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Probing the Interaction Mechanism between Air Bubbles and Bitumen Surfaces in Aqueous Media Using Bubble Probe AFM Lei Xie, Chen Shi, Xin Cui, Jun Huang, Jingyi Wang, Qi Liu, and Hongbo Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02693 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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Probing the Interaction Mechanism between Air Bubbles and Bitumen Surfaces in Aqueous Media Using Bubble Probe AFM

Lei Xie, Chen Shi, Xin Cui, Jun Huang, Jingyi Wang, Qi Liu, Hongbo Zeng* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada *Email: [email protected], Phone: +1-780-492-1044, Fax: +1-780-492-2881

ACS Paragon Plus Environment

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ABSTRACT Surface interactions involving deformable air bubbles have attracted tremendous interest in a wide range of engineering applications, such as mineral flotation and bitumen extraction. In this work, for the first time, the interaction forces between air bubbles and bitumen surfaces in complex aqueous media of varying pH, salinity and salts were directly measured using a bubble probe atomic force microscope (AFM) technique. The AFM topographic imaging reveals that bitumen surface tends to be rougher and form distinct domains at high NaCl concentration or under strongly alkaline environment. The force measurements demonstrate the critical role of ionic strength and solution pH in bubble-bitumen interaction and attachment, which could be well described by a theoretical model based on Reynolds lubrication theory and augmented Young-Laplace equation by including the effect of disjoining pressure. In 1 mM NaCl, the electrical double layer (EDL) repulsion inhibited bubble-bitumen attachment, and such repulsive effect could be further strengthened with increasing solution pH. In 500 mM NaCl, the hydrophobic attraction could lead to bubble-bitumen attachment, while a high solution pH could weaken the hydrophobic interaction. The addition of calcium ion in 500 mM NaCl could enhance the hydrophobic interaction and facilitate the bubble-bitumen attachment, most likely attributed to the bridging effect between calcium ions and the functional groups (e.g., carboxyl group) of interface-active molecules on bitumen surfaces thus leading to higher surface roughness and hydrophobic moieties/aggregates on bitumen as confirmed by AFM imaging. Our results provide quantitative information on the interaction mechanism between air bubbles and bitumen surfaces in complex aqueous solutions at the nanoscale, which has useful implications on many related interfacial interactions in industrial processes such as oil production, oil-water separation and wastewater treatment.


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1. INTRODUCTION The colloidal interaction involving deformable air bubbles has attracted tremendous interest from both fundamental and practical viewpoints, for its ubiquitous presence in a wide range of biological and technological applications,1-3 such as froth flotation,4 drug and gene delivery,5 and microfluidic devices.6 Air bubbles, due to the intrinsic hydrophobicity, can readily attract hydrophobic or partially hydrophobic materials in an aqueous medium and collect them at the air/water interface.7-9 Therefore, materials with different degrees of surface hydrophobicity can be selectively separated on the basis of their attachment propensities to air bubbles, which is the fundamental principle of industrial froth flotation process.4,

10-11

The bubble-surface

attachment is governed by the drainage process of the intervening water film under the combined influences of hydrodynamic pressure and disjoining pressure attributed to surface interactions.1214

Generally, repulsion arising from van der Waals (vdW) and electrical double layer (EDL)

interactions can impede thin water film drainage and prevent bubble-surface attachment, whereas strong hydrophobic attraction can lead to the rupture of thin water film and bubble-surface attachment. The Athabasca oil sands located in Alberta, Canada are the third largest oil reserves in the world, and water-based bitumen extraction is currently used to recover bitumen from mined oil sands.15 During this process (illustrated in Figure 1A), bitumen is aerated to form bitumen-air aggregates, floating to the top of slurry in the form of bitumen-rich froth. The collected bitumen froth is treated to provide clean bitumen products, in which extremely stable emulsions are commonly encountered but generally undesirable.15-17 Recent studies have revealed that bubblebitumen attachment during aeration is one of the most critical steps to determine the bitumen recovery and product quality.15 Hence, a quantitative understanding of the interaction mechanism


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and thin film drainage process between air bubble and bitumen surface under various aqueous conditions, which determines the formation of bitumen-air aggregates, is of both fundamental and practical importance to elucidate the bubble-bitumen attachment mechanism and optimize the current water-based bitumen extraction process. Over the last few years, significant progresses have been made to unravel the interaction mechanism involved in the water-based bitumen extraction process.18-21 Nanomechanical tools such as surface forces apparatus (SFA) and atomic force microscope (AFM) have been widely used to quantitatively measure the interaction forces of solid surfaces.22-26 Quantitative force measurements involving bitumen and asphaltenes (an interface-active component of bitumen) showed that the measured forces were dependent on aqueous conditions (e.g., pH, salinity, and salts), organic solvent and temperature.20-21,

