Water-in-Crude Oil Emulsion Stabilization: Review and Unanswered

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Water-in-Crude Oil Emulsion Stabilization: Review and Unanswered Questions Peter K. Kilpatrick* Department of Chemical and Biomolecular Engineering, University of Notre Dame, 257 Fitzpatrick Hall of Engineering, Notre Dame, Indiana 46656, United States ABSTRACT: Emulsions of oil droplets in water but more typically water droplets in oil are common in the production, transportation, and refining of petroleum and related products. Emulsions of water in petroleum or petroleum-derived liquids can be stabilized by a variety of surface-active compounds and components present in petroleum and also with which petroleum comes into contact. These include asphaltenes, carboxylic organic acids of various types, fine inorganic particles, and combinations of these three types of materials. Several of these means of emulsion and thin-film stabilization of emulsions in petroleum−water mixtures are reviewed here, as well as some of the key physical and chemical means of characterizing both the materials that stabilize the emulsions and the means of characterizing the oil−water interface and the emulsion films themselves. Some of the key unanswered questions surrounding the study of emulsion stability in petroleum systems are also reviewed.

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distinct molecules.35 Many of these molecules that report to the asphaltene fraction are surface-active, although many actually destabilize W/O emulsions, while others stabilize them. Therefore, it is clear that not all asphaltene molecules are responsible for emulsion stabilization but rather a subfraction. Of course, a key unanswered goal in crude oil emulsion research is to elucidate the nature of that subfraction, how to predict its presence in petroleum, and to predict the nature and severity of possible emulsion challenges in the processing of a particular petroleum feedstock. Many of the molecules in crude oil are acidic and can ionize at the oil−water interface to form the anion of the acid and dramatically lower the interfacial tension (IFT). This can lead to stabilized interfaces and emulsion stability.36−55 Some of these acid components are simple alkyl carboxylic acids; some are alkylbenzene carboxylic acids; some are naphthenic acids; and some are fused aromatic ring acids. In addition, some may be large molecular species with a combination of fused aromatic rings, alkyl side chains, and other chemical moieties with carboxylic acid pendant groups. It is well-appreciated that a number of these chemical species will “report” to the asphaltene fraction when the crude oil is precipitated with an alkane, but it is more precise and informative to describe the stabilization of water in oil emulsion droplets by these acids and their anions as crude oil acid stabilization rather than asphaltene stabilization. Indeed, some of these acids are known to be relatively high-molecular-weight acidic species. In addition, a mechanism whereby precipitates of high-molecular-weight acidic species form calcium soaps at the oil−water interface has been definitively demonstrated to cause severe emulsion challenges over the past 5−8 years. The tetra-acids of carbon

INTRODUCTION An emulsion is a mixture of two immiscible liquids in which droplets of one of the liquids become stably suspended in the other liquid because of either very slow coalescence or a barrier to coalescence.1−4 An emulsion is thus thermodynamically unstable but kinetically stable. Emulsions are common in a host of industrial products, such as foods, cosmetics, agricultural products, paints and coatings, and pharmaceuticals. Emulsions are also common in the petroleum industry, where they are typically (although not always) undesirable. Because emulsion droplets can dramatically increase the viscosity of a liquid, water-in-petroleum emulsions can dramatically increase pumping costs. Water in oil emulsions also enhance corrosion (because of the inclusion of salt and brine in the petroleum), reduce throughput, and can lead to equipment failure. Understanding their causes, both physically and chemically, and predicting their formation and how to mitigate them are, thus, very important objectives in the petroleum industry. There are a number of different physicochemical mechanisms whereby petroleum emulsions are stabilized, and we briefly summarize these here before describing in more detail in the main body of this paper. It has been well-appreciated for many decades that asphaltene-containing and related heavy crude oils often combine with water or brine to produce stable water-in-oil (W/O) emulsions, and this has led to the widespread and popular notion that asphaltenes are one of the primary causes of emulsion stability in petroleum mixtures.5−34 Indeed, there are many hundreds of papers on the topic, and a great effort has been made to characterize the properties of asphaltenic films and emulsions prepared in model systems in which the asphaltenes are isolated from the crude oil by alkane precipitation and then used to prepare emulsions in the laboratory. Asphaltenes are defined as the toluene-soluble but heptane- or pentane-insoluble fraction of petroleum. Therefore, the definition is as a solubility class, and, as a result, asphaltenes contain hundreds of thousands if not millions of chemically © 2012 American Chemical Society

Special Issue: Upstream Engineering and Flow Assurance (UEFA) Received: February 25, 2012 Revised: May 28, 2012 Published: May 30, 2012 4017

