Dielectric properties of asphaltene solutions

Dielectric E. Y. Sheu,

properties

of asphaltene

solutions

M. M. De Tar and D. A. Storm

Texaco R&D Department, (Received 29 May 1992;

PO Box 509, Beacon, NY 12508, revised 18 December 1992)

USA

The conductivity and dielectric response of asphaltene solutions were investigated for frequencies up to 7MHz. The conductivity behaviour indicated that the asphaltene colloids were well dispersed and the charges move much more freely within the colloids than between colloids. For asphaltene concentrations from 20 to 50 wt%, and a temperature range from 10 to WC, a well-defined dielectric loss peak was observed, again indicating that the asphaltene colloids are well dispersed. The loss spectra were interpreted by the Cole-Cole equation, where the average dielectric relaxation time and the frequency spread were obtained. The dielectric relaxation time obtained was on the order of microseconds for all cases and the frequency spread (or the stretch exponent) was less than unity, signifying a non-Debye relaxation process. Another phenomenon observed was a second peak arising at lower frequencies, either with increasing concentration or upon cooling. This arises from the interparticle correlation, which results in a correlation peak in the dielectric loss spectrum. Combining the observed conductivity and the dielectric response along both temperature and concentration axes, we conclude that the self-association of asphaltenes is likely due to the charge transformation between asphaltene molecules. (Keywords: asphaltene; dielectric response; conductivity)

Asphaltenes have been known to form colloidal-like particles in various organic solvents via self-association’-5. The resulting colloidal solutions behave similarly to a self-assembled micellar solution, as far as the association dissociation kinetics and reversibility are concerned’. Many studies have been dedicated to the study of this self-association phenomenon6-*. The ultimate goal of these studies is to understand the origin of this selfassociation, i.e. what causes the self-association? For micellar solutions, the equilibrium thermodynamic theory or the mass action model are often used to study

the energies that are involved in the self-association process. However, the asphaltene solution is not exactly the same as a micellar solution, which is conventionally defined in an aqueous phase. Instead of an aqueous solution, asphaltene solutions are usually in hydrophobic solvents (such as pyridine, toluene, cyclohexane, etc.). It is thus more appropriate to equate an asphaltene solution to a reversed micellar system, which has a hydrophobic continuous phase. In principle, the theories used for a micellar solution can still be applied to the reversed micellar cases, provided the free energies involved in the self-association are known. For conventional micellar or reversed micellar solutions, the self-association is dominated by the hydrophilic-hydrophobic imbalance. However, the asphaltene self-association is different. Up to date, the energies that initiate the self-association of asphaltenes are not yet known. What has been known about asphaltene solutions is that neither the hydrophobic energy nor the London force (the induce-induced dipole force) is the dominant energy. Otherwise, the asphaltene aggregates will dissociate at a moderate temperature’. On the contrary, they were found to remain as aggregates for temperatures as high as 167”C’“. It is 00162361/94/01/0045~6 (‘: 1994 Butterworth-Heinemann

Ltd

thus interesting to find out what causes asphaltenes to self-associate in organic solvents. In addition, the incentive of this study also comes from the debating of the existence of a reversed micelle system. In many claimed reversed micellar systems, the possibility of having some water was never excluded. If this is the case, these systems are microemulsions, rather than reversed micelle systems. Since asphaltene solutions contain no water, and selfassociate in hydrophobic solvents, it demonstrates that a reversed micelle-like solution can exist, if the selfassociation is not due to the hydrophilic-hydrophobic imbalance. This makes it an important issue to find out what is the dominant energy that initiates the selfassociation. Of course, one may also question the existence of water molecules in an asphaltene solution. However, it is not possible for asphaltene solutions to contain water molecules. First, the asphaltenes used here come from vacuum residue, which is the residual material under high temperature (+ 538°C) and vacuum condition. Second, the processes used for producing asphaltenes and the solutions were all well under humidity control (see Experimental). At least two works have studied the origin of the asphaltene self-association. First, Sheu et al. lo found that the asphaltene aggregates do not dissociate at temperatures up to - 167°C in a good solvent like toluene, but the polydispersity of the aggregates does decrease. This indicates that the micellization energy must originate from a molecular force much stronger than the hydrophobic energy. Also, there may be more than one association mechanism. Some mechanisms involve lower association energies, which make the aggregates dissociate at 167°C thereby lowering the polydispersity”. Another

