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Colloidal forces between emulsified water droplets in toluene-diluted bitumen
Colloids and Surfaces A: Physicochemical and Engineering Aspects 149 (1999) 577–583
Colloidal forces between emulsified water droplets in toluene-diluted bitumen X. Wu a, T.G.M. van de Ven b, J. Czarnecki a,* a Syncrude Canada Ltd., Edmonton Research Centre, Edmonton, Alberta, Canada, T6N 1H4 b Paprican and Department of Chemistry, Pulp and Paper Research Centre, McGill University, Montreal, Canada, H3A 2A7 Received 27 August 1997; accepted 19 February 1998
Abstract Interaction forces between two stable micron-sized water droplets in toluene-diluted bitumen have been studied by colloidal particle scattering, a method recently developed to determine surface forces between colloidal particles. Our results show that electrostatic forces contribute little or nothing to droplet stabilization. Instead, the stabilization mechanism is proposed to be steric repulsion between rough and non-homogeneous stabilizing layers on droplet surfaces. The range of possible thicknesses of the stabilizing layer is determined. The force–distance relationships reflecting this surface roughness are also plotted. © 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Oil/water emulsions have been widely used in many industrial processes such as paint making, food processing, sizing in papermaking, medicine formulation, oil recovery, and road surfacing [1]. Knowing the interaction forces between emulsified droplets helps in gaining a better understanding of rheological behavior and stability of various emulsion systems. The most commonly used force measuring instruments are the surface forces apparatus (SFA) and the atomic force microscope (AFM ). As a result of technical difficulties, neither of these two methods have been used to determine interaction forces between two soft surfaces. Recently, technical improvements have been achieved in the AFM technique to measure forces * Corresponding author.
between a solid sphere and an air–water [2,3] or an oil–water [4,5] interface. Meanwhile, the SFA technique has been applied to study interactions between vesicle-forming bilayers coated on mica [6 ]. True liquid–liquid interactions were successfully measured by Aveyard et al. with a micropipet technique [7] between one oil droplet and a macroscopic oil–water interface. It is, in principle, possible to use this technique to measure droplet– droplet interactions, provided that the tip of the micropipet is ‘‘soft’’ enough to detect a much smaller force between the droplets. Liquid–liquid interactions can also be estimated by studying the drainage of a thin liquid film between approaching droplet–droplet surfaces [8] or oil–water interfaces [9]. Based on similar principles, a more sophisticated device, which is capable of controlling and measuring disjoining pressure, has been built [10,11], and the interaction forces between two liquid film interfaces have been determined [12].
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X. Wu et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 577–583
In general, studies on liquid–liquid interactions are very limited, especially in the area of measuring surface forces between microscopic droplets, which are encountered in many emulsion systems. Recently a new force measuring technique, colloidal particle scattering (CPS), was developed to measure surface forces between micron-sized latex particles [13,14]. The apparatus, with which the measurements are performed, is called a microcollider and is capable of determining a force of 10−14–10−12 N which is several orders of magnitude smaller than those detected by the SFA, AFM or micropipet methods. Furthermore, there is no inherent difficulty in applying this technique to study liquid droplet interactions (as discussed below). In this study we performed experiments to measure interaction forces between two 6.5 mm water droplets in toluene containing a small amount of bitumen. The study was initiated because of the interest in the stabilization mechanism of a very stable water-in-oil emulsion formed while processing bitumen extracted from Athabasca oil sands deposits in Northern Alberta.
Brownian motion and van der Waals attraction. A mobile particle (droplet) of the same size is then brought to collide with the selected stationary one by a precisely controlled shear flow (cf. Fig. 1). Technically speaking, manipulating a liquid droplet with shear is almost identical to manipulating a solid particle. However, there are some differences between particle–particle collision and droplet–droplet collision. Our main concern is droplet deformation during a collison. With a simple analysis, the magnitude of the deformation can be estimated. It is known that the hydrodynamic driving force during a collision is of the same order of magnitude as the Stokes drag force: F #F =6pmau, hydr drag
(1)
where m is the viscosity of the medium (6× 10−4 Pa s), a is the droplet radius (3.7 mm) and u is the droplet velocity (typically 15 mm s−1). This force has to be balanced by the pressure difference inside and outside the droplet owing to its curvature according to the Laplace equation:
2. Theory
2c F =DpA= pr2, hydr a
The basic principles of CPS are described in detail in Ref. [13]. In essence, the method is based on generating collisions between two micron-sized particles (droplets) under well-controlled conditions and extracting the force–distance relationships by analysing the asymmetry of collision trajectories before and after the collision. To generate these collisions, we first searched under a microscope for a particle (droplet) attached to a smooth glass. This occurs randomly due to
where Dp is the pressure difference, A and r are the area and the radius of flattened surface, respectively, and c is the interfacial tension between water and oil, which, in our case, is between water and toluene containing small amount of bitumen and is assumed to be approximately 10 mN m−1. Combining Eqs. (1) and (2) yields r#6 nm, which corresponds to a change in the separation distance between the two drop surfaces due to flattening, i.e. the height of the crushed spherical segment, of
Fig. 1. A diagram of droplet–droplet collision in a microcollider sample cell.