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molecular structures may soon be forthcoming, at least for portions of asphaltene fractions.78−80 The molecular aggregates formed by asphaltenes, depending upon the solvent conditions and concentrations, vary all of the way from small oligomeric aggregates (a few to several molecules) to very large microparticulates (containing hundreds of thousands of molecules). Barre and co-workers have summarized nicely the recent work in characterizing asphaltenic aggregates in solution, much of it by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) work.81 The nanoscale aggregates (3−10 nm in characteristic dimension) are typically ellipsoidal and polydisperse and entrain significant solvent within the aggregate interior.81−91 A careful recent study by Barre and his collaborators at IFP and Grenoble confirm the oblate cylindrical model.91 The scattering curves of asphaltenic aggregates in solution are well-fit by mass fractal models with dimensions on the order of 1.8−2.3, suggesting roughened surfaces. There has been less work characterizing the microparticulates formed by asphaltenes, although it is known that both diffusion- and reaction-limited aggregation to form microparticulates occurs under conditions in which asphaltenes aggregate and precipitate and the crossover in the kinetics is sensitive to the asphaltene concentration.92−94 From SANS studies on emulsion droplets formed by asphaltenic solutions and crude oils, the stabilizing interfacial film in asphaltenic emulsions is on the order of the size scale of the asphaltenic aggregates, typically 7−20 nm.95−97 These studies also revealed that there is significant oil and water entrained in the interfaces and that the volume fraction of asphaltenes in the interface can vary substantially from 0.1 to 0.3. Asphaltenic emulsion films formed in “poorer” solvents, such as decalin, appear to lead to denser interfaces and more stable emulsions.96 There is also clear evidence of densification and molecular rearrangement of asphaltenic molecules once adsorbed at an oil−water interface.98−101 This evidence comes from many sources, including the many dozens of hours required for many properties of oil−water interfaces, to which asphaltenes have adsorbed to reach a steady state, including IFT, dilatational elastic modulus, interfacial shear modulus, and other properties. These properties continue to change over this time scale, even when all of the bulk asphaltenes in the oil phase have been replaced by pure solvent, indicating that the change is attributable to the consolidation or molecular rearrangement of the adsorbed asphaltenes. These asphaltenic films are also observed, by the same experimenters, to be very strongly adsorbed. This suggests that the asphaltenes are firmly anchored to the interfacial film and to each other, likely through multiple intermolecular interactions per asphaltene molecule. Considering the chemical composition of asphaltenes, the intermolecular interactions are likely π bonds between aromatic groups, hydrogen bonds, charge-transfer interactions, multipolar forces, and van der Waals interactions. These interactions confer on adsorbed asphaltenic films elasticity, rigidity, and enhanced emulsion stability. Role of Solvency and Chemical Structure in Asphaltene Emulsion Stability. Asphaltene nanoscale aggregates are mostly surface-active under solvent conditions in which they are still soluble but close to a precipitation phase behavior.23,24 Dependent upon the chemical identity of the individual and collective asphaltene molecules, this precipitation phase boundary can occur at Hildebrand solubility parameters varying from 16 to 18 MPa1/2. Moreover, the amount of asphaltenes

number 80 that have been given the name ARN acids are a particular case in point of this mechanism.56−61 A third mechanism for stabilizing crude oil emulsions is actually probably an enhancement mechanism, namely, by the surface modification of very small inorganic solid particles, such as clay fines, silicas, iron oxides, and other inorganics, through the adsorption of polar species from the crude oil, such as resins or asphaltenes, to render the surface-modified inorganic particles interfacially active.62−69 This mechanism can lead to dramatically enhanced emulsion stability, and indeed, this particle mechanism can play a dominant role in some systems in which there is a substantial inventory of such particles that are suitably modified. A final emulsion stabilization mechanism that is very closely related to the crude oil acid mechanism is the formation of lamellar liquid crystalline films and possibly other smectic liquid crystalline phases at the oil−water interface.43−46,70−73 It is well-known that acids and their soaps form these phases at sufficiently high concentrations and that adsorption at the oil− water interface serves as a means to concentrate these species. Many reports in the last several years have documented this mechanism, and it appears to occur with a very broad range of different types of acids and their soaps.

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ROLE OF ASPHALTENES AND THEIR SUBFRACTIONS IN WATER IN CRUDE OIL EMULSION STABILIZATION Definition, Composition, and Molecular Aggregation of Asphaltenes. As mentioned above, asphaltenes are defined as the toluene-soluble and n-heptane-insoluble fraction of petroleum [American Society for Testing and Materials (ASTM) and IP-143; see ref 74]. The great irony of this definition is that there are individual molecular species in asphaltenes that are known to be insoluble in toluene75 and there are molecular species that report to the asphaltene fraction that are actually soluble in n-heptane.76 However, because the physical state of the asphaltenes in toluene, heptane, and the crude oil itself is as an aggregated, molecularly self-assembled, lyophilic (solvent-loving) colloidal aggregate, the solubility behavior of the individual molecular species comprising the asphaltenes does not become apparent unless they are either diluted to very high dilution or separated from the other asphaltenic species. Thus, even with the solubility definition, which yields a very large number of molecular species, there are individual molecules that do not fit the definition. Nonetheless, the molecular and functional group composition of asphaltene fractions suggests that at least a substantial proportion of the molecules in such fractions are highly aromatic (with hydrogen/carbon atomic ratios in the range of 1−1.2), possess heteroatoms (N, S, and O) in amounts varying from 2 to 10% (w/w), possess a range of polar functional groups capable of hydrogen bonding (including pyrrole, pyridine, carboxylic, quinone, thiophene, sulfhydryl, sulfonyl, and related functional groups), possess trace amounts [parts per million (ppm) to parts per trillion (ppt)] of vanadium, nickel, and iron, suggesting porphyrin and porphyrin-like functional groups, and alkyl bridges and pendant groups.3 This molecular composition can be modeled with small numbers of particular molecules only to provide a sense of the types of molecules likely to be present in asphaltenes.77 Recent high-resolution mass spectrometric experiments using Fourier transform ion cyclotron resonance (FT-ICR) at high field and with high resolving power hold promise that precise 4018

