Water Interfaces

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Adsorption of Bituminous Components at Oil/Water Interfaces Investigated by Quartz Crystal Microbalance: Implications to the Stability of Water-in-Oil Emulsions Lamia Goual, Ge´za Horva´th-Szabo´,* Jacob H. Masliyah, and Zhenghe Xu Department of Chemical and Materials Engineering, University of Alberta, Edmonton AB T6G 2G6, Canada Received February 4, 2005. In Final Form: June 16, 2005 Silica-gel-coated QCM crystals oscillating in a thickness shear mode are used to measure adsorption of bituminous components in water-saturated heptol (1/1 vol ratio of a heptane/toluene mixture) at the oil/water interface. In addition to the viscoelasticity of the adsorbed film, the effects of the bulk liquid density and viscosity as well as the liquid trapped in interfacial cavities are taken into account for the calculation of adsorbed mass. Asphaltenes in heptol adsorb continuously at the oil/water interface, while resins (the surface-active species in maltenes) show adsorption saturation in the same solvent. For Athabasca bitumen in heptol, two adsorption regimes are observed depending on concentration. At low concentrations, a slow, non-steady-state, and irreversible adsorption takes place. At high concentrations, a steady-state adsorption with limited reversibility results in a quick adsorption saturation. The threshold concentration between these adsorption regimes is 1.5 wt % and 8 wt % for oil/water and oil/gold interfaces, respectively. The threshold concentration, the total adsorbed amount, and the flux of non-steady-state adsorption depend on the resin-to-asphaltene ratio. The threshold concentration is related to the earlier reported critical bitumen concentration characterizing the rigid-to-flexible transition of the interfacial film. We propose a new mechanism based on the change of the effective resin-to-asphaltene ratio with dilution to explain both the adsorption behavior and emulsion stability.

1. Introduction Adsorption of bituminous components from organic phases at oil/water or oil/metal interfaces has long been a focus of interest because of its relation to two problematic areas in crude oil production, namely, (i) the stability of water-in-oil emulsions, which leads to serious corrosion problems in bitumen processing especially when bitumen is separated from oil sands by Clark’s water extraction, and (ii) the deposition of crude oil fractions on metals, which can cause fouling. In this work, we study the adsorption of bituminous components at oil/water (o/w) interfaces, while the adsorption at oil/gold (o/g) interfaces will be given for comparison. The properties of o/w interfaces affect the stability of emulsions. In this respect, the presence and the role of bituminous asphaltenes,1-4 resins,5,6 natural surfactants,7,8 liquid crystals,9-11 and solids12-16 has been well * Corresponding author. E-mail: [email protected]. Phone: 780492-3712. Fax: 780-492-2881. (1) Abraham, T.; Christendat, D.; Karan, K.; Xu, Z.; Masliyah, J. Ind. Eng. Chem. Res. 2002, 41 (9), 2170. (2) Taylor, S. D.; Czarnecki, J.; Masliyah, J. J. Colloid Interface Sci. 2002, 252, 149. (3) Zhang, L. Y.; Lawrence, S.; Xu, Z.; Masliyah, J. H. J. Colloid Interface Sci. 2003, 264, 128. (4) Horva´th-Szabo´, G.; Masliyah, J. H.; Elliott, J. A. W.; Yarranton, H. W.; Czarnecki, J. J. Colloid Interface Sci. 2005, 283, 5. (5) Friberg, S. E.; Yang, H. F.; Midttun, O.; Sjo¨blom, J.; Aikens, P. A. Colloids Surf., A 1998, 136, 43. (6) Wu, X. Energy Fuels 2003, 17, 179. (7) Gu, G.; Xu, Z.; Nandakumar, K.; Masliyah, J. H. Fuel 2002, 81, 1859. (8) Horva´th-Szabo´, G.; Masliyah, J. H.; Czarnecki, J. J. Colloid Interface Sci. 2003, 257, 299. (9) Horva´th-Szabo´, G.; Czarnecki, J.; Masliyah, J. H. J. Colloid Interface Sci. 2001, 236, 233. (10) Horva´th-Szabo´, G.; Masliyah, J. H.; Czarnecki, J. J. Colloid Interface Sci. 2001, 242, 247. (11) Horva´th-Szabo´, G.; Czarnecki, J.; Masliyah, J. H. J. Colloid Interface Sci. 2002, 253, 42760.

documented over the past years. While asphaltenes have traditionally been the primary focus of research, recent studiesson bitumen as well as crude oilssshow that their interactions with resins, solids, etc. are key to understanding emulsion stability.17-21 The preferential adsorption of different bituminous components at the o/w interface under given conditions generates interfaces with variable viscoelasticity. The origin of changing rigidity of o/w interfaces with bitumen or crude concentration has not been grasped yet, although a strong correlation exists between emulsion stability and interfacial rigidity. The first observation of rigid or skinlike o/w interfacial films was reported more than fifty years ago.22 Since then several authors have investigated the skin-forming phenomena.23-27 Dabros et al.,28 who worked with Athabasca coker feed bitumen, described a (12) Yan, N. X.; Masliyah, J. H. J. Colloid Interface Sci. 1994, 168 (2), 386. (13) Yan, N. X.; Masliyah, J. H. Colloids Surf., A 1995, 96 (3), 229. (14) Long, Y.; Dabros, T.; Hamza, H. Fuel 2002, 81, 1945. (15) Long, Y.; Dabros, T.; Hamza, H. Fuel 2004, 83, 823. (16) Gu, G.; Zhou, Z.; Xu, Z.; Masliyah, J. H. Colloids Surf., A 2003, 215, 141. (17) McLean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 189, 242. (18) Gafonova, O. V.; Yarranton, H. W. J. Colloid Interface Sci. 2001, 241, 469. (19) Ekholm, P.; Blomberg, E.; Claesson, P.; Auflem, I. H.; Sjo¨blom, J.; Kornfeldt, A. J. Colloid Interface Sci. 2002, 247, 342. (20) Sullivan, A. P.; Kilpatrick, P. K. Ind. Eng. Chem. Res. 2002, 41, 3389. (21) Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K. Colloids Surf., A 2003, 220, 9. (22) Bartell, F. E.; Neiderhauser, D. O. In Fundamental Research on Occurrence and Recovery of Petroleum, 1946-1947; American Institute of Petroleum: New York, 1949; pp 57-80. (23) Dodd, C. G. J. Phys. Chem. 1960, 64, 544. (24) Kimbler, O. K.; Reed, R. L.; Silberberg, I. H. Soc. Pet. Eng. J. 1966, 6, 153. (25) Strassner, J. E. J. Pet. Tech. 1968, 20, 303. (26) Kim, Y. H.; Wasan, D. T.; Breen, P. J. Colloids Surf., A 1995, 95, 235.

