Impact of Asphaltenes and Naphthenic Amphiphiles on the Phase Behavior of Solvent−Bitumen−Water Systems

Published: March 25, 2011

r 2011 American Chemical Society 2223 dx.doi.org/10.1021/ef1016285 | Energy Fuels 2011, 25, 2223–2231

ARTICLE

pubs.acs.org/EF

Impact of Asphaltenes and Naphthenic Amphiphiles on the PhaseBehavior of Solvent�Bitumen�Water SystemsSumit K. Kiran,† Samson Ng,‡ and Edgar J. Acosta*,†

†Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto,Ontario M5S 3E5, Canada‡Edmonton Research Centre, Syncrude Canada Limited, Edmonton, Alberta T9H 3H5, Canada

ABSTRACT: A limiting factor impacting the quality and recovery of bitumen from oil sand operations is the formation of stablewater-in-oil (w/o) and/or oil-in-water (o/w) emulsions during froth treatment. In a previous study, the impact of asphaltenepartitioning on oil�water phase separation from the resulting emulsified phases (rag layers) was evaluated as a function of thesolvent�bitumen�water ratio, temperature, and solvent aromaticity. In this work, the added effect of naphthenic amphiphiles atconcentrations of 3 and 10 wt % on oil�water phase separation from similarly formulated rag layers is assessed. The observed phasebehavior of these rag layers is discussed in view of interfacial coadsorption mechanisms proposed in the literature. A major finding isthat, under alkaline process conditions, an increase in the concentration of sodium naphthenates (NaNs), produced as a result ofnaphthenic amphiphile dissociation, promotes a shift in emulsion morphology from w/o to o/w. The resulting transition fromasphaltene- to NaN-controlled properties significantly limits oil�water phase separation as a result of an increase in the surface areato volume ratio of dispersed droplets and an enhancement of interfacial asphaltene partitioning. Contrary to NaN-free systems, itwas also observed that both the temperature and solvent aromaticity have a minimal effect on the phase behavior of NaNformulations. Furthermore, undissociated naphthenic amphiphiles, referred to as naphthenic acids, are capable of promotingoil�water phase separation under acidic formulation conditions.

’ INTRODUCTION

Bitumen is a heavy crude oil that surrounds both quartz sandparticles and naturally bound thin films of water-in-oil sanddeposits.1 The recovery and conversion of such an unconven-tional crude oil into high-quality liquid fuels is essential as theworld reserve of light crude oil is now becoming limited becauseof the increasing global demand at an average rate of 1.42% perannum.2 To successfully liberate bitumen from oil sand ores,commercial water-based extraction processes are utilized.1,3,4 Amajor operational issue associated with these processes is theformation of stable water-in-oil (w/o) and oil-in-water (o/w)emulsions that limit bitumen�water separation and lead tofouling of the processing equipment. The term “rag layer” istypically reserved to describe such emulsion phases.5,6

The extent of rag layer formation and stabilization is severelyimpacted by the presence of surface-active species found natu-rally within heavy crude oil. One such component, whichconstitutes approximately 18 wt % bitumen, is asphaltene.7 As-phaltenes may be classified as highly aromatic structures thatsolubilize in aromatic hydrocarbons (e.g., benzene and toluene)and precipitate upon mixing with paraffinic solvents (e.g., pentaneand heptane).7,8 The degree of surface activity of asphaltenesvaries according to their solubility. Previous researchers havedemonstrated that asphaltenes undergo amonomer-to-aggregatetransition under partially soluble conditions, a transition that ismarked by an enhanced affinity for the oil�water interface.7,9�11

The resulting asphaltene skins, which are lipophilic in nature,promote the formation of w/o emulsions and act as a rigid energybarrier that adjacent water droplets must overcome in order tocoalesce.12�15 A contributing factor affecting the physical

properties of asphaltene skins is the oil�water interfacial area.11

The provision of larger interfacial areas tends to limit thethickness of these multilayer structures.

Naphthenic amphiphiles, which are present in heavy crude oilat concentrations up to 4 wt %, also notably impact the rag layerproperties.16 These endogenous surfactants, which have a pKa of∼6, may be categorized as either naphthenic acids (NAs) ornaphthenate salts [e.g., sodium naphthenates (NaNs)] accordingto the pH of the system.17 Under acidic conditions, where the pHis less than the pKa of naphthenic amphiphiles,NAs are the dominantsurrogate. This term encompasses all alkyl-substituted cycloaliphaticcarboxylic acids (R-COOH) presentwithin the crude oil.18Althoughmore hydrophilic than asphaltenes, NAs remain oil-soluble and thusalso promote the formation of w/o emulsions.15 Alkaline treatmentof bitumen, as is required in oil sand processing and enhanced oilrecovery, results in a pH greater than the pKa of naphthenicamphiphiles. Consequently, NaNs are produced through the asso-ciation of dissociated carboxylic acid anions (COO�) in the oil phasewith sodium cations (Naþ) dissolved in the aqueous phase.19 Theresults obtained by Havre et al. indicate that these metallic soapsfacilitate emulsification at increased pH values by significantlyreducing the oil�water interfacial tension (γo/w).

