Adsorption Kinetics of Asphaltenes at the OilâWater Interface and

Article pubs.acs.org/Langmuir

Adsorption Kinetics of Asphaltenes at the Oil−Water Interface and Nanoaggregation in the Bulk Jayant P. Rane,†,‡,∥ David Harbottle,†,‡ Vincent Pauchard,§,∥ Alexander Couzis,‡ and Sanjoy Banerjee*,†,‡,∥ †

Energy Institute and ‡Department of Chemical Engineering, City College of New York, New York, New York 10031, United States § Flow Technology Group, Department of Process Technology, SINTEF Materials and Chemistry, Trondheim, Norway ∥ FACE, The Multiphase Flow Assurance Innovation Center, Norway S Supporting Information *

ABSTRACT: Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop− drop coalescence. As a result, asphaltenes unfavorably impact water−oil separation processes and consequently oil production rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water−oil interfaces. This study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barriercontrolled regime rather than to a conformational relaxation regime. Turning first to the molecular scale, it is unclear whether the whole asphaltene fraction adsorbs at the water−oil interface. The fraction of heteroatoms in asphaltenes (usually with less than 1% oxygen and less than 10% sulfur) could barely lead to an average of one polar group per large hydrocarbon tail (25− 50 carbons).6 Even if all the heteroatoms were extremely polar (carboxylic or sulfonic acid), the corresponding HLB values would still give rise to low average interfacial activity. With these observations in mind, model asphaltenes with sufficient interfacial activity have been synthesized by grafting four carboxylic functions on the lateral chains of polyfunctional

1. INTRODUCTION Formation of fine water-in-oil (w/o) emulsions in flow restrictions (choke valves, electrical submersible pumps, etc.) hinders gravity separation because settling velocity depends strongly on droplet diameter. Adsorption of naturally occurring oil components such as asphaltenes, resins,1,2 solids-like waxes, and inorganic particles3−5 tends to increase the stability of these emulsions, further hindering the gravity separation process. Among these stabilizers, asphaltenes are of increasing importance because of the asphaltenic nature of much of the recently discovered reserves (oil sands, heavy oil). Despite many published results, considerable effort is still required in order to elucidate the mechanisms by which asphaltenes adsorb at the water−oil interface, modify the properties of the interface, and affect drop−drop coalescence. © 2012 American Chemical Society

Received: April 6, 2012 Revised: June 5, 2012 Published: June 8, 2012 9986

dx.doi.org/10.1021/la301423c | Langmuir 2012, 28, 9986−9995

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polyaromatic hydrocarbons (PAH).7 However, investigations using polarized spectroscopy have shown that these model asphaltenes adsorb lying perpendicular to the interface, whereas “real” asphaltenes lie flat at the interface.8 Furthermore, studies using the FT-ICR mass spectroscopy technique have failed so far to identify a specific chemical subfraction of asphaltenes that would exhibit specific surface activity and, in particular, any polyaromatic hydrocarbons (PAH). Mass spectra show that interfacial material extracted from asphaltenic emulsions contains a significant amount of sulfur/oxygen (S1O2, S1O3) substituted polyaromatic hydrocarbons,9−11 but most of those sulfur/oxygen functions have no acidic nature. Most of the sulfur/oxygen components seem to be more likely sulfoxides of low surface activity.12 It is therefore difficult to explain adsorption of asphaltenes as simple amphiphiles. On the other hand, the interfacial activity of asphaltenes seems to be correlated to their bulk solubility.13,14 This could be in agreement with a nanoparticle type of adsorption, which depends on the wetting angle (in turn largely dependent upon the same diffuse van der Waals forces as flocculation). SANS data actually show structural similarities between the interfacial layer and the nanoaggregates present in the bulk, which may suggest that asphaltenes adsorb as nanoaggregates and not individual molecules.15 FTIR spectroscopy on asphaltene films also shows similar alkyl adsorption bands as in nanoaggregates: in both cases, lateral alkyl chains of asphaltenes seem to be “crystallized”.16 Nanoaggregates adsorption is however contradicted by the CMC-like behavior of the concentration dependence of equilibrium interfacial tension. Below what is interpreted as the critical nanoaggregation concentration (CNAC), interfacial tension decreases with an increase in asphaltene concentration (mass fraction) while it levels off above. In other words, asphaltene nanoaggregates do not seem to contribute to the decrease in interfacial tension. Furthermore, analyzing the equilibrium interfacial tension values below the CNAC by means of the Gibbs plot gives cross-sectional areas of ca. 3 nm2, which is more in line with the adsorption of monomers.17 There appears to be no consensus about the nature and kinetics of the interfacial tension reduction in the presence of asphaltenes: explanation ranging from diffusion-dominated kinetics,18,19 kinetics of reorganization/relaxation of asphaltene molecules at the interface,20−22 diffusion kinetics followed by reorganization/relaxation,22,23 etc. The reorganization/relaxation process is often assumed to lead to formation of a cohesive gel exhibiting high elastic modulus and opposing coalescence.10,24 ‘Solid-like’ interfaces resisting droplet contraction are observed as the 2D structure approaches the gelling point.10,24 Langmuir trough/Brewster angle microscope observations also show heterogeneous domains upon compression/decompression of asphaltenes films that are said to be cohesive domains. Nevertheless, these observations were often made at the water/air interface, and conclusions differ from one case to the other (complete monolayer vs multilayered patchy assemblies).25 The present study contributes to a larger project on droplet coalescence and water separation in asphaltenic emulsions. The objective for this work, for a given fluid system used in other studies, was to answer “simple” questions, such as what are the forms of the asphaltenes that do adsorb, what is controlling the kinetics of adsorption, and what is the form of the adsorption isotherm? To address these questions, the methodology adopted was to analyze dynamic interfacial tension data obtained for various asphaltenes mass fractions and various

solvent viscosities and compare the insights gained with those related to similar issues obtained using other techniques, such as NMR and DLS, that probed the size of asphaltenes and their physical state.

