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Fatty acid-asphaltene interactions at oil/water interface Xi Wang a , Erica Pensini a , Yin Liang a , Zhenghe Xu a,∗ , M. Sharath Chandra b , Simon Ivar Andersen b , Wael Abdallah c , Jan J. Buiting d a

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 1H9, Canada Schlumber-Doll Research, One New Hampshire Street, Cambridge, MA 02139, USA Schlumberger Dhahran Carbonate Research Center, Dhahran, East Province, 31942, Saudi Arabia d Saudi Aramco, Dhahran 31311, Saudi Arabia b c

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Studied interactions of asphaltene and model naphthenic acid [stearic acid] at oil-water interface. • Calculated and investigated excess work of compression for the asphaltene-acid mixed film from their Langmuir trough pressure-area isotherms. • Identified a decrease in the rigidity of asphaltene interfacial film by stearic acid addition. • Visualized morphology changes in asphaltene interfacial films with the increase in stearic acid amount using in situ Brewster angle microscope.

a r t i c l e

i n f o

Article history: Received 6 July 2016 Received in revised form 28 September 2016 Accepted 13 October 2016 Available online xxx Keywords: Interfacial film Molecular interaction Asphaltenes Stearic acid Langmuir trough BAM

a b s t r a c t Asphaltenes have been shown to stabilize water-in-oil or oil-in-water emulsions by forming a viscoelastic interfacial film via molecular aggregation at oil-water interfaces. Natural carboxylic acids (“naphthenic acids”) and their anions present in crude oil are able to compete with asphaltenes to adsorb at the crude oil-water interface, decreasing significantly the crude oil-water interfacial tension. In this study, we designed a group of experiments to systematically probe the molecular interactions of naphthenic acid with asphaltenes at the oil-water interface by studying the Langmuir interfacial isotherms. Stearic acid as a representative naphthenic acid alone was not able to form rigid films at the toluenewater interface, in contrast to rigid interfacial films of asphaltenes. Upon mixing of asphaltenes with stearic acids, non-ideal (non-additive) behavior of interfacial isotherms was observed. Stearic acid was found to associate strongly with asphaltene molecules at the interface and render the films more expanded and flexible. The reduction in rigidity of interfacial film was found to be directly proportional to the amount of the stearic acid present in the system. Washing experiments by replacement of the top phase with fresh solvent showed irreversible adsorption of both asphaltenes and stearic acids at the toluene-water interface. The softening of interfacial film by stearic acid led to reduced compression energy of the interface, measurable quantitatively by defining the excess work of compression (Wexcess ) as a measure of molecular interactions at the oil-water interface. The calculated Wexcess was found to increase

∗ Corresponding author. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.colsurfa.2016.10.029 0927-7757/© 2016 Elsevier B.V. All rights reserved.

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with increasing stearic acid concentrations in the mixture as a result of softened interfacial films. In-situ Brewster angle microscopy visualization revealed a progressive reduction of molecular aggregates at the toluene-water interface with increasing addition of stearic acids. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Over the past several decades, increasing consumption of liquid fossil fuels has led to a swift decrease in conventional oil reserves, which prompted the development of unconventional crude oil resources such as shale gas and oil sands deposits [1]. Unlike the conventional oil, unconventional oil reserves are made of heavily biodegraded petroleum, typically with high content of naphthenic acids and/or asphaltenes [2]. The presence of high concentrations of these heavy oil components causes great difficulties in oil production, such as formation of highly stable water-in-oil emulsions. Such emulsions are problematic to downstream operations, which are problematic to downstream operations, such as scaling/fouling and corrosion of equipment in refineries [3–5]. More effective separations of W/O emulsions can help reduce the processing costs and improve the productivity and quality of final products. It is therefore highly desirable if not essential to understand the cause of emulsion stabilization, which allows prediction and mitigation of corresponding adverse effect. Despite their poor definition as a solubility class of petroleum material that are insoluble in n-heptane but soluble in toluene, asphaltenes have been considered as the main component that is responsible for stabilizing W/O petroleum emulsions. Previous studies have shown that upon mixing of water with heavy oil, asphaltenes were the main compounds to produce extremely stable W/O emulsions [6,7]. A popular and well-appreciated notion is that the surface-active asphaltenes are able to adsorb irreversibly at the oil-water interface to form a rigid and protective film surrounding the dispersed water droplets [8,9]. Studies on Langmuir-Blodgett films confirmed the formation of two-dimensional asphaltene layers at the oil-water interface. The asphaltene layers at oil-water interfaces were found to be rigid in nature and of measurable mechanical strength. Once asphaltenes adsorbed at the oil-water interface, the equilibrium becomes heavily shifted in favor of irreversible adsorption due to intermolecular association at the interface. Previous studies have shown such irreversible adsorption of asphaltenes at the toluene-water interface that even replenishing the top bulk toluene with pure toluene does not lead to a noticeable desorption [10,11]. Recent studies further suggested the cross-linking of asphaltene aggregates as the cause of the rigidity of asphaltene films, providing mechanical strength that prevents the coalescence of emulsified water droplets and hence increasing emulsion stability [12–14]. Apart from asphaltenes, the indigenous organic acids together with their corresponding soaps are also known to be surface and interfacially active, contributing to stabilizing W/O petroleum emulsions [15]. In heavily biodegraded crude oil, the acid contents are much higher with species ranging from simple alkyl carboxylic acids to fused ring aromatic acids [2]. Several mechanisms have been discovered to be responsible for the stabilization of water-incrude oil emulsions. First, the acidic molecules could ionize at the oil-water interface and dramatically lower the interfacial tension (IFT) [16]. Studies have shown that the formation of layered lamellar crystalline films could decrease the probability of water droplets coalescence and stabilize W/O emulsions [17–19]. Another mechanism that has been proved by several studies is the presence of high molecular weight acid species as calcium soaps at oil-water

