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Droplet coalescence and spontaneous emulsification in the presence of asphaltene adsorption Simone Bochner de Araujo, Maria C. Merola, Dimitris Vlassopoulos, and Gerald G. Fuller Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02638 • Publication Date (Web): 09 Sep 2017 Downloaded from http://pubs.acs.org on September 10, 2017
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Droplet coalescence and spontaneous emulsification in the presence of asphaltene adsorption Simone Bochner de Araujo1 , Maria Merola1 , Dimitris Vlassopoulos2 , Gerald G Fuller*1 1
2
Department of Chemical Engineering, 443 Via Ortega, Stanford University, Stanford, CA 94305, USA Institute of Electronic Structure & Laser, FORTH, and Department of Materials Science & Technology, University of Crete, 100 Nikolaou Plastira str., Vassilika Vouton, Heraklion, Crete 70013, Greece * Corresponding author:
[email protected], Stanford University
Abstract In a refinery, undesired high levels of salt concentration in crude oils are reduced by the contact of water with crude oil, where an emulsion is formed. Later, the separation of the water from the desalted oil is essential for the quality of both wastewater discharge and refined oil. However, complex components of crude oil such as asphaltenes may stabilize these emulsions causing difficulties in separation efficiency. Here we show the coalescence inhibition caused by asphaltene adsorption for both water-in-oil and oil-in-water emulsions, where the oil phase consists of a simple model of asphaltenes dissolved in toluene. We find that oil-in-water emulsions are less stable than water-in-oil emulsions by using a newly developed instrument where controlled experiments for the coalescence time of a single droplet against an oil/water interface can be measured as a function of asphaltene aging (associated with the adsorption process of asphaltene molecules onto interfaces) and asphaltene concentration. Furthermore, we find that the coalescence time for water droplets exhibits a maximum due to a spontaneous emulsification at the oil/water interface that produces droplets consisting of asphaltene-laden water droplets. Spontaneous emulsification, Asphaltene adsorption, Desalting process, Coalescence inhibition
Introduction The desalting process is among the first unit operations encountered in a refinery and serves the purpose of removing corrosive salt from the incoming crude oil. This is accomplished through a liquid-liquid extraction process where water is mixed with crude oil. Then, this mixture is brought to the desalting vessel where the oil and water streams are separated. However, complex components of crude oil such as asphaltenes may delay the coalescence process required for efficient separation, causing difficulties in refining operations. A successful separation is crucial for the quality of the wastewater discharge and the desalted oil. Asphaltenes are complex, high molecular weight polar components of crude oils 1 and are defined by their solubility in different solvents; they represent the crude oil fraction that is insoluble in alkanes (usually heptane or pentane) and soluble in toluene 2,3 . In addition, they have a propensity to adsorb onto oil/water interfaces to form viscoelastic layers 4–6 because they have both non-polar hydrocarbon structure with polar functional groups, a combination that makes them surface active. This can result in a strong stabilization of the formed emulsion causing difficulties for the separation process. Hence, a fundamental understanding of the role of asphaltenes on the coalescence of water and oil emulsion droplets, which is currently lacking, will provide the ingredients necessary for optimizing the desalting process. A common approach to understand the behavior of asphaltenes at oil/water interfaces is to correlate emulsion stability with interfacial rheology properties. Several techniques are used such as pendant drop dilatation and contraction 4,5,7 , shear rheology 6,8–11 , and surface pressure isotherms 12 . Scattering techniques such as small angle neutron scattering (SANS) 13,14 , small angle X-ray scattering (SAXS) 15 and dynamic light scattering (DLS) 16 are often used to
characterize size, structure and aggregation of asphaltenes. A more recent approach to study asphaltene aggregation in the bulk is to use confocal microscopy 17–19 . Confocal microscopy has also been used to study the phenomenon of spontaneous emulsification but those studies have been restricted to simpler lipophilic surfactants 20 . To the best of our knowledge, this technique has not been used to investigate the possibility of spontaneous emulsification in the presence of asphaltenes. This paper examines not only the coalescence process of both water and oil droplets against oil/water interfaces, but also the evolution of this interface in the presence of asphaltenes. The present work is distinguished from previous studies in two ways. First, a newly developed instrument allows us to fundamentally study the coalescence process of a single droplet in a controlled manner. Second, the use of confocal microscopy allows to investigate how hydrocarbon/water interfaces evolve over time due to the presence of asphaltenes. The results show that both asphaltene concentration and aging time strongly affect the coalescence dynamics for water droplets. On the contrary, the coalescence dynamics of oil droplets are much faster and are largely insensitive to asphaltene concentration and aging time. In addition, the contact of water against asphaltene-laden oil solutions induces spontaneous emulsification where micron-sized water droplets, stabilized by asphaltenes, appear.