2. MATERIALS AND METHODS The water used throughout the study is deionized Milli-Q grade with a conductivity of approximately 0.05 μS/cm. This water is premixed with 43 g/L of NaCl and 7 g/L of CaCl2, and the final pH was adjusted to 7 with 0.1 M NaOH. The asphaltenes were extracted by precipitation with 20 volumes of n-heptane to 1 volume of crude oil from the Norwegian continental shelf, stirring overnight at room temperature, followed by filtration and rinsing with n-heptane. For the pendant droplet and DLS experiments the asphaltenes were dispersed in a model oil phase containing 15% by weight toluene (reagent grade, Aldrich Chemical Co. Inc.) and 85% aliphatic base oil. The base oil of Nexbase 2000 series from Neste Oil, Finland is chosen for two reasons. The first is ability to stabilize emulsions with limited asphaltenes mass fraction in order to keep the fluids transparent at lower mass fractions, which is important for future experiments; second, the oil phase viscosity can be adjusted to higher values without changing the chemical nature of the oil since they are additive free and consist of oligomers of decane with different mass weights. The viscosities of different Nexbase oils with 15% toluene, measured using a 2° cone and plate configuration on a TA Instruments AR 2000 ex rotational rheometer at room temperature (298 K), are listed in Table

Table 1. Viscosities of Different Nexbase 2000 Series Oils with 15% Toluene oil 85% 85% 85% 85%

Nexbase Nexbase Nexbase Nexbase

2002 2004 2006 2008

viscosity (Pa·s) + + + +

15% 15% 15% 15%

toluene toluene toluene toluene

0.0065 0.0163 0.0201 0.0280

1. For NMR experiments asphaltenes were dispersed in a mixture of deuterated toluene (toluene-d8 99.5%) and deuterated heptane (heptane-d16 98%) supplied by Cambridge Isotope Laboratories, Inc. 2.1. Solution Preparation. A stock solution of 1000 ppm asphaltenes in toluene is prepared by sonication for 5 min and stored in a sealed glass vial. The vial is wrapped in aluminum foil to prevent exposure to light. Before use, the stock solution is sonicated (SONICS vibra-cell, USA) for 5 min and the appropriate volume of stock solution is withdrawn, which is then diluted with toluene before further dilution with Nexbase oil 2000 series to obtain the desired asphaltene mass fraction in a 10 mL sample. The prepared asphaltenes solution is then sonicated for a further 1 min. Asphaltene solutions are discarded after 24 h in order to avoid bias due to long-term phase segregation. 2.2. Interfacial Tension. The dynamic interfacial tension of the water−model oil interface containing asphaltenes is investigated using the pendant drop technique (Attention Theta tensiometer, Biolin Scientific, Finland). The tensiometer is a real-time droplet analyzer that allows continuous droplet profile extraction. An inverted 16-gauge needle is submerged in the aqueous phase such that the tip is visible in the frame of capture. A gastight syringe [1 mL] (Hamilton Co., USA) is mounted in a microsyringe pump (Harvard Apparatus) to ensure instantaneous creation of a droplet of a preset volume. Before the droplet is formed, the image capture software is triggered, collecting images at 2 frames/s for the first 10 min and 1 frame/min thereafter, for a total aging time of 10 000 s (2.78 h). Edge detection is used to identify the droplet shape, with the interfacial tension determined using the Young−Laplace equation.26 Experimental runs of 10 000 s are chosen. It became difficult to maintain a constant drop volume for longer times. The change in volume of the drop is perhaps due to the pressure applied by the 9987


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aqueous phase in the cuvette on the drop which may cause a very weak backflow in turn causing the drop to shrink in volume over long times. The acceptable volume change over 10 000 s is 2% of the original volume. This leads to maximum error of ±2.5% in interfacial tension

Figure 2. Chemical shift NMR spectrum of 100 ppm asphaltene solution in toluene-d8 and heptane-d16 solvent (top) and that without asphaltenes (bottom). impurities (possibly water). The integrated intensity between 1.16 and 1.18 ppm is hereafter referred to as “asphaltene monomer signal”. To confirm that the measured signal actually corresponds to the monomeric asphaltenes, similar experiments were performed with deuterated toluene solutions. The global trend is a linear increase up to 200 ppm and a progressive leveling up to 800 ppm, Figure 3. The graph is very close to the one previously published.27 Interpreting the ‘kink’ as the CNAC gives a value close to published data obtained with various techniques.27−30

Figure 1. Repeatability of dynamic interfacial tension (3 runs).

