On the Stabilization Mechanism of Water-in-Oil Emulsions in

Our own measurements of interfacial tension, on emulsified micrometer-sized water droplets in Athabasca bitumen dissolved in a 50:50 mixture of heptan...
0 downloads 0 Views 186KB Size
Download PDF
2074

Energy & Fuels 2005, 19, 2074-2079

On the Stabilization Mechanism of Water-in-Oil Emulsions in Petroleum Systems Jan Czarnecki* and Kevin Moran Syncrude Canada, Ltd., Edmonton Research Centre, Alberta, Canada Received May 11, 2005. Revised Manuscript Received July 14, 2005

We present a model explaining the mechanism of water-in-oil (W/O) emulsion stabilization in petroleum systems. According to the model, W/O petroleum emulsions are stabilized by at least two types of chemicals: one is a small subfraction of asphaltenes, and the other is a low-molecularweight, surfactant-like material. Their competition for the oil/water interface is based on adsorption kinetics, rather than on differences in adsorption energies. The asphaltenic material adsorbs slowly and irreversibly and forms rigid skins. Surfactant-like species adsorb fast, reaching equilibrium. Both are effective emulsifiers; however, emulsion breaking requires different strategies, depending on the stabilizer.

Introduction According to a generally accepted paradigm, it is believed that persistent water-in-oil (W/O) emulsions in petroleum industrial systems “are almost exclusively stabilized by asphaltenes” (see, e.g., refs 1-3). Despite the tremendous industrial importance of the subject, the physicochemical mechanism of petroleum emulsions stabilization is not well understood yet. Most of the research conducted on the stability of petroleum emulsions is based on the removal of one or several oil fractions and the determination of how the ability of the oil to form stable emulsions is affected. It is most frequently performed following various modifications of asphaltene precipitation, as in the first step of saturates, aromatics, resins, and asphaltenes (SARA) analysis, where a crude oil is fractionated based on the solubility of the individual components in aliphatic solvents. As an alternative to the aforementioned paradigm, below we present a model of W/O emulsion stabilization in petroleum systems that is based on our experience with Athabasca Oil Sands processing. Figure 1 shows the basic steps in oil sands surface mining and upgrading operation. Bitumen is extracted from mined ores using a modified flotation process, yielding a froth, which typically contains 60% bitumen, 30% water, and 10% solids. Before extracted bitumen can be upgraded to salable liquid fuels or shipped to an off-site refinery, the water and solids must be removed from the froth. This is achieved in an operation called froth treatment. Because bitumen density is almost the same as the * Author to whom correspondence should be addressed. Telephone: (780) 970 6825. Fax: (780) 970 6805. E-mail address: czarnecki.jan@ syncrude.com. (1) Kilpatrick, P. K.; Spiecker, P. M. Asphaltene Emulsions. In Encyclopedic Handbook of Emulsion Technology; Sjoblom, J., Ed.; Marcel Dekker: New York, Basel, 2001; p 707. (2) McLean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 189, 242. (3) Sjoblom, J.; Johnsen, E. E.; Westvik, A.; Ese, M.-H.; Djuve, J.; Auflem, I. H.; Kallevik, H. Demulsifiers in the Oil Industry. In Encyclopedic Handbook of Emulsion Technology; Sjoblom, J., Ed.; Marcel Dekker: New York, Basel, 2001; p 595.