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of the asphaltenes (40−50% toluene). It is thus clear from these data what a crucial role the chemistry of the specific asphaltenes plays in stabilizing emulsion. Indeed, it is clear from a variety of recent studies that only a select small fraction of the asphaltenes and associated acids play key roles in stabilizing emulsion.104−108 Thus, a important goal of current and future research in crude oil emulsions is the clear elucidation of those asphaltenic structures responsible for the stability of emulsions. The work on fractionating asphaltenes into fine fractions of very differing properties, the observation that asphaltenes of differing chemistry behave very differently in stabilizing emulsion, and the observation that some asphaltene molecules are actually insoluble in toluene but are only solubilized by a very different fraction of the asphaltenes suggest that there are specific chemistries and chemical functional groups (and possibly topologies) in asphaltene molecules that engender not only interfacial activity at the oil−water interface but may also be primary, perhaps essential, in facilitating the stability of W/O emulsions by asphaltenes. Clearly, there are polar, relatively insoluble asphaltene molecules that are more prone to self-assemble and aggregate that appear to be the most interfacially active. These molecules likely have multiple functional groups in each molecule that are capable of forming intermolecular bonds with neighboring asphaltene molecules. In addition, those intermolecular bonds of greatest importance are undoubtedly those with the greatest temporal lifetime, such as hydrogen bonds and metal−multipole intermolecular bonds. Indeed, there are a number of researchers who have recognized that interconnecting asphaltene molecules must be present to explain the rheological and emulsion-stabilizing potential of adsorbed asphaltenic films.98−101 What is the precise chemistry of these polar, emulsion-stabilizing asphaltenes? Recent work by Stanford et al.106,107 and commentary by Czarnecki108 suggest that these asphaltenes may contain oxygen-rich acidic functional groups, be of relatively low double bond equivalent (DBE) values, and contain multiple sites for binding to other asphaltene molecules. One might imagine that these oxygenrich molecules contain more than one chemical moiety for hydrogen bonding. One might also imagine that these molecules contain a topological structure for assuming many configurations to bind to multiple nearby molecules. This last point seems consistent with the low DBE value of the interfacially active material identified in the study by Wu104 and discussed by Czarnecki. Interfacial Properties of Asphaltenic Emulsion Films. The interfacial properties of asphaltenic films at the oil−water interface have been probed by dynamic IFT, interfacial shear rheology, interfacial dilatational rheology, SANS and SAXS, and a number of other methods.5,8,29,95−101,105−115 The picture that emerges is that (1) asphaltenes rearrange their conformation at a molecular and intermolecular level over time scales of hours to days, (2) the interfacial rheology gradually approaches that of an elastic solid if the inventory of asphaltenes is sufficiently high, (3) the stability of emulsions correlates with the dilatational elastic modulus, and (4) the thickness of asphaltenic emulsion films is on the order of 8−20 nm, and the composition of those films is denser (less entrained solvent) than the corresponding nanoscale aggregates that adsorbed to comprise the film. As mentioned above, these learnings do not speak to the precise chemistry of the adsorbing molecules, which is still being understood through a variety of experiments using both the Wu D2O droplet method104−107

that precipitate from solution can vary appreciably from crude oil source to source. It is common to lower the solubility parameter (or equivalently the refractive index of the solvent) by adding n-alkane (typically an alkane longer than 4−5 carbon atoms). Gawrys et al.102 studied the variation in elemental composition, trace metal content, and aggregation potential of asphaltene fractions precipitated in narrow bands of solvent space between a composition of n-heptane and toluene of 90% toluene/10% n-heptane (v/v) and 0% toluene/100% n-heptane (v/v). The variations in the hydrogen/carbon and heteroatom/ carbon ratios were relatively complex for asphaltenes isolated from three different asphaltenes from three differing crude oils. Asphaltenes isolated from Canadon Seco, an Argentine crude oil, were fairly aromatic (overall H/C ratio of 1.11), contained relatively low metal concentrations, and could be judged to be less polar than other crude oil asphaltenes studied. Consequently, emulsions formed from Canadon Seco asphaltenes (and from Safaniya or Arab Heavy asphaltenes, another nonpolar asphaltene fraction) were relatively less stable than those formed from polar asphaltenes, such as B6, Hondo, or Athabasca bitumen asphaltenes. This is shown in Figure 1,103 in