27-28

Nevertheless, direct measurement of surface

forces between air bubbles and bitumen surfaces at the nanoscale with theoretical analysis of the associated thin film drainage process have not been reported, probably due to the practical difficulties in precise manipulation of deformable bubbles and interpretation of the measured results.29-31 Recently, the bubble/drop probe AFM has been developed and applied to study the interaction mechanisms involving deformable gas bubbles and emulsion drops, demonstrating the combinatory effects of hydrodynamic interaction, surface forces and bubble/drop deformation on the interaction.8,

11, 32-41

The measured force results could be successfully

described by a theoretical model based on Reynolds lubrication theory and augmented YoungLaplace equation. The film drainage process during interaction was also directly visualized by simultaneously measuring the force and separation using the bubble probe AFM combined with reflection interference contrast microscopy (RICM), which has validated the above theoretical


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model.32 Using the bubble/drop probe AFM technique, various surface forces at air/water and oil/water interfaces were precisely quantified, such as structural and hydrophobic interactions, with sub-nN resolution.14, 42-43 One of the most recent applications of this bubble/drop probe AFM technique was to measure the interaction mechanism of oil drops in water in the presence of asphaltenes, indicating that asphaltenes stabilized the oil-in-water emulsion by strengthening the repulsive EDL interaction and inducing steric repulsion.36 In this study, the bubble probe AFM technique was employed to quantitatively measure the interaction forces between air bubble and bitumen surface in complex aqueous media, and the measured force results were analyzed using the theoretical model based on Reynolds lubrication theory and augmented Young-Laplace equation. The surface energy of bitumen was measured using three-probe-liquid method, and the effects of solution conditions (e.g., ionic strength and solution pH) on the morphology of bitumen surface were also investigated. Our results for the first time provide valuable quantitative information on the interaction mechanism between air bubbles and bitumen surfaces at the nanoscale, with useful implications for related interfacial processes in heavy oil production and wastewater treatment.


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Figure 1. (A) Illustration of bubble-bitumen-solid interactions in complex aqueous medium in water-based bitumen extraction process to recover bitumen from mined oil sands. (B) Schematic of force measurements between air bubble and bitumen surface in an aqueous solution using the bubble probe AFM technique. It is noted that the bubble size typically ranges from tens of µm to several mm in industrial bitumen extraction process (Figure 1A),10 and the typical bubble size in AFM force measurements can be varied from tens to hundreds of um (Figure 1B).

2. MATERIALS AND METHODS 2.1. Materials. Sodium chloride (NaCl, ACS reagent grade) and calcium chloride (CaCl2, ACS reagent grade) were purchased from Fisher Scientific and used as received without further purification. All aqueous solutions were prepared using Milli-Q water (Millipore deionized, 18.2 MΩ·cm resistivity). Hydrochloric acid (HCl, ACS reagent grade) and sodium hydroxide (NaOH, ACS reagent grade) purchased from Fisher Scientific were used to adjust solution pH. 2.2. Preparation of Bitumen Surface. Bitumen solution was prepared by adding bitumen (provided by Shell) into toluene at a concentration of 3 mg/mL and then centrifuging at 7000 rpm (or ~600 times the force of gravity) for 30 min to remove possible fine solids. After filtration with 0.2 µm PTFE membrane, the bitumen solution was spin coated on silica wafers at 2000 rpm for 40 s to ensure the full and uniform bitumen coverage on silica wafers; thereafter, the prepared bitumen surface was dried overnight to remove the remaining toluene. 2.3. Surface Energy Measurement. Three-probe-liquid method was used to determine the surface energy of bitumen. A contact angle goniometer (ramé-hart instrument Co., NJ, USA) was used to measure the contact angles of three probing liquids, including one nonpolar (i.e., diiodomethane) and two polar (i.e., water and ethylene glycol) liquids, on bitumen surface using a sessile drop method. For the same type of liquids, at least two different surfaces and three different positions on each surface were tested, and the average contact angle was reported.


