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Gold Core Nanoparticle Mimics for Asphaltene Behaviors in Solution and at Interfaces Jeoffrey Ollinger, Amin Pourmohammadbagher, Arthur D. Quast, Mildred Becerra, Jennifer S. Shumaker-Parry, and John M. Shaw Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01770 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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Gold Core Nanoparticle Mimics for Asphaltene Behaviors in Solution and at Interfaces Jeoffrey Ollinger1, Amin Pourmohammadbagher1, Arthur D. Quast2, Mildred Becerra1, Jennifer S. Shumaker-Parry2, and John M. Shaw1* 1. Department of Chemical and Materials Engineering, University of Alberta, 12-360 Donadeo Innovation Centre for Engineering, Edmonton, Alberta, CANADA, T6G 1H9 2. Department of Chemistry, University of Utah, 315 S 1400 E, Rm 2020, Salt Lake City, UT 84112-0850 USA *corresponding author: [email protected]

ABSTRACT Asphaltenes are a poorly defined class of self-assembling and surface active molecules present in crude oils. The nature and structure of the nanoaggregates they form remain subjects of debate and speculation. In this exploratory work, the surface properties of asphaltene nanoaggregates are probed using electrically neutral 5 nm diameter gold-core nanoparticles with alkyl, aromatic, and alkanol functionalities on their surfaces. These custom synthesized nanoparticles are characterized, and their enthalpies of solution at near infinite dilution, and the interfacial tensions of solutions containing these nanoparticles are compared with the corresponding values for Athabasca pentane asphaltenes. The enthalpies of solution of these asphaltenes in toluene, heptane, pyridine, ethanol and water is consistent with the behavior of gold-alkyl nanoparticles. The interfacial tension values of these asphaltenes at toluene – water and (toluene + heptane) – water interfaces is consistent with the behavior of gold-biphenyl nanoparticles as are the tendencies for these asphaltenes and gold-biphenyl nanoparticles to “precipitate” in toluene + heptane mixtures. Gold-alkyl nanoparticles are minimally surface active at toluene – water and (toluene + heptane) – water interfaces and remain dispersed in all toluene + heptane mixtures. The behavior of these asphaltenes in solution and at interfaces is inconsistent with the behavior of 1 ACS Paragon Plus Environment

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gold-n-alkanol nanoparticles. The outcomes of this formative work indicate potential roles for aromatic submolecular motifs on aggregate surfaces as a basis for interpreting asphaltene nanoparticle flocculation and interfacial properties, while alkyl submolecular motifs on aggregate surfaces appear to provide a basis for interpreting other aspects of asphaltene solution behavior. A number of lines of inquiry for future work are suggested. Key words: asphaltenes, nanoaggregates, gold, nanoparticles, ligands, enthalpy of solution, interfacial tension

1. INTRODUCTION Asphaltenes are a poorly defined class of self-assembling and surface active molecules present in crude oils.1-4 Basic properties, such as the mean molar mass and representative molecular structure(s) of this class of molecules continue to be debated in the literature. In addition to variations by method of measurement, results such as their mass fraction in a hydrocarbon sample and elemental composition vary depending on the provenance and even the preparation method applied to the same sample. The tendency of asphaltenes to self-aggregate and to adsorb on surfaces poses operating challenges during hydrocarbon resource production, transport and refining.5-8 The technical uncertainties and economic risks associated with asphaltene behaviors remain significant. Classic Spectroscopic characterization techniques such as Ultra Violet-Visible (UV-Vis) and Fourier Transform Infrared (FTIR) underscore the presence of aromatics rings in asphaltene molecules, along with alkanes chains, and the presence of S, N, O substituted groups.9 Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) provides additional details regarding their elemental constituents and molar mass distribution.10, 11 Naphthenic motifs would 2 ACS Paragon Plus Environment

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appear to be missed in typical analyses,12 but their presence has been reported in a recent SAXS + SANS analysis.13 Two contrasting average molecular models for asphaltenes, the “island” or “continental” model and the “archipelago” model co-exist in the literature.14 The differences between these average molecular prototypes are important because perceived reaction chemistries during refining and aggregation mechanisms differ greatly. For the island model, aggregation is driven by π – π stacking interactions complimented by steric repulsion. For the archipelago model, the spatial mix of attractive and repulsive interactions includes heteroatom interactions, hydrogen bonding and possibly entanglement in addition to π – π stacking. The literature continues to be split with evidence supporting the island structural model,3,15 especially laser desorption

ionization

mass

spectrometry16

and

time-resolved

fluorescence

depletion

measurements.17 Theoretical considerations18 and experimental work related to pyrolysis and coking19 identify small alkane and aromatic motifs that underscore the presence of molecules with an archipelago structure. Ongoing development and improvements to the FTICR-MS technique are contributing to this debate as new classes and subclasses of compounds are identified.20 Irrespective of the perspective one has on the average nature/structure/size of asphaltene molecules, they do form small and tightly bound aggregates at ppm level concentrations (50 to 200 mg/l) in solvents including toluene.21, 22 Small Angle X-Ray Scattering (SAXS) and Small Angle Neutron Scattering (SANS) indicate that 2 nm aggregates that by definition comprise a limited number of molecules are present at these low concentrations.23, 24 As with average molecule structure, the literature on nanoaggregation is also split. From the island molecular model perspective, asphaltenes aggregate by π – π stacking of polynuclear aromatic cores and aggregation is limited by repulsion of aliphatic chains on external surfaces.15, 17,25

4 nm to 30 nm nanoaggregates form depending on the details of the surrounding medium.26-