Figure 1. Emulsion stability as judged by percent water resolved versus percent toluene in the oil phase for asphaltenes from four different crude oils: Arab Heavy (AH), Canadon Seco (CS), B6, and Hondo (HO). Emulsions were prepared by emulsifying mixed equal volumes of water and oil, with the oil comprising a mixture of n-heptane and toluene into which 0.5% (w/w) of the asphaltenes has been added. The emulsions were prepared, and their stability was gauged by the centrifugal separation method described by Spiecker et al.33 The solubility limit of the asphaltenes is roughly 45−50% toluene, although the precise phase boundaries for three of the asphaltenes can be found in the study by Gawrys et al.102

which the percent water resolved profiles (a measure of inverse emulsion stability; i.e., the higher the percent water resolved by centrifugation, the lower the emulsion stability) versus heptane and toluene concentrations for these four crude oil asphaltenes103 are presented. The polar asphaltenes (B6 and HO) form very stable emulsions (by this test) up to nearly 100% toluene, as judged by the low percent water resolved (10−20%) over a large range of solvent conditions. Thus, even in a very good solvent, such as pure toluene, these polar asphaltenes adsorb strongly at the oil−water interface and stabilize emulsion. The relatively nonpolar asphaltenes (AH and CS) form weak emulsions at high toluene concentration and only stabilize emulsion with any efficacy at toluene concentrations close to the solubility boundary for the majority 4019

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in a number of West African and North Sea crude oils. Baugh and co-workers,56,57 Sjoblom and co-workers,58−61 and others have studied these materials, and while they appear to be common, they are not ubiquitous and are certainly not the sole cause of emulsions in the oil field by acids and their soaps. Nonetheless, they are an important means whereby actual petroleum emulsions are stabilized. There are also acidic components of asphaltenes, that are very effective at both adsorbing to water−oil interfaces and being capable of dramatically stabilizing W/O emulsion films. Verruto, Le, and Kilpatrick101 have probed the effective pKa of asphaltenes from two distinct heavy petroleum sources: Hondo, an offshore Californian crude oil, and bitA, an Athabasca bitumen, by dynamic IFT. Hondo asphaltenes have an effective pKa close to 7, and bitA asphaltenes have an effective pKa closer to 3 (i.e., more acidic). Thus, under alkaline conditions with both Hondo and bitA asphaltenes, these researchers observed interfacial transfer from the interface to bulk water of the anions of some of the asphaltenic acids. They also observed an initial maximum in dilatational elasticity at short times in the adsorption of both bitA and Hondo asphaltenes at alkaline pH in pure water, followed by a decline in elasticity over time. In contrast, with added electrolyte, there was less evidence from dynamic IFT of interfacial transfer into the aqueous phase of charged anions of asphaltenic acids and, over a long time period and after an initial decay in elasticity with added salt, a slow growth in elasticity, indicating asphaltenic consolidation at the interface. It thus appears that, under appropriate conditions of charging of acids, typically pH values in excess of 9 or 10, acidic species in the asphaltene fraction compete with other less charged or amphoteric asphaltenic species for the interface. The presence of salt screens electrostatic repulsion between anions at the interface and enables the mixture of anionic and less charged asphaltenes to interact and consolidate slowly over time, rendering a stronger emulsion interface. In summary, linear alkanoic acids, naphthenic acids, tetraacids (such as ARN), and asphaltenic acids can all adsorb strongly at water−oil interfaces to aid in the stabilization of both O/W but more commonly W/O emulsions. In the case of naphthenic acids, the mechanism appears to be, at least in the strongest emulsion interfaces, through the formation of layered lamellar liquid crystalline films. There is growing evidence for this. In the case of ARN acids, the mechanism appears to be through the formation of a cohesive film that can be strongly anchored to the interface through the binding of divalent cations, such as calcium. Finally, it appears that acids and asphaltenes can interact strongly with each other and, under appropriate conditions, form integrated films of high dilatational elasticity.

and a variety of chemical and molecular characterization methods.