Figure 1. Basic operations in the oil sands industry.

density of water, froth treatment is not possible without the addition of a light hydrocarbon diluent (e.g., naphtha). Diluting the froth with a light hydrocarbon reduces the oil phase density, making separation of water from diluted bitumen possible. Gravity settlers, centrifuges, and/or cyclones are used in all operating plants. The addition of a diluent also reduces the oil phase viscosity, further assisting in the removal of the water and solids. Thus, the addition of a diluent is a crucial operation in oil sand processing. This step makes it distinctly different from most conventional and heavy oil recovery processes. It also provides limited control over the composition of the oil phase. Therefore, it is not surprising that extensive research has been conducted to determine how the nature and amount of the diluent affects the ability of the oil phase to stabilize emulsions. In the mid-1990s, it was discovered that, if a paraffinic diluent was used above a diluent-to-bitumen (D/B) ratio of ∼2, a very clean and dry product could be produced in froth treatment. This was a breakthrough discovery that led to the development of the so-called “paraffinic froth treatment technology”.4 The results of extensive studies on the impact of the amount and nature of the diluent added can be summarized as follows. There is a critical D/B ratio at which many of the system properties change dramatically.5-8 (4) Shelfantook, W. E.; Long, Y. C.; Tipman, R. N. Solvent Process for Bitumen Separation from Oil Sands Froth. Canadian Patent No. CA 2217300; filed Sep. 29, 1997, issued Aug. 20, 2002. (5) Xu, Y.; Dabros, T.; Hamza, H.; Shelfantook, W. E. Pet. Sci. Technol. 1999, 17, 1051.

10.1021/ef0501400 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/23/2005

Water-in-Oil Emulsion Stability

Energy & Fuels, Vol. 19, No. 5, 2005 2075

Table 1. Summary of Emulsion Properties below and above Critical Dilutiona

a

below critical D/B (high bitumen concentration)

above critical D/B (low bitumen concentration)

W/O emulsions are formed easily flexible oil-water interface naphtha-based froth treatment operating regime

oil-water interface is rigid droplets crumple very dry and clean product can be made using paraffinic diluent operating in this regime

The photographs were taken from ref 6.

Some of these changes are listed in Table 1. The critical dilution ratio is dependent on the diluent composition; it increases with increasing diluent aromaticity. It was also found that the critical dilution coincides with the onset of asphaltene precipitation in the system.9 The critical dilution for a paraffinic diluent is ∼1.8-2.5,9 For naphtha used as a diluent in Syncrude’s commercial operation, this critical D/B ratio is ∼4.7 Experiments with individual emulsion droplets manipulated with micropipets showed that, above a critical dilution, the oil/water interface is rigid.6 Below a critical dilution (i.e., at high bitumen concentrations), the oil/water interface is flexible8 and emulsions are formed easily. This is the regime in which the conventional froth treatment process, using naphtha at a D/B ratio of ∼0.6-0.7, operates. Paraffinic froth treatment operates above the point of critical dilution. As the addition of a diluent increases, at critical dilution, the system seems to undergo a sudden transition from one type of behavior to another. It has been found that the composition of material at the oil/water interface also changes abruptly at the critical dilution.10 Such behavior is very difficult to explain, based on thermodynamic considerations, assuming reversible adsorption processes, although several attempts at such interpretation have been made.11,12 Below, we present an alternative model, which assumes competition between a subfraction of asphaltenes and low-molecularweight surfactants; this is a competition based on the irreversibility of asphaltenic material adsorption rather than on differences in the adsorption affinities of the competing species. The Model It is assumed that the surface competition is based on differences in adsorption rate and reversibility, rather than on differences in adsorption energies. According to the model, a small subfraction of asphaltenes, referred to as “asphaltenic surface-active material,” adsorbs on the oil/water interface through a slow, irreversible process. The asphaltenic surface-active material competes for the interface with low-molecular(6) Yeung, A.; Da¸ bros´, T.; Czarnecki, J.; Masliyah, J. Proc. R. Soc. London, A 1999, 445, 3709-3723. (7) Yang, X.; Czarnecki, J. Colloid Surf., A 2002, 211, 213. (8) Da¸ bros´, T.; Yeung, A.; Masliyah, J.; Czarnecki, J. J. Colloid Interface Sci. 1999, 210, 222. (9) Shelfantook, W. E. Can. J. Chem. Eng. 2004, 82, 704. (10) Wu, X. Energy Fuels 2003, 17, 179. (11) Wu, X.; Czarnecki, J. Energy Fuels 2005, 19, 1353-1359. (12) Horva´th-Szabo´, G.; Masliyah, J. H.; Elliott, J.; Yarranton, H.; Czarnecki, J. J. Colloid Interface Sci. 2005, 283, 5-17.