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Surface energy γ with Lifshitz-vdW γ LW and Lewis acid-base γ AB (electron acceptor γ + and electron donor γ − ) components can be derived by a method developed by van Oss et al. in eq 1.44-45

γ = γ LW + γ AB = γ LW + 2 γ +γ −

(1)

The relation between liquid contact angle on bitumen surface θ and surface energy components of bitumen surface and liquid can be given by eq 2, where the subscript B or L represents bitumen or liquid, respectively.44-45

γ L (cosθ + 1) = 2( γ BLW γ LLW + γ B+γ L− + γ B−γ L+ )

(2)

To determine surface energy components of bitumen surface, γ BLW , γ B+ and γ B− , three different probing liquids of known surface energy components should be used for contact angle measurements, and then three equations can be obtained based on eq 2. Thus, the energy components of bitumen surface can be obtained by eq 3, where L1, L2 and L3 represent three different probe liquids.

 γ LW  B  γ B+   γ−  B

   γ LW    L1  = 2  γ LLW2       γ LW    L3

γ

− L1

γ

− L2

γ L−3

γ   γ L+2   γ L+3   + L1

−1

 γ L1 (cosθ1 + 1)   γ (cosθ + 1)  2  L2   γ (cosθ + 1)  3  L3 

(3)

2.4. Surface Characterization. Tapping mode imaging was applied to characterize the topography and roughness of bitumen surface both in air and aqueous solution with MFP-3D AFM system (Asylum Research, Santa Barbara, CA, USA). The silicon probe (RTESP-150, Bruker) and silicon nitride probe (SCANASYST-FLUID, Bruker) were respectively used for tapping mode imaging in air and liquid. A Zetasizer Nano (Malvern Instruments Ltd., United Kingdom) was used to measure the zeta potential of bitumen drop in 1 mM NaCl at different pHs.


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2.5. Force Measurements. The interaction between air bubble and bitumen surface was measured using bubble probe AFM technique. Air bubbles were generated and immobilized on the glass disk of a fluid cell by purging air through an ultra-sharp glass pipette into aqueous solution. A custom-made rectangular silicon cantilever (400 × 70 × 2 µm) with a circular gold patch was used to pick up an air bubble of suitable size (typically 60-90 µm radius) to create a bubble probe. Prior to creating the bubble probe, the glass disk and AFM cantilever were hydrophobized following an established method.8, 11, 32 The spring constant of the cantilever was determined to be 0.3-0.4 N/m using the Hutter and Bechhoefer method.46 The cantileveranchored bubble was positioned over bitumen surface and then driven to approach/retract toward the surface until a fixed deflection of the cantilever was reached or bubble attachment occurred. The force measurements were conducted at a fixed driving velocity of 1 µm/s to minimize the hydrodynamic effect unless the effect of hydrodynamic condition on the bubble-bitumen interaction was considered. The movement of the cantilever with anchored air bubble and the corresponding interaction forces were recorded as a function of time by AFM software. After each force measurement, the surface tension of water was measured. The measured surface tension remained unchanged before and after the experiments, indicating the possible interfaceactive components (e.g., asphaltenes) in bitumen had negligible influence on the force measurements in this work. A schematic of typical experiment setup using the bubble probe AFM is illustrated in Figure 1B. It is noted that in common practice of froth flotation, chemical reagents such as surfactants (as frother) can be adsorbed at air/water interface, changing the air/water interfacial properties (e.g., interfacial tension) and thereby affecting the bubble-bubble and bubble-substrate interactions. In this work, we only considered a model system without the addition of chemical reagent.


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2.6. Theoretical Model. The measured force results between air bubble and bitumen surface was analyzed by a theoretical model based on Reynolds lubrication theory coupled with augmented Young-Laplace equation.

The drainage process of confined thin water film between air bubble and bitumen surface is described by the Reynolds lubrication theory.8, 11, 32-41

∂h 1 ∂  3 ∂p  =  rh  ∂t 12µ r ∂r  ∂r 

(4)

where h(r, t) is the thin film thickness, µ is the dynamic viscosity of water, and p(r, t) is the excess hydrodynamic pressure in the film relative to the bulk solution. Consistent with recent reports, immobile boundary condition was assumed at air/water and bitumen/water interfaces.8, 11, 14, 32-36, 42-43, 47

The bubble deformation under the combined effect of hydrodynamic pressure and disjoining pressure is described by the augmented Young-Laplace equation.8, 11, 32-41