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Dielectric

properties

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E. Y. Sheu

study by Shiffert et al.” indirectly pointed out that the dominant energy may be the charge exchange between asphaltene molecules. This is an important result. If this is indeed the case, a dielectric response measurement may provide more direct information. We thus decided to perform the dielectric relaxation measurement to investigate this point. The measurements were performed for 20 to 50 wt% asphaltenes in toluene at various temperatures. The dielectric relaxation spectra all exhibited a pronounced loss peak, suggesting that there are induced dynamic dipoles in the asphaltene aggregates. This means that the charges within the aggregates move rather freely, which provides a direct indication that the charge transformation may play an important role in the asphaltene self-association. In order to quantitatively characterize the dielectric response data, we analysed the dielectric loss data using the Cole-Cole equation to extract the mean relaxation time and the stretch exponent. The average relaxation time is on the order of 100 to 1000 ms, comparable to the diffusion relaxation of a typical colloidal suspension. This is to say that the asphaltene aggregates (or the dipole objects) are about the same size as typical colloids (about 1.0 to 100 nm)“. The particular asphaltene colloids studied here were found to be about 6.0nm diameter2. The stretch exponent (1-B) extracted from the analysis were much less than unity for all cases. This indicates that the interparticle interactions affect the process of the dielectric relaxation appreciably at these concentrations and temperatures. Finally a second peak at - lo5 Hz was observed in the dielectric loss spectrum, when temperature is low or when concentration is high. It may be attributed to the formation of a strongly correlated colloidal structure, which collectively creates a dielectric loss mode at a lower frequency, corresponding to the correlated volume.

EXPERIMENTAL Sample preparation

Asphaltenes were extracted from Ratawi vacuum residue (neutral zone), which is the residual material after +538”C vacuum distillation. The vacuum residue was mixed with heptane (1 g of vacuum residue with 40cm3 of heptane) under nitrogen at room temperature. Solutions were stirred overnight and filtered using Whatman No. 5 filter paper. The extracted insolubles were then dried to constant weight under a dry nitrogen controlled environment. Mass balances were performed to ensure complete solvent removal. The asphaltene fraction (representing about 25% of the vacuum residue) was then redissolved in toluene at various concentrations (20 wt%, 30 wt% and 50 wt%). The mixtures were then sealed and left in a sonicator at 30°C overnight to ensure complete dissolution. Instrumentation

and measurement

Dielectric relaxation measurements were performed using an HP4192A, low frequency impedance analyser. The instrument has a working frequency range from 5 Hz to 13 MHz. The sample cell used was a cylindrical cup, filled with >40cm3 of the sample. A liquid immersiontype cell from Rosemont Analytical was used to measure the capacitance, c(w), (w = frequency) and the dissipation factor, D(o), from which the dielectric storage, E’(O),and

46

Fuel 1994 Volume 73 Number 1

et

al. the dielectric loss, s”(c.&were calculated according to the following equation: E,(O)= E’(O)- k”(0) = [tc(o)]/(AEJ

(1)

.s’(~)=~,(~)cos{tan-‘[D(w)]}

(2)

E”(W)= E,(O)sin{ tan - ‘[D(o)]}

(3) where E,(O) is the relative dielectric permittivity, t is the inter-plate spacing (= 1 mm), E, = 8.854 x lo- I4 F cm- ’ is the dielectric permittivity of vacuum, and A is the total plate area ( =9cm2). The electrode plates were coated with platinum black to prevent electron polarization near the plates. For each asphaltene concentration (in toluene), the measurements were made as a function of frequency for various temperatures from 10 to 60°C. Although the maximum frequency was 13 MHz, the data for frequencies greater than 7 MHz may not be reliable, due to the working limit of the electrodes. The analysis was thus performed for frequencies up to 7 MHz only. During the data acquisition period, the samples were stirred gently to maintain sample isotropy.