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For the island model the distinction between nanoaggregates and flocs (objects comprising

multiple nanoaggregates) is important because the chemistry and kinetics of flocculation are not governed by the same phenomena as the formation of nanoaggregates.30 From the Archipelago model perspective, the mechanisms for nanoaggregation are not well defined but it is thought that heteroatoms and the diversity of the molecular structures present permit aggregation among molecules.31 Acid-base interactions, metal complexes and hydrogen bonding play important roles.32 The distinction between nanoaggregation and flocculation is not delineated within this conceptual framework. The link between either of these aggregation mechanisms and triggers that lead to the formation of flocs and deposits is unclear. Pressure and temperature variation, and changes of the chemical composition of the surrounding crude oil alter the stability of asphaltene nanoaggregates and affect aggregation kinetics33 in parts of the hydrocarbon phase diagram. At pressures near the bubble pressure, fluids are prone to flocculation.34, 35 The reverse process, disaggregation of flocs, is also a subject of debate,36 with data suggesting that partial reversibility adjacent to conditions where flocs form is possible37 but the use of chemical treatments is frequently found to be necessary.38 Given the uncertainty related to the fundamental properties and behaviors of asphaltenes, there is a clear need for hypothesis testing using well-defined probes. In this work, the details of the internal structure and composition of nanoaggregates formed at low concentration are hypothesized to be irrelevant to floc formation. Interactions among nanoaggregates and between aggregates and liquids and interfaces are hypothesized to be dictated by molecular motifs at or protruding from nanoaggregate surfaces. Physical models for asphaltene nanoaggregates comprising electrically-neutral gold-core nanoparticles (AuNPs) to which alkyl, primary alcohol, and alkyl aromatic organic ligands are attached were prepared. Their solution calorimetry at near


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infinite dilution and their interfacial properties in diverse solvents and interfaces are then compared with the properties of Athabasca pentane asphaltenes both qualitatively and quantitatively. The objective of this work is to identify robust NP mimics for asphaltene nanoaggregate behaviors.

2. EXPERIMENTAL SECTION 2.1 Materials. Toluene HPLC grade, with a purity of 99.9%, was obtained from Fisher Scientific. Borane tert-butylamine, dodecanethiol and chloro(triphenylphosphine)Au(I) were acquired from Sigma-Aldrich with a purity of 97.0%, 99.2% and 99.9% respectively. Ethanol was purchased from the Chemistry Department Store at the University of Alberta, with a purity greater 99.5%. Pentane (C5) precipitated asphaltenes were obtained from Athabasca bitumen supplied by Syncrude Canada Ltd. Pentane HPLC grade, with a purity of 99.6%, was obtained from Fisher Scientific. C5 Athabasca asphaltenes were prepared at the University of Alberta using a precipitation procedure described elsewhere.39 Composition analyses of C5 Athabasca asphaltenes and the composition of the parent hydrocarbon resource (Athabasca bitumen) are shown in Table 1.39 Asphaltenes tend to be enriched in heteroatomic species (oxygen, nitrogen, sulfur, nickel and vanadium) relative to the parent resource.40 Octanethiol functionalized AuNPs in toluene, 2% (w/v), purchased from Sigma-Aldrich have a D90 of 3.61nm (reported by Sigma Aldrich, using dynamic light scattering). . Table 1. Chemical analysis of Athabasca bitumen and its asphaltene content39 Element (wt.%)

Bitumen

Asphaltenes

C

83.2

77.1 5

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H

9.7

7.4

N

0.4

1.2

S

5.3

8.1

O

1.7

1.0

H/C (atomic) Pentane Asphaltene (wt. %)

1.39 18.6

1.14 100

2.1.1 Glassware Cleaning Solutions

Prior to use, glassware used for AuNP syntheses was cleaned using Aqua Regia (3:1 HCl:HNO3) to remove all metal contaminants. Glassware used for silanization was cleaned further using piranha solution (4:1 H2SO4:30% H2O2). Caution: Aqua Regia and Piranha solutions are strong oxidizing agents which have been known to detonate spontaneously upon contact with organic material and should be handled with extreme care using proper personal protective equipment.

2.1.2 Au Core Nanoparticles Ligand selection for Au core nanoparticles Asphaltenes possess a range of chemical functionalities. They have a carbon skeleton that includes a mix of alkane chains and aromatic groups. From Nuclear Magnetic Resonance (NMR) and Raman spectroscopy measurements, 50% to 60% of the carbon is aromatic.41,

42

The

heteroatoms O, S and N present a range of functionalities. For example, nitrogen is mostly found among aromatic or cyclic carbon species as pyrrolic or pyridinic nitrogen.43,

44

Amine type

functional groups are largely absent. Sulfur is present predominantly as thiophenes, followed by sulfides or sulfoxides45 depending on the maturity of the oil.46 For oxygen, phenolic OH groups predominate,47 but carboxyl and ketone groups are also present. One oxygen functional group is 6 ACS Paragon Plus Environment

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found per ~ 1600 amu of asphaltenes.48 Porphyrin rings are also present in very low mass fractions and contribute to aggregation.49-52 The strategy to functionalize AuNPs is based on the Au-sulfur bond. This dictates the presence of thiol functionalities at the particle surface. The feasibility of preparing AuNPs with specific ligands imposes additional constraints. The island molecular prototype, with C8-C12 alkane chains17 hypothesized to be present on the exterior surface of nanoaggregates, and the archipelago molecular prototype, with diverse functionalities hypothesized to be present on the exterior surface of aggregates, also provide a framework for ligand selection. Au-alkylthiol nanoparticle preparation Octanethiol and dodecanethiol functionalized AuNP synthesis largely followed a procedure by Zeng et al.53 The ligands comprise an n-alkyl chain tethered to the AuNP surface via thiolate bonding. The main change implemented was the use of toluene instead of benzene as the reaction medium. In a clean 200 mL glass vial, 100 mL of toluene and 2.174 g (25 mmol) of borane tertbutylamine were added. The vial was then sonicated for 10 minutes. Meanwhile, 100 mL of toluene was added to a 400 mL glass vial followed by addition of 1.25 mL of dodecanethiol and 1.236 g (2.5 mmol) of chloro(triphenylphosphine)Au(I). The vial was stirred under medium shear for 5 minutes and was then placed in a 55 °C water bath for 5 minutes. The reducing solution was then added using a 50 mL burette at ~1 mL/s. After 25 minutes of reaction, the dark red dispersion (~5 nm diameter particles)54 was removed from the bath and cooled at ambient temperature for 25 minutes. Then, 200 mL of ethanol was added. The solution was refrigerated for 24 hours, and then centrifuged at 20000 m/s2 for 10 minutes. The remaining solid was sonicated and centrifuged twice in a 1:1 ratio of toluene/ethanol under the same conditions in order to remove the remaining reagents. The sizes of the particles were verified using 7 ACS Paragon Plus Environment