Experimental
Materials Asphaltenes represent the toluene-soluble fraction of crude oils. The materials used in this study are extracted from a heavy crude oil with a density of 988 kg.m−3 at 200 C, which corresponds to 11.60 API gravity. The ASTM method D-3279-97, 2001 (“Asphaltene preparation method” or standard ASTM D4312 or D4072 test methods) is used for the asphaltene extraction, as reported in Merola et al. 21 . The asphaltene content is approximately 22% wt. ACS Paragon Plus Environment 1
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of the raw crude oil. The aqueous phase used is deionized water. As for the oil phase, we dissolve asphaltene in toluene at a desired concentration. The toluene used is from Fisher Chemicals (purity = 99.9%). The glass vial containing the oil solution is closed with a lid and sealed with parafilm. The solution is sonicated for 30 minutes prior to each experiment. To perform confocal experiments, two fluorescent dyes are used, fluorescein (a water soluble fluorophore) and nile red (an oil soluble fluorophore). The fluorescein is obtained from Sigma-Aldrich (Fluorescein sodium salt), and we create a stock solution of 0.3 mg/g fluorescein in deionized water, which is kept at room temperature. The nile red is dissolved in a 1 mg/ml asphaltene in toluene solution at a concentration of 0.14 mg/g and is kept at room temperature. The nile red is obtained from ThermoFisher Scientific. SANS experiments Small Angle Neutron Scattering (SANS) is a useful technique to determine submicron particle size. The SANS measurements reported have been conducted at the Paul Scherrer Institute (PSI) in Villigen (Switzerland). During the SANS experiments the source-to-sample distance is varied from 2 to 18 m. Asphaltene solutions at three different concentrations in deuterated toluene are prepared: 0.18, 1 and 3 mg/ml. Deuterated toluene is used to enhance contrast for measurements. It is purchased from c (anhydrous, ≥ 99.6% chemical purity). Sigma-Aldrich The samples appear homogeneous and no precipitation is evident. They are measured at 200 C in rectangular quartz cells with a path length of 1 mm. The scattering intensity I(q), where q is the scattering wavevector, is obtained from the total detector counts corrected for the neutron transmission through the sample, the background radiation (d-toluene), and scattering from the empty cell and water.
Figure 1: I-DDiaSS setup assembled to study the behavior of water droplets. A schematic drawing for the oil droplet study is reported in SI. Volumes of water and oil solutions are drawn out of scale, the used ratio is 1:1.5 o/w.
against a flat interface. Alternatively, the instrument can be re-configured to visualize the behavior of oil droplets against a flat interface (see Figure S1 in the Supporting Information section, SI). The square glass sample chamber (1in x 1in x 1.5in), where the flat oil/water interface is created, is contained in an outer chamber made of delrin. The droplet is formed at the tip of a capillary that is connected to a syringe pump by a tubing (OD 1/8 in). For the water-droplet case, the capillary enters the glass chamber from the top. Alternatively, for the oil-droplet case, the bottom of the glass chamber is made of viton rubber pierced with a hole allowing the capillary to enter the chamber from below. A pressure transducer is connected to the tubing between the capillary and the syringe pump, and records the pressure inside the droplet. Besides the pressure information, Interfacial rheology experiments The viscoelastic properties of the asphaltene layer are there are also two cameras that record the experiments: measured at the water/oil interface. We use the stress- a side camera that monitors the droplet pressing against controlled Discovery Hybrid Rheometer (DHR-3) from the interface, and a lower (Figure 1)/ upper (see Figure TA Intruments (USA) equipped with a du No¨ uy ring and S1) camera that records interference patterns as the in22 the double-wall Couette flow-cell geometry . The latter terstitial liquid drains to thicknesses comparable to the is made of a glass external cylinder and a Teflon internal wavelength of light. Finally, the outer chamber holder is placed on a motorized stage, and by moving the stage it cylinder. The rheological study is conducted using the following is possible to bring the flat interface against the droplet protocol: 1) amplitude sweep at ω = 1 rad/s, explor- interface at velocities ranging from 0.01 - 0.15 mm/s. For both water-droplet and oil-droplet coalescence exing the strain range from 0.01 to 10%; 2) time sweep at periments, the glass chamber is filled with 