphaltenes to become unstable and precipitate out of crude ~ i l . ~Because J of the extremely complex composition of crude oil, it is not feasible to isolate asphaltene compounds with specific chemical structures. Therefore, for the sake of convenience, asphaltenes were defined as the crude oil fraction that is soluble in aromatic solvents but insoluble in aliphatic solvents. Indeed, asphaltenes separated from crude oil by this definition include the most polar and the highest molecular weight species of crude Asphaltenes are generally composed of polyaromatic nuclei carrying aliphatic chains or rings and other elements including sulfur, oxygen, nitrogen, and
* To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, May 1, 1994. (1) Leontaritis, K.J.; Amaefule, J. 0.;Charles, R. E. Presented at the SPE International Symposium on Formation Damage, Lafayette, LA, 1992; SPE Paper No. 23810. (2) Pfeiffer, J. PH.; Saal, R. N. J . Phys. Chem. 1940, 44 (2), 139. (3) Burger, J. W.; Li, N. C. Chemistry of Asphaltenes; Advances in Chemistry Series195;AmericanChemicalSociety:Washington,DC, 1981. (4) Long, B. R. In Chemistry of Asphaltenes;Burger, J. W., Li, N. C., Eda.; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1981; p 17. Q
metals such as vanadium and ni~kel.~16 These extra elemental atoms (i.e., S, 0, N, and metals) account for a variety of polar groups of the asphaltene molecules, such as aldehyde, carbonyl, carboxylic acid, and amide. Using a homolog of alkyl aromatic molecules as asphaltene solvents, it was found that the asphaltene solubility increases linearly with the Hildebrand solubility parameter ( 6 ) of ~olvents.~ This 6 parameter is usually defined as the square root of a solvent’s cohesive energy density, i.e., (AUv/Vm)1/2,where AUV and Vm are the molar internal energy of vaporization and the molar volume of the solvent, respectively. In previous research, the 6 parameter has been included in the x parameter of the Flory-Huggins equation to model the phase behavior of asphaltenes in both solvents and crude oil by the following e x p r e s s i ~ n : ~ ~ ~
The Flory-Huggins model was widely used because of its relative simplicity for simulating the phase behavior of solutions containing macromoleculeslike asphaltenes. The Flory-Huggins x parameter represents the difference of the intermolecular interaction between solvent molecules and asphaltene molecules. Increasing the difference between the solubility parameter of solvents (gsolvent(crude)) (5) Yen, T. F. In Chemistry of Asphaltenes; Burger, J. W., Li, N. C.,
as.Advances ; in Chemistry Series 195; American Chemical Society:
Washington, DC, 1981; p 39. (6) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.;Marcel Dekker; New York, 1991; Chapter 11. (7) Mitchell, D. L.; Speight, J. G. Fuel 1973,52,149. (8) Hirschberg, A,;ddong, L. N. J.; Schipper,B. A.;Meijer, J. G. SPE J. 1984, June, 283. (9) Kawanaka, S.;Park, S. J.; Mansoori, G. A. SPE Reseruoir Eng. 1991, May, 185.
0743-7463/94/2410-1749$04.50/00 1994 American Chemical Society
1750 Langmuir, Vol. 10, No. 6,1994
and asphaltenes (6aspwtene) can increase the x value. If this x value exceeds 0.5, separation into two phases will occur. The 6 values of aliphatic and aromatic solvents are about 7.5 (cal/cm3)Oe5 (heptane) and 8.9 (cal/cm3)0.5 (toluene), respectively.’ The 6 value of asphaltenes was unable to be measured but deduced to be about 9.5 (cal/ ~ m ~from ) ~the. titration ~ of crude oil with light alkane solvents.* Though the solubility parameter concept can account for the phase behaviors of asphaltenes, it does not take into account the complete mechanism of asphaltene stabilization in crude oil. It was well recognized that asphaltenes contain not only aromatic compoundsbut also a variety of acidic and basic functional groups. In crude oil, asphaltene molecules may perform these acid-base interactions as well as T-T orbital associations to form micellar-like aggregate^.^ In addition, these asphaltene micelles are stabilized in crude oil mainly by another polar fraction of crude oil, called resin^.^^^ These natural resin molecules also contain various polar groups, as asphaltenes do, yet are soluble in the apolar aliphatic solvents. It was found that once resins were removed from crude by the method of adsorption chromatography, the remaining It has crude oil could no longer solubilize asphaltene~.~ been proposed that resins are essential to the peptization of asphaltenes because they attach to asphaltene micelles with their polar groups and stretch their aliphatic groups outward to form a steric-stabilization layer around asphaltenes.1° In fact, the evidences of the mutual interactions between asphaltenes and resins have been identified qualitatively using various methods, such as infrared spectroscopyll and calorimetry.12 However, it is extremely difficult to quantify the interaction between asphaltenes and resin molecules because both species contain a distribution of molecules with complex chemical structures. Moreover, spectroscopic analysis has revealed that the polar groups of asphaltenes and resins are very similar, leading to difficulties in using spectroscopic methods to study the asphaltene-resin intera~ti0n.I~ On the basis of the above considerations, we believe it is feasible to unveil the stability of asphaltenes and the mechanism of asphaltene-resin interactions using a series of polar molecules with well-defined chemical structure as a model system of natural resins. Gonzalez et al. have studied the peptization of asphaltenes in aliphatic solvents by various oil-soluble amphiphilesincluding long-chain alkylbenzene, alkyl alcohol, alkylamine,andp-alkylphen01.l~They found that the effectiveness of amphiphiles on asphaltene stabilization was influenced by the head group of the amphiphiles. Among the amphiphiles they used, palkylphenol appeared to be the most effective asphaltene stabilizer. However, their study did not quantify nor disclose the mechanism of asphaltene-amphiphile interaction. In this study, we used a series of alkylbenzene-derived amphiphiles to investigate this asphaltene-amphiphile interaction as well as the stability behavior of asphaltenes in amphiphile/alkane solutions. The amphipbiles chosen for this study represent a diversity in commercial products and their relative effectiveness to stabilize asphaltenes in alkane solvents. As shown in Figure 1, while all of the amphiphiles have a common alkyl benzene group, they (10) Leontaritis, K. J. Presented at the SPE Production Operations Symposium, Oklahoma City, OK, 1989; SPE Paper No. 18892. (11) Suryanarayana, I.; et al. Fuel 1990,69, 1645. (12) Anderson, S. I.;Birdi, K. S. J. ColloidZnterfaceSci. 1991,142 (2), 497. (13) Christy, A. A.; Dahl, B.; Kvalheim, 0. M. Fuel 1989,68, 430. (14) Gonzalez, G.; Middea, A. Colloids Surf. 1991,52,207.
