Applications of colloidal force measurements using a microcollider apparatus to oil sand studies

Colloids and Surfaces A: Physicochemical and Engineering Aspects 174 (2000) 133 – 146 www.elsevier.nl/locate/colsurfa

Applications of colloidal force measurements using a microcollider apparatus to oil sand studies X. Wu a,*, I. Laroche b, J. Masliyah b, J. Czarnecki a, T. Dabros c b

a Edmonton Research Center, Syncrude Canada Ltd., 9421 17th A6enue, Edmonton, Alta, Canada T6N 1H4 Department of Chemical and Materials Engineering, Uni6ersity of Alberta, Edmonton, Alta, Canada T6G 2G2 c CANMET, One Oil Patch Dri6e, PO Bag 1280, De6on, Alta, Canada T0C 1E0

Abstract Two colloidal force measurement techniques have been applied to oil sand studies both using the same apparatus called the microcollider. The techniques are colloidal particle scattering method for repulsion-dominant systems and hydrodynamic force balance method for attraction-dominant systems. The former is based on calculating the colloidal forces from the magnitude of particle–particle collision trajectory deflections. The latter is based on separating a colloidal doublet with increasing shear and calculating the maximum attractive force at the onset of the doublet breakup. Both methods involve the microscopic observation on two individual particles. The methods are also well suited to emulsion systems where the interaction forces as small as 10 − 13 N between two droplets can be detected. In oil sand research, two stable emulsions: water-in-diluted bitumen and bitumen-in-water have been receiving considerable attention because of their detrimental effects on the industrial process. The droplet – droplet forces and the emulsion stability mechanism were determined for both emulsions using either one of the techniques. For the w/o emulsion, steric repulsion is the main contributor to the emulsion stability. For the o/w emulsion, all interactions are exclusively DLVO-type forces and the electrostatic force plays a major role in stabilizing the emulsion. Isolated protrusions of tens of nanometers in thickness have also been detected on bitumen surfaces. The protrusions enhance the repulsive force at a long distance but reduce it near the energy barrier, a useful feature that might help in destabilizing the system. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Colloidal force measurements; Oil sand studies; Microcollider

1. Introduction In oil sand industry, bitumen is recovered in the form of froth from oil sands by hot water mixing and flotation [1]. The froth is then diluted with naphtha to remove most of the water and the coarse solids. Finally naphtha is recovered from * Corresponding author.

the diluted bitumen to yield a relatively dry bitumen product, ready for upgrading. When certain types of oil sands are processed, a stable bitumenin-water emulsion is formed prior to the formation of the froth. In this emulsion, bitumen droplets are approximately one order of magnitude smaller than the ideal size of several hundred microns in diameter for effective aeration and high bitumen recovery. Since stable droplets do

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not coalesce or aggregate to make larger droplets or clusters and small droplets have low collision efficiency with air bubbles, the bitumen recovery of these oil sands is inevitably low. After naphtha dilution, stable micron-sized water droplets have been found in diluted bitumen. These water droplets usually contain chlorides and solids, posing corrosion and fouling danger to the upgrading unit. The detrimental effects caused by these two emulsions can be eliminated by destabilizing the emulsions. Effective emulsion breaking requires the knowledge of the stability mechanism, which can be elucidated by the measurement of the colloidal forces between two individual emulsion droplets. Determining colloidal forces between deformable liquid droplets poses a challenge to many force measurement techniques. The problem becomes especially acute for many force apparatus [2–4] that are only capable of detecting 10 − 9 N or larger forces. In a typical industrial system containing micron-sized droplets in a moderate shear field, the hydrodynamic force exerted on a droplet is of the order of 10 − 12 N (or 1 pN). Therefore, to break an emulsion, only the colloidal forces of the same magnitude can be possibly balanced or overcome by the hydrodynamic force. This makes the determination of the piconewton forces of greater significance. To stretch the detection limit using the less sensitive instruments, the general strategy is to amplify the colloidal forces by employing larger particles or droplets (tens of microns) and/or macroscopic surfaces. As will be discussed later, a large liquid interface is more susceptible to deformation, which may seriously hamper accurate force measurements. Techniques capable of determining forces less than 1 pN include total internal reflection microscopy [5], micropipet with red-blood cells as force transducers [6], differential electrophoresis [7], colloidal particle scattering (CPS) [8] and hydrodynamic force balance (HFB) [9]. For liquid– liquid interaction measurements, the first two techniques require adhesion of the target liquid film or droplet to a solid surface or a membrane. This might limit their applicability to some systems. The limitation of differential electrophoresis is its requirement of different zeta

potentials of the two particles or droplets in a doublet. Breaking up doublets in a non-aqueous medium poses a problem as well. In the preceding papers [9–11], we have demonstrated that the colloidal forces between water droplets in diluted bitumen and between bitumen droplets in water can be determined using the CPS and the HFB techniques, which are based on the observation and analysis of droplet collision trajectories and shear-induced doublet breakups under an optical microscope. Both techniques are adaptable to emulsion systems without technical difficulties. Their limitation, although not a severe handicap in the present study, is that particles or droplets must be neutrally buoyant in the medium. Usually a chemically inert, high-density agent is added to achieve the neutral buoyancy. In the present paper, previous data obtained from the CPS and the HFB experiments are summarized. The data on untreated bitumen interactions in water are compared with new experimental data using the HFB technique on a treated bitumen system. 2. Theory

