Rheology of asphaltene-toluene/water interfaces

The stability of water-in-crude oil emulsions is frequently attributed to a rigid asphaltene film at the water/oil interface. The rheological properties of these films and their relationship to emulsion stability are ill defined. In this study, the interfacial tension, elastic modulus, and viscous modulus were measured using a drop shape analyzer for model oils consisting of asphaltenes dissolved in toluene for concentrations varying from 0.002 to 20 kg/m(3). The effects of oscillation frequency, asphaltene concentration, and interface aging time were examined. The films exhibited viscoelastic behavior. The total modulus increased as the interface aged at all asphaltene concentrations. An attempt was made to model the rheology for the full range of asphaltene concentration. The instantaneous elasticity was modeled with a surface equation of state (SEOS), and the elastic and viscous moduli, with the Lucassen-van den Tempel (LVDT) model. It was found that only the early-time data could be modeled using the SEOS-LVDT approach; that is, the instantaneous, elastic, and viscous moduli of interfaces aged for at most 10 minutes. At longer interface aging times, the SEOS-LVDT approach was invalid, likely because of irreversible adsorption of asphaltenes on the interface and the formation of a network structure.     

Water-in-Hydrocarbon Emulsions Stabilized by Asphaltenes at Low Concentrations

The role of Athabasca asphaltene particles and molecules in stabilizing emulsions was examined by measuring the surface area of water-in-toluene/hexane emulsions stabilized by various asphaltene fractions, each with a different proportion of soluble and insoluble asphaltenes. The stabilized interfacial area was found to depend only on the amount of soluble asphaltenes. Furthermore, the amount of asphaltenes on the interface was consistent with molecular monolayer coverage. Hence, at low concentrations, asphaltenes appear to both act as a molecular surfactant and stabilize emulsions. The effect of the hexane : toluene ratio on emulsion stability was examined as well. At lower hexane : toluene ratios, more asphaltenes were soluble but the surface activity of a given asphaltene molecule was reduced. The two effects oppose each other but, in general, a smaller fraction of asphaltenes appeared to stabilize emulsions at lower hexane : toluene ratios. The results imply that the emulsifying capacity of asphaltenes is reduced but not eliminated in better solvents. Copyright 2000 Academic Press.     

Adsorption kinetics of asphaltenes at the oil-water interface and nanoaggregation in the bulk

Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop-drop coalescence. As a result, asphaltenes unfavorably impact water-oil separation processes and consequently oil production rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water-oil interfaces. This study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barrier-controlled regime rather than to a conformational relaxation regime.     

Study on the Mechanism of Asphaltenes Reducing Oil-Water Interfacial Tension

As high polar components of crude oil, asphaltenes play a significant role in reducing oil-water interfacial tension(IFT). In this paper, the effects of asphaltenes on reducing IFT in the presence of surfactant were compared, and the mechanism of asphaltenes reducing the IFT was studied by the dynamic interfacial tension(DIFT) equation. Whether asphaltenes were added to the oil or 2,5-dimethyl-4-(4-dodecyl) benzene sodium sulfonate(p-S14-4) was added to the water phase, either of all results in the IFT reducing and the IFT is related to the coverage and the mass of asphaltenes adsorption at the interface. In the presence of asphaltenes, the adsorption of the active substances to the interface is not entirely dependent on diffusion, and the process can be divided into three regions. Region I: the IFT rapidly reducing, this process is controlled by diffusion of surfactant; Region II: the IFT reducing slowly, resulted from the lower diffusion rate that is limited due to the aggregates formed by the interaction of asphaltene-asphaltene; Region III: the interaction of asphaltene-asphaltene is broken by the interaction of surfactant-asphaltene. The asphaltene aggregates are reduced and adsorbed rapidly at the interface. Furthermore, the results reveal that the asphaltenes concentration affects the coverage rate and adsorption at the interface.     