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STABILIZATION OF W/O EMULSIONS BY CARBOXYLIC ACID, NAPHTHENIC ACIDS, AND THEIR SOAPS Another important mechanism whereby emulsions are stabilized in crude oil systems is through the adsorption of carboxylic acids and their anions, depending upon conditions of pH. It has long been known that acids in crude oil can adsorb to a crude oil−water interface and dramatically lower the IFT.36−40 Indeed, this mechanism has been exploited by many to devise enhanced oil recovery processes, using either simply added alkali or a combination of surfactant-assisted and/or polymer-assisted alkaline flooding.36,41,42 What has also been observed in experiments in which crude oil or model oils with components from petroleum are contacted with water phases of varying pH is that, following a dramatic reduction in IFT, sometimes to values as low as from 10−2 to 10−3 mN/m, the tension often rises again. This has been interpreted as a transfer of interfacially bound material, presumably the anion or the acid of the carboxylic acids in the crude oil that has adsorbed from the interface into the aqueous phase. Indeed, researchers have observed the buffering effect that this interfacial transfer has on the water phase emulsified into an acidic crude oil following resolution of the emulsion into a bulk water phase. The carboxylic acids capable of lowering IFT and stabilizing emulsion appear to vary quite broadly in chemical structure and molecular weight. While some crude oils have a modest concentration of linear n-alkanoic acids, it is more common to find what are sometimes collectively known as naphthenic acids in crude oil. These acids are called naphthenic because they have at least one cylic alkane functional group, sometimes more than one, and alkyl side chains onto which the carboxylic acid functional group is often attached.116−120 A number of excellent recent studies have described in great detail the chemical structure of naphthenic acids and the distribution of such structures, in particular crude oils (see ref 117 and references cited therein). One well-studied analogue of a naphthenic acid is 5-β(H)-cholanic acid, a steroidal acid that is commercially available. Ostlund et al. performed pulsed field gradient spinecho nuclear magnetic resonance (NMR) diffusion measurements and determined that β-cholanic acid interacts with asphaltenes.119 Ese and Kilpatrick demonstrated that β-cholanic acid can stabilize both oil-in-water (O/W) and W/O emulsions, depending upon the pH and concentration and appears to do so through the formation of a lamellar liquid crystalline phase.48 Horvath-Szabo, Masliyah, and Czarnecki have demonstrated that both commercial sodium naphthenates as well as naphthenates formed by contacting Athabasca bitumen with alkaline solutions from lamellar liquid crystalline phases, which stabilize water-in-crude oil emulsions,43−46 similar to the liquid crystalline stabilizing mechanism reported earlier by Friberg and co-workers in surfactant−oil−water mixtures.70−72 Another important mechanism whereby emulsions are stabilized by naphthenates is through the formation of the alkaline earth soaps, particularly calcium naphthenate, by polyvalent acids. Among these are tetra-acids of roughly 1230 atomic mass units that have come to be known as ARN acids.56−61 These acids and their calcium soaps adsorb at oil− water interfaces, yielding IFTs of 11−19 mN/m and dilatational elasticities of 30−60 mN/m, and appear to be common

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STUDIES ON INTERACTIONS OF ASPHALTENES AND ACIDS IN THE STABILIZATION OF W/O EMULSIONS The interactions of asphaltenes and either naphthenic acids, linear acids, or fused aromatic ring acids are an area of research that is garnering increased attention. Pauchard et al.55 and Muller et al.120 describe a recent study of a low total acid number (TAN)/high asphaltene crude oil that forms particularly stable and troublesome emulsions. By carefully isolating the interfacially adsorbed material and characterizing it by Fourier transform infrared (FTIR) spectroscopy, FT-ICR mass spectrometry, and other means, these researchers determined that the strongly bound interfacial material that 4020

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Figure 2. Emulsion stability as judged by percent brine resolved using the centrifugal separation method described by Spiecker et al.33 The emulsions were prepared by emulsifying equal volumes of water and oil, with the oil phase containing equal volumes of n-heptane and toluene into which 0.5% (w/w) Hondo asphaltenes have been dissolved. Varying amounts of β-cholanic acid were also included in the oleic phase, and the pH of the aqueous phase was adjusted to the designated pH using NaOH as described in the study by Ese and Kilpatrick.48

would not dissolve or wash off in toluene comprised a dicarboxylic acid of varying chain length, while weaker bound material appeared to be very similar to asphaltenes (almost certainly some fraction of the asphaltenes). Therefore, it appears from their work that integrated films of diprotic acids and asphaltenes can form particularly troublesome field emulsions. There has also been work showing both the enhancement and diminution of the stability of asphaltene-stabilized W/O emulsions upon the addition of model carboxylic acids of varying H/C ratios.121 As mentioned above, Ese and Kilpatrick demonstrated that β-cholanic acid (with an H/C ratio of 1.696 and a model naphthenic acid molecule) can stabilize both O/W and W/O emulsions depending upon the concentration of acid and the pH of the oil.48 In subsequent work,121 they showed that W/O emulsions stabilized by Hondo asphaltenes could be destabilized upon the addition of a very small amount of βcholanic acid (see Figure 2), as little as a few hundred ppm. In this experiment (Figure 2), Hondo asphaltenes (0.5%, w/w) were dissolved in a mixture of n-heptane and toluene (50:50, v/ v), varying amounts of β-cholanic acid were added (0−1.5%, w/ w), and the entire oleic phase was emulsified with water, which had been pH-adjusted with NaOH to have an aqueous phase pH between 10.0 and 13.2. The emulsion stability was then assessed by the centrifugal separation method.33 As shown in Figure 2, very small amounts of β-cholanic acid (300−1000 ppm or 0.03−0.1 wt %) at pH 12.0 significantly destabilize the Hondo asphaltene-stabilized emulsions. This indicates that the charged soap of β-cholanic acid is much more surface-active than the HO asphaltenes and tiny amounts can adsorb at the W/O interface and prevent Hondo asphaltenes from anchoring in that interface and stabilizing the emulsion droplets. However, when the β-cholanic acid concentration was increased from a few hundred ppm to 1−2% (w/w) and when the range of pH of the water phase was between 11.5 and 12.5, the liquid crystalline mechanism described above dominates and W/O emulsions are again very stable. Thus, two clearly different mechanisms of W/O emulsion stabilization are seen in the

