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Determination of Charge on Asphaltene Nanoaggregates in Air Using Electrostatic Force Microscopy Ravi Gaikwad, Aharnish Hande, Siddhartha Das, Sushanta K. Mitra, and Thomas Thundat Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503968v • Publication Date (Web): 17 Dec 2014 Downloaded from http://pubs.acs.org on December 18, 2014
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Determination of Charge on Asphaltene Nano-aggregates in Air Using Electrostatic Force Microscopy Ravi Gaikwad,1 Aharnish Hande,1 Siddhartha Das,2 Sushanta K. Mitra,3 and Thomas Thundat1 1
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada 2
Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA 3
Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, Canada.
In this paper, we provide measurement of charge of asphaltene nano-aggregates in air using electrostatic force microscopy. We obtain the average surface charge density of the nano-aggregates as 43.7 nC/cm2. Among the different aspects of asphaltene, one of the least known ones is its charge and the effect of solvent and compositional variability (of asphaltene) in dictating this charge. For aqueous systems, asphaltene charge demonstrates a strong dependence on the pH and the salt concentration, indicating that a possible ionization of the surface groups leads to this charging. On the contrary, for asphaltene in non-polar media (e.g., toluene and heptane) it is believed that asphaltene native charge is central in dictating this charging. This native charge is the solvent-independent charge or the asphaltene charge in air. Our measurements, therefore, provide the first direct quantification (i.e., a quantification of charge not from the measurement of the asphaltene mobilities, which in turn requires specification of the non-uniform asphaltene size distribution) of this asphaltene native charge by conducting the measurements in air. Same measurements in a solvent may introduce a solvent-dependent value, thereby forbidding not only the exact quantification of this native charge, but also the understanding of the specific role of the solvent. This measurement, therefore, will provide a useful starting point to
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quantify the mechanism of asphaltene charging in non-polar solvents with important ramifications in deciphering the role of asphaltene in transport and handling of crude and heavy oils.
I.
Introduction:
Understanding the micro-nanoscale dynamics, characteristics and morphologies of petroleum asphaltenes hold the key for ensuring better recovery, transport, and usability of crude and heavy oil.1,2 Therefore, asphaltene research in a way has become synonymous with high priority research on alternative forms of energy. Thus over the years there have been sustained efforts to quantify several fundamental aspects of aspahltene. Some of the better-established ideas regarding asphaltene behaviour include its transition from molecular-to-nano-aggregated-to-flocculated states in the appropriate conditions,2–4 its “solubility class” nature making it soluble in solvents like toluene and insoluble in solvents like n-alkane,3,5,6 formation of fractal structure whose dimensions depend on the nature of the solvent,2,7,8 etc. On the contrary, the issue of charge on asphaltene molecules has remained extremely debated and controversial. There are two primary reasons for such a controversy. Firstly, the charging of the asphaltene molecules is largely dictated by the exact molecular structure of the asphaltene, which is still debated and has been known to vary drastically depending on the source of asphaltene.3,5,9–12 Secondly, it is still not known for certain how the different functional groups of the apshaltene molecules behave in presence of different solvents, which is essential to quantify asphaltene charge in different media. Most of these studies attempting to resolve this issue have predicted asphaltene surface charge indirectly based on charge-dependent phenomenon such as electrophoretic transport of the asphaltene,13–15 which provides implicit quantification of parameters such as zeta potential and surface charge density16 [see Das et. al (Ref. 18) for a detailed review of the relevant literature]. These studies primarily include quantification of asphlatene charge in aqueous medium or
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water-oil interfaces, and these results predict drastically varying asphaltene charge (from very highly positive to very highly negative) as a function of buffer pH.17 Challenges with these studies stem from the fact that they inevitably necessitate assumption about the electric double layer (EDL) structure around the asphaltene aggregates (without the knowledge of the EDL structure it is not possible to quantify the electrophoretic mobility). Unfortunately with the exception of the recent study by Das et al.18 there has been very little theoretical modelling of electric double layer in charging of asphaltene. The problem regarding understanding of asphaltene charge in non-polar media, such as toluene or heptane, is even more serious. Unlike in an aqueous medium, the charging of asphaltene in non-polar solutions cannot be attributed due to a mechanism controlled by the ionization of the asphaltene surface groups. On the contrary, the charging of asphaletene in the non-polar media has been conjectured primarily due to the native charge of the asphaltene,19,20 which is the solvent-independent charge of the asphaltene, or in other words, the charge of asphaltene in air. This charge is the outcome of the π-bonding in asphaltene – this π-bonding is also responsible for the fact that asphaltene is insoluble in all solvents except aromatic compounds,21 Quantification of this asphaltene charge in air is crucial in understanding the charging of asphaltene in non-polar media, which in turn is central to virtually all the issues related to role of asphaltene in crude oil and heavy oil recovery. The most important question related to this native charge is: How can one measure it? Quite remarkably, the native charge of asphaltene has invariably been quantified by measuring the electrophoretic mobility of asphaltene in the non-polar medium itself.19,22,23 There are two key limitations to this approach. First and foremost this is an indirect charge quantification procedure in a sense that to obtain the charge from the electrophoretic mobility measurement one will also require the quantification of the asphaltene radius. This is often difficult, given the fact that asphaltene is typically in a poly-dispersed state in an organic solvent.7 The second issue is the fact that this procedure forbids pinpointing of any possible solvent-specific influence on asphaltene charge. In other words, in case the asphaltene charge
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results from the superposition of the native charge and this solvent-specific influence, none of the two effects can be separately quantified from this measurement of electrophoretic mobility in a solvent. In this context, it is vitally important to have a study that provides a direct quantification of this apshaltene native charge in air (i.e., when the effect of any solvent is completely absent). To the best of our knowledge, while there is perhaps only one study that provides such direct quantification24, there is absolutely no study that provides the quantification in air. In this study, we provide the first direct quantification of asphaltene charge in air – thus our study provides possibly the first direct quantification of the highly sought asphaltene native charge, without necessitating indirect procedure of electrophoretic mobility measurements. We use Electrostatic Force Microscopy (EFM)25–29 to determine the density of asphaltene native charge to approximately 43.7 nC/cm2. This is the central result of this paper. EFM has been known to provide quantification of surface charge for different samples;28,30–32 however, this is possibly the first study where EFM is applied to measure asphaltene surface charge in air. The asphaltene samples are in aggregated state, obtained from evaporation of toluene drop containing dissolved asphaltene. Such evaporation-mediated asphaltene aggregates has been previously employed to illustrate many effects, such as formation of fractals,33–35 temperature-dependent new aggregation dynamics,36 etc.
