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2 Journal of Colloid and Interface Science 310 (2007) Abstract Effect of interfacial rheology on model emulsion coalescence I. Interfacial rheology H.W. Yarranton, D.M. Sztukowski, P. Urrutia Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, T2N 1N4, Canada Received 20 October 2006; accepted 17 January 2007 Available online 2 February 2007 Interfacial elasticity and dynamic surface pressure isotherms were measured for interfaces between a dispersed water phase and a continuous phase of asphaltenes, toluene, and heptane. The interfacial modulus is a function of asphaltene concentration and in all cases reached a maximum at an asphaltene concentration of approximately 1 kg/m 3. The modulus increased significantly as the interface aged and slightly as the heptane content increased to a practical limit of 50 vol%. The modulus was approximately the same at 23 and 60 C. The modulus correlated with the inverse of the initial compressibility determined from surface pressure isotherms. The surface pressure isotherms also indicated that a phase transition occurred as the interface was compressed leading to the formation of low compressibility films. Crumpling was observed upon further compression. The phase transition shifted to a higher film ratio with an increase in heptane content and interface age. Asphaltene concentration and temperature (23 and 60 C) has little effect on the surface pressure isotherms. The surface pressure and elasticity measurements are consistent with the gradual formation of a cross-linked asphaltene network on the interface Elsevier Inc. All rights reserved. Keywords: Asphaltenes; Emulsions; Interfacial rheology; Elasticity; Interfacial films; Emulsion stability 1. Introduction Asphaltenes are known to contribute to the stability of oilfield water-in-oil emulsions [1 3]. However, the mechanism by which they stabilize emulsions is not well established and therefore optimum treatment methods are not always identified. Emulsions are more stable when settling, flocculation, or coalescence is inhibited. With water-in-oil emulsions, the electrostatic forces are usually weak because the continuous oil phase has a low dielectric constant and a correspondingly low ion concentration. Hence, the water droplets usually do not remain dispersed but are free to flocculate and/or settle. The stability of oilfield emulsions is believed to arise in part from a steric barrier to coalescence attributed to adsorbed asphaltenes at the interface. * Corresponding author. address: hyarrant@ucalgary.ca (H.W. Yarranton). An example of a steric barrier is a polymer-coated interface. When two polymer coated water droplets collide, the polymers begin to entangle at distances greater than 10 nm from the water surface [4]. Theentanglementpreventsclosecontactand subsequent coalescence. However, asphaltene interfacial films appear to be in the order of 2 10 nm thick [3,5]; too small to provide a traditional steric barrier. Instead, it has been speculated that asphaltenes cross-link on the interface to form a highly elastic or rigid film and it is the rigidity of the film that leads to high emulsion stability [2]. Note that a rigid interface has high elasticity (high surface energy provides a strong driving force to restore deformation). Elastic asphaltene films have been observed and their visco-elastic properties quantified using the Langmuir film balance technique [6 8], shear viscometry [9 12], and recently oscillatory drop shape analysis [13 17]. Asphaltenes likely create an elastic film because they adsorb irreversibly on water/hydrocarbon interfaces [13]. Since the asphaltenes are trapped on the interface, an asphaltene film will resist compression and deformation. Surface pressure isotherms /$ see front matter 2007 Elsevier Inc. All rights reserved. doi: /j.jcis

3 H.W. Yarranton et al. / Journal of Colloid and Interface Science 310 (2007) confirm that the compressibility of these films decreases dramatically when the area of the film is decreased [7,11,18]. Buckling of totally rigid asphaltene films has been observed upon sufficient compression in asphaltene solvent systems [8] and diluted bitumen systems [19]. To understand how a rigid film can inhibit coalescence, consider the steps in coalescence [4]. As two droplets approach each other, their surfaces dimple to form a planar region at the point of closest contact. Fluid drains from this region as the droplets continue to approach. As well, the surface active material on the interface is spread more thinly when the surface deforms creating openings for water to bridge the gap between the droplets. When the gap is