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Transmission Electron Microscopy and UV-Visible spectroscopy. The presence of ligands was validated by comparison with solution calorimetry and FTIR measurements for Au-SC8 NPs purchased form Sigma-Aldrich. Au-SC11OH nanoparticle preparation 11-Mercapto-1-undecanol functionalized nanoparticle synthesis followed a method by Sardar and Shumaker-Parry55 and were prepared at the University of Utah. These ligands comprise a primary alcohol tethered to the Au surface via a thiolate bond. First a new one liter Erlenmeyer flask was washed with piranha solution in order to remove organic contaminants from the surface of the glass. The inside glass surface was then silanized using a solution of ~5 mM octadecyltrichlorosilane in acetonitrile for 24 hours to render it hydrophobic56 which greatly reduced material loss due to AuNP adsorption onto an otherwise hydrophilic surface. 800 mL of acetonitrile and 200 mL of toluene were then added and mixed. 1.6 mmol (0.544 grams) of chlorotriethylphosphine Au (I) and then 16 mmol (1.76 grams) of 11-mercapto-1-undecanol were dissolved in the solution and stirred for 20 minutes. The solution was then immerged into a 60 °C water bath for 10 minutes. Next, addition of 10 mL of 0.5 M of 9-borabicyclo[3.3.1]nonane and 0.2 mL of trioctylamine led to a dark red coloration of the solution for the first two minutes of reaction, followed by the discoloration of the solution. The solution was left in the water bath for a total of 20 minutes and then cooled at room temperature for 30 minutes. After this, the slurry was refrigerated overnight to permit particles to sediment. Most of the solvent was decanted and the rest was removed by centrifugation. Then the particles were washed with the same solvent mixture that was used for the synthesis. The process was repeated four times, yielding ~100 mg of black powder. Mixing this powder with ethanol or methanol gave a red color characteristic of


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NP dispersions. These mixtures were stored in sealed containers at ~ 5 oC and no sedimentation was observed after two weeks. Gold-Biphenyl-4-thiol (biphenyl) nanoparticle preparation Au core NPs with thiol (biphenyl) ligands were synthesized by ligand exchange based on triphenylphosphine (TPP) AuNPs first prepared and stabilized with a phosphorus ligand that is readily replaced with the stronger thiolate bond.57, 58 These ligands comprise a biphenyl group tethered to the Au surface via a thiolate bond. In the first step of the synthesis a 6 L Erlenmeyer flask was washed with Aqua Regia and filled with a mixture of 4:1 toluene to acetonitrile at room temperature. Then, 2.04 g of chlorotriethylphosphine Au(I) was added, along with 9.345 g of TPP. This mixture was stirred for 1 hour.

The addition of 10 mL of 0.5 M 9-

borabicyclo[3.3.1]nonane in tretrahydrofuran followed. Over the course of 30 minutes the reaction mixture color gradually changed from clear and colorless to yellow and then brown. Then the whole solution was placed in a rotavapor and the solid was washed in hexane twice at 7000 rpm. For the exchange step, the solid was dispersed in 3 liters of methylene chloride to give a dark purple solution. A 1:1 mass ratio of biphenyl-4-thiol to AuNPs was injected in the solution, which was then stirred for 24 hours. After this, the solution was filtered to remove large aggregates. The solution was again placed in a rotavapor and washed twice with hexane at 7000 rpm. 2.2 Characterization 2.2.1 Nanoparticle Size Particle size measurements were carried out with a CM20 FEG TEM operated at 200kV (University of Alberta) and with a JEOL 1400 Plus TEM operated at 120kV (University of Utah). Solid NP samples were dispersed in a solvent prior to measurement. A concentration of 0.05 9 ACS Paragon Plus Environment

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mg/mL or less is recommended to avoid formation of a multilayer of particles for TEM analysis. A ~2 µL drop was put on a TEM grid and dried under atmospheric conditions. Toluene was used as the solvent for Au-SC8/C12 and Au-biphenyl, and ethanol for Au-SC11OH NPs. As AuNPs exhibit a localized surface plasmon resonance,54 UV-Visible spectroscopy is also useful to monitor the size of particles and their behaviors in solvents. Monodispersed 5 nm AuNPs have an absorbance peak at ~520 nm. Dispersions appear red in color. For these complimentary measurements, the NPs were dispersed in the same solvents used for TEM measurements and placed in a Varian Carey 50 UV-Visible spectrometer that registers absorption signals at 1 nm intervals between 800 nm and 350 nm. Pure solvent spectra were also measured and subtracted from the NP solution spectra.

2.2.2 Surface Ligand Characterization Fourier Transform Infrared Spectroscopy (FTIR), X-Ray photoelectron spectroscopy (XPS) and Scanning Transmission Electron Microscopy (S/TEM) were used to characterize the surface chemistry and morphology of the NPs. FTIR experiments were performed on a Perkin Elmer Spectrum 100 with DiffusIR attachment from PIKE at the University of Utah (from 4000 cm-1 to 600 cm-1) and on a Diffuse Reflectance Infrared Fourier Transform (DRIFT), Thermo Nicolet 670 FT-IR, at the University of Alberta (from 4000 cm-1 to 700 cm-1) with a resolution set to 1 cm-1 in both cases. For each experiment, NPs or ligands were mixed with potassium bromide (KBr) powder for DRIFT measurements. XPS measurements were carried out at the University of Utah with a Kratos Axis Ultra DLD system. Monochromatic Al K-alpha X-Rays were used (1486.6 eV), with a power of 150 W (10 mA emission current, 15 kV on the anode). The base pressure in the analysis chamber was 8 10-10 Torr. All spectra were collected over an analysis 10 ACS Paragon Plus Environment