as shown in Figure 1. Droplet volume change at longer times have previously been reported by Bauget et al.20 Just after droplet creation, the water−oil interfacial tension is measured around 40 ± 2 mN/m. This corresponds to the clean model oil−water interfacial tension measured without asphaltenes. The repeatability of the measurement for the same asphaltene mass fraction and the same organic phase is shown in Figure 1. As mentioned, scatter in the experimental data is ±2.5%. This is thought to be reasonable given the potentially numerous causes for discrepancy: slight drift of aqueous phase pH over time due to ambient carbon dioxide, temperature variation, differences in droplet expansion dynamics prior to measurements. Those processes seem to impact both the initial clean interface values and the decay kinetics. 2.3. NMR. Liquid-state 1H NMR has been previously reported to enable determination of CNAC27 of asphaltenes by monitoring the signal corresponding to the aliphatic hydrogen between 0.8 and 2 ppm in toluene solutions. It has been observed that the aliphatic signal increases linearly with increasing asphaltenes mass fraction below 200 ppm and levels off above 200 ppm (see Figure 3). The signal loss has been interpreted as a mobility loss of side alkyl chains of asphaltenes in nanoaggregates, which is supported by the observation of “crystalline” alkyl peaks in FTIR spectra of asphaltenes aggregates.16 Similar experiments using extracted asphaltenes from Norwegian crude oil in deuterated Heptol (85:15) [toluene-d8 (15%) and heptane-d16 (85%)] are performed. Deuterated heptane (85%) is used to match the solubility of the asphaltenes in the oil phase that has been used in the interfacial tension experiments. Spectra are measured 30 min after sample preparation using a NMR 500-Varian Inova Unity spectrometer operating at a 1H resonance frequency of 500 MHz. The following conditions are used to collect data: pulse of 45°, relaxation delay of 1s, acquisition time of 1.892s, data point of 8K, tube diameter of 5 mm, spectral width of 10 ppm, and at least 200 scans. As deuterated heptane also has peaks in the 0.8−2 ppm range, the challenge was to identify and differentiate the asphaltene peak from the heptane-d16 peaks. Figure 2 shows the difference of NMR peaks between pure solvent (Figure 2, bottom spectra) and 100 ppm asphaltenes solution (Figure 2, top spectra) in the range from 0.4 and 1.4 ppm. A difference is observed between the two spectra with the oil phase containing asphaltenes showing a peak at 1.16−1.18 ppm. Small peaks at 1.64 and 0.74 ppm are also seen with asphaltenes but are difficult to extract from the background. The peak around 0.5 ppm is observed in the solvent with no asphaltenes and can be attributed to

Figure 3. Concentration dependence of the NMR asphaltene signal in toluene shows CNAC at 200 ppm.

2.4. Dynamic Light Scattering. To further investigate the nature of the asphaltenes in solution, the dynamic light scattering technique has been used.31 Zetasizer Nano ZS (Malvern Instruments, U.K.) is used in forward scatter for mass fractions below 100 ppm and in backscatter mode (173°) for higher mass fractions due to high absorbance of the solutions. The radius of the particles in solution is measured using the Stokes−Einstein equation D=

kBT 6πηr

(1)

where the diffusion coefficient D is measured from the autocorrelation function of the scattering intensity signal, kB is the Boltzmann constant, η is the viscosity of the oil phase at temperature T, and r is the hydrodynamic radius of the asphaltene molecule. 9988


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Experiments are performed at 298 K with a measurement every 7 min for 200 min starting from solution preparation. Each data point is an average of 10 experimental runs. One should note that for solid particles the scattered intensity scales up as the particle radius to the power 6. As a result, DLS will be more sensitive to large aggregates than to asphaltenes monomers. 2.5. Pendant Droplet as a Langmuir Trough. To investigate the relationship between the interfacial tension and the surface coverage in various adsorption conditions, the pendant droplet apparatus is used as a Langmuir trough following a method developed for surfactant-stabilized systems.32,33 After a certain adsorption time the droplet is rapidly expanded to the point of droplet detachment. The interfacial tension is measured as a function of the interfacial area. With rapid expansion (few seconds) it is reasonable to assume that no asphaltenes are exchanged between the bulk and the interface. Therefore, the amount of asphaltenes on the droplet surface Γ(t)·A(t) is a constant, where Γ(t) is the interfacial coverage of asphaltenes at time t and A(t) is the surface area of the droplet at the same time. If a reference area is chosen one can plot the interfacial tension vs relative coverage [Γ(t)/Γ(Aref) = Aref/A(t)] for each test condition. To compare different test conditions (different adsorption times, different asphaltenes mass fraction) a common reference interfacial tension γref is chosen that dictates the choice of the reference area for each test Aref = A(γref). If the interfacial tension is a unique function of surface coverage then Γ(Aref) is constant. In turn, all curves of the interfacial tension vs relative coverage [Γ(t)/Γ(Aref) = Aref/A(t)] will overlap. On the contrary, if adsorbed species undergo relaxation/reorganization over time, curves will not overlap; instead, they will intersect at Γ(t)/ Γ(Aref) = 1. We verified with a pure oil phase that expansion of the droplet does not cause any change in interfacial tension that could arise from the viscous stresses due to the high viscosity of the oil phase and high volume change rate.

80, 100, 200, and 500 ppm) the interfacial tension appears to asymptotically approach a value of 20 mN/m with time. At first sight this could be interpreted as an indication of an abrupt CNAC at approximately 50 ppm. Such an interpretation would however be contradicted by the above-mentioned observation that the initial rate of reduction in interfacial tension still increases above 50 ppm. Moreover, it is clear from the experimental data that interfacial tension does not reach a constant value after 10 000 s. Even above 50 ppm interfacial tension continues to slowly decrease over the time interval of interest. In other words, above 50 ppm there is a late stage for which decay kinetics appear to be independent of asphaltene mass fraction. A possible explanation might be that after completion of adsorption interfacial tension continues to decrease due to conformational relaxation/reorganization of the asphaltenes. An alternative explanation would be that the diffusion-controlled regime is followed by an adsorption barrier-controlled regime. Above 50 ppm the surface coverage would rapidly reach a value for which further adsorption would be slowed by steric hindrance. At an extreme, the characteristic time scale for adsorption of an additional asphaltene would depend only on the interfacial mobility through the time necessary for local surface coverage fluctuations to open up a free area larger than the cross-section area of an asphaltene. Late adsorption kinetics would therefore largely be independent of asphaltene mass fraction, as observed. Such a scenario however relies on several working hypotheses, which are discussed in the current study. 3.2. Initial Diffusion-Controlled Kinetics. The interfacial tension measurements will first be considered for the period in which they change rapidly. As now discussed the controlling process appears to be diffusion-controlled kinetics. To proceed, the short time approximation for dynamic interfacial tension is given by a coupled Gibbs−Duhem and diffusion equation as