(2)
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about 0.01 nm, which is well below the experimental error (which is in the range 1–20 nm). The collision trajectories are recorded on video and digitized. Each trajectory is presented by a pair of x, z-coordinates. These coordinates represent the initial position of the mobile particle (droplet) before the collision, (x , z ), and the final i i position of the same particle (droplet) after the collision, (x , z ) (cf. Fig. 1). When many pairs of f f x, z-coordinates are plotted in one graph (a so-called scattering diagram), a specific ‘‘scattering pattern’’ will appear. This pattern can be analyzed by comparing the experimental (x , z )s with the f f ones calculated from hydrodynamic theory [15], assuming different force parameters. The best match between experimental and theoretical final particle positions yields the optimum set of parameters or the optimum force–distance profile.
Athabasca oil sand. Stability tests show that the presence of dibromobenzene has very little effect on the stability of the emulsion after water is added. However, the requirement of neutral buoyancy is still the major restriction of the current microcollider. A new vertical microcollider is presently under construction which does not have such a restriction. To enhance the contrast in subsequent microscopic observations, a 0.01% fluorescein isothiocyanate aqueous solution (pH 8) was used instead of pure water to form the aqueous phase (~3 vol.%) of the emulsion. Fluorescent water droplets of 1–10 mm in size started to form after the system was sonicated for 2 min. The sample cell was subsequently filled with this emulsion and standard procedures for operating the microcollider were followed [13,14].
3. Experimental
4. Results and discussion
To make water droplets neutrally buoyant in the medium, we mixed 71 wt.% toluene with 29 wt.% dibromobenzene (density 1.95 g ml−1) supplied by Aldrich Chemical. The organic phase also contained 0.04 wt.% bitumen extracted from
The experimental results are summarized in a scattering diagram (cf. Fig. 2). The open circles represent initial positions of the mobile droplet (x , z ) and the filled circles represent final positions i i (x , z ). Due to the poor light transmission in a f f
Fig. 2. A scattering diagram of water droplet collisions in 0.04% bitumen/toluene solution. The open and filled circles represent the experimental initial and final positions, respectively (numbered from collision 1 to collision 7). The semicircle in the middle is the projection shadow of the stationary droplet. The open triangles stand for the theoretical initial positions on ‘‘grid points’’. The filled triangles represent the calculated final positions from the theoretical initial positions assuming electrostatic force is the repulsive force. Each set of final positions are connected by solid lines (I ), or dotted lines (II ), based on different surface potential: (I ) 25 mV or (II ) 22 mV.