bridged, the droplets rapidly merge. A highly elastic film will resist deformation. As well, there will be a greater driving force for surfactant to diffuse to the depleted region between the droplets. Both factors will inhibit coalescence. As well, a decrease in film compressibility could resist the reduction in surface area that occurs upon coalescence inhibiting further coalescence. The purpose of this work is to determine if and how emulsion stability correlates to interfacial rheological properties. In Part I, rheological properties are examined, including the elastic and viscous moduli of the interface as well as surface pressure isotherms of interfacial films (a measure of the compressibility of the interface). The experiments are performed on model systems of water, asphaltenes, toluene, and heptane at 23 and 60 C. The effect of interface age is also studied over intervals of up to 24 h. 2. Experimental methods 2.1. Materials Athabasca coker-feed bitumen was obtained from Syncrude Canada Ltd. and been treated to remove most of the sand and water in preparation for upgrading. Reagant-grade n-heptane and toluene were purchased from Van Waters & Rogers Ltd. Reverse osmosis water was supplied by the University of Calgary water plant. Asphaltenes were precipitated from the bitumen and separated from any non-asphaltene solids using a previously established procedure [5,20]. Note that two batches of Athabasca bitumen were required to complete the experiments, as summarized in Table 1. Bitumen 1 was used for the elasticity measurements and Bitumen 2 was used for the surface pressure isotherms. Table 1 Asphaltenes and solids content of Athabasca bitumen Component Mass percent Bitumen 1 Bitumen 2 Asphaltene solids 15.1 a 17.0 a Asphaltenes (fraction of asphaltene solids) Solids (fraction of asphaltene solids) Solids (fraction of bitumen) a Asphaltene yield from bitumen (mass fraction) Interfacial elasticity measurements Interfacial elasticity, ε, is defined as follows: ε = dγ (1) dlna, where γ is the interfacial tension and A is the interfacial area. Elasticity has both a real and imaginary component defined for an oscillating area as follows: ε = ε d + iωη d, where ε d is the dilational elasticity, η d is the interfacial viscosity, and ω is the frequency of the oscillations. The dilational elasticity and interfacial viscosity can also be expressed in terms of an elastic modulus, ε, and a phase angle, φ,asfollows: ε d = ε cos φ and ωη d = ε sin φ. Interfacial tension and elasticity were measured using an IT Concept Drop Shape Analyzer. A mixture of Athabasca Bitumen 1 asphaltenes and solvent was prepared beforehand and loaded into a syringe. The mixture was injected through a U-shaped needle into an optical glass cuvette containing water. The profile of the droplet was captured using a CCD camera and analyzed using a video image profile digitizer board connected to a personal computer. Interfacial tension was determined from static measurements of the drop shape. To measure the elasticity, the volume of the drop was manipulated so that the area of the droplet oscillated sinusoidally. The interfacial tension and droplet surface area were both measured and the elasticity was calculated using Fourier analysis. When the experiments were performed at non-ambient temperatures, the cuvette was placed in a jacket with windows between the light source and the camera. The temperature of the jacket was controlled with a water bath to ±0.5 C. Elasticity measurements were taken after various intervals from 10 min to 24 h but most measurements were made after 4 h. To obtain an elasticity measurement, the drop size was oscillated periodically rather than continuously. The duration of each set of oscillations ranged from 20 to 500 s and the interval between sets of oscillations was 15 min. More details are provided in the supporting material Dynamic surface pressure isotherms A typical surface pressure isotherm is a plot of the equilibrium surface pressure (the pure solvent water interfacial tension less the measured interfacial tension) versus the average area a molecule occupies on the interface. In this work, the interfacial tension and the surface area of the drop were measured as fluid was withdrawn from the drop in steps. A dynamic or non-equilibrium surface pressure was calculated and plotted versus film ratio, where the film ratio is the ratio of the current (2) (3) (4)