interface, leading to formation of stable W/O emulsions. One particular case is the tetra-acids with a carbon number of eighty (ARN) [20]. Research has found that calcium naphthenates formed from ARN could accumulate at oil-water interface, leading to the formation of highly stabilized emulsion systems [21,22]. Comparing to the mechanisms of ionization and soap precipitation described above, understanding of acid-asphaltene interactions is less developed. Pauchard and Muller et al. showed that diprotic acids and asphaltenes form integrated films with high dilatational elasticity that stabilize crude oil emulsions [23,24]. However, other studies showed both enhancement and diminution of W/O emulsion stability by different acids. Studies by Gao et al. for example showed a significant reduction in the asphaltene-containing toluene-water interfacial tension and softening of the rigid asphaltene films by naphthenic acid addition that destabilized W/O emulsions [25]. Ese and Kilpatrick demonstrated that ␤ − cholanic acid could either stabilize or destabilize water-in asphaltene-containing oil emulsions, depending on the pH of the aqueous phase and concentration of the acids in oil [15]. They concluded that naphthenic acids seem to enhance the stability of emulsions at the neutral pH and decrease the stability at the alkaline pH; while asphaltene-like acids could stabilize emulsions under all conditions [26]. It becomes clear that asphaltene-acid interactions at oil-water interface are complex in nature and emulsions could be either stabilized or destabilized with various mechanisms and under different conditions by indigenous surfactants. As a result, asphaltene-acid interactions have to be treated carefully on a case-to-case basis. In this study, we are presenting experimental results showing interactions between asphaltenes and stearic acid at toluene-water interface. Stearic acid is one type of linear NAs containing 18 carbon atoms. It was chosen to represent naphthenic acid for the following reasons. First, stearic acid has high occurrence in nature with simple molecular structure, which simplifies the explanation of the observed isotherms [27,28]. The linear fatty carboxylic acids were found preferentially adsorb at the oil-water interface than other NAs containing cyclic components, indicating that the stearic acid is more likely the component of crude oil that interacts with asphaltenes at oil-water interface [29]. Since this research is focused on the interactions between asphaltenes and natural acid at the oil-water interface, choosing the linear stearic acid to represent NAs would be a good option. In contrast to other NAs with shorter hydrocarbon chains, stearic acid of long carbon chains has minimal solubility in the water subphase. Therefore, the concentration of acid in water phase can be considered negligible, which greatly simplifies the analysis. Pendant drop tensiometer was used to determine equilibrium time needed for the prepared interface to reach a dynamically stabilized state. The change in the interfacial property of asphaltenes films with the addition of stearic acid at various concentrations was studied using Langmuir interfacial trough technique. Film compression energy was calculated from Langmuir interfacial isotherms to evaluate the degree of acid-asphaltene interactions at oil-water interface. Calculations were based on the assumption that the adsorption of individual components at the interface was solely dependent on the partial concentration of the components in the bulk solution. The variation in compression energy can therefore be considered as a measure of the change in interfacial film properties. Washing experiments

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were conducted to study the reversibility of asphaltene-stearic acid adsorption at toluene-water interface, while Brewster angle microscopy is used to show in situ the morphology change of interfacial films. Our objective is to determine asphaltene-stearic acid interactions at solvent-water interface and to gain more insights into the mechanisms of emulsion stabilization by asphaltenes. 2. Materials and methods 2.1. Materials Optima-grade toluene (99.8%) and HPLC-grade acetone were all purchased from Fisher Scientific (Canada) and used as received without further purification. Ultrapure water (resistivity ∼ 18.2 M cm) was freshly collected from a Millipore Milli-Q system and used as the subphase. Stearic acid was purchased from Sigma-Aldrich (≥98.5%). Asphaltenes from crude oil were extracted using modified ASTM D4124 method. 2.2. Pendant drop tensiometer A set of interfacial tension measurements were first performed for asphaltene and stearic acid solutions in toluene using a pendant drop tensiometer (Attension Theta, Biolin Scientific, Espoo, Finland). In each measurement, a droplet of toluene solution (25 ␮l) with asphaltenes or stearic acid at 0.1 g/L or 0.0001 g/L concentrations was formed at the tip of an inverted needle immersed in a glass cuvette filled with deionized water. The tensiometer first recorded the silhouette of the droplet using a fast speed CCD camera. The images were then digitized and fitted to the Young-Laplace equation. This fitting allowed calculation of the curvature of the droplet and then surface/interfacial tension at ±0.1 mN/m accuracy. The obtained interfacial tension results were plotted against aging time of the interface. For dynamic interfacial tension measurements, images were recorded at 0.6 frame/s (fps) rate for a total aging time of 60 min. The volume of the droplet was kept constant 25 (␮l) and several runs were taken for the same condition to ensure good repeatability. It should be noted that for measurements longer than 90 min, it becomes more difficult to maintain a constant volume of the drop. This might be due to the aqueous pressure acting on the droplet that causes backflow or shrinkage in volume over longer aging time. 2.3. Langmuir trough Asphaltenes and stearic acid stock solutions of 1 g/L concentration in optima-grade toluene were prepared. Langmuir trough experiments were performed with a computer controlled KSV Langmuir Interfacial Trough (KSV Instruments, Finland). The trough has two compartments: a lower compartment of 120 ml to hold water as subphase and an upper compartment of 100 ml that is designed for holding lighter oil phase such as toluene. The trough was equipped with two movable barriers and was placed in an enclosure on an anti-vibration table. Prior to each measurement, the trough and the barriers were thoroughly cleaned by rinsing with toluene, acetone and Milli-Q water. The surface of the trough was then wiped and dried with Texwipe wipers. A Wihelmy plate of filter paper Whatman 1 CHR was used to measure the interfacial pressure. The Wilhelmy plate was wetted and cleaned with MilliQ water prior to each measurement. The interfacial pressure (␲) is defined as the difference of interfacial tensions of the oil-water interface in the absence (0 ) and presence () of any interfacial active materials, i.e.,  = 0 − 