Chang and Fogler
Figure 1. Schematic of the chemical structure of alkylbenzenederived amphiphiles used in this study.
have at least one of the following three structural differences: (1)different polar head groups, (2) different lengths of alkyl tails,and (3)the existence of extra groups including the side group or the modified tail. Consequently, the effect of the amphiphile’s chemical structure of the stabilization of asphaltenes can be systematically investigated. In this paper (i.e., part 11, the effect of the molecular structure of the amphiphiles and the size of solvent molecules on the amphiphile’s ability to stabilize asphaltenes is described. The relationships between the amphiphile’s effectiveness on asphaltene stabilization and the asphaltene-amphiphile attraction were clarified by measuring the extent of amphiphile adsorption to asphaltene surfaces. In a second paper (i.e., part 2, following paper in this issue), the results of Fourier transform infrared (FTIR) spectroscopy measurements will be used to describe the molecular interactions, more specifically the acid-base interactions, between asphaltenes and amphiphiles. In addition, small-angle X-ray scattering (SAXS)techniques also verified the association between asphaltenes and amphiphiles and quantified the asphaltene-amphiphile associated structure.
Experimental Section Preparation and Characterization of Asphaltenes. Asphaltene sample powders in this study were initially provided by MobiI Research & Development, and later prepared by us from Mobil crude oil according to the procedures in ASTM 2007D.16 Both asphaltene samples were the n-pentane-insoluble fraction of the same Mobil crude oil. The exact chemical structure of an asphaltene sample is difficult to determine. Nevertheless,several methods, which are describedhere, were employed to characterize the composition, size, and chemical structure of asphaltenes. The elemental composition of our asphaltene sample was characterized to be 84 wt % C, 8 w t % H, 2.7 wt % N, and 5.3 wt 5% other elements, including 0,S, and metals (by difference). The molecular weight of asphaltenes in tetrahydrofuran (THF) was measured by size exclusion chromatography (Waters Associates Co.) using polystyrene standards and a UV detector (254 nm). It was found that this asphaltene sample had a broad molecular weight distribution with M. = 1099,M, = 12 170, and M,= 78 302.l6 From small-angleX-ray scattering measurements, the physical dimensions of asphaltenes in the toluene solution were found to range up to 13 nm with the radius of gyration about 4 nm (see part 2). The chemicalstructure of thisasphaltene sample was investigated by Fourier transform infrared (FTIR) spectroscopy using carbon tetrachloride (CC4) and carbon disulfide (CS2) as solvents. These solvents were used because CC4 and CS2 have a low IR abeorption background above and below 1400 cm-l, respectively. As illustrated by the FTIR spectrum in Figure 2, strong absorption of CH2 symmetric and asymmetric stretching bands located at 2850 and 2920 cm-1, respectively, indicates that this asphaltene sample contains a major portion of CHa groups. However, this asphaltene also contains a significant portion of aromatics and/or C = C bonds as illustrated by the absorption peak at 1600 cm-1. The broad bands rangingfrom 1700to lo00 cm-l suggestthat this asphaltene sample may have various functional groups. These functional groups are very likely to form hydrogen bonds as illustrated by the broad hydrogen-bonding band in the range of 3100-3600 (15) AnnwlBook ofASTMStandards,ASTM Philadelphia,PA,1983; Vol. 05.02, p 158. (16) The molecular weight distributionsare defined as follows: M. = EiMiNJEiNip Mw = EiM?NilEiMsNi, Mz = E@?NJEiM?Ni.
Effect of Amphiphiles on Asphaltene
Langmuir, Vol. 10, No. 6, 1994 1751
Table 1. Alkylbenzene-Derived Amphiphiles Used in This Study name
abbrev
C
source Aldrich
108.14
99
p-ethylphenol
EP
Aldrich
122.17
99
p-(n-buty1)phenol
BP
Kodak
150.22
97
p-(sec-buty1)phenol
SBP
Aldrich
150.22
96
p-(n-hexy1)phenol
HP
Kodak
178.28
97
p-(n-octy1)phenol
OP
Kodak
206.33
91
p-(tert-octy1)phenol
TOP
Aldrich
206.33
95
p-(n-nony1)phenol
NP
Aldrich
220.36
p-(n-dodecy1)phenol
DP
Pfaltz
262
96.5
n-nonylbenzene
NB
Fluka
204.36
97
p-[(hydroxyethoxy)ethoxyl-n-nonylbenzene
NBDO
Aldrich
308.47
91
p-(n-hepty1oxy)phenol
HOP
Aldrich
208.3
97
4-(n-dodecyl)resorcinol
DR
Aldrich
278.44
97
p-(n-dodecyl)benzenesulfonic acid
DBSA
Pfaltz
326
97
0.6
0.4
.