II.
Methods and Materials:
Crude Athabasca asphaltene sample obtained from IOSI (Institute for Oil Sands Innovation, University of Alberta) were converted into 100% pure asphaltene in powdered form following several steps described in our previous paper.36 The pure asphaltene was then dissolved in toluene and a drop of the solution was placed on a silicon oxide wafer (obtained from University wafers, Boston, MA) and evaporated. Post-evaporation, after all the toluene had disappeared, we were left with precipitated solid asphaltene
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sample adhering to the SiO2 wafer. Details of the process of evaporation-triggered formation of solid asphaltene sample are described in reference 36. The SiO2 wafer with evaporation-precipitated asphaltene was subsequently mounted on a stage for imaging by Atomic Force Microscope.
Figure 1: Schematic of the sample preparation and steps involved in the AFM measurements: (a) A SiO2 wafer (1 cm2 in size) is cleaned with ethanol and dried with N2 gas, (b) a 10 μL drop of toluene containing 0.005wt% of dissolved 100% pure asphaltene is deposited on the cleaned SiO2 wafer, (c) toluene is allowed to evaporate for 30 min at room temperature resulting in solid asphaltene aggregates on SiO2 and (d) the sample is mounted on a stage for AFM analysis for simultaneous topography and EFM data collection. The inset shows a model of the electrostatic interaction between the AFM tip (of radius a) and the aspahltene sample (assumed to be a circular disc of radius R). For the EFM signal, the AFM is in the lift mode with the tip at a distance z above the sample. The sample is assumed to be negatively charged, with uniform surface charge density of σ (thus the net charge for an asphaltene element of area drdx, located at a distance r from the point vertically below the AFM tip, is σdrdx). The sample was grounded and a bias of 0.5V was applied to the conducting AFM probe during the lift mode. A schematic of the procedure is shown in Figure 1. In this configuration, topographic and EFM imaging were performed in the ‘Tapping mode’ and the ‘Interleave (Lift) mode’ using Bruker Icon AFM (Santa Barbara, CA) system. Electrically conducting SCM-PIT probe (Bruker, Santa Barbara, CA) with
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resonance frequency 64.1 KHz, spring constant of 2.8N/m, tip of radius of curvature (a) of 20nm, and a quality factor of approximately 200 was used as the probe. The topography and the EFM images were processed using Nanoscope analysis software (V1.40, Bruker)
III.
Results and Discussions:
Figure 2: (a) AFM topography of asphaltene aggregates and (b) Corresponding EFM image at a lift height of 35 nm. (c) and (d) provide the cross section of the topography and EFM at the same location corresponding to the white horizontal line shown in (a) and (b) respectively
Figures 2a and 2b show the representative qualitative topography and EFM images of the asphaltene aggregates obtained at a lift height of z=35 nm. The topography represents a fractal-like structure and is very similar to the one obtained in our previous study, where asphaltene aggregation is studied as a
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function of temperature.36 Electrostatic signal is detected by the AFM probe only at locations where asphaltene is present. As a result, the EFM profile of the aggregates (expressed in terms of the corresponding frequency shift) is commensurate with the topographic profile; for example peaks in the topography image corresponds to the valleys (indicating maximum frequency shift of the AFM cantilever) in the EFM profile.
