Energy & Fuels 2009, 23, 1237–1248
1237
Adsorption of Crude Oil on Surfaces Using Quartz Crystal Microbalance with Dissipation (QCM-D) under Flow Conditions† Adewunmi Abudu and Lamia Goual* Department of Chemical and Petroleum Engineering, UniVersity of Wyoming, Department 3295, 1000 East UniVersity AVenue, Laramie, Wyoming 82071 ReceiVed July 30, 2008. ReVised Manuscript ReceiVed October 30, 2008
Adsorption of crude oil on surfaces is successfully measured with a quartz crystal microbalance with dissipation (QCM-D) under flow conditions. The amounts and thicknesses of adsorbed films are determined with good accuracy using liquid loading corrections. Measurements are performed in solvents where the degree of asphaltene stability is high (toluene) and poor (n-alkanes and heptane/toluene mixtures). In toluene, Langmuirtype adsorption is recorded with saturation film thicknesses of 3-4 nm and limited desorption after rinsing. The size of adsorbing species is also determined at early times where adsorption is diffusion-controlled up to 3 wt % crude oil (or 835 ppm asphaltenes) in toluene. The primary asphaltene species that adsorb on the crystal surface are molecules with a diameter of 0.5-1.6 nm at 139-278 ppm asphaltenes and nanoaggregates with a diameter of 2.6-5.6 nm at 835 ppm asphaltenes in toluene. In n-alkanes (nC7, nC10, and nC12), saturation plateaus are not observed within the experimental time scale. Film thicknesses recorded after 3.5 h are all higher than those in toluene and increase with increasing n-alkane carbon number, mainly because of increasing polarity of aggregates. Atomic force microscopy (AFM) analyses reveal that the size of adsorbed aggregates decreases with n-alkane carbon number. Aging effects show that, with time, the adsorbed films become more rigid in toluene and more viscoelastic in n-alkanes. In heptane/toluene mixtures, a significant increase in the dissipation factor is observed close to the flocculation threshold. Adsorption on various surfaces from toluene shows a high affinity of asphaltenes to hydrophilic surfaces. On the other hand, asphaltenes are almost amorphous in n-alkanes. X-ray photoelectron spectroscopy (XPS) analyses of the adsorbed films confirm these observations.
1. Introduction Adsorption of petroleum heavy ends at solid/liquid interfaces may arise at different stages of oil production and cause wettability alteration, fouling, and coking. Among heavy ends in petroleum, asphaltenes are probably the most problematic because of their chemical nature and their tendency to aggregate and deposit under certain pressure, temperature, and composition conditions. Asphaltenes are defined as the fraction of petroleum that is insoluble in n-alkanes but soluble in aromatics. They consist of polyaromatic structures with aliphatic side chains. The presence of heteroatoms and metals in their structure confers to them the highest dipole moments in a petroleum fluid.1 Asphaltenes have a distribution of molecular masses in the range of 400-1500 amu.2 They start to associate at a concentration of about 50 mg/L in toluene14 and form nanoaggregates above 100 mg/L ((50%) in toluene,3,4 whose size is constant within a certain concentration range.5 The mechanism of asphaltene deposition on surfaces has not been well-established as yet, probably because compositional variations directly affect the aggregation state of adsorbing species. A fundamental understanding of asphaltene-solid † Presented at the 9th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. E-mail: lgoual@ uwyo.edu. (1) Goual, L.; Firoozabadi, A. Measurement of asphaltenes and resins, and their dipole moment in petroleum fluids. AIChE J. 2002, 48 (11), 2646– 2663. (2) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and structure. J. Phys. Chem. A 1999, 103, 11257.