0.3
.
-0.1 600
chemical structure
Mw
p-cresol
I
root,
3800
O
,
,
,
,
I
'
purity(wt%)
"
I '
' ' A
CGH,. .CHI.
strelching Aromatic or
141Q
1400
C-c stretckdw
2200
3000
Wavenumber (cm")
0
20
40
60
80
J 100
Val% of heptane
Figure 2. FTIR spectrum of asphaltenes [in CC4 (above 1400 cm-1) and in CS2 (below 1400 cm-l)] sample 2 w t % asphaltenes, path length 0.15 mm).
Figure 3. Percentage of asphaltenes stabilized in the binary heptane/toluene solution.
cm-1. The appearance of a sharp peak located at 3480 cm-l should be due to the vibrational stretching of the free N-H group of asphaltenes. In this study, we quantify asphaltene concentration in the solution from the absorbance of light at a wavelength of 400 nm using a Varian DMS200 UV/vis spectrophotometer. Light absorbance at this wavelength was found to vary linearly with the concentration of asphaltenes using toluene as the solvent. Therefore,concentrated samples of asphaltenes were diluted with toluene in the subsequent measurements. The polarity of asphaltene sampleswas estimated by the solubility of asphaltenes in the binary toluene/heptane solution. The percentage of asphaltenes solubilized in the solution was quantified by the absorbanceof the filtrate. It is shown in Figure 3 that asphaltenes start to precipitate when the composition of heptane attains 3040 vol % which corresponds to 6 = 8.3-8.4 ( c a l / ~ m ~ ) ~ . ~ . Amphiphiles Used for Study. The chemical structure, abbreviation, source, and purity of the oil-soluble amphiphiles used for this study are listed in Table 1. These oil-soluble amphiphiles were used in the experiments without further purification. Measurement of Asphaltene Stability. Experiments listed
in Table 2a were conducted in order to quantify the effect of the chemical structure of amphiphiles on the solubilization of asphaltenes in alkane solvents. First, the minimum concentration of each amphiphile needed for completeasphaltene solubilization in alkane solvents was estimated from a preliminary study, and an amphiphile stock solution which contained a sufficient amount of amphiphiles to totally solubilize asphaltenes was prepared. The asphaltene stock solution was prepared by dissolving 2 wt % asphaltene powders in each amphiphile stock solution. Samples were prepared by mixing the amphiphile stock solution, the asphaltene stock solution, and pure alkane by different proportions. Afterward, samples were gently shaken at room temperature for 6-12 h and then filtered by 0.22-rm Millipore filter papers. The concentration of asphaltenes in the filtrate was measured by the absorbance of light at the wavelength of 400 nm. This general experimental procedure for preparing the asphaltene stock solution was modifiedunder specificconditions. For example, the amphiphiles p(hepty1oxy)phenol and 4-dodecylresorcinol were only solubilized in alkanes at elevated temperatures (50 and 100 O C , respectively). Their asphaltene stock solutions were prepared using toluene as solvent rather than the amphiphile/alkane solution. The asphaltene stock solutions of
1752 Langmuir, Vol. 10, No. 6, 1994
Chang and Fogler
Table 2. Descriution of the AsDhaltene Stabilization Experiment and Amuhiuhile Adsomtion Exwriment (a) Asphaltene Stabilization Experiment asphaltene variable amphiphiles used solvents used concentration (wt 7 % ) amphiphile head group NB, NBDO, NP, DBSA heptane 0.15 amphiphile tail length C, EP, BP, SBP, HP, OP, TOP, NP, DP heptane 0.15 amphiphile extra group HP, HOP, DR dodecane 0.15 pentane, hexane, heptane, dodecane, hexadecane 1 alkane solvent NP, DBSA ~
variable amphiphile head group
amphiphile tail length alkane solvent
(b) Amphiphile Adsorption Experiment amphiphile amphiphiles used concentration (mol/dm3) NB 0.490 NBDO 0.382 DP 0.382 DBSA 0.307 C, EP, HP, DP all 0.382 DP 0.382
solvents used decane
decane pentane, heptane, decane
Table 3. List of the Infrared Absorption Bands of Amphiphiles Used for Quantifying Amphiphile Concentrations by FTIR SWCtrOSCODY ~
amphiphile NB NBDO p-alkylphenol DBSA
wavenumber (cm-1) 697 1516 1516 1173
band assignment out-of-planebneding vibration of the monosubstituted benzene ring semicircle stretching - of -para-disubstituted benzene ring same as above symmetric SO2 stretching