same system, as well as antagonism between the two that results in destabilization. Somewhat similar experiments were performed121 in which two blended crude oils, EB and GCB, known to stabilize W/O emulsions by forming asphaltenic films, were modified by the addition of 0.5% (w/w) of a carboxylic acid of varying molecular structure and then the stability of emulsions prepared from these modified crude oils by emulsifying them with water of two differing pH values were measured by the critical electric field (CEF) method.122 Both of the crude oils contained substantial amounts of asphaltenes (C7 insolubles for EB = 4.7 wt % and for GCB = 9.1 wt %) and very small amounts of acids (TAN for EB = 0.15 and for GCB = 0.12). At neutral pH of the water phase, the addition of acid had no effect on emulsion stabilization as judged by CEF for the most alkane-like acids (high H/C ratio): β-cholanic acid, undecylenic acid, and pentane−cyclohexanoic acid. However, for fused aromatic ring acids, such as anthracene carboxylic acid, naphthoic acid, and tetrahydro-naphthoic acid, the addition of 0.5% acid was able to double or triple the value of the CEF (see Figure 3), suggesting a dramatic increase in the emulsion stability. In identical experiments but with the pH elevated to 12 in the water phase, the addition of the asphaltene-like acids (lower H/C) to the oils yielded W/O emulsions of dramatically higher CEF and stability, while the addition of naphthenic acids, such as βcholanic acid, heptylbenzoic acid, and pentane−cyclohexanoic acid, to the oils yielded greatly destabilized W/O emulsions as gauged by CEF.121 These results indicate that both neutral and charged asphaltene-like acids can integrate with asphaltenes at water−oil interfaces to form even more stable emulsions than with the original asphaltenes. It should be noted that, in the experiments performed in Figure 3 and described in this paragraph, the water also contained 1% sodium chloride. These results are consistent with the interfacial rheology experiments reported by Verruto et al.101 and described above for Hondo and bitA asphaltenes at water interfaces and at alkaline pH with added electrolyte. Therefore, the emerging picture of the ways in which both asphaltenes and naturally occurring acids of varying chemical structure interact at 4021

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surface-active materials can alter the wettability of the particles and the stability of the emulsions. Sullivan and Kilpatrick66 described in detail the ways in which asphaltenes and resins can adsorb and modify inorganic solid particles and then studied the modification of fine particles (0.026−1.45 μm size) of iron oxide, the clay kaolin, and the clay montmorillonite by the adsorption of surfaceactive materials (asphaltenes and resins) from three crude oils. They concluded that W/O emulsions could be very strongly stabilized by these fine inorganic particles when suitably modified through asphaltene and resin adsorption. They also demonstrated the strong role that particle size plays in emulsion stability, with the smallest particles yielding the most stable emulsions. They also demonstrated the role that the solubility state of the asphaltene plays in its ability to adsorb to fine solids by varying the resin/asphaltene ratio, which, in turn, varies the size of the asphaltenic aggregates in solution. Sztukowski and Yarranton studied the role of clay platelets and other oil sand solids in enhancing the emulsion stability of water-in-model oil (heptane and toluene) droplets stabilized by asphaltenes and resins.67 They found that clay platelets of 100− 500 nm were very effective at enhancing stability and occupied up to 50% of the interfacial area of the droplets. They also determined that the clay platelet thickness at the interface was on the order of asphaltenic film thicknesses (8 nm). Hannisdal et al. elegantly reaffirmed the critical role of the adsorption of crude oil components (resins and asphaltenes) in modifying the wetting and interfacial adsorption properties of nanoscale silica and the subsequent role in stabilizing both O/W and W/O emulsions.68 The mechanism of stabilization of water-in-crude oil emulsions by inorganic solid particles thus appears to be primarily through the enhancement of asphaltene and/or resin or crude oil acid-stabilized emulsions through the adsorption of surface-modifying components in the crude oil to the particles that render them interfacially active. There appears to be a limit to the surface coverage of inorganic particles that can effectively stabilize micrometer-sized droplets, with particles in the tens to hundreds of nanometer size scale being most effective.

Figure 3. Emulsion stability as judged by critical electric field (CEF) as described in the study by Sullivan et al.123 The oleic phase was either EB (blend of European oils) or GCB (a blend of Gulf Coast oils). Into each oil, 0.5% of an acid was dissolved, and then the oil was emulsified according to the procedure by Spiecker et al.,33 in which the water/oil ratio was 30:70. The higher the CEF, the more stable the W/O emulsion. The acid with the lowest H/C (0.64) was 2-anthracene− carboxylic acid, with the order of increasing H/C: naphthoic acid (0.70), tetrahydronaphthoic acid (1.1), 1-heptylbenzoic acid (1.46), βcholanic acid (1.696), undecylenic acid (1.90), and pentane− cyclohexanoic acid (1.91). Note that the H/C ratio is computed only for the hydrophobic portion of the acid molecule and does not include the carboxylic moiety.