interactions through adsorption studies can provide valuable insight into the mechanism of asphaltene deposition. The adsorption kinetics of asphaltenes on minerals and metals has been the subject of several studies in the past; however, most of them were limited to a few solvents or surfaces. Moreover, the interpretation of adsorption behavior was often based on separated asphaltenes rather than asphaltenes in crude oils. Several investigators have studied the adsorption of asphaltenes on various minerals. The shape of isotherms describing asphaltene adsorption depends upon the nature of the solvent containing asphaltenes and the concentration of asphaltenes in the solvent. Langmuir-type isotherms were found in most cases with asphaltene-in-toluene systems and Berea sandstones.6-9 Comparative studies between static and dynamic adsorption of (3) 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. (4) Sheu, E. Y.; Long, Y.; Hamza, H. Asphaltene self-association and precipitation in solventssAC conductivity measurements. In Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007. (5) Freed, D. E.; Lisitza, N. V.; Sen, P. N.; Song, Y.-Q. Molecular composition and dynamics of oils from diffusion measurements. In Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007. (6) Collins, S. H.; Melrose, J. C. Adsorption of asphaltenes and water on reservoir rock. Proceedings of the Society of Petroleum Engineers (SPE) International Symposium on Oilfield and Geothermal Chemistry, Dallas, TX, 1983; pp 249-256. (7) Crocker, M. E.; Marchin, L. M. Wettability and adsorption characteristics of crude oil asphaltene and polar fractions. J. Pet. Technol. 1988, 470–474.
10.1021/ef800616x CCC: $40.75 2009 American Chemical Society Published on Web 12/10/2008
1238 Energy & Fuels, Vol. 23, 2009
Abudu and Goual
Table 1. Physical Properties and SARA Composition of Tensleep Crude Oil28
a
property
value
density at 20 °C (g/cm3) gravity (°API) refractive index at 20 °C molecular weight (g/mol) viscosity (mPa/s) acid number (mg of KOH/g of oil) base number (mg of KOH/g of oil) saturates (wt %) aromatics (wt %) resins (wt %) nC7 asphaltenes (wt %) wax content by DSC (wt %)
0.8684 31.2 1.4876 271 14.3 0.16 0.96 68.3 17.1 9.4 3.2 7.8a
Performed at Western Research Institute.
asphaltenes from toluene onto formation rocks revealed a continuous adsorption of asphaltenes under dynamic conditions.10The dynamic tests also proved the existence of competitive and consecutive processes.9 The presence of clays in mineral rocks renders fluid-solid interactions even more complex and will not be considered in this study. Multilayer adsorption on silica surfaces was reported for several asphaltenes in toluene.11-13 Isotherm inflection was attributed to the adsorption of large aggregates over long periods of time.14,15 Irreversible second-order adsorption kinetics was recorded from absorbance studies.16 Partially reversible adsorption was also observed in similar systems using a quartz crystal microbalance.12,17 The irreversibly adsorbed mass from 1 wt % asphaltenes in toluene was rigidly attached to silica and had a thickness of ca. 6 nm.12 Higher thicknesses (20-300 nm) were found on glass by ellipsometry from 200 to 10 000 mg/L asphaltenes in toluene and increased with time, confirming multilayer formation.13 In studies with other minerals, some reported higher amounts of asphaltenes adsorbed on alumina as compared to calcite and dolomite,8 while others found similar adsorption amounts on quartz, feldspar, and calcite.18 Gonzalez and Moreira19 observed (8) Dubey, S. T.; Waxman, M. H. Asphaltene adsorption and desorption from mineral surfaces. SPE ReserVoir EVal. Eng. 1995, 389–395. (9) Rayes, B. H.; Pernyeszi, T.; Lakatos, I.; To´th, J. Comparative study of asphaltene adsorption on formation rocks under dynamic and static conditions. SPE Paper 80265 Presented at the SPE International Symposium on Oilfield Chemistry, Houston, TX, 2003; pp 1-12. (10) Piro, G.; Canonico, L. B.; Galbariggi, G.; Bertero, L.; Carniani, C. Asphaltene adsorption onto formation rock: An approach to asphaltene formation damage prevention. SPE Prod. Facil. 1996, 1, 156–160. (11) Castillo, J.; Goncalves, S.; Fernandez, A.; Mujica, V. Applications of photothermal displacement spectroscopy to the study of asphaltenes adsorption. Opt. Commun. 1998, 145, 69–75. (12) Hannisdal, A.; Ese, M.-H.; Hemmingsen, P. V.; Sjoblom, J. Particlestabilized emulsions: Effect of heavy crude oil components pre-adsorbed onto stabilizing solids. Colloids Surf., A 2006, 276, 45–58. (13) Labrador, H.; Fernandez, Y.; Tovar, J.; Munoz, R.; Pereira, J. Ellipsometry study of the adsorption of asphaltene films on glass surface. Energy Fuel 2007, 21, 1226–1230. (14) Acevedo, S.; Castillo, J.; Ferna´ndez, A.; Goncalvez, S.; Ranaudo, M. A. A study of multilayer of asphaltenes on glass surfaces by photothermal surface deformation. Relation of this adsorption to aggregate formation in solution. Energy Fuels 1998, 12, 386–390. (15) Acevedo, S.; Ranaudo, M. A.; Garcı´a, C.; Castillo, J.; Ferna´ndez, A.; Caetano, M.; Goncalvez, S. Importance of asphaltene aggregation in solution in determining the adsorption of this sample on mineral surfaces. Colloids Surf., A 2000, 166, 145–152. (16) Acevedo, S.; Ranaudo, M. A.; Garcı´a, C.; Castillo, J.; Fernandez, A. Adsorption of asphaltenes at the toluene-silica interface: A kinetic study. Energy Fuels 2003, 17 (2), 257–261. (17) Dudasova, D.; Silset, A.; Sjo¨blom, J. Quartz crystal microbalance monitoring of asphaltene adsorption/desorption. J. Dispersion Sci. Technol. 2008, 29 (1), 139–146. (18) Gonza´les, G.; Louvisse, A. M. T. Adsorption of asphaltenes and its effect on oil production. SPE Prod. Facil. 1993, 91–96.