2.1. Basic principle of colloidal particle scattering Extracting interaction force data from elementary particle collisions can be traced back to Rutherford’s a-particle scattering experiment almost a century ago [12]. Recently, a similar idea of determining colloidal forces from colloidal particle collisions has evolved to a force measurement technique called colloidal particle scattering [8,13]. For colloidal collisions occurring in a simple shear flow, the hydrodynamic force makes the initial and final portions of the collision trajectory parallel to the flow direction or y-axis (cf. Fig. 1). These two portions of the trajectory can thus be described by x and z coordinates only: (xi, zi) for the initial part and (xf, zf) for the final part. If the colloidal force–separation distance profile is known, (xf, zf) can be calculated from (xi, zi) by solving the trajectory equation [8], dr = M(r)·[Fhydr(r)+ Fcoll(h)], dt

(1)

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Fig. 1. Basic principle of colloidal particle scattering.

where r is the position vector of the mobile particle, t is the time, M is the mobility tensor, Fhydr is the hydrodynamic driving force, Fcoll is the colloidal force and h is the separation distance between two particles (cf. Fig. 1). The construction of the components in M was described in Ref. [14]. Fhydr is calculated by solving Stokes equation and continuity equation with no-slip boundaries on particle and wall surfaces, m927= 9p

(2)

9·7 =0

(3)

where m is the fluid viscosity, 7 is the fluid flow velocity and p is the pressure. In practice, xf can be directly measured from a digitized microscopic image. zf can be calculated from the determined particle velocity and the known shear rate using the hydrodynamic theory [15–18]. However, the force – distance profile, Fcoll(h), is unknown. A hypothetical force model with adjustable parameters is used to calculate the th theoretical final position, (x th f , z f ), and to fit the experimental position, (xf, zf). The best fit yields the optimum parameters and force curve. The typical forces determined using this method are in the order of 0.1 pN, which correspond to interaction energies of several kT. More detailed description can be found in the literature [8,19]. The CPS technique can also be applied to emulsion systems. However, two problems that might affect the measurement accuracy must be addressed. The first problem is that the no-slip boundary conditions used in the trajectory calculation may not be valid for some liquid drops. In the bitumen-in-water emulsion, because of the

high viscosity of bitumen ( 400 Pa s [20]), no significant slipping would occur on bitumen droplet surface. In the water-in-diluted bitumen emulsion, it has been confirmed after the initial CPS experiment [10] that the oil–water interface is completely covered with rigid interfacial materials [21], which would prevent slipping of medium fluid molecules. For any systems that slipping is known to exist, a correction factor, similar to the factor of 2/3 used for a gas bubble–in–water system to correct Stokes drag law [22], must be introduced in the trajectory calculation. The second problem is the deformation of droplets during a collision. A simple model to estimate the deformation of a droplet is shown in Fig. 2. The compression force creates a pressure on a small area of the droplet surface, which is balanced by the higher pressure inside a spherical droplet. According to the Laplace equation, the pressure balance can be expressed as, Fcomp 2g = , a pb 2

(4)

Fig. 2. Spherical droplet under compression. The sphere radius is a. The radius of the flattened surface is b. hf is the height of the crushed segment. The deformation is exaggerated to show the geometry.

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Fig. 3. Basic principle of hydrodynamic force balance. Particle coordinates are scaled by the radius, a.

where Fcomp is the compression force or hydrodynamic driving force in CPS experiments, b is the radius of flattened surface (defined in Fig. 2), g is the interfacial tension of the droplet surface and a is the droplet radius. If typical values for Fcomp, a and g (1 pN, 3 mm and 10 mN m − 1, respectively) are assumed, Eq. (4) yields b =7 nm. According to geometry shown in Fig. 2, the height of the crushed spherical segment, hf, was found to be less than 0.01 nm. Therefore, the droplet deformation in CPS experiments can be completely neglected. As we mentioned in the Introduction, some less sensitive force-measurement techniques require larger Fcomp and a for the colloidal forces to be detected. According to Eq. (4), b increases with increasing either Fcomp or a, i.e. deformation becomes more pronounced in those force measurements.