10.1021/la050333f CCC: $30.25 © 2005 American Chemical Society Published on Web 08/04/2005

Adsorption of Bituminous Components

rigid o/w interfacial film below, and a flexible film above, 1 vol % bitumen concentration in heptane/toluene solvents. Although several experimental studies2,6,29-30 have been made recently to gain a better understanding of this phenomenon, a convincing account has not been presented yet. In this paper we suggest a reasonable explanation supported by experimental evidence. A fundamental knowledge of the role and behavior of the interfacial layer must include quantitative information about the adsorption of bituminous components at the o/w interface. To the best of our knowledge, the adsorption behavior of crude oil or bitumen at o/w interfaces from organic solvents has not been investigated. The modelbased prediction of adsorption kinetics is restricted to binary systems. For multicomponent systems, adsorption can be measured by various methods such as the quartz crystal microbalance (QCM), which is becoming increasingly popular especially for nontransparent systems. Early applications of QCM have been presented by Sauerbrey31 in gas systems for uniform, rigid, and sufficiently thin adsorption layers. The principle consists of monitoring the change in resonant frequency of a quartz crystal, which was found by Sauerbrey to vary linearly with the adsorbed mass density on the crystal. Extension of this method to liquid systems opened a way for in situ adsorption studies of proteins, polymers, etc. on various surfaces. However, deviations from the Sauerbrey equation have been reported for the adsorption of soft viscoelastic films.32-36 In this case, the amounts adsorbed are overestimated by the Sauerbrey equation. The viscoelastic effects depend on film thickness and shear modulus and may contribute significantly to the decrease of resonant frequency.33,36 Viscoelasticity can be estimated from either the frictional loss induced by energy loss of the oscillating crystal in the form of a dissipation factor32 or the increase in the resistance of the crystal.34 It can also be calculated from the complex shear modulus of the viscoelastic film.35,36 Using QCM with dissipation factor monitoring, Ekholm et al.19 measured the adsorption of asphaltenes and resins in toluene and heptol (1/1 vol ratio of a heptane/toluene mixture) at an oil/gold (o/g) interface. The dissipation factors of asphaltenes at low concentrations and resins were small enough to suggest a rigid attachment to the surface of the crystal. However, at higher asphaltene concentrations, the dissipation factor was not negligible. The same authors reported a competitive adsorption of asphaltene aggregates and resins at an o/g interface and found that the adsorption of inner layers of asphaltenes and resins is irreversible. In another work, the kinetics of asphaltene adsorption on different metals was measured with a QCM device.37 QCM applications appear to be scarce in petroleum science and have been limited so far to the study of asphaltene and resin species in solvents. Because of the complex interactions involved in petroleum systems, direct (27) Freer, E. M.; Svitova, T.; Radke, C. J. J. Pet. Sci. Eng. 2003, 39, 137. (28) Dabros, T.; Yeung, A.; Masliyah, J.; Czarnecki, J. J. Colloid Interface Sci. 1999, 210, 222. (29) Yeung, A.; Dabros, T.; Masliyah, J.; Czarnecki, J. Colloids Surf., A 2000, 174, 169. (30) Yang, X.; Czarnecki, J. Colloids Surf., A 2002, 211, 213. (31) Sauerbrey, G. Z. Phys. 1959, 155, 206. (32) Rodhal, M.; Kasemo, B. Sens. Actuators, A 1996, 54, 448. (33) Keller, C. A.; Kasemo, B. Biophys. J. 1998, 75, 1397. (34) Zhou, T.; Marx, K. A.; Warren, M.; Schulze, H.; Braunhut, S. J. Biotechnol. Prog. 2000, 16, 268. (35) Kankare, J. Langmuir 2002, 18, 7092. (36) Voinova, M. V.; Jonson, M.; Kasemo, B. Biosens. Bioelectron. 2002, 17, 835. (37) Xie, K.; Karan, K. Energy & Fuels 2005, 19, 1252.

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measurements from bitumen or crudes may present more realistic adsorption behavior of species at o/w interfaces. One should also note that the Sauerbrey equation is valid only for measurements in the gas phase. In the liquid phase, the shear motion of the crystal generates a shear wave that penetrates into the contacting liquid. This wave is dampened by energy dissipation due to the viscosity and density of the liquid resulting in a further decrease in the resonant frequency of the crystal. In a liquid of density F and viscosity η, the resonant frequency decreases linearly with the square root of Fη,38 while the resonance damping described as a motional resistance increases linearly with the square root of Fη.39 Martin et al.40 described the QCM by an electronic circuit and related the circuit elements to physical properties of the QCM, surface mass, and contacting liquid. An additional mechanical damping of the resonant frequency by liquid arises from the trapping of liquid by interfacial cavities and pores. The response of resonant frequency to liquid trapping was established by Martin et al.41 and was found to depend only on liquid density. Bund and Shneider42 reported that surface roughness affects the resonant frequency at early stages of adsorption and viscoelastic effects become important for higher layer thicknesses. Ha et al.43 found that the mass calculated from the Sauerbrey equation is from 3 to 6 times larger than the actual value obtained from fluorescence measurements, probably due to omission of liquid loading and surface roughness. It is clear from the above literature assessment that the evaluation of adsorbed mass from frequency shift data could be a formidable task for solutions with a high concentration of dissolved or dispersed components. In this case, a more precise characterization of the liquid phase and the electrode surface is needed. For example, corrections with the bulk viscosity and density are usually neglected in the literature because the solutions studied are usually diluted. However, for adsorption studies of bituminous components only the high-concentration region has practical importance. Therefore, corrections are unavoidable. Correcting for structured interfaces is especially difficult. In practice, this problem is usually avoided by working with polished electrode surfaces. However, in this work we are taking advantage of the problems associated with structured interfaces by using them to entrap water. In this manner we can develop a unique application of the QCM technique by using it for studying adsorption at liquid-liquid interfaces. Because our coated electrode surface is structured, we need to characterize it in order to correct for surface roughness. The density and viscosity corrections used in the literature are based on calibration curves obtained with solutions of low molar mass components (for instance glycerin dissolved in water). All these practices are based on the silent assumption that the dissolved components have no adsorption at the interface; therefore, their effects can be considered through their influence on the bulk viscosity and density. This assumption, however, cannot be necessarily upheld when the adsorption is measured with ng/cm2 resolution especially at inhomogeneous interfaces. One way to avoid this problem is to apply (38) Kanasawa, K. K.; Gordon, J. G. Anal. Chim. Acta 1985, 175, 99. (39) Muramatsu, H.; Tamiya, E.; Karube, I. Anal. Chem. 1988, 60, 2142. (40) Martin, S. J.; Granstaff, V. E.; Frye, G. C. Anal. Chem. 1991, 63, 2272. (41) Martin, S. J.; Frye, G. C.; Wessendorf, K. O. Sens. Actuators, A 1994, 44, 209. (42) Bund, A.; Shneider, M. J. Electrochem. Soc. 2002, 149 (9), E331. (43) Ha, T. H.; Kim, S.; Lim, G.; Kim, K. Biosens. Bioelectron. 2004, 20, 378.

