Influence of Aqueous-Phase Ionic Strength and Composition on the

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Influence of Aqueous-Phase Ionic Strength and Composition on the Dynamics of Water−Crude Oil Interfacial Film Formation Mehrnoosh Moradi†,‡ and Vladimir Alvarado*,§ †

Department of Petroleum Engineering and §Department of Chemical Engineering, University of Wyoming, Laramie, Wyoming 82071, United States ABSTRACT: In this paper, we examine the impact of aqueous-phase ionic strength and ionic composition on viscoelastic properties of the water−crude oil interfacial film by conducting interfacial shear rheological measurements under controlled water chemistry and aging time. A double-wall ring geometry is used in oscillatory mode to measure the film viscoelastic moduli. To further elucidate mechanisms controlling the kinetics of film formation, temperature-dependent interfacial rheological behavior is investigated as a function of time. Our results reveal that low-ionic-strength conditions are more conducive to film formation and, consequently, engender a more viscoelastic interface. This observation is consistent with our previous results on emulsion stability; i.e., increasing the ionic strength of the aqueous phase leads to the formation of less stable emulsions. On the other hand, the temperature dependence of the viscoelasticity buildup demonstrates that adsorption of polar materials onto the interface can be modeled as a diffusion-controlled process, affected by the temperature in two different ways: first, through viscosity temperature dependence and, second, directly through the diffusivity dependence. Our results provide insights into emulsion stability mechanisms as well as multiphase fluid interfacial processes.

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interface as a result of the adsorption of NAs promoting film elasticity.28−30 In contrast, it has also been shown that adsorption of NAs on the interface softens the interfacial film.25,31 Despite the uncertainty on the role of NAs on the film rheological properties, positive effects of asphaltenes have always been remarked in the literature.32−34 As aforementioned, evaluating properties of the interfacial film has been a subject of significant research efforts for many years. In general, interfacial studies include two types of measurements: static analysis, which focuses on the determination of interfacial tension (IFT) at equilibrium, and dynamic analysis, which measures interfacial mechanical moduli under transient or nonequilibrium conditions. The latter provides information on dynamic IFT changes as well as viscoelastic properties of the interface and can be determined through dilatational or shear rheology. Many studies have been conducted to better understand the effect of asphaltenes on viscoelastic properties of the crude water−crude oil interface and the effect of parameters such as the temperature, aging time, pH, and water composition on film formation.35−38 In addition, several researchers have dedicated efforts to elucidate the mechanisms through which the interfacial film forms.5,39 In a seminal study, Dodds studied rheological properties of the film at crude water− crude oil interfaces using a lab-designed interfacial viscometer.13 He studied the effect of the aging time, temperature, and water pH and chemistry on the rheological behavior of the film. Results of the subsequent investigations on film formation mechanisms suggest that two different phenomena are involved in asphaltene adsorption and film formation.40,41 The first phenomenon is the bulk-to-surface diffusion of asphalthenes, and the second

INTRODUCTION The interfacial rheological properties of crude water−crude oil interfaces have been of interest for many years.1−3 This area of research has been stimulated by the need to understand the mechanisms controlling the stability of the emulsions formed during crude oil production.2,4−7 The formation of stable waterin-crude-oil emulsions poses major challenges for production, transportation, and processing of crude oil.8,9 The elevated dynamic stability of these emulsions has been attributed to the formation of a mechanically rigid water−crude oil interface.10−12 The presence of the viscoelastic interfacial film, isolation and characterization of the film material, and properties of such films have been the subject of many studies for an extended period of time.10,13−17 Efforts to characterize the film material contemplate the introduction of indigenous components in crude oil, e.g., asphaltenes and naphthenic acids, as constituents of the interfacial film.18 Asphaltenes are a class of components in crude oil that are soluble in aromatics, e.g., toluene, but are insoluble in low-molecular-weight alkanes, such as n-pentane or n-heptane. Several studies show that slow and irreversible adsorption of asphaltene on the water−crude oil interface leads to the formation of viscoelastic interfacial films.19−22 Naphthenic acids (NAs), another fraction of crude oil, comprise all alkylsubstituted cyclic and cycloaliphatic carboxylic acids present in the crude oil with a general formula of R−COOH.23,24 NAs are somewhat hydrophilic; therefore, they have a tendency to accumulate at the water−crude oil interface and also partition and dissociate in the aqueous phase. The dissociated NAs can react with the cations present in the aqueous phase to form naphthenate salts, which are also amphiphilic materials that can accumulate at the water−crude oil interface.25,26 There is ambiguity in the literature with regard to how NAs affect the mechanical behavior of the interfacial film.27 Several studies indicate that formation of lamellar liquid crystalline phases at the © XXXX American Chemical Society