γ ∂  ∂h 

2γ − p−∏ r  = r ∂r  ∂r  R

(5)

where γ is the air/water interfacial tension, R is the bubble radius, and Π(r, t) is the overall disjoining pressure contributed from surface forces such as vdW, EDL, and hydrophobic interactions. The disjoining pressure attributed to vdW, EDL and hydrophobic interactions ΠvdW, ΠEDL and ΠHB can be given by eqs 6, 7 and 8, respectively, where AA−W −B is the Hamaker constant for air-water-bitumen, κ −1 is the Debye length, ψ A and ψ B are the surface potential of air bubble


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and bitumen surface, respectively, D0 is the decay length of hydrophobic interaction and C is a constant (N/m). 8, 11, 32, 48

∏vdW = −

∏ EDL =

AA−W − B 6π h3

(6)

2ε 0εκ 2 ( e+κ h + e−κ h )ψ Aψ B − (ψ A2 +ψ B2 ) 

∏ HB = −

(e

+κ h

− e −κ h )

2

C γ (1 − cos θ ) − h / D0 e − h / D0 = − e 2π D0 D0

(7)

(8)

The overall interaction between an air bubble and a bitumen surface F(t) is theoretically calculated by integrating p(r, t) and Π(r, t) based on Derjaguin approximation (eq 9).8, 11, 32-41 ∞

F (t ) = 2π ∫ ( p ( r , t ) + ∏ ( h ( r , t )))rdr

(9)

0

By fitting the measured force results with the theoretical model of Reynolds lubrication theory coupled with augmented Young-Laplace equation, parameters such as surface potential ψ and decay length D0 can be determined, and the evolution of confined thin film thickness h(r, t) with time can also be calculated.

3. RESULTS AND DISCUSSION 3.1. Surface Energy. The contact angles of three probing liquids (i.e., diiodomethane, water and ethylene glycol) on bitumen surface were determined using a sessile drop method. Based on the measured contact angle values and known surface energy components of these three probing liquids, the surface energy components of bitumen were determined. As shown in Table 1, the nonpolar liquid (e.g., diiodomethane) showed a lower contact angle on bitumen surface than the polar liquid (e.g., water), indicating the natural hydrophobicity of bitumen


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surface. The surface energy of bitumen was determined to be γ = 40.32 mJ/m2, comprising the Lifshitz-vdW component γ LW of 40.31 mJ/m2 and Lewis acid-base component γ AB of 0.01 mJ/m2. The extremely low γ AB value reveals that bitumen surface is almost non-polar. Table 1. Measured contact angle values (°) of probing liquids (diiodomethane, water and ethylene glycerol) on bitumen surface, literature reported surface energy components (mJ/m2) of probing liquids44-45 and calculated surface energy components (mJ/m2) of bitumen

γ+

γ LW

γ−

γ

γ AB

Measured contact angle on bitumen surface

Diiodomethane

50.80

0.00

0.00

0.00

50.80

38.6 ± 1.3

Water

21.80

25.50

25.50

51.00

72.80

93.2 ± 1.5

Ethylene glycerol

29.00

1.92

47.00

19.00

48.00

61.5 ± 0.9

Bitumen

40.31

0.00

0.90

0.01

40.32

(calculated)

The Lifshitz-vdW and Lewis acid-base interactions could be considered based on the free energy of interaction between air and bitumen in water. The free energy of Lifshitz-vdW

∆GALW−W −B and Lewis acid-base ∆GAAB−W −B interactions for air-water-bitumen is respectively calculated to be 15.69 and -92.36 mJ/m2 based on eqs 10 and 11.44-45 ∆GALW−W − B = 2 γ WLW ( γ BLW − γ WLW )

(10)

∆G AAB−W − B = 2[ γ W+ ( γ B− − γ W− ) + γ W− ( γ B+ − γ W+ )]

(11)

where the subscript A, B or W represents air, bitumen or water, respectively. The positive

∆GALW−W −B suggests that the repulsive vdW interaction inhibits the attachment between air and AB bitumen in water. The negative ∆GA−W −B reveals that the attractive interaction, including

hydrophobic interaction, facilitates the attachment between air and bitumen in water.


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The Hamaker constant for two identical materials interacting in vacuum could be 2 estimated using γ ≈ A / 24π l0 , where l0 = 0.165 nm is the typical cut-off separation.48 Thus, the

Hamaker constant of bitumen ABB is calculated to be 8.3 × 10-20 J. Based on the Hamaker constant of water AWW = 3.7 × 10-20 J, the Hamaker constant for air and bitumen interacting across water AA-W-B is estimated to be -1.8 × 10-20 J.48 From thermodynamic consideration, the interaction energy per unit area of two planar half-spaces in water should approach their work of adhesion with separation h decreasing to 0. Therefore, the constant C in eq 8 is determined to be 0.41 N/m. 3.2. Surface Morphology. The AFM topographic image of bitumen surface in air is shown in Figure 2. The root-mean-square (rms) roughness is about 0.45 nm, which indicates that a very smooth bitumen surface can be obtained by the spin coating process.