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isotopes of the solvent as solutes (for example D2O in H2O). We can then be sure that no artifact is introduced by the unwanted adsorption, because both solvent and isotope molecules have the same adsorption affinity to the interface. There is no perfect solution to correct the viscoelasticity of the interfacial film. One possibility is to characterize the viscoelastic behavior of the film by measuring the resonance curves at multiple frequenciessan expensive technique, which is not easily available. The other, but not perfect, solution is to use a commercial microbalance capable of characterizing the motion resistance of the crystal and introduce corrections. We chose the latter solution because we think that application developments are still worthwhile. Our correction is similar but not identical to that used by Snellings et al.44 who introduced a viscoelastic correction based on the energy dissipation of the adsorbed film. The main objective of this study is to measure the adsorption of bitumen on hydrophilic surfaces such as gold and water using the QCM technique. The effects of mass loading, viscous entrainment, and trapping of liquid on the QCM response are all taken into account. The technique allows the study of adsorption on gold surfaces, and in this work an approach is devised to measure adsorption on water surfaces as well. This study is structured into five parts. In the first part, the kinetics of adsorption of diluted bitumen at o/g and o/w interfaces are measured at different bitumen concentrations in heptol. Original findings are reported, which may explain the interfacial rigidity observed at low bitumen concentrations. In the second part, the effect of asphaltenes and maltenes on bitumen adsorption at the o/w interface is investigated. The role of these species is further discussed. In the third part, the kinetics of adsorption of asphaltenes and maltenes at the o/w interface are measured at different concentrations in heptol in order to quantify the individual behavior of these fractions. In the fourth part, several experiments are conducted to test the reversibility of adsorption at low and high bitumen concentrations. Finally, in the last part, results are compared and discussed and a model is proposed to explain interfacial rigidity and its connection to emulsion stability against flocculation. 2. Materials and Methods 2.1. Materials. Materials include a naphtha-diluted bitumen feed supplied by Suncor (0.947 g/cm3 density, 10 wt % asphaltenes), toluene 99.9% HPLC grade, deuterized toluene 99% (Aldrich), heptane 99.6% HPLC grade, acetone 99% certified ACS grade, sodium hydroxide 99.8% certified ACS grade, dioctyl sulfosuccinate sodium salt 99% (Aldrich), sulfuric acid 96% (Aldrich), Millipore Q water, heavy water 99.8% (Acros), and LUDOX TM-50 colloidal silica (Aldrich). The heptane/toluene mixture with a volume ratio of 50/50 is referred to as heptol. 2.2. Cleaning Procedure for the QCM Crystals. To remove organics adsorbed on crystals, the crystals are soaked in toluene for several days, sonicated for 5 min in a 2 wt % solution of dioctyl sulfosuccinate sodium salt in toluene, then thoroughly washed with toluene and acetone, and finally dried with acetone. To remove the silica coating, the crystals are left overnight in an aqueous solution of 5 wt % sodium hydroxide, then sonicated for 5min, washed with water and acetone, and dried with acetone. 2.3. Formation of the Silica-Supported Water Layer. A spin coater model WS 400A- 4NPP/Lite (Laurell Technologies Co.) is used to coat the QCM crystals with a silica suspension (10 wt % in distilled water). The suspension is first filtered with 0.2 µm pore size Whatman paper to remove any large-size silica (44) Snellings, S. L.; Fu¨ller, J.; Paul, D. W. Langmuir 2001, 17, 2521.

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Figure 1. Experimental setup of the RQCM. aggregates; the size of silica particles in the filtrate is about 16 nm. The filtrate is then placed in excess amounts on the gold surface of the crystal. The crystal is rotated at high speed to spread the suspension by centrifugal force. For uniform coating, the spinner is programmed to rotate at 500, 1000, 2000, 3000, 4000, and 5000 rpm, for 15 s each step, then 6000 rpm for 2 min. The resulting silica gel film does not dissolve in heptol, and the motion resistance of the crystal remains constant after coating. The stability of the gel film is due to the formation of Si-O-Si bonds between silica particles, which has been noted in the fabrication of QCM moisture sensors by spin coating with silica suspensions.45 2.4. Water Content in the Silica-Supported Water Layer. The following procedure is used to determine the amount of water in the silica gel: (1) The crystal is first spin coated with a silicain-water suspension. (2) The crystal is then placed inside the cell saturated with water vapor, which was in equilibrium with a small amount of Millipore Q water. A 10433 Hz decrease in the resonant frequency compared with the original frequency is observed corresponding to the adsorption of water vapor on the crystal. (3) The water inside the cell is replaced with 96% sulfuric acid desiccating agent resulting in a 2366 Hz frequency increase due to desorption of water from the crystal. From these data we find that the silica gel contains about 60 vol % water. 2.5. Research Quartz Crystal Microbalance. The setup presented in Figure 1 consists of an AT-cut (optimized for 25 °C) 9 MHz quartz crystal (Maxtek Inc. No. 149272-1) sandwiched between two gold electrodes. The diameter of the crystal is 1 in., while the piezo-active area of the gold electrode facing the solution is 3.419 × 10-5 m2. The crystal is placed in a CHK-100 kynar crystal holder (Maxtec Inc. No. 184204) allowing the active crystal face to be in contact with the solution. The holder is put inside a custom-made water-jacketed cell. The cell is mounted on top of a magnetic stirrer and is connected to a thermostatic bath set to 25.00 ( 0.01 °C. The crystal is connected to a high-performance phase lock oscillator circuit (Maxtek Inc. RQCM No. 603200-2) with 0.03 Hz sensitivity, which provides measurement stability and accuracy over a frequency range of 5.1-10 MHz. The microbalance is connected to the crystal holder and to a computer for data acquisition. When an ac voltage is applied between the electrodes, the crystal oscillates in a thickness shear mode at its resonant frequency. The resonant frequency depends on the mass of the crystal including the mass adsorbed from the liquid phase. Thus, QCM monitors the adsorbed mass with a 0.4 × 10-9 g·cm-2 resolution by measuring the frequency shift of the crystal. The frequency and resistance data are collected with a time resolution of 1 Hz. 2.5.1. Principle of Operation. When the baseline (i.e., frequency versus time) is determined in a solvent, the adsorption mass density, Γ [g·cm-2], on a crystal oscillating at the fundamental resonance frequency is calculated in SI units from the frequency shift, ∆f [Hz], of the crystal in contact with liquid (i.e., solvent (45) Hong, S.; Lee, K. S.; Bank, H.; An, I.; Kim, O.; Nham, S. H. Ungyong Mulli 1999, 12 (6), 531.


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Figure 2. Mechanical model of the vibrating electrode/ interface. The elastic adsorbed layer represents the mass loading;31 the coupling between the electrode and the bulk liquid results in a viscously entrained liquid layer;38,40 and the surface roughness results in a trapped liquid layer with effective thickness h.41 and solute), according to the following:31,38,40,41

∆f ) -Cf[Γ + X + h(Fl - Fs)]

(1)

where

Cf )

2f02

(2)

(Fqµq)1/2

and

X)

( ) ( ) Flηl 4πf0

1/2

-

Fsηs 4πf0

1/2

(3)

(4)

Figure 3A represents the variations of Y versus (Fl - Fs) on gold and water surfaces. From the slopes of the straight lines, the effective thickness of the trapped liquid h, according to eq 4, is 4 × 10-6 cm or 40 nm on gold, which is in a good agreement with the results of Yang and Thompson46 who found by SEM that the average depth of the cracks and gaps on the polished gold electrode surface is below 50 nm. The effective thickness, h, of the trapped organic liquid on water-saturated silica gel is found to be 2 × 10-5 cm or 200 nm. (46) Yang, M.; Thompson, M. Anal. Chem. 1993, 65, 1158.

The change in the damping of resistance, ∆RTHEOR, that arises from liquid loading compared to that of the solvent is expressed by the following:41

∆RTHEOR ) CrX

Here f0 is the resonant frequency of the unloaded crystal (9 MHz), Fq is the density of quartz (2.648 g·cm-3), µq is the effective shear modulus of quartz (2.947 × 1011 g·cm-1·s-2), Cf is a fundamental property of the crystal, called the sensitivity factor (0.1834 Hz·cm2·ng-1 for a 9 MHz quartz crystal at 25 °C), Fl and Fs are the densities of liquid (bitumen solution in heptol) and solvent (heptol), respectively, ηl and ηs are the viscosities of liquid and solvent, respectively, and h is the effective thickness of the trapped liquid. The first term on the right-hand side of eq 1 represents mass loading,31 the second term is due to viscous entrainment of liquid compared to that of the solvent,38-40 and the last term arises from the trapping of liquid as compared to solvent, due to surface roughness (see Figure 2).41 The effective thickness of the trapped liquid, h, is determined by measuring the frequency shifts of the gold-coated crystal at different concentrations of heavy water (D2O, F25 ) 1.097 g·cm-3) in water, as well as the frequency shifts of the silica-gel-coated crystal at different concentrations of deuterized toluene (C7D8, F25 ) 0.932 g·cm-3) in toluene (watersaturated mixture). Since Γ is zero in eq 1 for (water-heavy water) and (toluene-deuterized toluene) systems, we can define the term Y by Y ) -∆f/Cf - X, leading to the following:

Y ) h(Fl - Fs)

Figure 3. Determination of the effective thickness of the trapped liquid h (the slope of Y vs (Fl - Fs) according to eq 4) and the parameter Cr (the slope of ∆R vs X according to eq 5) on water (i.e., water-saturated silica) and gold surfaces. h (or Cr) is determined by measuring the frequency (or resistance) shifts of the gold crystal at different bulk concentrations of D2O in water, as well as the frequency (or resistance) shifts of the silica-gel-coated crystal at different concentrations of deuterized toluene in toluene (water-saturated mixture).