a distinct adsorption of asphaltenes on hematite as compared to fluorite. Adsorption isotherms of asphaltenes on quartz, dolomite, calcite, and some pure oxides indicated Freundlichtype adsorption, with inflections characteristic of multilayer adsorption and surface reorientation.20 Langmuir-type isotherms were found with silica, alumina, and some pure oxides, and the amounts adsorbed on silica were usually the highest.17 Very few adsorption studies on minerals exist with asphaltenes in heptane/toluene mixtures (also called heptol). Pernyeszi et al.21 recorded a two-stage isotherm on quartz and reservoir rocks because of the increase in asphaltene aggregation propensity. Dusadova et al.17 reported a Langmuir-type isotherm with amounts higher in heptol (50/50) than in pure toluene. The study of adsorption kinetics of asphaltenes onto metallic surfaces has been documented in the literature. Ekholm et al.22 were the first to apply a quartz crystal microbalance with dissipation (QCM-D) to investigate the adsorption of crude oil and its polar fractions on gold surface. Unlike resins, asphaltenes irreversibly adsorbed in multilayers from toluene and heptol (50/ 50). Viscoelastic effects were also detected in crude oil systems. More recently, Xie and Karan23 used a research quartz crystal microbalance (RQCM) to study the adsorption kinetics in a flow cell of asphaltenes from heptol (50/50) on gold. Asymptotic analysis according to a methodology proposed by Filippov24 was adopted and indicated that the initial adsorption process is controlled by the diffusion of asphaltenes from the bulk solution to the surface. The equivalent diameters of asphaltenes (10-200 ppm in heptol) fell in the range of 300-1200 Å. More recently, Langmuir (type I) isotherms of asphaltenes in toluene generated by RQCM and XPS were analyzed and compared.25 The estimated thickness of adsorbed asphaltene varied between 8 and 12 nm. The adsorption of asphaltenes on powdered metals (stainless steel, iron, and aluminum) was also investigated by means of UV-vis spectrophotometry.26 Langmuir (type I) isotherms were observed, indicating that asphaltenes saturated the available surface area for adsorption. The thicknesses of saturation films from toluene were of the same order of magnitude as those found on minerals. In all cases, the amounts adsorbed on stainless steel were higher than the amounts adsorbed on iron and aluminum. These amounts also increased with increasing heptane/toluene ratios, and the adsorption near the onset of precipitation was almost double that in pure toluene. At a different scale, Wang et al.27 studied the adsorption of supersaturated mixtures of crude oils and n-alkanes on stainlesssteel capillary tubings from pressure drop measurements. (19) Gonza´lez, G.; Moreira, M. B. C. The wettability of mineral surfaces containing adsorbed asphaltenes. Colloids Surf. 1991, 58, 293–302. (20) Marczewski, A. W.; Szymula, M. Adsorption of asphaltenes from toluene on mineral surface. Colloids Surf., A 2002, 208, 259–266. (21) Pernyeszi, T.; Patzko, A.; Berkesi, O.; Dekany, I. Asphaltene adsorption on clays and crude oil reservoir rocks. Colloids Surf., A 1998, 137, 373–384. (22) Ekholm, P.; Blomberg, E.; Claesson, P.; Auflem, I. H.; Sjoblom, J.; Kornfeldt, A. A quartz crystal microbalance study of the adsorption of asphaltenes and resins onto a hydrophilic surface. J. Colloid Interface Sci. 2002, 247, 342–350. (23) Xie, K.; Karan, K. Kinetics and thermodynamics of asphaltene adsorption on metal surfaces: A preliminary study. Energy Fuels 2005, 19 (4), 1252–1260. (24) Filippov, L. K. Kinetic-diffusive-convective adsorption in TIRF flow cells. J. Colloid Interface Sci. 1995, 174 (1), 32–39. (25) Rudrake, A. Investigation of asphaltene-metal interactions. M.S. Thesis, Queen’s University, Ontario, Canada, 2008. (26) Alboudwarej, H.; Pole, D.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W. Adsorption of asphaltenes on metals. Ind. Eng. Chem. Res. 2005, 44, 5585–5592. (27) Wang, J.; Buckley, J. S.; Creek, J. L. Asphaltene deposition on metallic surfaces. J. Dispersion Sci. Technol. 2004, 25 (3), 287–298.
