Langmuir 1999, 15, 7653-7657
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Study of the Adsorption of Alkyl Benzene-Derived Amphiphiles on Asphaltene Particles Olga Leo´n,*,† Estrella Rogel,† Argelia Urbina,† Angel Andu´jar,‡ and Andre´s Lucas‡ Departamento de Produccio´ n, PDVSA-INTEVEP, Apdo. 76343 Caracas 1070A Venezuela, and Escuela de Ingenierı´a Quı´mica, Universidad Metropolitana, Apdo. 76819 Caracas 1070A Venezuela Received September 14, 1998. In Final Form: June 3, 1999 Adsorption isotherms of alkylbenzene-derived amphiphiles were determined experimentally and correlated with their activity as stabilizers of asphaltenes. The isotherms of these amphiphiles indicate an adsorption process in two steps: in the first step, the amphiphiles are adsorbed through the interactions with the surface of the asphaltene particles; in the second step, the interactions between adsorbed amphiphiles become predominant and the formation of aggregates on the surface begins. A direct correlation was also found between the maximum surface excess concentration of the amphiphile and its activity as stabilizer. This indicates that the aggregation of amphiphiles on the asphaltene surface plays a very important role in the inhibition of the precipitation of asphaltenes.
Introduction One of the most common problems related to crude oil production is asphaltene deposition. Reservoir damage, reduction of well productivity, and clogging of the tubing and production facilities are some of its consequences. The asphaltene fraction is the most aromatic and highest molecular weight portion of crude oil and is defined as the fraction that is soluble in toluene or benzene and insoluble in low boiling alkanes such as n-pentane or n-heptane. The composition of the asphaltene fraction is not completely known at present, but asphaltene molecules are supposed to be an associated system of polyaromatic sheets with different substitutions of functional groups and alkyl chains on the edges.1 Usually, crude oils are considered to be colloid-disperse systems. The asphaltene deposition is the consequence of the instability of this colloidal system. The asphaltenes are the disperse phase. This disperse phase is maintained in solution by resins, another polar fraction of the crude oil. The resins act as peptizing agents and their separation from the crude oil originates the precipitation of asphaltenes.2 In this sense, the resins seem to provide a transition between the most polar (asphaltene) and the relatively nonpolar (oils) fractions in petroleum, making it possible to maintain the asphaltenes in solution. Evidence of the strong interaction between asphaltenes and resins has been found.3,4 Some substances are used to prevent the deposition of asphaltenes. Among these substances, some amphiphiles have been tested as asphaltene stabilizers.5-7 They act in a similar way to the resins, peptizing the asphaltenes and * To whom correspondence should be addressed. † PDVSA-INTEVEP. ‡ Universidad Metropolitana. (1) Andersen, S. I.; Birdi, K. S. Fuel Sci. Technol. Int. 1990, 8, 593. (2) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149. (3) Suryanarayana, I.; Rao, K. V.; Duttachaudhury, S. R.; Subrahmanyan, B.; Saikie, B. K. Fuel 1990, 69, 1546. (4) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497. (5) Gonzalez, G.; Middea, A. Colloids Surf. 1991, 52, 207. (6) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1749. (7) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1758.
keeping them in solution. The stability of the crude oils is considered to be a function of the concentration of the peptizing agent in the solution, the fraction of heavy organic particle surface sites occupied by the peptizing agent, and the equilibrium conditions between the peptizing agent in solution and on the surface of heavy organic particles.8-11 Gonzalez et al.5 studied the activity of various oil-soluble amphiphiles in n-heptane as asphaltene precipitation inhibitors. They found that the activity of the amphiphile is related to the headgroup.5 More recently, Chang and Fogler6,7 used a series of alkylbenzene-derived amphiphiles to investigate the asphaltene-amphiphile interactions and connected their results with the relative effectiveness of the amphiphiles to stabilize asphaltenes in alkane solvents. Their results showed that the effectiveness of the amphiphiles to stabilize asphaltenes is primarily controlled by two factors: the strength of the asphalteneamphiphile interactions, which is related to the polarity of the amphiphile’s headgroup, and the capacity of the amphiphiles to form a steric-stabilization layer, which is related to the length of the amphiphile’s alkyl tail. In the present work, the interest is focused on the first factor. The adsorption of a set of alkylbenzene-derived amphiphiles on asphaltene particles was studied. In this work, the adsorption of amphiphiles on asphaltene particles from n-heptane was quantified using the UV spectrophotometric technique and correlated with measurements of flocculation points. As a result, it was possible to obtain information about the adsorption mechanism, the amphiphile orientation on the surface, and the relationship between this adsorption and the activity of the amphiphiles as asphaltene stabilizers. (8) Ray, R. B.; Witherspoon, P. A.; Grim, R. E. J. Phys. Chem. 1957, 61, 1296. (9) Leontaritis, K. J.; Mansoori, G. A. SPE Int. Symp. On Oil Field Chem.; San Antonio, Texas; Society of Petroleum Engineers, Inc.: Richardson, TX, 1987; Paper No. SPE 16258. (10) Park, S. J.; Mansoori, G. A. Energy Sources 1988, 10, 109. (11) Mansoori, G. A. Third Latin American/Caribbean Petroleum Engineering Conference; Buenos Aires, Argentina, Society of Petroleum Engineers, Inc.: Richardson, TX, 1994; Paper No. SPE 27070. (12) March, J.; Advanced Organic Chemistry: Reactions, Mechanisms and Structure, 4th Ed.; John Wiley & Sons: New York, 1992; p 528.
