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Langmuir 2005, 21, 2728-2736
High-Q Ultrasonic Determination of the Critical Nanoaggregate Concentration of Asphaltenes and the Critical Micelle Concentration of Standard Surfactants Gae¨lle Andreatta, Neil Bostrom, and Oliver C. Mullins* Schlumberger-Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877 Received June 2, 2004. In Final Form: October 15, 2004 Asphaltenes are known to be interfacially active in many circumstances such as at toluene-water interfaces. Furthermore, the term micelle has been used to describe the primary aggregation of asphaltenes in good solvents such as toluene. Nevertheless, there has been significant uncertainty regarding the critical micelle concentration (CMC) of asphaltenes and even whether the micelle concept is appropriate for asphaltenes. To avoid semantic debates we introduce the terminology critical nanoaggregate concentration (CNAC) for asphaltenes. In this report, we investigate asphaltenes and standard surfactants using high-Q, ultrasonic spectroscopy in both aqueous and organic solvents. As expected, standard surfactants are shown to exhibit a sharp break in sonic velocity versus concentration at known CMCs. To prove our methods, we measured known surfactants with CMCs in the range from 0.010 g/L to 2.3 g/L in agreement with the literature. Using density determinations, we obtain micelle compressibilities consistent with previous literature reports. Asphaltenes are also shown to exhibit behavior similar to that of ultrasonic velocity versus concentration as standard surfactants; asphaltene CNACs in toluene occur at roughly 0.1 g/L, although the exact concentration depends on the specific (crude oil) asphaltene. Furthermore, using asphaltene solution densities, we show that asphaltene nanoaggregate compressibilities are similar to micellar compressibilities obtained with standard nonionic surfactants in toluene. These results strongly support the contention that asphaltenes in toluene can be treated roughly within the micelle framework, although asphaltenes may exhibit small levels of aggregation (dimers, etc.) below their CNAC. Furthermore, our extensive results on known surfactants agree with the literature while the asphaltene CNACs reported here are one to two orders of magnitude lower than most previously published results. (Previous work utilized the terminology “micelle” and “CMC” for asphaltenes.) We believe that the previously reported high concentrations for asphaltene CMCs do not correspond to primary aggregation; perhaps they refer to higher levels of aggregation or perhaps to a particular surface structure.
I. Introduction Asphaltenes are the most aromatic and most enigmatic component of crude oil.1-4 They have enormous impact in all phases of petroleum resource utilization and, thus, are of extreme interest. Their definition is operational (e.g., insoluble in n-heptane, soluble in toluene) in part because of the difficulty of assigning to them a proper chemical classification. This definition ensures that a single asphaltene sample is polydisperse in any property of interest (e.g., molecular weight, polarity, number of fused aromatic ring). Nevertheless, this definition does provide sufficient chemical restriction to be useful in a chemical sense in addition to an operational sense. Predictive petroleum science rests in part on understanding constituent molecular structures. As Crick advises,5 “to understand function, study structure.” A central focus in our lab has been to establish asphaltene molecular structures to then understand asphaltene function. Many of the major molecular structural issues of asphaltenes are now understood. In our first-principles approach, we now launch our investigation into the primary aggregation of asphaltenes. Many molecular structural properties of asphaltenes have been sorted out in recent years. Asphaltenes are (1) Chilingarian, G. V., Yen, T. F., Eds. Bitumens, Asphalts, and Tar Sands; Elsevier Scientific Publishing Co.: New York, 1978. (2) Bunger, J. W., Li, N. C., Eds. Chemistry of Asphaltenes; American Chemical Society: Washington, DC, 1984. (3) Sheu, E. Y., Mullins, O. C., Eds. Asphaltenes: Fundamentals and Applications; Plenum Pub. Co.: New York, 1995. (4) Mullins, O. C., Sheu, E. Y., Eds. Structures and Dynamics of Asphaltenes; Plenum Pub. Co.: New York, 1998. (5) Crick, F. Phys. Today 2000, 53, 19.
mostly carbon and hydrogen (atomic ratio ∼1:1.2), and ∼40% of the carbon is aromatic. Asphaltenes also possess heteroatoms (a few percents), mostly nitrogen and sulfur. The characterization of sulfur6 and nitrogen7 moieties in asphaltenes has been established using X-ray absorption near-edge structure. Infrared spectroscopy elucidates the oxygen moieties.8 The basic ring geometries of the aromatic ring system have been shown using 13C NMR8 and X-ray Raman spectroscopy.9 The type of alkane substituent has been explored via NMR and IR8 and by pyrolysis gas chromatography.10 The size of the aromatic ring systems has been determined in numerous ways; scanning tunneling microscopy has been used to obtain direct images of the aromatic portions of asphaltene molecules.11 Highresolution transmission electron microscopy has also been used to image aromatic ring systems in asphaltenes.12 Optical absorption and emission spectroscopy13 coupled with molecular orbital calculations14 has been used to interrogate this same issue. Finally, time-resolved fluorescence depolarization (FD) studies also constrain the (6) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3182. (7) Mitra-Kirtley, S.; Mullins, O. C.; Chen, J.; van Elp, J.; George, S. J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252. (8) Scotti, R.; Montanari, L. In ref 4, chapter 3. (9) Bergmann, U.; Groenzin, H.; Mullins, O. C.; Glatzel, P.; Fetzer, J.; Cramer, S. P. Chem. Phys. Lett. 2003, 369, 184. (10) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355. (11) Zajac, G. W.; Sethi, N. K.; Joseph, J. T. Scanning Microsc. 1994, 8, 463. (12) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. C. Energy Fuels 2002, 16, 490. (13) Mullins, O. C. In ref 4, chapter 2. (14) Ruiz-Morales, Y. J. Chem. Phys. A 2002, 106, 11283.