Received: July 27, 2016 Revised: October 17, 2016 Published: October 17, 2016 A

DOI: 10.1021/acs.energyfuels.6b01841 Energy Fuels XXXX, XXX, XXX−XXX

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probe and the bulk phase. As found out from the definition, using the geometry with the maximal perimeter for a given contact area increases Bo, and as a result, interfacial forces dominate these measurements. The other concern in using interfacial probes is the limited dynamic range with respect to deformation and frequency. Among the aforementioned probes, a magnetic rod provides the best Bo value but the greatest dynamic limitation, while biconical geometry is less sensitivity but can afford better dynamic range. Du Noüy ring has high sensitivity as a result of the reduced contact area exposed to the bulk phase but has a number of issues; e.g., contribution of the surface inside the ring is not taken into account, or the rounded cross section is not ideal for pinning the interface.54 Our motivation in this study originates from our previous results showing that a low ionic strength of the aqueous phase is conducive to more stable emulsions.56,57 Our hypothesis is that a low ionic strength of the aqueous phase expedites the attraction of polar materials indigenous to crude oil toward the interface. In addition, it has been shown in our earlier investigation that the solubility of organic acids in the aqueous phase, which, in turn, can affect the adsorption of asphaltenes at the water−crude oil interface, alters the salinity of the brine.58 A higher concentration of organic components was detected in the aqueous phase at a higher ionic strength. We believe that the lower stability of the water-in-crude oil emulsions at a higher ionic strength of the aqueous phase results from the higher concentration of naphthenic components in the brine. Because these components are interfacial-active materials, their adsorption at the water− crude oil interface serves as a barrier for adsorption of asphaltene onto the interface, which, in turn, reduces the rigidity of the interface and, consequently, lowers emulsion stability. In this research, rheological properties of the interfacial film between crude oil and water are studied using a double-wall ring (DWR) interfacial probe. DWR is a newly developed geometry that combines the advantages of the different types of interfacial probes. It provides the maximal perimeter/contact area ratio (high Bo) for a given viscosity ratio between the interface and the bulk phase. Also, this device offers a satisfying dynamic range as well as a sharp edge to pin the interface. The effect of the water chemistry, specifically ionic strength of the aqueous phase, on the rheological behavior of the interfacial film is investigated in this research. The adsorption kinetics of interfacially active materials is also demonstrated through results regarding the effect of the temperature on the kinetics of film formation.

phenomenon is interfacial diffusion or molecular rearrangement at the interface. We argue that asphaltenes can diffuse through the bulk phase and, subsequently, adsorb on the interface. As the adsorption of asphaltene progresses, the interface becomes increasingly occupied, and as a result, adsorbed species undergo interfacial rearrangement.42 Hence, at least two kinetics control film formation: bulk diffusion and interfacial rearrangement or interfacial diffusion. It has been shown in several studies that at least a couple of hours is required for the formation of a relatively elastic film.43 In most of the aforementioned studies, the socalled model oil, i.e., a mixture of asphaltenes precipitated from crude oil, heptane, and toluene, has been used. In model fluids, structures of asphaltenes might differ substantially from their native state in crude oil. When crude oil is used, in addition to asphaltenes, NAs and their salts have a tendency to occupy the interface as well. Therefore, competitive adsorption of both asphaltene and naphthenic components control the interface mechanical properties.42 To the best of our knowledge, a small number of investigations have focused on the effect of water chemistry on the rheological properties of crude water−crude oil interfaces. Several studies demonstrate the effect of water pH on dynamic IFT. Sheu et al.44 and Poteau et al.45 show that the dynamic IFT reduces at pH values both below and above neutral pH. At intermediate pH, the highest value of IFT is observed. This behavior is attributed to the presence of acidic−basic moieties in asphaltene structures. In other studies, Strassner46 reveals that acidic pH favors the formation of a rigid asphalthenic film. Varadaraj et al.18,47 propose that synergistic interactions between asphaltenes and NAs lead to the formation of acid−base complexes that accumulate at the interface and reduce IFT. In addition, it has been shown that adding salt to the aqueous phase retards the formation of an elastic film.21 This is attributed to the shielding effect of ions with respect to the electrostatic attraction between the interfacial layer and asphaltenes in the bulk crude oil phase. Several apparatuses and measuring probes have been used to evaluate interfacial rheological properties. In general, rheological measurements can be performed applying either dilatational or shear deformation.40,43,48−50 The major difference between these two techniques is that, in dilatational deformation, the interfacial area changes in contrast to shear deformation; this means that, in shear rheology, the concentration of the interfacial material is not affected by deformation. A pendant drop apparatus is the system most frequently used to perform dilatational rheology. For interfacial shear rheology, different types of measuring probes have been used, such as a magnetic rod,51 knife-edge rheometer,52 bicone geometry43 and Du Noüy ring.53 The main challenge in designing the interfacial probe is the coupling between the velocity profile in the bulk phase and at the interface, which means that the total force applied on the probe is the combination of the force exerted from the bulk and the interface.54,55 To determine the significance of the surface force over the bulk force, a dimensionless number called the Boussinesq number (Bo) is defined as follows:

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V

ηs L P1 interface drag Bo = = V1 bulk drag η L As s

MATERIALS AND METHODS

Sample Preparations. A crude oil from Wyoming referred to as TC is used in this study. Basic crude oil properties are dynamic viscosity of 64 cP, density of 0.91 g/mL at 25 °C, and 5% hexane−asphaltene content. TC crude oil has been found to produce stable water-in-oil macroemulsions. We attribute this observation to high interfacial activity, including its dynamic response. Aqueous solutions are prepared by dissolving analytical-grade Na2SO4, CaCl2, and NaCl in deionized water. Salts are selected such that the impact of different cations (monovalent and divalent) and anions (sulfate and chloride) can be investigated. The ionic strength of the brines is selected to be the same as the synthetic reservoir water used in our previous research and represents reservoir water having an ionic strength value of 0.6724 M. In this study, the aqueous phase with ionic strength of 0.6724 M is labeled 100% salinity brine for simplicity. The 100% solution is then diluted by the addition of deionized water to obtain an aqueous solution with 0.1, 1, 5, 10, and 100% salinity or ionic strength. Interfacial Shear Rheology. An AR-G2 (TA Instruments) rheometer equipped with a DWR fixture is used to perform interfacial shear rheology experiments. The DWR setup includes a sample holder

(1)

where η and ηs are the bulk and interface shear viscosities in steady shear flow, respectively. L1 and Ls are the characteristic length scales, P1 is the contact perimeter between the interface and the measuring probe, and As in the contact area between the B


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Electrorheology. Emulsion stability as a function of the brine ionic strength is investigated using electrorheology. In this technique, the critical electric field (CEF) at which current percolation occurs through the sample is measured and is correlated to emulsion stability. Stainlesssteel 50 mm parallel plates are set up in an AR-G2 rheometer (TA Instrument). The plates are connected to an electrical amplifier, which, in turn, is connected to a signal generator. This allows the plates to be polarized while conducting rheological experiments as well as measuring electric current between plates and through the sample. Dynamic or steady-state experiments can be conducted at a given selected voltage drop. When water-in-oil emulsions are loaded in the geometry, as soon as the direct current (DC) electric field initiates, water droplets are polarized, leading to an increase in the emulsion viscosity. At a given value of the electric field (or voltage drop), current percolation occurs and a typical decrease in viscosity is observed. This value of the electric field is denominated CEF. The higher CEF corresponds to the more stable the emulsion. A 1 mm gap is kept between plates, and the linear viscoelastic region is determined, before CEF experiments. In our experiments, a typical strain of 1% at 1 rad/s is used in most experiments.

(trough) and a ring, depicted in Figure 1. The ring is connected to the rheometer to shear the interface and to measure torque. The sample

Figure 1. Schematic of the cross section of the DWR setup.