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area of approximately 300 x 700 microns (as defined by the hybrid/slot settings on the systems lens mode and aperture settings). Spectra were collected with a hemispherical analyzer operating in FAT (Fixed Analyzer Transmission) mode. Charging artifacts in the spectra were minimized using a systems charge neutralization system. Wide energy region survey spectra and highresolution regional spectra were collected using a pass energy of 160 eV and 40 eV, respectively. Around 10 mg of material were placed on a carbon vacuum tape and placed under vacuum at least 48 hours before measurement. Images in Figure 1b and 1c were obtained using a JEOL 2800 S/TEM operating at an accelerating potential of 200 kV with a hot field emission gun (FEG) source. 2.2.3 Interfacial Tension Measurements Interfacial tension measurements were performed by the pendant-drop method using a Ramé-Hart goniometer (Model 250) and the “Advanced DROPimage” software from the same company.59 Drops with volumes in the range of 20 to 30 µL were used. Interfacial tension values were calculated every 2 seconds over an interval of up to 2000 s unless volume variation with time became too large to obtain meaningful measures of surface tension.60 NP dispersions were prepared just prior to measurement. Between 0.5 mg and 10 mg of NPs were weighed and dispersed in 10 mL of solvent and sonicated for 15 minutes. 2.2.4 Solution Calorimetry Measurements Isothermal solution calorimetry measurements were performed using a precision solution calorimetry module (SolCal) from TA Instruments. The module was inserted into a TAM III thermostat with a temperature uncertainty of 1 µ°C. SolCal is a semiadiabatic system with a short-term noise of < 10 µ°C/5 min. The procedure is described in detail elsewhere.61, 62

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3.1 Nanoparticle Size Characterization Analysis of TEM images for Au-SC12 (Figure 1a), Au-SC11OH, (Figure 1b) and Au-biphenyl (Figure 1c) yield respective mean sizes of 5.3, 4.0, and 3.8 nm with standard deviations of 0.7, 2.3, and 2.1 nm. At ~100 mg/L, Au-biphenyl NP sedimentation was not observed after one week, but at ~1000 mg/L aggregates, dispersible by sonication, could be found on the bottom of the vial after few days. These images and observations indicate that gold core to gold core interactions are of secondary importance. Although the NP dispersions possess similar mean sizes and size distributions, suspensions of the three particles appear different to the naked eye. Au-SC8 and Au-SC12 NPs in toluene and Au-SC11OH NPs in ethanol appear red; Au-biphenyl NPs appear purple. This difference is quantified in the UV-Visible absorption spectra shown in Figure 2. AuSC11OH and Au-SC12 NPs spectra are comparable, while the principal peak for Au-biphenyl NPs is red-shifted and two minor peaks appear between 700 nm and 800 nm. These minor peaks may indicate the presence of nanoaggregates in solution and or larger nanoparticles and cannot be interpreted without complementary data. (a)

(b)

(c)

Figure 1. TEM images of Au-SC12 (a), Au-SC11OH (b), and Au-biphenyl NPs (c).


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1.2

λmax 510 nm

1

0.8

λmax 539 nm

0.6

0.4

λmax 506 nm

Normalized absorbancy A

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0.2

0 350

400

450

500

550 600 650 Wavelength (nm)

700

750

800

Figure 2. UV-Vis absorption spectra of synthesized AuNPs. Au-SC11OH in ethanol λmax= 510 nm (solid curve), Au-SC12 in toluene λmax= 506 nm (dotted curve), Au-biphenyl in toluene λmax= 539 nm (dashed curve).

3.2 Nanoparticle Surface Composition FTIR results for the AuNPs are presented in Figure 3 (Au-SC8), Figure 4 (Au-SC11OH) and Figure 5 (Au-biphenyl). Regions of interest are highlighted and used to compare the compositions of the particles with relevant standards. The shaded region (3000 cm-1 – 2800 cm-1) indicates the presence of linear saturated carbons in the sample. The peaks seen in this region correspond to symmetric and asymmetric CH2/CH3 stretching bands. Measurements for both synthesized and purchased Au-SC8 NPs are presented in Figure 3. Their spectra are qualitatively and quantitatively similar and show that in-house preparation meets expected standards. For the Au-SC11OH NPs, Figure 4, only two peaks, corresponding to antisymmetric and symmetric 13 ACS Paragon Plus Environment

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methylene stretches, are seen in this wavenumber range because the ligand, 11mercaptoundecanol, does not possess a terminal CH3 group. The terminal OH group appears in the shaded region (3500 cm-1 – 3200 cm-1). For the Au-biphenyl NPs, a small signal in the 3000 cm-1 – 2800 cm-1 wave number range is observed in the spectra in Figures 5a and b, suggesting the presence of an external contaminant containing aliphatic carbons. The typically weak S-H stretch mode at ~2600 cm-1, clearly observed in the thiol reagents, is absent from the synthesized particles. This absence is likely due to a significant change in concentration between reagents and NPs and the formation of a thiolate bond with the surface of the AuNPs.63, 64 Aromatic C-H stretching peaks (3200–3000 cm-1) and aromatic C-C breathing modes (~1500 cm-1) are visible in Figures 5a, b and c because both triphenylphosphine and biphenyl-4-thiol include aromatic carbons. Three peaks around 800 cm-1 assigned as out of plane C-H and C-C bending for the aromatic rings present in the Au-biphenyl NPs and in the biphenyl-4-thiol ligand.64

(a)

(b)

CH2 – CH3 alkanes

3800 3400 3000 2600 2200 1800 1400 1000

600

-1

Wavenumber cm

Figure 3. FTIR spectra of Au-SC8 NPs: (a) synthesized in this work, (b) purchased from SigmaAldrich. 14 ACS Paragon Plus Environment

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CH2 alkanes (a)

OH group (b)

SH group

4000 3600 3200 2800 2400 2000 1600 1200

800

-1

Wavenumber cm

Figure 4. FTIR spectra for (a) Au-SC11OH NPs synthesized in this work, and (b) 11-mercaptoundecanol.

(a) Aromatic groups

Alkanes

Aromatic groups

(b)

(c) SH group

4000 3600 3200 2800 2400 2000 1600 1200

800

-1

Wavenumber cm

Figure 5. FTIR spectra: (a) triphenylphosphine, (b) synthesized Au-biphenyl NPs and (c) biphenyl-4-thiol.