Multivariate Screening Analysis of Water-in-Oil Emulsions in High External Electric Fields as Studied by Means of Dielectric Time Domain Spectroscopy

The effect of crude oil resins with various polar characters on the stability of w/o model emulsions containing asphaltenes is investigated using a mixture design. The resins were extracted using an adsorption-desorption technique. One asphaltene fraction and four different resin fractions from one European crude oil were used. The stabilities are measured using time-domain dielectric spectroscopy in high external electric field. It is found that resins with different polar character have different effects on the emulsion stability. At asphaltene/resin ratios of 1 and 5 : 3 the resins in some cases lead to an emulsion stability higher than that of a similar emulsion stabilized by asphaltenes only, while at low asphaltene/resin ratios ( approximately 1 : 3) the emulsion stability is reduced by the resins. The effect on emulsion stability of combining two different resin fractions depended on the resin types combined as well as the relative amount of resins and asphaltenes. Also, an increase in the stability of some of the emulsions containing resins and asphaltenes for a period of 50-300 min after the emulsification was observed. This time-dependence of emulsion stability is attributed to the mobility of resins at the oil-water interface and the slow buildup of a stabilizing interfacial film consisting of resins and asphaltenes. Copyright 2000 Academic Press.     

Influence of Interfacial Rheological Properties on Stability of Asphaltene-Stabilized Emulsions

A series of oscillating droplet measurements have been performed on asphaltenes at the oil/water interface, in order to correlate the interfacial rheological behavior to their ability to stabilize emulsions. In the concentration sweep, the elastic modulus goes through a maximum around an asphaltene concentration of 0.05–0.10 g/l. This behavior was not in good correspondence with emulsion stability, which increased consistently from low to high concentrations. The decrease above 0.10 g/l was most likely an effect of diffusion of asphaltenes in the bulk to the interface, which became more significant at higher bulk concentrations. The rheology data as a function of concentration has been fitted to Butler's surface equation of state and the Lucassen–van den Tempel model. A decent correlation was found between emulsion stability and elasticity for both the effect of solvent aromaticity and pH. The elastic modulus displayed a gradual increase when xylene was mixed with heptane as the solvent, as was seen with emulsion stability. This was not caused by a significant increase of the adsorbed amount of asphaltene at the interface, as shown by a quartz crystal microbalance (QCM), but a more efficient reorganization of the already adsorbed asphaltenes. The ability asphaltenes displayed in stabilizing emulsions was significantly increased at both low and high pH, according to a previous study. The elastic modulus, on the other hand, only showed a very weak increase at pH 2, but a better correlation with emulsion stability above pH 8. From this it would appear that the dissociation of acid groups in the asphaltene structure at high pH has a bigger impact on the interfacial activity than the protonation of bases at low pH, while their effect on emulsion stability was the same.      

Adsorption isotherms of associating asphaltenes at oil/water interfaces based on the dependence of interfacial tension on solvent activity

In the Gibbs adsorption equation, the application of solvent activity for the calculation of the surface/interfacial excess is proposed for nonideal or associating or pseudocomponents such as asphaltenes. For the aforementioned systems, only the mass-based phenomenological interfacial excess can be determined based on interfacial tension versus activity data. The use of the mole fraction is compared to the use of the activity when the adsorbed amount of associating asphaltenes is calculated at a water/toluene interface. Langmuir-type isotherms describe the adsorption of asphaltenes at toluene/water interfaces. Asphaltenes were treated to remove the resins and natural surfactants using cyclic precipitation and dissolution of asphaltenes at a fixed aliphatic/aromatic ratio. Different fractions of asphaltenes were obtained by changing the aliphatic/aromatic ratio of the precipitating solvent. The limiting molar masses of asphaltenes measured by vapor pressure osmometry are different for fractions precipitated at different heptane to toluene ratios. The mass-based adsorbed amounts at the water/toluene interface, at a 0.1 asphaltene-to-toluene mass-ratio, varied in the range of 0.8-2.8 mg/m(2), depending on the molar mass of asphaltenes.     

