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Energy & Fuels 2000, 14, 38-42
The Critical Micelle Concentration of Asphaltenes As Measured by Calorimetry Simon Ivar Andersen* and Steen D. Christensen Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark Received June 10, 1999. Revised Manuscript Received September 30, 1999
Micellization of asphaltenes in solution has been investigated using a micro calorimetric titration procedure (Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497). The method uses the analysis of heat of dissociation and dilution of asphaltene micelles when a pure solvent (or solvent mixture) is titrated with a solution of asphaltene in the same solvent. The asphaltene concentration of the injected solution is at a level above the critical micelle concentration (CMC). In the present paper the procedure is applied in investigation of asphaltenes as well as subfractions of asphaltenes obtained by solvent extraction with toluene/heptane mixtures. These subfractions get more complex in structure as the toluene content of the extraction solvent increases and at the same time the CMC of the asphaltenes in pure toluene decreases. There is an indication that the presence of “lower” asphaltene species leads to less association in solution due to the interaction between these and the heavier asphaltenes. Asphaltenes of different origin have also been analyzed as a function of toluene/heptane solvent composition. It confirms the early finding that CMC decreases upon heptane addition. However, the relative decrease depends on the nature of the asphaltenes. Results from n-heptane asphaltenes from eight different locations are presented.
Introduction Association of asphaltenes has been the subject of several studies in the past, basically focusing on revealing the true molecular size of the components in the fraction. In the past decade or so, more work has been directed toward an understanding of the association behavior and the apparent micellization of asphaltenes in solution as was pointed out at a very early stage by Pfeiffer and Saal.1 A number of studies have been directed toward determinations of the critical micelle concentration of asphaltenic material in solution as well as the effect of solvent on CMC.1,3,4 The determination of CMC is based on analogy with simple systems such as detergents in aqueous solutions, as well as an assumption that the system can be approximated by a binary surfactant-solvent system. In simple systems the concentration of the monomer grows proportional to the concentration as this increases, but as the CMC is reached the monomer concentration remainssto a good approximationsconstant, as all addition monomers enter into micellar type structures. The detection of CMC is based on a change in a measurable property such as interfacial tension. In the present case the procedure outlined in an earlier work of ours3 using * Corresponding author. Phone: +45 45 25 28 67. Fax: +45 45 88 22 53. E-mail:
[email protected]. (1) Pfeiffer, J. Ph.; Saal, R. N. J. J. Phys. Chem. 1940, 44, 130. (2) Rogacheva, O. V.; Rimaev, R. N.; Gubaidullin, V. Z.; Khakimov, D. K. Colloid J. U.S.S.R. 1980, 490. (3) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497. (4) Sheu, E. Y.; De Tar, M. M.; Strom, D. A.; DeCanio, S. J. Fuel 1992, 71, 299.
determination of heat of dilution of an asphaltene solution into a pure solvent is used. The heat evolved upon dilution of a micelle solution is dependent upon the total surfactant, or in this case asphaltene, concentration of the resulting solution:
Below CMC: ∆hdilution,total ) ∆hdilution,micelle(1) + ∆hdemicellization(2) + ∆hdilution,monomer(3) + Q (1) The various contributions come from initial dilution of the micelles (1), then a demicellization contribution when micelles dissociate (2), and finally a contribution from the dilution of resulting monomers (3). Q is a heat caused by external influences such as heat from the stirring, frictional effects during liquid injection, as well as minor differences in solvent composition caused by evaporation from the syringe tip. The latter can be minimized but has been found to give a minor contribution in solvent-into-solvent experiments.
Above CMC: ∆hdilution,total ) ∆hdilution,micelle + Q2
(2)
When the final solution has an asphaltene monomer concentration above CMC the monomer concentration remains constant as the micelles added to the pure solvent do not dissociate. Hence only the dilution of micelles is taking place. In the experiment small aliquots of a surfactant solution of concentration above CMC are added to
10.1021/ef990122g CCC: $19.00 © 2000 American Chemical Society Published on Web 12/17/1999
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Energy & Fuels, Vol. 14, No. 1, 2000 39
Figure 1. Heat trace from calorimetric titration of toluene with N1 asphaltenes in solution. Manual injection.