1.5 ml of deionγ = 0.01% and ω = 1 rad/s until the moduli reach their ized water and 1 ml of asphaltene/toluene solution on plateau values (approximately 2 hours). top. To minimize evaporation and dust contamination, I-DDiaSS experiments the glass chamber is covered. Then, droplets of 0.9 µl are To investigate the stability of oil/water emulsions, the produced (water droplets descending from a syringe tip Interfacial Drainage Dilatational and Stability Stage (I- from above and oil droplets ascending from a syringe tip DDiaSS) developed in the Fuller laboratory is used. A from below). schematic of the device is shown in Figure 1. The IExperiments proceed using the motorized stage to transDDiaSS is used to study the drainage and stability of late the flat interface towards the droplet to a position thin aqueous films laden with complex structures that so that the distance between the two interfaces is one are entraped between two liquid-liquid interfaces. The droplet-radius (R). At this point, the droplet and the first is a flat liquid-liquid interface and the second is the interface are both aged for a specified amount of time, interface of a droplet. There are two possible configu- referred to as the aging time. During the aging time, rations for the I-DDiaSS setup. The first configuration, asphaltene molecules adsorb onto both interfaces. FolFigure 1, is used to study the behavior of water droplets lowing the work of Alvaraz et al. 23 , the minimum aging ACS Paragon Plus Environment 2
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time is chosen as 17 minutes. After aging, the flat interface is brought against the droplet, which results in a thin film of liquid being entrapped. This thin film eventually drains and ultimately ruptures, which is identified by a sharp drop of the pressure recorded at the pressure transducer (see Figure S2). The time from the initiation of drainage to film rupture is recorded as the coalescence time. The lower camera (or upper camera in the case of an oil droplet) acquires images of the drainage dynamics. All experiments presented here are performed at room temperature. Confocal experiments: Spontaneous emulsification analysis over time To study the evolution of the spontaneous emulsification over time a Nikon spinning disk confocal microscope is used. This microscope scans the interface between water and asphaltene/toluene solutions that are contained within a cylindrical glass cell (20x20 mm - Diameter x Height), as shown in Figure S3. To prepare the experiment, approximately 100 µl of water is added inside the glass cell. Then, a hollow glass piece (inner diameter 3.8 mm) is gently inserted inside the glass cell until it reaches a step at the bottom. Finally, 30 µL of toluene/asphaltene solution is added in the hollow region of the glass insert and on top of the water solution. The confocal microscope is operated in the transmitted light mode, with a 20x air objective. Images with a spatial resolution of 2048x2048 pixels of the sample are taken at intervals of a few minutes. The images are analyzed using the software ImageJ. A second set of experiments is conducted using an inverted Zeiss LSM 780 multiphoton laser scanning confocal microscope. In these experiments a fluorescein/water solution at a concentration of 0.3 mg/g is used onto which a nile red/asphaltene/toluene solution is added. The concentration of nile red in the oil solutioon is 0.14 mg/g. The volume of fluorescein/water solution is chosen so that the interface is located at the working distance of the objective. The interface is allowed to age briefly for 5 minutes so that spontaneous emulsified droplets appear, allowing one to focus at the interface. In this experiment a 40x water objective is used. Images with a spatial resolution of 1024x1024 pixels are taken at intervals of a few minutes.
well solubilize asphaltenes 27 . In the Porod region, i.e. at high q’s, the power-law exponent of the scattered intensity is −2, which suggests a flexible disk-like configuration of the asphaltene clusters. The results do not show any further aggregation over time (days). We conclude that toluene promotes the dissolution of small asphaltene clusters which remain stable for long time. Finally, it is worth mentioning that the obtained size corresponds to an asphaltene cluster, according to the Yen-Mullins model 28 .
Figure 2: SANS results for three asphaltene concentrations in dtoluene: 0.18 mg/ml (red circles), 1 mg/ml (blue squares) and 3 mg/ml (green triangle). In the Guinier region (low q), a schematic illustration of the asphaltene cluster is shown (off-scale), according to the Yen-Mullin model 28 .