Adsorption of Crude Oil on Surfaces
Energy & Fuels, Vol. 23, 2009 1239
Table 2. Verification of Liquid Loading Equations with 1-Methyl Naphthalenea C (wt %)
2 9 17 a
d23 (g/cm3)
n23 (mPa s)
∆Dliqload (measured) (×10-6)
0.863825
0.545
130.00
0.866895 0.877175 0.887895
0.563 0.614 0.665
∆Dliqload (calculated) (×10-6)
∆fliqload (measured) (Hz)
Toluene 113.00
1-Methyl Naphthalene in Toluene 2.11 2.04 6.83 7.91 13.40 13.60
∆fliqload (calculated) (Hz)
2∆fliqload/ 3∆Dliqload (MHz)
-847 -16.5 -51.0 -102. 0
-15.3 -59.3 -101.6
5.2 5.0 5.1
The third overtone is considered for ∆f and ∆D.
Unexpectedly, the adsorption amounts were found to increase with an increasing n-alkane carbon number. Moreover, the deposits formed from asphaltene solutions were very different from those produced by the parent oil. This paper investigates crude oil-surface interactions through characterization of the adsorption behavior on gold and other surfaces using QCM-D under flow conditions. Measurements are performed in solvents where the degree of asphaltene stability is high (toluene) and poor (n-alkanes). In this way, notable differences can be recorded in the amounts and viscoelastic properties of the adsorbed films. This in turn will help explain the unusual adsorption trends observed in the past with asphaltenes in n-alkanes. This study is structured into five
Figure 1. Changes in frequency and dissipation factor because of liquid loading with 2, 9, and 17 wt % 1-methyl naphthalene in toluene. Rinsing is performed after each concentration to define the baseline shift.
parts. In the first part, liquid loading equations for frequency and dissipation changes are verified with a model chemical in toluene before being applied to crude oil systems. In the second and third parts, adsorption of crude oil in toluene and n-alkanes is measured in terms of adsorption kinetics and isotherms. The D-f plots provide insight into liquid loading effects (implicit concentration) as well as viscoelasticity effects (implicit time) in different solvents. We also investigate the size of asphaltenes at a concentration range where adsorption from toluene is diffusion-controlled. In the last part, adsorption of crude oil on hydrophilic and hydrophobic surfaces allows for the identifica-
Figure 2. Adsorption kinetics of 1 and 10 wt % Tensleep crude oil in toluene.