initially a pure solvent of the same type as the solution, and the heat of dilution is measured. At each step of addition the resulting titration gets closer to CMC. The calorimetric trace of a typical experiment is depicted in Figure 1. Beyond a certain number of injections the heat evolved drops as described above. Three procedures can be used in the analysis of the data to evaluate the existence of and the magnitude of the critical micelle concentration: (1) The cumulated heat evolved is plotted versus the total concentration after each injection. The curve is approximated by two linear sections, and the intersection of these defines CMC.3 (2) The heat/g or mol of surfactant per injection is plotted versus total concentration after injection, and CMC is taken as the point where as significant drop in heat/g is observed.5 (3) The heat change difference per injection dQ/dC is plotted as a function of total concentration; CMC is defined by the minimum in this curve. Due to the nature of the general heat signals for asphaltenes presented in this work, procedure 3 was not found adequate for the analysis. Hence this is not used herein. Procedures 1 and 2 are outlined in Figures 2a and 2b for a sodium dodecyl sulfate (SDS) solution at 35 °C measured in this work. CMC was determined as 2.54 g/L in agreement with the literature. In Figure 2a the correction made from subtraction of eq 2 from eq 1 is also shown in order to enhance the detection of a break point in the curve. As for surfactants in organic nonaqueous solution, the reversed micelles formed must have small aggregate numbers and a significant polydispersity and the reversed micelle is more likely to stem from a stepwise association than from a phase separation. Hence, we can expect the micelle behavior of these systems to be complex and the CMC region must therefore be diffuse. The latter also is the case for many pure surfactants in both aqueous and nonaqueous solvents.5 Experimental Section Pure solvent phase (1.50 mL) was placed in a LKB perfusion cell, which was brought to thermal stability in a LKB 2277 bio activity monitor water-thermostated to 35.0 °C within 0.01 (5) Paula, S.; Su¨s W.; Tuchtenhagen, J.; Blume, A. J. Phys. Chem. 1995, 99, 11742.
Figure 2. (a) Evaluation according to procedure 1 of CMC from calorimetric titration of sodium dodecyl sulfate (SDS) in water at 35 °C (CMC ) 2.54 g/L). (b) Evaluation of calorimetric titration data of SDS in water according to procedure 2. K. Small aliquots (5-20 µL) of concentrated solution of asphaltene (C . CMC, in the range of 30 g/L) in the same solvent was injected (by a Hamilton dispenser) into the cell under continuous stirring, and the heat evolved, due to the process described above, was recorded both on a PC and on a chart recorder. The injection system was operated either manually or automatically by the PC. Asphaltenes were precipitated using a modified IP143 as described elsewhere. In a number of cases the heptane precipitant was substituted by heptane/toluene mixtures in order to change the asphaltene composition toward the onset composition. In some cases solid asphaltenes were extracted by solvent mixtures resulting in a soluble and an insoluble fraction. The different procedures are described elsewhere.6 Molecular weights of asphaltenes were determined using vapor pressure osmometry of toluene solutions. The elemental analysis of C, H, N, S, and O was determined using standard techniques at DBlab A/S, Denmark. Solvent mixtures used were prepared volumetrically at room temperature. All reagents were >99% in purity and used as received and the toluene contained 0.013% water by Karl Fischer titration.
Results and Discussion For complex micelle-forming molecules such as bile salts in water a diffuse transition from monomers to micelles with very low aggregation numbers has been reported in the literature.5 Hence it is not surprising that a similar observation of the transition region is observed in the investigation of reversed asphaltene micelles. (6) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12, 1551.
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Figure 3. Analysis of asphaltene titration according to procedure 1. Data from Figure 1. Asphaltene N1 in toluene at 35 °C.
Andersen and Christensen
Figure 5. Analysis of Boscan n-heptane asphaltenes (three different samples) according to procedure 2. Table 1. Some Properties and CMC of n-Heptane Asphaltenesa H/C NSO % w/w MWa CMCc g/L toluene
N1
A95
Boscan
L1
L
Kuwait
1.016 6.19 2100 4.9
1.125 5.88 n.d. 4.6
1.225 11.09 2000 4.0
1.122 7.97 3100 3.6
1.101 10.12 n.d. 3.4
1.128 9.82 4300 3.2d
a MW in Toluene Using VPO. b CMC by procedure 1 (see text for details). c At 35.05 °C. d Ref 3 25.02 °C.