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High-resolution XPS measurements for Au, sulfur, phosphorus and carbon are presented in Figure 6, for Au-SC11OH NPs, in Figure 7, for Au-biphenyl NPs, and in Figure 8 for Au-SC12 NPs. For the Au-SC11OH NPs, the presence of some residual triethylphosphine from the starting material salt is possible. However, the length of the desired undecanol ligand compared to the length of triethylphosphine, the relative ratio between sulfur and phosphorus and the dispersive behavior of the particles in alcohol compared to pure alkanes is sufficient to attribute the behavior of these particles to the presence of the OH group. Phosphorus is not detected on the surface of Au-biphenyl NPs, as shown in Figure 7. The absence of phosphorus on the surface indicates the absence of triphenylphosphine (ligand used before the exchange). This outcome illustrates the efficiency of the ligand exchange process where phosphorus is removed in favor of the biphenyl ligand. XPS measurements performed for Au-SC12 NPs confirm the presence of SC12 ligands on the surface. 3.3 Solution Calorimetry 3.3.1 Enthalpy of Solution at Infinite Dilution for Molecular Solutes in Solvents Illustrative enthalpy of solution values, h12, from this work and from the literature for binary mixtures are shown in Table 2. The reported values are on a per unit mass of solute basis, for component 1 (solute) in component 2 (solvent) at infinite dilution. For n-alkanes the values are positive in alcohols, water, pyridine, aromatics, and n-alkanes. Available values for aromatics in water, alcohols and n-alkanes, shown in Table 2 and elsewhere67 are also positive. For alcohols, the available values in n-alkanes and aromatics are positive, and trend to larger values with increasing temperature. The values for alcohols in water are negative.


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Figure 6. High resolution XPS spectra of the Au-SC11OH NPs for the region associated with (a) P 2p, (b) Au 4f, (c) C 1s, and (d) S 2p.


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Figure 7. High resolution XPS spectra of the Au-biphenyl NPs for the region associated with (a) P 2p, (b) Au 4f, (c) C 1s, and (d) S 2p.


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Figure 8. High resolution XPS spectra of the Au-SC12 NPs for the region associated with (a) P 2p, (b) Au 4f, (c) C 1s, and (d) S 2p.


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Table 2. Enthalpies of solution, h12, for component 1 (solute) in component 2 (solvent) at infinite dilution Component 1

Component 2

h12, J/g

Temp, °C

Lai et al.65

Hexane

n-Octanol

13.6

35

Pourmohammadbagher and Shaw66

Heptane

Water

56

60

Octane

Toluene

19

60

Octane

Heptane

2

60

Octane

Pyridine

57

60

Heptane

Toluene

18

25

Heptane

Octane

1.5

60

Heptane

Octane

5

80

Shinoda and Fujihira67

Benzene

Water

31

60

Pourmohammadbagher and Shaw66

Toluene

Water

60

60

Toluene

Heptane

9.5

25

Toluene

Octane

10.5

80

Benzene

1-hexanol

30.3

25

Benzene

2-methyl-2-butanol

32.5

25

Toluene

2-methyl-2-butanol

28

25

Ethyl benzene

Octane

17

–

Ethyl benzene

Dodecane

17.5

–

Ethyl benzene

Hexadecane

18.8

–

n-Octanol

Hexane

28.3

25

n-Octanol

Hexane

29.4

35

n-Octanol

Hexane

44

45

n-Butanol

Toluene

59.6

25

n-Butanol

Toluene

64.9

35

n-Butanol

Toluene

70

45

n-Propanol

Water

-197

25

Source

This work

This work

Hsu and Clever68

Arenosa et al.69

Mrazek and Van Ness70

Lama and Lu71


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3.3.2 Nanoparticle Solution Calorimetry The enthalpy of solution, at near infinite dilution, is sensitive to the nature of the ligands, and the number of ligands per unit mass, which is in turn sensitive to both the surface coverage (number of ligands/unit area) and particle size (area/unit mass). A sensitivity analysis for closely related nalkyl ligands is presented in Figure 9. The reported values were measured at a concentration of 1000 mg/L, with an absolute uncertainty of less than 1 J/g. The enthalpies of solution of the purchased and synthesized Au-SC8 NPs, Figure 9a, agree to within measurement uncertainty in toluene, heptane, pyridine and water. Both sets of NPs have comparable mean size, and ligand surface densities. Similarly, solution enthalpies for Au-SC8 and Au-SC12 NPs synthesized using the same procedure are also indistinguishable within measurement uncertainty (Figure 9b) suggesting that small differences in chain length have little impact on solution chemistry. The enthalpies of solution of synthesized Au-SC8, Au-SC11OH and Au-biphenyl NPs in toluene, heptane, pyridine, ethanol and deionized water, are reported in Figure 10. Measurements were made at 25 °C (Figure 10a), and at 60 °C (Figure 10b). The dataset at 60 °C is more limited because the boiling point of ethanol is too low, and the amount of Au-biphenyl NPs was limited. At 25 °C (Figure 10a) the enthalpies of solution of these three NP types in toluene, heptane, ethanol and water cannot be distinguished, within experimental uncertainty. However, the value for Au-biphenyl NPs in pyridine is readily distinguished from the other NP types. At 60 °C, the behaviors of Au-SC8 and Au-SC11OH NPs differ significantly in heptane, pyridine and water (Figure 10b). In heptane, the difference in their enthalpies of solution exceeds 10 J/g, and their enthalpies of solution possess opposing signs in pyridine and water. The strong temperature dependence of the enthalpy of solution values of NPs was unexpected. As the NPs are dispersed and do not dissolve, the measured values reflect particle-solvent interactions and their variation