1240 Energy & Fuels, Vol. 23, 2009
Abudu and Goual Table 3. Adsorption of Crude Oil in Toluene
C (wt %)
d23 (g/cm3)
n23 (mPa s)
∆D (measured) (×10-6)
0
0.86383
0.545
130.00
0.5 1 3 5 8 10 20 30
0.86380 0.86398 0.86402 0.86400 0.86407 0.86401 0.86407 0.86426
0.559 0.574 0.584 0.612 0.657 0.681 0.798 1.010
error %
1.10 1.84 3.75 6.18 9.98 11.60 23.50 40.00
∆Dliqload (calculated) (×10-6)
∆f (measured) (Hz)
error %
Pure Toluene 113.00 6 25 14 15 15 10 15 15
Crude Oil in Toluene 1.54 -67.01 3.01 -81.65 3.94 -85.49 6.75 -113.7 11.10 -146.5 13.40 -155.7 23.70 -240.0 40.90 -366.0
tion of interaction forces involved at asphaltene/solid interfaces under different stability conditions in the medium. 2. Materials and Methods 2.1. Materials. Materials include Millipore Q water, toluene (99.9% HPLC-grade, Fisher Scientific), n-heptane (98.7% HPLCgrade, Fisher Scientific), n-decane (99%, Alfa Aesar), n-dodecane (99+%, Alfa Aesar), acetone (99.4% Ultra Resi-Analyzed, J.T. Baker), 1-methyl naphthalene (97% GC, Fluka), and Tensleep crude oil from Wyoming (see Table 1 for physical properties).28 The oil
∆fliqload (calculated) (Hz)
∆fads (Hz)
∆Γ (ng/cm2)
h (nm)
-55.43 -59.06 -55.93 -63.06 -63.27 -55.53 -62.16 -59.02
327.06 348.46 329.99 372.07 373.27 327.62 366.72 348.19
3.27 3.48 3.30 3.72 3.73 3.28 3.67 3.48
-847.11 9 14 9 3 5 3 5 5
-11.58 -22.59 -29.56 -50.64 -83.24 -100.17 -177.85 -306.99
was filtered with a 20-25 µm pore-size Whatman filter paper (Fisher Scientific) prior to measurements. The onset of asphaltene precipitation is measured by mixing crude oil in heptol solutions with an increasing heptane/toluene ratio and recording the ratio at which precipitation starts from filtration with 0.2 µm pore-size Whatman filter paper (Fisher Scientific). For Tensleep oil, we find that the onset of precipitation occurs at 50:50 (vol/vol) heptane/ toluene. 2.2. Density and Viscosity. Density, F, is measured with DMA 4500 laboratory density meter (Anton Paar, Ashland, VA), with a repeatability of 0.000 01 g/cm3. The principle of measurement is based on the oscillating tube method and requires calibration with
Figure 3. Adsorption isotherm of Tensleep crude oil in toluene. Figure 5. Adsorption kinetics of 1 wt % crude oil in toluene at 50, 75, and 100 µL/min flow rates.
Figure 4. D-f plot for Tensleep crude oil in toluene. The increase in the frequency shift with time suggests that the films are becoming more rigid, especially at lower concentrations.
Figure 6. Early time analysis for 1 wt % crude oil in toluene.
Adsorption of Crude Oil on Surfaces
Energy & Fuels, Vol. 23, 2009 1241
Table 4. Effect of the Crude Oil Concentration in Toluene on the Size of Adsorbing Particles C (wt %) 0.5 0.5 0.5 1 1 1 3 3 3
Casph (kg/m3) 0.13913 0.27827 0.83478
slope (kg m-2 s-1/2) 10-6
4.40 × 3.50 × 10-6 5.67 × 10-6 9.60 × 10-6 8.62 × 10-6 1.29 × 10-6 1.13 × 10-5 1.61 × 10-5 1.65 × 10-5
D (m2/s)
d (nm)
7.9 × 10-10 5.0 × 10-10 1.3 × 10-9 9.3 × 10-10 7.5 × 10-10 1.7 × 10-9 1.4 × 10-10 2.9 × 10-10 3.1 × 10-10
1.01 1.60 0.61 0.85 1.06 0.47 5.58 2.74 2.60
air and water. A built-in Peltier thermostat maintains the measuring temperature at 23 ( 0.01 °C. Dynamic viscosity, η, is calculated from kinematic viscosity measurements using calibrated capillary viscometers (size 25/50, Cannon Fenske, State College, PA) according to American Society for Testing and Materials (ASTM) D 445. The method involves measuring the time of flow of Newtonian liquids in a capillary with an accuracy of 1 s. The viscometer is immersed in a thermostatic bath set at 23 ( 0.1 °C. 2.3. QCM-D. The E4 model (Q-sense AB, Sweden) was used with a four-sensor chamber containing four flow modules mounted in parallel configuration. The flow modules are made of aluminum and titanium, with a height of 37 mm, width of 35 mm, and depth of 63 mm. AT-cut crystals (5 MHz) with a diameter of 14 mm are mounted inside the flow modules. The volume of liquid above the sensor crystal and in the flow channel is 40 and 100 µL, respectively. We used highly resistant o-rings (Chemraz), sealing
Figure 7. Adsorption kinetics of 1 and 10 wt % Tensleep crude oil in nC7.