4 248 H.W. Yarranton et al. / Journal of Colloid and Interface Science 310 (2007) Fig. 1. Surface pressure isotherms of asphaltene films (a) data for 1 kg/m 3 asphaltenes in 50/50 heptol surrounded by water at 23 C, plotted versus film ratio; (b) comparison with similar Langmuir trough experiments [8], plotted versus area per molecule. surface area to the initial surface area of the droplet (A/A 0 ). This dynamic surface pressure is usually not a thermodynamic property but is a measure of the time dependent film properties and may be more relevant to emulsion stability over finite times. An equilibrium surface pressure can be obtained if sufficient time is allowed between steps. To perform an experiment, a hydrocarbon droplet consisting of Athabasca Bitumen 2 asphaltenes, toluene and heptane was formed at the tip of a capillary and aged in the surrounding water for a given time. The initial drop diameter was approximately 1.5 mm. The drop was retracted stepwise into the capillary by reversing the direction of the drive motor of the Drop Shape Analyzer apparatus. After each step, the interface was allowed to stabilize for an interval of 0 or 5 min, and then the interfacial tension and droplet surface area were measured. In most cases, the experiment ended when the film crumpled upon further compression. Asphaltene concentrations of 1, 5, and 10 kg/m 3 and aging times varying from 1 min to 4 h were considered. A typical surface pressure isotherm is shown in Fig. 1a. To confirm the validity of the technique, this surface pressure isotherm was compared with Langmuir trough experiments conducted by Zhang et al. [8] also using Athabasca asphaltenes. The measured droplet surface areas were converted to an area per molecule as follows. The area per molecule for an undisturbed drop in a given solvent at 23 C was determined from the Gibbs adsorption isotherm: RT A n =, (5) dγ/dlnc A where A n is the surface area per molecule on the interface, R is the universal gas constant, T is temperature, and C A is the asphaltene molar concentration. The asphaltene concentration was converted from mass to molar concentration using measured asphaltene molar masses, as described by [5]. The calculated area per molecule was assumed to apply at the initial condition of the surface pressure isotherm; hence, the area at any film ratio is given by A n (A/A 0 ). The results compare well with Zhang s, as shown in Fig. 1b. 3. Results and discussion The experiments were performed using solutions of asphaltenes in heptane and toluene. For convenience, a solution of X vol% heptane and Y vol% toluene is denoted as X/Y heptol. Heptane contents greater than 50 vol% were not considered because asphaltenes would precipitate Interfacial elasticity at 23 C To relate interfacial rheology to emulsion stability, the effects of asphaltene concentration, solvent composition, and temperature are of interest. However, when interfacial elasticity is measured with the oscillating drop method, the measured elasticity also depends on the frequency of the oscillations. For example, consider the expansion of the droplet at low and high frequency. At low frequency, the freshly created expanding interface is exposed to the bulk phase for a longer time than during a high frequency oscillation. Surface-active components can diffuse from the bulk phase or from within the interface and thereby reduce the interfacial tension and the measured elasticity. At a sufficiently high frequency, the diffusion is negligible and the intrinsic or instantaneous elasticity is measured. Fig. 2a shows the effect of frequency on the measured interfacial elasticity of asphaltenes in toluene over a range of asphaltene concentrations at 23 C. As expected, at each concentration, the measured elasticity increases with frequency and approaches a plateau, the instantaneous elasticity. The trends in elasticity with concentration are approximately the same at any frequency, as shown in Fig. 2b, and therefore, all further measurements were made at one arbitrary frequency, Hz. Fig. 3 shows the effect of asphaltene concentration on the total and viscous moduli for hydrocarbon phases of asphaltenes in 50/50 heptol, 25/75 heptol, and pure toluene (0/100 heptol) at 23 C after 4 h of aging. At low asphaltene concentrations, the interfaces are completely elastic. At higher concentrations, a viscous modulus appears as diffusion begins to affect the measured elasticity. The elastic and total modulus decrease at higher concentrations as diffusion increasingly affects the measure-