10.1021/la048640t CCC: $30.25 © 2005 American Chemical Society Published on Web 02/25/2005
Asphaltene CNACs and Standard Surfactants CMCs
proposed size of asphaltene ring systems.15-18 Mass spectroscopy is also providing stringent requirements for proposed asphaltene structures.19,20 All results from these disparate methods are in accord, providing a robust answer. The number of fused rings in asphaltenes varies from ∼4 to 10. The molecular weight of asphaltenes was previously one of the biggest unresolved issues in asphaltene science. Because of agreement sited above about the basic unit or subunit of asphaltene molecules, the debate over asphaltene molecular weight essentially has been whether asphaltenes are monomeric or polymeric. One proposed asphaltene structure showed a polymer consisting of about 10 monomers,10 although the author of that paper advised that his molecular weight determination was uncertain. In fact, the molecular weight of asphaltenes had been the subject of debate spanning decades, both in time and in numerical magnitude. This debate is now resolved; FD studies have shown that asphaltenes are small molecules, essentially monomeric, not polymeric systems.15-18 Furthermore, by knowing the proper molecular weight, one can relate the molecular structure to function both for asphaltene solubility17 and for molecular changes with feedstock cracking.18 Mass spectroscopy has weighed on this topic. Initial field ionization mass spectral studies19,20 showed that petroleum asphaltenes are about 700 g/mol with a width in the mass distribution of about a factor of 2. Subsequently, many mass spectral studies using many ionization techniques have corroborated these results.21-25 There are, however, some mass spectral reports utilizing laser desorption that are at variance with all other mass spectral methods.26 By comparing and contrasting crude oil and petroleum asphaltenes, FD studies have shown that the key molecular parameters controlling asphaltene solubility are intermolecular binding of the aromatic ring systems versus steric disruption of this binding by alkane substituents.17 Furthermore, a direct correlation exists between solubility, molecular size, and ring size.15-18,27 Index-of-refraction studies clearly show the importance of van der Waals interaction in the binding in asphaltene solubility.28,29 Within this point of view, the Yen model for primary and secondary aggregation of asphaltenes is reasonable. In this picture, primary aggregation occurs when aromatic ring systems of different asphaltene molecules stack more or less as pancakes to form primary nanoaggregates with corresponding high binding energy. (We are attempting to change the terminology per the reviewers’ request. These nanoaggregates were called asphaltene “micelles” in many previous reports.) However, alkane steric hin(15) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237. (16) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (17) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972. (18) Buch, L.; Groenzin, H.; Buenrostro-Gonzalez, E.; Andersen, S. I.; Lira-Galeana, C.; Mullins, O. C. Fuel 2003, 82, 1075. (19) Boduszynski, M. M. In ref 2, chapter 2. (20) Boduszynski, M. M. Energy Fuels 1988, 2, 597. (21) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12, 1290. (22) Yang, M.-G.; Eser, S. ACS Reprints, 281th ACS National Meeting, New Orleans, LA,Aug 22-26, 1999; American Chemical Society: Washington, DC, 1999; p 768. (23) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492. (24) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 4145. (25) Cunico, R. I.; Sheu, E. Y.; Mullins, O. C. Pet. Sci. Technol. 2004, 22, 787. (26) Suelvas, I.; Islas, C. A.; Millan, M.; Galmes, C.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Fuel 2003, 82, 1. (27) Groenzin, H.; Mullins, O. C.; Eser, S.; Mathews, J.; Yang, M.-G.; Jones, D. Energy Fuels 2003, 17, 498.