gaskets (Kalrez), and pump tubings (Tygon). The instrument monitors in real time the series resonant frequency and dissipation or damping response of the freely oscillating crystal by numerically curve fitting the decay voltage to an exponentially damped sinusoidal when the power is disconnected. 2.3.1. Principle of Operation. There are three main contributions to the frequency and dissipation of the crystals because of the adsorption of a rigid film: (1) mass loading,29 (2) liquid loading,30 and (3) liquid trapping.31 The equations describing each contribution are Mass loading:
∆fads ) -
2nf02 ∆m n∆Γ )Fqνq A C
(1)
Liquid loading:
∆Dliqload )
3/2
n f0 ( Fη - F η ) π Fqνq √ l l √ s s
(2)
1/2 1 2f0 (√Flηl - √Fsηs) √nπ Fqνq
(3)
2f02 h (F - Fs) Fqνq l l
(4)
∆fliqload ) -
Liquid trapping:
∆fliqtrap ) -
where f0 is the fundamental resonant frequency (f0 ) 5 × 106 Hz), n is the overtone number (n ) fn/f0 ) 1, 3, 5, 7, and 11), ∆m is the adsorbed mass, ∆Γ is the adsorbed mass density, A is the active area of the crystal (0.785 cm2), Fq is the specific density of quartz (2650 kg/m3), νq is the shear wave velocity in quartz (3340 m/s), νq ) (µq/Fq) ) 2f0hq, µq is the shear modulus of quartz (2.947 × 1010 Pa), hq is the thickness of the quartz crystal (3.37 × 10-4 m), hl is the thickness of trapped liquid, and C is the constant of the quartz crystal (17.7 ng Hz-1 cm-2 for a 5 MHz crystal). Subscripts s and l refer to the solvent and liquid mixtures, respectively. Changes in the resonant frequency of the crystal ascribed to mass loading in the gas phase can be correlated to changes in adsorbed mass density according to the Sauerbrey equation29 provided that the adlayer is thin and rigid and does not slip at the interface. When operated in a Newtonian liquid phase, the liquid becomes coupled to the crystal oscillation and the increase in density and/or viscosity of the medium leads to a rise in both frequency (in absolute values) and dissipation factor.30 This liquid loading behavior manifests by a linear variation of ∆D versus ∆f, hereafter referred to as D-f plots. The D-f plot offers the advantage of eliminating the concentration as an explicit parameter. Also, the slopes provide information about the kinetic regimes and conformational changes and how these features evolve over time. Additional shifts may arise with surface roughness because of liquid trapping by interfacial cavities and pores.32,33 This contribution is usually small, and liquid trapping could be neglected by using smooth surfaces. An interesting observation is that - 2∆f/∆D ) nf0 is always valid when liquid (28) Lord, D. L.; Buckley, J. S. An AFM study of nanoscale features affecting at crude oil-water-mica interfaces. Colloids Surf., A 2002, 206, 531–546. (29) Sauerbrey, G. Verwendung von schwingquarzen zur wagung dunner schichten und zur mikrowagung. Z. Phys. 1959, 155, 206–222. (30) Kanazawa, K. K.; Gordon, J. G. Frequency of a quartz microbalance in contact with liquid. Anal. Chem. 1985, 57, 1770–1771. (31) Rodahl, M.; Kasemo, B. On the measurement of thin liquid overlayers with the quartz crystal microbalance. Sens. Actuators, A 1996, 54, 448–462. (32) Martin, S. J.; Granstaff, V. E.; Frye, G. C. Characterization of a quartz crystal microbalance with simultaneous mass and liquid loading. Anal. Chem. 1991, 63, 2272–2281. (33) Cho, N.-J.; D’Amour, J. N.; Stalgren, J.; Knoll, W.; Kanazawa, K.; Frank, C. W. Quartz resonator signatures under Newtonian liquid loading for initial instrument check. J. Colloid Interface Sci. 2007, 315, 248–254.