the less effective amphiphiles, including nonylbenzene and p-[(hydroxyethoxy)ethoxy]nonylbenzene, were also made of toluene. The final concentration of toluene in these samples was approximately 5 vol % which did not have any influence on our experimental results. Measurement of Amphiphile Adsorption. As listed in Table 2b, experiments on measuring the adsorption of amphiphiles from alkane solutions to asphaltene surfaces were conducted to investigate the asphaltene-amphiphile interaction. In contrast with the asphaltene stabilization experiment, excessive amounts of asphaltene powders (up to 4 wt 5%) were mixed with the alkane solution which contained only 1 wt % amphiphiles. At this low amphiphile concentration, amphiphiles cannot stabilize most asphaltenes; instead they are adsorbed to the surface of insoluble asphaltenes. After 6-12 h of contact, samples were centrifuged a t 3000 rpm for 20 min in a Beckman TJ-6 centrifuge (relative centrifugal field 1300g). Next, the concentration of the amphiphiles in the supernatant was measured by a single-beamMatterson CYGNUS 100FTIR spectrophotometer equipped with a wide-band mercury-cadmium-telluride (MCT) detector. An FTIR sample holder with a pair of zinc selenite (ZnSe) windows (from Harrick Scientific) was used, and FTIR measurements used triangular apodization. The infrared ab: sorption peaks listed in Table 317 were used to quantify the concentration of amphiphiles in the supernatant because they were specific to amphiphiles but not to asphaltenes and alkanes. In the experiment listed in Table 2b where volatilepentane solvent was used, decane was used to replace pentane in the supernatant after the supernatant was separated from the precipitated asphaltenes. First, pentane was evaporated gently and completely from the supernatant a t 30 O C . From the weight decrease of the supernatant, the exact volumeof pentaneremoved was calculated. Then, the same volume of decane was added to the supernatant fraction to completely dissolve the dried amphiphiles. In order to obtain a significant adsorption of amphiphiles, asphaltenes were pretreated into extremely fine powders with large surface area by freeze-drying the benzene solution containing approximately 0.2 wt 7% dissolved asphaltenes. Besides, Teflon tubes (Nalge Co.) were used to accommodate samples because of their weak amphiphile adsorption and solvent swelling. The concentration of asphaltenes in the supernatant was also determined by the absorbance of supernatants a t the wavelength of 400 nm. (17)Colthup,N.B.;Dely,L.H.;Wiberley,S.E.lntroduction tolnfrared and Raman Spectroscopy, 3rd ed.; Academic Press: New York, 1990.
ot""""""'''"': 0
2
quantification method peak area peak area peak area peak height
4 6 8 Wt% of nmphiphiles
10
Figure 4. Percentage of asphaltenes stabilized in the heptane solutions containing amphiphiles with different head groups.
Results and Discussion Effect of the Amphiphile's Head Group. Four different kinds of amphiphileswere used to study the effect of the head group polarity on the amphiphile's ability to stabilize asphaltenes and on the asphaltene-amphiphile interactions. These amphiphiles were p-dodecylbenzenesulfonic acid (DBSA),p-nonyl (andpdodecy1)phenol p-[(hydroxyethoxy)ethoxylnonylbenzene (N(D)P), (NBDO),and nonylbenzene (NB).The reason for choosing these amphiphiles is because their head groups have significantly different acidities (Le., polarities), making it feasible to examine systematically the influence of the amphiphile's polarity on asphaltene stability. The polarities of the amphiphiles are in the following order: DBSA (with sulfonic acid (S03H) group) > N(D)P (with phenol group) > NBDO (with (hydroxyethoxy)ethoxy(OCzH&OH) group) > NB (without polar group). In addition, all of these amphiphiles have sufficiently long alkyl tails (C9C12) that the effect of the amphiphile's tail becomes insignificant (see the results on the amphiphile's tail length). The experimental results are presented in Figures 4-6. Figure 4 shows that the effectiveness of amphiphiles to stabilize asphaltenes follows the order DBSA > NP > NBDO > NB. Without a polar head group, NB does not have any ability whatsoever to stabilize asphaltenes; the concentration of solubilized asphaltenes in a heptane solution containing up to 6 w t 5% NB remains almost as
Langmuir, Vol. 10, No. 6,1994 1753
Effect of Amphiphiles on Asphaltene
..
.. ... 0.1
a
670
680 690
700
710
720
1500
1480
Wavenumber (cm-')
1520
1540
Wavenumber(cm-1)
Wavenumber (cm-1)
Wavenumber (em-')
Figure 5. FTIR spectra of amphiphiles with different head groups in the supernatantof the centrifuged amphiphile/decane solutions containing different amounts of asphaltenes.