THE COLE-COLE

EQUATION

In a dielectric relaxation measurement as a function of frequency, the relaxation spectrum can be analysed using the Cole-Cole equation I3 . This equation is applicable when the solution contains well-dispersed particles suspended in a continuous dielectric medium. The ColeCole equation reads: &JO)=&, +A&/[1 +(io~)‘-~] (4) where E, is the permittivity of the sample at the infinite frequency, AE is the difference between the static permittivity and the infinite frequency dielectric constant, i=,6: z is the dielectric relaxation time, and (1-B) is the stretch exponent. The stretch exponent (1- /?) represents the frequency spread of the relaxation, generally indicative of the interparticle interactions and the particle polydispersity. When /3=0, it is called the Debye relaxation. This is the case when there is no frequency spread in relaxation (or the relaxation time of every individual particle is identical). This situation usually occurs at dilute concentration or at high temperature when the system approaches the ideal condition. Since E,(O) is in the complex plane, it can be divided into a real part and an imaginary part. The real part represents the dielectric storage and the imaginary part represents the dielectric loss, assuming that the conductivity loss is negligible. In principle, one can analyse the storage data (the real part) or the loss data (the imaginary part) to characterize the dielectric properties of the sample. In many dispersion systems, the dielectric loss data showed a well defined peak. This peak represents the average relaxation mode directly. In fact, the particle size can be roughly estimated from the position of the peak, using the Stoke-Einstein equation. Since the dielectric loss data are more sensitive (as far as fitting Cole-Cole equation to the data is concerned) we only analysed the loss data. To get the dielectric loss data, we rearranged Equation (4) to yield: A+$ -fl cos(27$/2) E”(W)= 1 -2(oz)‘-Bsin(7c~/2)+(or)2-28

Dielectric

I

“2

3

4 Log (0)

Figure 1 asphaltene 60°C (+)

5

6

(Hz)

Conductivity, 6, as a function of frequency, w, for 20% in toluene at 10°C (0) 20°C (O), 35°C (A), 50°C (0) and

50-

properties

of asphaltene

solutions:

E. Y. Sheu et al.

rise of the conductivity means that the particles are well dispersed, and the charges move much more freely within the particles than between particles. The temperature independence means that the particle size remains more or less the same within the measured temperature range. This is consistent with the recent small angle scattering of Sheu et al.“, where the colloidal size was found to be nearly constant for temperatures up to 167°C and for concentrations up to 5%. This conductivity result qualitatively shows that charge transformation within asphaltene colloids does occur and may be responsible for the asphaltene self-association. In order to further characterize the dielectric dynamics of the colloids, we examined their dielectric loss spectra. Figure 2 shows a typical dielectric loss spectrum for 20% asphaltene in toluene at 50°C as a function of o. The solid line is a Cole-Cole equation fit. The data fitting was reasonable. For most of the samples studied, the quality of the fit was about the same, except in the case of high asphaltene concentrations, or for low temperature samples where a second peak became apparent (see Figure 3a and b). To analyse rigorously the two-peak data, a summation of two Cole-Cole equations is required, which will involve six free parameters. This would likely cause ambiguity, such as non-unique convergence. We thus decided to fit these data using a single ColeeCole equation. 60

ooo

50 w

; s

0

40

0

0

00°

0

aoo 0

Log(w) o in Hz Figure 2 Dielectric loss, E” (o), for 20% asphaltene in toluene at 50°C. The solid line is the Cole-Cole fit. The extracted parameter values are given in Tahk I

a

10 0

Equation (5) was the equation we used for fitting. As, /?, and z were the free parameters.