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dark bitumen solution, only seventeen collisions were observed. After following a standard data screening procedure [14], seven of them, all between one pair of water droplets (obtained by reversing the flow field after each collision), were retained for further analysis. The scattering diagram shows that the final positions do not follow a ‘‘ring’’ pattern, normally observed in previous studies of latex interactions [13,14] where the force–distance relationship is consistent for all collisions. When droplet surfaces are highly heterogeneous, for each collision the mobile droplet ‘‘sees’’ a surface region of different properties on the stationary drop and thus ‘‘experiences’’ different interaction forces. To illustrate this point, 14 arbitrary initial positions (open triangles in Fig. 2) were taken and the theoretical final position after a collision with different values of one force parameter were then calculated. Our experimental final positions were subsequently compared to these ‘‘theoretical’’ final positions. The maximum and minimum possible values of the force parameter used to calculate these ‘‘theoretical’’ positions and their errors can then be estimated. It has been known that water droplets in a bitumen/toluene solution are stable. Since the van der Waals forces between two water droplets are always attractive, a repulsive force is required to stabilize the system. The most common repulsive forces are electrostatic and steric repulsion. The electrostatic interaction in an organic medium is not fully understood. In this paper we assume it is a double-layer interaction similar to that in an aqueous system. This assumption is based on the finding that the conductivity of our system is about 30 mS/m. In an aqueous sytem, this conductivity can be interpreted in terms of the ionic strength of a typical 1:1 salt (e.g. NaCl ) of concentration 2×10−6 M. In an organic medium, however, ions are considerably smaller than their hydrated counterparts in water. This increases the equivalent conductivity of ions, so the argument above overestimates the actual ionic strength in an organic medium. Hence, we chose a slightly smaller value, 1×10−6 M, as the ionic strength of our system. It will be seen shortly that varying this value by one or two orders of magnitude does not change the final results qualitatively. A ka (k being the recipro-
cal Debye length and a the drop radius) of 70 can then be calculated based on this assumed ionic strength. The large ka value suggests that the double-layer interaction theory might still apply to this non-aqueous system. In this paper we use a modified Gouy–Chapman theory to calculate the electrostatic force. Van der Waals forces are calculated based on Hamaker’s theory with a retardation function introduced by Schenkel and Kitchener [16 ]. The Hamaker constant of water–toluene–water is taken from Ref. [17]. Both the Hamaker constant and the retardation wavelength are kept constant throughout. Fig. 2 gives two theoretical final position ‘‘rings’’ with surface potentials of 22 and 25 mV (signs unspecified ). When the surface potential is below 22 mV, our calculations indicate that the two colliding droplets should form a doublet and no final positions can be plotted. It is obvious that the experimental final positions, which are mostly located close to the origin (0.0,1.0), cannot be explained by this theoretical prediction no matter how we adjust the surface potential. Similar results were found if we vary the ionic strength by one or two orders of magnitude, since it is not a very sensitive parameter at a value around 10−6 M. Similar to the electrostatic mechanism, the steric repulsion mechanism in our system is also difficult to model. A bitumen/toluene solution itself is a very complex system containing high molecular weight asphaltenes, natural surfactants and ultrafine particles. These components are very likely to be adsorbed on the water/toluene interface. As a result of this complexity, it is hard to model the adsorption layer with a single elastic modulus, as we did for the analysis of polyethylene oxide adsorption layers on latexes [18]. However, all steric forces resemble hardwall interactions. They can be approximately modeled by high-order polynomial functions. In this case we used a simple expression: c F = , steric h7
(3)
where h is the separation distance between two
X. Wu et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 577–583
droplets and c is a constant chosen in such a way that it makes F equal to the non-retarded steric van der Waals force at h=2L (L being the adsorps s tion layer thickness). The power of h is arbitrary. Eq. (3) describes a steric force that is zero slightly beyond L and increases sharply as the distance s L is reached. It has been found that the final s results are not sensitive to this power as long as it remains reasonably high. Based on this model, in Fig. 3 we plot theoretical ‘‘rings’’ using the same initial positions as shown in Fig. 2. The scattering diagram shows that the experimental final position of collision 1 is located on ring A and the final position of collision 5 is on ring D. Other final positions located between ring A and ring D can also be predicted by varying L between 7.5 and s 40 nm. Therefore, we may state that our experimental results are consistent with the theoretical prediction based on the assumption of steric repulsion being the stabilization mechanism of the system. The steric layer has a non-uniform thickness. This hypothesis is also consistent with direct microscopic observation of dark fuzzy layers surrounding 10 mm water droplets in a more concentrated 10 wt.% bitumen–toluene solution (M. Tychkowsky, private communication). Another experimental finding by Yoon et al. [19] indicates that by using a surface forces apparatus the interaction forces in an inverse system, i.e. bitumen–water–bitumen, resemble polymer–poly-
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mer interactions. If we assume these interactions are caused by hydrophilic parts of the surfactants adsorbed on bitumen–water interface, it is no surprise to observe the steric interactions between hydrophobic parts of the same surfactants in a water–bitumen–water system. The error in determining the stabilizing layer thickness can be estimated by comparing the theoretical final positions of collisions 1, 2, 4 and 5 (denoted by filled squares in Fig. 3) with the experimental final positions of the same collisions (denoted by filled circles). Since the error is random [14], the tangential deviation of each collision shown in Fig. 3 also reflects its radial deviation. It can be seen that the deviation of collision 1 is comparable to the separation distance between ring A and ring B, which were calculated from L =40 nm and L =20 nm, respectively. This indis s cates that the error in the maximum value of L is s around 50%. Similarly, by studying the deviation of collision 5 we can conclude that the error in the minimum value of L is around 1% because of the s large separation distance between ring C and ring D, which was calculated from two L values, 8 nm s and 7.5 nm, which are very close. After the maximum and minimum L values s have been determined, we can plot the forcedistance curves of these two extreme cases (cf. Fig. 4). Other possible force profiles leading to any final position between ring A and ring D in
Fig. 3. The same scattering diagram as Fig. 2, except the filled triangles which represent the calculated final positions assuming steric force is the repulsive force. Rings A to D were calculated based on four adsorption layer thicknesses, 40, 20, 8 and 7.5 nm, respectively. Four filled squares stand for theoretical final positions calculated from the experimental initial positions of collisions 1, 2, 4 and 5.