3. RESULTS AND DISCUSSION 3.1. Initial Observations and Working Hypotheses. Dynamic interfacial tension curves are shown in Figure 4 for

γ(t ) = γ0 − 2RTC

Dt π

(2)

where γ is the interfacial tension at any time t, γo the clean interfacial tension at time t = 0, R the universal gas constant, T the temperature, C the bulk concentration of adsorbing species, and D the diffusion coefficient. Figure 5 confirms that at early times (less than 10 s for the highest asphaltene mass fraction and up to more than 100 s for the lowest) interfacial tension scales linearly with √t, with the slope increasing with bulk asphaltene mass fraction. 3.3. Aggregation and Diffusion of Surface-Active Asphaltene Fraction. Figure 6 shows the bulk concentration dependency of the early time slopes extracted from Figure 5. There appear to be two regions. At an asphaltene mass fraction of approximately 80 ppm we observe a break in the trend of the slopes with asphaltene mass fraction. As indicated in eq 2, the slopes are directly proportional to the square root of the diffusion coefficient and also to the mass fraction of the species adsorbing onto the interface. The decreased slopes at a mass fraction of above 80 ppm therefore indicate a reduction in either the diffusion coefficient or the mass fraction of the adsorbing species. If the adsorbing species is postulated to be asphaltene monomer and their diffusion coefficient at such low mass fractions is considered to be independent of concentration then the results could be explained by formation of aggregates of monomer above 80 ppm. Such aggregates being much larger than the monomer would diffuse much more slowly. The

Figure 4. Dynamic interfacial tension for different asphaltene mass fractions [10−500 ppm] in Nexbase 2002.

asphaltene mass fractions between 10 and 500 ppm in Nexbase 2002 and toluene as an organic phase (each mass fraction repeated several times). For all mass fractions studied the interfacial tension measured shows a rapid initial decrease followed by a progressive reduction of the decay rate. The interfacial tension measured for the samples with mass fractions in the range of 10−50 ppm continues to decrease even after 10 000 s. For samples with higher asphaltene mass fractions (50, 9989


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Figure 7. Mass fraction dependence of the NMR asphaltenes signal in deuterated Heptol shows CNAC approximately 80 ppm.

the sum of the contributions of aggregates and monomers evolve the same way for NMR and adsorption. On a more quantitative basis, theoretical calculation of the slope 2RTC(D/ π)1/2 can be done assuming that all asphaltenes are in the monomer form (molecular weight of 800 g/mol and a radius of gyration of 1 nm) and that all monomer asphaltenes adsorb. For 80 ppm, the theoretical slope is equal to −1.3 mN·m−1·s−1/2, whereas the experimental slope is equal to −0.53 mN·m−1·s−1/2, which is of the right order but the discrepancy will be further addressed in the following study. Calculations can also be done assuming all asphaltenes are in the form of small aggregates. If this is done for asphaltene mass fractions of 500 ppm (with a molecular weight of 20 000 g/mol and a radius of gyration of 3 nm15,34) the calculated slope is 2 orders of magnitude lower than the experimental values. If large aggregates (with a molecular weight of 100 000 g/mol and a radius of gyration of 10 nm) are assumed then the predicted value is further reduced by 1 order of magnitude. From these calculations it would appear that nanoaggregates do not contribute to the initial decay of interfacial tension as they diffuse too slowly. This may explain, at least in part, why both the slope of the interfacial tension vs √t and the NMR signal are proportional to the amount of asphaltenes monomers below and above the CNAC. From the NMR signal definition (alkyl carbon relaxation) it seems that the ratio between aromatic and aliphatic carbon in the population of monomer asphaltenes is constant below and above the CNAC. This in turn implies that aggregation in this medium is not selective in terms of the chemical structure of monomer asphaltenes. Similarly, aggregation in this medium does not seem to be selective in terms of surface activity. This appears different from the previously reported correlations35,36 between solubility, chemical structure, and interfacial activity. It is noteworthy that in Figure 3 the asphaltenes signal at 1000 ppm in toluene is about one-half of what it should be following the linear dependency observed below 200 ppm. From the hypothesis mentioned above, that the NMR asphaltenes signal is proportional to the monomer concentration, it would mean that one-half of the asphaltenes present in the stock solution used for preparing the desired asphaltene mass fraction are already present in nanoaggregates. During making of solutions of various asphaltene mass fractions from

Figure 5. Dynamic interfacial tension vs square root of time for different asphaltene mass fractions of (10−500 ppm) in Nexbase 2002. (Inset) Linear behavior for all asphaltene mass fractions.