(5)

where

Cr )

π 4K′ 2C0(µqFq)1/2

(6)

Here, K′ is the electromechanical coupling factor for quartz and C0 is the static capacitance of the QCM. Similar to the calculation of the effective thickness of the trapped liquid, the parameter Cr is determined by measuring the resistance shifts of the crystal at different concentrations of heavy water in water and deuterized toluene in toluene. According to eq 5 and Figure 3B, the slopes, Cr, of the straight lines obtained by plotting ∆RTHEOR versus X are equal to 7.0 × 106 m4·s-3·A-2 on gold and 3.0 × 106 m4·s-3·A-2 on water-saturated silica gel. 2.6. Density Measurements. Densities are measured at 25 °C with a PAAR-DMA 45 instrument (Anton Paar) with a precision of 0.001 g·cm-3. The density meter measures the resonant frequency of an oscillating U-tube of known volume, which is inversely proportional to the square root of its mass. The apparatus is calibrated with air and water prior to density measurements. 2.7. Viscosity Measurements. Viscosities are measured in a thermostatic bath at 25 °C using a capillary viscometer, which employs a glass capillary tube attached to a bulb of known volume. The time of flow, t, of the heptol solution (density Fl, viscosity ηl) through the bulb is compared to the time taken by toluene (Fs ) 0.867 g·cm-3, ηs ) 5.6 × 10-3 g/cm·s). The viscosity is then calculated from the expression ηl/ηs ) (Fltl)/(Fsts). 2.8. Separation of Asphaltenes and Maltenes. The procedure for separation of asphaltenes and maltenes from bitumen is as follows: Bitumen and n-heptane are first mixed in a volume ratio of 1/40. After 1 h of stirring, the solution is left overnight, then filtered with a 0.2 µm pore size Whatman filter paper. The filter cake is extensively washed with n-heptane until the filtrate is colorless. Asphaltenes are recovered from the filter cake by

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dissolution in toluene; toluene is further evaporated and the asphaltenes are dried. Once the asphaltenes are recovered, the liquid filtrate is transferred into a rotary evaporator in order to remove n-heptane from the maltenes. Following this procedure, we find that the bitumen contains 10 wt % asphaltenes and 63 wt % maltenes. Note that this bitumen was used as received and contains about 27 wt % naphtha. 2.9. Apparent Molecular Weight of Asphaltenes and Maltenes. The apparent molecular weight of asphaltenes and maltenes is measured with a vapor pressure osmometer Model 232A (Corona Westacan) at 50 °C in toluene. Benzil is used for calibration. The limiting (extrapolated to zero concentration) apparent molecular weight of asphaltenes is 4000 dalton. The extrapolation is based on six molecular weight data points within a 0-0.02 range of solvent to solute mass ratio. The apparent molecular weight of asphaltenes increases with concentration reaching 7000 dalton at 0.02 solvent to solute mass ratio. The apparent molecular weight of maltenes is 500 dalton and is independent of concentration within a 0-0.02 range of solvent to solute mass ratio. 2.10. Preparation of Asphaltene and Maltene Mixtures with Bitumen. The asphaltenes (or maltenes) are first mixed with toluene, then added to bitumen. This bitumen contains some solids that have not been removed. The mixture is left overnight with continuous shaking, then heptane is added in the same volume as toluene, and the solution is stirred for about 15 min prior to injection into the cell. 2.11. Experimental Procedure. To prepare water-saturated heptol, 10 mL of Millipore water is added to 4 L of heptol, the mixture is shaken for about 1 min, and left to stand for 2 days. The experiment is initiated by injecting 200 g of water-saturated heptol (1/1 vol ratio of heptane/toluene mixture) inside the cell in order to establish the baseline. The weight of solvents and solution are measured with a precision of 0.01 g. Crystal capacitance cancellation is performed before each experiment to reduce error caused by the parasitic capacitance of the crystal, cables, and holder. Once the baseline is established, a given amount of solute (bitumen, asphaltene, maltene, or their mixtures) dissolved in water-saturated heptol is introduced into the cell. The liquid mixture inside the cell is referred to as oil. The adsorption process at the oil/water-saturated silica gel interface or o/g interface is followed by monitoring the change in frequency and resistance as a function of time (one recording per second). The oil viscosity is measured at the end of each experiment.

3. Results and Discussion 3.1. Water Film Thickness on the Silica-Coated QCM Crystal Versus Water Saturation of the Solvents. We used water-saturated organic solvents throughout this investigation for two reasons. First, it has been predicted by molecular mechanics calculations that water forms bridging H-bonds between the heteroatoms of asphaltenes with a notable span in energies; thus, the presence of water has an effect on asphaltene association.47 These predictions agree well with recent experimental findings about asphaltene-water interactions.48,49 Second, the silica-gel-coated crystal should possess a water layer having similar physicochemical properties to those of the bulk water to ensure that the experimental findings are characteristics to o/w interfaces. That is, the hydrophilic groups of the adsorbing components should find similar water properties at oil/silica gel interfaces as at o/w interfaces. In water-saturated organic solvents, the chemical potential of water is identical to that of the watersaturated vapor (cf. section 2.4.). Therefore, the water content of the silica gel measured in water-saturated vapor (47) Murgich, J.; Merino-Garcia, D.; Andersen, S. I.; Del Rio, J. M.; Lira-Galeana, C. Langmuir 2002, 18, 9080. (48) Andersen, S. I.; Del Rio, J. M.; Khvostitchenko, D.; Shakir, S.; Lira-Galeana, C. Langmuir 2001, 17, 307. (49) Horva´th-Szabo´, G.; Masliyah, J.; Czarnecki, J. J. Can. Eng. 2004, 82, 1089.