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RESULTS AND DISCUSSION Emulsion Stability. Figure 2 shows the CEF versus the salinity of the aqueous phase. Sodium sulfate was the salt used in

holder is placed onto the Peltier plate that controls the temperature through simultaneous water circulation and heating. A detailed description of advantages and disadvantages of this geometry can be found in the study by Vandebril et al.54 The main merits regarding this geometry are the high Bo and the square shape of the cross section of the ring that enables it to create a planar interface. Combining the sensitivity of the DWR and transducers assembled in the AR-G2 rheometer enables this system to perform measurements at low frequency and low torque values. In interfacial shear rheology in oscillatory mode, a sinusoidal shear deformation or strain (γ) is applied to a constant area interface. γ = γ0 sin(ωt )

(2)

If the strain amplitude (γ0) is small enough, the shear stress response of the interface is proportional to the strain amplitude, which varies sinusoidally as well. The stress response of the interface can be represented as (3)

Figure 2. CEF as a function of brine salinity for Na2SO4 aqueous solutions.

The term in phase with the strain and proportional to G′(ω) is called the storage modulus, while the term proportional to G″(ω) and out of phase with the strain is denominated the loss modulus. The loss modulus represents the viscous energy dissipation, while the storage modulus reflects the storage of elastic energy; i.e., G′(ω) and G″(ω) show how solid-like or liquid-like the interface is, respectively.59 The experimental procedure is as follows: The Pt/Ir ring is flamed prior to every experiment to remove any organic contamination. The trough is placed onto the Peltier plate, and the denser phase (water) is poured in the sample holder to a level defined by a ledge on the wall of the trough. This system is designed such that, if the level of the lower phase reaches the ledge and the ring is positioned at the geometry gap, which is defined in the design, the ring will be placed accurately enough at the interface. After positioning the ring, a 10 cm3 volume of crude oil is pipetted carefully onto the brine phase. Efforts are made at this step to prevent interfacial agitation. The experimental temperature is controlled by the Peltier plate underneath the sample holder. Initially, a strain sweep test is performed on the individual samples to determine the strain value at which the measurement is maintained in the linear viscoelastic region. Time sweep tests at constant frequency and strain values are performed afterward to measure the rheological behavior of the crude water−crude oil interface after a predetermined aging time. In this study, two sets of experiments are conducted. In the first set, the goal is to evaluate the effect of the water chemistry on interfacial properties of the film. To meet this goal, brines with different compositions and ionic strength values are used as the aqueous phase. Three different salts are used in these tests, as mentioned in the previous section. The second set of experiments aims at the investigation of the different kinetics involved in film formation at different temperature values. In these tests, one type of brine at a certain ionic strength is selected and time sweep tests are performed at different temperatures after a predetermined aging time.

this test. The first observation is that the CEF value increases monotonically as the salinity decreases. Once the salinity decreases to a value of 1% or lower, the CEF seems to grow more slowly. These results are consistent with the hypothesis that lower brine salinity is conducive to more stable emulsions. We relate this stability response to a stiffer water−crude oil interface. Dynamic Interfacial Rheology. The effect of brine chemistry and ionic strength is investigated in this study using Na2SO4, CaCl2, and NaCl. Salinity values of 0.1, 1, 10, 5 and 100% are examined for all of the salts. To evaluate the interfacial viscoelasticity of the film, time sweep tests are performed in the linear viscoelastic region. To determine this region, a strain sweep test is performed at a constant frequency of 1 rad/s in oscillatory mode after 24 h of aging the interface for a couple of the samples. Figure 3 shows an example of these tests. In all of the examined cases, at a strain value lower than 1.3%, both storage and loss moduli remain almost constant, which indicates a linear viscoelastic response. G′ reduces at the strain of 1.3%, but G″ begins to decrease at a larger strain value of roughly 4%. Hence, 1% strain and a frequency of 1 rad/s are selected for time sweep tests to remain in the linear viscoelastic region. It has been demonstrated by several authors that film formation is a time-dependent phenomenon, because different kinetics, such as diffusion, adsorption, and rearrangement of interfacial active components, control the rate of film develop-

σ(t ) = γ0[G′(ω)sin(ωt ) + G″(ω)cos(ωt )]

C


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Figure 3. Strain sweep test after 24 h of aging at the frequency of 1 rad/s for 1% Na2SO4.