10.1021/la9812370 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/08/1999
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Table 1. Characteristics of the Crude Oil OL1 and Its Asphaltenes crude oil OL1 saturates (%)
aromatics (%)
resins (%)
asphaltenes (%)
density °API
29.9
44.3
17.9
7.9
22.4
asphaltenes OL1 %C
%H
%S
%N
molecular weight
84.40
6.75
3.50
1.31
2100
Table 2. Alkylbenzene-Derived Amphiphiles Used in the Experimental Study amphiphile
abbreviation
p-(sec-butyl)phenol p-(tert-butyl)phenol p-(tert-octyl)phenol p-(n-nonyl)phenol p-(n-dodecyl)phenol p-(n-nonyl)phenol ethoxylated (6) p-(n-dodecyl)benzene sulfonic acid
source
purity (wt %)
SBP TBP TOP NP DP NPE6
Aldrich Aldrich Aldrich Kodak Industrial Texaco
98 99 95 98 98 90
DBSA
prepared according to ref 12
99
Figure 1. Adsorption isotherms for amphiphiles with linear tails.
Experimental Methods Materials. The asphaltene used as adsorbent throughout this study was extracted from a Venezuelan crude oil (OL1) with a 40:1 volume ratio of n-heptane to crude oil according to the preparation method described in IP-143/90. Table 1 shows the main characteristics of the crude oil (OL1) and asphaltenes. The amphiphiles used as solutes are shown in Table 2. Toluene and n-heptane were purchased from OmniSolv (purity > 99%). All of the compounds were used without further purification. Adsorption Isotherms. Solutions of a known concentration of each amphiphile were prepared in n-heptane. A 10 cm3 aliquot was added to 50 mg of asphaltenes in a flask. The flask was covered and the suspension was stirred in a thermostated bath at 25 °C during 16 h. After equilibrium was reached, the supernatant liquid was separated by centrifugation and its absorbance was measured on a Perkin-Elmer UV-vis spectrophotometer using 1 cm path length cells at the wavelength of 283 nm. The absorbance allowed calculation of the equilibrium amphiphile concentration and, as a consequence, the total amount of amphiphile adsorbed on the asphaltene surface. Onset of Flocculation. Because of the complex nature of crude oils, the evaluation of the factors that influence the stability of these systems is very difficult. An indirect way of evaluating the factors involved in such stability is to determine the incipient precipitation or flocculation onset upon addition of a nonsolvent. A number of methods are used to estimate the point at which asphaltene flocculation or precipitation starts. In this work, the activity of the amphiphiles as asphaltene stabilizers was evaluated by means of flocculation measurements. The flocculation points were determined by a titration method: n-heptane is added at a constant rate (1 cm3/min) to a solution of crude oil (OL1) and amphiphile in toluene under intensive stirring. The titration is monitored by means of a Guided Wave NIR spectrophotometer at the wavelength of 768 nm. The flocculation point is defined as the amount of n-heptane needed to obtain the maximum of the light intensity, which corresponds to the beginning of aggregation and coagulation of colloids. The activity of the amphiphiles was determined by titration of 5 mL of 20 vol % solution of crude oil OL1 in toluene. The concentration of amphiphile ranged from 0 to 16 wt % with respect to asphaltene. Temperature, titration rate, stirring speed, time of sample preparation, and crude oil concentration were the same in all titrations. Solutions without amphiphile were used as references.