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drance would restrict the number of molecules in the stack. Further addition of asphaltene to the solution results in new nanoaggregates but not in the size increase of a single nanoaggregate. Eventually, at very high concentrations, the nanoaggregates undergo clustering; the binding energy of one nanoaggregate to another is much lower than the binding interior to the nanoaggregate. This picture is roughly in accord with the temperature dependence of small-angle neutron scattering (SANS); the smallest aggregates are always observed independent of concentration.30 However, the smallest particle detected by SANS (∼5 nm diameter) is larger than one might expect for the nanoaggregates giving rise to some uncertainty. The huge viscosity variation with temperature of asphaltic systems such as roofing tar shows that the disruption of higher asphaltene aggregation is not that high in binding energy.31 Particle size analysis for asphaltene flocculation was produced by solvent addition32 and live oil depressurization.33 Using some micellar formalisms for the primary asphaltene aggregation is plausible from a molecular structural point of view. In particular, the concept that nanoaggregate growth shuts off after reaching a certain small size due to steric hindrance is consistent with substantial observations; it needs to be tested. Surface tension measurements have been employed to measure the “asphaltene critical micelle concentration (CMC)” in pyridine.34 A clear break in the surface tension data occurred at ∼400 mg/L. Other surface tension studies have reported “CMCs of asphaltene in toluene” to be much higher in concentration, 10 g/L in one study35 and 1.7 g/L in another study.36 There are kinetic issues associated with surface tension measurements that may help explain the large range of values reported for the asphaltene CMC. Thermo-optical techniques suggested a possible “CMC” for asphaltenes in toluene at ∼50 mg/L.37 Microcalorimetry has also been applied to asphaltenes; reported “asphaltene CMCs in toluene” are in the range of several grams per liter.38 A recent report using microcalorimetry illustrates effects from water on aggregation and labels the observed transitions at ∼3 g/L asphaltene in toluene as apparent “CMCs”.39 The authors also state they do not detect a CMC without water being present. High-Q, high-resolution ultrasonic spectroscopy is perhaps the most direct and sensitive method to test the formation of micelles. The speed of sound is a direct probe of the bulk and so is not sensitive to surface issues, and one can easily exclude transients in the measurements. High-Q ultrasonic measurements have been used successfully to monitor many types of phase transitions in solution.40-46 (28) Buckley, J. Energy Fuels 1999, 13, 328. (29) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J.-X.; Gill, B. S. Pet. Sci. Technol. 1998, 16, 251. (30) Sheu, E. Y. In ref 3, chapter 1. (31) Lin, M.-S.; Chaffin, J. M.; Davison, R. R.; Glover, C. J.; Bullin, J. A. In ref 4, chapter 9. (32) Anisimov, M. A.; Yudin, I. K.; Nikitin, V.; Nikolaenko, G.; Chernoustan, A.; Toulhoat, H.; Frot, D.; Briolant, Y. J. Phys. Chem. 1995, 99, 9576. (33) Joshi, N. B.; Mullins, O. C.; Jamaluddin, A.; Creek, J.; McFadden, J. Energy Fuels 2001, 15, 979. (34) Sheu, E. Y. J. Phys.: Condens. Matter 1996, 8, A125. (35) da Silva Ramos, A. C.; Haraguchi, L.; Nostripe, F. R.; Loh, W.; Mohamed, R. S. J. Pet. Sci. Eng. 2001, 32, 201. (36) Bouhadda, Y.; Bendedouch, D.; Sheu, E. Y.; Krallafa, A. Energy Fuels 2000, 14, 845. (37) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Fernandez, A.; Perez, P.; Caetano, M. Fuel 1999, 78, 997. (38) Andersen, S. I.; Christensen, S. D. Energy Fuels 2000, 14, 38. (39) Andersen, S. I.; del Rio, J. M.; Khvostitchenko, D.; Shakir, S.; Lira-Galeana, C. Langmuir 2001, 17, 307.
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Here, high-Q ultrasonic measurements are performed on aqueous and toluene solutions containing standard surfactants and are compared against known literature values when available to validate the methodology. The governing equations for the micelle phase equilibrium model41,42 are given, and all data presented here are interpreted within this framework. The surfactants used included SDS in water, C16TAB in water, Tween 80 in water and separately in toluene, and Brij 35 in toluene. (Surfactant names are explained in the Experimental Section.) CMC determinations via ultrasonic spectroscopy are shown to agree well for known surfactants over a broad range of CMCs. In particular, surfactants with very small values of CMCs are treated without difficulty using high-Q ultrasonic measurements. We employ these ultrasonic techniques to study several asphaltene-toluene systems up to concentrations of several grams per liter. In addition, with density measurements, the ultrasonic results provide a direct measure of monomer and micelle compressibilities. Thus, asphaltene compressibilities can be compared with those of standard surfactants. For all of these solutions, density measurements were made enabling the determination of micellar or nanoaggregate compressibilities in solution and, in some case, monomer compressibilities in solution. Comparisons are emphasized between standard surfactants and asphaltenes.
The density of the solution is deduced from the previous equations (3 and 4):
F ) F0 + (1 - v˜ 1F0)c1 + (1 - v˜ mF0)cm
where F0 is the density of the solvent, c1 is the weight concentration of surfactant in the monomeric form, and cm is the weight concentration of surfactant in the micellar form. If assumed that the phase equilibrium model is valid here, then
for c < cmc, c1 ) c and cm ) 0 for c > cmc, c1 ) cmc and cm ) c - cmc where c is the weight concentration of surfactant in the solution and cmc is the numerical value at the CMC. That is, at CMC, cm ) 0 but any new increase in surfactant concentration corresponds to increasing the cm. For c < cmc
u)
1 xFκ
F ) F0 + (v˜ m - v˜ 1)F0cmc + (1 - v˜ mF0)c
(7)
To obtain the adiabatic compressibility of the solution κ as a function of the concentrations of surfactants in the solution, the density is differentiated with respect to pressure P at constant entropy.