2.2. Basic principle of hydrodynamic force balance The above-mentioned CPS method requires a stable colloidal system. If particles or droplets coagulate after a collision, the doublet has to be separated through large shear for the CPS experiment to continue. Under a gradually increasing external force, doublet cleavage is actually a direct force measurement experiment by itself since the maximum attractive force between the two particles in a doublet is precisely balanced by the known external force at the onset of breakup. This external force could be an electrophoretic displacement force [7], which, as mentioned in the

Introduction, is restricted to systems containing two particles with different zeta potentials in a doublet. For a more general application, hydrodynamic force would be used for doublet cleavage. Early attempts of applying hydrodynamic force to colloidal doublets in a travelling microtube apparatus and a plate rheoscope have achieved only limited success [23,24]. Quantitative determination of the forces was not possible using those techniques. Recently, a new method, called hydrodynamic force balance technique [9], has been developed to precisely determine the hydrodynamic force for doublet cleavage. It works as follows. A doublet as shown in Fig. 3 is first artificially created by a particle–particle collision and is subsequently broken by an increasing wall shear. At the onset of the breakup, the hydrodynamic force (or drag force) is balanced by the colloidal force according to the following formula, yb Fcoll = F2y (zb) 2

y b + (zb − 1)2 + F2z (zb)

zb − 1

y + (zb − 1)2 2 b

(5)

where yb and zb are coordinates of the free particle position at the onset of a breakup (cf. Fig. 3) and F2y and F2z are components of the hydrodynamic force in y and z axes. The hydrodynamic force is calculated by solving Eqs. (2) and (3), the same method used in the CPS trajectory calculation. The colloidal force, Fcoll, is the maximum attractive force in a force minimum or energy well. The typical range of the determined forces is

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0.1–10 pN. For the three coordinates of the free particle position, only zb has a significant effect on the hydrodynamic force 9. It is calculated in the same way as zf described in Section 2.1. The particle velocity is determined after the breakup. Note that Eq. (5) gives a dimensionless force scaled by 6pmGba 2, where Gb is the shear rate at the onset of a breakup and is determined by image analysis on microscopic video recordings. Detailed theoretical description of the HFB method can be found in [9]. Unlike the CPS method, the HFB technique does not require any force model to determine the colloidal forces. However, one might be more interested in surface structure or force mechanism rather than the value of a force. Hence, a force model with adjustable parameters is used in the HFB experiment as well to interpret the determined forces. Main concern about the HFB application to emulsion systems is the slipping boundaries on droplet surfaces. As mentioned above, no significant slipping would occur in either the water-indiluted bitumen system or the bitumen-in-water system. Therefore, in the following calculation, no correction factor is used and liquid droplets are treated as solid particles.

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3. Water-in-diluted bitumen emulsion

3.1. Materials and experimental procedure The bitumen used was Syncrude coker-feed bitumen extracted from Athabasca oil sand. Deionized water with 0.01% fluorescein isothiocyanate (pH 8) was emulsified in 0.04 wt% diluted bitumen using an ultra-sonic bath. The solvents used for dilution are 71 wt% toluene and 29 wt% dibromobenzene (density 1.95 g ml − 1), supplied by Aldrich Chemical Co. The mixture has a density close to that of water. Water droplets of 1–10 mm in diameter were generated following the procedure above. They were studied using the CPS technique. The apparatus for CPS experiments is called a microcollider. The key part of the instrument is a narrow gap of 120 mm between a fixed glass window and a mobile bottom plate driven by two-axis motors (cf. Fig. 4). The emulsion was filled in the gap. Some droplets attached the top glass window were usually observed under the microscope after a waiting period of several minutes. One of them was chosen as the stationary droplet. A mobile droplet of the same size was then selected and maneuvered to the vicinity of the stationary droplet to collide with it using a shear flow generated by moving the bottom plate of the cell. The collision trajectory was recorded with a CCD camera mounted on top of the microscope. The recorded images were processed using a frame grabber plugged in a computer. The resulting x coordinates and droplet velocities in y-axis were used to calculate (xi, zi) and (xf, zf).

3.2. Colloidal force model of water droplets in bitumen

Fig. 4. Key part of a microcollider: the gap region between the top glass surface and the motorized bottom plate, where droplet collisions and doublet breakups occur.

As discussed in Section 2.1, a colloidal force model must be established to calculate the trajectories. Since this was the first attempt to determine the colloidal forces between water droplets in diluted bitumen, very little was known about the nature of the repulsive force. Two tentative mechanisms were assumed: electrostatic repulsion and steric repulsion.

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Fs,w − w =

Fig. 5. Steric layer model for water droplets in diluted bitumen. The droplet radius is a and the steric layer thickness is L s.