Results and Discussion Adsorption Isotherms. Figures 1 and 2 show the adsorption isotherms determined at 25 °C for the am-
Figure 2. Adsorption isotherms for amphiphiles with branched tails.
phiphiles studied. All of the isotherms obtained could be classified as two-plateaus-type (LS) or S-type (S). These kinds of isotherms can be related to a two-step adsorption mechanism where the amphiphile molecules are first adsorbed through interactions with the solid surface and then, in the second step, through the interaction with the adsorbed amphiphiles. During the last step, the adsorption increases dramatically and it is supposed that hemimicelles are formed on the solid/liquid interface. Jaoui et. al.13 also found an adsorption mechanism in two stages onto the asphaltene surface. They studied the adsorption of phenol and 4-chlorophenol from water onto asphaltenes deposited on silica and attributed the adsorption in two steps to two different organizations of solute molecules at the surface. They also found that the adsorption of the same solutes on asphaltenes flocculated in bulk from water corresponds to a Freundlich isotherm mechanism. To fit the adsorption data, the adsorption equation derived by Zhu and Gu14 was used:
Γ ) Γ∞ k1C (n-1 + k2Cn-1)/[1 + k1C(1 + k2Cn-1)] (1) This equation is based on the law of mass action of a two-step adsorption mechanism similar to the one de(13) Jaoui, M.; Achard, C.; Hasnaoui, N.; Rogalski, M. Rev. Inst. Fr. Pet. 1998, 52, 35. (14) Zhu, B. Y.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3813.
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Table 3. Fitted Zhu and Gu Parameters for Amphiphiles and Other Characteristic Parameters of the Adsorption Isotherms amphiphile SBP TBP TOP NP DP NPE6 DBSA
Γ∞
k1
0.0026 520 0.0035 850 0.0070 160 0.0032 190 0.0017 188 0.00083 7800 0.0058 35000
k2
n
1.7 × 1018 1.7 × 1028 1.0 × 1028 1.5 × 1038 2.6 × 1051 7.0 × 1020 2.7 × 1043
7.5 11.0 12.0 14.0 20.0 9.5 14.7
hmc -∆G°hm (mol/L) (kJ/mol) 0.0014 0.0013 0.0020 0.0012 0.0021 0.0031 0.0007
13.9 14.6 13.3 15.6 14.7 12.5 16.9
scribed above. In this equation, n is the mean aggregation number of the adsorbed hemimicelles, Γ∞ is the maximum coverage of the surface, k1 and k2 are the equilibrium constants of the first and second adsorption step, and C is the equilibrium concentration of monomer in solution. The equation by Zhu and Gu has been successfully used to describe the adsorption of n-decanol on graphite from n-heptane,15,16 which includes reverse surface micelle formation. In this work (see Figures 1 and 2), the equation fit adequately the experimental data. The fitted parameters are shown in Table 3. Chang and Fogler6 found that the extent of the amphiphile’s adsorption to a Mobil asphaltene in n-decane follows the order: DBSA > NP > DPE2 > nonylbenzene, where DBSA is p-(n-dodecyl)benzene sulfonic acid, NP is p-(n-nonyl)phenol, and DPE2 is p-(n-dodecyl)phenol ethoxylated (2). These results are in agreement with the values of maximum coverage of the surface found in the present work for asphaltenes OL1 (Figure 1). In fact, using the adsorption isotherms, we found that the amount of amphiphile adsorbed follows a similar order: DBSA > NP > NPE6, where NPE6 is p-(n-nonyl)phenol ethoxylated (6). According to previous studies,5-11 the peptizing agents attach to asphaltene micelles and form a steric-stabilization layer around asphaltenes that protects them from flocculation. In the present work, using the results obtained from the adsorption isotherms, it was possible to determine the number of amphiphile molecules adsorbed per asphaltene molecule. The molecular weight of the asphaltene was determined by vapor pressure osmometry using dichloromethane as solvent (2100 g/mol). The results obtained here indicate that, at the first plateau, the ratio of amphiphile molecules per asphaltene molecule is very low, less than one molecule of amphiphile per molecule of asphaltene. At the second plateau, the values calculated range from 1.7 to 14.7 molecules of amphiphile per asphaltene molecule. Chang and Fogler7 reported for asphaltene samples from a Mobil crude oil, that the acid-base interaction DBSA/ asphaltene has a stoichiometry of about 1.8 mmol of DBSA/g of asphaltene. For DP, they found a stoichiometry of 1.6-2.0 mmol of DP/g of asphaltene. Using the mean molecular weight of the asphaltene reported by them,7 we found that approximately 1.8-2.2 DP molecules interact per asphaltene molecule. For the DBSA amphiphile, 2.0 DBSA molecules interact per asphaltene molecule. These values are lower that those obtained in the present work (3.6 molecules of DP and 12.2 molecules of DBSA per asphaltene molecule). On the other hand, Gonzalez and Middea5 reported that 250 molecules of NP are involved in the peptization of each asphaltene molecule, compared to the 6.7 molecules of NP per asphaltene molecule obtained in the present work. The differences found in (15) Zhu, B. Y.; Gu, T. Colloids Surf. 1990, 46, 339. (16) Zhu, B. Y.; Gu, T. Adv. Colloid Interface Sci. 1991, 37, 1.