0 0

O0

0

c@y

3

4

I

1

5 Log(w) w in Hz

6

1

7

6oy-86 RESULTS AND DISCUSSION Before showing the dielectric loss data and its analysis, we first examine the conductivity behaviour. Figure 1 shows the conductivity as a function of the applied field frequency, for the 20% asphaltene case at various temperatures. In the static region (i.e. < 1OOHz) the conductivity remained fairly constant for all cases, but a rise occurred at -1OOOOHz. For o>l.Ox 105Hz, the conductivity becomes more or less temperature independent. The interpretation of these observed phenomena is as follows. At the low frequency, the conductivity measurement probes the larger length scale. The low conductivity indicates that the asphaltene colloids are well dispersed, thus the electric charges do not transfer (or hop) between particles. As the frequency increases to a threshold value corresponding to the length scale of the particle size, the conductivity observed will essentially be the charge movement within a particle. Therefore, the

-u

50

3 40

0

0

0 o”o

s

2

3

4 Log(o)

5 o in Hz

0

6

Figure 3 Conductivity, o, as a function of frequency, w, for (a) 20% asphaltene at 10°C and (b) 50% asphaltene at 30°C. The shoulder on the low frequency wing of the main peak likely results from the effect of caging (see text)

Fuel 1994 Volume 73 Number 1

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Dielectric

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1 Extracted parameter Cole-Cole equation Table

Temperature (“C)

Concentration (WV?/)

10 20 30 30 35 35 50 50 50 60 60 60

20 20 30 50 20 30 20 30 50 20 30 50

solutions:

E. Y. Sheu et al.

values from the E”(O) fit using the

AE

Relaxation time (P)

(l-8)

57.5 57.5 59.9 55.3 58.5 57.0 56.6 55.7 56.8 56.0 56.6 56.2

1.785 1.629 1.512 2.631 1.466 1.435 1.317 1.369 1.879 1.250 1.365 1.749

0.5957 0.6588 0.6770 0.5051 0.7296 0.7196 0.7778 0.7203 0.5795 0.7993 0.7332 0.6208

1

0

.4

10

I 20

I 30 Temperature

1 40

1 50

Cl

60

(“C)

Stretch exponent, (1 -b), as a function of temperature for three concentrations. 0: 20% asphaltene, A: 30% asphaltene, and 0: 50% asphaltene

Figure 4

Table I tabulates the extracted AE,(1 -B), and z values. A.E remained fairly constant as a function of both concentration and temperature. This means that both the static permittivity (i.e. at o=O) and the high frequency permittivity (i.e. o= co) of the material (solvent and asphaltenes) are similar, and are more or less temperature independent. The stretch exponent (1 -p) was found to increase gradually from N 0.5 to - 0.8 (see Figure 4), with increasing temperature. This indicates that the relaxation process evolved toward the Debye relaxation process [( 1 - 8) = l] as temperature increases. Of course, it is hard to predict how high the temperature should be raised, in order to enter the true Debye relaxation region. However, one should expect the system to evolve toward an ideal solution at higher temperatures. Within our measuring temperature range (10 to 6O”C), (1 -p) values were all less than unity. It means that the systems are still very non-ideal, either due to the particle polydispersity or to the interparticle correlation, or to a combination of both. The effect of polydispersity is obviously present”, and it definitely will contribute to the frequency spread. The interparticle correlations, on the other hand, may or may not be present, depending on the asphaltene concentration. When the asphaltene concentration is low, the interparticle distance is long. In this dilute case, if