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Fig. 4. Interaction force as a function of separation distance between two droplets. Curves 1 and 2 represent the force as a sum of van der Waals forces and steric force with one adjustable parameter L (L =7.5 nm for curve 1 and L =40 nm for curve s s s 2). Curve 3 represents the force as a sum of van der Waals forces and electrostatic force (the surface potential was assumed to be 22 mV ). The force unit is pN (10−12 N ).
requirement. A well-screened double-layer force would also satisfy this condition at certain separations h and the resulting shallow minimum is called secondary minimum in DLVO theory. However, this requires a high ionic strength (~0.01 M ) which is not possible in our system. If the actual ionic strength of our system is several orders of magnitude smaller than the assumed value (10−6 M ), the electric double layers cannot be fully developed and it is inappropriate to calculate the electrostatic force with a double-layer interaction equation. Although we do not know exactly what form the force equation will take, we know the expression of the Coulomb force which is the extreme case of electrostatic interactions without screening. It does not decay faster with increasing h than the van der Waals forces. For this reason, it seems that the electrostatic force is unlikely to be the main cause of stabilization in the system studied, although it may play a minor role in addition to steric stabilization.
5. Conclusion Fig. 3 must fall within these two bounds. It is of interest to note that the shallow force minimum (~1 pN at about 20 nm) before the hard wall-like repulsion seen in curve 1 allows the final position of the mobile droplet in a scattering diagram to be close to the origin, meanwhile keeping the droplet from being captured by the stationary drop during the collision. In the case of a double-layer force (cf. curve 3), depending on the height of the force barrier, the mobile droplet is either being pushed exclusively by a repulsive force and repelled far from its original position, or it is being pulled strongly to the stationary droplet by a large attractive force in the deep minimum of curve 3, and eventually being captured. Therefore, the most important message from the current experiments is that any mechanism which allows the force profiles to resemble curves 1 and 2 would explain our experimental data. In order to take the shape of curve 1, the repulsive part of the force must be a fast decaying one since the attractive part (the van der Waals force) is well defined and is proportional to h−2 without retardation or h−3 with retardation. A steric force obviously meets this
Results of droplet–droplet interaction force measurements indicate that emulsified water droplets in toluene-diluted bitumen are probably stabilized by steric repulsion between adsorbed layers on the droplet surfaces. It has been further found that these adsorption layers are not uniform. The layer thickness varies from 7.5 to 40 nm on a single droplet.
Acknowledgments The authors wish to thank Dr Tadek Dabros and Dr Tony Yeung for their helpful discussions. This work was supported by NSERC in the form of Industrial Research Grant #194390-1996 to X. Wu.
References [1] D.F. Evans, H. Wennerstrom, The Colloidal Domain: Where Physics, Chemistry, Biology and Technology Meet, VCH, New York, 1994, pp. 455.