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must be close to the water content of the gel in watersaturated organic solvents. Therefore, the measured 60% water content of the gel in water-saturated vapor can be applied to the gel in water-saturated organic solvents as well. From this water content and from the 16 nm primary radius of silica particles,50 the average thickness of the water layer on the particles is 5 nm corresponding to about 20 layers of molecular water. Therefore, the o/w interface of this thick water layer can be well considered as a bulk o/w interface. This is crucial in our case because the first layer of bound water on silica surface has distinctly different chemical properties from that of bulk water.51,52 Nevertheless, one has to consider the flip side of the question as well. This water layer should be sufficiently thin to oscillate with the silica matrix, otherwise an unwanted apparent viscous component might be introduced into the properties of the adsorbed layer when QCM technique is used. Previous ultrasonic studies53 revealed that 64% water kinetically moves together with silica nanoparticles at 10 MHz frequency changing by this way the apparent compressibility and density of colloidal particles calculated from Urick’s equation.54 Thus, the 5 nm thick water layer in the silica gel is a mechanically bound water layer from the perspective of the QCM technique, while the very same water layer in contact with the organic phase can be considered as having bulk properties from the perspective of adsorption behavior. The former statement is supported as well by the approximately 1 µm penetration depth of 9 MHz shear waves in water.55 Hence from this point on, the oil/watersaturated silica gel interface will be referred to as the o/w interface. 3.2. Elastic and Viscoelastic Properties of the Adsorbed Film. Figure 2 presents a mechanical model of the vibrating electrode/solvent interface according to Martin’s mathematical description.41 The electrode has an uneven surface covered with a perfectly elastic adsorbed film with mass density Γ. The uneven surface traps liquid. Both the trapped liquid and the elastic film oscillate simultaneously with the electrode. Due to the viscous coupling between these bodies (that is, electrode, film, and trapped liquid) and the bulk liquid, an extra liquid layer next to the electrode is also involved in the oscillation movement. The latter viscously entrained liquid layer is the only unit of the model that dissipates energy. This dissipation can be characterized by the resistance shift of the equivalent circuit model, which is either calculated from the bulk properties according to eqs 3 and 5 (∆RTHEOR) or directly measured experimentally (∆REXP). The resistance shift calculated either theoretically or experimentally should result in exactly the same frequency shift for systems corresponding to Martin’s model.41 However, when the adsorbed film is viscoelastic, there will be a difference between the theoretical and experimental resistance. This is because the film contributes to the energy dissipation as well, and the frequency shifts calculated by the two resistances will be different. Therefore, ∆REXP can offer a correction for the film viscoelasticity, which was not included in the original model. In this work, we shall use ∆REXP in the calculation (50) Magual, A. M.; Horva´th-Szabo´, G.; Masliyah, J. H. Langmuir, in press. (51) Baacom, W. D.; Timmons, R. B. J. Phys. Chem. 1972, 76, 3192. (52) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (53) Horva´th Szabo´, G.; Høiland, H. J. Colloid Interface Sci. 1996, 177, 568. (54) Urick, R. J. J. Appl. Phys. 1947, 18, 983. (55) Thompson, M.; Kipling, A. L.; Duncan-Hewitt, W. C.; Rajakovic, L. V.; Cavic-Vlasak, B. A. Analyst 1991, 116, 881.


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Figure 4. Adsorption kinetics of 10 wt % bitumen in heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) at o/g and o/w interfaces. The solid curves represent data points collected with 1 point per second resolution. The dotted lines are theoretical fits according to eq 8.

Figure 5. Adsorption kinetics of 0.3 wt % bitumen in heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) at o/g and o/w interfaces. The solid curves represent data points collected with 1 point per second resolution. The dotted lines are theoretical fits according to eq 10.

of the frequency shift of the loaded crystal, according to the following:

[

∆f ) -Cf Γ +

]

∆REXP + (Fl - Fs)h Cr

(7)

3.3. Adsorption of Bitumen at Oil/Gold and Oil/ Water Interfaces. 3.3.1. Adsorption Kinetics. Figure 4 shows the time dependence of the adsorbed mass density when gold or water surfaces are in contact with 10 w% bitumen dissolved in water-saturated heptol. At steady state, when adsorption saturation in time is reached, the mass density at the o/w interface is almost three times higher than that at the oil/gold (o/g) interface. Since a Langmuir-type adsorption has been observed for bituminous components at o/w4 and oil/metal interfaces,37 Langmuir adsorption kinetics56 is assumed here as well:

KC [1 - exp(-(kaC + kd)t)] Γ(C,t) ) Γ∞ 1 + KC

(8)

where C is the concentration in the bulk phase, Γ∞ is the adsorbed mass density when the conditions t f ∞ and C f ∞ are satisfied, ka and kd are the rate constants of adsorption and desorption, respectively, and K ) ka/kd is the equilibrium constant of adsorption. At t f ∞, eq 8 becomes the Langmuir isotherm:

KC Γ(C,t f ∞) ) Γ∞ 1 + KC

(9)

The values of Γ(C, t f ∞) and (kaC + kd) can be determined by fitting the experimental data to eq 8 as shown in Figure 4. Figure 5 presents the adsorption behavior of 0.3 wt % bitumen in water-saturated heptol at both o/g and o/w interfaces. Surprisingly, steady-state adsorption over the tested period is not reached at both o/g and o/w interfaces. Instead, the adsorbed amount increases in time with a constant slope. This increase lasts for a very long period of time as shown in Figure 6, suggesting a multilayer adsorption. Indeed, steady-state adsorption of 10 wt % (56) Huang, M.; Shen, D.; Chow, L. M.; Yang, M. Analyst 2002, 127, 940.

Figure 6. Adsorption kinetics of 0.3 wt % bitumen in heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) at the o/g interface for longer times. The curve represents data points collected with 1 point per second resolution.

bitumen at the o/g interface yields a mass density at saturation of 0.6 µg·cm-2 (see Figure 4). On the other hand, adsorption of 0.3 wt % bitumen at the same interface yields a mass density of 4 µg·cm-2 after 8 h (see Figure 6), which corresponds to the adsorption of about seven steady-state layers. It appears that the adsorption kinetics follows two different regimes depending on bitumen concentration in heptol: (1) a steady-state regime above an adsorption threshold concentration of bitumen, and (2) a non-steadystate regime below the adsorption threshold concentration of bitumen where a continuous interfacial build-up results in multilayer adsorption. A modified Langmuir model may, as a first approximation, describe the adsorption kinetics at low bitumen concentration in the non-steady-state regime:

Γ(C,t) ) Γ(C,t f ∞)[1 - exp(- (kaC + kd)t)] + F(C)t (10) where F(C) is the slope of the non-steady state adsorption kinetics at relatively large time (above 800 s) or the flux

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Figure 7. Flux of non-steady-state adsorption of bitumen in heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) at o/g and o/w interfaces.

of non-steady-state adsorption. The fit of eq 10 is presented in Figure 5. 3.3.2. Adsorption Threshold Concentration (Ct). We define Ct as the lowest concentration of a component or pseudocomponent in a given solvent at which the flux, F(C), of non-steady-state adsorption in eq 10 is zero for a given interface. The adsorption threshold concentration of bitumen is 1.5 wt % for the o/w and 8 wt % for the o/g interface in water-saturated heptol (see Figure 7). 3.3.3. Flux of Non-Steady-State Adsorption Versus Bitumen Concentration. The flux of non-steady-state adsorption (in the concentration range below the adsorption threshold concentration) in eq 10 depends on the concentration of bitumen in heptol and on the nature of the interface as shown in Figure 7. The initial flux is higher on water than on gold but falls sharply with increasing bitumen concentration. According to mass transfer laws, the adsorption flux is expected to increase linearly with concentration. This is not the case for bitumen and denotes the complexity of the multicomponent system studied. 3.3.4. Adsorption Isotherms. The adsorbed amounts at large time (above 800 s) are found to be constant and equal to 0.6 µg/cm2 at the o/g interface within the 8-10 wt % concentration range and 1.8 µg/cm2 at the o/w interface within the 1.5-10 wt % concentration range, with a standard deviation of 5%. 3.4. Effect of Asphaltenes and Maltenes on Bitumen Adsorption at the Oil/Water Interface. 3.4.1. Flux of Non-Steady-State Adsorption Versus Bitumen Concentration. Figure 8 reports the flux of non-steady-state adsorption of bitumen mixtures with asphaltenes (and maltenes) at the o/w interface. When bitumen is mixed with asphaltenes in a weight ratio of 9/1, adsorption kinetics yields a higher flux of non-steady-state adsorption than that with bitumen. Furthermore, the non-steadystate regime with additional asphaltenes spans a larger concentration interval as the threshold concentration increases from 1.5 wt % to 1.8 wt % (see Figure 8). Conversely, when bitumen is mixed with maltenes in a weight ratio of 3/1, the flux of non-steady-state adsorption with additional maltenes is found to be smaller than that with bitumen; also, the adsorption threshold concentration is reduced to 1.4 wt % (see Figure 8). 3.4.2. Adsorption Isotherms. Figure 9 presents the adsorption isotherm of bitumen and asphaltene (or maltene) mixtures. While asphaltenes increase the total adsorption amount, maltenes decrease it. The effects of

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Figure 8. Effect of addition of asphaltenes and maltenes on the flux of non-steady-state adsorption of bitumen in heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) at the o/w interface. B+A ) bitumen (B) mixed with asphaltenes (A) in a weight ratio of 9/1. B+M ) bitumen (B) mixed with maltenes (M) in a weight ratio of 3/1.