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with temperature where net repulsion and net attraction yield positive and negative enthalpy of solution values respectively. Contributions from solvent sorption (negative) on electrically neutral NP surfaces partially covered with ligands are expected to be small. Not all of the NP enthalpies of solution conform with expectation based on the behaviors of related pure compounds in the same solvents at near infinite dilution. For example, the enthalpy of solution for the Au-SC8 NPs in water was not expected to decrease with increasing temperature. Hypotheses related to the significant temperature dependence not observed with pure compounds, except for alcohols, warrant further exploration and study. 3.3.3 Asphaltene Solution Calorimetry Asphaltene behaviors in crude oils and solvents are complex. Due to the importance of asphaltene impacts on hydrocarbon resource production, transport and refining, understanding the behavior of asphaltenes in organic media and at interfaces is important. Treating asphaltenes as either soluble species or aggregates neglects the possibility of having different states of aggregation and different portions of the asphaltenes in different states depending on the temperature and the surrounding chemical environment.72 Nikooyeh and Shaw reviewed simple theoretical models based on molecular solubility and colloidal behaviors that have been used to describe the physical and phase states of asphaltenes in diluents and showed that they have had only limited success.73 Experimental measurements on the state of aggregation of asphaltenes15, 17, 21, 22 provide insights regarding the scale of aggregation and the nature of the asphaltene-asphaltene interactions, but do not give direct measures of the interaction between asphaltenes and the surrounding media. Enthalpy of solution measurements give information on the changes of state and the interactions that species undergo when combined with other species. Titration calorimetry and solution calorimetry at near infinite dilution are commonly used. In titration calorimetry the entire range 22 ACS Paragon Plus Environment

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of solute and diluent compositions is surveyed. Asphaltenes precipitation74, association75 and sub fraction characterization76 have been carried out using titration calorimetry. In solution calorimetry, milligrams of solute are added to grams of solvent providing summative information regarding dissolution and solute-solvent interaction at low solute concentrations. Zhang et al.77 and Nikooyeh and Shaw61 confirmed some behaviors of asphaltene + diluent mixtures, and Nikooyeh et al.78 challenged the application of regular solution theory to asphaltene + diluent behavior. Care must be taken in interpreting the temperature independent enthalpy of solution data presented in Figures 10 a and b. The enthalpies of solution are positive in all solvents except water. The values include impacts of asphaltene dissolution (positive), solvent-asphaltene attraction (negative) or repulsion (positive), and solvent sorption (negative). The impacts of asphaltene aggregation state variation, positive for disaggregation and negative for aggregation are negligible.61 3.3.4 Nanoparticle vs. C5 Athabasca Asphaltene Solution Calorimetry By comparing the calorimetric behavior of well-defined Au + ligand NPs with asphaltene behaviors, as shown in Figure 10, hypotheses regarding the roles of functional groups in asphaltene nanoaggregate - diluent interactions can be tested. Even though partial dissolution of asphaltenes is a confounding variable, testing and comparing interactions with diverse diluents affords the possibility of understanding whether or not one type of interaction is a better mimic for asphaltene behaviors and whether or not controls for asphaltene behaviors can be identified. The results obtained with the synthesized NPs are compared with C5 Athabasca asphaltenes at 25 °C, Figure 10a, and at 60 °C, Figure 10b, under comparable experimental conditions. The enthalpies of solution obtained for asphaltenes are positive in solvents other than water, at both temperatures. The Au-biphenyl NPs possess a positive enthalpy of solution in water and a 23 ACS Paragon Plus Environment

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negative enthalpy of solution in pyridine at 25 °C and thus have an opposing qualitative behavior. At 60 °C, the enthalpy of solution for Au-SC11OH NPs in water and in pyridine have opposing signs relative to C5 Athabasca asphaltenes. Only the enthalpies of solution of Au-SC8 NPs possess the correct sign, within experimental uncertainty, and trends with temperature vis-à-vis C5 Athabasca asphaltenes. Au-SC8 NPs are the best mimic for C5 Athabasca asphaltene enthalpies of solution, of the NP types tested. This is an important and arguably an unexpected result depending on ones perspective on the nominal structure of asphaltene nanoaggregate surfaces. However, other untested ligands or mixes of ligands may exhibit equally good qualitative trends. NPs may exhibit variation in the state of aggregation in each solvent, a negligible contributor to enthalpy of solution,61 and variation in ligand-solvent interaction, a significant contributor to enthalpy of solution values. Enthalpies of solution of asphaltenes include these effects plus the potential for dissolution, an unknown and variable positive contributor to the enthalpy of solution. Thus, direct quantitative comparison between values for the NPs and asphaltenes is precluded.

3.4 Interfacial Tension Measurements The pendant drop technique is a reliable technique for interfacial tension measurement between immiscible liquids. For saturated liquids with small mutual solubilities, in the absence of chemical impurities, and significant variation in drop volume, repeatability uncertainties less than 0.10 mN/m can be obtained. Such idealized conditions are not possible to obtain in a multiuser facility. For this work, measurement repeatability (within experiments) is ± 0.25 mN/m and measurement reproducibility (among experiments) is ± 1 mN/m. Further, when a drop is injected into an immiscible liquid, for example toluene + NPs into water, the initial value of the interfacial 24 ACS Paragon Plus Environment

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tension should be the same as for toluene + water because diffusion of NPs to the interface takes time. Composition and time dependent formation of structures within sorbates at liquid-liquid interfaces can also impact the steady state interfacial tension values obtained. Care must be taken in interpreting the absolute values of measurements, the differences among measurements, and the changes in interfacial tension measurements with time. Different uncertainties apply to each case. Repeatability uncertainties are used to interpret changes in interfacial tension over time within an experiment. Reproducibility uncertainties are used to compare outcomes among experiments. No individual solvents or mix of solvents were identified where both Au-SC11OH NPs and asphaltenes disperse. Au-SC11OH NPs disperse in alkanols and pyridine; asphaltenes do not. Consequently, Au-SC11OH NPs were excluded from the comparative interfacial tension part of this study. Some interfacial tension measurements, reported in the Supporting Information, were made with mixtures including methanol and dodecane, but the interfacial tension values (~3 mN/m) were too low to distinguish differences among measurements including background interfacial tension measurements. 3.4.1 Nanoparticles at Toluene-Water and Toluene + Heptane-Water Interfaces Au-SC12 and Au-biphenyl NPs disperse in toluene and heptane. However, Au-biphenyl NPs aggregate within minutes of sonication in heptane. The impacts of Au-SC12 and Au-biphenyl NPs on the interfacial tension of toluene and toluene + heptane-water mixtures at 23 °C and atmospheric pressure are shown in Figures 11 and 12, respectively. For Au-SC12 NPs, variations with composition and time fall at or within the relevant uncertainties and are not readily discriminated from background measurements for toluene-water or toluene + heptane-water


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interfaces. Au-SC12 NPs are minimally surface active. By contrast, variations with composition and time for the Au-biphenyl NPs are evident.