between the two moduli becomes more significant after 10 h of aging, but this occurs after 1 day for salinities higher than 5%. This fact demonstrates that a higher ionic strength of the aqueous phase retards film formation as opposed to low-salinity brine. Considering G′ as an indicator of interfacial film growth, we focus on the elastic modulus of the interface in this study. Despite our focus on elastic modulus, we believe that the interface should exhibit viscous behavior as well; otherwise, the interface would be brittle. Figure 6 shows the phase shift, which is a ratio of viscous modulus/elastic modulus, as a function of time. Because the interfacial elasticity is maximal at 1% salinity and minimal in the presence of 100% salinity brine, these two salinity values are selected to analyze the phase shift. As depicted, the minimum phase shift is about 0.4. This fact demonstrates that, despite a high elastic modulus, the interface always exhibits a viscous energy dissipation mechanism that suppresses interfacial fragility. To better understand the effect of the ionic strength on the kinetics of film formation, we sought a function that best fit the experimental data. Results of curve fitting are presented in Figure 7. It is found that a saturation function with the following definition is the best fit:

ment. In addition, interfacial moduli can be representative of the physical state of the interfacial film; i.e., a higher elastic modulus corresponds to a more rigid film, and a higher viscous modulus is linked to a more liquid-like film. On the basis of the aforementioned facts, in these sets of experiments, rheological properties of the crude water−crude oil interface are measured as functions of time and ionic strength of the aqueous phase. Figure 4 shows results for the elastic modulus at 1% NaCl brine.

G′(t ) = a − be−t / c Figure 4. Interfacial elastic modulus at 1% strain and 1 rad/s frequency for 1% NaCl.

(4)

where G′(t) is the elastic modulus as a function of time, t is the aging time (h), and a, b, and c are the fitting parameters. Parameter a represents the saturation value of the elastic modulus (Gf), and c relates to the characteristic time (τ). A smaller value of τ corresponds to faster film formation. Figures 8 and 9 show the dependence of τ and Gf on the ionic strength of the aqueous phase and water composition for three different brine solutions studied in this work. It should be noted that a longer τ corresponds to higher ionic strength, except for a 5% salinity value. This observation shows that, in general, addition of salt lowers the rate of film formation. In addition, Figure 9 shows that the plateau value of the elastic modulus is maximal at 1% salinity for all of the brine solutions. Furthermore, it can be observed that, at lower salinities, the presence of sulfate ions delays film buildup (larger τ). However, sulfate induces a more elastic interface at the extended aging time (higher elastic modulus). According to the results of characteristic time and final elasticity, an interpretation for the kinetics observed in Figures 8 and 9 follows. It has been proposed previously that electrostatic

Repeatability of the data can be appreciated by noting how small the error bars are. Figure 5 shows the elastic and loss moduli of the interface in the presence of Na2SO4, CaCl2, and NaCl at different salinity values over 7 days of aging time. No error bars are shown to avoid clutter and because not all measurements were repeated. As depicted in the figure, G′ is always higher than G″, which is an indicator of the formation of a predominantly elastic film, except for 5% NaCl salinity, a case for which G″ can be larger than G′. The larger viscous modulus could be the result of contributions of sodium naphthenate, which is known to behave more fluid-like. It should be pointed out that the difference between G′ and G″ becomes more significant gradually over time and also in the presence of low-ionicstrength brine. The first measurement in these tests is performed after 5 h of aging time, and as the data show, G′ is slightly higher than G″; however, the difference is within experimental uncertainty. For salinity values equal or below 5%, the difference D


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Figure 5. Interfacial viscoelastic modulus at 1% strain and 1 rad/s frequency for (a) Na2SO4, (b) NaCl, and (c) CaCl2.

Figure 6. Tangent of the phase shift as a function of the aging time.

interactions and diminish the rate of film consolidation.21 Therefore, one expects shorter τ at a lower ionic strength. Results in Figure 8 follow this trend, except at the salinity value of 5%. To explain this different response, it is worth recalling that

interactions between the double layer in the aqueous phase and the polar materials, such as asphaltenes, affect interfacial consolidation kinetics.21,58 It has been shown that higher ionic strength of the aqueous phase can shield these electrostatic E


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Figure 7. Curve fitting through experimental data for (a) Na2SO4, (b) NaCl, and (c) CaCl2.

naphthenic component adsorption. Also, we showed that, as the ionic strength of the aqueous phase increases, more NAs partition and dissociate in water, leading to a higher concentration of naphthenic components in the water phase.

naphthenic components are also known to be surface-active materials and compete with asphaltenes to adsorb onto the interface. In our previous study,58 we speculated that the interfacial response is a combined effect of both asphaltene and F


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Figure 8. Characteristic time of film formation versus salinity of the aqueous phase.