For the electrostatic mechanism, no well-established theory is available to calculate the droplet interactions in a non-aqueous medium. In this study, we assumed that the double-layer interaction theory developed for an aqueous system is still valid in a non-aqueous system. The ionic strength was estimated from the conductivity of the solution to be 10 − 6 M [10]. The surface potential of the droplet, c0, is an adjustable parameter to be determined by comparing experimental and theoretical trajectories. According to modified Gouy – Chapman theory with Derjaguin approximation [25], the double-layer force can be expressed as, Fdl,w − w = Bpak − 1e − k(h − 2Ls),

(6)

where B= 32 tanh2(ec0/4kT)o0ok 2(kT/e)2, e is the electronic charge, o0 is the permittivity of vacuum, o is the dielectric constant of the medium, k is the reciprocal Debye length and Ls is the steric layer thickness (cf. Fig. 5). The electrostatic force in a non-aqueous medium will be further discussed in the next section. The steric repulsion mechanism was mostly used in polymer-coated particle systems. The exiting theories developed from thermodynamic laws [26] are more suited to static interactions instead of dynamic interactions in a collision. For the dynamic interactions between water droplets in diluted bitumen, very little is known besides the common knowledge that all steric repulsion gives a steeply increasing force – distance curve. For this reason, we used a simple high-order polynomial expression to model the force [10],

c , h7

(7)

where c is a parameter to be adjusted so that the force becomes zero or close to zero when h]2Ls. Here we define c= (8/3)Aw – t – waL 5s , where Aw-t-w is the Hamaker constant of water–toluene–water [27] since the diluted bitumen composed of 71% toluene. This expression makes the sum of steric repulsion and unretarded van der Waals attraction (an approximation) zero at a distance of 2Ls. The parameter to be determined from experimental trajectories is Ls.Both mechanisms incorporate the same van der Waals forces calculated based on Hamaker’s method modified by Schenkel and Kitchener for the retardation effect [28], Fvdw,w − w =

Aw − t − wa f1(p), 12h 2

(8)

where f1(p)=

1+3.54p , p51; (1+ 1.77p)2

f1(p)=

0.98 0.434 0.067 − 2 + 3 , p\1; p p p

p= 2ph/l (l being the retardation wavelength, fixed at 100 nm [28]). Eq. (8) neglects the contribution of van der Waals forces from the steric layers assuming the steric layers mainly comprise molecules of the medium. The assumption is based on the findings that in many polymeric systems, a large part of a steric layer is composed of long polymer tails with a low segment density [26].

3.3. Results and discussion The CPS data are usually presented in a scattering diagram (cf. Fig. 6). In the diagram, (xi, zi)s and (xf, zf)s are represented by open and filled circles, respectively. Both x and z coordinates are scaled by droplet radius, a. Theoretical final posith tions, (x th f , z f )s, assuming different force parameters are represented by shaded triangles, squares, diamonds and inverted triangles, which normally form rings of different radii. A ring with a large radius indicates that a strong repulsive force is present in a collision and vice versa. If a (xf, zf)

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falls between two rings, the optimal value of the force parameter for that data point lies between the two numbers marked beside the rings. Ideally, th (x th f , z f )s should be calculated from the observed initial positions, (xi, zi)s. However, in this experiment, there are only seven experimental points, not enough to show a complete ring. Imaginary initial positions (open triangles) covering most of the area where the experimental initial positions were found were used to calculate the ring positions (cf. Fig. 6a). The dotted semicircle represents the projection of the stationary droplet with its center located at (0.0, 1.0) in the graph. In Fig. 6a, the electrostatic mechanism was assumed to be the cause of emulsion stability. The

Fig. 6. Scattering diagram of seven collisions between two 7.3 m water droplets in toluene-diluted bitumen, reproduced from ref. [10]. The repulsive force in this system was assumed to be (a) electric double-layer force; (b) steric force. The adjustable parameters are (a) droplet surface potential, c0 and (b) steric layer thickness, Ls. Open and filled circles represent initial and final positions of the mobile droplet. Shaded triangles and squares in (a) represent the theoretical final positions of the mobile droplet assuming c0 = 22 and 25 mV, respectively. Shaded triangles, squares, inverted triangles and diamonds in (b) represent the theoretical positions assuming Ls = 7.5, 8, 20 and 40 nm, respectively. All shaded symbols were calculated from the initial positions represented by open triangles in (a). Open squares in (b) are theoretical final positions calculated from the four selected experimental initial positions. All coordinates are scaled by droplet radius, a.