Water-in-model oil emulsions studied by small-angle neutron scattering: interfacial film thickness and composition

The ever-increasing worldwide demand for energy has led to the upgrading of heavy crude oil and asphaltene-rich feedstocks becoming viable refining options for the petroleum industry. Traditional problems associated with these feedstocks, particularly stable water-in-petroleum emulsions, are drawing increasing attention. Despite considerable research on the interfacial assembly of asphaltenes, resins, and naphthenic acids, much about the resulting interfacial films is not well understood. Here, we describe the use of small-angle neutron scattering (SANS) to elucidate interfacial film properties from model emulsion systems. Modeling the SANS data with both a polydisperse core/shell form factor as well as a thin sheet approximation, we have deduced the film thickness and the asphaltenic composition within the stabilizing interfacial films of water-in-model oil emulsions prepared in toluene, decalin, and 1-methylnaphthalene. Film thicknesses were found to be 100-110 A with little deviation among the three solvents. By contrast, asphaltene composition in the film varied significantly, with decalin leading to the most asphaltene-rich films (30% by volume of the film), while emulsions made in toluene and methylnaphthalene resulted in lower asphaltenic contents (12-15%). Through centrifugation and dilatational rheology, we found that trends of decreasing water resolution (i.e., increasing emulsion stability) and increasing long-time dilatational elasticity corresponded with increasing asphaltene composition in the film. In addition to the asphaltenic composition of the films, here we also deduce the film solvent and water content. Our analyses indicate that 1:1 (O/W) emulsions prepared with 3% (w/w) asphaltenes in toluene and 1 wt % NaCl aqueous solutions at pH 7 and pH 10 resulted in 80-90 A thick films, interfacial areas around 2600-3100 cm (2)/mL, and films that were roughly 25% (v/v) asphaltenic, 60-70% toluene, and 8-12% water. The increased asphaltene and water film composition at pH 10 versus pH 7, along with unique dynamic interfacial tension profiles, suggested that the protonation state of carboxylic moieties within asphaltenes impacts the final film properties. This was further supported when we characterized similar asphaltenic emulsions that also contained 9-anthracence carboxylic acid (ACA). Addition of this aromatic acid led to slightly thinner films (70-80 A) that were characteristically more aqueous (up to 20% by volume) and 5-6% (v/v) ACA. This unique in situ characterization (deduced entirely from SANS data from emulsion samples) of the entire film composition calls for further investigation regarding the role this film-based water plays in emulsion stability.     

Asphaltene aggregation in organic solvents

Asphaltenic solids formed in the Rangely field in the course of a carbon dioxide flood and heptane insolubles in the oil from the same field were used in this study. Four different solvents were used to dissolve the asphaltenes. Near-infrared (NIR) spectroscopy was used to determine the onset of asphaltene precipitation by heptane titration. When the onset values were plotted versus asphaltene concentrations, distinct break points (called critical aggregation concentrations (CAC) in this paper) were observed. CACs for the field asphaltenes dissolved in toluene, trichloroethylene, tetrahydrofuran, and pyridine occurred at concentrations of 3.0, 3.7, 5.0, and 8.2 g/l, respectively. CACs are observed at similar concentrations as critical micelle concentrations (CMC) for the asphaltenes in the solvents employed and can be interpreted to be the points at which rates of asphaltene aggregations change. CMC values of asphaltenes determined from surface tension measurements (in pyridine and TCE) were slightly higher than the CAC values measured by NIR onset measurements. The CAC for heptane-insoluble asphaltenes in toluene was 3.1 g/l. Thermal gravimetric analysis (TGA) and elemental compositions of the two asphaltenes showed that the H/C ratio of the heptane-insoluble asphaltenes was higher and molecular weight (measured by vapor pressure osmometry) was lower.     

Effects of petroleum resins on asphaltene aggregation and water-in-oil emulsion formation