(1)

3

When the interfacial pressure (␲) was continuously monitored and recorded as a function of the trough area (A), the interfacial pressure-area (␲ − A) isotherms were generated. In this study, spreading protocol was used to prepare the interfacial films of asphaltenes and stearic acid. This method was adopted to investigate asphaltenes and stearic acid interactions as well as their film behaviors (individually and combined) by ‘forcing’ the molecules at the oil-water interface. As an alternative to the spreading protocol, a diffusion method was also used, where molecules were allowed to diffuse freely from the top phase to the interface over 3 h. However the results obtained using diffusion protocols will be reported in a separate publication with more focused objectives.

2.3.1. Spreading method In this method, 120 ml Milli-Q water was first poured into the lower compartment of the trough. To test the cleanliness of the trough and the subphase, the pressure isotherm at air-water interface was first collected. The system was considered clean when the collected isotherm has a constant pressure reading of within ±0.1 mN/m during an area change from 170 cm2 to 12.5 cm2 . The barriers were then fully expanded and the balance was zeroed at clean air-water interface prior to spreading any solution. On the water subphase, a given volume of oil samples (toluene stock solutions of asphaltenes, stearic acid, or pre-mixed solutions of the two) were spread dropwise and evenly using a gastight Hamilton syringe, followed by evaporation of the spreading solvent (toluene) for 20 min. After complete evaporation of the solvent, 100 ml optima grade toluene was carefully introduced to the upper compartment of the trough with the aid of a glass funnel and rod to avoid disturbance of the film. The system was then left to equilibrate for 30 min, which was determined by IFT measurements. At the end of the equilibration period, compression isotherms were collected by moving both barriers at a speed of 10 mm/min. Table 1 summarizes the spreading amounts of asphaltenes and stearic acid in these experiments. In this study, a precisely known amount of asphaltenes and/or stearic acid was spread at the air-water interface, which simplifies interpretation of the results on molecular interactions at the interface by eliminating the effect of mixing and partition in the top phase [30–32]. This approach corresponds well with the focus of current study to determine whether the addition of stearic acid could alter the asphaltene interactions at the interface. For asphaltenes/total mass ratios of 0.2, 0.3 and 0.5, the total mass of asphaltenes and stearic acid spread on the interface was kept at 60 ␮g. To investigate the softening effect of stearic acid on asphaltenes films, two additional experiments were conducted where only the spreading amount of asphaltenes was kept constant at 30 ␮g, while the amounts of stearic acid increased from 40 ␮g to 60 ␮g.

2.3.2. Washing experiments In order to understand whether the equilibrium of interfacial active materials weighs heavily in favor of irreversible adsorption, washing tests were conducted for all the experiments by exchanging the top phase with pure toluene. In washing experiments, the toluene top phase was carefully removed with a pipette after the barriers were at minimum separation, followed by the addition of 100 ml fresh toluene to the top phase. The barriers were then slowly expanded at 5 mm/min. After the system was left for another 30 min to equilibrate, the newly formed toluenewater interface was compressed at 10 mm/min to complete the first washing experiment. For each system, two consecutive washing tests were conducted to study the reversibility of adsorption.

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Table 1 Spreading amounts of asphaltenes (Asp.) and stearic acids in each experiment. Sample