Figure 3: Plot of experimental data for the variation of frequency shift with lift height z (in nm), corresponding to a tip bias of 0.5 V. The red bold line shows the theoretical fit to the experimental data using the expression
[see eqs.(5,6)], where
. Best fit
was obtained for R=62 nm (R is the asphaltene disc radius) and M=15.62 Hz. For these parameters (R=62 nm and M=15.62 Hz), we get the best value of coefficient of determination (equal to 0.9521), and this allows to choose the specified values of R and M. Using this value of M, we get the charge density of 43.7 nC/cm2 [using a=20 nm, f=64086.51 Hz, k=2.8 N/m and ε0=8.85×10−12 C/(V-m)]
The frequency shift obtained from the EFM measurements can be related to the surface charge as illustrated below. Let us consider that the AFM tip (of radius a) is at a height z from the asphaltene
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sample (see the inset and caption of Fig. 1), which is assumed to be a circular disc (of radius R) of uniform negative surface charge density of σ. Consider an asphaltene element of area drdx, located at a distance r from the point on the asphaltene sample that is vertically below the AFM tip. This element contains a net charge of dq=σdrdx. This element, therefore, will induce a charge dq’ at the tip of the AFM (of radius a), such that dq′ = − ( dq )
a
(r
2
+z
)
2 1/2
a
= − (σ drdx )
(r
2
+z
)
2 1/2
.30 Here we assume that
the charge is localized at the tip, as has been considered for metallic AFM tips in other studies.28 Therefore, the total induced charge (q’) on the AFM tip due to the entire asphaltene sample will be
q′ = ∫
q′
0
R a dq ′ = −σ ∫ 0 r 2 + z2
(
)
1/2
dx dr = −2πσ a R 2 + z 2
(∫ ) 2π r
0
(
)
1/2
− z .
(1)
Consequently, the vertically downward force on the AFM tip due to the electrostatic interactions of this net induced charge (on the AFM tip) and the charge on the asphaltene sample can be obtained as:
F
F = ∫ dF = 0
σ z q′ 1 R ( 2πσ rdr ) ( q′ ) cosθ = ∫ 2 2 4πε 0 0 2ε 0 r +z
(
)
∫
R
0
(r
( rdr ) 2
+ z2
)
3/2
π aσ 2 R 2 + 2z 2 σ q′ z 1− , = = − 2z 2ε 0 ( R 2 + z 2 )1/2 ε 0 ( R 2 + z 2 )1/2
(2)
where ε0 is the permittivity of air. The gradient of the force in the vertical direction, therefore, can be computed as:
dF π aσ 2 4 = dz ε 0 ( R / z ) 1+ ( z / R)2
{
}
1/2
−
1+ 2 ( z / R)
{
2
( R / z) 1+ ( z / R )
8
}
2 3/2
− 2 .
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Also, we can relate the magnitude of the gradient of the force to the magnitude of frequency shift ( ∆f ) as:28,37
∆f dF = 2k , f dz
(4)
where k is the cantilever spring constant and ݂ is the resonance frequency of the cantilever. Therefore, from eqs.(3) and (4) we can get the charge density, in terms of the frequency shift, as 1/2
2kε ∆f 1 σ = 0 , π a f g ( z / R)
(5)
where g(z/R) can be expressed as:
g ( z / R) =
4
( R / z ) {1+ ( z / R)2 }
1/2
−
1+ 2 ( z / R )
2
( R / z ) {1+ ( z / R)2 }
3/2
− 2.
(6)
Fig. 3 provides the experimental result for the variation of the ∆f with z/R corresponding to a tip bias of +0.5 V. We fit this experimental data with the theoretical expressions [see eqs.(5,6)], and from the best fit, we get the magnitude of the charge density as 43.7 nC/cm2. In order to quantify the best fit, we computed the Coefficient of Determination parameter ( CDR2 ) expressed as: n
CDR2 = 1−
∑ ( ∆f ) i=1
e,i
− ( ∆f )c,i
2
1 n ∑ ( ∆f )e,i − n ∑ ( ∆f )e,i i=1 i=1 n
2
(7)
.
In eq.(7), n is the total number of lift height values (z) where the frequency shift ∆f has been obtained experimentally, ( ∆f )e,i is the mean value of the experimentally obtained frequency shift at a given lift
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height value (zi) and ( ∆f )c,i is the value of the frequency shift at the same zi obtained from the equations [eqs.(5,6)]. The idea is to find that combination of the fitting parameters M and R, which provides the highest (i.e., closest but always less than unity) value of CDR2 . We find that for R=62 nm and M=15.62 the corresponding value of CDR2 is 0.9521. From this value of M, we calculate the average charge density as 43.7 nC/cm2. In order to ensure that the value of the obtained charge density is independent of the polarity of the tip bias, we conducted experiments with tip bias of -0.5 V: the resulting surface charge density was 39.5 nC/cm2, i.e., the value was quite similar to the value with tip bias of 0.5 V. Ideally, in EFM the tip is ଵ ௗ
sensitive to the gradient of the capacitive attractive force (ܨ = ଶ ௗ௭ ܸ ଶ) associated with the tipsubstrate capacitance C.38,39 The tip bias is applied to increase the sensitivity of the EFM signal and to minimize additional contribution from capacitive forces arising from the SiO2 substrate.30 This is evident from the experimental data shown in Figure 3. In EFM the charge, capacitive and image charge force gradients compete with each other; but in case of asphaltene which exhibits intrinsic charges, the EFM signals predominate over the image charge contributions which is proportional to Q2 and hence to the charge density σ as shown in equation 5. This signal is slightly enhanced when the charged sample is prepared on a good insulator like SiO2. This can be seen from the increase in the surface charge density of 43.7 nC/cm2 on SiO2 from 39.4 nC/cm2 as observed on Si wafer with native oxide of about 2nm. This charge density can be quantified as the density of native charge of the asphaltene sample, which as discussed previously, is important in characterizing asphaltene in various non-polar media. Please note that the charge determination through the present procedure only provides the magnitude of the charge, and not the sign. We have discussed in the Introduction the relevance of the present measurements – they are useful to quantify the asphaltene native charge, which in turn, is vitally important to quantify the