Figure 2. AFM topographic image (5 × 5 µm2) of spin-coated bitumen surface in air

The morphology of bitumen surface in aqueous solutions with various ionic strengths and pH conditions was also investigated. As illustrated in Figure 3, the bitumen surface became noticeably rougher than that in air and some distinct domains were observed on bitumen surface, which coincided with the protrusion structure observed on bitumen droplet in water from freeze-


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fracture scanning electron microscope image.18, 49 Interestingly, the size of aggregation domains formed in 1 mM NaCl (Figures 3A, 3B and 3C) was found to be evidently smaller than that in 500 mM NaCl (Figures 3D, 3E and 3F). It is worth noting that all the bitumen substrates were imaged after being immersed in aqueous solutions for at least 15 min, after which the morphologies remained almost unchanged even after several hours. It is noted that the formation of nanobubbles on bitumen surface can be ruled out in this study based on AFM imaging due to the continuous aggregation patterns on bitumen surfaces, similar module of the aggregation domains and surrounding areas, and the increased size of aggregation domains with increasing pH and salt concentration, which are very different from the observed nanobubbles on hydrophobic substrates reported previously.35

Figure 3. The morphology (5 × 5 µm2) of bitumen surface in 1 mM NaCl at pH 4.0 (A), pH 5.8 (B) and pH 8.5 (C) and 500 mM NaCl at pH 4.0 (D), pH 5.8 (E) and pH 8.5 (F).

In 1 mM NaCl, the rms roughness of bitumen surface increased from 0.53 nm to 0.57 and 0.59 nm with solution pH increasing from 4.0 to 5.8 and 8.5. It is widely recognized that bitumen


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contains large amounts of natural interface-active components (e.g., asphaltenes) with functional groups (e.g., carboxyl and phenolic groups) that can be deprotonated with the addition of alkaline.20,

50

Therefore, the deprotonation of polar functional groups and conformational

rearrangement of the molecules at bitumen/water interface at higher pH could lead to larger roughness. As solution pH increased from 4.0 to 5.8 and 8.5 in 500 mM NaCl, the rms roughness of bitumen surface raised from 1.28 nm to 1.50 and 2.07 nm. The enhanced surface roughness at high salt concentration is mainly attributed to the decreased electrostatic repulsion among the polar functional groups on bitumen molecules and/or aggregates. 3.3. Bubble-bitumen Interaction in 1 mM NaCl. Figure 4 shows the interaction force profiles measured between air bubble and bitumen surface in 1 mM NaCl at different pH conditions at v = 1 µm/s. It is worth noting that no bubble attachment could be observed during the approach-retraction cycle under all the pH conditions tested. With the cantilever-anchored bubble approaching bitumen surface, strong repulsion was registered that prevented the bubble from attaching to bitumen surface. During retraction of the cantilever, the measured repulsion gradually decreased until an attractive maximum was achieved due to the hydrodynamic suction effect.51-52 For the air-water-bitumen system, the vdW interaction is repulsive at any separation, and the EDL interaction between negatively charged air bubble and bitumen surface is also repulsive. In 1 mM NaCl, the Debye length is calculated to be 9.6 nm, and the relatively longrange EDL repulsion could inhibit the bubble attachment on bitumen surface. The surface potentials of air bubble in 1 mM NaCl at different pH conditions were obtained by conducting the bubble-bubble interaction measurements (Figure S1 in supporting information) and compared with the literature values in Table 2. The aforementioned theoretical model incorporating surface forces was applied to analyze the measured forces (open symbols) and the fitted results were


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shown as the red curve in Figures 4A, 4B and 4C. The fitted surface potentials of bitumen surface in 1 mM NaCl at different pH conditions are summarized in Figure 5, which are consistent with the measured zeta potentials and literature values.20, 50 As solution pH increased from 4.0 to 5.8 and 8.5, surface potential of bitumen changed from -35 ± 5 mV to -64 ± 8 and 80 ± 10 mV, thereby strengthening the EDL repulsion between air bubble and bitumen surface. The change of surface potential of bitumen is most likely due to the increased adsorption of hydroxide ion and the deprotonation of functional groups (e.g., carboxyl and phenolic groups) of interface-active components at bitumen/water interface with the addition of alkaline.