Figure 6. Slope of curves (interfacial tension vs square root time) against asphaltenes mass fractions in Nexbase 2002.

decrease in interfacial tension would then be essentially due to the diffusion and adsorption of the monomer that remains nonaggregated. Whether this inference is true or not requires validation by more direct evidence of monomer aggregation above 80 ppm mass fraction in solution. To this end, NMR data have been obtained as now discussed. In Figure 7, the asphaltene monomer signal is plotted against the bulk mass fraction of asphaltenes in deuterated Heptol. It is observed that the asphaltene signal increases linearly with mass fraction until 80 ppm. At mass fractions higher than 80 ppm the signal continues to increase but more slowly up to the maximum mass fraction tested which was 500 ppm. The slope of NMR signal vs asphaltene mass fraction and the slope of interfacial tension vs √t are very similar both below and above 80 ppm, which is thought of as the CNAC. This is perhaps coincidental as there is no readily apparent physical reason why 9990


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∂c ∂ 2c = D 2 (z > 0, t > 0) ∂t ∂z

the stock solution, upon mixing in the aliphatic Nexbase oil, those nanoaggregates are unlikely to redissolve readily because of their tendency to flocculate and precipitate. At low asphaltene mass fractions (below the apparent CNAC in the aliphatic solvent, i.e., below 80 ppm), no further aggregation is expected. On the contrary; at high mass fractions (above 80 ppm) more aggregates will most likely form and/or initially present aggregates flocculate. Those assumptions are largely confirmed by DLS data shown in Figure 8. Immediately after

(3)

where D is the diffusion coefficient of asphaltene molecules in the bulk oil phase. The boundary condition at the subsurface layer, which is an imaginary layer just near the interface, is given by ∂Γ ∂c =D ∂t ∂zz = 0

(4)

where Γ is the interfacial coverage of asphaltene on the interface. On the other side, in the absence of depletion (due to the small area to cover compared to the volume of solution) c(∞ , t ) = c∞

(5)

Integrating those equations yields the well-known Ward− Tordai equation Γ(t ) = Γ(0) +

D⎡ ⎢2c∞ t − π ⎣

∫0

t

c(0, τ )dτ ⎤ ⎥ t−τ ⎦

(6)

Solving the Ward−Tordai equation together with the adsorption isotherm (Γe = f(c)) yields the evolution of surface coverage Γ(t). Inserting the surface coverage into the equation of state (γ = f(Γ)) gives the evolution of interfacial tension γ(t). However, some issues remain. First, the Ward−Tordai equation does not have an exact analytical solution, except in a specific case using the Henry isotherm. Furthermore, those calculations require a detailed description of the system thermodynamics (equation of state and adsorption isotherm), which is not yet available for asphaltenes. However, one can consider the case where the only change in the adsorption conditions is the diffusion coefficient (i.e., through the viscosity of the solution as in the Stokes−Einstein formulation). In the diffusion equations, time can then be replaced by a rescaled time t* = √t/η, producing a unique “Ward−Tordai” equation for all viscosities. Solution of this equation should be a unique function Γ(t*) leading to unique dynamic interfacial tension decay γ(t*) whatever the viscosity. This applies for all adsorption isotherms and equations of state. Experimentally this can be achieved by switching from the Nexbase 2002 oil to one of its homologues of higher viscosity (2004, 2006, 2008). The increase in length of the poly alpha-olefin should not significantly change the solubility of asphaltenes and their adsorption behavior. Figure 9 shows experimental data obtained for a 10 ppm bulk mass fraction of asphaltenes. Interfacial tension decay with time is seen to be slower with increasing viscosity. For the three higher viscosities tested, the dynamic interfacial tension curves collapse over the whole time interval (10 000 s) if plotted vs t scaled with viscosity specifically against (√(time/viscosity)) (see Figure 10). However, for the lowest viscosity the interfacial tension decay rate curve diverges after about 1700 s (∼30 min). This experiment at the lowest viscosity was repeated to exclude the possibility of experimental error. Besides supporting the initial diffusion-controlled mechanism discussed earlier, the data also provides some insight into the later transition to the nondiffusion-controlled regime. If the nondiffusion-controlled regime were to correspond to conformational relaxation of already adsorbed species (like unfolding for some proteins), it should happen from the start of experiments and its kinetics should be largely independent of bulk viscosity.

Figure 8. Mean radius of asphaltenes (intensity based) vs time for different asphaltene mass fractions.

mixing, all solutions contain particles of a size (∼75 nm) largely independent of mass fraction. With time, the average particle size increases for mass fractions above 80 ppm while remaining almost constant below that mass fraction. The measured size range is higher than published data for crude oil15 but lower than published data for flocculation conditions (alkane dissolution).35,37 An important issue relates to how the time required for the nanoaggregates to form might affect the experimental results. To study this a series of experiments was performed with different times between sonication and the start of the surface tension decay with time measurements for 100 ppm bulk aspahaltene mass fraction. The results indicate very little effect (as shown in the last figure in the Supporting Information). Similarly, surface tensions measured for 500 ppm bulk mass fractions appeared to be unaffected by the time between sonication of the samples and the experiments. When considering that only 50% of asphaltenes are initially in the monomeric form in the aliphatic solutions even at low mass fractions, the theoretical slope 2RTC(D/π)1/2 becomes −0.65 mN·m−1·s−1/2 for 80 ppm. It only differs from the experimental slope by 20%. Thus, nearly one-half of the asphaltenes molecules present in the solution are in the form of nanoaggregates initially. 3.4. Transition to Nondiffusion-Controlled Kinetics. Starting from the finding that the initial decay rate in interfacial tension is diffusion controlled, one can analyze the long-term data. This requires more detailed analysis than has been presented until now. In the diffusion-controlled adsorption regime on a flat interface (z = o), the bulk concentration is c. Mass transport in the z (interface-normal) direction can be written as 9991