1242 Energy & Fuels, Vol. 23, 2009
Abudu and Goual Table 5. Adsorption of Crude Oil in n-Alkanes ∆Dliqload (calculated) (×10-6)
∆f (measured) (Hz)
C (wt %)
d23 (g/cm3)
n23 (mPa s)
∆D (measured) (×10-6)
0
0.68157
0.39
98.90
84.40 Crude Oil in nC7 4.10 -95.85 8.75 -128.65 14.90 -184.98 29.60 -270.08
error %
error %
Pure nC7
1 5 10 20
0.68326 0.69010 0.69977 0.71585
0.42 0.46 0.52 0.67
3.24 7.41 13.10 25.10
0
0.72770
0.82
146.00
1 5 10 20
0.72908 0.73367 0.74066 0.75333
0.88 0.92 1.04 1.36
5.19 7.82 18.60 39.20
0
0.74699
1.33
182.00
1 5 10 20
0.74771 0.75194 0.75791 0.76873
1.40 1.49 1.69 2.02
3.87 13.90 21.20 38.30
27 5 7 7
5 5 5 5
12 7 5 5
20 5 5 5
loading occurs alone. Deviations from this equation indicate the existence of liquid trapping effects.31 In some systems, the amount of liquid associated with the adsorbed film is significant and viscoelastic effects become important. In this case, viscoelastic loading can be characterized by measuring the resonance curves at multiple frequencies according to the Voight model.34 This model provides a general solution to the wave equation describing the propagation and decay of acoustic shear waves in a single uniform viscoelastic adsorbed film in contact with a semi-infinite bulk Newtonian liquid under no-slip conditions.34 In petroleum fluids, viscoelastic effects may become significant at longer times, as will be shown in this study. 2.3.2. Procedure. Clean crystals are first mounted inside the flow modules, and then the system is assembled. The chamber is temperature-controlled at 23 ( 0.02 °C. The crystals are first checked in air and then in liquid solvent. The liquid is introduced through a 60 cm long tubing, with a diameter of 0.75 mm. The system is left to stabilize for an hour to establish the baseline of the measurement. When using n-alkanes as a solvent, samples are sonicated for 5 min prior to injection. Samples are continuously mixed on a stirring plate and then introduced by using a peristaltic pump at a constant rate of 75 µL/min. It takes about 4 min to observe changes in frequency and dissipation factor from the time
Figure 8. Adsorption isotherm of Tensleep crude oil in n-alkanes after 3.5 h.
∆Γ (ng/cm2)
h (nm)
-30.74 -65.61 -112.12 -221.37
-65.12 -63.04 -72.87 -48.72
384.19 371.94 429.72 287.42
3.84 3.72 4.30 2.87
-90.80 -114.90 -107.33 -88.76
535.75 677.90 633.25 523.66
5.36 6.78 6.33 5.24
-140.56 -152.41 -114.19 -120.31
829.3 899.21 673.74 709.83
8.29 8.99 6.74 7.10
-955.11 5 7 5 5
Pure nC12 164.00 Crude Oil in nC12 4.43 -173.82 10.30 -229.71 22.60 -283.59 41.10 -428.76
∆fads (Hz)
-633.10
Pure nC10 127.00 Crude Oil in nC10 4.22 -122.46 7.82 -189.57 17.40 -237.85 39.0 -381.37
∆fliqload (calculated) (Hz)
-31.66 -74.67 -130.52 -292.62 -1229.56
5 23 10 5
-33.26 -77.30 -169.40 -308.45
the tubing is transferred from the solvent to the solution. The instrument records all 11 harmonics from each sensor crystal, and
Figure 9. (a) D-f plot for Tensleep crude oil in n-alkanes after 3.5 h. Each point corresponds to a different crude oil concentration. (b) D-f plot for Tensleep crude oil in nC12. The increase in ∆f and ∆D with time suggests that the films are becoming more viscoelastic.