X. Wu et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 577–583 [2] H.-J. Butt, J. Colloid Interface Sci. 166 (1994) 109. [3] W.A. Ducker, Z. Xu, J. Israelachvili, Langmuir 10 (1994) 3279. [4] P. Mulvaney, J.M. Perera, S. Biggs, F. Grieser, G.W. Stevens, J. Colloid Interface Sci. 183 (1996) 614. [5] B.A. Snyder, D.E. Aston, J.C. Botg, Langmuir 1 (1997) 590. [6 ] S. Chiruvolu, J.N. Israelachvili, E. Naranjo, Z. Xu, J.A. Zasadzinski, E.W. Kaler, K.L. Herrington, Langmuir 11 (1995) 4256. [7] R. Aveyard, B.P. Binks, W.-G. Cho, L.R. Fisher, P.D.I. Fletcher, F. Klinkhammer, Langmuir 12 (1996) 6561. [8] L.R. Fisher, E.E. Mitchell, N.S. Parker, J. Colloid Interface Sci. 128 (1989) 35. [9] G.W. Stevens, H.R.C. Pratt, D.R. Tai, J. Colloid Interface Sci. 136 (1990) 470. [10] A. Sheludko, Adv. Colloid Interface Sci. 1 (1967) 391.
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[11] O.D. Velev, G.N. Constantinides, D.G. Avraam, A.C. Payatakes, R.P. Borwankar, J. Colloid Interface Sci. 175 (1995) 68. [12] D. Exerowa, D. Kashchiev, D. Platikanov, Adv. Colloid Interface Sci. 40 (1992) 201. [13] T.G.M. van de Ven, P. Warszynski, X. Wu, T. Dabros, Langmuir 10 (1994) 3046. [14] X. Wu, T.G.M. van de Ven, Langmuir 12 (1996) 3859. [15] T. Dabros, T.G.M. van de Ven, J. Colloid Interface Sci. 149 (1992) 493. [16 ] J.H. Schenkel, J.A. Kitchener, Trans. Faraday Soc. 56 (1960) 161. [17] J. Visser, Adv. Colloid Interface Sci. 3 (1972) 331. [18] X. Wu, T.G.M. van de Ven, J. Colloid Interface Sci. 183 (1996) 388. [19] R.-H. Yoon, D. Guzonas, B.S. Aksoy, Processing of Hydrophobic Minerals and Fine Coal, Proceedings of the First UBC-McGill Bi-Annual Intenational Symposium, 1995, p. 277.
Colloidal forces between emulsified water droplets in toluene-diluted bitumen X. Wu a, T.G.M. van de Ven b, J. Czarnecki a,* a Syncrude Canada Ltd., Edmonton Research Centre, Edmonton, Alberta, Canada, T6N 1H4 b Paprican and Department of Chemistry, Pulp and Paper Research Centre, McGill University, Montreal, Canada, H3A 2A7 Received 27 August 1997; accepted 19 February 1998
Abstract Interaction forces between two stable micron-sized water droplets in toluene-diluted bitumen have been studied by colloidal particle scattering, a method recently developed to determine surface forces between colloidal particles. Our results show that electrostatic forces contribute little or nothing to droplet stabilization. Instead, the stabilization mechanism is proposed to be steric repulsion between rough and non-homogeneous stabilizing layers on droplet surfaces. The range of possible thicknesses of the stabilizing layer is determined. The force–distance relationships reflecting this surface roughness are also plotted. © 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Oil/water emulsions have been widely used in many industrial processes such as paint making, food processing, sizing in papermaking, medicine formulation, oil recovery, and road surfacing [1]. Knowing the interaction forces between emulsified droplets helps in gaining a better understanding of rheological behavior and stability of various emulsion systems. The most commonly used force measuring instruments are the surface forces apparatus (SFA) and the atomic force microscope (AFM ). As a result of technical difficulties, neither of these two methods have been used to determine interaction forces between two soft surfaces. Recently, technical improvements have been achieved in the AFM technique to measure forces * Corresponding author.
between a solid sphere and an air–water [2,3] or an oil–water [4,5] interface. Meanwhile, the SFA technique has been applied to study interactions between vesicle-forming bilayers coated on mica [6 ]. True liquid–liquid interactions were successfully measured by Aveyard et al. with a micropipet technique [7] between one oil droplet and a macroscopic oil–water interface. It is, in principle, possible to use this technique to measure droplet– droplet interactions, provided that the tip of the micropipet is ‘‘soft’’ enough to detect a much smaller force between the droplets. Liquid–liquid interactions can also be estimated by studying the drainage of a thin liquid film between approaching droplet–droplet surfaces [8] or oil–water interfaces [9]. Based on similar principles, a more sophisticated device, which is capable of controlling and measuring disjoining pressure, has been built [10,11], and the interaction forces between two liquid film interfaces have been determined [12].