Asphaltenes from four crude oils were fractionated by precipitation in mixtures of heptane and toluene. Solubility profiles generated in the presence of resins (1:1 mass ratio) indicated the onset of asphaltene precipitation occurred at lower toluene volume fractions (0.1–0.2) than without resins. Small-angle neutron scattering (SANS) was performed on solutions of asphaltene fractions in mixtures of heptane and toluene with added resins to determine aggregate sizes. Water-in-oil emulsions of asphaltene–resin solutions were prepared and separated by a centrifuge method to determine the vol.% water resolved. In general, the addition of resins to asphaltenes reduced the aggregate size by disrupting the π–π and polar bonding interactions between asphaltene monomers. Interaction of resins with asphaltenic aggregates rendered the aggregates less interfacially active and thus reduced emulsion stability. The smallest aggregate sizes observed and the weakest emulsion stability at high resin to asphaltene (R/A) ratios presumably corresponded to asphaltenic monomers or small oligomers strongly interacting with resin molecules. It was often observed that, in the absence of resins, the more polar or higher molecular weight asphaltenes were insoluble in solutions of heptane and toluene. The addition of resins dissolved these insolubles and aggregate size by SANS increased until the solubility limit was reached. This corresponded approximately to the point of maximum emulsion stability. Asphaltene chemistry plays a vital role in dictating emulsion stability. The most polar species typically required significantly higher resin concentrations to disrupt asphaltene interactions and completely destabilize emulsions. Aggregation and film formation are likely driven by polar heteroatom interactions, such as hydrogen bonding, which allow asphaltenes to absorb, consolidate, and form cohesive films at the oil–water interface.     

A small angle neutron scattering study of the adsorbed asphaltene layer in water-in-hydrocarbon emulsions: structural description related to stability

We have developed a specific protocol to study with SANS measurements, the structure of the interfacial film layer in water-in-oil emulsions stabilized by asphaltene. Using the contrast matching technique available for neutron scattering, we have access to both the composition and the quantity of interface. The results obtained give us a view of the asphaltene aggregates in the interfacial film, which are structured as a monolayer and show a direct correlation between the size of asphaltene aggregates in solution and the thickness of the film layer. The organization of the interface has been studied as a function of several parameters such as the quantity of resins, i.e., the size of aggregates, the pH of the aqueous phase, and the aging time of the emulsions and the consequences of these variations on the macroscopic stability of these emulsions. We show that the key parameter for the stability is the inter-asphaltene aggregate interaction inside the film layer. Changing the attractive/repulsive balance between the aggregates in the film at the microscopic scale, by changing the aggregate's size or the aggregate's ionization, has a direct incidence on the quantity of water recovered after centrifugation: the stronger the attraction between aggregates in the film, the more stable the emulsion is.     

Roles of Various Bitumen Components in the Stability of Water-in-Diluted-Bitumen Emulsions

An experimental study was conducted to evaluate the effectiveness of the various components of Athabasca bitumen in stabilizing water-in-diluted-bitumen emulsions. The solvent used to dilute the very viscous bitumen was a mixture of 50:50 by volume of hexane and toluene. The various bitumen components studied were asphaltenes, deasphalted bitumen, and fine solids. It was found that asphaltenes and fine solids were the main stabilizers of the water-in-diluted-bitumen emulsions. Individually, the two components can stabilize water-in-diluted-bitumen emulsions. However, when both are present the capacity of the diluted bitumen to stabilize water emulsions is greatest. Emulsion stabilization tests indicated that whole bitumen had less capacity to stabilize water emulsions than asphaltenes and solids. This would indicate that the presence of the small molecules within the whole bitumen tends to lower the emulsion stability. Deasphalted bitumen acts as a poor emulsion stabilizer. Although deasphalted bitumen led to the least emulsion stabilization capacity, interfacial tension measurements showed that diluted deasphalted bitumen gave a greater decrease in the interfacial tension of water with diluent.     

Oilfield solids and water-in-oil emulsion stability

Model water-in-hydrocarbon emulsions consisting of toluene, heptane, water, asphaltenes, and native solids were used to investigate the role of native solids in the stability of oilfield emulsions. The solids were recovered from an oil-sands bitumen, a wellhead emulsion, and a refinery slop oil. The solids were clay platelets and fell into two size categories: (1) fine solids 50 to 500 nm in diameter and (2) coarse solids 1 to 10 microm in diameter. Emulsions stabilized by fine solids and asphaltenes were most stable at a 2:1 fractional area ratio of asphaltenes to solids. It appears that when the asphaltene surface coverage is high, insufficient solids remain to make an effective barrier. When the solids coverage is high, insufficient asphaltenes remain on the interface to immobilize the solids. Treatments that weaken the interface, such as toluene dilution, are recommended for emulsions stabilized by fine solids. Emulsions stabilized by coarse solids were unstable at low solids concentrations but became very stable at solids concentrations greater than 10 kg/m(3). At low concentrations, these solids may act as bridges between water droplets and promote coalescence. At high concentrations, layers of coarse solids may become trapped between water droplets and prevent coalescence. Treatments that flocculate the solids, such as heptane dilution, are recommended for emulsions stabilized by high concentrations of coarse solids. It is possible that emulsions containing both types of solids may require more than one treatment, or even process step, for effective water resolution.     