Figure 3. Critical hemimicellar concentration as a function of the solubility parameter of amphiphiles.
the values reported can be attributed to the different active areas of the samples studied. Critical Hemimicellar Concentration. The critical hemimicellar concentration (hmc) is defined as the concentration above which the formation of hemimicelles begins. The hmcs of the amphiphiles were calculated from the adsorption isotherms as the equilibrium concentration with the maximum ∂Γ/∂C. The standard free energy of hemimicellization (∆G°hm) is obtained from the equation:
(∆G°hm) ) -(1/n) RT ln k2
(2)
The hmc and ∆G°hm are also reported in Table 3. The calculated standard free energies of hemimicellization are similar in magnitude to those reported for the adsorption of n-decanol on graphite from n-heptane.15 In this work, the hmc was related to the solubility parameter of the amphiphile. As can be seen from Figure 3, these values show a clear dependence on the solubility parameter of the amphiphile. In Figure 3, the DBSA is excluded because of the absence of information about its solubility parameter. However, it can be supposed that, because of its strong polar headgroup, its solubility parameter must be higher than those of the other amphiphiles reported here. This assumption is compatible with the lowest hmc for the DBSA reported in Table 3. According to these results, the hmc seems to be related to the solubility of the amphiphile in the solvent. Amphiphile Orientation on the Surface. It was found experimentally that for the phenol-derived amphiphiles studied, the coverage at the first plateau is proportional to the inverse of the area of the long axis of the amphiphile (Figure 4). This indicates that these amphiphiles lie parallel to the asphaltene surface. In Figure 4, NPE6 was excluded because of the shape of the isotherm, which makes it impossible to determine accurately the amount adsorbed at the first plateau. Other authors have found the same behavior for similar systems. They reported17-19 the formation of closely packed monolayers with molecules lying with their long axes parallel to the surface. These reports included alcohols and carboxylic acids adsorbed from organic solutions onto graphite, as well as large alkanes adsorbed from organic (17) Rabe, J. P.; Bucholz, S. Science 1991, 253, 424. (18) Groszek, A. J. Proc. R. Soc. London 1970, A314, 473. (19) Castro, M. A.; Clarke, S. M.; Inaba, A.; Dong, C. C.; Thomas, R. K.; J. Phys. Chem. B 1998, 102, 777.
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Figure 4. Amount of adsorbed amphiphile at the first plateau as a function of the inverse of the area and polarizability of the amphiphiles.
solutions onto graphitized carbon.20 Lamellar structures have been identified17 for fatty acids with extended carbon chains oriented parallel to a lattice axis in the graphite basal plane. The parallel orientation of the phenol-derived amphiphiles at the first plateau indicates that the interactions between asphaltenes and these molecules are mainly of a dispersive type. In fact, there is a linear relationship between the amount of amphiphile adsorbed at the first plateau and the polarizability of the amphiphile as illustrated in Figure 4. For DBSA, the experimental coverage determined is proportional to the inverse of the area of the headgroup, which means that this amphiphile is in perpendicular position to the surface (Figure 4). Because of its polar head, DBSA has a larger dipole moment (4.5 D) compared to the other amphiphiles (1.3-1.6 D) and, as a consequence, it is reasonable to suppose that it prefers to interact through the polar head with the alkyl chain extended toward the solvent. It is important to note that the polarizabilities and dipole moments reported for the amphiphiles were calculated using the semiempirical method MOPAC. When the results from the coverage of the first plateau were used, an active surface area of 42 m2/g for the asphaltenes was estimated. This area is similar to the areas reported for graphitized carbon blacks.21 Flocculation Onset and Its Relation with the Maximum Surface Excess Concentration (Γ∞). The effectiveness of the amphiphiles studied as asphaltene stabilizers was determined by flocculation measurements. For this study, only those amphiphiles with the longest tails were used. This selection was made based on the finding reported by Chang and Fogler6 that only amphiphiles with a carbon number of the tail larger than six can increase significantly the stabilization of the asphaltenes. Figure 5 shows the activity of the amphiphiles (NP, p-(tert-octyl)phenol (TOP), DP, NPE6, and DBSA) measured as the volume of n-heptane needed to begin flocculation at different amphiphile concentrations. The straight line represents the flocculation onset of the crude oil without amphiphile. At the lowest concentrations (1 and 4%) the activity is low and very similar for all the amphiphiles. However, at highest concentrations (8 and 16%) a clear tendency is observed. At these concentrations, (20) Findenegg, G. H.; Liphard, M. Carbon 1987, 25, 119. (21) Kern, H. E.; Findenegg, G. H. J. Colloid Interface Sci. 1980, 75, 346.