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control be denoted by γd and td, respectively. Then, for the fastest diffusion, it can be concluded that γc > γd and τr< td. Also, if γc = γd then τr ≪ td, and conversely, if τr = td then γc ≫ γd. Combining those two conditions one can conclude conservatively that for slower diffusion the effect of reorganization should be apparent at least with a delay δt = td from the moment t when γ(t) = γd. For the data presented in Figures 9 and 10, deviation of the Nexbase 2002 curve occurs after 1700 s and at 33.5 mN/m. For Nexbase 2004 and 2006, the 33.5 mN/m value is reached after 4300 and 5300 s, respectively. However, upon rescaling time by viscosity, the Nexbase 2006 curve shows a good overlap with the Nexbase 2004 curve up to 10 000 s. This means no reorganization occurred for an additional 4700 s in Nexbase 2006. Then, except if interfacial reorganization is strongly dependent upon bulk viscosity, such a process can be excluded for the present data. Another important observation relies on the sign of the deviation from the diffusion-controlled regime. From Figure 10 it can be seen that the transition to the nondiffusion regime for Nexbase 2002 (lowest viscosity) corresponds to a much slower decay rate than predicted by viscosity rescaling. If any relaxation/reorganization was to cause a further decrease over long times, the decay rate observed for the lowest viscosity should be faster than predicted by the viscosity rescaling. Both observations, (i) transition time and (ii) sign of the deviation from diffusion-controlled regime, are compatible with barrier adsorption. Barrier adsorption should appear when hindrance due to already adsorbed molecules causes a delay of similar characteristic time as diffusion. Transition from diffusion-controlled kinetics would then be delayed for the highest viscosities because the time necessary to reach the critical coverage will be longer. At the extreme, if the time necessary to cross the potential barrier remains small compared to the characteristic diffusion time (i.e., at low asphaltene mass fraction and high viscosity) then no transition will be observed before the surface coverage almost reaches equilibrium. One can also note that in the case of a transition to barrier adsorption regime late kinetics should be slower than predicted by viscosity rescaling. Similar observations are made at higher mass fraction (100 ppm as shown in Figures 11 and 12). These show an early

Figure 9. Dynamic interfacial tension for different viscosities of Nexbase oil for 10 ppm asphaltene solution.

Figure 10. Dynamic interfacial tension versus time rescaled with viscosity as the square root of time/viscosity for the 10 ppm asphaltene solution.

When the only change between experiments is viscosity, the coupling of the two regions of interfacial decay rate can be rewritten as a sum of a diffusive term and a relaxation term γ(t ) = γdiffusion(t *) + γrelaxation(t )

(7)

For a given t*, a smaller t corresponds to a smaller relaxation contribution. If, as often assumed, the interfacial relaxation of asphaltenes is a lengthy process then the faster diffusion kinetics should dominate the lesser contribution of relaxation to the total decay rate of interfacial tension. Consequently, the lower the bulk viscosity, the longer the dynamic interfacial tension decay should follow diffusion control. This is the opposite of our observations. The lowest viscosity case deviates first. Similar considerations apply to the case of a collective reorganization of asphaltenes (e.g., gelling) that would only occur when surface coverage reaches a critical value (i.e., asphaltenes are close enough to interact). Even if the critical surface coverage (or corresponding critical interfacial tension γc) and characteristic time for observing a measurable impact of reorganization (τr) are not known, they can be bounded. Let the interfacial tension and time at deviation from diffusion

Figure 11. Dynamic interfacial tension for different viscosities of Nexbase oil for 100 ppm asphaltene solution. 9992


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Figure 12. Dynamic interfacial tension with rescaled x axis to the square root of time/viscosity for 100 ppm asphaltene solution.

diffusion-controlled regime and transition to a slow nondiffusion-controlled regime. The transition time is again shorter for lower viscosities. By comparison between 10 and 100 ppm, it also appears that transition occurs earlier for the higher mass fraction. Both observations are compatible with a transition to an adsorption barrier regime. However, when it does occur, this transition corresponds to interfacial tension values in the range from 35 to 30 mN/m. This could correspond to a critical surface coverage for which asphaltenes−asphaltenes interaction hinders further adsorption. 3.5. Interfacial Tension Isotherm. Droplet expansion experiments have been conducted for Nexbase 2002 at 25, 50, and 60 ppm (below CNAC) and 200 ppm (above CNAC) to elucidate the effect of nanoaggregates. Aging times in these experiments ranged from 30 min to 6 h (well into the nondiffusion-controlled regime of adsorption and also in the kinetic regime independent of mass fraction) to check whether reorganization/relaxation occurred. The area of the droplet vs time and interfacial tension vs area plots are shown in Figure 13. It should be noted that in addition to asphaltene mass fraction and time, our experimental variables include initial droplet volume and expansion rate. For further analysis, one requires a reference interfacial tension (γref) that is consistent for all tests. From Figure 13b, γref is chosen to be 26.58 mN/m and reference areas A(γref) are obtained for each of the five experimental curves. The interfacial tension as a function of the relative interfacial coverage A(γref)/A(t) is shown in Figure 14. It is clearly seen that all five curves are superposed as relative coverage changes by a factor 4, i.e., the interfacial tension appears to be a unique function of surface coverage. In more detail, it can be seen that for a given asphaltene mass fraction (50 ppm) the interfacial tension isotherm is independent of time (from 30 min to 6 h). In other words, if any relaxation/ reorganization occurs it is either much more rapid than 30 min or much slower than 6 h. Within this range of times, the observed nondiffusion-controlled kinetics is not due to relaxation/reorganization. On the other hand, for a given time (3 h) the interfacial tension isotherm is independent of asphaltene mass fraction below and above CNAC. This means that the relative amounts of monomers and nanoaggregates have no impact on the interfacial tension isotherm. Aggregates probably do not have any impact on the measurement. Inserting the measured aggregates sizes (together with an

Figure 13. (a) Droplet area versus time during expansion, and (b) corresponding interfacial tension versus droplet area.