20 Being water-soluble,NaNs induce a shift in the rag layermorphology fromw/o too/w.15 It has been proposed that the principal mechanism by whichnaphthenic amphiphiles stabilize emulsions is via formation oflamellar liquid crystals.21,22 These mesomorphic phases, which

Received: December 1, 2010Revised: February 26, 2011

2224 dx.doi.org/10.1021/ef1016285 |Energy Fuels 2011, 25, 2223–2231

Energy & Fuels ARTICLE

are typically produced under concentrated surfactant regimes,spontaneously spread across the oil�water interface and reduceits mobility and bending ability. The effects of electrostaticrepulsion also become relevant for o/w dispersions.23

Despite recent advances in the understanding of the individualrole of the above surfactants on emulsification in crude oilsystems, their synergistic behavior remains relatively unexplored.Insight into this matter for asphaltene�NaN mixtures wasoffered by Wu and Czarnecki using a thermodynamic modelingapproach.24 These authors proposed a bilayer structure todescribe the competitive adsorption of asphaltenes and NaNsat the oil�water interface. In this hypothesized structure, NaNsoccupy the primary adsorbed layer and therefore act to suffi-ciently reduce γo/w and control the rag layer morphology. Themakeup of the secondary, or “floating”, layer is predominantlyasphaltenes. Because of the minimal interaction of this layer withthe oil�water interface, it mainly serves to enhance the emulsionstability. Elements of this interfacial model were justified experi-mentally in a follow-up study by Moran and Czarnecki.25 By acomparison of the γo/w isotherms of an aqueous NaN solutionwith a highly diluted bitumen sample (0.1%) and a syntheticsolvent (Heptol), it was observed that NaNs completely displaceasphaltenes from direct oil�water interfacial adsorption at aconcentration of 0.1 wt %. Furthermore, using droplet interac-tion experiments, these researchers showed that emulsionsprepared using diluted bitumen, instead of Heptol, exhibit amore rigid interface because they are able to withstand coales-cence over prolonged contact periods. The nature of asphalte-ne�NA films is less well-defined. Upon analyzing of theirmolecular adsorption characteristics, Varadaraj and Brons con-cluded that these components likely adsorb at oil�water inter-faces as either mixed aggregates or mixed monolayers.26 Theimpact of the resulting surfactant mixture on the stability ofdispersed water droplets was assessed indirectly by Poteau et al.via coalescence studies involving the addition of maltenes(containing NAs) to diluted asphaltene solutions.27 Theseauthors observed that asphaltene�maltene mixtures consider-ably enhance the emulsion stability. In contrast to these indirectobservations, Gao et al. suggested that NAs soften the oil�waterinterface and promote emulsion coalescence.28

A critical gap in the literature that this work aims to address ishow naphthenic amphiphiles impact the phase behavior ofsolvent�bitumen�water systems under formulation conditionscompatible with bitumen extraction processes. On the basis ofthe literature above, it is hypothesized that the separation ofbitumen�water emulsions is influenced by the presence ofnaphthenic amphiphiles when using formulation conditions thatpromote their adsorption at the oil�water interface and that thenature of this influence may vary depending on the interaction ofthe naphthenic amphiphile with asphaltenes. To evaluate thisinfluence, the oil�water separation, asphaltene partitioning, andrag layer properties will be characterized as a function of therelative solvent�bitumen�water ratios, solvent aromaticity, pH,and temperature at different naphthenic amphiphile concentra-tions. The resulting phase behaviors are discussed in light of theinterfacial coadsorption mechanisms currently available in theliterature.

’MATERIALS AND METHODS

Materials. All materials were used as received: anhydrous toluene(99.8%) and naphthenic acids (NAs; technical-grade extract) were

purchased from Sigma-Aldrich Canada, heptane (g99%) and hexane(reagent grade) were obtained from Caledon Laboratory Chemicals,coker feed bitumen (16.3 wt % saturates, 39.8 wt % aromatics, 28.5 wt %resins, and 14.7 wt % asphaltenes according to SARA analysis29) wasdonated by Syncrude Canada Ltd., NaNs (90%) were supplied byEastman Kodak, and HCl (6 N) and NaOH (10 N) were acquired fromVWR. A saline solution was prepared by dissolving 25 mmol/L NaCl,15 mmol/L NaHCO3, 2 mmol/L Na2SO4, 0.3 mmol/L CaCl2, and0.3 mmol/L MgCl2 in deionized water to simulate typical watercompositions in oil-field operations.4 The pH of this solution isapproximately 7.5.Formulation. Homogeneous oil phases were prepared by mixing

Heptol and bitumen together at mass ratios of 1:1.5, 1:1, 3:1, 4:1, and10:1 in 250 mL glass jars overnight using a wrist-action shaker with astroke length and frequency of approximately 4 cm and 180 strokes/min, respectively. Heptol blends tested include 80 vol % heptane�20vol % toluene (Heptol 80/20) and 50 vol % heptane�50 vol % toluene(Heptol 50/50). Aqueous solutions consisting of 3 and 10 wt % NaNsdissolved in saline water were added to the diluted bitumen samples atmass ratios of 1:10, 1:3, 1:1, and 3:1 in 15 mL flat-bottomed glasscentrifuge tubes.Using a previously establishedbatch emulsification�separa-tion protocol, these formulations were mixed (VWR Vortex Mixer) at3200 rpm for 2 min and then centrifuged (IEC Clinical Centrifuge) atapproximately 500 Gs for 1 min at room temperature.11 The tempera-ture effects on oil�water phase separation were evaluated by placing theabove formulations in a hot water bath at 80 �C prior to mixing andcentrifugation. To modify the formulation pH, 6 N HCl and 10 NNaOH were added dropwise to the aqueous phase until the respectiveend points pH 4 and 10 were detected using an Oakton Benchtop pH/ion 510 meter. As a result of the onset of NaN precipitation upon HCladdition, NAs were predissolved within the oil phase at concentrationsof 3 and 10 wt %.Microscopy. Mixing and centrifugation of the formulations