Interfacial rheology: Stiffness of asphaltene - decorated interfaces The interfacial rheology of asphaltene solutions at two different concentrations is studied: 1 mg/ml and 3 mg/ml. In the linear regime (strain amplitudes γ0 < 0.2% and γ0 < 0.8% for concentrations of 1 mg/ml and 3 mg/ml, respectively), where the interfacial viscoelastic moduli are independent of strain, the asphaltene layer is solid-like for both concentrations (as shown in Figure 3). For higher strain values, a transition from the solid-like to the liquidlike behavior of the interface occurs through a yielding phenomenon, where the elastic modulus G0s equals the viscous modulus G00s . Moreover, this transition is characterized by an overshoot in G00s , which means that the interfacial layer yields plastically (a similar behavior is seen in Results other systems such as in globular protein-surfactant mixSANS: Structure of asphaltene clusters tures 29 ). The yield point is reached at higher strain as The scattering intensity I(q) is obtained as a function of the concentration of asphaltene increases (γ0 ≈ 0.9% and the wavevector q for three different concentrations of as- γ0 ≈ 2% for concentrations of 1 mg/ml and 3 mg/ml, rephaltenes. Figure 2 shows that the three curves collapse spectively). Finally, in the nonlinear regime the slopes of on a master-curve when the scattering intensity is scaled G0 and G00 with strain amplitude have a ratio of about 2, s s with the asphaltene volume fraction φ. At low q values, which is consistent with the value reported in the literawhere the Guinier region can be extrapolated, the parti- ture for materials that yield 30,31 . cle or aggregate size is determined. In this region, using In the linear regime, higher asphaltene concentration the Guinier fit, we obtain the following apparent radii of exhibits slightly higher elastic moduli. However, when gyration: 5.9 nm ± 0.12 nm for the 0.18 mg/ml solution, approaching the nonlinear regime, the effect of asphaltene 6.5 nm ± 0.13 nm for the 1 mg/ml solution, and 5.8 nm concentration becomes more pronounced where higher con± 0.12 nm for the 3 mg/ml solution. Similar values of centration of asphaltenes leads to higher moduli and postasphaltene aggregates in crude oil 24,25 and model oil 25,26 pones the onset of yielding. Additionally, time sweep are found in the literature. Additionally, increasing con- measurements indicate that the interfacial rheological becentration does not reveal any increase in the sizes of the havior is established after approximately 40 minutes of agclusters, suggesting that they are stable. This stability is ing for an asphaltene concentration of 1 mg/ml (as shown related to the solvent chosen (toluene) and its ability to in Figure S4). On the other hand, for a higher asphaltene ACS Paragon Plus Environment 3
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concentration of 3 mg/ml the interfacial properties are established immediately. Harbottle et al. 11 studied the shear rheological response as a function of time for different concentrations of asphaltene (0.1 and 0.4 mg/ml) and solvents. Similar to our results, they found a much faster film aging as the concentration of asphaltene increased. I-DDiaSS: Coalescence dynamics The coalescence time τ of water droplets is measured for different concentrations and aging times of asphaltene in toluene, as shown in Figure 4. The error bars represent the standard error calculated for up to 8 separate experiments. For a concentration of 0.066 mg/ml the coalescence time is independent of aging time and the system is extremely unstable (τ is much less than a minute). 12 10 8
0.066mg/ml 1 mg/ml 1.5 mg/ml 3 mg/ml 3.5 mg/ml 5 mg/ml
function of aging time, and it reaches a maximum coalescence time τmax that varies with concentration. As the concentration of asphaltenes increases, τmax is reached at shorter aging times. This behavior is believed to be related to the unexpected appearance of micron-size droplets that spontaneously form at the asphaltene - toluene/water interface, as discussed below. This phenomenon, known as spontaneous emulsification 43 , is a function of asphaltene concentration and aging, where small droplets spontaneously appear at the interface as both concentration of asphaltene and aging time increase. The spontaneous emulsification process is set by thermodynamics where the film curvature minimizes its free energy 43 (this mechanism is discussed in some detail further). This process commences as soon as the interface is initially formed. However, in its early stages it does not affect the coalescence process. In fact, the coalescence time initially increases with aging due to a strong viscoelastic network at the interface. Over time, however, spontaneous emulsification becomes increasingly relevant and interferes with the coalescence process by creating bridging points between the descending water droplet and the oil/water interface, which yields the non-monotonic behavior in coalescence time. The number of spontaneous emulsified droplets also increases with concentration, hence the peak in coalescence time is reached at shorter aging times as the concentration of asphaltene increases. The short coalescence time and lack of an observed maximum at the highest concentration of 5 mg/ml suggest that this process of spontaneous emulsification may have saturated the interface. 70 1 mg/ml 1.5 mg/ml 3 mg/ml
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Figure 3: Effect of amplitude of oscillatory deformation on viscous and elastic moduli for two different concentrations of asphaltene: 1 mg/ml (black circles) and 3 mg/ml (red squares). Filled symbols represent the elastic modulus and open symbols represent the viscous modulus. In the linear regime, the asphaltene layer is solidlike (G0 > G00 ) for both concentrations. In the nonlinear regime, a transition to a liquid-like behavior of the interface occurs through a yield point, where G0 equals G00 . The yield point (indicated by arrows) is reached at higher strain as the concentration of asphaltene increases.