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asphaltene dynamics in non-polar media. The present measurements are in air. However, the basic measurement principle employed here is equally valid even if the asphaltene sample is in a solvent which does not dissolve asphaltene. This implies that in a solvent where asphaltene is not completely dissolved and maintains a solid aggregated form strongly adhered to the substrate (this ensures the existence of a solvent-asphaltene interface for a finite time), the same frequency-response-based approach of the EFM may be implemented to measure the asphaltene surface charge if we employ polymer shielded probes with better tip dynamics in liquid medium. Of course, there will be certain modifications in order to account for the solvent-specific effects. For example, in polar solvent like water, presence of ions in the bulk liquid will lead to the development of an electric double layer at the asphaltene-water interface.18 This double layer will screen the asphaltene surface charge over the distance equal to the Debye length, which typically varies from 1 to 100 nm (depending on the bulk ion concentration). Therefore, if the Debye layer thickness is smaller than the mean height of the AFM tip from the asphaltene sample, the AFM tip will not sense any charge of the asphaltene sample (since all the charge has been screened within the Debye layer). Hence, in order to ensure that the proposed method becomes applicable to polar solvents like water, one must choose the mean AFM-tip-sample separation distance in a manner such that it is substantially smaller than the corresponding Debye layer thickness. On the other hand, for non-polar solvents one needs to account for issues such as adhesion of solid form of asphaltene on the substrate and extreme volatility of the solvent. The next important issue is to compare the obtained value of the asphaltene surface charge density with those reported previously. Asphaltene zeta potential and charge density have typically been measured in aqueous systems – in such systems, there is unanimity about the governing role of ionization of asphaltene surface charges, thereby yielding zeta potential and surface charge density values that are functions of the pH. Szymula et al.17 provided direct measurement of the surface charge density: they reported that depending on the salt concentration, the asphaltene surface charge density
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can vary from +10 µC/cm2 at pH=4 to −45 µC/cm2 at pH=10. There are also other studies16 that provided zeta potential values for the asphaltene as a function of buffer pH and electrolyte salt concentration. Employing the condition,40 σ (ζ ) =
eζ 2 ε 0 ε r k BT κ sinh (here it is assumed that the asphaltene e 2kBT
surface charges are monovalent; also kB is the Boltzmann constant, T is the temperature in Kelvin, e is the electronic charge, κ is the inverse of the EDL thickness calculated on the basis of the concentration of electrolyte salt), we can get a somewhat lesser surface charge density values of ± 0.5 µC/cm2. Therefore, our estimate of the asphaltene native charge density is substantially smaller than the charge density values witnessed in polar media. Hence one can neglect this native charge for aqueous systems; however, as has been discussed in details before, this native charge becomes important for asphaltene charging in non-polar media.
Conclusions: In this paper, we have provided the first possible direct determination of asphlatene native charge in air by the use of EFM. It is well recognized that this native charge is central to dictating the charge dynamics of asphaltene in non-polar media. The state of the art in quantifying this native charge is an indirect procedure of quantifying the asphaltene electrophoretic mobility in the non-polar media itself – the associated limitations are the requirement of separate quantification of the size distribution of asphaltene in a possible polydispersed state and the inability to segregate the native charge from any possible solvent-specific charging. In this light, the direct quantification capability of the proposed method is a significant step forward towards deciphering fundamental characteristics of asphaltene. Author information The authors declare no competing financial interest.
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Corresponding Author *(Ravi Gaikwad – [email protected]) Acknowledgements: This work was supported by the Canadian Excellence Research Chair (CERC) program for the Oil sands at the University of Alberta. We would also like to thank members of our Nano-interfaces and molecular engineering (NIME) group for their fruitful discussion and feedbacks. References:
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Hashmi, S. M.; Firoozabadi, A. Tuning Size and Electrostatics in Non-Polar Colloidal Asphaltene Suspensions by Polymeric Adsorption. Soft Matter 2011, 7, 8384.
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Hashmi, S. M.; Firoozabadi, A. Controlling Nonpolar Colloidal Asphaltene Aggregation by Electrostatic Repulsion. Energy & Fuels 2012, 26, 4438–4444.
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Drelich, J.; Long, J.; Yeung, A. Determining Surface Potential of the Bitumen-Water Interface at Nanoscale Resolution Using Atomic Force Microscopy. Can. J. Chem. Eng. 2008, 85, 625–634.
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Leng, Y.; Williams, C. C. Electrostatic Characterization of Biological and Polymeric Surfaces by Electrostatic Force Microscopy. Colloids Surfaces A Physicochem. Eng. Asp. 1994, 93, 335–341.
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Martin-Olmos, C.; Stieg, A. Z.; Gimzewski, J. K. Electrostatic Force Microscopy as a Broadly Applicable Method for Characterizing Pyroelectric Materials. Nanotechnology 2012, 23, 235701.
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Qiu, X. H.; Qi, G. C.; Yang, Y. L.; Wang, C. Electrostatic Characteristics of Nanostructures Investigated Using Electric Force Microscopy. J. Solid State Chem. 2008, 181, 1670–1677.
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Sokolov, I.; Smith, D. S.; Henderson, G. S.; Gorby, Y. A. Cell Surface Electrochemical Heterogeneity of the Fe(III)-Reducing Bacteria. Environ. Sci. Technol. 2001, 35, 341–347.