Figure 9. Effect of addition of asphaltenes and maltenes on adsorption isotherms of bitumen in heptol (1/1 vol ratio of watersaturated heptane/toluene mixture) at the water interface. B+A ) bitumen (B) mixed with asphaltenes (A) in a weight ratio of 9/1. B+M ) bitumen (B) mixed with maltenes (M) in a weight ratio of 3/1.

asphaltenes and maltenes on the adsorbed amount are clearly opposite. Maltenes, due to their resin content, enhance the solubility of asphaltenes in the bulk phase, and therefore, the adsorbed amounts are relatively small. In this work, we refer to resins as all the surface-active materials present in maltenes. The adsorption threshold concentration and the continuous build-up of the interfacial layer observed at low bitumen concentration seem to be controlled by the resinto-asphaltene (R/A) mass ratio in bitumen (see Figures 8 and 9). 3.5. Adsorption of Asphaltenes and Maltenes at the Oil/Water Interface. 3.5.1. Flux of Non-Steady-State Adsorption Versus Concentration. The adsorption kinetics of asphaltenes in water-saturated heptol at the o/w interface exhibits no steady-state regimes in the concentration range of 60-5500 ppm (see Figure 10). In other words, the adsorption threshold concentration, if it exists,


Figure 10. Flux of non-steady-state adsorption of asphaltenes and bitumen in heptol (1/1 vol ratio of water-saturated heptane/ toluene mixture) at the o/w interface.

Figure 11. Adsorption isotherms of maltenes and bitumen in heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) at the o/w interface.

is higher than 0.55 wt %. The flux of the non-steady-state adsorption of asphaltene is higher than that of bitumen and increases drastically with asphaltene concentration, as seen in Figure 10. Ekholm et al.19 did not report on the continuous increase of asphaltene adsorption at the o/g interface with increasing asphaltene concentration in heptol. Note that when the solvent is toluene instead of heptol, the kinetics of asphaltene adsorption is different. In a separate studysnot presented heresthe flux of nonsteady-state adsorption of asphaltenes at the o/g interface was found to be zero, which is in agreement with previous work.19,37 For maltenes, there are no fluxes of non-steadystate adsorption in the 0.3-10 wt % concentration range. 3.5.2. Adsorption Isotherms. The adsorption kinetics of maltenes in water-saturated heptol at the o/w interface exhibits only steady state regimes. The threshold concentration for maltenes is zero or very close to zero. The adsorption equilibrium or quasi-equilibrium state is reached slowly at low maltene concentrations (2 h for 0.3 wt % maltenes) but much faster at higher concentrations (2 min for 3 wt % maltenes). Figure 11 presents the adsorption isotherm of maltenes where the adsorbed amount first increases then stabilizes with maltene concentration. The adsorbed amount is substantial, almost

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Figure 12. Irreversible adsorption of 1 wt % bitumen in heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) at the o/w interface (B ) bitumen). The curves represent data points collected with 1 point per second resolution.

equal to the values from bitumen. Thus, at concentrations higher than the threshold concentration, the interfacial layer may contain a large proportion of maltenes (or resins). 3.6. Reversibility. Reversibility of bitumen adsorption is first explored below the adsorption threshold concentration in the non-steady-state regime (see Figure 12). The adsorbed mass density versus time is plotted as a dotted line for 2 wt % bitumen and as a solid line for 1 wt % bitumen at o/w interfaces in water-saturated heptol. After sufficiently long time, bitumen concentration in the bulk is increased from 1 to 2 wt %. Consequently, if adsorption were reversible the adsorbed amount would decrease to reach a saturation value of 1.8 µg/cm2 obtained with 2 wt % bitumen (see the dotted line). Instead, adsorption at 2 wt % bitumen concentration increased first and then stabilized to a saturation value of 2.3 µg/ cm2. Therefore, bitumen adsorption from heptol at the o/w interface is irreversible at concentrations lower than the adsorption threshold concentration. Next, reversibility is investigated at high bitumen concentration in the steady-state regime. A straightforward approach would be to test the reversibility after reducing the bulk concentration of bitumen. However, this was not possible because the system is on the plateau of Langmuir adsorption isotherm where the adsorbed amount, within the experimental uncertainty, is independent of the bulk concentration (see Figure 11). Instead, reversibility is tested from the effect of maltenes on adsorption. Figure 13 shows the adsorption kinetics of 4 wt % bitumen in water-saturated heptol at the o/w interface. After addition of maltenes in the same amount as bitumen, bitumen concentration in heptol drops to 3.6 wt % and the adsorption slightly decreases (see bold line). However, the final saturation adsorption does not reach (within the investigated time scale of 10 min) the one obtained with the same solution, that is, 3.6 wt % bitumen and 3.6 wt % maltenes in heptol (see the dotted line). Only 6% of the adsorbed layer is dissolved back to the oil phase instead of 17% as expected if adsorption were reversible. The reversibility of bitumen adsorption in this case seems to be limited. To corroborate this result, another reversibility test is performed on 4 wt % bitumen in watersaturated heptol at the o/w interface. After a certain time, the crystals are transferred from oil into pure toluene, and the adsorption density decreased from 1.7 µg/cm2 to

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Figure 13. Limited reversibility of 4 wt % bitumen adsorption in heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) at the o/w interface after maltene addition (B ) bitumen, M ) maltene). The curves represent data points collected with 1 point per second resolution.

Figure 14. Limited reversibility of bitumen and maltene adsorption at the o/w interface after transfer of crystals from heptol (1/1 vol ratio of water-saturated heptane/toluene mixture) to water saturated toluene (B ) bitumen, M ) maltene). The curves represent data points collected with 1 point per second resolution.

1 µg/cm2 as seen in Figure 14. Although asphaltenes and maltenes are completely soluble in toluene, only 40% of the adsorbed layer desorbs back to the oil phase. Therefore, adsorption is partially reversible at high bitumen concentration in the steady-state regime. When the same test is performed with 4 wt % maltenes instead of bitumen, about 80% of the maltenes desorbs back to toluene (see Figure 14), which may indicate that the limited reversibility observed with bitumen is due to presence of asphaltenes. The limited reversibility of bitumen adsorption reveals that the elemental kinetic processes of adsorption and desorption are not frozen despite the fact that the adsorbed amounts are independent of concentration. Thus, the conditions of thermodynamic equilibrium can still be satisfied in this regime, as was demonstrated recently.4 3.7. Interpretation of the Results. 3.7.1. Adsorption Behavior. It is accepted that both asphaltenes and resins are surface-active components of crudes. It is also known