0927-7757/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0 9 2 7- 7 7 5 7 ( 9 8 ) 0 03 1 2 - 4
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X. Wu et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 577–583
In general, studies on liquid–liquid interactions are very limited, especially in the area of measuring surface forces between microscopic droplets, which are encountered in many emulsion systems. Recently a new force measuring technique, colloidal particle scattering (CPS), was developed to measure surface forces between micron-sized latex particles [13,14]. The apparatus, with which the measurements are performed, is called a microcollider and is capable of determining a force of 10−14–10−12 N which is several orders of magnitude smaller than those detected by the SFA, AFM or micropipet methods. Furthermore, there is no inherent difficulty in applying this technique to study liquid droplet interactions (as discussed below). In this study we performed experiments to measure interaction forces between two 6.5 mm water droplets in toluene containing a small amount of bitumen. The study was initiated because of the interest in the stabilization mechanism of a very stable water-in-oil emulsion formed while processing bitumen extracted from Athabasca oil sands deposits in Northern Alberta.
Brownian motion and van der Waals attraction. A mobile particle (droplet) of the same size is then brought to collide with the selected stationary one by a precisely controlled shear flow (cf. Fig. 1). Technically speaking, manipulating a liquid droplet with shear is almost identical to manipulating a solid particle. However, there are some differences between particle–particle collision and droplet–droplet collision. Our main concern is droplet deformation during a collison. With a simple analysis, the magnitude of the deformation can be estimated. It is known that the hydrodynamic driving force during a collision is of the same order of magnitude as the Stokes drag force: F #F =6pmau, hydr drag
(1)
where m is the viscosity of the medium (6× 10−4 Pa s), a is the droplet radius (3.7 mm) and u is the droplet velocity (typically 15 mm s−1). This force has to be balanced by the pressure difference inside and outside the droplet owing to its curvature according to the Laplace equation:
2. Theory
2c F =DpA= pr2, hydr a
The basic principles of CPS are described in detail in Ref. [13]. In essence, the method is based on generating collisions between two micron-sized particles (droplets) under well-controlled conditions and extracting the force–distance relationships by analysing the asymmetry of collision trajectories before and after the collision. To generate these collisions, we first searched under a microscope for a particle (droplet) attached to a smooth glass. This occurs randomly due to
where Dp is the pressure difference, A and r are the area and the radius of flattened surface, respectively, and c is the interfacial tension between water and oil, which, in our case, is between water and toluene containing small amount of bitumen and is assumed to be approximately 10 mN m−1. Combining Eqs. (1) and (2) yields r#6 nm, which corresponds to a change in the separation distance between the two drop surfaces due to flattening, i.e. the height of the crushed spherical segment, of
Fig. 1. A diagram of droplet–droplet collision in a microcollider sample cell.
(2)
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about 0.01 nm, which is well below the experimental error (which is in the range 1–20 nm). The collision trajectories are recorded on video and digitized. Each trajectory is presented by a pair of x, z-coordinates. These coordinates represent the initial position of the mobile particle (droplet) before the collision, (x , z ), and the final i i position of the same particle (droplet) after the collision, (x , z ) (cf. Fig. 1). When many pairs of f f x, z-coordinates are plotted in one graph (a so-called scattering diagram), a specific ‘‘scattering pattern’’ will appear. This pattern can be analyzed by comparing the experimental (x , z )s with the f f ones calculated from hydrodynamic theory [15], assuming different force parameters. The best match between experimental and theoretical final particle positions yields the optimum set of parameters or the optimum force–distance profile.
Athabasca oil sand. Stability tests show that the presence of dibromobenzene has very little effect on the stability of the emulsion after water is added. However, the requirement of neutral buoyancy is still the major restriction of the current microcollider. A new vertical microcollider is presently under construction which does not have such a restriction. To enhance the contrast in subsequent microscopic observations, a 0.01% fluorescein isothiocyanate aqueous solution (pH 8) was used instead of pure water to form the aqueous phase (~3 vol.%) of the emulsion. Fluorescent water droplets of 1–10 mm in size started to form after the system was sonicated for 2 min. The sample cell was subsequently filled with this emulsion and standard procedures for operating the microcollider were followed [13,14].