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Dokukin, M.; Olac-Vaw, R.; Guz, N.; Mitin, V.; Sokolov, I. Addressable Photocharging of Single Quantum Dots Assisted with Atomic Force Microscopy Probe. Appl. Phys. Lett. 2009, 95, 173105.
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Taylor, D. M. Molecular Nanostructures and Their Electrical Probing. Thin Solid Films 1998, 331, 1–7.
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Melin, T.; Zdrojek, M.; Brunel, D. Electrostatic Force Microscopy and Kelvin Force Microscopy as a Probe of the Electrostatic and Electronic Properties of Carbon Nanotubes. In Scanning Probe microscopy in Nanoscience and Nanotechnology; Bhushan, B., Ed.; Springer: Verlag Berlin Heidelberg, 2010; pp. 89–128.
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Mélin, T.; Deresmes, D.; Stiévenard, D. Charge Injection in Individual Silicon Nanoparticles Deposited on a Conductive Substrate. Appl. Phys. Lett. 2002, 81, 5054.
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Toulhoat, H.; Prayer, C.; Rouquet, G. Characterization by Atomic Force Microscopy of Adsorbed Asphaltenes. Colloids Surfaces A Physicochem. Engg Asp. 1994, 91, 267–283.
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Mehranfar, M.; Gaikwad, R.; Das, S.; Mitra, S. K.; Thundat, T. Effect of Temperature on Morphologies of Evaporation-Triggered Asphaltene Nanoaggregates. Langmuir 2014, 30, 800– 804.
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Determination of Charge on Asphaltene Nanoaggregates in Air Using Electrostatic Force Microscopy Ravi Gaikwad, Aharnish Hande, Siddhartha Das, Sushanta K. Mitra, and Thomas Thundat Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503968v • Publication Date (Web): 17 Dec 2014 Downloaded from http://pubs.acs.org on December 18, 2014
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Determination of Charge on Asphaltene Nano-aggregates in Air Using Electrostatic Force Microscopy Ravi Gaikwad,1 Aharnish Hande,1 Siddhartha Das,2 Sushanta K. Mitra,3 and Thomas Thundat1 1
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada 2
Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA 3
Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, Canada.
In this paper, we provide measurement of charge of asphaltene nano-aggregates in air using electrostatic force microscopy. We obtain the average surface charge density of the nano-aggregates as 43.7 nC/cm2. Among the different aspects of asphaltene, one of the least known ones is its charge and the effect of solvent and compositional variability (of asphaltene) in dictating this charge. For aqueous systems, asphaltene charge demonstrates a strong dependence on the pH and the salt concentration, indicating that a possible ionization of the surface groups leads to this charging. On the contrary, for asphaltene in non-polar media (e.g., toluene and heptane) it is believed that asphaltene native charge is central in dictating this charging. This native charge is the solvent-independent charge or the asphaltene charge in air. Our measurements, therefore, provide the first direct quantification (i.e., a quantification of charge not from the measurement of the asphaltene mobilities, which in turn requires specification of the non-uniform asphaltene size distribution) of this asphaltene native charge by conducting the measurements in air. Same measurements in a solvent may introduce a solvent-dependent value, thereby forbidding not only the exact quantification of this native charge, but also the understanding of the specific role of the solvent. This measurement, therefore, will provide a useful starting point to
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quantify the mechanism of asphaltene charging in non-polar solvents with important ramifications in deciphering the role of asphaltene in transport and handling of crude and heavy oils.
I.
Introduction:
Understanding the micro-nanoscale dynamics, characteristics and morphologies of petroleum asphaltenes hold the key for ensuring better recovery, transport, and usability of crude and heavy oil.1,2 Therefore, asphaltene research in a way has become synonymous with high priority research on alternative forms of energy. Thus over the years there have been sustained efforts to quantify several fundamental aspects of aspahltene. Some of the better-established ideas regarding asphaltene behaviour include its transition from molecular-to-nano-aggregated-to-flocculated states in the appropriate conditions,2–4 its “solubility class” nature making it soluble in solvents like toluene and insoluble in solvents like n-alkane,3,5,6 formation of fractal structure whose dimensions depend on the nature of the solvent,2,7,8 etc. On the contrary, the issue of charge on asphaltene molecules has remained extremely debated and controversial. There are two primary reasons for such a controversy. Firstly, the charging of the asphaltene molecules is largely dictated by the exact molecular structure of the asphaltene, which is still debated and has been known to vary drastically depending on the source of asphaltene.3,5,9–12 Secondly, it is still not known for certain how the different functional groups of the apshaltene molecules behave in presence of different solvents, which is essential to quantify asphaltene charge in different media. Most of these studies attempting to resolve this issue have predicted asphaltene surface charge indirectly based on charge-dependent phenomenon such as electrophoretic transport of the asphaltene,13–15 which provides implicit quantification of parameters such as zeta potential and surface charge density16 [see Das et. al (Ref. 18) for a detailed review of the relevant literature]. These studies primarily include quantification of asphlatene charge in aqueous medium or