Figure 4. Interaction forces (A-C), calculated thin film profiles at maximum force load (D-F) and calculated disjoining pressure profiles (G-I) between air bubble and bitumen surface in 1 mM NaCl at pH 4.0 (A, D and G), pH 5.8 (B, E and H) and pH 8.5 (C, F and I) at v = 1 µm/s (open symbols for experiment results and solid curves for theoretical calculations). The


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theoretical calculations were based on the interactions of a bubble vs. a flat substrate. The bubble radius R is 82 µm at pH 4.0, 81 µm at pH 5.6, and 81 µm at pH 8.5.

The calculated profiles of thin water film confined between air bubble and bitumen surface at maximum force load are illustrated in Figures 4D, 4E and 4F. The central portion of bubble surfaces was flattened due to the EDL disjoining pressure that balanced the Laplace pressure inside the bubbles. With solution pH increasing from 4.0 to 5.8 and 8.5, the central separation between air bubble and bitumen surface dramatically increased from 17.4 nm to 28.7 and 32.2 nm, suggesting that the enhanced EDL repulsion at higher solution pH could effectively impede thin water film drainage and prevent air bubble from getting close to bitumen surface. It is worth noting that surface roughness plays an important role in surface force measurements in fluids.53, 54 The rms roughness of bitumen surfaces under different pH conditions in Figure 4 was almost the same, thus the effect of surface roughness on the comparison of bubble-bitumen surface interactions under various solution conditions can be neglected here. It should be also noted that all the theoretical calculations were based on the interactions of a bubble vs. a flat substrate. The calculated disjoining pressure profiles between air bubble and bitumen surface are shown in Figures 4G, 4H and 4I. The EDL repulsion is the dominant interaction and the hydrophobic attraction is too weak to trigger the bubble-bitumen attachment.

Table 2. Comparison of theoretically fitted surface potential values of air bubble and the literature values in 1 mM NaCl55-57 pH

Fitted value (mV)

Literature value (mV)

4.0

-20 ± 5

-15


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-10 5.8

-32 ± 6

-35 -37

8.5

-45 ± 8

-50 -65

Figure 5. Comparison of theoretically fitted surface potential and measured zeta potential values of bitumen in 1 mM NaCl with literature reported zeta potential values in 1 mM KCl.20, 50

3.4. Bubble-bitumen Interaction in 500 mM NaCl. Figure 6 shows the interaction forces between air bubble and bitumen surface in 500 mM NaCl at various pH conditions at v = 1 µm/s. As shown in Figure 6A, a sudden “jump-in” behavior was observed during approach when the measured force reached ~6.5 nN, indicating that the bubble was attached on bitumen surface, which was observed from the optical microscope. In 500 mM NaCl, the EDL interaction is significantly screened and vdW interaction is repulsive, so the observed bubble attachment was induced mainly by the attractive hydrophobic interaction which was incorporated into the


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aforementioned theoretical model. By fitting the measured forces (open symbols) with the theoretical model (red curve), the decay length of hydrophobic interaction was determined to be D0 = 0.95 ± 0.05 nm. The calculated profile of thin water film confined between air bubble and bitumen surface in Figure 6D shows that the critical central separation just before bubble attachment was calculated to be 9.2 nm. In Figures 6B and 6C, the “jump-in” behavior was also observed at the measured force of ~7.5 nN at pH 5.8 and ~8.2 nN at pH 8.5. The theoretical calculation shows that the decay length of hydrophobic interaction was predicted to be D0 = 0.78 ± 0.05 nm at pH 5.8 (Figure 6B) and D0 = 0.65 ± 0.07 nm at pH 8.5 (Figure 6C), and the critical central separation just before bubble attachment was calculated to be 8.3 nm at pH 5.8 (Figure 6E) and 6.9 nm at pH 8.5 (Figure 6F), both of which indicates the weakened hydrophobic attraction with the addition of alkaline. The hydrophobic interaction is believed to be closely correlated to the entropic effect originating from the disruption of the hydrogen bonding network between water molecules adjacent to the hydrophobic surface.48, 58-59 The bitumen wettability was evaluated by captive bubble method, where the solid substrate was immersed in aqueous solution with coated bitumen facing downward and a bubble was injected beneath the bitumen surface. In the captive bubble method, the bitumen surface properties would be the same as that in the AFM force measurements under the respective solution conditions. By analyzing the bubble profile, the water contact angle at pH 4 was measured to be 77.1° ± 2.0°, which decreased to 71.7° ± 1.5° at pH 5.8 and 66.8° ± 1.7° at pH 8.5. The decreased surface hydrophobicity is most likely due to the deprotonation of functional groups (e.g., carboxyl and phenolic groups) of interface-active molecules in alkaline environment that enables higher mobility of water molecules around bitumen surface. The change of bitumen wettability may alter the mobility of water molecules and water correlations at the vicinity of bitumen/water interface, and in response