Figure 9. Final elasticity of the interface versus aqueous-phase salinity.

We propose that, at 5% salinity, on one hand, electrostatic attractions are stronger than at 10 and 100% salinities, which results in faster film formation, i.e., shorter τ. On the other hand, the concentration of naphthenic components is higher than in cases at 0.1 and 1% salinities, which means more competition for asphaltenes to adsorb onto the interface. Naphthenic components would occupy the interface and reduce the available surface area for asphaltene adsorption, which results in lower elasticity of the interface. This fact is confirmed by Figure 9, in which Gf is higher at the brine salinities of 0.1 and 1% compared to higher salinities. Also, considering that, at lower ionic strength, the electrostatic attraction force is stronger, we propose that fast adsorption of asphaltene onto the interface can prevent proper molecular stacking and a stable conformation of the interfacial film that, in turn, can induce molecular rearrangement at the interface. This fact can explain a lower G′ value at 1% salinity compared to 5% salinity at a shorter aging time. In addition, it should be pointed out that simultaneous adsorption of naphthenic components and asphaltenes saturates the interface gradually to the point that it could induce rearrangement of adsorbed components or desorption of interfacial components back to the bulk phase as well. Recalling the higher concentration of naphthenic components at a higher ionic strength, we propose that, at 5% brine salinity compared to 0.1 and 1%, the concentration of both surface-active materials is high, because,

on one hand, asphaltene attraction is high compared to 10 and 100% and, on the other hand, the naphthenic component concentration is higher than in cases of 0.1 and 1% salinities. We speculate that, as a result of the different kinetics of asphaltene adsorption and acid dissociation/adsorption, the interfacial film grows faster initially in the presence of 5% salinity brine, because of the sufficient driving force to attract asphaltene onto the interface. However, the adsorption of naphthenic components is still significant but may occur at a different time scale that seems to be longer. This means that asphaltenes are probably the primary adsorbent and naphthenic components are the secondary adsorbent. Therefore, interfacial film growth slows gradually and reaches a stable value at the extended period of aging time. This behavior is not observed at 0.1 and 1% salinities, because the most significant kinetics is the asphaltene adsorption, and as a result, a gradual increase in interfacial elasticity is observed, which continues even longer than the aging time tracked in this study. Considering the proposed interfacial molecular reorganization as a result of conformational arrangement at the interface at a lower ionic strength, simultaneous adsorption of asphaltenes and napthenic components, and consequent molecular rearrangement as a result of saturation of the interface, we believe that an optimum salinity value exists at which the collective effect of these different kinetics leads to the maximum interfacial G


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Figure 10. Curve fitting through experimental data at three different temperatures after a predetermined period of aging time of (a) 48 h, (b) 30 h, and (c) 26 h.

To investigate the kinetics of film formation, interfacial rheology tests are performed at three different temperature values of 25, 35, and 50 °C. Na2SO4 at 1% salinity is selected to perform this test. Temperature values are selected on the basis of Figure 11, which shows the dependence of the crude oil viscosity

elasticity. This proposed optimum salinity may vary in response to crude oil properties. For the case of TC crude oil, results show that 1% salinity provides the optimum ionic strength. Effect of the Temperature. It has been demonstrated in several studies that interfacial film formation is a diffusioncontrolled phenomena at short aging time but becomes reactioncontrolled in the long term.35 To further test this hypothesis, the effect of the temperature on the rate of film formation is investigated in this study. In reality, we cannot separate different diffusion processes or interfacial restructuring; therefore, our analysis is somewhat crude. The analysis lumps all effects affected by the temperature. We also tested the response at only three temperature values; thus, the analysis is mostly qualitative in nature. To simplify the problem, we assume that diffusion of the polar material through the oil phase is similar to the diffusion of spherical particles and obeys the Stokes−Einstein equation as follows:

D=

kBT 6πμr

Figure 11. Viscosity of TC crude oil as a function of the temperature.