κ)
(2)
Furthermore,
V ) w0v0 + w1v˜ 1 + wmv˜ m
(6)
(1)
The compressibility of the liquid can be considered as adiabatic because the compressions and decompressions in ultrasonic waves are too fast for heat dissipations.43 For dilute surfactant solutions, the ultrasonic velocity and the density of the solution can be expressed as functions of the concentration of surfactants.41 In general, surfactant molecules in solution coexist in monomeric and micellar forms. Here, we follow the treatment given in ref 41. Considering a volume V, if there are w0 g of solvent, w g of surfactant, w1 g of surfactants in the monomeric form, and wm g of surfactants in the micellar form,
w ) w1 + w m
F ) F0 + (1 - v˜ 1F0)c and for c > cmc
II. Theory The ultrasound velocity u in liquid (solution) is related to the density of the liquid (solution) F and the adiabatic compressibility of the solution κ by the relationship
(3)
(5)
1 ∂F F ∂P
( )
(8)
S
Differentiating eq 5,
|
|
|
∂F0 ∂(1 - v˜ 1F0) ∂c1 ∂F + ) + c1 + (1 - v˜ 1F0) ∂P S ∂P S ∂P S ∂P ∂cm ∂(1 - v˜ mF0) (9) cm + (1 - v˜ mF0) ∂P S ∂P
|
If it is considered that the concentration of monomers c1 and the concentration of micelles cm change with pressure only through the changes in the volume of the solution, it follows that
∂c1 ) c1κ ∂P
(10)
The adiabatic compressibility of the solvent is defined by where v0 is the specific volume of the solvent, v˜ 1 is the apparent specific volume of the monomeric form, and v˜ m is the apparent specific volume of the micellar form. The weight of the surfactant solution is equal to
FV ) w0 + w1 + wm
(4)
(40) Kudryashov, E.; Kapustina, T.; Morrissey, S.; Buckin, V.; Dawson, K. J. Colloid Interface Sci. 1990, 203, 59. (41) Zielinski, R.; Ikeda, S.; Nomura, H.; Kato, S.; J. Colloid Interface Sci. 1987, 119, 398. (42) Blandamer, M. J.; Cullis, P. M.; Soldi, L. G.; Engberts, J. B. N. F.; Kacperska, A.; Van Os, N. M.; Subha, M. C. S. Adv. Colloid Interface Sci. 1995, 58, 171. (43) Buckin, V.; Smyth, C. Semin. Food Anal. 1999, 4, 113. (44) Mukerjee, P.; Mysels, K. J. NSRDS-NBS 36; U.S. Department of Commerce: Washington, DC, 1971.
∂cm ) cmκ ∂P κ0 )
( )|
1 ∂F0 F0 ∂P
(11) (12)
S
The apparent adiabatic compressibility of the surfactant in the monomeric form is defined by
κ˜ 1 ) -
( )|
1 ∂v˜ 1 v˜ 1 ∂P
S
(13)
And the apparent adiabatic compressibility of the sur-
Asphaltene CNACs and Standard Surfactants CMCs
Langmuir, Vol. 21, No. 7, 2005 2731
Figure 1. Chemical structure of Tween 80.
factant in the micellar form is defined by
κ˜ m ) -
( )|
1 ∂v˜ m v˜ m ∂P
(14)
S
Then, from eq 9 and using the definitions 10-14,
Fκ ) F0κ0 + κ[(1 - v˜ 1F0)c1 + (1 - v˜ mF0)cm] + F0c1v˜ 1(κ˜ 1 - κ0) + F0cmv˜ m(κ˜ m - κ0) (15) From eqs 5 and 15, the adiabatic compressibility of the solution is given by
κ ) κ0 + (κ˜ 1 - κ0)v˜ 1c1 + (κ˜ m - κ0)v˜ mcm
(16)
Equation 1 relates the ultrasonic velocity in liquid to density and adiabatic compressibility. For dilute solutions (c1 , 1 and cm , 1),
u ) u0 +
[( ) ]
u0 κ˜ 1 v˜ 1 2 - v 0 c1 + 2 κ0 u0 κ˜ m v˜ m 2 - v0 cm (17) 2 κ0
[(
) ]
where v0 is the specific volume of the solvent and is equal to the inverse of F0. For c < cmc,
u ) u0 +
[( ) ]
u0 κ˜ 1 v˜ 1 2 - v0 c 2 κ0
(18)
And for c > cmc, c1 ) c, and cm ) 0
u ) u0 +
[( ) (
)]
u0 κ˜ 1 κ˜ m v˜ 1 2 - v˜ m 2 cmc + 2 κ0 κ0 κ˜ m u0 v˜ m 2 - v0 c (19) 2 κ0
[(
) ]
It can be concluded from eqs 18 and 19 that the ultrasonic velocity can be modeled by two straight lines in the plot of ultrasonic velocity versus concentration; one line segment below the CMC, the other above.41,44-46 The apparent compressibilities of the monomeric form and the micellar form can be deduced from ultrasonic and density measurements.41,42,44-46 III. Experimental Section The different surfactants used here are sodium dodecyl sulfate (SDS) from Sigma-Aldrich Chemicals, purity > 99%, hexadecyltrimethylammonium bromide (C16TAB) from Sigma-Aldrich, purity > 99%, polyoxyethylene 23 lauryl ether (Brij 35; C12E23) from Acros Chemicals, purity 99%, and polyoxyethylene sorbitan monooleate (Tween 80) from Acros Chemicals, purity > 99%. N-heptane asphaltenes UG8 and BG5 from Kuwaiti crude oils were used; the extraction procedure is described elsewhere.17 The aqueous solutions were made in Milli-Q water for the density measurements and in distilled water for the ultrasound measurements. The organic solutions were prepared in toluene 99.8%