Figure 14. Interfacial tension against relative coverage for all runs showing collapse of all curves to a single master curve.

estimate of their molar weight) into linearized approximation of interfacial tension decay indicates that the maximum impact aggregates could have on interfacial tension over 3 h would be extremely low (≪1 mN/m). It cannot however be excluded from the present data that with much higher asphaltene mass fractions (typically of a few mass % as in crude oil) and under 9993


Langmuir

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(2) Spiecker, P.; Kilpatrick, P. Interfacial rheology of petroleum asphaltenes at the oil-water interface. Langmuir 2004, 20 (10), 4022− 4032. (3) Bridie, A.; Wanders, T.; Zegveld, W.; Heijde, H. v. d. Formation, prevention and breaking of sea water in crude oil emulsions ’chocolate mousses’. Mar. Pollut. Bull. 1980, 11 (12), 343−348. (4) Papirer, E.; Bourgeois, C.; Siffert, B.; Balard, H. Chemical nature and water oil emulsifying properties of asphaltenes. Fuel 1982, 61 (8), 732−734. (5) Rondon, M.; Bouriat, P.; Lachaise, J.; Salager, J. Breaking of water-in-crude oil emulsions. 1. Physicochemical phenomenology of demulsifier action. Energy Fuels 2006, 20 (4), 1600−1604. (6) Durand, E.; Clemancey, M.; Quoineaud, A. A.; Verstraete, J.; Espinat, D.; Lancelin, J. M. 1H Diffusion-Ordered Spectroscopy (DOSY) Nuclear Magnetic Resonance (NMR) as a Powerful Tool for the Analysis of Hydrocarbon Mixtures and Asphaltenes. Energy Fuels 2008, 22 (4), 2604−2610. (7) Nordgård, E. L. k.; Sjö blom, J. Model Compounds for Asphaltenes and C80 Isoprenoid Tetraacids. Part I: Synthesis and Interfacial Activities. J. Dispers. Sci. Technol. 2008, 29 (8), 1114−1122. (8) Groenzin, H.; Mullins, O. C. Molecular Size and Structure of Asphaltenes from Various Sources. Energy Fuels 2000, 14 (3), 677− 684. (9) Rodgers, R. P.; Marshall, A. G. Petroleomics: Advanced Characterization of Petroleum-Derived Materials by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) Asphaltenes, Heavy Oils, and Petroleomics: Springer: New York, 2007; pp 63−93. (10) Pauchard, V.; Sjöblom, J.; Kokal, S.; Bouriat, P.; Dicharry, C.; Müller, H.; al-Hajji, A. Role of Naphthenic Acids in Emulsion Tightness for a Low-Total-Acid-Number (TAN)/High-Asphaltenes Oil†. Energy Fuels 2008, 23 (3), 1269−1279. (11) Czarnecki, J. Stabilization of Water in Crude Oil Emulsions. Part 2. Energy Fuels 2008, 23 (3), 1253−1257. (12) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Determination of the chemical environment of sulphur in petroleum asphaltenes by X-ray absorption spectroscopy. Fuel 1992, 71 (1), 53− 57. (13) Fossen, M.; Kallevik, H.; Knudsen, K. D.; Sjoblom, J. Asphaltenes precipitated by a two-step precipitation procedure. 1. Interfacial tension and solvent properties. Energy Fuels 2007, 21 (2), 1030−1037. (14) Yarranton, H. W.; Hussein, H.; Masliyah, J. H. Water-inHydrocarbon Emulsions Stabilized by Asphaltenes at Low Concentrations. J. Colloid Interface Sci. 2000, 228 (1), 52−63. (15) Jestin, J.; Simon, S.; Zupancic, L.; Barré, L.; Small Angle, A Neutron Scattering Study of the Adsorbed Asphaltene Layer in Waterin-Hydrocarbon Emulsions: Structural Description Related to Stability. Langmuir 2007, 23 (21), 10471−10478. (16) Mullins, O. C. Review of the molecular structure and aggregation of asphaltenes and petroleomics. SPE J. 2008, 13 (000254430400005), 48−57. (17) Moran, K.; Czarnecki, J. Competitive adsorption of sodium naphthenates and naturally occurring species at water-in-crude oil emulsion droplet surfaces. Colloids Surf., A: Physicochem. Eng. Aspects 2007, 292 (2−3), 87−98. (18) Sheu, E.; Storm, D.; Shields, M. Adsorption kinetics of asphaltenes at toluene/acid solution interface. Fuel 1995, 74 (10), 1475−1479. (19) Sheu, E.; Detar, M.; Storm, D. Interfacial properties of asphaltenes. Fuel 1992, 71 (11), 1277−1281. (20) Bauget, F.; Langevin, D.; Lenormand, R. Dynamic surface properties of asphaltenes and resins at the oil-air interface. J. Colloid Interface Sci. 2001, 239 (2), 501−508. (21) Freer, E. M.; Radke, C. J. Relaxation of asphaltenes at the toluene/water interface: diffusion exchange and surface rearrangement. J. Adhes. 2005, 80 (6), 481−496. (22) Jeribi, M.; Almir-Assad, B.; Langevin, D.; Henaut, I.; Argillier, J. Adsorption kinetics of asphaltenes at liquid interfaces. J. Colloid Interface Sci. 2002, 256 (2), 268−272.

advection smaller nanoaggregates may possibly contribute significantly to interfacial properties.