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water-oil interfaces, and these results predict drastically varying asphaltene charge (from very highly positive to very highly negative) as a function of buffer pH.17 Challenges with these studies stem from the fact that they inevitably necessitate assumption about the electric double layer (EDL) structure around the asphaltene aggregates (without the knowledge of the EDL structure it is not possible to quantify the electrophoretic mobility). Unfortunately with the exception of the recent study by Das et al.18 there has been very little theoretical modelling of electric double layer in charging of asphaltene. The problem regarding understanding of asphaltene charge in non-polar media, such as toluene or heptane, is even more serious. Unlike in an aqueous medium, the charging of asphaltene in non-polar solutions cannot be attributed due to a mechanism controlled by the ionization of the asphaltene surface groups. On the contrary, the charging of asphaletene in the non-polar media has been conjectured primarily due to the native charge of the asphaltene,19,20 which is the solvent-independent charge of the asphaltene, or in other words, the charge of asphaltene in air. This charge is the outcome of the π-bonding in asphaltene – this π-bonding is also responsible for the fact that asphaltene is insoluble in all solvents except aromatic compounds,21 Quantification of this asphaltene charge in air is crucial in understanding the charging of asphaltene in non-polar media, which in turn is central to virtually all the issues related to role of asphaltene in crude oil and heavy oil recovery. The most important question related to this native charge is: How can one measure it? Quite remarkably, the native charge of asphaltene has invariably been quantified by measuring the electrophoretic mobility of asphaltene in the non-polar medium itself.19,22,23 There are two key limitations to this approach. First and foremost this is an indirect charge quantification procedure in a sense that to obtain the charge from the electrophoretic mobility measurement one will also require the quantification of the asphaltene radius. This is often difficult, given the fact that asphaltene is typically in a poly-dispersed state in an organic solvent.7 The second issue is the fact that this procedure forbids pinpointing of any possible solvent-specific influence on asphaltene charge. In other words, in case the asphaltene charge
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results from the superposition of the native charge and this solvent-specific influence, none of the two effects can be separately quantified from this measurement of electrophoretic mobility in a solvent. In this context, it is vitally important to have a study that provides a direct quantification of this apshaltene native charge in air (i.e., when the effect of any solvent is completely absent). To the best of our knowledge, while there is perhaps only one study that provides such direct quantification24, there is absolutely no study that provides the quantification in air. In this study, we provide the first direct quantification of asphaltene charge in air – thus our study provides possibly the first direct quantification of the highly sought asphaltene native charge, without necessitating indirect procedure of electrophoretic mobility measurements. We use Electrostatic Force Microscopy (EFM)25–29 to determine the density of asphaltene native charge to approximately 43.7 nC/cm2. This is the central result of this paper. EFM has been known to provide quantification of surface charge for different samples;28,30–32 however, this is possibly the first study where EFM is applied to measure asphaltene surface charge in air. The asphaltene samples are in aggregated state, obtained from evaporation of toluene drop containing dissolved asphaltene. Such evaporation-mediated asphaltene aggregates has been previously employed to illustrate many effects, such as formation of fractals,33–35 temperature-dependent new aggregation dynamics,36 etc.
II.
Methods and Materials:
Crude Athabasca asphaltene sample obtained from IOSI (Institute for Oil Sands Innovation, University of Alberta) were converted into 100% pure asphaltene in powdered form following several steps described in our previous paper.36 The pure asphaltene was then dissolved in toluene and a drop of the solution was placed on a silicon oxide wafer (obtained from University wafers, Boston, MA) and evaporated. Post-evaporation, after all the toluene had disappeared, we were left with precipitated solid asphaltene
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sample adhering to the SiO2 wafer. Details of the process of evaporation-triggered formation of solid asphaltene sample are described in reference 36. The SiO2 wafer with evaporation-precipitated asphaltene was subsequently mounted on a stage for imaging by Atomic Force Microscope.
Figure 1: Schematic of the sample preparation and steps involved in the AFM measurements: (a) A SiO2 wafer (1 cm2 in size) is cleaned with ethanol and dried with N2 gas, (b) a 10 μL drop of toluene containing 0.005wt% of dissolved 100% pure asphaltene is deposited on the cleaned SiO2 wafer, (c) toluene is allowed to evaporate for 30 min at room temperature resulting in solid asphaltene aggregates on SiO2 and (d) the sample is mounted on a stage for AFM analysis for simultaneous topography and EFM data collection. The inset shows a model of the electrostatic interaction between the AFM tip (of radius a) and the aspahltene sample (assumed to be a circular disc of radius R). For the EFM signal, the AFM is in the lift mode with the tip at a distance z above the sample. The sample is assumed to be negatively charged, with uniform surface charge density of σ (thus the net charge for an asphaltene element of area drdx, located at a distance r from the point vertically below the AFM tip, is σdrdx). The sample was grounded and a bias of 0.5V was applied to the conducting AFM probe during the lift mode. A schematic of the procedure is shown in Figure 1. In this configuration, topographic and EFM imaging were performed in the ‘Tapping mode’ and the ‘Interleave (Lift) mode’ using Bruker Icon AFM (Santa Barbara, CA) system. Electrically conducting SCM-PIT probe (Bruker, Santa Barbara, CA) with
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resonance frequency 64.1 KHz, spring constant of 2.8N/m, tip of radius of curvature (a) of 20nm, and a quality factor of approximately 200 was used as the probe. The topography and the EFM images were processed using Nanoscope analysis software (V1.40, Bruker)
III.