Figure 2. Ultrasonic spectrum of water at 25 °C. from Acros and Sigma-Aldrich. The chemical structure of Tween 80 is shown in Figure 1. The ultrasonic measurements were performed on the HRUS 102 high-resolution ultrasonic spectrometer from Ultrasonic Scientific, Ltd. The speed of sound is determined using a resonance technique in the range of frequencies between 2 and 20 MHz; we used ∼5 MHz for our experiments, and the spectrometer can measure the speed of sound to one part in 5 × 10-6. The measurements are made using two identical cells filled with a volume from 1 to 2 mL, one filled with the analyzed solution and the other with the solvent (water or toluene). Both cells are fixed together in the same block and are thermostated at 25 ( 0.1 °C, enabling small differences in ultrasonic velocity to be determined.43 Each cell consists of a resonance cavity with a resonant glass chamber built with two lithium niobate transducers on two opposite sides of the cell; one transducer is used as the signal source, and the other is the receiver. Two main factors determine the resolution of the measurements: the quality of the resonance (including a high factor of quality Q and the absence of satellites of the resonance peaks) and the stability of the resonances. The first factor is ensured by having a high precision in the parallel alignment of the cells and the quality of the lithium niobate piezotransducers. The second factor requires a special construction of the resonator, insisting on keeping the distance between the two transducers constant. To maintain a perfect geometry, the resonator chamber has been built separately from the transducers. Of course, frequencies can be measured to very high accuracies.43 In fact, for our measurements exquisite precision is needed more than exquisite accuracy. The ultrahigh-Q resonator coupled with the dual beam experimental configuration provides this objective. The conversion of frequency to sound speed is performed using the equation
δu δfn ) u fn
(20)
where u is the speed of sound and fn is the frequency of the nth resonance. This equation presumes plane wave propagation.43,48 A typical ultrasonic spectrum of water is shown in Figure 2. Several of the sharp ultrasonic resonances are shown in Figure 3 at the frequency range used for this study. Figures 4 and 5 show the comparable spectra for toluene. In these figures, the amplitude of the output signal is shown as a function of the frequency of the acoustical signal. In addition to the narrow acoustic cell resonances, Figures 2 and 4 show broad resonances (at ∼4, 7, and 10 MHz, for example) that are associated with (45) Bloor, D. M.; Gormally, J.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1915. (46) Sua´rez, M. J.; Lo´pez-Fonta´n, J. L.; Sarmiento, F.; Mosquera, V. Langmuir 1999, 15, 5265.
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Figure 3. Several acoustic cell resonances in the ultrasonic spectrum of water at 25 °C.
Figure 4. Ultrasonic spectrum of toluene at 25 °C.
Figure 6. (a) Ultrasonic titration of SDS in aqueous solution at 25 °C. (b) Density measurements of SDS in aqueous solution at 25 °C. Table 1. Speeds of Sound, Densities, and Adiabatic Compressibilities at the Temperature of 25 °C of the Solvents Used in This Study
Figure 5. Several acoustic cell resonances in the ultrasonic spectrum of toluene at 25 °C. ultrasonic resonances in the glass walls of the cells and with the transducers. These spectral regions were avoided in all of our experiments. For distilled water at 25 °C, the speed of sound is u0 ) 1496.7 m/s with a resolution of 0.0075 m/s. In toluene at 25 °C, the speed of sound is u0 ) 1307.1 m/s with a resolution of 0.0065 m/s. All ultrasonic spectra were acquired by diluting solutions from the highest concentrations, with stepwise concentration reductions. Each curve consisted of approximately at least 15-25 points. For each point, a quantitative dilution was performed; the solution was stirred and allowed to equilibrate for 10-15 min prior to recording the ultrasonic frequency for that concentration. Integrated runs were typically 4-6 h. No difference in the spectrum was observed if the equilibration time was increased or decreased by a factor of 2. The reproducibility of the measurements was checked and found to be very good. All the ultrasonic titrations exhibit a break between two straight-line segments in the curves, as expected from eqs 18 and 19. The CMCs of the different surfactants were given by the intersection of two straight-line segments. The densities were measured at 25 ( 0.01 °C with an Anton Paar DMA 4500 densitometer with a resolution of 5 × 10-5 g/cm3. Each measurement was done twice and averaged. The densities of milli-Q water and toluene were found to be close to the expected
solvent
speed of sound (m/s)
density (g/cm3)
v0 (cm3/g)
κ0 adiabatic compressibility (10-5/bar)
water toluene
1496.7 1307.1
0.997 04 0.862 22
1.002 97 1.1598
4.48 6.79
values: 0.997 04 and 0.862 22 g/cm3, respectively. The apparent specific volumes of the monomer and of the micelle of SDS, C16TAB, and Tween 80 in water and Tween 80 and Brij 35 in toluene were calculated using eq 7, using a straight-line segment of the points at concentrations above the CMC, which are more reliable than the points at concentrations lower than the CMC because of the resolution of the densitometer. For Tween 80 in water and asphaltene in toluene, the apparent specific volume of the micelle was determined from the slope of the density versus concentration graph but the apparent specific volumes of monomer have not been calculated because of insufficient measurement resolution at the very low concentrations. Table 1 presents the ultrasonic speed of sound and densities measured for our two solvents, water and toluene. The derived compressibility from eq 1 is also given.