Asp + Stearic Acid Mixture Asp Stearic Acid

Asp amount (␮g)/Total materials (␮g) 0.2

0.3

0.5

0.33

0.43

60 ␮g (12 ␮g Asp + 48 ␮g Acid) 12 ␮g 48 ␮g

60 ␮g (18 ␮g Asp + 42 ␮g Acid) 18 ␮g 42 ␮g

60 ␮g (30 ␮g Asp + 30 ␮g Acid) 30 ␮g 30 ␮g

90 ␮g (30 ␮g Asp + 60 ␮g Acid) 30 ␮g 60 ␮g

70 ␮g (30 ␮g Asp + 40 ␮g Acid) 30 ␮g 40 ␮g

2.4. Brewster angle microscopy Brewster angle microscopy is utilized to obtain images showing the morphology of interfacial films in situ, without the need to transfer them to a solid substrate. In this technique the interfacial films were hitby an incident light at an angle known as Brewster’s angle (␪B ), at which only s-polarized light was reflected from the interface to the detector. At this angle, the light was reflected solely by the interface, and not by the bulk phases below or above the interface, thus collecting images at the interface. Brewster angle microscopy (Model EP3, Accurion GmbH, Germany) equipped with light guides and a CCD camera was used to image films at the liquid-liquid interface. Images were collected at a magnification of 5X using EP3View2.x software (Accurion GmbH, Germany). The trough used for these experiments was not equipped with mobile barriers and had a fixed area of 28 cm2 . Following the addition of 30 ml of aqueous phase to the trough was the spreading of asphaltenes (160 ␮l of 1 g/L stock solution) at the toluene-water interface. The concentration of asphaltenes and stearic acid was chosen in such a way that high quality BAM images could be obtained. After 20 min evaporation, either 100 ml toluene or stearic acid-in toluene solution (0.035 wt% or 3.5 wt%) was gently added as the top phase, with great care being taken to prevent any disturbances of the interface. This procedure was chosen to understand the morphological change of asphaltene films at the interface while controlling the stearic acid concentration in the bulk phase. In addition, if high concentration of stearic acid was spread directly at the interface, rigid skins would form immediately due to gelation/crystallization, which would interfere severely with BAM imaging. The images were captured after 30 min equilibration. All images of the film at the toluene-water interface were obtained using 48◦ of incidence angle, while the polarizer and analyzer were set to 10◦ .

3. Results and discussion 3.1. Interfacial tension measurement Dynamic interfacial tension of toluene solutions of two different asphaltenes or stearic acid concentrations in contact with water is shown in Fig. 1. All the data are averages of at least three repeating measurements. For 0.1 g/L asphaltenes and stearic acid, the interfacial tension decreases rapidly with time t up to ∼10 min, after which only a marginal further decrease in the interfacial tension is observed. For 0.1 g/L asphaltene, the rate of interfacial d() tension decrease after 10 min is d(t) = −2.77x10−2 mN m /min, and for stearic acid at the same concentration,

d() d(t)

= −1.51x10−2 mN m /min.

At a lower concentrations of 0.0001 g/L, asphaltene and stearic acid solutions do not show significant variation in interfacial tension over the measurement period. This finding suggests that by the time Langmuir trough compression started 30 min after film preparation, the rapid diffusion-controlled adsorption of asphaltene and/or stearic acid molecules at the interface would have reached a dynamic equilibrium state. The small change in the interfacial tension is likely due to reorganization of asphaltene or stearic

acid molecules at toluene-water interface. During Langmuir trough experiments, the toluene-water interface can be considered at a dynamic steady state, where interfacial pressure change caused by re-adsorption of materials is minimized. Therefore, we could conclude that the measured Langmuir trough isotherm with 30 min aging reflects the true compressible nature of the interfacial film. Similar conclusions were made by Rane, Zarkar et al. [33,34], where they measured a strong dependence of interfacial tension on surface coverage or the bulk asphaltene concentrations, but not on time. For 20–500 ppm asphaltene in toluene solutions, the interfacial tension decreases rapidly for the initial 5 min and starts to reach near-equilibrium values after 30 min of measurements. The results in this experiment demonstrate that 30 min equilibrium time in the Langmuir trough experiments is sufficient for the formation of stable and consistent interfacial films. 3.2. Langmuir trough isotherms Typical pressure-area (␲ − A) isotherms were obtained when the trough area was compressed at a constant speed. The isotherms were then plotted by subtracting the isotherm of pure toluenewater interface over the entire trough area. The correction of the isotherms allowed better comparison among different systems of asphaltenes, stearic acid and their mixtures. Fig. 2 shows the interfacial ␲ − A isotherms of spreading various amounts of asphaltenes at toluene-water interface. The lack of a clear phase transition in the resultant ␲ − A isotherms indicates the complex, multi component nature of asphaltene molecules [35]. Due to the relatively low concentration of deposited asphaltenes, the buckling of the interface even under high degree of compression was not observed. Despite the fact that asphaltene concentrations at the given toluene-water interfaces varied, the initial interfacial pressures obtained at maximum trough area were essentially the same, indicating a less surface active nature of asphaltenes than common surfactants. As demonstrated by previous research, the compressibility properties of Langmuir monolayer can be determined from the slope of ␲ − A isotherms [36,37]. Therefore, with increasing amount of asphaltenes spread, the gradual upward shift in ␲ − A isotherms suggests an increasing rigidity of the interface. In addition, with increasing amounts of asphaltenes spread at the interface, a larger surface area is required for the molecules to stay beyond the separation distance where significant intermolecular interactions come into effect. The arrows in Fig. 2 point to the largest area on the ␲ − A isotherm where the pressure starts to increase, indicating 2-D molecular interactions at the interface. This critical interfacial area occurs at ∼ 58 cm2 , 90 cm2 and 120 cm2 for spreading 12 ␮g, 18 ␮g and 30 ␮g of asphaltenes, respectively. By taking a current consensus of average molecular weight of asphaltenes in the range of 750 g/mol [38], 12 ␮g asphaltenes correspond to 1.6 × 10−8 moles of asphaltene molecules at the interface. At ∼ 58 cm2 trough area, asphaltene molecules reached a full coverage with a detectable increase in the surface pressure upon initial compression. Assuming a monolayer adsorption/spreading at this amount led to a calculated mean molecular area (MMA) of 0.62 nm2 /asphaltene molecule (62 Å2 /molecule). It should be noted that the above calculations are valid only when asphaltenes form

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Fig. 1. Dynamic interfacial tension of 0.1 g/L and 0.0001 g/L asphaltenes or stearic acid in toluene solutions in contact with water: a) asphaltenes and b) stearic acid.