weight, surfactant-like material, which is assumed to adsorb quickly, reaching an equilibrium state. (See Figure 2.) It is further assumed that the affinity of the asphaltenic surface-active material to the oil/water interface (or its adsorption free energy) is lower than that of surfactant-like material.3 Asphaltenic surface-active material adsorption is only possible when the other competing substance or substances (i.e., surfactant-like material, which is adsorbing through a reversible process reaching equilibrium) is present in relatively low quantities, such that the interface is not fully covered with surfactant-like material. Thus, at high dilutions (or at low concentrations) of all competing chemicals, the oil/water interface, initially, is only partially covered with surfactant-like material. Asphaltenic surface-active material molecules can get attached to those parts of the oil/water interface that are free of surfactant-like molecules. They are likely forming aggregates with other asphaltenic surfaceactive material molecules at the interface, interacting with each other through π-electron and hydrogen bonding. This process is slow and irreversible, yielding a thick, rigid layer, which, in extreme cases, forms a separate phase, visible as a rigid, crumpling skin. As the predominantly asphaltenic skin grows larger, occupying an increasing fraction of the oil/water interface, the surfactant-like chemicals, which obey equilibrium, are pushed out of the interface. After a sufficiently long time, the interface would be completely dominated by asphaltenic materials, despite their lower affinity to the interface. In short, irreversibly adsorbing material displaces material that adsorbs reversibly, reaching equilibrium. At low dilutions (or high concentrations) of both competing materials, the interface is totally covered by

Figure 2. Schematic of the model depicting the competition between subfractions of asphaltenes and low-molecular-weight surfactants at the oil/water interface.

2076

Energy & Fuels, Vol. 19, No. 5, 2005

the reversibly adsorbing surfactant-like material. Under such conditions, asphaltenic material, which has a lower affinity to the interface, cannot adsorb at the interface, because it would require displacing surfactant-like molecules forming full coverage. This cannot happen due to the difference in their adsorption energies. Therefore, not being able to gain a hold on the interface, asphaltenic surface-active material molecules cannot bond with each other and the interface is completely dominated by reversibly adsorbed surfactant-like species. Such interface would remain flexible, showing no visible accumulation of a rigid skin. The model can explain all basic observations, including formation of visible, crumpling skins at high dilutions (i.e., above critical dilution) and flexible interfaces below the critical dilution, with corresponding changes in the composition of surface material. The proposed model is based on three major assumptions: (i) irreversibility of asphaltenic material adsorption, (ii) blockage of the interface with surfactant-like molecules, and (iii) a drastic change in surface material composition at approximately critical dilution. Let us now review experimental evidence in support of these three assumptions.

Czarnecki and Moran

Figure 3. Creeping changes in interfacial tension for Athabasca bitumen solutions in Heptol against water.

Irreversibility of Asphaltenes Adsorption. Irreversible adsorption is not a new phenomenon. The irreversible adsorption of proteins was studied by Langmuir and Waugh.13 They reported the formation of rigid, crumpling skins on the water/air interface that are very similar to the skins we have observed in petroleum systems. The skin-forming proteins were transformed to a new, denatured form, in which some of their original properties, especially their water solubility and biological activity, were diminished or eliminated. Langevin and co-workers14 studied the adsorption kinetics of asphaltenes at liquid interfaces, finding them similar to that of proteins, including the formation of rigid skins over a very long period of time. Their results implied that a “true equilibrium” might never be reached in petroleum systems. Radke et al. have studied the irreversible adsorption of asphaltenes in the context of reservoir wettability.15 Using the oscillating pendant drop method, Freer and Radke studied the relaxation mechanisms of asphaltenes that are adsorbed at the toluene/water interface.16 Their surface rheology data fitted a combination of diffusion and surface rearrangement model. After the interface was washed with toluene, the oil-phase diffusion component of the frequency response disappeared and the relaxation time of the adsorbed film increased by an order of magnitude. The conclusion was that “most of the surface-active asphaltenic molecules are irreversibly adsorbed from the oil phase”.15 Irreversible behavior can be observed by dynamically tracking the tension at an interface. Our own measure-