IV. Results and Discussion: Ionic Surfactants. Two well-known ionic surfactants were studied here in aqueous solutions: SDS (anionic) and C16TAB (cationic). The CMCs were deduced from the ultrasonic velocity versus concentration plots, were taken as the intersections of the two straight-line segments (above and below the CMC), and were found to be close to the CMC given in the literature.40,44,49 Figure 6a gives the solution ultrasonic velocity versus SDS concentration, (47) De Lisi, R.; Milioto, S.; Verrall, R. E. J. Solution Chem. 1990, 19 (7), 665.
Asphaltene CNACs and Standard Surfactants CMCs
Langmuir, Vol. 21, No. 7, 2005 2733
Figure 7. (a) Ultrasonic titration of C16TAB in aqueous solution at 25 °C. (b) Density measurements of C16TAB in aqueous solution at 25 °C.
Figure 8. (a) Ultrasonic titration of Tween 80 in water at 25 °C. (b) Density measurements of Tween 80 in water at 25 °C.
Table 2. Values of the CMCs at 25 °C of the Ionic Surfactants Used in This Study
The results are in excellent agreement with the references.40,45-47,49 κ˜ 1 is large and negative for the ionic surfactants SDS and C16TAB in water40,41,46,47 while κ˜ m is large and positive.40,41,45-47 This high molar compressibility for the surfactants in the micellar state can be associated with the compressibility of the internal core of micelles and is close to the adiabatic compressibility of pure hydrocarbon liquids with the same length of the hydrocarbon chain.40 Nonionic Surfactants in Aqueous and Organic Solvents. Tween 80 (see Figure 1) is a nonionic amphiphile composed of 20 oxyethylene groups on an oxocyclopentane core which are the hydrophilic part of the molecule, while the hydrocarbon chain is the hydrophobic part of the molecule. It was studied here both in water and in toluene. Brij 35 is a polyoxyethylene dodecyl ether, which is a nonionic surfactant with a C12 hydrophobic alkyl chain and a hydrophilic chain of 23 polyoxyethylene subunits. It was studied in toluene. Figure 8a shows the solution ultrasonic velocity of Tween 80, and Figure 8b shows the corresponding density curve. At the very low concentration of 8 mg/L, Tween 80 exhibits a clear change in the ultrasonic velocity curve. This agrees with the literature determination of the CMC.51 Thus, the ultrasonic method for CMC determination is established over 2.5 orders of magnitude in concentration. Two nonionic surfactants were run in toluene; Figure 9a shows for Tween 80 the solution ultrasonic velocity versus Tween 80 concentration; Figure 9b shows the corresponding density curve. Figure 10a shows for Brij 35 the solution ultrasonic velocity versus
surfactant
CMC (this work) (g/L)
SDS
2.573
C16TAB
0.335
CMC [refs] (g/L) 2.393 [44] 2.408 [49] 0.328 [44] 0.334 [40]
Table 3. Apparent Specific Volumes and Compressibilities of the Monomeric and Micellar Forms of the Ionic Surfactants Used in This Study at a Temperature of 25 °C surfactant SDS (this work) SDS [refs]
v˜ 1 (cm3/g)
v˜ m (cm3/g)
κ˜ 1 (10-5/bar)
κ˜ m (10-5/bar)
0.822 0.873 -1.97 0.813 [50] 0.854 [50] -1.93 [46]
4.02 4 [46] 4.3 [45] 4.33 4.28 [40] 4.24 [47]
C16TAB (this work) 0.864 0.983 -1.64 C16TAB [refs] 0.962 [40] 0.989 [40] -0.039 [40] 0.964 [47] 0.988 [47] -1.25 [47] 1.002 [50]
and Figure 6b gives the solution density versus SDS concentration. Figure 7a gives the solution ultrasonic velocity versus C16TAB concentration, and Figure 7b gives the solution density versus C16TAB concentration. Table 2 presents the CMCs obtained here at 25 °C for SDS and C16TAB and also gives literature values for these CMCs. Note the excellent agreement. The density measurements provide apparent specific volumes; these combined with the ultrasonic measurements allow us to derive the apparent adiabatic compressibilities of the monomer and micelle. Our apparent specific volumes and derived compressibilities for SDS and C16TAB are given in Table 3 and are compared with literature values. Note the generally excellent agreement for all parameters.
(48) Mason, W. P.; Thurston, R. N. Physical Acoustic. In HighFrequency Continuous Wave Ultrasonics; Bolef, D. I., Miller, J. G., Eds.; Academic Press: New York, 1971; Vol. VIII, chapter 3. (49) Priev, A.; Zalipsky, S.; Cohen, R.; Barenholz, Y. Langmuir 2002, 18, 612.