Fig. 2. Interfacial pressure-area (␲ − A) isotherms of asphaltenes spread at the toluene-water interface, showing increasing adsorption at the interface with a larger amount of asphaltenes spread. Arrows indicate where molecular interaction starts to affect the compression at different concentration.

Fig. 3. Interfacial pressure-area isotherms of stearic acid spread at the toluenewater interface, showing the absence of interfacial activity of the acid when the spreading amount is 30 ␮g. Increasing interfacial pressures were observed under compression when higher amounts of stearic acid were spread at the interface.

monolayers at toluene-water interfaces [3,39,40]. To justify this assumption, further tests were conducted by spreading a different amount (18 ␮g and 30 ␮g) of asphaltenes at the toluene-water interface. The calculated MMA value is now 65 Å2 /molecule and 54 Å2 /molecule, respectively. Therefore, these calculated values of MMA at ∼65 Å2 /molecule suggest that pure asphaltene molecules adsorb perpendicular to the toluene-water interface. Fig. 3 shows ␲ − A isotherms of pure stearic acid spread at the toluene-water interface. Compared with isotherms of asphaltenes shown in Fig. 2, stearic acid exhibits a much lower interfacial pressure upon compression (note much narrower scale of pressure). With 30 ␮g of stearic acid spread at the interface, no noticeable interfacial pressure increase can be observed, with the interfacial isotherm obtained being almost identical to that of pure toluenewater interface. If the stearic acid molecules formed a monolayer, a complete packing (at approx. 0.2 nm2 /molecule) would occur at 127 cm2 for 30 ␮g of acid at the interface. As a result, one would expect a fast increase in the interfacial pressure upon further compression. Such an interpretation is clearly contradictory to the above-mentioned observation of negligible pressure increase upon compression to the trough area well below 127 cm2 , suggesting that stearic acid by itself does not form rigid films at toluene-deionized water interface and the oily molecules would prefer migrating to the top toluene phase upon compression. Due to the long aliphatic chain in stearic acid molecule with respect to its small polar head group, it is possible that the equilibrium is heavily shifted in favor of desorption of stearic acid into the bulk oil phase. Since a low interfacial pressure corresponds to a high interfacial tension, the results reveal that only a small amount of stearic acid molecules

were present at the interface during compression. When more acid molecules were spread, the interfacial pressure-area isotherms only show an upward shift due most likely to the increased initial concentration of stearic acid at the toluene-water interface. Even though stearic acid is known to stabilize emulsions and decrease interfacial tension, it lacks the ability to remain anchored at the interface during compression, unless a favorable condition exists in the aqueous phase to significantly increase its interfacial activity. This can be obtained by increasing the electrolyte content and changing the composition of the water phase. Fig. 4 shows the isotherm of spreading a mixture of 18 ␮g asphaltenes and 42 ␮g stearic acid (0.3 asphaltene/total (asphaltene + stearic acid) mass ratio) at toluene-water interface, in comparison to the isotherms obtained by spreading an equivalent amount of individual components. All isotherms are plotted after correction for the pure toluene-water surface pressures, so that the effect of film softening by stearic acid could be easily visualized. In addition, a calculated isotherm corresponding to an ideal mixing condition is included as a dashed line to illustrate molecular interactions of asphaltenes with stearic acid at the interface. The ideal mixing condition assumes no interaction between asphaltenes molecules and stearic acid so that the pressure-area isotherm of the mixture is simply the summation of the isotherms of pure stearic acid (a) and pure asphaltene (b) over the entire compressed area. To further address the rationales behind summation, one should recognize that the ideal adsorption from solution assumes that the ratio of species in the bulk remains the same as in the interfacial film. Therefore, the direct summation made herein is facilitated by the fact that the partial mass concentration of com-

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Fig. 4. Interfacial pressure-area isotherms of spreading: 42 ␮g stearic acid (a); 18 ␮g asphaltenes (b), and their mixture (c) at the toluene-water interface, in comparison with the hypothetic isotherm calculated based on the assumption of additive contributions from each individual components to the total interfacial pressure change (d), showing significant deviation of prediction (d) from the measured (c) isotherms as a result of strong interactions between stearic acid and asphaltenes at the toluene-water interface.

Fig. 5. Interfacial pressure-area isotherms of spreading: 30 ␮g stearic acid (a), 30 ␮g asphaltenes (b) and their mixture (c) at the toluene-water interface, in comparison with the isotherm calculated based on the additive assumption that individual components contribute at their corresponding levels to the total interfacial pressure change (d). The diagram shows a close match between the measured (c) and calculated (d) pressure-area isotherms of the mixtures.