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adjustable force parameter in the force model is the droplet surface potential (sign uncertain). It can be seen that most of the experimental positions (filled circles) are far from the two rings plotted in the figure. The trajectory calculation indicates that the surface potential of 22 mV is the minimum value required to prevent a coagulation after the collision. In other words, the theoretical ring cannot be made smaller to be closer to the experimental points assuming the electrostatic force mechanism. Because of the hypothetical nature of applying the double-layer theory to a non-aqueous system, one might consider the argument above inadequate. Trajectory analysis [10] shows that the observed experimental positions can only be explained by a repulsive force that decays faster than the retarded van der Waals forces ( h − 3). Although the exact form of an electrostatic force in an organic solution is unknown, two extreme cases of opposite nature: double-layer interaction and Coulomb interaction are well defined. The Coulomb force for two point charges is proportional to h − 2, which cannot explain the experimental data either. In Fig. 6b, the steric repulsion mechanism was assumed to the cause of the emulsion stability. The adjustable parameter is the steric layer thickness, Ls. The scattering diagram shows that all experimental positions (filled circles) lie between Ls = 7.5 and 40 nm, a reasonable range for most steric layers. The contrasting results of two hypotheses strongly indicate that the steric repulsion mechanism is the real cause of the water-in-diluted bitumen emulsion stability. The steric mechanism was later supported by direct microscopic observation of a rigid film on water–dilute bitumen interface [21] and direct measurement of the layer thickness (32 nm) using a thin liquid filmpressure balance [29]. However, none of the experimental evidences can completely eliminate the possibility that the electrostatic repulsion coexists with the steric force and plays a minor role. The determined force–distance curve based on the Ls values obtained above can be found in Ref. [10]. The experimental errors in these Ls values were evaluated using the following method. The colloidal forces, regardless of the force parameter values, always act through the centers of two

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droplets. Therefore, the observed tangential deviath tions of (xf, zf)s from corresponding (x th f , z f )s, which were marked by double-headed arrows in Fig. 6b, can only be explained by droplet surface roughness and/or Brownian motion of the mobile droplet during a collision, which are two main sources of errors in any CPS experiments. The error in the force parameter, being the normal deviation, is not apparent in the figure. However, it can be assumed that the effects of the rough droplet surfaces in both normal and tangential directions are identical. The typical tangential deviation in most CPS experiments is approximately 0.1a, which is equivalent to an error of less than 0.5 nm in Ls at its lower limit (7.5 nm) and an error of about 20 nm in Ls at its higher limit (40 nm). The noticeable difference of the errors in small and large Ls values is due to the proximity of theoretical rings with large Ls values.

4. Bitumen-in-water emulsion

4.1. Materials and experimental procedure Two bitumen samples were used in this study. The first sample, referred to as untreated bitumen, is the Syncrude coker-feed bitumen without further purification. The second sample is referred to as filtered bitumen. It was prepared by diluting the untreated bitumen to 1 wt% solution with toluene, filtering the diluted bitumen with a 0.025 m filter (Millipore MF series, VSWP04700) and evaporating the solvent in a fume hood at room temperature. Chemical assay on the filtered bitumen showed a reduction of asphaltene content by approximately 30% [30]. Most of the asphaltenes removed by filtration are asphaltene aggregates, which are normally considered highly surface-active species in the bitumen system [31,32]. Either bitumen sample (untreated or filtered) was spread on the bottom of a 20 ml glass vial. A solution containing 0.05 M (for CPS experiments) or 0.1 M (for HFB experiments) KCl, 30% D2O and deionized water was added to the vial, which was subsequently placed in a Fisher FS6 ultrasonic bath filled with water at 50 – 60°C for 80 s. D2O was added to make the bitumen droplets

neutrally buoyant in the medium. The pH of the solution was unadjusted and was found to be 7. The zeta potential of bitumen droplets was determined to be − 85 mV using a Pen Ken Lazer Zee Meter Model 501. The droplets prepared in this method are typically 6 mm in diameter. The bitumen-in-water emulsion was added to the sample cell shown in Fig. 4. The untreated bitumen sample was studied using both the CPS and the HFB techniques. The filtered bitumen sample was only studied using the latter. A microcollider was used for experiments with both techniques. The procedure for the CPS experiment is identical to that described in the previous topic. The procedure for the HFB experiment is as follows. A doublet was first created by a droplet– droplet collision similar to that in CPS experiments. Following the formation of a doublet, however, the computer, which controls the twoaxis motors on the bottom of the sample cell, was programmed to generate a linearly increasing shear in the gap until either the doublet was broken or the speed limit of the motors was reached. The breakup process was recorded with a VCR (SONY SVO-9500MD). The videocassette was played frame by frame after the experiment to locate the breakup frame, i.e. the last frame before a noticeable position change of the free droplet. The time when breakup occurred was then used to calculate the exact shear rate at that moment, Gb. The zb coordinate was determined from the droplet velocity after the breakup. Finally the hydrodynamic force was calculated following the procedure described in Section 2.2.