Figure 4. Evaluation of asphaltene titration according to procedure 2. Same data as in Figure 3.
The data given in Figure 1 is analyzed according to procedure 1 in Figure 3 and procedure 2 in Figure 4. The use of procedure 2 indicates that the micellization of asphaltenes is stepwise more than a process happening above a certain threshold value. Comparing the analysis of the simple SDS in water and the asphaltenes shows that the constant and composition-independent heat generated per injection below CMC is not observed for asphaltenes. Here the change is a steady decrease in heat/injection and at a certain level the slope is changed. In a few cases a steady level was observed at very low concentrations followed by a sudden decrease and then a change to a more moderate slope similar to the surfactant systems. Hence the conclusion is that indeed a transition takes place from a less associated state to a more associated state in the concentration range examined, but apparently the mechanism is not equivalent to the classical concept of surfactant monomer-micelle transition. The stepwise mechanism followed by a steady level above the apparent CMC is in agreement with HPLC size exclusion chromatographic data of Boscan asphaltenes which indicate that there is a gradual change up to a certain concentration, but not above a concentration which is expected from a classical surfactant behavior.7 This was seen as an increase in intensity of the peak related to associated species, relative to the low molecular peaks up to approximately 10 g/L toluene and then a constancy of (7) Andersen, S. I. J. Liq. Chromatogr. 1994, 17, 4065.
the ratio between these peaks for concentrations up to about 20 g/L. From the plot according to procedure 2 one may not be able to locate CMC as indicated on Figure 2a, but only the concentration where the slope changes. Hence procedure 1 is more likely to give an adequate determination of a CMC related to surfactant theory. In Figure 5 experiments of three different samples of Boscan heptane asphaltenes are given according to procedure 2. As seen a CMC determination cannot be performed following the procedure by Paula et al.5 Two linear segments are fitted to the two parts of the curve of the average heat signals of the three experiments although an exponential fit was also possible. Note the good agreement of the three separate experiments. The steps observed in two of the experiments (open circles and solid circles) could be due to some specific steps in the association mechanism. However, taking the large composition variation into account this may as well be due to changes in concentration caused by accumulation of material on the syringe tip. The stepwise association has recently also been shown for various asphaltenes in toluene using analysis of thermooptical diffusivity as a function of concentration.8 Three factors are expected to affect the apparent CMC, namely, primarily the composition of the asphaltenes, the solvent affinity (or solvent power), and the temperature. Of these the two later can be controlled easily whereas the composition may be varied either by source or by changing the precipitation procedure. The change in source did not reveal a great difference in CMC (3.2 to 4.9 g/L toluene) although the H/C ratio did span from around 1 to 1.2, and the NSO content did vary significantly (Table 1). One could assume that the latter had an influence on the polarity and hence the ability to form aggregates. Several correlations of the (8) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Ferna´ndez, A.; Pe´rez, P.; Caetano, M. Fuel 1999, 78, 977.
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Energy & Fuels, Vol. 14, No. 1, 2000 41
Table 2. CMC in Toluene at 35.05 °C of Fractions of Asphaltenesa,b Boscan precipitation solvent % vol
CMC g/L
Kuwait extraction solvent % vol
CMC g/L
n-heptane asphaltenes 10% tol in n-C7 20% tol in n-C7 30% tol in n-C7
4.0 1.7 0.7 2.6
n-heptane asphaltenes 10% ins/20% sol 30% ins/40% sol 40% ins
3.2 ca. 4c 4.1 3.2
c
a ins: insoluble; sol: soluble. b % is vol/vol toluene in n-heptane. Very diffuse transition.