Figure 9. Comparisons between enthalpies of solution of synthesized Au-SC8 NPs (light grey) and (a) purchased Au-SC8 NPs (horizontal stripes) at 60 °C and 1000 mg/L, and (b) synthesized Au-SC12 NPs (diagonal stripes) at 25 °C, and 1000 mg/L.

Figure 10. A comparison between the enthalpies of solution of Au core NPs and asphaltenes, at 1000 mg/L, in diverse solvents at (a) 25 °C and (b) 60 °C. Au-SC8 (light grey), Au-SC11OH (white), Au-biphenyl (gridded), asphaltenes (black).


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3.4.2 Asphaltenes at Toluene-Water and Toluene + Heptane-Water Interfaces C5 Athabasca asphaltenes disperse in toluene and heptane. However, they aggregate/flocculate within minutes of sonication in toluene:heptane 1:3 and in heptane. The impacts of C5 Athabasca asphaltene on the interfacial tension of toluene-water and toluene + heptane-water mixtures at 23 °C and atmospheric pressure are included in Figures 11 and 12. These asphaltenes clearly exhibit interfacial activity but care must be taken in interpreting the outcomes because even though the effect is well known,79 there are disagreements on the attribution. For example, the presence of polar and nonpolar functionalities is cited as the reason for their interfacial activity, but the low frequency and distributed nature of these functionalities is thought to be inconsistent with their high surface activity.80 Sorption is treated as molecular, in some cases,81 even though asphaltenes aggregate in “good” solvents like toluene at ppm level concentrations.4 Czarnecki et al.82 also distinguish the roles of aggregates vs. flocks at interfaces in the behavior of oil water emulsions. The behavior of water drop interfaces in Athabasca bitumen + mixtures of toluene and heptane has been subdivided into two regimes based on asphaltenes solubility.83 Rane et al.80 found that the adsorption of asphaltenes at synthetic oil - water interfaces is slow; after 60 h, steady state was not attained in some cases. This suggests that a slow exchange process occurs at such interfaces.84 A denser/thicker/multi-layer of asphaltenes develops at the drop interface as asphaltenes interact with one another.85 Irrespective of the details of the attributions, from the perspective of this comparative work, both the kinetics of interfacial tension change and the steady state values attained are of importance.


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Figure 11. Measured interfacial tension values for toluene-water + NPs and asphaltenes at 23 °C: a) 100 mg/L, b) 500 mg/L, c) 1000 mg/L. Symbols: Au-SC12 (diamonds), asphaltenes (open squares), Au-biphenyl (triangles), toluene-water interfacial tension this work (dashed line).


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Figure 12. Measured interfacial tension values for (toluene + heptane)-water + NPs and asphaltenes at and 23 °C: a) toluene:heptane 3:1 by volume, b) toluene:heptane 1:1 by volume, c) toluene:heptane 1:3 by volume, d) heptane. Symbols: Au-SC12 (diamonds), Au-biphenyl (triangles), asphaltenes (open squares), toluene + heptane mixture (open circles).

3.4.3 Nanoparticle vs. C5 Athabasca Asphaltene Behaviors at Steady State The interfacial tension values for toluene-water interfaces with Au-SC12, and Au-biphenyl NPs and C5 Athabasca asphaltenes are compared at 100, 500 and 1000 mg/L in Figures 11a-c respectively. At all three concentrations, the interfacial tension values for asphaltenes fall between those of Au-SC12 and Au-biphenyl NPs but the asphaltenes more closely track the interfacial tensions of the Au-SC12 NPs than the Au-biphenyl NPs. By progressively adding heptane to the toluene, where the ratios of toluene to heptane are 3:1, 1:1, 1:3 and pure heptane, 29 ACS Paragon Plus Environment

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Figure 12a-d, the behavior of the C5 Athabasca asphaltenes shifts toward and then beyond the track for the Au-biphenyl NPs. Au-SC12 NPs disperse in both toluene and heptane, and have no clear impact on the interfacial tension values vis-à-vis the solvent. By contrast, C5 Athabasca asphaltenes and Au-biphenyl NPs aggregate within minutes of sonication in toluene:heptane 1:3 and pure heptane. Their composition in the drops is uncertain, and the Au-biphenyl NPs and asphaltene aggregates/flocks are likely to accumulate at the base of drops – affecting their shape and hence the measured interfacial tension values. The behaviors of the Au-biphenyl NPs and the C5 Athabasca asphaltenes are qualitatively similar. From a quantitative perspective, the Aubiphenyl NPs are more surface active than the C5 Athabasca asphaltene nanoaggregates while the impacts of flocks are comparable at steady state on a mass basis. More detailed investigation and comparative analysis of the properties of interfacial layers formed comprise aspects of future work.

3.4.4 Comparative Kinetics of Au-Biphenyl NPs and C5 Athabasca Asphaltene Behaviors Parity plots showing the interfacial tensions of Au-biphenyl NPs and C5 Athabasca asphaltenes at fixed time provide information about the relative kinetics of specie accumulation at an interface. The parity plots shown in Figures 13a-d are illustrative. At low concentration in toluene, Figure 13a, asphaltenes and the Au-biphenyl NPs have comparable kinetic behaviors. At higher concentrations, Figure 13b and c, there are asphaltene components that impact interfacial tension more quickly (at short times) and more slowly (at longer times) than the Au-biphenyl NPs. This behavioral difference is evident in toluene + heptane mixtures as well, Figure 13d. At short times, the asphaltene components appear to sorb more quickly than the Au-biphenyl NPs and the asphaltenes continue to sorb after the NPs appear to have reached a steady state value.