ments of interfacial tension, on emulsified micrometersized water droplets in Athabasca bitumen dissolved in a 50:50 mixture of heptane and toluene (or Heptol), showed slow creeping of the interfacial tension for weeks in highly diluted systems (Figure 3). Similar behavior has been noted for the adsorption of protein macromolecules17 and asphaltenes14 at water/oil interfaces. It must be emphasized that such a tension reduction in the highly diluted system is not simply a diffusioncontrolled phenomenon. In a conservative estimate, utilizing the Einstein-Smoluchowski equation, diffusion-controlled processes may dominate for ∼10 hours (and likely for a much shorter duration), whereas the creeping of the interfacial tension was observed for days, if not weeks. For a higher bitumen concentration (0.1%), which is still below the critical value, a practically constant interfacial tension was observed over the same time period. However, note that the final interfacial value was not dependent on the bitumen concentration. These results, which are shown in Figure 3, are in agreement with the notion of irreversible adsorption of asphaltenes. In the aforementioned work, interfacial tension data were obtained via a suction pressure technique using micropipets.6,18 As such, individual emulsion droplets are aspirated up a near-micrometersized capillary tube. The pressure required to aspirate the droplet is equated to its geometry through the Young-Laplace relation. In these experiments, the interfaces were aged by simply storing an emulsion for a given period of time in a sealed container. After appropriate aging, a sub-sample of the emulsion was obtained and the droplets contained within were used for measurement. Further evidence is provided by Langmuir trough studies by Masliyah et al.19 They deposited a monolayer of asphaltenes on water spreading a small amount of asphaltene solution in toluene. After evaporation of the solvent (toluene), they carefully poured a layer of clean toluene on top of the water that covered the asphaltene monolayer. Several compression runs revealed the existence of a monolayer. This is a surprising finding as the monolayer-forming material was deposited from the toluene solution; thus, prior to formation of the

(13) Langmuir, I.; Waugh, D. F. J. Gen. Physiol. 1938, 21, 75. (14) Jeribi, M.; Almir-Assad, B.; Langevin, D.; Henaut, I.; Argillier, J. F. J. Colloid Interface Sci. 2002, 256, 268. (15) Freer, E. M.; Svitova, T.; Radke, C. J. Pet. Sci. Eng., J. 2003, 39, 137. (16) Freer, E. M.; Radke, C. J. J. Adhes. 2004, 80, 481.

(17) Beverung, C. J.; Radke, C. J.; Blanch, H. W. Biophys. Chem. 1998, 70, 121-132. (18) Moran, K.; Yeung, A.; Masliyah, J. Can. J. Chem. Eng. 2000, 78, 625-634. (19) Zhang, L. Y.; Lopetinsky, R.; Xu, Z.; Masliyah, J. Energy Fuels 2005, 19, 1330-1336.