5 H.W. Yarranton et al. / Journal of Colloid and Interface Science 310 (2007) Fig. 2. Effect of oscillation frequency (a) and concentration (b) on the total modulus of the interface between water and solutions of asphaltenes and toluene after 4 h of aging at 23 C. The open symbols in (b) are the reciprocal of the phase 1 compressibilities from surface pressure isotherms. Fig. 3. Effect of asphaltene concentration on the elastic (closed symbols) and viscous (open symbols) moduli of the interface between water and solutions of asphaltenes and toluene, 25/75 heptol, and 50/50 heptol after 10 min of aging at 23 C and an oscillation frequency of Hz. ments. Note that the emulsions considered in this work were prepared at asphaltene concentrations from 2 to 20 kg/m 3.In this region, the elastic and viscous moduli both decrease with concentration. Fig. 3 also shows that the viscous and elastic moduli both increase as the heptane content increases. An increase in the total modulus as the heptane fraction in heptol increases is consistent with the observations of [9]. It is speculated that the asphaltenes cross-link on the interface and form a more rigid film more readily in a poor solvent. Fig. 4 shows the effect of time on the elastic and viscous moduli for asphaltenes in toluene at 23 C. At most concentrations, the elastic modulus increases as the interface ages. The increase is most significant at concentrations between approximately 0.01 and 1 kg/m 3. At low concentrations, the elasticity is too low to register any changes with aging. At concentrations above 20 kg/m 3, it appears that diffusion reduces the measured elasticity to the point where no aging effects register. Interfacial Fig. 4. Effect of aging time on the elastic (closed symbols) and viscous (open symbols) moduli of the interface between water and solutions of asphaltenes and toluene at 23 C. aging appears to have little impact on the viscous modulus. The total modulus at different aging times was measured for typical asphaltene concentrations in the model emulsions (5, 10, and 20 kg/m 3 ) in toluene, 25/75 heptol, and 50/50 heptol. Data are provided in the supporting information. The elasticity at very low aging times was modeled with the Lucassen van den Tempel (LDVT) model, a surface EOS with bulk-to-interface diffusion effects [21]. The model successfully accounted for the effects of concentration and oscillation frequency. However, the model failed at higher aging times as would be expected if the asphaltenes form a rigid film on the interface over time. The mechanical strength of this film results in a higher interfacial elasticity than predicted by the model. However, it is not clear why this mechanical effect does not alter the viscous modulus Surface pressure isotherms at 23 C Fig. 5 shows two surface pressure isotherms measured with 10 kg/m 3 asphaltenes in 25/75 heptol, the first with 10 min

6 250 H.W. Yarranton et al. / Journal of Colloid and Interface Science 310 (2007) Fig. 5. Detection of low compressibility film formation for 10 kg/m 3 of asphaltenes in 25/75 heptol after 60 min of aging at 23 C and with 30 s intervals between compression steps on (a) Cartesian and (b) semilog coordinates. of aging before compression and the second after 60 min of aging. At 10 min of aging time, the film can be compressed until the droplet becomes very small and the measurement becomes invalid (high scatter region). At 60 min of aging time, the film experiences a change in compressibility (an apparent phase change) at a film ratio of approximately The film crumples at a film ratio of It appears that at low aging time, the film is reversible or nearly reversible but that at higher aging times, at least some of the asphaltenes are irreversibly adsorbed. [8,13,19] also observed irreversible adsorption of asphaltenes in toluene solutions. At either aging time, the interface is highly compressible at high film ratios. The compressibility of the interfacial film can be expressed analogously to bulk compressibility as follows: c I = 1 ( ) ( ) da dlna =, (6) A dπ T dπ T where c I is the compressibility of the interfacial film. The initial (phase 1) compressibilities of these asphaltene in 25/75 heptol films were determined from the slopes in Fig. 5b and are 0.49 (mn/m) 1 after 10 min of aging to 0.30 (mn/m) 1 after 60 min of aging. Note that this apparent compressibility is not a true thermodynamic property because the number of molecules on the interface is not fixed. In other words, asphaltenes may be free to leave the interface upon compression. A thermodynamically valid compressibility can be measured only when all of the asphaltenes are irreversibly adsorbed. However, the apparent compressibility may be a more useful measure for emulsion stability studies because asphaltenes are not necessarily bound to the interface in an emulsion; some may be reversibly adsorbed and some may be reversibly associated in an asphaltene aggregate only part of which is adsorbed on the interface. Also note that the apparent interfacial compressibility is