ponent X/interfacial area is the same in both pure and mixture solutions. The direct comparison between the experimental and calculated isotherms of mixtures can be made since there exists no clear liquid-to-solid type phase transition in the pressure-area isotherms of individual components. In the following discussions, asphaltene will be considered as a single component in the analysis despite its polydisperse nature. The pressure-area isotherm (a) of pure stearic acid in Fig. 4 rises slowly, indicating a soft interface with only a small amount of stearic acid present. The ␲ − A isotherm (b) for pure asphaltenes shows a much faster increase in the interfacial pressure under compression, reaching ∼13 mN/m when the trough area was compressed from 170 cm2 to 18 cm2 . This finding indicates the formation of a more rigid interfacial film by asphaltenes. This can be attributed to stronger intermolecular association among asphaltene molecules, forming an interconnected networks or elastic films at the toluene-water interface. Surprisingly, the pressurearea isotherm of the mixture (c) is lower than the isotherm of asphaltenes (b), but higher than the isotherm of stearic acid alone (a) obtained despite the same amount of asphaltenes being spread. Previous studies showed preference of asphaltenes to stay at the toluene-water interface while stearic acid dissolves in the bulk toluene phase. If there were no interactions, the two species would behave at toluene-water interface based on their own interfacial activity upon compression. This would cause an additive increase in the measured interfacial pressure. In reality, the obtained compression isotherm for the mixture exhibits a more relaxed interfacial film. The calculated MMA value also decreases from ∼ 65 Å2 /molecule to 50 Å2 /molecule (based on the fact that sharp increase in the slope of the mixture’s isotherm occurs at 72 cm2 ). This finding clearly demonstrates a strong interaction between the stearic acid and asphaltene molecules at the toluenewater interface. Due to this interaction, the original rigid structure of asphaltene films is altered, resulting in a softer film. In addition, the presence of stearic acid in competition with asphaltenes at the interface also contributes to the reduced interfacial pressure as compared with the isotherm of asphaltenes alone. Acosta et al. obtained similar results that increasing the amount of naphthenic acids decreases the stability of asphaltene films [41]. With the addition of naphthenic acids, the interfacial film properties transited gradually from asphaltenes-controlled to naphthenic acid-controlled, showing an intermediate behavior. Their work illustrated the ability of naphthenic acids to significantly weaken asphaltene films and corresponds well with our conclusions. To

explain such an effect, one would have to assume that stearic acid and asphaltenes become inter-connected at the interface, facilitating the retention of acid upon film compression. The stearic acid introduced participates in the lateral interaction of asphaltene molecules in the interfacial film and collectively forms a more flexible film. To quantitatively evaluate such interactions, the pressure-area isotherm of the mixture was calculated by assuming additive contributions from asphaltenes and stearic acid to the total interfacial pressure change. The calculated pressure-area isotherm shown as the dashed line in Fig. 4(d) predicts a much higher interfacial pressure than the measured under any given compression, clearly indicating a non-additive nature for asphaltenes and stearic acid that resulted in altered interfacial film properties. It appears that the stearic acid prevented the formation of rigid network by asphaltene molecules, resulting in a more expanded and flexible film. This behavior is consistent with the observations in our previous study that indigenous naphthenic acids in bitumen can destabilize the water-in-oil emulsion by lowering the interfacial tension, reducing the rigidity and promoting the coalescence of water droplets [42]. Similar results were obtained for the mixtures of 20%, 33% and 43% asphaltenes (isotherms not shown). The deviation of the experimental isotherm from the additive assumption was found to become much less significant with increasing asphaltene contents in the mixture. As an example, Fig. 5 shows ␲ − A isotherms of spreading a mixture containing 30 ␮g asphaltenes and 30 ␮g stearic acid (0.5 Asp/total material mass ratio) in comparison to the isotherms obtained with each single component at the same corresponding concentrations. Since stearic acid lacks the interfacial activity and ability to form rigid films at toluene-water interface, the spread acid becomes quickly dissolved into the top phase after introducing toluene, resulting in a negligible interfacial pressure change upon compression, as shown in Fig. 5(a). It is interesting to note the overlap between the ␲ − A isotherm for asphaltenes in Fig. 5(b) with that for the mixture shown in Fig. 5(c). It is clear that with only small amount of acid present, the interfacial behavior is completely dominated by asphaltenes and the softening effects of acid is minimal. Fig. 6 shows interfacial ␲ − A isotherms of 30 ␮g asphaltenes spread with various amounts of stearic acid at toluene-water interface. Since the typical errors in pressure-area isotherm measurement is less than ±0.2 mN/m, corresponding to a relative error of less than 5%, the following energy calculations were based on

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Fig. 6. Interfacial pressure-area isotherm of 30 ␮g asphaltenes spreading with: 0 ␮g (a), 30 ␮g (b), 40 ␮g (c) and 60 ␮g (d) of stearic acid at the toluene-water interface, showing a minimum content of 40 ␮g stearic acid is needed to soften the asphaltene films. Arrows indicate potential phase transitions in isotherm (c) such as azeotropy or a eutectic point.