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alter the magnitude and range of hydrophobic interaction. The calculated disjoining pressure profiles between air bubble and bitumen surface in Figures 6G, 6H and 6I shows the hydrophobic attraction is the driving interaction for the bubble attachment on the bitumen surface. It should be noted that in this work the water contact angles on bitumen surfaces measured using the captive bubble method in aqueous media and the sessile drop method in air show good agreement with the values reported previously.20, 50, 60 The difference of the water contact angles measured using these two methods was because that the bitumen surfaces were immersed in aqueous solution for the captive bubble method and the functional groups of natural interface-active components could be deprotonated and exposed at bitumen/water interface, thereby changing the surface morphology and wettability.


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Figure 6. Interaction forces (A-C), calculated thin film profiles at maximum force load (D-F) and calculated disjoining pressure profiles (G-I) between air bubble and bitumen surface in 500 mM NaCl at pH 4.0 (A, D and G), pH 5.8 (B, E and H) and pH 8.5 (C, F and I) at v = 1 µm/s (open symbols for experiment results and solid curves for theoretical calculations). The theoretical calculations were based on the interactions of a bubble vs. a flat substrate. The bubble radius R is 73 µm at pH 4.0, 74 µm at pH 5.6, and 77 µm at pH 8.5.

3.4. Effect of Calcium Ion. In industrial processes such as oil production from the Athabasca oil sands, the process water or recycled water of high salinity generally contains a certain amount of divalent cations such as Ca2+. It is important to understand the influence of calcium ions on the surface properties (e.g., morphology) of bitumen and bubble-bitumen interaction. The morphologies of bitumen surface in 500 mM NaCl at pH 8.5 with the addition of 1 mM and 10 mM CaCl2 are shown in Figures 7A and 7B, respectively. In contrast to the morphology without CaCl2 addition in Figure 3F, the aggregation domains observed on bitumen surface were much larger, and the rms roughness increased from 2.07 nm (Figure 3F) to 3.41 and 4.28 nm after the addition of 1 mM and 10 mM CaCl2, respectively (Figures 7A and 7B). The larger aggregation domains formed on bitumen surface in the presence of calcium ion is most likely through the strong interaction between calcium ion and the functional groups of interfaceactive molecules.


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Figure 7. The morphologies (5 × 5 µm2) of bitumen surface in 500 mM NaCl at pH 8.5 in presence of 1 mM (A) and 10 mM (B) CaCl2.

Figures 8A and 8B show the interaction between air bubble and bitumen surface in 500 mM NaCl at pH 8.5 with the addition of 1 mM and 10 mM CaCl2, respectively. The “jump-in” behavior was observed during approach when the measured force reached ~5.8 nN and ~4.7 nN. The theoretical calculation shows that the decay length of hydrophobic interaction was fitted to be D0 = 1.07 ± 0.07 nm and D0 = 1.20 ± 0.10 nm with the addition of 1 mM and 10 mM CaCl2, respectively, and the corresponding critical central separation just before bubble attachment was calculated to be 11.1 nm and 12.2 nm as illustrated in Figures 8C and 8D. In contrast to the interaction without CaCl2 addition (Figure 6C), the critical force load required for bubblebitumen attachment in the presence of calcium ion is much weaker, and the decay length of hydrophobic interaction and calculated central separation are higher. It is likely that calcium ion may bridge the carboxyl groups of interface-active components, which increases the surface roughness (Figure 7) and leaves the hydrophobic moieties exposed to the aqueous solution and thereby strengthens the apparent hydrophobic interaction between air bubble and bitumen surface. The calculated disjoining pressure profiles between air bubble and bitumen surface in Figures 8E and 8F shows the dominant role of hydrophobic attraction.