4.2. Model of bitumen droplet surface The goal of investigating the bitumen droplets in water is to understand the repulsive force mechanism and bitumen surface structure that affects the force. As we mentioned above, the force or structure parameters, e.g. c0 and Ls, can be determined by interpreting the observed collision trajectories or attractive forces with an assumed force–distance curve. In the preceding paper [11], a micron-sized bitumen droplet (untreated) in water was studied un-

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Fig. 7. Disk-sphere model used to calculate the interactions between bitumen droplets covered with isolated protrusions in water. The droplet radius is a. The radius and thickness of the disk-shaped protrusion are r and La, respectively.

der a freeze-fracture scanning electron microscope (SEM). Multiple bumps of 50 – 100 nm in horizontal diameter and around 200 nm for the average spacing were observed. The interactions between two bitumen droplets were therefore modeled as two spheres carrying two disk-shaped protrusions facing directly to each other (cf. Fig. 7). The disk radius is r and the thickness is La. h is the separation distance between two spheres. Other protrusions are neglected in the model because of the relatively large spacing between them [9]. If the interactions between bitumen droplets are entirely DLVO-type forces (to be confirmed later), the electric double-layer force can be expressed as the sum of the sphere – sphere and the plate–plate (or disk–disk) interactions [25], assuming that the disk–sphere interaction is negligible due to the fast-decay feature of the force, Fdl,b − b = Bpak − 1e − kh +Bpr 2[e − k(h − 2La) −e − kh] (9) where B and k were defined in Eq. (6). The second term in brackets was used to eliminate the overestimation of the double-layer force between two uncharged surfaces behind the disks. To minimize the number of adjustable parameters, c0 in the definition of B was assumed to be identical to the measured zeta potential of the droplet ( −85 mV) and r was fixed at the average value found in the SEM picture (38 nm). Hence, La became the only adjustable parameter to be determined in CPS and HFB experiments. The van der Waals force calculation is slightly more complicated because the exact composition and consequently the Hamaker constant of the

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protrusion are unknown. Here, only two extreme cases of the protrusion composition are considered. Case A assumes that the protrusion is entirely composed of water, thus the van der Waals forces reduce to the form of Eq. (8) with a bitumen-water-bitumen Hamaker constant, Ab – w – b, instead of Aw – t – w. Case B assumes that the protrusion is entirely composed of bitumen. The Hamaker constants of protrusion–water–protrusion and protrusion–water–bitumen are therefore identical to that of bitumen–water–bitumen, Ab – w–b. Disk–disk and disk–sphere interactions were simplified as plate-infinite plate and platehalf space interactions. The force to the infinite parts of the plate and half space should be small because of the fast-decaying feature of the van der Waals forces. The total van der Waals forces as a sum of sphere–sphere, disk–sphere and disk–disk interactions include many terms that cancel each other [9]. The final expression is, A ba Fvdw,b − b = − b − w − f1(p)− Ab − w − br 2 12h 2 f (h−2La) f2(h) × 2 − 3 , (10) (h− 2La)3 h where ps ps 3 f2(s)= 0.168−0.093 + 0.019 l l



 

− 0.005

ps l

n



4

,

f1(p) was defined in Eq. (8) and p=2ph/l (l fixed at 100 nm). f1(p) and f2(s) are correction functions accounting for retardation effect between two spheres [28] and two plates [33], respectively. The sum of Fdl,b – b and Fvdw,b – b gives rise to a force–distance curve with an interesting feature when it is compared with the curve predicted by the classical DLVO theory for two smooth spheres. Reductions of both the maximum repulsive force on an energy barrier and the maximum attractive force in the secondary energy minimum are observed (cf. Fig. 8). The same feature was also reported in the literature [34,35]. In the present study, since the typical hydrodynamic forces used in the experiments are still not large enough to overcome the reduced repulsion on a barrier, the effect of surface roughness becomes

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merely a repulsion enhancement. This additional repulsion should not be confused with the steric force, e.g. the Fs in Eq. (7). The disk-sphere model shown in Fig. 7 or other rough surface models do not predict a surface – surface contact and compression. The additional repulsion originates from the rearrangement of separation distances for van der Waals and electrostatic force components.

4.3. Results and discussion 4.3.1. Hamaker constant of bitumen– water – bitumen The scattering diagram of 38 collisions between two 6.6 mm untreated bitumen droplets is shown in Fig. 9. All symbols used in this diagram are identical to those in Fig. 6. Because of the rela-

Fig. 8. Typical force–distance and energy–distance profiles in bitumen-in-water systems. Solid curves represent the profiles for smooth droplet interactions predicted by classical DLVO theory. Dotted curves represent the profiles calculated from the disk – sphere model assuming r= 30 nm and La =10 nm. Other parameters used in the calculation are salt concentration= 0.1 M, a= 3 m, c0 = − 85 mV and Ab – w – b =3 × 10 − 21 J. The protrusion was assumed to be composed of 100% bitumen (case B calculation). The heights of energy barriers and force barriers are shown in the insets.