single experimental results, which could be cautioned to bear a 5% relative error when interpreting the results in Fig. 6. With stearic acid content below 30 ␮g (b), the rigidity of the interfacial film is dominated by asphaltenes (a), as shown by the overlap of the isotherms. Increasing stearic acid content to 40 ␮g caused a significant deviation of the resultant isotherm from the isotherm of asphaltenes alone. It seems that more stearic acids become incorporated into the interfacial film structures, diminishing interactions among asphaltene molecules while forming the film of decreased rigidity. A further increase in the stearic acid content to 60 ␮g did not show any further decrease in film rigidity. It seems that 40 ␮g of stearic acid has already saturated the film, where the film reached its flexibility limit. This result reveals that in the case of 30 ␮g asphaltenes, a maximum amount of stearic acid of 30 ␮g exists that can retain the inter-connected asphaltene films at the interface. Further increasing stearic acid to 40 ␮g prevented the formation of rigid asphaltene network and increased the compressibility of the interfacial films. Furthermore, the isotherm of mixture containing 40 ␮g stearic acid shows some resemblance of liquid-liquid phase transitions, as observed by more pronounced change in the slope d␲/dA between 40 cm2 and 100 cm2 compressed trough area. This observed phase transition can be attributed to an enhanced degree of molecular interactions at the interface, where loosely attached molecules become more closely packed with each other to form a well-mixed interfacial film. Similar observations have been reported for binary mixtures containing stearic acid and various other surface active compounds [42]. It has also been suggested in the literature that the observed changes in film properties are due to azeotropy or presence of eutectic points in the phase diagram of the blend. As the collapse pressure was not reached at final compression in the current study, the exact mechanism of these changes cannot be determined. To quantitatively understand asphaltenes-stearic acid interactions observed in Fig. 4, the work or net energy associated with mechanically compressing the film at the oil-water interface is calculated. With the interfacial pressure measured experimentally using Langmuir Trough, the energy of compression (W) can be determined by integrating ␲ − A isotherms over the entire compression area (A) as



W=

dA

(2)

The same equation has been used in literature to evaluate energy barriers of protein desorption from the oil-water interface [43]. For our systems, this method calculates the net energy (work) associ-

7

Fig. 7. Energy of compressing toluene-water interface containing a total mass of 60 ␮g with varying asphaltenes wt% in the mixed system.

ated with the process of film compression. If no interaction exists between asphaltenes and stearic acid, the two types of molecules, upon compression should behave based on their own interfacial activity at the toluene-water interface. Therefore, the energy used to compress the mixed system should be equal to the sum of the compressional energy for compressing each individual species [44–47], i.e.: Wcalc. = Wasp. + Wacid

(3)

where Wasp and Wacid are the energy of compression for the toluene-water interface formed with asphaltenes alone and stearic acid alone, respectively. The obtained results are shown in Fig. 7. The experimental energy of compression for the corresponding mixture is shown as separate bars for comparison. As shown in Fig. 7, for a given total mass of 60 ␮g, increasing mass ratio of asphaltenes in the mixture increases the energy of compression due to higher concentrations of asphaltenes at the interface, as anticipated. In fact, when half of the mixture is asphaltenes, much more energy was required in order to compress the interface to the same extent, indicating a more rigid film due to minimal effect of added stearic acid. In this case the calculated and measured compressional energy matches with each other. At lower asphaltene to stearic acid mass ratio, the lower compressional energy is required as anticipated due to low concentration of asphaltenes at the toluene-water interface. More interestingly, the compressional energy of the interface determined from the measured ␲ − A isotherms is much lower than that from the calculated ␲ − A isotherms, more so at lower asphaltene to stearic acid mass ratio. The significant deviation from ideal (indifferent) behavior is caused by molecular interactions, where the film properties of the mixture do not depend linearly on the monolayer composition. This result suggests a strong interaction between stearic acid and asphaltene molecules at toluene/water interface [48–51]. The excess energy (Wexcess ) arising from the interactions between the stearic acids and asphaltenes molecules could be calculated by Wexcess = Wcalc. − Wexp.

(4)

where Wexp represents the energy of compressing the toluenewater interface formed with acid and asphaltene mixture. Positive Wexcess for all cases indicates attractive interactions between asphaltenes and stearic acid, leading to porous network structures that are easier to compress. Fig. 8 summarizes the results of the two sets of spreading experiments, separated by a dashed line, the results on the left of the dividing line are from the experiments where the total amount of materials are kept constant at 60 ␮g;

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Fig. 8. Excess energy of compression (Wexcess ) determined for two sets of spreading conditions: left for a constant total asphaltenes-stearic acid mass of 60 ␮g and right for a fixed asphaltenes amount of 30 ␮g with varying stearic acid content.

Fig. 9. Effects of toluene washing on the interface formed by spreading 12 ␮g asphaltenes at the interface.

while on the right the amount of asphaltenes was kept constant but the mass of stearic acid varied. Increasing the amount of asphaltenes decreases the excess energy of compression, indicating an increasingly dominant behavior of asphaltenes at the toluene-water interface due to insufficient amount of stearic acid to modify the asphaltene network. It can be concluded that there exists a minimum stearic acid content to modify the structure of asphaltene interfacial films, below which no film softening can be observed. Overall, the results confirm that stearic acid is able to participate in the lateral structures of interfacial film and weaken the molecular interactions among asphaltene molecules. 3.3. Washing experiments To further confirm whether the stearic acid remains at the interface with asphaltene molecules, washing experiments were conducted. The primary goal of the washing experiments was to investigate whether there indeed exists ‘irreversible’ adsorption of asphaltene molecules at the toluene-water interface. By replacing the top phase with fresh toluene, free asphaltenes and/or stearic acid in the bulk phase were removed. If the adsorption were indeed reversible, this procedure would eventually lead to depletion of both species from the interface. On the contrary, when the adsorption is irreversible, the species will prefer to stay at the interface, showing identical compression isotherms despite repetitive washing. Generally speaking, ‘irreversibly’ adsorbed surface active materials are more detrimental, since they tend to create more stable emulsions during petroleum processing. Fig. 9 shows the interfacial pressure-area isotherms of spreading 12 ␮g asphaltenes at the toluene-water interface after multiple washings. It can be observed that after the first washing, a more compressible film was obtained. However, the highest interfacial pressure obtained at lowest trough area remained almost the same, indicating an identical final state of the interfacial films. The results collectively show the removal of some loosely attached asphaltene molecules from the toluene-water interface into the toluene top phase upon the first compression, leaving behind the ‘irreversibly’ attached asphaltene molecules at the interface. Similar characteristics of the ␲ − A isotherms were obtained after washing interfacial films formed from different amount of asphaltenes. These observations are in good agreement with the results in our previous studies [4,35]. Fig. 10 shows interfacial pressure-area isotherms after two consecutive washings of the interfaces formed by spreading a mixture of 12 ␮g asphaltenes and 48 ␮g stearic acid at the toluene-water interface. Similar to the effect of washing the interfaces formed by