0
I
2 3 4 Wt% of total asphaltenes
-e-- DP (amphiphile) -mNBDO (amphiphile) --e NB (amphiphile) --c DBSA (amphiphile)
5
+DP
(asphaltene) NBDO (as haltene) NB (asphaftene) --8- DBSA (asphaltene)
--8-
Figure 6. Percentage of amphiphiles with differenthead groups (and asphaltenes)remaining in the supernatantof the centrifuged amphiphile/decane solutions containing different amounts of asphaltenes. low as that in pure heptane. NP at a concentration of 7 wt 5% can completely solubilize asphaltenes, but 7 w t 9% NBDO solubilizes only 50% asphaltenes. That is, the effectiveness of NP as an asphaltene stabilizer is approximately twice that of p- [(hydroxyethoxy)ethoxylnonylbenzene (NBDO) though both amphiphiles' head groups contain the same hydroxyl (OH) group. The most effective amphiphile in this study was p-dodecylbenzenesulfonic acid (DBSA). With 1-2 wt % DBSA, asphaltenes can be totally solubilized in heptane solvent. The adsorption of amphiphiles to asphaltenes was studied using FTIR spectroscopy. Figure 5 shows the FTIR spectra of four amphiphiles (NB, NBDO, DP, and DBSA) in the supernatants containing different amounts of asphaltenes. The greater the decrease of the amphiphile's FTIR absorption intensity, the greater the number of amphiphile molecules adsorbed from solutions onto asphaltene surfaces, and the stronger the attractive interaction between amphiphiles and asphaltenes. The amount of amphiphiles and asphaltenes in the supernatants was quantified and plotted in Figure 6. The results clearly demonstrate that the extent of the amphiphile's adsorption to asphaltenes follows the order DBSA > DP > NBDO > NB. This trend is exactly the same as that shown in Figure 4, suggesting that the effectiveness of
these amphiphiles on the stabilization of asphaltenes relies mainly on the direct attractive interactions between amphiphiles and asphaltenes. As illustrated in Figure 6, the concentration of nonylbenzene (NB) in the supernatant remains unchanged even up to 4 wt % asphaltenes added, indicating that the benzene group itself cannot adsorb to asphaltenes. In contrast, DBSA shows the exact opposite trend by the close match of the asphaltene curve and the amphiphile curve. The percentage of asphaltenes stabilized in the supernatant is only slightly less than the percentage of DBSA that remained in the supernatant, which indicates that almost all of the DBSA molecules are attached to the asphaltene molecules. This strong attachment of DBSA to asphaltenes explains why DBSA molecules are a strongly effective asphaltene stabilizer. Figure 6 also shows that the addition of large amounts of asphaltenes are required to obtain significant adsorption ofp- [ (hydroxyethoxy)ethoxylnonylbenzene(NBDO) and p-dodecylphenol (DP) amphiphiles, indicating the attractions of these two amphiphiles to asphaltenes are quite weak. Nevertheless, DP still shows a higher extent of asphaltene attachment than NBDO. Figures 4 and 6 show that the effectiveness of the amphiphile's head group on the asphaltene stabilization and on the asphalteneamphiphile interaction is in the order sulfonic acid group > phenol group > (hydroxyethoxy)ethoxy group. This trend also follows the strength of these groups' acidity, suggesting that asphaltenes are stabilized by these amphiphiles through the acid-base interaction (note, hydrogen-bonding belongs to a weak acid-base interaction). DBSA is such a strong acid that it can readily undergo an almost irreversible acid-base reaction by donating its proton to the C=C bonds and/or specific basic groups of asphaltenes (see part 2). This strong acid-base interaction results in the attachment of the amphiphile's sulfonic acid group irreversibly to asphaltenes and makes DBSA a very effective asphaltene stabilizer. For phenol, its aromatic benzene group can delocalizethe electrons bonded between 0 and H elements of the hydroxyl group to make the phenol more acidic than the common alcohol. It is known that in water the equilibrium acidity constant Ka of phenol is higher than that of alcohol by a factor of lo8 (10-l0 versus 10-18 in HzO at 25 OC).18 On the other hand, for NBDO amphiphiles with the structure CsH19Ph(OC2H&OH, the separation of the hydroxylgroup from the benzene group by the linear (hydroxyethoxy)ethoxy group makes NBDO still a common alcohol. This lower acidity of NBDO than N(D)P causes NBDO to be a weaker asphaltene stabilizer than N(D)P. Though the ethoxy group (OCzH4) can easily perform hydrogen-bonding with water (hydrophilic), it appears not to be very attractive toward asphaltenes. The ineffectiveness of (hydroxyethoxy)ethoxyto interact with asphaltenes could be due to the fact that the basic ethoxy group and the acidic hydroxyl group of different NBDO molecules can associate favorably in the apolar alkane solvents (instead of water) and diminish the advantage of the NBDO's (hydroxyethoxy)ethoxy group to associate with asphaltenes. Results similar to Figure 4 were also obtained by Gonzalez et al., indicating the mechanism of asphaltene stabilization by amphiphiles through acid-base interactions should be quite general and independent of the asphaltenes used. However, they did not measure the actual attraction between asphaltenes and amphiphiles as demonstrated by Figure 6. This experiment also (18)Solomona,T.W.G.OrganicChemietry,5thed.;Wiley:NewYork, 1992.
Chang and Fogler
1754 Langmuir, Vol. 10, No. 6,1994 100
80
0 0
4
20
8 12 16 Wt % of alkyl phenol
Figure 7. Percentage of asphaltenes stabilized in the heptane solutions containing p-alkylphenol amphiphiles with different tail lengths (en,n = carbon number of the alkyl tail). 2*5*25
c 20 15
1.5
m
c
P
1 0.5 L
0.0-0 0
2
4 6 8 1 0 1 2 1 4 Carbon number of alkyl tail
Figure 8. Minimum concentration of p-alkylphenolamphiphiles with different tail lengths for completely dissolving asphaltenes in heptane.
4. ,.: I
0
+C1
-m-
-0-A-
2 3 4 Wt% of total asphaltenes
(amphiphile)
C2 (amphiphile) C6 (amphiphile) C12 (amphiphile)
-0-
-e-e--A-
5
C1 (asphaltene) C2 (asphaltene) C6 (asphaltene) C12 (asphaltene)
Figure 9. Percentage of p-alkylphenol amphiphiles with different tail lengths (and asphaltenes) remaining in the supernatant of the centrifuged amphiphile/decane solutions containing different amounts of asphaltenes (Cn,n = carbon number of the alkyl tail).