3. Experimental
4. Results and discussion
To make water droplets neutrally buoyant in the medium, we mixed 71 wt.% toluene with 29 wt.% dibromobenzene (density 1.95 g ml−1) supplied by Aldrich Chemical. The organic phase also contained 0.04 wt.% bitumen extracted from
The experimental results are summarized in a scattering diagram (cf. Fig. 2). The open circles represent initial positions of the mobile droplet (x , z ) and the filled circles represent final positions i i (x , z ). Due to the poor light transmission in a f f
Fig. 2. A scattering diagram of water droplet collisions in 0.04% bitumen/toluene solution. The open and filled circles represent the experimental initial and final positions, respectively (numbered from collision 1 to collision 7). The semicircle in the middle is the projection shadow of the stationary droplet. The open triangles stand for the theoretical initial positions on ‘‘grid points’’. The filled triangles represent the calculated final positions from the theoretical initial positions assuming electrostatic force is the repulsive force. Each set of final positions are connected by solid lines (I ), or dotted lines (II ), based on different surface potential: (I ) 25 mV or (II ) 22 mV.
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dark bitumen solution, only seventeen collisions were observed. After following a standard data screening procedure [14], seven of them, all between one pair of water droplets (obtained by reversing the flow field after each collision), were retained for further analysis. The scattering diagram shows that the final positions do not follow a ‘‘ring’’ pattern, normally observed in previous studies of latex interactions [13,14] where the force–distance relationship is consistent for all collisions. When droplet surfaces are highly heterogeneous, for each collision the mobile droplet ‘‘sees’’ a surface region of different properties on the stationary drop and thus ‘‘experiences’’ different interaction forces. To illustrate this point, 14 arbitrary initial positions (open triangles in Fig. 2) were taken and the theoretical final position after a collision with different values of one force parameter were then calculated. Our experimental final positions were subsequently compared to these ‘‘theoretical’’ final positions. The maximum and minimum possible values of the force parameter used to calculate these ‘‘theoretical’’ positions and their errors can then be estimated. It has been known that water droplets in a bitumen/toluene solution are stable. Since the van der Waals forces between two water droplets are always attractive, a repulsive force is required to stabilize the system. The most common repulsive forces are electrostatic and steric repulsion. The electrostatic interaction in an organic medium is not fully understood. In this paper we assume it is a double-layer interaction similar to that in an aqueous system. This assumption is based on the finding that the conductivity of our system is about 30 mS/m. In an aqueous sytem, this conductivity can be interpreted in terms of the ionic strength of a typical 1:1 salt (e.g. NaCl ) of concentration 2×10−6 M. In an organic medium, however, ions are considerably smaller than their hydrated counterparts in water. This increases the equivalent conductivity of ions, so the argument above overestimates the actual ionic strength in an organic medium. Hence, we chose a slightly smaller value, 1×10−6 M, as the ionic strength of our system. It will be seen shortly that varying this value by one or two orders of magnitude does not change the final results qualitatively. A ka (k being the recipro-
cal Debye length and a the drop radius) of 70 can then be calculated based on this assumed ionic strength. The large ka value suggests that the double-layer interaction theory might still apply to this non-aqueous system. In this paper we use a modified Gouy–Chapman theory to calculate the electrostatic force. Van der Waals forces are calculated based on Hamaker’s theory with a retardation function introduced by Schenkel and Kitchener [16 ]. The Hamaker constant of water–toluene–water is taken from Ref. [17]. Both the Hamaker constant and the retardation wavelength are kept constant throughout. Fig. 2 gives two theoretical final position ‘‘rings’’ with surface potentials of 22 and 25 mV (signs unspecified ). When the surface potential is below 22 mV, our calculations indicate that the two colliding droplets should form a doublet and no final positions can be plotted. It is obvious that the experimental final positions, which are mostly located close to the origin (0.0,1.0), cannot be explained by this theoretical prediction no matter how we adjust the surface potential. Similar results were found if we vary the ionic strength by one or two orders of magnitude, since it is not a very sensitive parameter at a value around 10−6 M. Similar to the electrostatic mechanism, the steric repulsion mechanism in our system is also difficult to model. A bitumen/toluene solution itself is a very complex system containing high molecular weight asphaltenes, natural surfactants and ultrafine particles. These components are very likely to be adsorbed on the water/toluene interface. As a result of this complexity, it is hard to model the adsorption layer with a single elastic modulus, as we did for the analysis of polyethylene oxide adsorption layers on latexes [18]. However, all steric forces resemble hardwall interactions. They can be approximately modeled by high-order polynomial functions. In this case we used a simple expression: c F = , steric h7