Fig. 10. Effect of toluene washing on the interface formed by spreading 12 ␮g asphaltenes and 48 ␮g stearic acid.

asphaltenes, the maximum pressure obtained at minimum trough area remained nearly unchanged after two consecutive washings. This finding indicates that the stearic acid was able to form a stable interfacial film with asphaltenes that resists stearic acid desorption from the interface upon further washing. The association between the two types of molecules provides a synergetic anchor that keeps the stearic acid at the interface and consequently changes the film permanently. More specifically, the incorporation of stearic acid through molecular interactions with asphaltenes plays a key role in decreasing the rigidity of interfacial asphaltene films at the toluenewater interface. By comparing the isotherms in Figs. 9 and 10, it can be observed that the isotherms formed with asphaltenes and stearic acid maintained the same level of surface pressure. If stearic acid were selectively desorbed from the interface by washing, the magnitude of the final compressed interfacial pressure after washing would approach the value obtained for pure asphaltene systems in Fig. 9. The unvaried final compressional pressure further proves the strong intermolecular association between stearic acid and asphaltenes at the toluene-water interface. 3.4. Brewster angle microscopy To study the role of stearic acids in reducing the rigidity of the interfacial asphaltene films, the morphology of asphaltene film formed at toluene-water interface in the presence of different amount of stearic acid was studied using a Brewster angle microscopy (BAM). As shown in Fig. 11, asphaltenes in the absence of stearic acid form a relatively homogenous film at the toluenewater interface with only a few visible molecular aggregates. With

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Fig. 11. BAM images of 160 ␮g asphaltenes at the toluene-water interface with: (a) 0 wt%, (b) 0.035 wt% and (c) 3.5 wt% of stearic acid in the top phase. The overall concentration of asphaltenes is 1.6 ␮g/ml. The size of the images is 801 ␮m × 801 ␮m.

the addition of 0.035 wt% stearic acid in the top phase, more and larger asphaltene aggregates appeared at the interface. The number and size of the aggregates increased significantly when stearic acid concentration in the top toluene phase increased to 3.5 wt%. The morphological differences of asphaltenes films formed in the presence of stearic acid in the toluene top phase clearly indicate the role of stearic acid in altering the network of asphaltene interfacial films. This result further confirms that strong interactions exist between stearic acid and asphaltenes. The addition of stearic acid alters the structure of original interfacial films leading to decreased observed rigidity. The results obtained in this study provide a scientific insight on the observed reduction in the elasticity of interfacial films when small surface-active molecules were added.

4. Conclusions Interactions of stearic acid with asphaltenes at the toluenewater interface were studied using pendant drop tensiometer, Langmuir trough and Brewster angle microscopy. Unlike asphaltenes that can irreversibly adsorb at the toluene-water interface, stearic acid alone at neutral pH of aqueous phase is only weakly adsorbed at the interface. The lower interfacial activity of stearic acid molecules makes them more likely to stay in the top toluene phase by themselves. However, upon mixing, stearic acid and asphaltenes interact strongly at the interface, leading to decreased film rigidity in comparison to the pure asphaltene films. The interaction between stearic acid and asphaltenes contributes to the decreased interfacial film rigidity. The compression energy of the interfacial films was calculated from the pressurearea isotherms of individual components and their mixtures of equivalent amounts. The results confirm that the presence of stearic acid decreases the energy required to compress the mixed interface to the same extent. On one hand, the decreased energy is attributed to acid participating in the lateral asphaltene network structure, leaving behind loosely attached asphaltene molecules at the interface. On the other hand, the various functional groups in asphaltenes provide anchoring points to stearic acid and together they lower the energy of the whole system. Washing experiments by exchanging the top phase for the mixture showed irreversible adsorption of asphaltenes and stearic acid at the toluene-water interface, which further proves strong molecular interactions of stearic acids with asphaltenes. The same method can be extended to other systems to understand molecular interactions at the interfaces. The Brewster angle microscopy images provide a more direct view on the enhanced aggregation of asphaltenes at the interface with the addition of stearic acid and the softening of the resulting interfacial film.

Acknowledgement The authors thank Saudi Aramco and Schlumberger for financial support and the permission to publish. We (XW and ZX) also like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) under Industrial Research Chair Program in Oil Sands Engineering with a partial support from Alberta InnovatesEnergy and Environmental Solutions.

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Please cite this article in press as: X. Wang, et al., Fatty acid-asphaltene interactions at oil/water interface, Colloids Surf. A: Physicochem. Eng. Aspects (2016), http://dx.doi.org/10.1016/j.colsurfa.2016.10.029