provides strong evidence that the acid-base interactions between asphaltenes and natural resins (i.e., adsorption) may account for the stabilization of asphaltenes in crude oil. Effect of the Amphiphile'sTail Length. The effects of tail length on the amphiphile's ability to solubilize asphaltenes and on the asphaltene-amphiphile interactions were studied using p-alkylphenols with tail lengths from one carbon to twelve carbons. The results are shown in Figures 7-9. Figure 7 showsthe percentage of solubilized asphaltenes in heptane solutions containing different p-alkylphenols. When the carbon number of the am-
phiphile's tail is less than 6, the amphiphile's ability to stabilize asphaltenes decreases significantly with decreasing tail length. The effectiveness of p-alkylphenols on asphaltene stabilization is in the order p-(n-hexy1)phenol > p-(n-buty1)phenol > > p-(sec-buty1)phenol > p-ethylphenol > p-cresol. The minimum amounts of palkylphenol amphiphiles for complete asphaltene stabilization are plotted in Figure 8, where one observes that the effectiveness of p-alkylphenols with alkyl chains longer than six carbons are quite similar. On the basis of the weight percentage of amphiphiles, the effectiveness of p-alkylphenols on asphaltene stabilization follows the order p-(n-hexy1)phenol > p-(n-nony1)phenol > p-(ndodecy1)phenol. We note that if the effectiveness is recalculated in terms of the molar concentrations of these p-alkylphenols, the molar amount of amphiphiles to stabilize asphaltenes still slightly increases with the tail length. This trend suggests that a tail with a carbon number greater than 6 can only contribute a very slight positive effect to the stabilization of asphaltenes. Overall, it seems that a minimum alkyl chain length is necessary for an amphiphile to provide sufficient ability to stabilize asphaltenes in the solution. The minimum chain length for p-alkylphenol amphiphiles is about six carbons. It was also found that the shape of the alkyl tail of p-alkylphenol amphiphiles affected the stabilization of asphaltenes. Figure 8 shows that the concentration of p-(sec-buty1)phenol (SBP) necessary to completely stabilize asphaltenes is greater than that of p-(n-buty1)phenol (BP), suggesting SBP is less effective than BP. The influence of the tail shape is much less for p-octylphenol even though p-(tert-octy1)phenol (TOP) still appears to be very slightly less effective in stabilizing asphaltenes than p-(n-octy1)phenol. Besides, we also found that SBP and TOP were not completely soluble in heptane but had solubility limits at 0.20 and 0.08 g/mL heptane, respectively. The smaller degree of miscibility of these two amphiphiles with heptane may account for their being slightly less effective in stabilizing asphaltenes. Experiments on the adsorption of p-alkylphenol amphiphiles to asphaltenes were carried out using FTIR spectroscopy. The percentage of p-alkylphenol remaining in the supernatant was quantified from the integrated absorbance of the 1516-cm-l peak. Figure 9 shows that the extent of the adsorption of p-alkylphenols to asphaltenes increases with decreasing tail length from C12 to C1. This result demonstrates that the decrease in the ability of p-alkylphenols to stabilize asphaltenes with decreasing tail length does not result from the decrease of the attractive interactions between asphaltenes andp-alkylphenols. Therefore, we can deduce that the major effect of the amphiphile's tail length is to provide a steric-stabilization effect to prevent asphaltenes from aggregating. The p-alkylphenols with short tails are not only much smaller than asphaltene molecules but also more polar throughout most of the molecular structure than the surrounding solvent molecules. Consequently, they cannot surround asphaltene particles but instead intercalate with, or are imbedded in, asphaltenes, thereby offering little hindrance to asphaltene flocculation. As the tail length of p-alkylphenols increases, the size of this apolar moiety becomes larger and tends less to be surrounded by asphaltene molecules. Therefore, the ability of the p-alkylphenols to stabilize asphaltenes successively increases fromp-cresol (Cl) to p-hexylphenol ((26). When the alkyl tail has six carbon atoms or more, this apolar moiety becomes sufficiently large to form a stable aliphatic layer to prevent asphaltene particles from
Effect of Amphiphiles on Asphaltene
Langmuir, Vol. 10,No. 6, 1994 1755
1
25
io
ep
-
0
0
1
2 3 4 5 Wt% of amphiphiles
6
0t 4
7
Figure 10. Percentage of asphaltenesstabilizedin the dodecane solutions containing amphiphiles with extra polar side groups (HP, p-hexylphenol, HOP, p-(hepty1oxy)phenol;DR, 4-dodecylresorcinol). aggregating. Supporting evidence for this tail effect is that p-cresol (Cl) can be solubilized (i.e., imbedded) in water by 2.3 g/lW g of HzO (at 25 OC),18 while palkylphenols with long alkyl tails are virtually insoluble in water. The fact that the adsorption of p-alkylphenols to asphaltenes successively decreases with increasing tail length from C1 to C12 can also be explained by the steric exclusion effect of amphiphile tails on the asphalteneamphiphile association and amphiphile-amphiphile association. The tail of an amphiphile molecule attached to an asphaltene particle can cover a portion of asphaltene surface and exclude other amphiphile molecules from attaching any other asphaltene polar groups in this area. As the length of the amphiphile tails increases, this steric exclusion effect becomes successively significant and, hence, the amount of amphiphiles adsorbed to the asphaltene surface is decreased. Overall, it seems that the effect of the amphiphiletail length on the solubilization of asphaltenes in apolar media has a certain extent of analogy to a water-in-oil microemulsion system if we correspond the amphiphile, asphaltene, and alkane solvent in our system to the surfactant, water, and oil in a