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that resins are the main surface-active components of maltenes. Consequently, competitive adsorption with the formation of a mixed adsorption layer of asphaltenes and resins can be anticipated from bitumen solutions. Any change in the bulk concentration of either of these components will have an effect on the total adsorption amount. The increased concentration of resins in the bulk, for instance, should result in an increased resin-toasphaltene ratio in the interfacial layer. Combination of these concepts with the fact that the specific surface area of asphaltenes is smaller in area/mass units than that of maltenes57 leads to the conclusion that above the adsorption threshold concentration, Ct, the total adsorbed mass density should increase with increasing asphaltene concentration and decrease with increasing maltene concentration, in agreement with the results in Figure 9. From the results graphed in Figure 8, similar statements can be made on the effect of asphaltene or maltene addition on the flux of non-steady-state adsorption. We want to emphasize that, because resins stabilize asphaltenes in the bulk by forming a composite kinetic unit, bitumen adsorption either on gold or water involves the simultaneous adsorption of both asphaltenes and the stabilizing resins. To interpret the two regimes of adsorption, first we define the number of kinetic units of a component in the system as the sum of the number of individual molecules and the number of groups of associated molecules of the said component; then we introduce the hypothesis of the increasing resin-to-asphaltene ratio (in kinetic units of asphaltenes and resins), in the bulk, with increasing bitumen concentration. This hypothesis is supported by the increase of asphaltene apparent molecular weight with concentrationsdue to aggregationsand the constant resin or maltene molecular weight with increasing concentration (cf. section 2.9.). According to the schematics presented in Figure 15, the number of kinetic units of resins (NR) and the number of kinetic units of asphaltenes (NA) changes with bitumen concentration. The NR/NA ratio increases with bitumen concentration in heptol because of asphaltene association. Using this hypothesis as well as the steric stabilization model of asphaltenes by resins,58 we can conclude that resins at higher bitumen concentration stabilize better the kinetic units of asphaltenes than at lower bitumen concentration because the specific surface of asphaltenes in the bulk decreases with increasing bitumen concentration due to association. Whether or not an asphaltene adsorption layer at the interface can subsequently grow further by deposition of a new layer is dependent on how well the steric layer of resins can stabilize asphaltenes. At the adsorption threshold concentration, Ct, which corresponds to a critical (NR/NA)CRIT ratio, the steric interaction between an asphaltene unit at the interface and a unit in the bulk is just sufficient to prevent the deposition of a second layer of asphaltenes. Therefore, it is at this concentration (i.e., the smallest bitumen concentration) where the flux of non-steady-state adsorption just becomes zero (see Figure 7). Above Ct, in the steady-state adsorption regime, where NR/NA > (NR/ NA)CRIT, the steric interaction between an asphaltene unit at the interface and a unit in the bulk prevents the deposition of a consecutive adsorption layer (see Figures 4 and 16B). Below Ct, in the non-steady-state adsorption regime, where NR/NA < (NR/NA)CRIT, asphaltenes are weakly stabilized both in the bulk and at the interface, (57) Ese, M. H.; Yang, X.; Sjo¨blom, J. Colloid Polym. Sci. 1998, 276, 800. (58) Pfeiffer, J. P.; Saal, R. N. J. J. Phys. Chem. 1940, 44, 139.


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Figure 15. Schematic variation of the number of resin and asphaltene kinetic units (NR and NA, respectively) as a function of bitumen concentration (solid lines). The threshold concentration Ct separates the steady-state and non-steady-state adsorption regimes. At the adsorption threshold concentration, Ct, which corresponds to the critical (NR/NA)CRIT ratio, the steric interaction between an asphaltene unit at the interface and a unit in the bulk is just sufficient to prevent the deposition of a second layer of asphaltenes. Above Ct, in the steady-state adsorption regime, where NR/NA > (NR/NA)CRIT, the steric interaction prevents the deposition of a consecutive adsorption layer. Below Ct, in the non-steady-state adsorption regime, where NR/NA < (NR/NA)CRIT, asphaltenes are weakly stabilized and a continuous build-up of the adsorption layer can be observed. As a result of resin addition (dotted line a) Ct shifts to Cta because NR1/NA2 = NR3/NA4. As a result of asphaltene addition (dotted line b) Ct shifts to Ctb because NR1/NA2 = NR5/ NA6.

and a continuous build-up of the adsorption layer can be observed (see Figures 5-7 and Figure 16A). This model also explains the dependence of the adsorption threshold concentration on the resin-to-asphaltene ratio. NR increases upon addition of resins to bitumen (see line a in Figure 15) and so does the NR/NA ratio. As a result, the adsorption threshold concentration of bitumen is lowered from Ct to Cta (see Figure 15), because now at this lower bitumen concentration the actual NR/NA ratio has already reached the critical value, (NR/NA)CRIT, necessary for the steric stabilization of asphaltenes, i.e., NR1/ NA2 = NR3/NA4. This explanation is in agreement with the observed decrease of the adsorption threshold concentration as a result of maltene addition (see Figures 8 and 9). On the other hand, the NR/NA ratio decreases with additional asphaltenes to bitumen because of the increase in NA (see line b in Figure 15). The (NR/NA)CRIT ratio necessary for stabilization will be reached only at a higher bitumen concentration because now NR1/NA2 = NR5/NA6; hence, the adsorption threshold concentration of bitumen increases from Ct to Ctb (see Figure 15). This is again in agreement with the observed increase of the adsorption threshold as a result of asphaltene addition (see Figures 8 and 9). The suggested hypothesis of the increasing resin-toasphaltene ratio with increasing bitumen concentration becomes a simple statement in the language of thermodynamics: The activity of the resins increases much more with bitumen concentration than the activity of asphaltenes does due to asphaltene association. The activity limitation of asphaltenes is similar to that of the aqueous micellar systems above the CMC. Therefore, at low bitumen concentration the resin-to-asphaltene activityratio is low, and the system becomes a poor solvent for asphaltenes. Under these conditions there is a high adsorption affinity toward the o/w interface resulting in a high adsorption amount.

Figure 16. Effect of bitumen concentration on water-in-oil emulsion stability against flocculation. (A) Continuous buildup of a rigid interfacial layer (nonstabilized asphaltenes) on a water droplet at low bitumen concentration. (B) The saturation layer (resin-stabilized asphaltenes) provides steric stabilization of the water droplet at high bitumen concentration. (C) Flocculation of two water droplets by asphaltene bridges at low bitumen concentration in solvent.

The adsorption behavior of bitumen at o/g and o/w interfaces is quite similar. In this respect, the difference between the adsorbed amounts does not even matter because the adsorbed amounts should be first corrected with the specific area of the interfaces, a topic that is beyond the scope of this work. The most important thing now is the existence of an adsorption threshold concentration for both o/g and o/w interfaces with the same bulk systems. Therefore, the existence of a threshold stems from the properties of bulk of the oil phase and not from the properties of the water or metal phases, in agreement with the relationship between steric stabilization in the

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bulk and the kinetics of adsorption we propose. Nevertheless, the difference between threshold concentrations for o/w and o/g interfaces should be associated with the differences in the attractive interactions between the adsorbing species in the bulk and the macroscopic phases (gold or water). From an adsorption perspective, it is the adsorption of resin-coated asphaltene units that increases the asphaltene content of the adsorption layer while the free resins are competing with these complex units for the interface. Since the diffusion coefficient of resins is higher than that of the associated asphaltenes, in the very first period of the adsorption process the resins adsorb at the o/w interface after its formation; then, these resins can subsequently be replaced by asphaltenes or asphaltene aggregates in agreement with the conclusions of electroacoustic spectroscopy studies.50 The observed continuous build-up of the interfacial film below Ct is due to the formation of bounds between sterically nonstabilized asphaltenes covered with a small amount of resins. The resulting structure possesses a remarkable mechanical strength because the multilayer adsorption forms a network of species and increases the number of bonds within the film. Therefore, it is very likely to generate rigid skins at the o/w interface, which has been described by Dabros et al.28 Thus, the adsorption threshold concentration of bitumen, Ct, between the nonsteady-state and steady-state regimes simply corresponds to the critical bitumen concentration of the rigid-to-flexible transition of the interfacial film, which was previously reported, among others, by Yeung et al.29 The rigidity at low concentration (see Figure 17, parts A and B) stems from the strong bonds between nonstabilized asphaltenes (see Figure 16A), while the flexibility of o/w interface above Ct (see Figure 17, parts D and E) is due to the steric repulsion between resin-stabilized asphaltenes at the interface (see Figure 16B). In addition, this work is to some extent in line with the results of Wu,6 who correlates the interfacial rigidity with the resin-to-asphaltene ratio of the interfacial material. We are also in agreement with Sjo¨blom and co-workers,59 who found by AFM imaging that resins modify the rigid structure of asphaltene films by creating more open fractal networks; furthermore, resins increase the compressibility of the interfacial film.57 3.7.2. Implications on the Stability of Water-in-Oil Emulsions. On the basis of the results of this study, a model is proposed to explain why water-in-oil emulsions are stable against flocculation at high, but unstable at low, bitumen concentration. At low bitumen concentration, a network of asphaltenes with a small amount of resins forms a rigid film at the o/w interface (see Figures 16A and 17, parts A and B). These asphaltenes at the interface are sterically not well stabilized with resins. Thus, they can accept either the formation of consecutive adsorption layers or the attachment of another water drop covered with similarly non-well-stabilized asphaltenes (see Figure 16C). Below the threshold bitumen concentration, Ct, the formation of asphaltene bridges between water droplets destabilize water-in-oil emulsions by flocculating them, which is in agreement with the observed flocculation of emulsions at low bitumen concentration in Figure 17C.6 Thus, dewatering of bitumen froth would be easier at low bitumen concentration. This is not the case at high bitumen concentration where a flexible interface is formed (see Figures 16B and 17, parts D and E) and a stable (59) Sjo¨blom, J.; Johnsen, E. E.; Westvik, A.; Ese, M. H.; Djuve, J.; Auflem, I. H.; Kallevik, H. In Encyclopedic Handbook of Emulsion Technology; Sjo¨blom, J., Ed.; Dekker: New York, 2001; p 595.