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Figure 13. Parity plots for the variation of C5 Athabasca asphaltenes and Au-biphenyl NPs: a) in toluene at 100 mg/L, b) in toluene at 500 mg/L, c) in toluene at 1000 mg/L, d) in toluene:heptane 3:1 at 500 mg/L.

4. CONCLUSIONS Understanding mechanisms that govern the behaviors of asphaltenes in solution and at interfaces is a core competence for the petroleum industry. In this work, electrically-neutral Au core + organic ligand NPs including alkyl, alcohol, and aromatic features that are thought to be present on asphaltene nanoaggregate surfaces were synthetized and characterized, and then used to probe asphaltene behaviors in solution and at interfaces. The solution and interfacial behaviors of these three NP types are shown to differ from one another both quantitatively and qualitatively. The 31 ACS Paragon Plus Environment

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signs and trends of the enthalpy of solution values for Au-SC8 NPs mimic the behaviors of C5 Athabasca asphaltenes in solution at near infinite dilution. The Au-SC11OH and the Au-biphenyl NPs do not mimic the solution enthalpy behaviors of asphaltenes at near infinite dilution. AuSC11OH NPs do not disperse in toluene or toluene + heptane mixtures and Au-SC12 NPs are minimally active at toluene–water or (toluene + heptane)-water interfaces. Au-SC12 NPs also do not form flocs in heptane or toluene + heptane mixtures. These behaviors contrast with the interfacial and flocculation behaviors of C5 Athabasca asphaltenes and Au-biphenyl NPs. Both disperse in toluene and form flocs in toluene + heptane mixtures; both are also active at toluene– water or (toluene + heptane)-water interfaces. In toluene and in toluene + heptane mixtures where asphaltenes and Au-biphenyl NPs do not aggregate significantly, Au-biphenyl NPs are more surface active than asphaltenes, on a mass basis. Under conditions where both the Au-biphenyl NPs and asphaltenes aggregate over time, asphaltenes are more surface active. The kinetics of adsorption of Au-biphenyl NPs and asphaltenes at these interfaces differ. Asphaltenes include species that sorb both more and less rapidly than the Au-biphenyl NPs. While it is possible that other Au core + individual ligand NPs may prove to be better mimics for the behavior of asphaltene nanoaggregates in solution and at interfaces, this survey shows that asphaltene nanoaggregate behaviors in solution and at interfaces are complex. Floc formation and interfacial properties are consistent with the behavior of gold core NPs with aromatic ligands, while solution behaviors, at near infinite dilution, are consistent with those of gold core NPs with alkly ligands. In future works we plan to prepare and then evaluate the solution and interfacial behaviors of individual Au core NPs with mixed aromatic/alkyl ligands and mixtures of Au core NPs with individual aromatic/alkyl ligands, and to extend the work to include depletion flocculation phenomena86 as we pursue this line of inquiry further.


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Acknowledgements The authors thank Dr. Hongbo Zeng and his research group at the University of Alberta, and the undergraduate students: Ifeanyichukwu (Bobby) Uyanwune (Co-op student) and Ramanish Singh (Mitacs Internship) for their assistance with interfacial tension measurements.

The authors

gratefully acknowledge financial support from the sponsors of the NSERC Industrial Research Chair in Petroleum Thermodynamics: the Natural Sciences and Engineering Research Council of Canada (NSERC), Alberta Innovates Energy and Environment Solutions, BP Canada, ConocoPhillips Canada Resources Corp., Nexen Energy ULC, Shell Canada Ltd., Total E&P Canada Ltd., and the Virtual Materials Group. Acknowledgement is made to the donors of the American Chemical Society Petroleum Research Fund for support of A.Q. in this research through grant ACS-PRF (54149-ND5).

Supporting Information The supplemental information includes: enthalpies of solution of AuNPs and asphaltenes in diverse solvents at 25 °C, Table S1 and 60 °C Table S2; and interfacial tension measurements for (methanol + Au-SC11OH) in dodecane at 23 °C, Figure S1. This information is available free of charge via the Internet at http://pubs.acs.org/.

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80. Rane, J. P.; Harbottle, D.; Pauchard, V.; Couzis, A.; Banerjee, S., Adsorption Kinetics of Asphaltenes at the Oil–Water Interface and Nanoaggregation in the Bulk. Langmuir 2012, 28, 9986-9995. 81. Rane, J. P.; Pauchard, V.; Couzis, A.; Banerjee, S., Interfacial Rheology of Asphaltenes at Oil–Water Interfaces and Interpretation of the Equation of State. Langmuir 2013, 29, 4750-4759. 82. Czarnecki, J.; Tchoukov, P.; Dabros, T., Possible Role of Asphaltenes in the Stabilization of Water-in-Crude Oil Emulsions. Energy Fuels 2012, 26, 5782-5786. 83. Wu, X., Investigating the Stability Mechanism of Water-in-Diluted Bitumen Emulsions through Isolation and Characterization of the Stabilizing Materials at the Interface. Energy Fuels 2003, 17, 179-190. 84. Bhardwaj, A.; Hartland, S., Dynamics of Emulsification and Demulsification of Water in Crude Oil Emulsions. Ind. Eng. Chem. Res. 1994, 33, 1271-1279. 85. Jeribi, M.; Almir-Assad, B.; Langevin, D.; Hénaut, I.; Argillier, J. F., Adsorption Kinetics of Asphaltenes at Liquid Interfaces. J. Colloid Interface Sci. 2002, 256, 268-272. 86. Pouralhosseini, S.; Eslami, F.; Elliott, J. A. W.; Shaw, J. M., Modeling the Phase Behavior of Asphaltene + Toluene + Polystyrene Mixtures—A Depletion Flocculation Approach. Energy Fuels 2016, 30 (2), 904-914.


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Energy & Fuels

Mimicking#Asphaltene#Nanoaggregate#ProperFes#with#Gold#Core#NanoparFcles# Gold(SC8/C12#

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Gold Core Nanoparticle Mimics for Asphaltene ... - ACS Publications

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