dimensionally the reciprocal of elasticity. The inverse compressibilities are also plotted in Fig. 2b for the sake of comparison and are similar to total moduli measured at Hz. Fig. 6 shows that the total modulus correlates reasonably well with the inverse compressibility. At low aging time, the inverse compressibility from a surface pressure isotherm is in general Fig. 6. Correlation between total modulus and inverse film compressibility for 1, 10, and 20 kg/m 3 asphaltenes in toluene, 25/75 heptol, and 50/50 heptol at 23 C (see supporting information for more details). somewhat lower than the total modulus probably because there is more time for diffusion to take place; it is equivalent to an oscillating droplet measurement at a lower oscillation frequency. The higher scatter at 240 min of aging may be caused by differences in cross-linked film formation in oscillating and continuously compressed systems. Part of the remaining scatter in the correlation may be attributed to differences in the asphaltene samples used for each type of experiment. Recall that the elasticity measurements were performed on asphaltenes from Bitumen 1 while the compressibility experiments were performed on asphaltenes from Bitumen 2. Overall, the total modulus and the inverse compressibility are reasonably well correlated; hence, the surface pressure isotherm data alone appears to be sufficient to provide asphaltene film properties. The additional information that the surface pressure isotherm provides is the change of compressibility upon contraction. The compressibility change is of interest because a reduction in compressibility may prevent coalescence and the destabilization of an emulsion. For example, for a film that was aged for

7 H.W. Yarranton et al. / Journal of Colloid and Interface Science 310 (2007) Fig. 7. Effect of asphaltene concentration on surface pressure isotherms in 25/75 heptol after 60 min of aging at 23 C and with 30 s intervals between compression steps. 60 min, the film compressibility decreased from 0.30 m/mn in phase 1 to approximately 0.06 m/mn in phase 2, a fivefold reduction in compressibility. A similar reduction in compressibility was observed whenever an apparent interfacial phase change took place. In almost all cases, further contraction leads to crumpling of the interface; that is, the compressibility approaches zero. Of relevance to emulsion stability is how much compression must occur before the low compressibility and zero compressibility films appear. If little compression is required, only a small amount of coalescence can occur before the low compressibility film appears and inhibits further coalescence. Therefore, the film ratio at which the incompressible film appeared was determined from the change in slope on the surface pressure isotherms as shown in Fig. 5. The film ratio at which the zero compressibility films appeared is always the point at the lowest reported film ratio shown for a given isotherm. The phase 1 and phase 2 compressibilities as well as the film ratios at which the phase changed and at which crumpling occurred are listed in the supporting information for all the experiments performed in this work. Fig. 7 shows the surface pressure isotherms of interfacial films of 1, 10, or 20 kg/m 3 asphaltenes in 25/75 heptol after 60 min of aging at 23 C. Asphaltene concentration had relatively little effect on the surface pressure isotherms. In general, for most solvents and aging times, the highest phase change film ratio was observed at an asphaltene concentration of 1 kg/m 3 and the lowest at 10 kg/m 3. In other words, low compressibility films formed more readily at the lowest concentration considered (1 kg/m 3 ) and less readily at the intermediate concentration of 10 kg/m 3. The lack of concentration effect suggests that the interfaces are nearly saturated with asphaltenes at the concentration range considered here. In other words, the molecular surface coverage is similar in all cases and therefore the interfacial compressibility is expected to be similar as well. In fact, Gibb s adsorption isotherms suggest that asphaltenes saturate water hydrocarbon interfaces at concentrations above 1 kg/m 3 [20]. Note that asphaltenes do self- Fig. 8. Effect of solvent on surface pressure isotherms for 10 kg/m 3 asphaltenes after 60 min of aging at 23 C and with 30 s intervals between compression steps. Fig. 9. Effect of aging on surface pressure isotherms for 10 kg/m 3 asphaltenes in toluene after aging at 23 C and with 30 s intervals between compression steps. associate into larger aggregates as the asphaltene concentration increases. However, the larger aggregates have been shown to