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Figure 5: Coalescence time of oil droplets as a function of aging time for different concentrations of asphaltene in toluene. For this configuration, the coalescence time is independent of aging time and asphaltene concentration.
Oil droplets, however, present a very different behavior, as shown in Figure 5 (error bars represent the standard error calculated for up to 6 separate experiments). For 0 0 20 40 60 80 100 120 140 this case, the results show that the coalescence time is independent of aging time and asphaltene concentration. Aging Time [min] Moreover, this configuration is very unstable giving maxFigure 4: Coalescence time of water droplets as a function of agimum coalescence times of 30 seconds compared to time ing time for different concentrations of asphaltene in toluene. For scale on the order of many minutes for water droplets. concentrations c ≥ 1 mg/ml, the coalescence time exhibits a maxiThis difference in behavior from the water droplets is mum. For c = 0.066 mg/ml the coalescence time is much less than believed to be related to the arrangement of asphaltene a minute. molecules at the oil/water interface and the resulting interFor higher concentrations of asphaltenes (1 mg/ml to face-interface interactions, as shown in Figure 6. In the 3.5 mg/ml) the coalescence time is a strong, non-monotonic ACS Paragon Plus Environment 4 2
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case of the water-droplet, the aliphatic portion of the asphaltene molecules extends into the oil layer leading to a strong hydrophobic repulsion of the tails 32 . Strong repulsive forces between asphaltene tails have been measured using a surface force apparatus and are reported in the literature 3 . In the case of oil-droplets, the aromatic portion of the asphaltenes is exposed to a draining water film producing an attractive interaction. This influence of interface asymmetry on coalescence time is also seen in particle-laden interfaces 33 where the particle contact angle takes the place of the hydrophilic-lipophilic balance of a surfactant.
Figure 6: Interaction between asphaltene molecules and the oil/water interface for each configuration. Volumes of water and oil solutions are drawn out of scale, the used ratio is 1:1.5 o/w.
Evidence of spontaneous emulsification Brightfield microscopy during coalescence experiments Figure 7 displays images taken from the lower camera of the I-DDiaSS setup as a water droplet approaches the interface at two different times (full video can be seen in SI). In this case the asphaltene concentration is 1 mg/ml and the aging time is 2 hours. Panel a) shows the initial interaction of the water droplet with the flat interface, where small droplets of approximately 2 µm in diameter are seen to have spontaneously appeared. Panel b) is a snapshot of the water droplet after it has been moved into closer proximity with the interface. The population of spontaneously emulsified droplets is seen to have been repelled from the lower apex of the water droplet. Also, appearing in panel b) are interference fringes arising from the thin draining oil film.