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Figure 8. Interaction forces (A and B), calculated thin film profiles at maximum force load (C and D) and calculated disjoining pressure profiles (E and F) between air bubble and bitumen surface in 500 mM NaCl at pH 8.5 with the addition of 1 mM (A, C and E) and 10 mM (B, D and F) CaCl2 at v = 1 µm/s (open symbols for experiment results and solid curves for theoretical calculations). The theoretical calculations were based on the interactions of a bubble vs. a flat substrate. The bubble radius R is 67 µm for 1 mM CaCl2 and 73 µm for 10 mM CaCl2. 3.5. Effect of hydrodynamic condition. The measured forces (open symbols) between air bubble and bitumen surface in 500 mM NaCl at pH 8.5 under higher hydrodynamic conditions (i.e., v = 5 and 20 µm/s) are shown in Figure 9. Compared to the bubble attachment observed during approach at v = 1 µm/s (Figure 6C), the bubble-bitumen attachment was observed during retraction at v = 5 µm/s (Figure 9A) and no bubble attachment was observed during the approach-retraction cycle at v = 20 µm/s (Figure 9B) under the same solution condition, indicating that the bubble attachment could be inhibited at higher approach velocity due to increased hydrodynamic force. The fitted decay length D0 = 0.65 ± 0.07 nm at v = 1 µm/s was used to predict the interaction force and bubble attachment behavior at v = 5 and 20 µm/s, and the theoretically


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calculated force results are shown as solid curves in Figure 9. It is obvious that theoretical calculation agrees very well with the measured force results, indicating that D0 = 0.65 ± 0.07 nm at pH 8.5 reflects the true hydrophobic interaction that is independent of hydrodynamic conditions. From the above results, it can be seen that the hydrodynamic force and surface forces (e.g., EDL and hydrophobic interactions) play critical roles in bubble-bitumen interaction and attachment. It is worth noting that the hydrophobic interaction was much less important than the hydrodynamic interaction at a higher velocity (see Figure S2 in Supporting Information).

Figure 9. Interaction forces between air bubble and bitumen surface in 500 mM NaCl at pH 8.5 at v = 5 µm/s (bubble radius R=75 µm) (A) and at v = 20 µm/s (bubble radius R=73 µm) (B) (open symbols for experiment results and solid curves for theoretical calculations).

CONCLUSIONS The colloidal interaction involving deformable air bubbles has attracted tremendous interest in a wide range of engineering applications. A bubble probe AFM technique was employed, for the first time, to directly measure the interaction forces between air bubbles and bitumen surfaces in complex aqueous media with varying solution salinity, pH and divalent electrolytes addition. The bitumen surface shows a rougher morphology with some distinct aggregation domains at higher ionic strength and/or higher solution pH. The force results reveal the critical role of ionic


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strength and solution pH in bubble-bitumen interaction and attachment, which could be analyzed by a theoretical model based on Reynolds lubrication theory and augmented Young-Laplace equation by including the effect of disjoining pressure. As solution pH increased from 4.0 to 8.5, the strengthened EDL repulsion could more readily inhibit bubble attachment in 1 mM NaCl. In 500 mM NaCl, the EDL repulsion was significantly suppressed and the hydrophobic attraction between bubble and bitumen played a critical role in the bubble-bitumen attachment. While it is also noted that the hydrophobic attraction between bubble and bitumen became weaker with increasing pH from 4.0 to 8.5 in 500 mM NaCl, which was mainly due to enhanced surface hydrophilicity of bitumen under higher pH condition. It was found that the presence of calcium ions in 500 mM NaCl could strengthen the hydrophobic attraction between air bubble and bitumen surface and facilitate the bubble-bitumen attachment, which was most likely due to the bridging interaction between calcium ions and the carboxyl groups of interface-active molecules on bitumen surfaces thereby leading to higher surface roughness and hydrophobic moieties/aggregates on bitumen as confirmed by AFM imaging. This work provides valuable quantitative information on the interaction mechanism between air bubbles and bitumen surfaces at the nanoscale, with important implications in heavy oil production. The methodology can be readily extended to many other interfacial interactions in industrial processes such as oil-water separation and wastewater treatment.

SUPPORTING INFORMATION Force profiles for bubble-bubble Interaction in 1 mM NaCl, and calculated profiles of disjoining pressure and hydrodynamic pressure under different interaction velocities.


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ACKNOWLEDGEMENTS We gratefully acknowledge the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Future Energy Systems under the Canada First Research Excellence Fund, the Canada Foundation for Innovation (CFI), the Alberta Advanced Education & Technology Small Equipment Grants Program (AET/SEGP) and the Canada Research Chairs Program (H. Zeng).


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