described above resulted in oil�water phase separation, which waslimited by the formation of an intermediate rag layer (Figure 1). Allobservable phases were sampled at three distinctive random pointsand analyzed using the combination of anOlympus BX-51microscopeat 50� magnification and an Olympus C-7070 wide-zoom digitalcamera set to its maximum magnification (4�).11 Three differentmicroscope configurations were used: optical (transmitted) light,which was used to differentiate between the solid and liquid phases;transmitted light with cross-polarized lenses, which was used to detectthe presence of liquid crystals; fluorescence microscopy, whichdifferentiated between the oil (green) and aqueous (black) phases.In comparison to the rag layer, only traces of emulsified phases weredetected in the fluorescent micrographs of the excess oil and aqueousphases. The purities of the excess oil (<1% water for all systems) andaqueous (<4% oil for all systems) phases were confirmed using

Figure 1. (a) Optical and (b) fluorescent micrographs taken fromsamples of the excess oil phase, excess aqueous phase, and rag layer for a3 wt % NaN formulation at 25 �C with a Heptol 80/20 to bitumendilution ratio of 10:1 and an aqueous phase to oil phase ratio of 1:3.

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Karl-Fischer titration (KAM PKF Portable Karl-Fischer MoistureAnalyzer) and total carbon analysis (Shimadzu TOC-VCPH/CPN),respectively. Furthermore, all cross-polarized images, except for thosetaken of systems at pH 4, were omitted from the analysis because noliquid crystals were detected.Material Balances. A detailed overview of the material balance

approach utilized in this study to track the composition of the rag layeris provided elsewhere.11 In short, the volume of each phase producedwas initially calculated from its relative height measurement. As aresult of the excess phases being essentially pure, the composition ofoil and water within the rag layer was approximated as the differencebetween their initial volumes added and measured excess volumesafter separation. Fluorescent images of the rag layer were analyzed, asfollows, using Scion Image Software in order to validate this materialbalance closure:30

(1) A droplet of the rag layer (<20 μL) was spread on top of a glassslide using a 22 mm� 22 mm glass coverslip in order to producea thin film of 10 ( 2 μm.

(2) The oil and water volumes within the rag layer were approxi-mated by converting the fluorescent rag layer micrographs into abinarymap of clear (oil) and dark (water) regions. The thresholdof such binary maps was adjusted such that the boundarybetween the oil and water areas was located in the middle ofthe fuzzy interface. This fuzzy interface is caused by the curvatureof the walls of the squeezed droplets.

(3) Measurement tools within the software package were used tocalculate the total area of the rag layer film (pixels2) along withthat of the oil and water phases (pixels2).

(4) The ratio of the oil and water area to the total area was used toapproximate the volume fraction of these phases within therag layer.

The rag layer composition measured using the material balance andfluorescent micrographs was in agreement within (1 vol %. Thestandard deviation of oil lost to the rag layer in replicate systems wase5 vol %.Asphaltene Losses. The UV�vis spectroscopic methodology

developed by Yang et al. to determine the concentration of asphal-tenes in heavy crude oil was recently adapted to measure the fractionof asphaltenes lost to the rag layer in solvent-bitumen-watersystems.7,11 In this technique, a calibration of the asphaltene absor-bance as a function of its concentration in toluene (expressed in termsof a bitumen to toluene ratio) was established at a wavelength of450 nm using an Ocean Optics spectrophotometer (model HR2000).This relationship is presented in Figure 2. At a given bitumen totoluene ratio, the difference in the absorbance between the initial oilphase and the excess (free) oil phase after emulsification and separa-tion was used to calculate the fraction of asphaltenes lost to the rag

layer:

asphaltenes lost ð%Þ ¼ absorbance of the initial oil� absorbance of the excess oilabsorbance of the initial oil

�100 ð1Þ

Here, the percentage lost represents both the fraction of asphaltenesadsorbed/segregated and precipitated. From the material balance per-spective, it should be noted that oil and asphaltene losses to the rag layerare treated independently. As a result, it is possible to observe a largefraction of oil lost to the rag layer and a near-zero fraction of asphalteneslost to the rag layer if the concentration of asphaltenes in the resultingfree oil phase is similar to that of the original oil.Interfacial Tension Measurements. For NaN formulations,

where γo/w < 1 mN/m, a spinning drop tensiometer manufactured byTemco Inc. (model 500) was utilized. In this technique, a borosilicateglass tube was first filled with the aqueous phase of interest. A 2 μLdroplet of the oil phase was then inserted, and the glass tube was spun atincreasing revolutions per minute (rpm) values until the oil dropletexpanded sufficiently such that its length was 4 times greater than itswidth. Once the oil droplet expansion reached an equilibrium value at agiven rpm, γo/w was calculated as follows:15

γo=w ¼ ΔFω2w3

4ð2Þ

In this equation, ΔF is the density difference between the heavy(aqueous) and light (oil) phases,ω is the rotational velocity, andw is thewidth of the oil droplet. Freshly prepared NaN and diluted bitumensolutions were used as the aqueous and oil phases, respectively, becauseof the poor phase separation results. The operating temperature wasadjusted via a thermocouple and a temperature controller.