(center region in Figure 8c shows larger droplets). On the other hand, droplets far from the center region are not sufficiently close and avoid coalescence. Data such as those shown in Figure 8 can be analyzed to reveal the kinetics of droplet formation and average droplet size. To appreciate the kinetics of droplet formation, the number of droplets as a function of size and time is measured by prescribing a circle encapsulating a large population of droplets near the meniscus center. The results for asphaltene concentration of 1 mg/ml are shown in Figure 9. The error bars represent the standard deviation calculated for two separate experiments. They show that the number of droplets increases significantly over time. Additionally, the droplet radii range from 1 to 10 µm and the mode (the most frequently-occurring value) shifts to larger radii as time evolves. For instance, the mode is [1 − 2] µm at t = 6 minutes, and then it appears to grow to [3 − 4] µm after t = 20 minutes through a process of coalescence. Similar behavior is seen for an asphaltene concentration of 1.5 mg/ml, as shown in Figure S5. However, at this concentration the radii distribution function shifts more rapidly towards larger values. Finally, for a higher asphaltene concentration of 3 mg/ml the spontaneous emulsification phenomenon occurs much faster than the coalescence between spontaneously emulsified droplets (Figure S6). Figure 10 presents the total number of droplets as a function of time and asphaltene concentration. The results show a notable increase in the number of droplets over time. Moreover, as the concentration of asphaltene increases more spontaneously emulsified droplets appear. In fact, at t = 20 minutes, for both 1.5 mg/ml and 3 mg/ml, multilayers of spontaneously emulsified droplets start to form (which impede an accurate measurement of the number of droplets). Fluorescence confocal microscopy imaging The results from the fluorescence confocal experiments for an oil phase concentration of 1 mg/ml are shown in Figure 11. The left photograph presents the fluorescent image where fluorescein is excited, the center photograph presents the brightfield image and the right photograph presents the fluorescent image where nile red is excited; all images are at the same location and time. Each phase can be identified by either the presence or lack of signal from the respective fluorescent dyes. For instance, the aqueous phase can be identified by the fluorescein signal (colored green) and the lack of nile red signal (colored red). In contrast, the oil phase can be identified by the opposite characteristics: presence of nile red and absence of fluorescein signals. From both fluorescent images, an oil droplet can be seen in the lower left corner. In the fluorescein-signal photograph it appears as a black circle due to the absence of fluorescein. On the other hand, in the nile red-signal photograph it appears as a bright red circle. The same oil droplet is also readily observed in the brightfield photograph in the center. Conversely, spontaneously created water droplets are readily identified in the nile red-signal photograph as black circles due to the absence of fluorescence signal. These water droplets are also readily identify within the brightfield photograph. From these images, it is evident that spontaneous emulsification produces both oil and water droplets but the water droplets
Confocal microscopy imaging To investigate how the spontaneous appearance of droplets develops over time, we perform confocal microscopy experiments. The oil/water interface in the sample container described in the Methods section is slightly curved due to the presence of a meniscus. During our experiments, we focus at the center of the meniscus for up to 2 hours. Figure 8 shows a time sequence as droplets are spontaneously created for an asphaltene concentration of 1.5 mg/ml in toluene. Two phenomena are apparent from this figure. First, the number of droplets increases over time. Second, the droplets accumulate towards the center of the meniscus. This latter observation suggests that these droplets are a denser phase. At the center of the curved interface, where the droplets are more concentrated due to gravitational settling, some degree of coalescence occurs between spontaneously created droplets ACS Paragon Plus Environment 5
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Figure 7: Evidence of spontaneous emulsification during coalescence experiments for a 1 mg/ml asphaltene/toluene solution after 2 hours aging are captured by the lower camera of the I-DDiaSS setup. a) descending water droplet approaching the flat interface: a layer of micron-size droplets are observed. b) droplet in contact with the interface: interference patterns are observed and spontaneously emulsified droplets are displaced due to repulsion interactions.
a ) t = 6 min
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Figure 8: Images of the spontaneous emulsification at the oil/water interface for a 1.5 mg/ml solution. Images are captured using a confocal microscope and they show an accumulation of droplets towards the center of the meniscus as a function of time, which indicates that the spontaneously formed droplets are heavier than the oil phase.