Effect of interfacial rheology on model emulsion coalescence I. Interfacial rheology

Interfacial elasticity and "dynamic" surface pressure isotherms were measured for interfaces between a dispersed water phase and a continuous phase of asphaltenes, toluene, and heptane. The interfacial modulus is a function of asphaltene concentration and in all cases reached a maximum at an asphaltene concentration of approximately 1 kg/m(3). The modulus increased significantly as the interface aged and slightly as the heptane content increased to a practical limit of 50 vol%. The modulus was approximately the same at 23 and 60 degrees C. The modulus correlated with the inverse of the initial compressibility determined from surface pressure isotherms. The surface pressure isotherms also indicated that a phase transition occurred as the interface was compressed leading to the formation of low compressibility films. Crumpling was observed upon further compression. The phase transition shifted to a higher film ratio with an increase in heptane content and interface age. Asphaltene concentration and temperature (23 and 60 degrees C) has little effect on the surface pressure isotherms. The surface pressure and elasticity measurements are consistent with the gradual formation of a cross-linked asphaltene network on the interface.     

疏水改性聚丙烯酰胺对原油组分界面扩张流变性质的影响

利用悬挂滴方法研究了疏水改性聚丙烯酰胺(HMPAM)对胜利采油厂高温高盐油藏采出原油中酸性活性组分和沥青质界面膜扩张流变性质的影响,考察了不同活性组分浓度条件下的界面扩张流变行为.实验结果表明:1750mg·L-1HMPAM能够在界面上形成网络结构,界面扩张模量数值高达100mN·m-1左右;油相中的酸性组分随着老化时间增加吸附到界面上,与HMPAM分子的疏水改性部分形成聚集结构,一方面通过快速的扩散交换过程大大降低扩张模量,另一方面通过与疏水改性部分的相互作用加强HMPAM分子间的缔合强度,增强网络结构的弹性.沥青质分子尺寸相对较大,分子间存在氢键等较强的相互作用,造成沥青质界面聚集体和HMPAM形成的网络结构共同决定界面膜性质,混合膜的扩张模量较单独HMPAM体系仅略有降低.     

沥青质及其亚组分与烷基苯磺酸盐溶液在降低界面张力中的协同作用

利用混合溶剂沉淀法将原油中的沥青质分为4个亚组分, 研究了含有各组分的模拟油与烷基苯磺酸钠(p-S14-4, p为对二甲苯; 14表示烷基链上有14个碳原子; S表示表面活性剂; 4表示芳基在长链烷基的4号碳原子上)水溶液间的动态界面张力(DIFTs). 结果表明, 沥青质及其亚组分的分子尺寸、 浓度和极性对DIFTs有显著影响. 分子尺寸较大的沥青质亚组分与p-S14-4分子之间难以形成混合吸附膜, 协同作用较弱, DIFTs曲线呈“L”形, 最小界面张力(IFT_min)受浓度和极性的影响较小. 在高浓度时, 分子尺寸较小的沥青质及其亚组分快速扩散至界面与p-S14-4分子形成紧密的混合吸附膜, 能够快速降低界面张力(IFT); 随着时间的延长, 界面层的分子发生重排, 导致DIFTs曲线呈“V”形, 且在这种情况下沥青质及其亚组分的极性越高, 降低IFT的协同作用越明显, IFT_min越低.     