(5)

where D is the diffusion coefficient, kB is the Boltzmann constant, T is the absolute temperature, μ is the viscosity of the bulk phase, and r is the radius of the diffusing particle. If we assume that all of the polar components have the same size, the rate of film formation should be proportional to the diffusion coefficient, which, in turn, means proportional to the ratio between the absolute temperature and viscosity. As aforementioned, the rate of the film formation is inversely proportional to the characteristic time of the system (τ), which can be found by fitting the saturation curve through the experimental data, as shown in Figure 10. Therefore, if film formation is a diffusion-controlled phenomenon, we expect a linear relationship between 1/τ and T/μ.

upon the temperature. The purpose of temperature selection is to sweep the temperature/viscosity ratio uniformly. In the case of our selection in this study at 35 and 50 °C, T/μ values are 1.3 and 2 times the value at 25 °C, respectively. Therefore, we expect that, if diffusion is one of the controlling mechanisms, the rate of film formation should be 1.3 and 2 times faster at 35 and 50 °C than at 25 °C. Figure 12 shows the correlation between the inverse of the characteristic time and the temperature/viscosity ratio. As depicted in Figure 10, it is apparent that, at the aging time of 30 h and less, saturation curves fit the best to the experimental data. This result confirms the observation by Sheu et al.34 that the rate of film formation is diffusion-controlled initially but reaction-controlled at an extended period of aging H


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Figure 12. Linear correlation between the inverse of the characteristic time and T/μ (1% Na2SO4).

time. To better investigate this fact, 1/τ is plotted versus T/μ in Figure 12 after 26, 30, and 48 h of aging. As the results show, despite the limited number of points, 1/τ varies linearly with T/μ up to 30 h of aging, which indicates a diffusion-controlling phenomena. However, after 48 h, a significant departure from the linear relationship is observed, which means that diffusion is not the dominant film-formation-controlling mechanism. It is apparent that different processes are not separable through this analysis, but instead, hints at the existence of diffusion-like processes, at least at the initial stages of film formation.

phenomena that are occurring simultaneously, such as (1) adsorption of asphaltene onto the interface, which we believe to be expedited at a lower ionic strength, provoking improper conformation of the interface, (2) competitive adsorption of naphthenic components, which is controlled by the ionic strength of the brine but in an opposite direction (i.e., a higher ionic strength appears to expedite the partitioning but retards the asphaltene adsorption), and (3) displacement and rearrangement of the adsorbed component as a result of the saturation of the interface with surface-active materials. We propose that, at a certain value of salinity, both kinetics are significant but occurring at different rates. This fact induces faster film formation at a early stage of aging with a slowed rate eventually as a result of the adsorption of naphthenic components. We believe that this synergistic effect occurs between 1 and 10%, as results show at 5% salinity, for the brines and crude oil used in our study. The results presented here confirm the complex interfacial interactions, particularly between NA and asphaltenes. We propose that an optimum ionic strength exists at which the collective effects of the naphthenic components and asphaltenes on the interfacial film would be in the direction of forming a more rigid interface. To further understand molecular reorganization in the solution and at the interface, one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopies of the crude oil and aqueous phase will be the focus of our future research.

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CONCLUSION A double-wall ring geometry has been used to determine the interfacial rheology of the water−crude oil system. The effect of the aqueous-phase ionic strength and temperature has been examined. The interfacial elastic modulus is measured as an indicator of the mechanical strength of the film. It can be concluded that the adsorption and reorganization of the polar materials to form a rigid water−crude oil interface with detectable elasticity takes at least a couple of hours. The results reveal that lower ionic strength favors the formation of rigid interfacial films. The retardation of elasticity growth as a result of the addition of salt is attributed to the shielding of the electrostatic attraction between the double layer at the interface and the polar materials as well as a higher concentration of the naphthenic component in the aqueous phase. The latter has been discussed in our previous study, in which we demonstrated that NAs tend to partition and dissociate more in the water with high salinity to form naphthenate salts. These components are also surface-active materials, and their adsorption at the water−crude oil interface serves as a barrier for adsorption of asphaltene onto the interface, which, in turn, reduces the rigidity of the interface. A discrepancy is observed in the rate of film formation at the salinity values lower than 5%. We speculate that this different behavior results from different kinetics of several interfacial

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-307-766-6464. Fax: +1-307-766-6777. E-mail: [email protected]. Present Address

‡ Mehrnoosh Moradi: ALCO Champion, An Ecolab Company, 7705 HIGHWAY 90-A, Sugar Land, Texas 77478, United States.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge TIORCO, LLC and PUC-Rio for financial support. The authors thank Griselda Garcia-Olvera and Xiao Wang for help with the rheological experiments and providing the rheogram of the crude oil, respectively. Also, the authors are indebted to Dr. Koichi Takamura for his insightful comments.

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REFERENCES

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