A KSV Sigma 700 tensiometer equipped with a platinum Du No€uyring probe was required for the γo/w measurement of pH 4 systemsbecause of their relatively large values (20�26 mN/m). In this techni-que, the DuNo€uy ring was initially immersed in 5mL of a given oil phasethat resided on top of 10 mL of acidified saline water. As the Du No€uyring was lowered through the oil�water interface, the resulting forceexerted on it was calculated using the following relationship:31

F ¼ ðFw � FoÞgVo ð3ÞHere, Fw and Fo are the densities of the aqueous and oil phases,

respectively, g is the gravitational acceleration (9.81 m/s2), and Vo is thevolume of oil pulled through the oil�water interface. γo/w was subse-quently determined as follows:31

γo=w ¼ F4πRr

fRr

3

Vo,Rr

Rw

!ð4Þ

Tabulated values of f, which is a dimensionless quantity that may beexpressed exclusively as a function of the probe’s ring (Rr = 9.545 mm)and wire (Rw = 0.185 mm) radii as well as Vo, are available throughoutthe literature.32�34 The Du No€uy ring was rinsed with toluene, ethanol,and deionized water prior to reuse. Because of the large volumes ofexcess aqueous and oil phases required for this methodology, the effectof the aqueous phase to oil phase ratios on the γo/w isotherms could notbe assessed in situ.Surface Pressure (π)�Area (A) Isotherms. The effect of NAs

on the collapse pressure and elasticity (ε) of asphaltene skins was testedusing a KSV Minitrough operated on a vibration-free table. Initially,asphaltenes were precipitated from bitumen via hexane dilution at ratios>40:1. Recovered asphaltenes were rinsed with additional quantities ofhexane in order to minimize maltene contamination.7 Surfactant mix-tures of 100% asphaltenes, 75% asphaltenes�25% NAs, 50% as-phaltenes�50% NAs, 25% asphaltenes�75% NAs, and 100% NAs

Figure 2. Calibration of the asphaltene absorbance as a function of thebitumen to toluene ratio at λ = 450 nm. The error in the y intercept wasdetermined using the built-in data analysis package available inMicrosoftExcel 2010.




were subsequently diluted with toluene to a concentration of 0.1%. Avolume of 45 μL of each of these surfactant solutions was spread on asubphase composed of pure deionized water with a surface tensionmeasurement comparable to that of saline water (∼71 mN/m). Afterwaiting 10 min to allow for solvent evaporation, the spreading phase wascompressed symmetrically at a constant rate of 8 mm/min from an areaof 256 to 23 cm2 using two interlinked surface barriers. The change in π(mN/m) during compression was measured using a Wilhelmy plate asfollows:35

π ¼ σo � σ ð5ÞIn this equation, σo and σ represent the surface tension (mN/m) in

the absence and presence of surface-active molecules, respectively. Thecollapse pressures of asphaltene�NA films were interpreted from theresulting π�A isotherms as the maximum attainable π. Furthermore,the resulting surfactant film ε value, which is a measure of their resistanceto compressibility, was calculated as follows:36

ε ¼ dπdðln AÞ �

Δπ

Δðln AÞ ð6Þ

Each compression test was followed by a cleaning cycle in which allinstrumentation was thoroughly rinsed with ethanol and deionizedwater. The cleanliness of the subphase was verified by ensuring that π< 0.30 mN/m prior to deposition of the spreading phase. In addition, itwas verified by spreading toluene alone onto pure deionized water thatthe solvent has a negligible impact on π. Although the π�A isothermsgenerated in this study are not a true reflection of those attainable at theoil�water interface, they provide an accurate depiction of the trends inchanges of the film stability for asphaltene�NAmixtures. This point was

illustrated in an earlier study by Zhang et al. in which similarities in thetrends of surface pressure and interfacial pressure isotherms of asphal-tene�demulsifier mixtures were observed.35

’RESULTS

Solvent�Bitumen�Water Systems. A detailed overview ofthe major results obtained from baseline solvent�bitumen�water phase behavior studies in the absence of naphthenicamphiphiles is published elsewhere.11 In short, larger oil andasphaltene losses to the rag layer are observed for systemsprepared at 25 �C with an aliphatic solvent (Heptol 80/20)upon an increase in both the bitumen dilution ratio and theaqueous phase concentration. Such an effect is observed as aresult of the enhanced stability of emulsion droplets produced viaan increase in the interfacial activity of asphaltenes. An increase inthe temperature to 80 �C helps mitigate oil losses to the rag layerdespite having little effect on the asphaltene partitioning beha-vior. In the presence of a more aromatic solvent (Heptol 50/50),low solvent and high water contents contribute to the stability ofemulsions. These trends are depicted in Figure 3a�f for systemsprepared at 50 and 9.1 wt % aqueous phase concentrations. All ofthe rag layers produced display a w/o morphology with no signsof liquid-crystal formation.Phase Behavior of Systems Containing Naphthenic Am-