Assumptions upon Which the Proposed Model is Based

Water-in-Oil Emulsion Stability

Energy & Fuels, Vol. 19, No. 5, 2005 2077

monolayer, it was toluene-soluble. Even after replacing the top layer of toluene with a fresh one, the asphaltene monolayer was not removed or affected. In other words, after an asphaltene monolayer is formed on water, it cannot be removed with toluene, despite the fact that the initial material from which the monolayer was formed was soluble in toluene. Obviously, the material was transformed to a new form, making the adsorption of asphaltenes irreversible. Three additional comments are worthy of mention. First, water is likely a reagent in the complex series of reactions that are occurring at the interface to form the skin. This is supported by Fourier transform infrared (FTIR) spectra of the skin picked up from the water/ toluene interface, using the Langmuir-Blodgett technique.19 Also, the process of skin formation was much faster at the oil/water interface than at the oil/air interface,14 possibly because of the limited supply of water from the air phase. Second, as was shown by the “washing” experiments of Dabros et al.,5 only a small fraction of the total asphaltenes is involved in formation of this skinlike layer. Dabros et al. made a series of W/O emulsions, using increasing amounts of water blended into diluted bitumen. These emulsions were centrifuged at 10 000 g, the bottom cake was discarded, and a constant amount of water (10%) was then emulsified into the supernatant. This secondary emulsion was centrifuged at 260 g, and the amount of water released was measured. Because, in the first centrifugation, the material capable of emulsion stabilization was removed with the cake, the stability of the second emulsion was used to estimate the amount of emulsion-stabilizing agent in the oil. Making conservative assumptions in all calculations steps, Dabros et al. estimated that no more than 2% of the total bitumen could be held responsible for the oil emulsion stabilization ability.5,21 Yet, the Athabasca bitumen contains 17%-20% asphaltenes, leading to the obvious conclusion that only a small subfraction of asphaltenes can be considered to be a “bad actor”. Third, the behavior of asphaltenic surface active material at the water/oil interface is obviously linked to the asphaltenes dispersion state in the bulk phase. As stated previously, the critical dilution coincides with the onset of asphaltene precipitation. The formation of a rigid “skin” by asphaltenic surface-active material (which obviously involves some type of two-dimensional aggregation) is observed above the point of critical dilution (i.e., at concentrations below the precipitation onset). Whether the mechanism of the skin formation is similar to that involved in asphaltenes “nanoaggregation” at high dilutions, as recently reported by Mullins and co-workers,20 remains an open and intriguing question. Blocking of Asphaltene Adsorption. If the mechanism described in “The Model” section mentioned previously is correct, the asphaltenic rigid skin cannot be formed if the oil/water interface is completely covered with low-molecular-weight, highly surface-active mate-

rial. We have performed such experiments in which sodium naphthenate (SN) served as a surfactant. The choice of SN was dictated by the fact that it is a naturally occurring surfactant in petroleum systems, it is highly surface active, and we have performed extensive studies on its phase behavior and emulsion stabilization ability.22,23 Practical-grade SN from Eastman Kodak was used in previous and current studies without purification. In attempts to “quantify” the nature of an interface, the tension at the water droplet surfaces was evaluated. Tension measurements were performed using the maximum droplet pressure technique with micropipets.18 Figure 4 shows interfacial tensions of SN solutions against Heptol (open circles) and 0.1 wt % solution of bitumen in Heptol (solid triangles). Surface tensions of aqueous SN solutions are also shown there (solid circles) to illustrate that the critical micelle concentration (CMC) of the surfactant is observed at a concentration of ∼1 wt %. Furthermore, as the concentration of SN is increased, the diluted bitumen-naphthenate solution tension isotherm maps onto that given by the pure fluids-naphthenates solution. Such a result suggests that the surfactant dominates the droplet surface. Here, it is sufficient to note that if the water phase had an SN concentration of 1 wt % or more, skin formation was prevented. Because, at CMC, full surface coverage with the surfactant is achieved, it is expected that the adsorption of asphaltenic material and its subsequent surface aggregation will be prevented. Without asphaltenic material, the interface is expected to be flexible. Indeed, this is exactly what was observed (cf. Figure 5), in full agreement with our model. Without the addition of SN to water, emulsified water droplets in 0.1% solution of bitumen (diluted in Heptol) crumpled at deflation, as our earlier experiments showed.6 (Also see the photo in the right column of Table 1.) Figure 5 shows the photographs of deflating water droplets in 0.1 wt % bitumen in Heptol below (Figure 5a) and above (Figure 5b) the SN CMC. Composition of the Adsorbed Material. Wu developed a technique to allow collection of the material