Leo´ n et al.
Figure 5. Activity of amphiphiles as a function of their concentration.
Figure 6. Activity of the amphiphiles as a function of their maximum surface excess concentration.
the activity of the amphiphiles seems to be related with the Γ∞ shown by the amphiphiles at the adsorption isotherms (Table 3). It is important to indicate that at the flocculation point, the amount of n-heptane in the system ranges from 72 to 78% in volume. For this reason, the adsorption behavior of the amphiphile at the flocculation point must be very similar to the behavior observed in the adsorption isotherms. However, some caution should be exercised in the comparison of both systems. In particular, resins and maltenes from the crude oil could compete with amphiphiles for the adsorption sites on the asphaltene particles. Nevertheless, the high activity as stabilizers shown by the amphiphiles indicates their preferential adsorption on the asphaltene particles. In fact, Figure 6 shows the activity of the amphiphiles as a function of Γ∞ at the highest concentrations (8 and 16%). With the increase of Γ∞, more n-heptane is needed to flocculate asphaltenes. According to this, the effectiveness of the amphiphile depends on its capacity to be adsorbed to the asphaltene surface. These results can be interpreted in terms of the thermodynamical colloidal model developed by Leontaritis and Mansoori.9 According to this model, a larger concentration of the amphiphile at the interface could produce a more effective stabilization layer to prevent the attractive interaction between asphaltene particles. The flocculation point can be considered as the
Adsorption of Alkyl Benzene-Derived Amphiphiles
point at which the amount of amphiphile adsorbed is not high enough to cover the surface of the asphaltene particles. In principle, it can be supposed that the activity of amphiphiles is related to the formation of hemimicelles, or at least, that it is related to the sharp increase of the adsorption corresponding to the second step of the adsorption mechanism. Similar tendencies are reported for adsorption of Aerosol-OT on graphite from cyclohexane by Somasundaran and Krishnakumar.22 They found that the sharp increase in adsorption at the hmc produces a decrease in settling rate. On the basis of the results obtained at higher concentrations, the effectiveness of the amphiphiles studied as asphaltene stabilizers follows the order: TOP > DBSA > NP > DP > NPE6. These results are quite comparable with those reported by Chang and Fogler,6 although the adsorbent and the method used to measure the effectiveness are different. Conclusions The adsorption of amphiphiles from n-heptane on asphaltene particles seems to be a process that occurs in (22) Somasundaran, P.; Krishnakumar, S. Colloids Surf., A 1997, 123-124, 491.
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two steps: in the first step, the amphiphiles are adsorbed individually on the asphaltene surface; in the second step, the interactions between adsorbed amphiphiles become predominant and the formation of hemimicelles on the surface begins. At the first plateau, all of the amphiphiles studied lie parallel to the asphaltene surface, except DBSA, which stands perpendicular to the surface. The active surface area for the asphaltenes precipitated in n-heptane is of the same order of magnitude as the values reported for graphitized carbon blacks. The activity of the amphiphiles as asphaltene stabilizers is related to the maximun amount of amphiphile adsorbed on the asphaltene surface. According to this, the larger the concentration of amphiphile at the asphaltene surface, the larger the volume of n-heptane needed to begin the flocculation of asphaltenes. Acknowledgment. The authors thank the support provided by IMP, Cenpes, ICP, and PDVSA-INTEVEP under the CODICID Research Project “Study of the asphaltene precipitation and its effects on crude oil production”. We also thank Alejandro Acevedo for DBSA isotherm data. LA9812370