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Andreatta et al. Table 4. CMCs for Nonionic Surfactants in Aqueous and Organic Solutions at 25 °C surfactant
solvent
CMC (our work; g/L)
CMC [refs] (g/L)
Tween 80 Tween 80 Brij 35
water toluene toluene
0.008 7.4 17
0.013 [51]
Table 5. Apparent Specific Volumes and Compressibilities of the Monomeric and Micellar Forms of the Nonionic Surfactants in Toluene and Water at 25 °C Used in This Study
Figure 9. (a) Ultrasonic titration of Tween 80 in toluene at 25 °C. (b) Density measurements of Tween 80 in toluene at 25 °C.
Figure 10. (a) Ultrasonic titration of Brij 35 in toluene at 25 °C. (b) Density measurements of Brij 35 in toluene at 25 °C.
Brij 35 concentration; Figure 10b shows the corresponding density curve. For these nonionic surfactants in toluene, it is not surprising that there is not a clear break in the
surfactant
v˜ 1 (cm3/g)
v˜ m (cm3/g)
κ˜ 1 (10-5/bar)
κ˜ m (10-5/bar)
Tween 80 in water Tween 80 in toluene Brij 35 in toluene
0.8895 0.9269
0.9227 0.9162 0.9309
4.38 5
1.37 3.52 3.94
velocity curves (even though we fit sections of the curve with straight lines.) We interpret the changes in slopes in the ultrasonic velocity curves as effective CMCs for Figures 9a and 10a. Nonionic surfactants in toluene do not exhibit a sharp limit to the size of the micelles; this leads to curvature of the ultrasonic plots (Figures 9a and 10a). Table 4 lists the CMCs determined here for standard nonionic surfactants. Using measured densities, apparent specific volumes and apparent compressibilities are obtained and are listed in Table 5. Comparison of the ultrasonic curves for ionic surfactants in water versus nonionic surfactants in water shows very different behavior. The differential quantity, the compressibility, is much more sensitive and, thus, accounts for this change much more than the integral quantity, the density. The ionic surfactants in water have much different apparent compressibilities than the nonionic surfactants in toluene. In particular, the ionic surfactants exhibit negative apparent compressibilities (in water) for the monomeric form with a very large increase in apparent compressibilities upon formation of the micelle. These micelles have a nonpolar core which is anticipated to be rather compressible. On the other hand, the nonionic surfactants in toluene exhibit very large positive apparent compressibilities in toluene for the monomeric form and show a reduction in apparent compressibilities upon micelle formation. The core for these micelles is polar and is anticipated to be more rigid. Simple heuristics account for these systematics and are useful for comparison with asphaltene results. Asphaltenes. UG8 Asphaltenes. UG8 Asphaltenes have been used by essentially every technique we have of analyzing asphaltenes. Their properties are common; thus, they represent “typical” asphaltenes. Figure 11a shows the solution ultrasonic velocity versus concentration for UG8 asphaltenes in toluene. A clear break in this curve is observed; this gives a “critical nanoaggregate concentration” (CNAC) for asphaltenes of 0.164 g/L. The velocity versus concentration curve for asphaltene is very similar to that of other nonionic surfactants in toluene, strengthening the CMC interpretation. Furthermore, at concentrations higher than the CMC, there is not another break in the curve even up to concentrations of 2 g/cm3 asphaltene in toluene. Either there is no other change in aggregates up to this concentration or any further change in aggregation has no effect on ultrasonic velocity (because the binding energy is too low to change the compressibility). From the density measurements (Figure 11b), we can calculate the apparent specific volume of nanoaggregate from the slope of the density versus concentration graph but we cannot get the apparent specific volume of monomer because the concentrations are too low for the accuracy
Asphaltene CNACs and Standard Surfactants CMCs
Figure 11. (a) Ultrasonic titration of asphaltenes UG8 in toluene at 25 °C. (b) Density measurements of asphaltenes UG8 in toluene at 25 °C. Table 6. Values of the CMC, Apparent Specific Volumes of the Nanoaggregates, and Apparent Adiabatic Compressibilities of the Nanoaggregates for Different Asphaltenes in Toluene at 25 °C asphaltenes
CMC (g/L)
v˜ m (cm3/g)
κ˜ m (10-5/bar)
UG8 BG5
0.164 0.048
0.8814 0.8582
3.95 3.45
of the densitometer. The adiabatic apparent compressibility in the nanoaggregate form is then calculated from the ultrasonic data with eq 19. The results are presented in Table 6. The apparent compressibility of the asphaltene nanoaggregate is close in magnitude to that of nonionic surfactants, again lending credence to the CNAC interpretation for the asphaltenes. BG5 Asphaltenes. BG5 Asphaltenes were obtained from Kuwait Burgan5 crude oil. They too have been subjected to many different kinds of investigation, and they too have “typical” characteristics. Asphaltenes from UG8 and BG5 might be called “vanilla” asphaltenes. Figure 12a shows the solution ultrasonic velocity versus concentration for BG5 asphaltenes. A break in this curve is evident; this gives a CNAC of 0.048 g/L. The CNAC for BG5 is lower than that of UG8 asphaltenes; nevertheless, a CNAC is evident in both cases. Again, no other change is observed in the ultrasonic velocity up to 3 g/cm3. As in the case of UG8 asphaltenes, we can calculate the apparent specific volume of the nanoaggregate from the slope of the solution density versus concentration (Figure 12b), but we cannot get the apparent specific volume of monomer because the concentration is too low. From the ultrasonic data, we can calculate the apparent adiabatic compressibility in the micellar form. The results are presented in Table 6. Again, we get agreement between asphaltene micelle apparent compressibilities with those of other nonionic surfactants.