tively large spacing between protrusions on the bitumen surface, smooth–smooth surface interactions would still occur in some collisions. The collision trajectories of these collisions should indicate a more attractive force for the reason explained above. Therefore, their experimental final positions (filled circles) should be closer to the reference center (0.0, 1.0) than others. Here, we assume the filled circle (partially covered by an open square) pointed by a thick arrow in Fig. 9 is the measurement of smooth–smooth surface interactions. Using Eqs. (9) and (10) with La =0, the Hamaker constant of bitumen–water–bitumen was calculated to be 3.0× 10 − 21 J for the theoretical final position (an open square) to match the experimental position. Three other points close to the reference center were also calculated using the same Hamaker constant. The calculated positions (open squares) are linked to their corresponding experimental positions by double-headed arrows. These three points, however, were not used to fit the experimental positions and calculate the Hamaker constant because of the noticeable tangential deviations. As explained in Section 3.3, tangential deviations are usually associated with experimental errors. If a general deviation (normal or tangential) of 0.1a on a scattering diagram is assumed for all data points including those with no tangential deviations, the error in the determined Hamaker constant is 90.3× 10 − 21 J. The maximum attractive forces between untreated and filtered bitumen droplets measured in HFB experiments were listed in Tables 1 and 2. Following the same argument mentioned above, we chose the largest attractive force observed in either sample as the smooth surface interaction force. The resulting Hamaker constants of untreated and filtered bitumen samples in water are 3.2× 10 − 21 and 2.9×10 − 21 J, respectively. The typical error in the determined forces is 0.1 pN [9]. This can be interpreted as an uncertainty of 9 0.05× 10 − 21 J for the determined Hamaker constants. The good agreement between the Hamaker constants of untreated bitumen in water determined from the CPS and the HFB experiments (3.0 × 10 − 21 vs. 3.2× 10 − 21 J) confirms the validity of

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Fig. 9. Scattering diagram of 38 collisions between two 6.6 m bitumen droplets in water, reproduced from ref. [11]. The definitions of most of the symbols are the same as in Fig. 6. The adjustable parameter here is La. Shaded triangles, squares, inverted triangles and diamonds represent the theoretical final positions calculated from the experimental initial positions (open circles) assuming La =1, 5, 20 (case B) and 20 nm (case A), respectively. Four open squares are theoretical positions assuming La =0. Table 1 Determined forces between untreated bitumen droplets (reproduced from Ref. [9]) No.

a (mm)

zb

Determined force and its error (pN)

Disk–sphere model calculation La (nm) Case A

1 2 3 4 5 6 7 8 9 10 11 12 13

3.9 3.6 3.2 3.7 2.2 3.7 3.7 3.7 2.6 2.2 3.0 2.5 3.5

B1.01 B1.01 1.05 1.24 1.09 1.75 1.70 1.65 1.94 1.35 1.59 B1.01 1.15

5.2 4.0 1.7 1.1 0.9 0.9 0.8 0.7 0.6 0.5 0.5 0.5 0.2

(−0.40) (−0.1+0.3) (9 0.1) (9 0.1) ( 9 0.1) (9 0.1) (9 0.1) ( 9 0.1) (9 0.1) (9 0.1) ( 9 0.1) (−0.10) (−0.10)

the measurements. Based on the data of typical hydrocarbons [36,37], the theoretically calculated Hamaker constant of bitumen – water – bitumen is 2.8× 10 − 21 J. Since non-DLVO forces, e.g. hydrophobic force, can significantly alter the apparent Hamaker constant from its theoretical value, the consistent results of theoretical and experimental Hamaker constants prove the validity of the DLVO theory in the bitumen system at a separation distance larger than 9 nm, the distance range probed in CPS and HFB experiments.

0 1 2 4 3 5 6 6 5 5 6 6 12

Case B 0 1 3 6 4 7 8 8 8 8 10 9 \50

4.3.2. Surface structure of a bitumen droplet in water Excluding the single point indicating a good fit to the prediction of the classical DLVO theory for smooth surfaces, most of the data points in Fig. 9 and Tables 1 and 2 show more repulsive interactions. Based on the SEM image, the additional repulsive forces can be explained by the isolated bumps on bitumen surface. The disk–sphere model described in Section 4.2 was used to interpret the observed final positions of trajectories

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Table 2 Determined forces between filtered bitumen droplets No.

a (mm)

zb

Determined force and its error (pN)

Disk–sphere model calculation La (nm) Case A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

3.7 3.7 2.9 2.4 2.4 2.4 2.4 2.4 2.4 2.4 3.7 3.7 4.2 3.7 3.7 4.2 2.4

1.01 B1.01 1.02 1.05 B1.01 B1.01 1.04 B1.01 1.02 1.06 1.03 1.03 1.12 1.05 1.03 1.07 1.14

4.3 3.1 2.8 2.3 2.0 2.0 1.9 1.6 1.5 1.4 1.3 1.3 1.3 0.7 0.5 0.4 0.4

(−0.1) (−0.1) (9 0.1) ( 9 0.1) (−0.1) (−0.1) (9 0.1) (−0.1) (9 0.1) (9 0.1) (9 0.1) ( 9 0.1) (9 0.1) ( 9 0.1) ( 9 0.1) (9 0.1) ( 9 0.1)