Model Compounds for Asphaltenes and C80 Isoprenoid Tetraacids. Part I: Synthesis and Interfacial Activities

Three model compounds for asphaltenes and two model compounds for the C₈₀ isoprenoid tetraacids (ARN) have been synthesized and their interfacial and solubility properties were investigated. All compounds exhibit high interfacial activities. The asphaltene models lowered the interfacial tension between toluene and pH 9 to around 5 mN/m at 12.5–35 μM and the tetraacid models gave a drop in the interfacial tension between chloroform and pH 9 to 13 mN/m at only 5 μM, which is consistent with previous findings for the natural occurring C₈₀ tetraacids. A sudden drop in the IFT over a very narrow concentration range was observed for two of three asphaltene models. NIR spectroscopy studies indicated an aggregation most likely a result of polar and hydrogen bond interactions. The IFT results also showed different behavior with only small changes in chemical structure. The tetraacid models have similar interfacial behavior as the C₈₀ tetraacids and will thus be suitable model compounds with their highly UV active and fluorescent properties.     

Water-in-Oil Emulsions Stabilized by Asphaltenes Obtained from Venezuelan Crude Oils

W/O emulsions were studied using asphaltenes as surfactants. Asphaltenes were obtained from three Venezuelan crude oils: “Lago Cinco,” “Rosa Mediano,” and “Ayacucho.” They were extracted using n-heptane as a precipitanting agent. The following variables were studied: concentration of asphaltenes in the oleic phase and pH of the aqueous phase. An increase in asphaltene concentration in the oleic phase increases emulsion stability. On the other hand, the most stable emulsions correspond to an alkaline aqueous phase. Likewise, emulsion stability was higher for asphaltenes obtained from “Lago Cinco” crude oil and lowest from Rosa Mediano asphaltenes.     

Effect of oil soluble surfactant in emulsions stabilised by clay particles

Although surfactants and particles are often mixed together in emulsions, the contribution of each species to the stabilisation of the oil-water interface is poorly understood. We report the results of investigations into the formation of emulsions from solutions of surfactant in oil and aqueous suspensions of laponite. Depending on the salt concentration in the aqueous suspensions, the laponite dispersed as individual disc-shaped particles, 30 nm in diameter, or flocculated into aggregates tens of micrometres in diameter. At the concentrations studied, the flocculated particles alone stabilized oil-in-water emulsions. Synergistic interactions between the particles and octadecylamine at the oil-water interface reduced the average emulsion drop size, while antagonistic interactions with octadecanoic acid enhanced coalescence processes in the emulsions. The state of particle dispersion had dramatic effects on the emulsions formed. Measurements of the oil-water interfacial tension revealed the origins of the interactions between the surfactants and particles.     

Stability of emulsions of water in mineral oils containing asphaltenes

Emulsions of water in mineral oils are stable if the oil phase contains asphaltenes which are near the condition of incipient flocculation. This condition is determined by the composition of the oil phase and by the nature of the asphaltenes. High aromaticity of the oil phase and the presence of deflocculants prevent flocculation of asphaltenes; the deflocculants may be interfacially active agents or asphaltene-like compounds with better solubility in the oil phase. Conditions of incipient flocculation of asphaltenes correlate very well with a considerable increase of rheological resistance of the interface between the oil phase and distilled water, determined according to the torsion oscillation method. Stabilization of the water-in-oil emulsions is therefore caused by the build-up of a coherent layer of asphaltenes in the water-oil interface in these cases. Deflocculants of asphaltenes in the oil phase destroy their stabilizing effect; however, the deflocculants themselves may stabilize the water-in-oil emulsions by adsorption on the water-oil interface and then the correlation between the condition of asphaltenes and emulsion stability does not hold, nor is the interfacial viscosity perceptibly increased. Under borderline conditions of emulsion stability a few percent of sodium chloride in the water phase counteracts the build-up of a stabilizing layer of asphaltenes in the water-oil interface and so do higher p_H values of a buffered water phase. At low p_H-values emulsion stability does not correlate with interfacial resistance. It can be concluded that asphaltenes stabilize water-in-oil emulsions if they accumulate on the water-oil interface. This interfacial layer may show a coherence, which is an indication of the presence of asphaltenes rather than a condition for stability of the emulsions.