interfaces suggests that naphthenic acids can modestly enhance asphaltene emulsion stability at neutral pH and dramatically decrease that stability at alkaline pH. Asphaltene-like acids, in contrast, containing fused aromatic ring moieties, appear to enhance the stability of W/O emulsions stabilized by asphaltenes at all pH, whether charged or not. Similarly, diand multiprotic acids can significantly enhance the stability of asphaltenic interfaces. Indeed, one wonders if these various acids (aromatic, fused ring aromatic, and multiprotic) naturally report to the asphaltene fraction and are typically intimately associated with asphaltenes in the crude oil medium. One could well imagine that the polar and hydrogen-bonding capability of these acids and the π-bonding capability of aromatic acids make them natural bridging agents and intermolecular cross-linkers of the other asphaltenes.

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SUMMARY AND SOME KEY UNANSWERED QUESTIONS It seems clear from the evidence in the literature that asphaltenes play an unambiguous and important role in water-in-crude oil emulsion stabilization. Even with the most nonpolar asphaltenes, it appears possible to create relatively stable emulsions, although it is also clear that asphaltene polarity and chemistry strongly impact the stability of W/O emulsions. It also seems clear that there are subfractions of asphaltenes that appear primarily responsible for emulsion stability. From a number of studies, the chemistry of those asphaltenes most responsible for W/O emulsion stability appear to be acidic, of relatively low DBE or aromaticity, and, by inference, highly flexible molecules. Recent studies also suggest that the acidity in these “key” molecules may be multiprotic, which further suggests that their role may be to anchor the stabilizing film in the interface or possibly to serve as intermolecular cross-linking agents for the asphaltenes. It is still a very important chemical, functional question to try and resolve the precise range of molecular identities of these molecules. Crude oil acids are also known to play an unambiguous and important role in water-in-crude oil emulsion stabilization. We have reviewed the stabilization of these emulsions by

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ROLE OF INORGANIC SOLID PARTICLES IN STABILIZING W/O EMULSIONS Among the inorganic fine solid particles that arise naturally in the production and processing of petroleum are silicas, clays, and metal oxides, such as iron. Early in the 20th century, researchers discovered the ability to stabilize emulsions of water and oil with finely divided solid particles. Finkle et al.124 were the first to relate this to the solid−liquid−liquid contact angle, observing that the phase that wets the solid better (i.e., for which the contact angle is less than 90°) is the continuous phase of the emulsion. Gelot et al.62 studied the emulsification of water in oil by the clays Ca−bentonite and Ca−kaolinite, as well as carbon black. They found that they could enhance the emulsion stability and change the wettability of the particles by adding the surfactant sodium dodecyl sulfate (SDS). Thus, the principles that emerged from this work and prior work was that surface modification of the solid particles by adsorption of 4022