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Figure 12. (a) Ultrasonic titration of asphaltenes BG5 in toluene at 25 °C. (b) Density measurements of asphaltenes BG5 in toluene at 25 °C.
Figure 13 shows an expanded scale of the ultrasonic velocity versus concentration to make the CNAC clear. Figure 13a expands the low concentration range of Figure 11a while Figure 13b expands the low concentration range of Figure 12a. The break in the ultrasonic velocity curve is quite clear; we interpret this break to be the CNAC. At higher concentrations than the CNAC, there is no change in the ultrasonic slope. This indicates that the nanoaggregates are not changing at these concentrations, just that there are more of them at higher concentration. That is, nanoaggregate growth shuts off. We also note that while a clear break is evident at the CNAC, the ultrasonic data cannot rule out formation of dimers or trimers at concentrations below the CNAC. Different asphaltenes exhibit CNACs at similar concentrations but with some variability in the exact value: CNACs ∼ 50-150 mg/L. The apparent compressibilities of the asphaltene nanoaggreagtes are similar to each other and similar to other apparent micelle compressibilities for other nonionic surfactants in toluene. The CNACs determined here are 1-2 orders of magnitude lower than those in literature reports for asphaltene-toluene systems obtained by other techniques. In our view, the other techniques are not recording proper CNACs. They may be recording some higher-level aggregation phenomenon. Many techniques do not measure an explicit parameter such as apparent nanoaggregate compressibility that can then be checked against known surfactants as we do. Rather, some of the other techniques only interpret a change in some property as the CNAC. If these techniques are not sufficiently sensitive to detect CNACs at 100 mg/ L, then corresponding data will be subject to misinterpretation. The governing chemical principles of asphaltenes that determine solubility and, thus, define asphaltenes have been shown to be van der Waals attraction of aromatic
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Andreatta et al.
VPO and other colligative methods are performed at concentrations that are significantly in excess of the asphaltene CNAC; consequently, VPO provides an aggregate weight. VPO is often in error by a factor of ∼5 for molecular weight determination; consequently, VPO along with our asphaltene CNAC results here show that the aggregation number in an asphaltene micelle is ∼5. It is important to note that for nonionic surfactants and asphaltenes in organic solvents trace water may influence results significantly.39 In this report we have not attempted to dry the toluene, so our results may be influenced by trace water. First, we note that most previous determinations of the asphaltene CMC by various techniques also did not dry the toluene. To obtain a direct comparison with our work and previous work, we have used similar methodology. Our CNACs differ substantially from these previous studies, as has been noted. Second, in natural systems, water is frequently present; we are interested in performing experiments on a system simple enough to hopefully understand while, at the same time, providing some indication of what is to be expected in natural systems. In the future, it will be important for us to assess the effect of dissolved water on our data.
Figure 13. (a) Ultrasonic titration of asphaltenes UG8 in toluene at 25 °C. (b) Ultrasonic titration of asphaltenes BG5 in toluene at 25 °C.
ring systems versus steric repulsion of alkane chains;17 essentially asphaltene molecules are shaped “like your hand” with the ring systems being the palm and the alkane chains being the fingers. We believe that the same forces are operative in determining the asphaltene nanoaggregate. The idea is that the first few asphaltene molecules to associate have fairly clear access to intermolecular interaction of the fused ring system. However, subsequent aggregation of more molecules becomes constrained by alkane substituents, thereby impeding further stacking. At some point, the interaction between the nanoaggregate and an additional molecule becomes rather weak, probably as a result of steric repulsion. At this point new nanoaggregates form with increasing concentration. These ideas are central to the Yen model; our data is in general in agreement with this well-known model.52 The low values of the CNACs help explain why colligative techniques such as vapor pressure osmometry (VPO) always record “molecular” weights that are too high.
V. Conclusions High-Q ultrasonic spectroscopy has proven to be a very valuable tool in the characterization of micelle formation for known surfactants and of nanoaggregate formation of asphaltenes. CMCs of ionic and nonionic surfactants are easily measured in high and low concentration ranges. For standard surfactants, we obtain excellent agreement between our measurements and literature values of CMCs, apparent specific volumes, and apparent compressibilities. Asphaltenes in toluene exhibit CNACs at ∼100 mg/L. The phase equilibrium model for micelles applies readily to all data presented here, surfactant and asphaltene data alike. Furthermore, derived parameters such as the compressibility of nanoaggregates of asphaltenes and of micelles of nonionic surfactants are very similar, thereby strengthening the conclusion that asphaltenes are nonionic surfactants that form nanoaggregates and exhibit CNACs. The literature reports with much larger concentrations for asphaltene CNACs (of CMCs) are probably not measuring CNACs but perhaps some higher order aggregation. Our asphaltene CNACs explain why VPO measurements of asphaltene molecular weights are consistently too high. LA048640T (50) Corkill, J. M.; Goodman, J. F.; Walker, T. Trans. Chem. Soc. 1967, 63, 768. (51) www.sigma-aldrich.com (accessed 2004). (52) Yen, T. F. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1990, 35, 314.