occupy a similar area on the interface as smaller aggregates and simply project further into the bulk phase [5]. Fig. 8 shows the surface pressure isotherms of interfacial films of 10 kg/m 3 asphaltenes in toluene, 25/75 heptol, or 50/50 heptol after 60 min of aging at 23 C. There is little difference between the surface pressure isotherms in toluene and 25/75 heptol. However, the films in 50/50 heptol show somewhat lower initial compressibility (higher elasticity) and form low compressibility films at high film ratios. Similar results were observed at most other asphaltene concentrations and aging times. A possible explanation is that asphaltenes are more likely to become irreversibly adsorbed in a poorer solvent leading to less compressible films. Fig. 9 shows the surface pressure isotherms of interfacial films of 10 kg/m 3 asphaltenes in toluene after aging from 10minto8hat23 C. Film compressibility decreases and higher phase change film ratios are observed in all cases with

8 252 H.W. Yarranton et al. / Journal of Colloid and Interface Science 310 (2007) increased aging. The complete data set is provided in the supporting information. The increase in phase change film ratio with aging is observed in all cases. The significant increase in the phase change film ratio in 50/50 heptol solutions is also apparent. Note that the film ratio at which crumpling occurred followed similar trends. As will be shown in Part II of this work, the amount of asphaltenes adsorbed on the interface changes little over time. Hence, the decrease in film compressibility over time is likely due to a rearrangement of the asphaltenes on the interface. The asphaltenes appear to initially adsorb as a monolayer of aggregates [5]. It is plausible that the aggregates reform into a three-dimensional network once confined on the interface Effect of temperature on interfacial rheology The rheological measurements presented thus far were performed at 23 C while the emulsion stability experiments were performed at 60 C. The interfacial tension of asphaltenes was observed to change little with temperature and hence elasticity was not expected to change significantly. However, the effect of temperature on the surface pressure isotherms was unknown. Both elasticity and film ratio measurements were repeated at 60 C. Increasing temperature from 23 to 60 C did not appear to have any significant effect on the rheological properties of the asphaltene films. Data are provided in the supporting information. 4. Conclusions The surface pressure and elasticity measurements are consistent with the gradual formation of a cross-linked network on the interface. Both measurements show that rigid films develop over time and form more readily in a poorer solvent. The total modulus from the oscillating droplet experiments correlates to the inverse of the initial compressibility from the surface pressure isotherms. Hence, surface pressure isotherm data appear to be sufficient to provide the asphaltene film properties. To our knowledge, this is the first time that a relationship has been demonstrated between a surface pressure isotherm and interfacial elasticity. If this equivalence holds, then surface pressure isotherms could prove to be a sufficient measurement of the rheological properties of interfacial films. Since the asphaltene film compressibility decreases as the film is compressed, the coalescence rate of the emulsion is expected to decrease as more coalescence occurs. Less coalescence is also expected with increasing aging time and heptane content. Changing the asphaltene concentration is expected to have relatively little effect. In Part II of this work, we show how coalescence rates can be predicted based on the surface pressure isotherm data. Acknowledgments The authors thank Syncrude Canada Ltd. for providing samples and financial support. We are also grateful to AERI and NSERC for financial support. Maryam Jafari at the University of Calgary helped to establish the surface pressure technique and Dr. Alain Cagna at IT Concept assisted in developing the procedure for the elasticity measurements. We also thank Dr. Kevin Moran at Syncrude and Dr. Shawn Taylor at Oilphase- DBR (Schlumberger) for helpful discussions. Supporting information The online version of this article contains additional supporting information. Please visit DOI: /j.jcis References [1] H.W. Yarranton, H. Hussein, J.H. Masliyah, J. Colloid Interface Sci. 228 (2000) 52. [2] J.D. McLean, P.K. Kilpatrick, J. Colloid Interface Sci. 189 (1997) 242. [3] S.D. Taylor, J. Czarnecki, J. Masliyah, J. Colloid Interface Sci. 252 (2002) 149. [4] P.C. 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