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(3) Kilpatrick, P. K.; Spiecker, P. M. Asphaltene emulsions. In Encyclopedic Handbook of Emulsion Technology; Sjoblom, J., Ed.; Marcel Dekker: New York, 2001; pp 707−730. (4) Sjoblom, J.; Johnson, E. E.; Westvik, A.; Ese, M.-H.; Djuve, J.; Auflem, I. H.; Kallevik, H. Demulsifiers in the oil industry. In Encyclopedic Handbook of Emulsion Technology; Sjoblom, J., Ed.; Marcel Dekker: New York, 2001; pp 595−619. (5) Blair, C. M. Interfacial films affecting the stability of petroleum emulsions. Chem. Ind. 1960, 20, 538. (6) Nellenstyn, F. J. J. Inst. Pet. Technol. 1924, 10, 311. (7) Pfeiffer, J. P.; Saal, R. N. J. Asphaltic bitumen as colloid system. J. Phys. Chem. 1940, 44, 139−165. (8) Kimbler, O. K.; Reed, R. L.; Silberberg, I. H. Physical characteristics of natural films formed at crude oil−water interfaces. Soc. Pet. Eng. J. 1966, 6, 153. (9) Berridge, S. A.; Thew, M. T.; Loriston, A. G. Formation and stability of emulsions of water in crude petroleum and similar stocks. J. Inst. Pet. 1968, 54, 333. (10) Strassner, J. E. Effect of pH on interfacial films and stability of crude oil−water emulsions. J. Pet. Technol. 1968, 20, 303. (11) Bridie, A. L.; Wanders, T. H.; Zegveld, W.; van der Heijde, H. B. Formation, prevention, and breaking of sea-water in crude oil emulsions chocolate mousses. Mar. Pollut. Bull. 1980, 11, 343−348. (12) Mackay, G. D. M.; McLean, A. Y.; Betancourt, O. J.; Johnson, J. I. Formation of water-in-oil emulsions subsequent to an oil spill. J. Inst. Pet. 1973, 59, 164−172. (13) Papirer, E.; Bourgeois, C.; Siffert, B.; Balard, H. Chemical nature and water oil emulsifying properties of asphaltenes. Fuel 1982, 61, 732−734. (14) Menon, V. B.; Wasan, D. T. Particle fluid interactions with application to solid-stabilized emulsions. 1. The effect of asphaltene adsorption. Colloids Surf. 1986, 19, 89−105. (15) Menon, V. B.; Wasan, D. T. Particle fluid interactions with application to solid-stabilized emulsions. 3. Asphaltene adsorption in the presence of quinaldine and 1,2-dimethylindole. Colloids Surf. 1987, 23, 353−362. (16) Eley, D. D.; Hey, M. J.; Symonds, J. D. Emulsions of water in asphaltene-containing oils. 1. Droplet size distribution and emulsification rates. Colloids Surf. 1988, 32, 87−101. (17) Eley, D. D.; Hey, M. J.; Symonds, J. D. Emulsions of water in asphaltene-containing oils. 2. Rheology. Colloids Surf. 1988, 32, 103− 112. (18) Sjoblom, J.; Urdahl, O.; Borve, K. G. N.; Li, M; Saeten, J.; Christy, A. A.; Gu, T. R. Water-in-crude oil emulsions from the Norwegian continental shelfCorrelation with model systems. Adv. Colloid Interface Sci. 1992, 41, 241−271. (19) Taylor, S. E. Resolving crude oil emulsions. Chem. Ind. 1992, 20, 770−773. (20) Shetty, C. S.; Nikolov, A. D.; Wasan, D. T. Demulsification of water in oil emulsions using water soluble demulsifiers. J. Dispersion Sci. Technol. 1992, 13, 121−133. (21) Acevedo, S.; Escobar, G.; Gutiérrez, L. B.; Rivas, H.; Gutiérrez, X. Interfacial rheological studies of extra-heavy crude oils and asphaltenesRole of the dispersion effect of resins in the adsorption of asphaltenes at the interface of water-in-crude oil emulsions. Colloids Surf., A 1993, 71, 65−71. (22) Fordedal, H.; Schildberg, Y.; Sjoblom, J. Crude oil emulsions in high electric fields as studied by dielectric spectroscopy. Influence of interaction between commercial and indigenous surfactants. Colloids Surf., A 1996, 106, 33−47. (23) McLean, J. D.; Kilpatrick, P. K. Effects of asphaltene solvency on stability of water-in-crude oil emulsions. J. Colloid Interface Sci. 1997, 189, 242−253. (24) McLean, J. D.; Kilpatrick, P. K. Effects of asphaltene aggregation in model heptane−toluene mixtures on stability of water-in-oil emulsions. J. Colloid Interface Sci. 1997, 196, 23−34. (25) Ese, M. H.; Yang, X.; Sjoblom, J. Film forming properties of asphaltenes and resins. A comparative Langmuir−Blodgett study of

naphthenates, which often appear to stabilize by a lamellar liquid crystalline mechanism. Multiprotic acids, such as ARN acids, also appear to stabilize W/O emulsions well, apparently by being multiply anchored in the interface, particularly when saponified and precipitated by alkaline earth counterions. As mentioned, there are apparently strong interactions between and among asphaltenes and multiprotic acids in the stabilizing interfacial film that can enhance emulsion stability. Similarly, aromatic and polyaromatic acids (presumably asphaltenic acids) can also greatly enhance the stability of asphaltene emulsions, again, likely through multiple points of interaction with the asphaltenes. It stands to reason that, if the stability of water-incrude oil emulsions is typically accompanied by high dilatational interfacial elasticity (as well as shear interfacial elasticity), any molecules that are able to enhance multiple intermolecular bonds among asphaltenes in the interface would also serve to enhance the stability of the emulsion droplet films. Interestingly, as shown in the review, acids can under alkaline conditions, in which they form anions, dramatically destabilize asphaltenic interfacial films. This has been shown for a host of monoprotic naphthenic acids. Is interfacial dilatational elasticity or gel strength an appropriate measure of emulsion stability? Recent data on this question seem to be mixed. If not elasticity, what is the best measure or predictor of emulsion stability? This question remains elusive for the full range of water-incrude oil emulsions observed. If multiprotic acids appear to be a common means of cross-linking asphaltenes in water-in-crude oil emulsions, to which functional groups in the asphaltenes are they binding? Other acidic or Lewis base groups? Answers to these questions might yield measures of functional groups or asphaltene chemistry that could correlate with TAN and a modified multiprotic TAN, which could provide important clues to emulsion stability. Finally, it seems clear that inorganic solid fines, particularly clays, silicas, and iron oxides, can play major roles in enhancing water-in-crude oil emulsion stability, particularly when modified through the adsorption of resins and asphaltenes to render them strongly interfacially active. Inorganic particle size and wettability are key variables in dictating how effective inorganic solids are in enhancing this stability, with the most effective particles being in the range of a few hundred nanometers in size.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Parts of this work were supported by grants from Exxon Mobil Research and Engineering [National Science Foundation (NSF) Grant TSE0124760] and an industrial consortium comprising ExxonMobil, Nalco, ConocoPhilips, Shell, and PetroBeam. The author gratefully acknowledges their support.

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dx.doi.org/10.1021/ef3003262 | Energy Fuels 2012, 26, 4017−4026