Results and Discussions:
Figure 2: (a) AFM topography of asphaltene aggregates and (b) Corresponding EFM image at a lift height of 35 nm. (c) and (d) provide the cross section of the topography and EFM at the same location corresponding to the white horizontal line shown in (a) and (b) respectively
Figures 2a and 2b show the representative qualitative topography and EFM images of the asphaltene aggregates obtained at a lift height of z=35 nm. The topography represents a fractal-like structure and is very similar to the one obtained in our previous study, where asphaltene aggregation is studied as a
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function of temperature.36 Electrostatic signal is detected by the AFM probe only at locations where asphaltene is present. As a result, the EFM profile of the aggregates (expressed in terms of the corresponding frequency shift) is commensurate with the topographic profile; for example peaks in the topography image corresponds to the valleys (indicating maximum frequency shift of the AFM cantilever) in the EFM profile.
Figure 3: Plot of experimental data for the variation of frequency shift with lift height z (in nm), corresponding to a tip bias of 0.5 V. The red bold line shows the theoretical fit to the experimental data using the expression
[see eqs.(5,6)], where
. Best fit
was obtained for R=62 nm (R is the asphaltene disc radius) and M=15.62 Hz. For these parameters (R=62 nm and M=15.62 Hz), we get the best value of coefficient of determination (equal to 0.9521), and this allows to choose the specified values of R and M. Using this value of M, we get the charge density of 43.7 nC/cm2 [using a=20 nm, f=64086.51 Hz, k=2.8 N/m and ε0=8.85×10−12 C/(V-m)]
The frequency shift obtained from the EFM measurements can be related to the surface charge as illustrated below. Let us consider that the AFM tip (of radius a) is at a height z from the asphaltene
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sample (see the inset and caption of Fig. 1), which is assumed to be a circular disc (of radius R) of uniform negative surface charge density of σ. Consider an asphaltene element of area drdx, located at a distance r from the point on the asphaltene sample that is vertically below the AFM tip. This element contains a net charge of dq=σdrdx. This element, therefore, will induce a charge dq’ at the tip of the AFM (of radius a), such that dq′ = − ( dq )
a
(r
2
+z
)
2 1/2
a
= − (σ drdx )
(r
2
+z
)
2 1/2
.30 Here we assume that
the charge is localized at the tip, as has been considered for metallic AFM tips in other studies.28 Therefore, the total induced charge (q’) on the AFM tip due to the entire asphaltene sample will be
q′ = ∫
q′
0
R a dq ′ = −σ ∫ 0 r 2 + z2
(
)
1/2
dx dr = −2πσ a R 2 + z 2
(∫ ) 2π r
0
(
)
1/2
− z .
(1)
Consequently, the vertically downward force on the AFM tip due to the electrostatic interactions of this net induced charge (on the AFM tip) and the charge on the asphaltene sample can be obtained as:
F
F = ∫ dF = 0
σ z q′ 1 R ( 2πσ rdr ) ( q′ ) cosθ = ∫ 2 2 4πε 0 0 2ε 0 r +z
(
)
∫
R
0
(r
( rdr ) 2
+ z2
)
3/2
π aσ 2 R 2 + 2z 2 σ q′ z 1− , = = − 2z 2ε 0 ( R 2 + z 2 )1/2 ε 0 ( R 2 + z 2 )1/2
(2)
where ε0 is the permittivity of air. The gradient of the force in the vertical direction, therefore, can be computed as:
dF π aσ 2 4 = dz ε 0 ( R / z ) 1+ ( z / R)2
{
}
1/2
−
1+ 2 ( z / R)
{
2
( R / z) 1+ ( z / R )
8
}
2 3/2
− 2 .
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Also, we can relate the magnitude of the gradient of the force to the magnitude of frequency shift ( ∆f ) as:28,37
∆f dF = 2k , f dz
(4)
where k is the cantilever spring constant and ݂ is the resonance frequency of the cantilever. Therefore, from eqs.(3) and (4) we can get the charge density, in terms of the frequency shift, as 1/2
2kε ∆f 1 σ = 0 , π a f g ( z / R)
(5)
where g(z/R) can be expressed as:
g ( z / R) =
4
( R / z ) {1+ ( z / R)2 }
1/2
−
1+ 2 ( z / R )
2
( R / z ) {1+ ( z / R)2 }
3/2
− 2.
(6)
Fig. 3 provides the experimental result for the variation of the ∆f with z/R corresponding to a tip bias of +0.5 V. We fit this experimental data with the theoretical expressions [see eqs.(5,6)], and from the best fit, we get the magnitude of the charge density as 43.7 nC/cm2. In order to quantify the best fit, we computed the Coefficient of Determination parameter ( CDR2 ) expressed as: n
CDR2 = 1−
∑ ( ∆f ) i=1
e,i
− ( ∆f )c,i
2
1 n ∑ ( ∆f )e,i − n ∑ ( ∆f )e,i i=1 i=1 n
2
(7)
.