(20) Gaelle, A.; Bostrom, N.; Mullins, O. C. Langmuir 2005, 21, 2728. (21) Czarnecki, J. Water-in-Oil Emulsions in Recovery of Bitumen from Oil Sands. In Encyclopedic Handbook of Emulsion Technology; Sjoblom, J., Ed.; Marcel Dekker: New York, Basel, 2001; p 497.

(22) Horva´th-Sza´bo, G.; Masliyah, J.; Czarnecki, J. J. Colloid Interface Sci. 2001, 242, 247. (23) Horva´th-Szabo´, G.; Masliyah, J.; Czarnecki, J. J. Colloid Interface Sci. 2003, 257, 299.

Figure 4. Blocking of the oil/water interface with sodium naphthenate (SN).

2078

Energy & Fuels, Vol. 19, No. 5, 2005

Czarnecki and Moran

Figure 5. (a) Crumpling interface below the critical micelle concentration (CMC). (b) Flexible interface above the CMC. Figure 7. Impact of the order of reagent addition on emulsion behavior. The amount of resolved water (as a measure of emulsion stability) as a function of diluent (naphtha) to bitumen ratio. (Data for the figure are taken from ref 24.)

Kinetics Rules

Figure 6. Hydrogen-to-carbon (H/C) ratio for interfacial material collected from emulsion droplets at different bitumen concentrations in the oil phase. (Data are taken from ref 10.)

adsorbed on emulsified water droplets with little contamination from the bulk phases in quantities sufficient for chemical analysis.10 In short, his experimental protocol was as follows: a heavy water (D2O) emulsion in bitumen diluted with Heptol was carefully placed on top of an ordinary water layer in a centrifuge cell and then centrifuged. Heavy water emulsion droplets, which had a density greater than that of water, were breaking through the oil/water interface in the cell and formed a cake in ordinary water at the bottom of the cell. The cake was then collected, dried, and analyzed. Figure 6 shows the hydrogen-to-carbon (H/C) ratio for the materials collected at various bitumen concentrations in diluted bitumen. At low bitumen concentrations, or above the point of critical dilution, the H/C ratio for the surface material is ∼1.13-1.16, which is, more or less, equal to the H/C ratio for Athabasca asphaltenes (∼1.16). At higher bitumen concentrations, or below the point of critical dilution, the H/C ratio for the surface material is much higher (∼1.3-1.32), which resembles the H/C ratio of resins rather than asphaltenes. It is worth noting that the low H/C ratio corresponds to a bitumen concentration range at which the oil/water interface is rigid and small droplets crumpled while deflating in micropipet experiments, whereas the high H/C values are observed in the region where the interface was flexible.6,8,21 Again, these observations are in full agreement with the proposed model.

If the competition for interfaces in petroleum systems is based on differences in adsorption rate and reversibility, rather than on differences in adsorption energies, the order in which reagents are added to a sample being studied can have a significant impact on measured properties. However, if the system follows equilibrium thermodynamics, the order of reagent addition is absolutely irrelevant. The system is expected to reach an equilibrium state in which measured properties are dependent on the component concentrations only and not on the order in which they were added. In this context, it is justified to quote our earlier emulsion stability measurements.24 Usually, when studying the stability of water in a diluted bitumen emulsion, bitumen is first diluted with a suitable solvent, and then water is added to the already diluted bitumen; the system then is agitated to make an emulsion and observations are made. For instance, in the experiments shown in Figure 7, the percentage of water released after a given time is recorded as a function of bitumen concentration in the oil phase. The same experiment was repeated adding water to undiluted bitumen, followed by addition of the solvent (in this case, naphtha), followed by agitation and measurement of the amount of water released. As Figure 7 shows, the two protocols yielded completely different results. The implications are 2-fold. First, if the objective of the experiment is to gain industry-relevant conclusions, it is extremely important to make the experimental protocol as similar to the industrial conditions as possible. Second, the difference in the results shown in Figure 7 provides further evidence that the formation of petroleum emulsions involves nonequilibrium, irreversible processes, the nature of which are not fully understood yet. Concluding Remarks We do not have absolute proof that our model is correct, nor do we intend to claim it is. Yet, the current paradigm that indicates that water-in-oil (W/O) emulsions are stabilized by asphaltenes seems to be such a (24) Yang, X.; Czarnecki, J. Colloids Surf. A 2002, 211, 213.