and the determined maximum attractive forces in an energy well. Two extreme cases of the protrusion compositions were assumed in the calculation. In Fig. 9, shaded diamonds and inverted triangles represent theoretical final positions for La = 20 nm using case A and case B calculations, respectively (see Section 4.2). No significant differences of case A and case B results were found for La = 1 and 5 nm (represented by shaded triangles and squares). The numbers of collisions bracketed between any two adjacent rings were listed in Table 3. Since all collisions were observed between one pair of droplets, the non-uniform La values indicate uneven thickness of the protrusions in different regimes of the droplet surface. In the HFB experiment for untreated bitumen, eight pairs of droplets were used to generate eleven breakups (cf. Table 1). Various protrusion thickness values were observed as well on surfaces of different droplets. The scarcity of data points showing La \20 nm in the HFB experiment is due to the fact that protrusions with large La

0 1 1 1 1 1 1 1 2 2 3 3 4 6 7 8 6

Case B 0 1 1 1 1 1 1 2 2 2 4 4 5 8 10 13 11

significantly reduce the secondary energy minimum and forbid the formation of a doublet. With the currently available data, we are unable to eliminate the possibility of either case A or case B. In other words, the protrusions might be composed of either 100% water or 100% bitumen or any mixture in between. However, the results do eliminate the possibility of protrusions made of silica or clay, represented by kaolinite, the common clay in oil sands. The Hamaker constants of silica (or kaolinite)–water–bitumen and silica– water–silica (or kaolinite–water–kaolinite) are 1.0×10 − 20 [36,37] and 3.8× 10 − 20 J [36–38], Table 3 Distribution of La values for untreated bitumen surface No. of collisions Case A 5 16 7 10

La (nm) Case B 5 16 3 14

B1 1–5 5–20 \20

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Table 4 Summary of all force measurements on oil sand samples and their references Methods

Water-in-diluted bitumen

Untreated bitumen-in-water

Filtered bitumen-in-water

CPS measurement CPS calculation

Seven collisions [10] Steric model [10]

HFB measurement HFB calculation

– –

38 collisions Disk–sphere Disk–sphere 13 breakups Disk–sphere

– – – 17 breakups (new) Disk–sphere model (new)

respectively. The relatively large values of the Hamaker constants would cause an increase of the attractive force in the secondary energy minimum, which is contradictory to the reduced attraction observed in both CPS and HFB experiments. Compared with the La values in Table 1, the results in Table 2 show thinner protrusions on filtered bitumen surface. The results may also be interpreted as smaller protruding disks since the value of the disk radius, r, was assumed to be the same as that of the untreated bitumen (38 nm), which was not verified by any SEM studies on the filtered bitumen surface. In either case, the removal of asphaltene aggregates larger than 25 nm by filtration does make the bitumen surface somewhat smoother. This suggests that the surface-active asphaltene aggregates might be one of the contributors of the protrusions. From the perspective of oil sand industry, the repulsion force observed in the CPS experiment at a salt concentration of 0.05 M, close to the ionic strength of process water, renders the desirable droplet–droplet coalescence and coagulation impossible. Although increasing the ionic strength does make the droplets aggregate as shown in HFB experiments, it is not an option for commercial operations because of the detrimental effects of salts on bitumen – sand separation. The reduced energy barrier predicted by a rough surface model is worth exploiting. The reduction becomes especially pronounced if spherical protrusions instead of flat disk-shaped protrusions are present. For example, attaching a 5 nm sphere with the same compositions as bitumen can lower the energy barrier to 30 kT. Attaching clay or silica particles with larger Hamaker constants can further reduce

[11] model case A [11] model case B (new) [9] model [9]

the barrier to about 20 kT. The energy barrier is completely eliminated if two 5 nm spherical protrusions directly opposing each other, causing the formation of a primary doublet. The study of clay attachment on bitumen surface will be the topic of future publications. 5. Conclusions All force measurements and calculation models were summarized in Table 4. Colloidal particle scattering technique has been applied to both non-aqueous and aqueous emulsion systems. In the first case, the trajectories of the collisions between two water droplets in toluene–diluted bitumen were analyzed and the main repulsive force responsible for emulsion stability was found to the steric repulsion rather than the electrostatic repulsion. In the second case, the surface structure of untreated bitumen droplets in water was studied through trajectory analysis. It has been found that the observed trajectories indicate a more repulsive force than the DLVO prediction for two smooth spheres and can be explained by a bitumen surface model incorporating isolated diskshaped protrusions of tens of nanometers in thickness. Similar conclusion has been reached after the interaction forces were determined using the hydrodynamic force balance technique. The HFB technique was also applied to the filtered bitumen droplets-in-water system. Smoother surfaces on those droplets were detected. The discoveries of both the stability mechanism in a water-in-diluted bitumen system and the bitumen surface structure in a bitumen-in-water system have brought us one step closer to the final goal of destabilizing these two emulsions.

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