are present in far greater number. It was determined that droplets). This result agrees with the finding that the approximately 99.7% ± 0.15% of the droplets consisted coalescence time for water droplets exhibits a maximum of water droplets. An explanation for this imbalance in at shorter aging times as the concentration of asphaltene number density between the two types of droplets could increases. be related to the lower stability of oil droplets against coalescence as discussed in the section I-DDiaSS: Coales- Discussion cence dynamics. We find that the coalescence time of water droplets against The reason why water droplets are not observed in the an oil/water interface is a strong function of asphaltene fluorescein signal photograph is that the excitation wave- concentration and aging time. This configuration delays length needed to induce fluorescence in the water droplets coalescence and a water droplet may take up to ten minis strongly atenuated through adsorption by the oil phase utes to coalesce. At concentrations at and above 1 mg/ml, (as shown in Figure S7). the coalescence time exhibits a maximum with respect to Combining this finding with the fact that these droplets aging time, which occurs at shorter aging times as the accumulate at the bottom of the meniscus, we conclude concentration of asphaltene increases. that the spontaneous emulsification phenomenon in the Oil droplets, on the other hand, are observed to coalesce presence of asphaltenes consists mainly of water droplets very quickly and in a manner that is insensitive to conin the oil phase. Moreover, the appearance of small spon- centration and aging time. We believe that this difference taneously emulsified droplets at the oil/water interface in behavior between the two types of droplets is related to encourages a faster coalescence process between the de- the arrangement of asphaltene molecules at the oil/water scending water droplet and the oil/water interface by cre- interface 32 and the resulting interface-interface interacating points of bridging between the two interfaces. Fi- tions (Figure 6). In the water-droplet case hydrophobic nally, the spontaneous emulsification in the presence of repulsion occurs due to the interaction between aliphatic asphaltenes is a dynamic phenomenon and is a strong tails 3 . In the oil-droplet case the aromatic portion of the function of asphaltene concentration (higher concentra- asphaltenes interacts producing attractive forces. tion of asphaltene yields more spontaneously emulsified During the water droplet coalescence experiments, miACS Paragon Plus Environment 6
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Range of radii [µm] Figure 9: Number of droplets as a function of time and droplet radii for an asphaltene concentration of 1 mg/ml. The results show that the number of droplets increase significantly over time. In addition, for this concentration, the mode increases to larger radii as time evolves.
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droplets is low and the interface supports a strong, viscoelastic network of the asphaltene nanoclusters. This interfacial viscoelasticity is sufficient to inhibit droplet coalescence. Over time, as the concentration of spontaneously emulsified water droplets increases, the coalescence process is affected. These emulsified water droplets are entrapped between the descending water droplet and the oil/water interface and create bridging points, pro1 mg/ml moting droplet coalescence. The spontaneous emulsifica1.5 mg/ml tion process is accelerated as the concentration of asphal3 mg/ml tene increases, and therefore the maximum in coalescence is reached at shorter aging times. 0 5 10 15 20 25 Spontaneous emulsification has been previously observed Time [min] to occur between immiscible liquid phases and is promoted by the presence of a third phase 40 , which could Figure 10: Total number of spontaneously emulsified droplets at be surfactants or, in some cases, alcohols have this efthe oil/water interface as a function of time and asphaltene concentration. The number of droplets increase with both time and fect (common examples being the alcoholic drinks ouzo asphaltene concentration. and Pastis 41 ). Even though this phenomenon is usually reported to occur at ultra-low interfacial tensions, cron-sized spheres with the appearance of droplets are it has also been observed at appreciable interfacial tenobserved at the asphaltene-toluene/water interface. It sions 42 , similar to the present case. We believe that this is known that the presence of water in organic solvents occurrence is related to a general phenomenon in statiscan affect asphaltene characteristics and drive agglom- tical thermodynamics, known as spontaneous curvature. eration 35–38 and deposition 39 . It is believed that water Spontaneous curvature occurs at flexible interfaces such acts as a bridge between asphaltene molecules encourag- as the interface between water and oil in the presence of ing them to agglomerate, as explained below. Using con- amphiphiles. Surfactant molecules have a tendency to adfocal microscopy to scan across asphaltene-toluene/water sorb at the interface and reduce its tension substntially, interfaces, it was determined that a process of sponta- however their packing density depends on their intrinneous emulsification producing micron-size droplets was sic shape and size, in addition to their amphiphilicity. occuring. The fluorescence imaging reported in Figure Hence, interfaces can spontaneously curve to accommo11 demonstrates that these droplets are primarily water date more surfactant molecules, while keeping the free droplets situated on the oil-side of the interface. energy at its minimal value. A surfactant-laden interAsphaltenes are observed to inhibit the coalescence of face may spontaneously curve towards either the aqueous water droplets but this delay in coalescence passes through phase or the oil phase, and this reflects the competition a maximum as aging times are increased. We propose between the packing area of the polar aromatic head and the following mechanism, which is connected to the oc- the aliphatic tails 43,44 . In the present case, asphaltene currence of spontaneous emulsification. Initially, when nanoclusters consist of a flat polar region and hydrophospontaneous emulsification is in its early stages, the areal bic tails (Figure 6). Additionally, according to the analyconcentration of the micron-sized asphaltene-laden water sis of de Gennes and Taupin 45 , we may use the concept of ACS Paragon Plus Environment 7 Number of droplets
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Figure 11: Images obtained with a fluorescence confocal microscope of the spontaneous emulsification at the interface between water and 1 mg/ml of asphaltene in toluene. The aqueous phase is tagged with fluorescein and the oil phase with nile red. The left photograph presents the fluorescent image where fluorescein is excited, the center photograph presents the brightfield image and right photograph presents the fluorescent image where the nile red is excited, at the same location and time. From both fluorescent images, an oil droplet can be seen in the lower left corner (black circle in the fluorescein-signal photograph and red circle in the nile red-signal photograph). From the nile red-signal photograph, the presence of several water droplets in the oil phase is evident. The results indicate that 99.7% ± 0.15% of the spontaneously emulsified droplets are water droplets.