phiphiles. Overlaying the baseline phase behavior results pre-sented in Figure 3a�f are those obtained using a NaNconcentration of 3 wt % at pH 7.5. For systems prepared usingHeptol 80/20 at 25 �C (Figure 3a,b), oil and asphaltene losses tothe rag layer are worsened at increased water concentrations. At awater content of 75 wt %, the entire oil phase is emulsified withinthe rag layer. For systems containing e50 wt % water, reducingthe solvent to bitumen dilution ratio significantly increases the oillosses to the rag layer. Despite observing similar trends inoil�water separation at 80 �C (Figure 3c) and when usingHeptol 50/50 as the solvent (Figure 3e), complete emulsificationof the oil phase within the rag layer is observed at a watercontent g50 wt %. As illustrated in Figure 4, acidifyingsolvent�bitumen�water systems to pH 4 significantly improvesoil recovery.With regards to the asphaltene partitioning behavior in the

above systems, parts b and d of Figure 3 show that an increase inthe Heptol 80/20 to bitumen dilution ratio promotes an increasein asphaltene losses to the rag layer. Such losses may beassociated with interfacial segregation because the heptaneconcentration in Heptol required to promote the onset of

Figure 3. Oil and asphaltene losses to the rag layer for systemsprepared with a 0 wt % (baseline) and 3 wt % NaN aqueous phase(AP) solution at pH 7.5. Formulation conditions: (a and b) Heptol80/20 and 25 �C, (c and d) Heptol 80/20 and 80 �C, and (e and f)Heptol 50/50 and 25 �C.

Figure 4. Oil losses to the rag layer for 0 wt % (baseline) and 3 wt %NAsystems as a function of theHeptol 80/20 to bitumen and aqueous phase(AP) to oil phase ratios. All systems were evaluated at 25 �C and pH 4.

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asphaltene precipitation is 85 vol %.11 Whereas asphaltene lossesare independent of the temperature, they are notably lower forHeptol 50/50 systems (Figure 3f) and decrease with increasedsolvent to bitumen dilution ratios. In all cases, the presence of 3wt % NaNs induces an increase in asphaltene losses relative tobaseline formulations. Asphaltene losses were not evaluated forsystems at pH 4. Here, the presence of NAs changed the visibleabsorbance spectrum of diluted bitumen. A similar shift observedby €Ostlund et al. in the near-IR spectra of bitumen�NAmixturessupports the idea of asphaltene�NA interactions.37

Transitions in the Rag Layer Morphology. Changes in therag layer morphology produced upon increases of the concentra-tion of naphthenic amphiphiles from 0 to 10 wt % in sol-vent�bitumen�water systems are illustrated in Figure 5a,b.For all NaN systems at pH g7.5, an increase in the surfactantconcentration results in a transition of the rag layer from w/o too/w. This transition is accompanied by a major decrease in thedroplet size from 0 to 3wt%NaNs, whereas such changes are lessdramatic from 3 to 10 wt % NaNs. At pH 4, where NAs aredominant, a w/o rag layer is continuously observed despite anincrease in the surfactant concentration. Traces of multipleemulsions also become evident. Of special interest for systemsprepared at pH 4 is the increased observance of liquid crystals atlarger NA concentrations. These phases, which appear as brightspots in Figure 5c, do not form continuous films adsorbed atoil�water interfaces, as suggested by other researchers.21 In-stead, they appear as aggregates dispersed throughout the oilphase. Differences in the resulting liquid-crystal structures maybe attributed to variations in the emulsification protocol.Interfacial Tension Isotherms. Baseline γo/w isotherms as

well as those obtained for systems prepared using naphthenicamphiphiles are presented in Figure 6a,b. At increased Heptol80/20 to bitumen dilution ratios, it is observed that NaNssignificantly reduce γo/w from ∼5�10 to 0.5�0.9 mN/m.

Introducing NAs into similar formulations at pH 4 results inan increase in γo/w from∼20�22 to 23�26 mN/m. In both ofthe above scenarios, increasing the naphthenic amphiphileconcentration from 3 to 10 wt % results in negligible changesin γo/w. Furthermore, varying the formulation temperature andsolvent aromaticity has little influence on all of the aboveisotherms. Equilibrium γo/w is obtained almost instantaneouslyin all of the above systems. This finding is in agreement withthat of Moran and Czarnecki, who suggested that all dynamicbehavior is lost at such large asphaltene and naphthenicamphiphile concentrations because of their relatively fastadsorption kinetics and high interfacial packing.25

Impact of NAs on Asphaltene Film Properties.The effect ofNAs on the collapse pressure and ε of asphaltene skins may beinterpreted from the π�A isotherms illustrated in Figure 7.According to these results, pure asphaltenes produce morestable films than pure NAs, as indicated by their larger collapsepressure. Mixtures of these two components show an inter-mediate behavior where the film stability is reduced at increasedNA concentrations. As shown in Table 1, such mixtures aregood representations of the spread in the relative asphalteneand NA compositions tested. Values of ε for asphaltene�NAfilms produced were calculated at π values between 0 and 10mN/m, as is the common linear regime among all π�Aisotherms generated. The results presented in Table 2 suggestthat all such films are prone to deformation. It is interesting to

Figure 5. Fluorescent micrographs representing the effect of (a) NaN(pH 7.5) and (b) NA (pH 4) concentrations on the rag layermorphology for solvent (Heptol 80/20)�bitumen�water systems.The oil and aqueous phases are green and black, respectively. (c)Cross-polarized images of the rag layer for systems prepared at pH 4revealing the presence of liquid crystals (bright spots) at larger NAconcentrations.