4. CONCLUSION The dynamic interfacial tension of asphaltenes at oil−water interface is measured using the pendant drop technique in the asphaltene mass fraction range of 10−500 ppm and for various solution viscosities. The initial interfacial tension decay rate appears to be due to diffusion-controlled adsorption, i.e., the interfacial tension varies linearly with the square root of time. Furthermore, the effect of viscosity can be accounted for by rescaling time with viscosity as the square root of (time/ viscosity), as would be expected in a diffusion-controlled regime. The data also suggest that the monomers start to aggregate, leading to a change in the dependence of the decay rate on asphaltene mass fraction in the diffusion-controlled regime. Transition to nondiffusion-controlled kinetics leads to a slowing down of interfacial tension decay. Transition time is strongly impacted by both the viscosity and the mass fraction of asphaltenes. Both observations do not appear to support relaxation/reorganization kinetics and rather favor barrier adsorption kinetics. This is confirmed by the invariance of the interfacial tension isotherm over time for a given asphaltenes mass fraction as observed during droplet expansion. Also, the interfacial tension is a unique function of surface coverage. The inferences from the pendant drop experiments are confirmed by the similarity of the initial rate of interfacial tension decay with asphaltene monomer signal from NMR. Both increase up to 80 ppm and then progressively level off. Thus, 80 ppm is considered to be the CNAC in the studied solutions from interfacial tension, NMR, and DLS measurements. The progressive nature of aggregation onset is attributed to the polydispersity of asphaltenes molecular structures.

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ASSOCIATED CONTENT

S Supporting Information *

Autocorrelation function data from DLS; dynamic interfacial tensions measured for 100 ppm asphaltene mass fraction after different premeasurement times. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed by the FACE Centre, a research cooperation between IFE, NTNU, and SINTEF. The Centre is funded by The Research Council of Norway and by the following industrial partners: Statoil ASA, GE Oil & Gas, Scandpower Petroleum Technology AS, FMC, CD-adapco, and Shell Technology Norway AS.

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REFERENCES

(1) Sjoblom, J.; Aske, N.; Auflem, I.; Brandal, O.; Havre, T.; Saether, O.; Westvik, A.; Johnsen, E.; Kallevik, H. Our current understanding of water-in-crude oil emulsions. Recent characterization techniques and high pressure performance. Adv. Colloid Interface Sci. 2003, 100−102, 399−473. 9994


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(23) Pauchard, V.; Roy, T. The Oil Field Chemistry Symposium, Tekna, 2012. (24) Bouriat, P.; El Kerri, N.; Graciaa, A.; Lachaise, J. Properties of a Two-Dimensional Asphaltene Network at the Water−Cyclohexane Interface Deduced from Dynamic Tensiometry. Langmuir 2004, 20 (18), 7459−7464. (25) Fan, Y.; Simon, S. b.; Sjöblom, J. Influence of Nonionic Surfactants on the Surface and Interfacial Film Properties of Asphaltenes Investigated by Langmuir Balance and Brewster Angle Microscopy. Langmuir 2010, 26 (13), 10497−10505. (26) Barnes, G.; Gentle, I. Interfacial science: an introduction; 2005; p 247. (27) Lisitza, N. V.; Freed, D. E.; Sen, P. N.; Song, Y.-Q. Study of Asphaltene Nanoaggregation by Nuclear Magnetic Resonance (NMR). Energy Fuels 2009, 23, 1189−1193. (28) Freed, D. E.; Lisitza, N. V.; Sen, P. N.; Song, Y.-Q. Molecular Composition and Dynamics of Oils from Diffusion easurements Asphaltenes, Heavy Oils, and Petroleomics; Springer: New York, 2007; pp 279−299. (29) Sheu, E.; Long, Y.; Hamza, H. Asphaltene Self-Association and Precipitation in SolventsAC Conductivity Measurements Asphaltenes, Heavy Oils, and Petroleomics; Springer: New York, 2007; pp 259−277. (30) Andreatta, G.; Bostrom, N.; Mullins, O. C. High-Q Ultrasonic Determination of the Critical Nanoaggregate Concentration of Asphaltenes and the Critical Micelle Concentration of Standard Surfactants. Langmuir 2005, 21 (7), 2728−2736. (31) Hiemenz, P.; Rajagopalan, R. Principles of colloid and surface chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (32) Kumar, N.; Couzis, A.; Maldarelli, C. Measurement of the kinetic rate constants for the adsorption of superspreading trisiloxanes to an air/aqueous interface and the relevance of these measurements to the mechanism of superspreading. J. Colloid Interface Sci. 2003, 267 (2), 272−285. (33) Pan, R.; Green, J.; Maldarelli, C. Theory and Experiment on the Measurement of Kinetic Rate Constants for Surfactant Exchange at an Air/Water Interface. J. Colloid Interface Sci. 1998, 205 (2), 213−230. (34) Simon, S.; Jestin, J.; Palermo, T.; Barre, L. Relation between Solution and Interfacial Properties of Asphaltene Aggregates. Energy Fuels 2009, 23 (1), 306−313. (35) Fossen, M.; Kallevik, H.; Knudsen, K. D.; Sjöblom, J. Asphaltenes Precipitated by a Two-Step Precipitation Procedure. 2. Physical and Chemical Characteristics. Energy Fuels 2011, 25 (8), 3552−3567. (36) Yarranton, H. W.; Sztukowski, D. M.; Urrutia, P. Effect of interfacial rheology on model emulsion coalescence-I. Interfacial rheology. J. Colloid Interface Sci. 2007, 310 (1), 246−252. (37) Marczak, W.; Dafri, D.; Modaressi, A.; Zhou, H.; Rogalski, M. Physical State and Aging of Flocculated Asphaltenes. Energy Fuels 2007, 21 (3), 1256−1262.

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