waterin-oil microemulsion solution, respectively. It is well known that increasing the length of the surfactant's alkyl tail can lead surfactant molecules toward lipophilic (i.e., decreases the surfactant's HLB value) and,favor the formation of stable water-in-oil microemulsions. Effect of the Amphiphile's Side Groups. p-(Hepty1oxy)phenol (HOP, CTHlsOPhOH) and 4-dodecylresorcinol (DR, C12H2$h(OH)z) were chosen to study the effect of the polar side group and the polar tail group on asphaltene solubilization. HOP has an extra ether (-0-) group between the benzene group and the alkyl chain while DR has an extra hydroxyl (OH) group attached to the benzene ring in the meta (1,3) position. These extra polar groups strongly reduce the solubility of HOP and DR amphiphiles themselves in the alkane solvents. This reduced solubility can be due to the fact that a distribution of multiple polar groups around the amphiphile's benzene groups makes these benzene groups strong enough to hydrogen-bond together by either stacking along the benzene flat side or bridging along the benzene edge side. The temperatures required to solubilize HOP and DR in dodecane are about 50 and 100 OC, respectively. In order to overcome this limited solubility, this experiment was carried out at 100 OC. Figure 10 shows that 3 wt 5% DR and 7 wt % HP are necessary to completely solubilize asphaltenes, suggesting DR is more than twice as effective as p-hexylphenol (HP). Because DR has twice as many acidic hydroxyl groups as HP, it is expected that DR has stronger acid-base interactions with asphaltenes than HP and, hence, stabilizes asphaltenes more effectively. This result proves that, by adding certain types of extra polar
'
I
6
8
I
'
l
,
l
10 12 14 16 Carbon number of alkane
18
Figure 11. Minimum weight percentage ofp-nonylphenol(NP) and p-dodecylbenzenesulfonic acid (DBSA)for completely dissolving 1 w t % asphaltenes in different alkanes.
-
L " " "
0
-e-&-
-e-
" '
I
J
2 3 4 Wt% of total asphaltenes
Pentane (amphiphiid heptane (amphiphile) decane (amphiphile)
--f -A-
-+-
5
pentane (asphaltene) heptane (asphaltenel decane (asphaltene)
Figure 12. Percentage of p-dodecylphenol (DP) (and asphaltenes) remaining in the supernatant of the different centrifuged alkane solutions with different amounts of asphaltenes. groups (e.g., hydroxyl group) to amphiphiles, the ability of amphiphiles to stabilize asphaltenes can be further improved. On the other hand, the effectiveness of HOP on asphaltene stabilization is similar to that of HP, suggesting the extra ether group does not significantly affect the stabilization of asphaltenes. This result is quite similar to the study on asphaltene stabilization using p-[(hydroxyethoxy)ethoxylnonylbenzene(NBDO)which contains a (hydroxyethoxy)ethoxygroup with two ether groups. Effect of Alkane Solvent. The effects of alkane solvents on the amphiphile's ability to stabilize asphaltenes and on asphaltene-amphiphile interactions are illustrated in Figures 11 and 12. Figure 11 shows the minimum concentrations of two oil-soluble amphiphiles, p-nonylphenol (NP)andp-dodecylbenzenesulfonicacid (DBSA), needed to completely stabilize 1 w t % asphaltenes in different alkanes. The minimum concentration of NP for complete asphaltene solubilization decreases successively from pentane (24 wt % 1, to hexane (12 wt % 1, to heptane (7 wt %), and to dodecane and hexadecane (both 6.5 wt % ). This trend shows that alkane solvents with different molecular weights can influence the ability of NP amphiphiles to stabilize asphaltenes. In order to clarify this solvent effect, an experiment on the adsorption of pdodecylphenol (DP) amphiphiles (similar to NP) to asphaltenes was studied using three different alkanes, pentane, heptane, and decane. Figure 12 shows that alkane solvents may affect the attractive interactions between asphaltenes and DP amphiphiles. The adsorption of DP amphiphiles to asphaltenes is perhaps slightly decreased when the solvent is changed from decane to pentane, suggesting the slightly
1756 Langmuir, Vol. 10, No. 6,1994 successive reduction of asphaltene-amphiphile interactions. Besides,we also found that the solubilitiesofp-(secbuty1)phenol (SBP) and @-tert-octy1)phenol(TOP) (which have limited solubilities in alkanes) decreased with increasing size of alkane solvents (for SBP, from 0.28g/mL pentane to 0.05 g/mL dodecane; for TOP, from 0.1 g/mL pentane to 0.027 g/mL dodecane). These solubility differences are caused by the size difference of the apolar alkane molecules rather than any specific (e.g., acid-base) interactions between alkanes and amphiphiles. It is known that the smaller alkane solvent molecules can give an amphiphile/alkane system a greater number of molecular arrangements than that of larger solvent molecules. This better miscibility between amphiphilesand lighter alkanes can perhaps draw amphiphiles away from asphaltene surfaces more easily. Hence, the ability of amphiphiles to stabilize asphaltenes is decreased with decreasing molecular weight of alkane solvents. The fact that lighter alkanes (less than C7) are less effective in stabilizing asphaltenes is consistent with the observation of previous studies that the amount of asphaltenes precipitated from crude oil increases with decreasing molecular weight of the alkane solvent added to crude oiL4v6 However, it appears that the smaller attraction between p-alkylphenols in lighter alkanes (see Figure 12) cannot completely explain the significant reduction of the effectiveness ofp-alkylphenols to stabilize asphaltenes (see Figure 11). Perhaps a simplified explanation of the reduction of p-alkylphenol effectiveness in light alkanes can be drawn from the expression of the x parameter in the Flory-Huggins equation given in eq 1. This x parameter represents the difference of the Hildebrand solubility parameter (6) between asphaltenes and alkanes. The 6 value of alkanes decreases with the (dodemolecular weight of alkanes from 8.0 (cal/~m~)O.