Adsorption of Crude Oil on Surfaces
Energy & Fuels, Vol. 23, 2009 1243
Figure 10. AFM images of asphaltene films adsorbed on gold from 1 wt % crude oil in n-alkanes.
the third harmonic is considered in this study. The data are processed with Q-tools software from Q-sense. The mass sensitivity is 1.8 ng/cm2 (0.1 Hz), and typical detectable change in D is 0.1 × 10-6. To investigate the effect of surface chemistry, we used crystals coated with gold, silica, stainless steel, and hydrophobic polystyrene, all available from Q-sense. 2.3.3. Size of Adsorbing Species. According to Ward and Tordai,35 the initial rate of change of surface mass density is a linear function of the inverse of the square root of time, as given by
d(Γ) D )C dt πt
1/2
( )
(5)
where C is the concentration of species in the bulk solution, Γ is the adsorbed mass density, D is the diffusion coefficient, and t is time. This theory was later verified by Filippov24 for diffusive adsorption in flow cells. The concentration of species accumulated at the interface at any time is obtained by integrating the above equation. The apparent diffusion coefficients may be calculated from the initial slopes, S, of ∆Γ/∆Γ0 versus t curves, according to
( Dπ )
S ) 2C
1/2
(6)
assuming particles with a spherical shape; their diameters d are calculated from the Stokes-Einstein equation for steady-state flow
d)
k BT 3πηsD
(7)
where kB is the Boltzmann constant (1.380 650 3 × 10-23 m2 kg s-2 K-1) and T is the temperature (296 K). 2.4. Cleaning Procedure of QCM-D Crystals and Flow Modules. A chemical treatment procedure is used to remove organic contamination from the gold crystal surfaces. The dry crystals go
first through UV light ozone (UVO) for about 5-10 min, and then they are placed in a crystal holder and immersed in a heated cleaning solution for 5 min. The cleaning solution consists of a 5:1:1 (15: 3:3 mL) mixture of mQ water, ammonia, and hydrogen peroxide heated to a temperature of 75 °C with a thermostatic bath. After 5 min, the crystals are rinsed with mQ water 3 times and then dried with nitrogen gas. The dried crystals are placed once again in the UV lamp for UV light ozone (UVO) treatment for about 5-10 min. Some crystals are sonicated for 1 min to remove any residuals left on the surface, then rinsed with mQ water, and dried with nitrogen. The flow modules are also cleaned after each test. The modules are flushed with toluene, dried with nitrogen, then cleaned with 2 wt % sodium dodecyl sulfate (SDS) in water, then rinsed with mQ water, and dried with air and nitrogen. 2.5. Atomic Force Microscopy (AFM). AFM imaging is performed using a NanoScope multimode atomic force microscope (Digital Instruments, Culver City, CA). Images are captured in air at room temperature using tapping mode. These images are scanned at 5 × 5 µm, and the image size is gradually reduced to 500 × 500 nm. An Olympus tapping mode cantilever with a length of 125 µm, a resonant frequency between 261.8 and 316.3 kHz, and a spring constant of ∼42 N/m was used. Both amplitude and height signal images are captured, and the diameters of adsorbing species are analyzed using the NanoScope IIIa software version 4.22r2.36 Scan rates are set at 0.9-1.2 Hz range depending upon the scale of observation. Substrate Preparation. The substrates are fixed by a piece of double-sided adhesive tape on a mica sample magnetically attached to the AFM stage. The diameter and height distribution of the adsorbing species are analyzed using the section and the depth (34) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Viscoelastic acoustic response of layered polymer films at fluid solid interfaces: Continuum mechanics approach. Phys. Scr. 1999, 59, 391–396.
1244 Energy & Fuels, Vol. 23, 2009
Abudu and Goual Table 6. XPS Peak-Fitted Parameters of C 1s, S 2p, N 1s, and O 1s of Adsorbed Films on Gold from 1 wt % Crude Oil in n-Alkanes element binding energy (eV) fwhm (eV) atomic % chemical bond C 1s S 2p N 1s O 1s
C 1s S 2p N 1s O 1s
C 1s S 2p N 1s O 1s
285.4 285.6 164.7 166.1 400.2 398.7 532.6 533.2 534.4
nC7 1.11 0.39 1.40 0.98 1.00 0.20 1.14 0.66 1.63
285.5 285.8 164.7 165.2 400.1 398.8 533.1 534.4
nC10 1.06 1.10 0.71 2.88 1.00 5.00 1.23 0.56
285.4 286.8 164.6 165.0 400.1 398.9 532.8 533.5
nC12 1.25 0.94 1.61 5.60 3.80 3.71 1.15 2.49
98.8 0.40