Figure 6. Baseline γo/w for (a) NaN systems prepared at variousaqueous phase concentrations and pH 7.5 and (b) NA systemsprepared at pH 4 as a function of the Heptol 80/20 to bitumen dilutionratio. The effects of added naphthenic amphiphile concentration are alsoevaluated. The temperature was maintained at 25 �C in all of the aboveformulations.

Figure 7. Surface pressure�area isotherms of asphaltene (A)�NAsurfactant mixtures.






note that asphaltene�NA films in the NA-rich domain show alower ε than that of pure NAs.The π�A isotherms for NaN systems were conducted by first

spreading 45 μL of a 0.1% asphaltene solution on pure deionizedwater. A concentrated NaN solution was then infused within thesubphase such that its final concentrations tested were 0.1%, 1%,and 3%. The resulting collapse pressures were measured as 25, 9,and 6mN/m, respectively. Although this data illustrate the abilityof NaNs to significantly weaken asphaltene films, it cannot beextrapolated to phase behavior studies because a liquid�liquidtrough is required to permit the formation of a secondaryasphaltene layer on top of the primary adsorbed NaN layer.Gao et al. successfully implemented the above recommendationfor interfacial films composed of asphaltenes and NaNs in a 1:1volume ratio.28 The resulting π�A isotherm of the bilayerstructure showed an intermediate behavior compared to pureasphaltenes and pure NaNs with a collapse pressure >30 mN/m.

’DISCUSSION

Interfacial Coadsorption of Asphaltenes and NaNs. Thebaseline phase behavior trends illustrated in Figure 3a,b areindicative of asphaltene-controlled interfacial properties in sol-vent�bitumen�water formulations prepared at 25 �C and pH7.5. Within this regime, oil losses to the rag layer are reduced atlow Heptol 80/20 to bitumen dilution ratios as a result of fullysoluble asphaltene monomers existing in their non-surface-activestate.11 Such conditions enable oil�water phase separationbecause a decreased energy input is required for emulsiondroplets to coalesce. This conclusion is in agreement with thatof �Alvarez et al., who effectively showed a decrease in the collapsepressure of asphaltene films at lower surface concentrations.38

Increasing the Heptol 80/20 to bitumen dilution ratio inducesthe self-assembly of asphaltenemonomers into partially insolublesurface-active aggregates. The tendency of such aggregates toenhance the emulsion stability results in larger oil losses to therag layer. Increasing the aqueous phase to oil phase ratio furthermagnifies rag layer formation and stabilization because of theincrease in the oil�water interfacial area produced upon mixingthat is available for asphaltenes to segregate to.In a comparison of the asphaltene- and NaN-controlled

regimes, trends depicting increased oil losses to the rag layer atincreased aqueous phase to oil phase ratios are maintained inaddition to the partitioning behavior of asphaltenes. Of specialinterest, however, is the increased magnitude of oil lossesobserved in NaN systems. To understand this phase behavior,the effect of the NaN concentration on emulsification must firstbe evaluated. The first step in establishing this link is to discussthe relationship between γo/w and the largest droplet diameter

(dmax) in the emulsion:

dmax � γo=wR ð7Þ

where R is 1 for laminar flow and 0.6 for turbulent flow.39

According to this relationship, any reduction in the interfacialtension produces a reduction in the drop size. The change in dmaxalso changes the surface area to volume (SA/V) ratio of dispersedemulsion droplets as follows:

SAV

¼ 6dmax

ð8Þ

From the above equations, it is apparent that γo/w is inverselyrelated to the SA/V ratio of a given emulsion droplet. Figure 8illustrates changes in γo/w measured experimentally for a givenoil phase as a function of the NaN concentration. According tothis data, γo/w decreases linearly up to a NaN concentration of 1wt % [Δγo/w/ΔNaN∼�6.2 (mN/m)/wt % NaN]. Holding allparameters constant in eq 7 other than γo/w, eq 8 reveals apotential increase in the SA/V ratio of emulsion droplets by afactor of 3 upon increasing the NaN concentration from 0 to 1 wt%. Emulsion stabilization is facilitated at such large interfacialareas as the potential for asphaltene adsorption is enhanced.Figure 3b shows the increase in asphaltene losses to the rag layer.At NaN concentrations >1 wt %, changes in the SA/V ratio ofemulsion droplets are less pronounced [Δγo/w/ΔNaN ∼ �0.3(mN/m)/wt % NaN]. Furthermore, it is here where thedominant morphology of the emulsion droplets switches fromw/o to o/w. This transition in the emulsion properties coincideswith the fact that the critical micelle concentration (CMC) ofNaNs is 1 wt %. A similar CMC value was previously reported byMoran and Czarnecki.25 Beyond this concentration, the sponta-neous increase in the total number of dispersed droplets further

Table 2. Film Elasticity Measurements

system ε (mN/m)

100% asphaltenes 73

75% asphaltenes�25% NA 34



100% NA 23

Figure 8. Interfacial tension between NaN solutions and Heptol 80/20at pH 7.5 (γo/w) as a function of the NaN concentration. The transitionin the rag layer morphology from w/o (NaN < CMC) to o/w (NaN >CMC) is demonstrated using fluorescent micrographs where the aqu-eous and oil phases are black and green, respectively. CMCNaN∼ 1 wt%.