Water-in-Oil Emulsion Stability

gross oversimplification that we offer this model as an alternative, in hopes of provoking further discussion on this important subject. As Dabros et al.’s washing experiments show,5 only a small fraction of the total asphaltenes is involved in emulsion stabilization. Asphaltenes are a solubility class that consists of a large number of different chemicals, only some of which (and only under certain conditions) are involved in emulsion stabilization. Therefore, the use of SARA terminology (including the terms such as asphaltenes, resins, and so on) is impeding the progress in understanding emulsion stabilization mechanisms in petroleum systems. Instead, we should try to identify the molecules, which are really responsible for emulsion stabilization. This is being made possible because of the recent progress in the chemical characterization of asphaltenes and other petroleum fractions.25 Highresolution mass spectroscopy (petroleomics),26,27 in combination with Wu’s surface material collection method,10 seems to be the most promising avenue to follow. The existence of a point of critical dilution shows that at least two different emulsion stabilization mechanisms are involved: those that dominate above and below the point of critical dilution. The chemicals responsible for stabilization above and below the point of critical dilution are different. They compete for the oil/water interface based on adsorption kinetics and reversibility, rather than on equilibrium surface activity. This is expected to be valid for conventional petroleum systems, in which no diluent is added. The natural composition of a crude may correspond to one of the two states discussed previously. (25) Buenostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972. (26) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 4145. (27) Rodgers, R. P.; Shaub, T. M.; Marshall, A. G. Anal. Chem. 2005, 77, 20A.

Energy & Fuels, Vol. 19, No. 5, 2005 2079

Our model does not consider the role of fine solids in emulsion stabilization. Although solids undoubtedly have an important role in petroleum emulsion stabilization,28 in our opinion, they do not seem to be the dominant factor. In a single unpublished experiment, Dabros29 determined that the emulsion stabilization ability of a diluted bitumen sample had not been visibly affected by ultracentrifugation at >200 000 g for 48 h, which should have removed all solids larger than ∼1 nm. The density of clay was assumed for the solids to make this estimation. Finally, we would like to stress again the importance of characterizing the asphaltenic materials that are involved in forming the rigid skins observed. How do the molecules react with each other and possibly with water to form the skins? What are the chemicals that cover the flexible water/oil interfaces? Moreover, what is the linkage of the surface phenomena discussed in this paper to the bulk properties of the oil, especially to the dispersion state of asphaltenes, and to the observed importance of the asphaltene-to-resins ratio in emulsion stabilization? There is much work yet to be done. Acknowledgment. We would like to thank all our colleagues and friends, especially Tad Da¸ bros´, Geza Horva´th-Szabo´, Jacob Masliyah, Yicheng Long, Bill Shelfantook, Alex Wu, Zhenghe Xu, and Xiaoli Yang for helpful discussions; their data have been used to illustrate the model presented. We would also like to thank the reviewers of this manuscript; their positive comments contributed to the final version of the paper. We regret that there is no mechanism in place to personalize our appreciation. EF0501400 (28) Sztukowski, D. M.; Yarranton, H. W. J. Dispersion Sci. Technol. 2004, 25, 299. (29) Dabros, T. Private communication.