Figure 6. CMC of different asphaltenes. Asph L (open circle); Asph L1 (solid square); Kuwait (solid circle). Kuwait data from Andersen and Birdi.3
type CMC ) f(H/C, NSO, MW) were investigated without finding an overall description of the phenomenon. The asphaltenes consist of a vast number of species, which differ significantly in composition within the fraction. Hence it should also be expected that the observed CMC is either an average property or more likely highly affected by the interaction between the constituents of the specific asphaltene sample. Like the composition variation in the standard n-heptane asphaltenes, one may also expect a similar variation in terms of affinity toward association. This was investigated in the present work on Boscan asphaltenes and on Kuwait asphaltenes. The asphaltenes were divided by precipitation of asphaltenes in mixtures of n-heptane and toluene, and by stepwise extraction of n-heptane asphaltenes with solvents of increasing toluene content. Hence the extraction process yields in example a fraction which is first insoluble in 10% toluene/heptane, but soluble in 20%toluene/heptane (denoted 10%ins/20%sol in Table 2). The two processes generate two types of material: the precipitation leads to material which always contains the heaviest asphaltene, whereas the extraction process yields discrete subfractions which basically contain a narrow distribution of individual components not dominating the other fractions. Several of the heat versus concentration curves showed very diffuse CMC transitions, or a smooth transition from one region to the other. Hence an accurate determination of CMC was not possible and there may be a large error in the numbers given in Table 2. For the precipitation solvents there is a decrease in CMC as the lighter components are removed in the first three fractions and then an increase. The first trend was expected if resin type components or lighter asphaltenes are expected to stabilize molecules in a pseudo-monomeric state. The increase observed for the 30%toluene/n-C7 sample
Table 3. Accumulated Heat at Micellization of n-Heptane Asphaltenes in Toluene at 303 K asphaltenes
accumulated heat mJ
CMC g/L toluene
Boscan (30% toluene in C7) L1 N1 CA VBH7 VBH6
55 40 15 5 3 5
2.6 3.6 4.9 3.2 4.6 4.6
Figure 7. CMC of n-heptane asphaltenes in toluene at 303 K. Oils of different origin.
remains speculative. The molecular weight of the Boscan asphaltene has been shown to increase with increasing toluene content in the precipitant.6 Comparing with the discrete fractions from the extraction experiments these seem to attain almost a constant magnitude throughout the fractions. In our previous work CMC of a single type of asphaltenes was found to decrease with increasing content of n-alkane in the toluene solvent3. It was also observed that with n-C7, n-C10, and n-C16 the CMCs obtained could be correlated with the solubility parameter of the resulting solvent mixture showing a clear relation between solvent power and CMC. Hence as the solvent gets poor the asphaltenes will tend to associate at a lower concentration using the CMC concept in agreement with the well-known effect of solvent dielectric constant in VPO.9 In the present work several different asphaltenes have been examined regarding this, and it has been found that different asphaltenes respond differently to the change in solvent. An example is given in Figure 6. There is no clear explanation of a relation between high initial CMC in toluene and a large effect of the heptane addition. From corrected curves according to procedure 1 the accumulated heat corresponding to CMC was recorded. This heat is not the heat of micellization, which preferably should be obtained by plots according to procedure 2, where the heat of micellization is obtained as the difference between the two constant levels before and after CMC. In this case the y-axis should be given in J/mass of asphaltene. As seen in Table 3 the heats differ significantly from 55 to 3 mJ whereas the CMC only varies between 2.6 and 4.9 g/L toluene. Boscan 30% is (9) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker Inc.: New York, 1999.
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the only exception from investigations of n-heptane asphaltenes. The VBH samples are from the same reservoir but from oils of significantly different gravity but asphaltenes have almost identical composition. Conclusions Calorimetric measurements of heat of dilution of asphaltene solutions have been performed to investigate the presence of an apparent critical micelle concentration (CMC) of asphaltenes. The change in heat signal as a function of concentration has been investigated using two different types of analysis of heat signals. These indicate that in a diffuse concentration region a change does occur in the signal, which in analogy with similar data of surfactant solutions may be interpreted as the presence of a CMC. However, the heat traces may as well be seen as a result of a stepwise mechanism
Andersen and Christensen
where a certain average structure is obtained at a certain concentration level. A relation between CMC and asphaltene composition has not been detected nor has the change in CMC as a function of solvent composition indicated any obvious relation with the asphaltene properties. However, whereas a large difference in heat of micellization was observed the CMCs fell in a very narrow range between 3.2 and 4.9 g/L toluene for n-heptane asphaltenes. These are compiled for comparison in Figure 7. Acknowledgment. We thank Mr. Z. Tecle for performance of part of the calorimetric work as well as Sarmad Shakir for the determination of the heat of micellization. EF990122G