Table 1. Relative Asphaltene and NA Compositions in For-mulations Tested

3 wt % NA 10 wt % NA

Heptol 80/20 to bitumen ratio % asphaltenes % NA % asphaltenes % NA

1:1.5 78 22 52 48

1:1 75 25 47 53

3:1 60 40 31 69

4:1 55 45 26 74

10:1 35 65 14 86




increases the overall oil�water interfacial area and asphaltenelosses to the rag layer. Under interfacial saturation conditions,which occurs at NaN concentrations > CMC, the bilayer modelproposed by Wu and Czarnecki is believed to upheld.24 Here,asphaltene skins must adsorb as a secondary layer at the oil�-water interface because of NaNs predominantly occupying theprimary adsorbed layer, as suggested from the γo/w isothermprovided in Figure 8. A modified schematic of this model isprovided in Figure 9a.Of the above mechanisms available for increasing the oil�-

water interfacial area, an increase in the SA/V ratio of emulsiondroplets is the primary factor leading to larger oil losses to the raglayer. This is supported by the data in Figure 3a for systemsprepared with 0 and 3 wt % NaNs, respectively. Contributionsfrom spontaneous emulsification are less significant becausechanges in oil losses to the rag layer for systems prepared with3 and 10 wt %NaNs were marginal. It should be noted thatΔγo/w/ΔNaN in Figure 8 below the CMC of NaNs is impactedsignificantly by formulation conditions. This is concludedfrom Figure 6a for systems prepared with 0 wt % NaNs whereγo/w is observed to vary according to changes in the Heptol80/20 to bitumen and aqueous phase to oil phase ratios as aresult of modifications in asphaltene partitioning within therag layer. Also depicted in Figure 6a is that the impact of theabove variables on γo/w at NaN concentrations > CMC is lessprominent.The decrease in oil losses to the rag layer at large bitumen

dilution ratios described in Figure 3a for 3 wt % NaN systems atlow aqueous phase compositions opposes theoretical principlesrelating asphaltene losses to the emulsion stability in NaN-freesystems.11 In the presence of NaNs, however, the ratio ofasphaltenes to NaNs decreases with an increase in the dilutionratio, thus producing thinner, or “softer”, asphaltene skins.Emulsion coalescence studies conducted by Gao et al. supportthis conclusion.28 These researchers showed that a criticalasphaltene to NaN ratio exists below which asphaltene skinscan no longer mask the softening effect of NaNs and emulsiondestabilization is promoted.Effect of the Temperature and Solvent Aromaticity on Oil

Recovery from Rag Layers. A more realistic depiction of thecrude oil formulation phase behavior that is in line with actualprocessing conditions is to evaluate oil recovery from rag layers at80 �C. As previously outlined for the baseline systems, increasingthe formulation temperature from 25 to 80 �C helps to reduce oil

losses to the rag layer from a maximum of 30 vol % to 10 vol %despite having a negligible influence on the asphaltene partition-ing characteristics. This effect is a direct consequence of the raglayer being composed of water droplets dispersed throughout acontinuous oil phase. Decreasing the viscosity of the oil phase atelevated temperatures, as was shown experimentally by Nouret al., facilitates drainage, and hence oil recovery, from the raglayer.11,40 This relationshipmay be interpreted theoretically fromrate models describing creaming/sedimentation and film-thin-ning processes.41�43 In both cases, the drainage rate of thecontinuous phase is inversely proportional to its viscosity. For 3wt % (and 10 wt %) NaN systems, increasing the formulationtemperature from 25 to 80 �C is of negligible benefit to oil�-water phase separation (Figure 3a,c) because the γo/w isothermpresented in Figure 6a remains unchanged. This observation issupported by Acosta et al., who found that the temperature haslittle influence on the surface activity of ionic surfactants.44 As aresult, the hydrophilic nature of NaNs induces a transition in therag layer morphology from w/o to o/w as depicted in Figure 5a,facilitating drainage of the aqueous phase. It should be empha-sized for NaN formulations that asphaltene losses to the rag layerremain independent of the temperature.The effect of the solvent aromaticity on the rag layer stability