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the interaction is short-ranged (for most oil continuous systems, the interaction is usually short-ranged14), the interaction may be negligible. However, when the concentration is increased, the interparticle correlations may become significant. For the concentrated cases, we can evaluate the correlation length and assign a hypothetical volume for the correlated particles. If the correlated particles do respond to the applied alternating field, then there will be a collective dielectric loss peak corresponding to the correlated volume. Assuming the correlation volume is not much larger than the particle itself, then the dielectric loss peak will appear near the particle peak, and become a shoulder. As a result, its contribution will be added onto the frequency spread of the particle peak. This is why the frequency spread of the particle peak may contain both particle polydispersity and interparticle correlation information. Figure 5 shows the average relaxation time, z, extracted from the fittings; it decreased as a function of temperature. This is again expected, because the thermal energy should make the relaxation of the particle faster. In Figures 4 and 5, the 50 wt% case exhibits a distinctive difference in both (1-p) and r. We speculate that this is due to the long range correlation effect, as described previously for the appearance of the second peak. This point will be returned to for the conductivity data observed. The second peak occurring at - 105Hz probably results from interparticle correlation, which creates a hypothetical collective particle. The error bars for the measurements are all smaller than the symbols that denote the data, and repeated measurements consistently showed the second peak, which leads to the conclusion that it is a true effect. To the best knowledge of the authors, there are three physical processes that can create such a hypothetical particle. The first process is the clustering of the colloids via flocculation. In this case, the particles are considered to have physical contact, and are essentially forming dispersed floes of larger volume, with the relaxation time corresponding to this volume. The second process is percolation, including static and dynamic percolation phenomena. This process is not as easy to define. Basically, it means that there exist certain physical parameters extending from an intraparticle scale to an interparticle scale (or from a microscopic scale to a macroscopic scale). A common example is conductivity percolation. At the percolation

3’o/ 0 0

A 1.01

I

10

20

B

I

I

I

30

40

50

Temperature Figure 5

ei

(“C)

The same plot as F@re 4 for relaxation time, 5

60

Dielectric 5 Cl 4-

0

3 33-

0

0

3

cl

%2s

0

0

0

A 0

A

lA

O-

10

I 20

I 30

Temperature

I 50

I 40

60

(“C)

Figure 6 Static conductivity, 0, as a function of temperature for 20% asphaltene (a), 30% asphaltene (O), and 50% asphaltene (A)

condition, a non-conducting, well-dispersed water-in-oil microemulsion will exhibit a sudden increase in conductivity of several orders of magnitude’ 5,16.In this case, the physical parameter is the conductivity, and the phenomenon can be called the percolation upon conductivity. On the other hand, a viscosity percolation means that the particles are correlated such that below the percolation threshold, the viscosity follows a certain hydrodynamic behaviour, while above the threshold it follows a distinctively different one. A dynamic percolation means that these above-described phenomena only occur dynamicallyi7. The last process that may result in the formation of hypothetical collective particles in a dielectric measurement, is the direct concentration increase. This concentration increase makes the system behave like a viscous liquid. In fact, it means there are no processes occurring, except shortening of the interparticle distance, which naturally enhances the correlation strength and length. Another possibility is network formation, but it will usually convert the system into a non-dispersed system (such as a bicontinuous structure), which is not the case here. We will come back to this point later. The first and the second processes in our case can be examined simultaneously. They can be achieved by measuring the conductivity as a function of both temperature and concentration. Both processes should not occur, if the conductivity does not show a sudden increase along both temperature and concentration. Figure 6 shows static conductivity as a function of temperature for various concentrations. From this conductivity behaviour, it is clear that the flocculation process should not occur, otherwise the conductivity will exhibit a sudden rise along either the temperature or the concentration axis when the blocs form a macroscopic object. As for the percolation, it is not as trivial. One has to check according to the following equations’*: (6)

where, TV is the conductivity, 4 is the chosen physical axis along which the conductivity percolation occurs. 4, is

properties

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solutions:

E. Y. Sheu et al.

the percolation threshold, and s and p are the critical exponents. s and p should be ~0.7 and 2 1.9, respectively, if the system truly percolates. In Figure 6 we found that conductivity increases as a function of temperature; not with a power law dependence described by Equation (6). Thus, percolation clearly does not occur, at least not for the temperature investigated. As for the concentration dependence, it indeed decreases, which can only occur when the particles are correlated to each other without flocculation or percolation. The network formation is also not possible, because it represents continuity of the particles extending from a microscopic length scale to a macroscopic scale. When it occurs, conductivity should also show a sudden rise along both concentration and temperature axes. Since Figure 6 does not show this behaviour, the networking phenomenon was not considered to occur. Based on conductivity behaviour, it is obvious that the second peak observed is nothing but an increase of the interparticle correlation, which makes the system a viscous liquid at high concentrations (up to 50%) or at low temperatures (- 30°C for 50% asphaltene). Other evidence concerning viscous liquid formation is from interparticle interactions. In general, the interparticle interactions are attractive when flocculation, percolation, or networking can occur. The asphaltene colloids, however, show short range repulsive interparticle potential’ 9; this is also an indication that the high concentration and low temperature cases are more like viscous liquids. Combining the dielectric relaxation data and the conductivity data, our physical picture of the system is that the charges within an asphaltene colloid exchange rapidly between the associated asphaltene molecules, but there is virtually no exchange between colloids (this is why the conductivity rises suddenly at a threshold frequency which corresponds to a length scale comparable to the size of the asphaltene colloids, see Figure I). This gives a good indication that the origin of the self-association is probably due to the charge transformation, as Shiffert et al.’ ’ speculated.