In eq.(7), n is the total number of lift height values (z) where the frequency shift ∆f has been obtained experimentally, ( ∆f )e,i is the mean value of the experimentally obtained frequency shift at a given lift
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height value (zi) and ( ∆f )c,i is the value of the frequency shift at the same zi obtained from the equations [eqs.(5,6)]. The idea is to find that combination of the fitting parameters M and R, which provides the highest (i.e., closest but always less than unity) value of CDR2 . We find that for R=62 nm and M=15.62 the corresponding value of CDR2 is 0.9521. From this value of M, we calculate the average charge density as 43.7 nC/cm2. In order to ensure that the value of the obtained charge density is independent of the polarity of the tip bias, we conducted experiments with tip bias of -0.5 V: the resulting surface charge density was 39.5 nC/cm2, i.e., the value was quite similar to the value with tip bias of 0.5 V. Ideally, in EFM the tip is ଵ ௗ
sensitive to the gradient of the capacitive attractive force (ܨ = ଶ ௗ௭ ܸ ଶ) associated with the tipsubstrate capacitance C.38,39 The tip bias is applied to increase the sensitivity of the EFM signal and to minimize additional contribution from capacitive forces arising from the SiO2 substrate.30 This is evident from the experimental data shown in Figure 3. In EFM the charge, capacitive and image charge force gradients compete with each other; but in case of asphaltene which exhibits intrinsic charges, the EFM signals predominate over the image charge contributions which is proportional to Q2 and hence to the charge density σ as shown in equation 5. This signal is slightly enhanced when the charged sample is prepared on a good insulator like SiO2. This can be seen from the increase in the surface charge density of 43.7 nC/cm2 on SiO2 from 39.4 nC/cm2 as observed on Si wafer with native oxide of about 2nm. This charge density can be quantified as the density of native charge of the asphaltene sample, which as discussed previously, is important in characterizing asphaltene in various non-polar media. Please note that the charge determination through the present procedure only provides the magnitude of the charge, and not the sign. We have discussed in the Introduction the relevance of the present measurements – they are useful to quantify the asphaltene native charge, which in turn, is vitally important to quantify the
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asphaltene dynamics in non-polar media. The present measurements are in air. However, the basic measurement principle employed here is equally valid even if the asphaltene sample is in a solvent which does not dissolve asphaltene. This implies that in a solvent where asphaltene is not completely dissolved and maintains a solid aggregated form strongly adhered to the substrate (this ensures the existence of a solvent-asphaltene interface for a finite time), the same frequency-response-based approach of the EFM may be implemented to measure the asphaltene surface charge if we employ polymer shielded probes with better tip dynamics in liquid medium. Of course, there will be certain modifications in order to account for the solvent-specific effects. For example, in polar solvent like water, presence of ions in the bulk liquid will lead to the development of an electric double layer at the asphaltene-water interface.18 This double layer will screen the asphaltene surface charge over the distance equal to the Debye length, which typically varies from 1 to 100 nm (depending on the bulk ion concentration). Therefore, if the Debye layer thickness is smaller than the mean height of the AFM tip from the asphaltene sample, the AFM tip will not sense any charge of the asphaltene sample (since all the charge has been screened within the Debye layer). Hence, in order to ensure that the proposed method becomes applicable to polar solvents like water, one must choose the mean AFM-tip-sample separation distance in a manner such that it is substantially smaller than the corresponding Debye layer thickness. On the other hand, for non-polar solvents one needs to account for issues such as adhesion of solid form of asphaltene on the substrate and extreme volatility of the solvent. The next important issue is to compare the obtained value of the asphaltene surface charge density with those reported previously. Asphaltene zeta potential and charge density have typically been measured in aqueous systems – in such systems, there is unanimity about the governing role of ionization of asphaltene surface charges, thereby yielding zeta potential and surface charge density values that are functions of the pH. Szymula et al.17 provided direct measurement of the surface charge density: they reported that depending on the salt concentration, the asphaltene surface charge density
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can vary from +10 µC/cm2 at pH=4 to −45 µC/cm2 at pH=10. There are also other studies16 that provided zeta potential values for the asphaltene as a function of buffer pH and electrolyte salt concentration. Employing the condition,40 σ (ζ ) =
eζ 2 ε 0 ε r k BT κ sinh (here it is assumed that the asphaltene e 2kBT
surface charges are monovalent; also kB is the Boltzmann constant, T is the temperature in Kelvin, e is the electronic charge, κ is the inverse of the EDL thickness calculated on the basis of the concentration of electrolyte salt), we can get a somewhat lesser surface charge density values of ± 0.5 µC/cm2. Therefore, our estimate of the asphaltene native charge density is substantially smaller than the charge density values witnessed in polar media. Hence one can neglect this native charge for aqueous systems; however, as has been discussed in details before, this native charge becomes important for asphaltene charging in non-polar media.
Conclusions: In this paper, we have provided the first possible direct determination of asphlatene native charge in air by the use of EFM. It is well recognized that this native charge is central to dictating the charge dynamics of asphaltene in non-polar media. The state of the art in quantifying this native charge is an indirect procedure of quantifying the asphaltene electrophoretic mobility in the non-polar media itself – the associated limitations are the requirement of separate quantification of the size distribution of asphaltene in a possible polydispersed state and the inability to segregate the native charge from any possible solvent-specific charging. In this light, the direct quantification capability of the proposed method is a significant step forward towards deciphering fundamental characteristics of asphaltene. Author information The authors declare no competing financial interest.
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Corresponding Author *(Ravi Gaikwad – [email protected]) Acknowledgements: This work was supported by the Canadian Excellence Research Chair (CERC) program for the Oil sands at the University of Alberta. We would also like to thank members of our Nano-interfaces and molecular engineering (NIME) group for their fruitful discussion and feedbacks. References:
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