was tested by decreasing Heptol’s heptane to toluene ratio from80/20 to 50/50. To understand the effect of such a change on theobserved phase behavior of baseline systems, the concept ofcritical transition concentration (CTC) should be reviewed.11 Inshort, the CTC refers to the solvent to bitumen dilution ratiobelow which the surface activity of asphaltene aggregates may beassociated with an increased tendency to form rag layers. Fromγo/w measurements, the CTC for Heptol 80/20 and 50/50systems was observed to correspond to solvent to bitumendilution ratios of 10 and 2�3, respectively. These results arestrongly correlated with the trends in oil and asphaltene losses tothe rag layer depicted in Figure 3a,b,e,f. In the presence of 3 wt %(and 10 wt %) NaNs, the effect of the solvent aromaticitybecomes less relevant. As illustrated in Figure 3a,e for solventsHeptol 80/20 and Heptol 50/50, respectively, oil losses to therag layer are quite similar. This is a result of emulsification beinggoverned by NaNs, which are water-soluble molecules whosesurface activity is independent of the solvent type. It is surprisingthat, despite the reduced surface activity of asphaltenes in Heptol50/50 systems (Figure 3f), sufficient surface coverage is achievedto promote the emulsion stability. The interfacial coadsorptionmodel proposed by Wu and Czarnecki is a useful tool tounderstand this phenomenon.24 These authors speculated thatthe secondary adsorbed asphaltene layer is predominantly an-chored into place via polar point contacts with the oil�waterinterface instead of cross-linking with the primary adsorbed NaNlayer. Therefore, the smaller sized asphaltene aggregates adsorbmore efficiently at the secondary layer of the interface. Suchefficient adsorption promotes larger oil losses to the rag layer forsystems prepared with an aqueous phase content of 50 wt %.Effect of the pH on Oil Recovery from Rag Layers. As

previously mentioned, the pKa value of naphthenic amphiphileshas been reported in the literature as∼6.17 Increasing the pH ofsolvent�bitumen�water formulations containing 3 wt % NaNsfrom pH 7.5 to 10 is therefore expected to have no substantialimpact on the observed phase behavior because the ionizedsurfactant already exists in its dissociated state. Although theresults for pH 10 are not presented, this hypothesis was indeed

Figure 9. (a) Bilayer model proposed byWu and Czarnecki to describethe interfacial coadsorption of asphaltenes and NaNs at the oil�waterinterface.24 (b) Coadsorption mechanisms proposed by Varadaraj andBrons for asphaltenes and NAs at the oil�water interface include (i)mixed monolayers and (ii) mixed aggregates.26.




validated experimentally using the procedures outlined in theMaterials and Methods section.As shown in Figure 4, reducing the pH of baseline solvent�

bitumen�water formulations results in improved oil�waterseparation. This behavior was unexpected because asphaltenesshould show an increase in the surface activity under suchconditions because of protonation of its basic functional groups,such as amines.45 Because of similarities in the γo/w isotherms(Figure 6b) and oil losses to the rag layer (Figure 4) for 0 and3 wt % NA systems at pH 4, it is suspected that a considerablefraction of endogenous NAs remain in the coker feed bitumenthat, at low pH, are less active at the oil�water interface,promoting higher interfacial tensions and improved oil�waterseparation. This theory may also explain why lower γo/w valuesare observed for solvent�bitumen�water systems under neutralconditions compared to other values published in the litera-ture.25,28 Although it cannot be confirmed from the air�liquidπ�A isotherms presented in Figure 7, it is likely that NAs help todestabilize emulsion droplets by weakening the asphaltene filmsadsorbed at the oil�water interface. The experimental observa-tions recently published by Gao et al. hint at a similar softeningeffect.28 Conflicting results obtained by Poteau et al. are likely aresult of constituents other than NAs present within the maltenesolution added to the oil phase.27 The possible mechanismshighlighted by Varadaraj and Brons to describe the coadsorp-tion of asphaltenes and NAs at the oil�water interface are providedin Figure 9b.26

’CONCLUSIONS

The results and ensuing discussions reveal that, consistent withthe initial hypothesis, the presence of naphthenic amphiphilesinfluences the separation of solvent�bitumen�water systems informulations that promote the adsorption of these species. For-mulations with highwater content and low interfacial tensions tendto produce smaller drop sizes and increase the oil�water interfacialarea during emulsification, magnifying the effect of naphthenicamphiphiles. For NaNs, stable o/w emulsions are produced informulations with high asphaltene and NaN contents (i.e., highbitumen and water contents). The increase in asphaltene losses inthe presence of NaNs suggests that NaNs improve the segregationof asphaltenes at the oil�water interface. Although indirect evi-dence suggests that NaNs produce softer films, this apparentsoftening effect does not compensate for the decrease in theinterfacial tension and drop size. In acidic environments, undisso-ciated NAs lead to high interfacial tensions that facilitate theseparation of the w/o emulsions produced during mixing. Onceagain, indirect evidence at the air�liquid interface suggests thatNAs soften asphaltene films. This observation is consistentwith theimproved separation obtained in NA-containing systems at pH 4compared with baseline systems at pH 7.5.

Interfacial coadsorption mechanisms proposed in the literatureare compatible with the results highlighted above. Although resultsobtained under acidic conditions show improved oil recovery,further studies are required to assess how NAs modify the proper-ties of asphaltene films adsorbed at the oil�water interface.

’AUTHOR INFORMATION

Corresponding Author*Tel.: 1-416-946-0742. Fax: 1-416-978-8605. E-mail: [email protected].

’ACKNOWLEDGMENT

We thank Syncrude Canada Ltd. for their financial supportand permission to publish this work. Additional funding from theNatural Science and Engineering Research Council of Canada isalso recognized.

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Documents

Impact of Asphaltenes and Naphthenic Amphiphiles on the Phase Behavior of Solvent−Bitumen−Water Systems