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Figure 17. Microphotographs about flexible and rigid interfaces (photos A, B and D, E adapted from Dabros et al.,28 with permission) and about stable and flocculated water-in-oil emulsions (photos C and F adapted from Wu,6 with permission). Photos A and B: contraction of a water droplet in 0.1 vol % bitumen in heptol (below Ct) by withdrawing water back into a micropipet; the interface crumples like a paper bag during contraction. Photos D and E: contraction of a water droplet in 10 vol % bitumen in heptol (above Ct) by withdrawing water back into a micropipet; the surface of the water droplet remains spherical during contraction. Photos C and F: water-in-diluted bitumen emulsions observed under a microscope. Photo C: unstable emulsion containing clustering water droplets; the bitumen concentration is 0.5 wt % in heptol (below Ct). Photo F: stable emulsion containing well-dispersed water droplets; the bitumen concentration is 5 wt % in heptol (above Ct).

water-in-oil emulsion can be observed as shown in Figure 17F.6 Here, resins and asphaltenes sterically stabilize water emulsions, and dewatering of bitumen froth by centrifugation requires high g-forces to yield clean resolved water.6,30 The interpretation of emulsion stability suggested here is also in agreement with the earlier established link between emulsion stability and asphaltene solubility. Here, we further elaborate this concept. In a previous survey, McLean and Kilpatrick17 concluded that “asphaltenes stabilize water-in-oil emulsions only if they are near or above the point of incipient flocculation, which suggests that their mode of action is to collect at the interface in the form of finely divided solid particles or aggregates”. They also suggested an emulsion stabilization model based on the steric stabilization of asphaltenes in the bulk and the steric stabilization of water droplets by asphaltenes with a protective resin layer. A strong point was also made about the resin-to-asphaltene ratio as one of the main parameters in emulsion stability through its effect on


asphaltene solubility.17,18,60 We have to point out that in this work we are making predictions on emulsion stability against flocculation only, while previous studies consider the amount of resolved water by centrifugation, which is related to the stability against both flocculation and coalescence. Nevertheless, the steric stabilization model of emulsions proposed by Kilpatrick and co-workers is in excellent agreement with our model. Furthermore, they observed that emulsions have their highest stability at an “intermediate” resin-to-asphaltene ratio21san observation which again is in agreement with our findings. According to our model, the asphaltenes at the interface should possess a dense enough resin-coating layer to be an effective stabilizer; a certain amount of resins is thus necessary for the stabilization. Too much resin, however, can reduce considerably the chances of asphaltene adsorption, leading to interfaces that are composed of resins only, and this was found to result in low emulsion stability.61 There is, however, one point, that needs further elaboration: it is difficult to explain, solely on the basis of the above cited asphaltene flocculation model, why the adsorption threshold concentration is apparently the same in 75 vol % toluene/25 vol % heptane mixtures (data are not presented here) which is definitely out of the range of asphaltene precipitation,21 especially if one considers the presence of resins in bitumen as well. A possible way to resolve this issue is the introduction of the above hypothesis of the increasing resin-to-asphaltene ratio (in kinetic units) with increasing bitumen concentration. Consequently, our emulsion stabilization model can be considered as a refinement of previous models. 4. Conclusions The main conclusions are as follows: (1) The RQCM is a simple and powerful tool that allows the study of bitumen adsorption at different interfaces. An approach, based on the use of silica-gel-coated QCM crystals in water-saturated solvents, is devised to examine adsorption at o/w interfaces. (2) Two distinct adsorption regimes are established at both oil/gold and o/w interfaces according to bitumen concentration: a steady-state regime at high concentration where adsorption saturation can be observed and a non(60) Kallevik, H.; Kvalheim, O. M.; Sjo¨blom, J. J. Colloid Interface Sci. 2000, 225 (2), 494. (61) Yan, Z.; Elliott, J. A. W.; Masliyah, J. H. J. Colloid Interface Sci. 1999, 220, 329.

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steady-state regime at low concentration where continuous build-up of the interfacial layer is detected. For heptoldiluted bitumen, the threshold concentration between the steady-state and non-steady-state regimes is 1.5 wt % for the o/w interface and 8 wt % for the oil/gold interface. (3) The threshold concentration between these regimes, the adsorption mass densities, and the flux of non-steadystate adsorption are determined by the resin-to-asphaltene ratio in bitumen. (4) Adsorption is irreversible in the non-steady-state regime (at low bitumen concentration) and partially reversible in the steady-state regime (at high bitumen concentration). Nevertheless, the elemental adsorption and desorption processes may still be in equilibrium in the steady-state regime. (5) In the 1.5-10 wt % bitumen concentration range in heptol, the adsorbed amount of bitumen at the o/w interface is 1.8 µg/cm2 and is independent of concentration. In the 8-10 wt % bitumen concentration range in heptol, the adsorbed amount of bitumen is 0.6 µg/cm2 at the oil/ gold interface. The adsorbed amount of maltenes from heptol increases sharply in the 0-1 wt % concentration range reaching a saturation value of approximately 1.5 µg/cm2 in the 1-10 wt % concentration range. (6) A model is suggested for both the interpretation of adsorption behavior at interfaces and the stability of water-in-oil emulsions in bitumen or in mixed systems of asphaltenes and resins. According to this model the following can be concluded: (i) The ratio of resin-toasphaltene kinetic units increases with bitumen concentration. (ii) There is a critical ratio above which the asphaltenes are sterically well stabilized by resins. (iii) Above the critical ratio, the well-stabilized asphaltenes prevent both the continuous asphaltene adsorption at o/w and oil/metal interfaces and the flocculation of water drops in petroleum fluids. (iv) Below the critical ratio, the sterically nonstabilized asphaltenes adsorb continuously at o/w and oil/metal interfaces and form rigid interfacial networks, which enhances the flocculation of water drops in petroleum fluids. Acknowledgment. The financial support through the NSERC Oil Sands Research Chair in Oil Sands Engineering (J. H. Masliyah) is gratefully acknowledged. The authors thank Darlene Mahlow at the Department of Chemistry, University of Alberta, for the VPO measurements. LA050333F

Water Interfaces

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