CONCLUSION The dielectric properties of asphaltene solutions, where asphaltenic molecules self-associate into colloidal sized particles, was studied. The dielectric loss, as a function of frequency, exhibited a pronounced peak indicating that the colloidal particles were well dispersed. The relaxation time observed was found to be on the order of microseconds, similar to a typical size colloid. The conductivity measurements indicated that the charges rapidly transfer within the colloids, but not between colloids. This finding suggests that the charge transfer energy may be responsible for the self-association of asphaltene. The Cole-Cole equation was used to analyse the dielectric loss spectra. The results indicate that the relaxation behaviour differs from the Debye relaxation process, because of the particle polydispersity and significant interparticle interactions. Additionally, the colloid was found to be strongly correlated to its neighbouring colloids at high concentrations and at low temperatures. The creation of a second peak on the dielectric spectra, corresponded to the correlated volume.

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REFERENCES Speight, J. G. and Moschopedis, S. E. ‘Chemistry of Asphaltenes’, (Eds J. W. Bunger and N. C. Li), American Chemical Society, Washington, DC, 1981, p. 1 Sheu, E. Y., Storm, D. A. and De Tar, M. M. J. Non-Cryst. Solids 1991, 131-133, 341 Storm, D. A., Decanio, S. J., De Tar, M. M. and Nero, V. P. Fuel 1990,69, 735 Overfield, R. E., Sheu, E. Y., Sinha, S. K. and Liang, K. S. Fuel Sci. Technol. Int. 1989, 7, 611 Sheu, E. Y., De Tar, M. M., Storm, D. A. and DeCanio, S. J. Fuel 1992, 71,299 Pfeiffer, J. P. and Saal, R. N. J. J. Phys. Gem. 1940, 44, 139 Sheu, E. Y., De Tar, M. M. and Storm, D. A. Fuel 1992,71,1277 Anderson, S. I. and Birdi, K. S. J. Colloid Interface Sci. 1991, 142,497 Sheu, E. Y., Liang, K. S. and Chiang, L. Y. J. Phys. (France)

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10 11 12

13 14 15 16 17

1989,50, 1279 Sheu, E. Y., Liang, K. S., Sinha, S. K. and Overfield, R. E. .l. Colloid Interface Sci. 1992, 153, 399 Shiffert, B., Kuczinski, J. and Papirer, E. J. Colloid Interface Sci. 1990, 135, 107 Hirtzel, C. S. and Rajagopalan, R. (Ed.) ‘Colloidal Phenomena: Advanced Topics’, Noyes Publications, Park Ridge, NJ, 1985 Cole, K. S. and Cole, R. H. J. Chem. Phys. 1941, 9, 341 Huang, J. S. J. Chem. Phys. 1985,82,480 Battacharya, S., Stokes, J. P., Kim, M. W. and Huang, J. S. Phys. Rev. Mt. 1985, 55, 1884 Huang, J. S. J. Surface Sci. Technol. 1989, 5, 83 Grest, G. S., Webman, I., Safran, S. A. and Bug, A. L. R. Phys. Rev. A 1986, 33(4), 2842

18 19

Ponton, A., Bose, T. K. and Delbos, G. J. Chem. Phys. 1991,94, 6879 Sheu, E. Y., De Tar, M. M. and Storm, D. A. Fuel Sci. Technol. Int. 1992, 10(4-6), 607