~ cane) to 7.1 (~al/cm~)O.~ (pentane). This decrease in 6 leads to the gradual increase of the x parameter value of lighter alkane solutions and, therefore, the lower solubility of asphaltenes in lighter alkanes. As opposed to NP, the effectiveness of DBSA amphiphiles to stabilize asphaltenes increases with increasing molecular weight of alkanes. Figure 11 shows that the minimum concentrations of DBSA to completely stabilize 1 wt 96 asphaltenes increase successively from pentane and heptane (both 3.5 wt % 1, to dodecane (5 wt % 1, and to hexadecane (11wt 96). This trend can be due to the extremely strong binding of DBSA to asphaltenes as well as among DBSA molecules themselves. We observed that DBSA molecules could donate their protons to the asphaltene’s basic groups and/or C=C bonds and, therefore, irreversibly attach to asphaltene surfaces (see part 2). The strong binding of DBSA to asphaltenes makes DBSA effective enough to sterically stabilize asphaltenes, even in light pentane. On the other hand, DBSA amphiphiles have a strong tendency to self-associatein apolar media (see part 2). If this association takes place between the DBSA molecules attaching different asphaltene particles, asphaltenes can flocculate and precipitate out of solution. As mentioned earlier, heavier alkanes provide fewer molecular arrangements for an amphiphile/alkane system than lighter alkanes. Therefore, it is possible that DBSA molecules associate stronger among themselves in the heavier alkane solution and cause asphaltenes to flocculate more easily through the adsorbed DBSA molecules. This study suggests amphiphiles containing both a strong polar head group and an alkyl tail with a proper carbon number could be the most favorable asphaltene
Chang and Fogler
stabilizers. The strong polar head group can strengthen the attachment of amphiphiles to asphaltenes, and the proper length of the alkyl tail can provide a stable amphiphile’s steric layer around asphaltenes. These observations may apply to other oil-soluble amphiphilic molecules. However, the minimum tail length for an amphiphile to support the asphaltene-amphiphile associated moiety can be influenced by the amphiphile’s head group. A more polar or larger head group may need a longer tail to make the amphiphile as well as asphaltenes stabilized in alkane solutions. This situation may not be applicable for amphiphiles with an extremelystrong polar head group because their tails will tend to crystallize if they are longer than C16420. Very strong amphiphiles (e.g., DBSA) may have an advantage over weak amphiphiles (e.g., NP), especially at low asphaltene concentrations and in lighter alkanes. Either higher asphalteneto-amphiphile ratios or heavier alkanes tend to precipitate amphiphiles containing a strong head group (e.g., DBSA) with asphaltenes. In addition, the head groups of amphiphilic molecules always associate with themselves more or less in apolar media. The strong attraction between head groups of amphiphiles can reduce their ability to interact with, and thus stabilize, asphaltene molecules. We found thep-alkylbenzoic acid which could form a stable dimer structure with its carboxylic acid (COOH) head group was not effective to stabilize asphaltenes in alkane media. The addition of polar groups to the amphiphile’s side or tail may also strengthen the association of amphiphiles themselves such as the 4-dodecylresorcinol amphiphile. Perhaps, only after the self-association of amphiphiles is overcome (e.g., by raising the temperature) is the positive effect of additional polar groups of amphiphiles on asphaltene stabilization observed.
Conclusions This study shows clearly that asphaltenes from crude oil can be stabilized by the oil-soluble amphiphiles. Two factors are found important to stabilize asphaltenes by amphiphiles, the adsorption of amphiphiles to asphaltene surfaces and the establishment of a stable steric alkyl layer around asphaltene molecules. The influence of the chemical structure of amphiphiles on the asphalteneamphiphile interaction can be summarized as follows: (1)Effect of amphiphile’s head group: Increasing the polarity (or more specifically, the acidity, in this study) of the amphiphile’s head group strengthens the attraction of amphiphiles to asphaltenes through acid-base interaction; therefore, the amphiphile’s effectiveness on asphaltene stabilization is increased. (2) Effect of amphiphile’s tail length Increasing the tail length of an amphiphile can improve its effectiveness to stabilize asphaltenes even though this increase may reduce their affinity to asphaltenes by a small amount. Amphiphileswith short chains cannot peptize asphaltene molecules by forming a steric-stabilization layer but will be imbedded in and coprecipitate with asphaltenes. (3) Effect of modification of the amphiphile structure: Addition of certain polar groups (e.g., hydroxyl group) to amphiphiles can increase their ability to stabilize asphaltenes. However, if these polar groups are at the amphiphile’s side or tail, they may reduce the stability of amphiphiles themselves in the solution. (4) Effect of solvent: Different alkane solvents can influence the ability of amphiphiles to stabilize asphaltenes. The effectiveness of weak amphiphiles (e.g., NP) to stabilize asphaltenes decreases in lighter alkane solutions due to the reduced asphaltene-amphiphile attractions and
Effect of Amphiphiles on Asphaltene
the stronger attractions among asphaltenes themselves. On the other hand, strong amphiphiles (e.g., DBSA) can remain effective in the extremely volatile pentane by strongly associating with asphaltenes. Acknowledgment. The authors thank the following sponsors of the industrial affiliate program of the Uni-
Langmuir, Vol. 10, No. 6, 1994 1757
versity 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 is greatly appreciated.