persistence length to describe the flexibility of interfaces. For length scales below the persistence length, the interface is essentially flat, whereas for values above the persistence length, the interface can bend. The persistence length, ξK , can be using the following equa estimated , which can be approximated by tion: ξK = a exp 2πK kB T ξK ≈ 10a, where a is the surfactant size, K is the rigidity constant, T is the temperature, and kB is the Boltzmann constant 45 . From our SANS measurements and following de Gennes, we estimate a persistence length of approximately 60 nm for our system, which is sufficiently small to allow the interface to wrinkle, and smaller than the radii of the smallest drops we observe to be spontaneously created. The fact the we observe the spontaneous creation of both oil and water droplets agrees with the expectation that the interface should curve to both sides. However, as results show, the asphaltene-laden water droplets are more stable than the asphaltene-laden oil droplets, hence the former persist without coalescing. Since asphaltene samples will be influenced by the extraction method that was used to separate them from the bulk crude oil (the relative solubilities of the solvents used, for example), the phenomenon of spontaneous emulsification can be expected to vary with the particular process that was used. In addition, asphaltenes extracted from different crude oil samples can also be expected to behave differently. Finally, the purity of the oil (which may contain small surfactants or charges) may play a role as well. Here we use analytic grade toluene. To the best of our knowledge, this is the first report of spontaneous emulsification through the presence of asphaltenes. We find that this time-dependent phenomenon is a strong function of asphaltene concentration, where both the rate of spontaneous emulsification and the resulting droplet size varies with concentration. Since droplets are formed more quickly as the concentration of asphaltene increases, the effect of the spontaneous emulsification on the coalescence process of water droplets occurs at shorter aging times, as data show. From interfacial rheology, we find a soft solid-like behavior of asphaltene-laden oil/water interfaces. Similar results can be found in the literature 4,5 .
Conclusions
We have shown that the presence of asphaltene molecules at a toluene/water interface causes a spontaneous emulsification, producing mainly water droplets at the oil side of the interface. This spontaneous emulsification triggers a faster coalescence between water droplets and an initially flat asphaltene-toluene/water interface for long aging times. The spontaneously emulsified droplets act as demulsifiers since they create bridges between the descending water droplet and the asphaltene-toluene/water interface. For short aging times, prior to the emergence of many spontaneous emulsified droplets, asphaltene molecules create a strong viscoelastic network that inhibits coalescence, and consequently coalescence times increase with increasing aging time. Hence the phenomenon can be tailored by a delicate balance of asphaltene concentration and aging time. In the case of oil droplets, coalescence times are measured to be very short and independent of aging time and asphaltene concentration. The fundamental understanding regarding the stability of water-in-oil and oil-in-water emulsions is extremely important for the quality of both wastewater discharge and refined oil in a refinery. In addition, our findings of spontaneous emulsification and its effects on coalescence can be expected to be of great importance in many unit operations such as the desalting process. During this process, the efficiency of separation and the effluent of the desalter could come with undesirable spontaneous emulsified droplets, where there could either be water droplets in the oil stream or oil droplets in the water stream.
Acknowledgements
This research was supported by NSF Award 1435683, by the Welltrailing Company, and by the Somatai Initial Training Network (ITN SOMATAI GA316866), financed by the EU’s 7th Framework Programme. We thank Mario Minale for providing asphaltene samples, Cedric Espenel for his help and useful discussions during confocal microscopy experiments. Mehmet Solyali and Karlheinz Merkle at the Varian Physics Machine Shop at Stanford University are thanked for their assistance on building the I-DDiaSS experimental setup. Jay Schwalbe is thanked for his help and useful discussions during UV-Visible experiments. Finally, we would like to thank Isabele MorACS Paragon Plus Environment 8
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eira for her help on I-DDiaSS coalescence experiments.
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Figure 12: Table of contents.