(3)
where h is the separation distance between two
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droplets and c is a constant chosen in such a way that it makes F equal to the non-retarded steric van der Waals force at h=2L (L being the adsorps s tion layer thickness). The power of h is arbitrary. Eq. (3) describes a steric force that is zero slightly beyond L and increases sharply as the distance s L is reached. It has been found that the final s results are not sensitive to this power as long as it remains reasonably high. Based on this model, in Fig. 3 we plot theoretical ‘‘rings’’ using the same initial positions as shown in Fig. 2. The scattering diagram shows that the experimental final position of collision 1 is located on ring A and the final position of collision 5 is on ring D. Other final positions located between ring A and ring D can also be predicted by varying L between 7.5 and s 40 nm. Therefore, we may state that our experimental results are consistent with the theoretical prediction based on the assumption of steric repulsion being the stabilization mechanism of the system. The steric layer has a non-uniform thickness. This hypothesis is also consistent with direct microscopic observation of dark fuzzy layers surrounding 10 mm water droplets in a more concentrated 10 wt.% bitumen–toluene solution (M. Tychkowsky, private communication). Another experimental finding by Yoon et al. [19] indicates that by using a surface forces apparatus the interaction forces in an inverse system, i.e. bitumen–water–bitumen, resemble polymer–poly-
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mer interactions. If we assume these interactions are caused by hydrophilic parts of the surfactants adsorbed on bitumen–water interface, it is no surprise to observe the steric interactions between hydrophobic parts of the same surfactants in a water–bitumen–water system. The error in determining the stabilizing layer thickness can be estimated by comparing the theoretical final positions of collisions 1, 2, 4 and 5 (denoted by filled squares in Fig. 3) with the experimental final positions of the same collisions (denoted by filled circles). Since the error is random [14], the tangential deviation of each collision shown in Fig. 3 also reflects its radial deviation. It can be seen that the deviation of collision 1 is comparable to the separation distance between ring A and ring B, which were calculated from L =40 nm and L =20 nm, respectively. This indis s cates that the error in the maximum value of L is s around 50%. Similarly, by studying the deviation of collision 5 we can conclude that the error in the minimum value of L is around 1% because of the s large separation distance between ring C and ring D, which was calculated from two L values, 8 nm s and 7.5 nm, which are very close. After the maximum and minimum L values s have been determined, we can plot the forcedistance curves of these two extreme cases (cf. Fig. 4). Other possible force profiles leading to any final position between ring A and ring D in
Fig. 3. The same scattering diagram as Fig. 2, except the filled triangles which represent the calculated final positions assuming steric force is the repulsive force. Rings A to D were calculated based on four adsorption layer thicknesses, 40, 20, 8 and 7.5 nm, respectively. Four filled squares stand for theoretical final positions calculated from the experimental initial positions of collisions 1, 2, 4 and 5.
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Fig. 4. Interaction force as a function of separation distance between two droplets. Curves 1 and 2 represent the force as a sum of van der Waals forces and steric force with one adjustable parameter L (L =7.5 nm for curve 1 and L =40 nm for curve s s s 2). Curve 3 represents the force as a sum of van der Waals forces and electrostatic force (the surface potential was assumed to be 22 mV ). The force unit is pN (10−12 N ).
requirement. A well-screened double-layer force would also satisfy this condition at certain separations h and the resulting shallow minimum is called secondary minimum in DLVO theory. However, this requires a high ionic strength (~0.01 M ) which is not possible in our system. If the actual ionic strength of our system is several orders of magnitude smaller than the assumed value (10−6 M ), the electric double layers cannot be fully developed and it is inappropriate to calculate the electrostatic force with a double-layer interaction equation. Although we do not know exactly what form the force equation will take, we know the expression of the Coulomb force which is the extreme case of electrostatic interactions without screening. It does not decay faster with increasing h than the van der Waals forces. For this reason, it seems that the electrostatic force is unlikely to be the main cause of stabilization in the system studied, although it may play a minor role in addition to steric stabilization.
5. Conclusion Fig. 3 must fall within these two bounds. It is of interest to note that the shallow force minimum (~1 pN at about 20 nm) before the hard wall-like repulsion seen in curve 1 allows the final position of the mobile droplet in a scattering diagram to be close to the origin, meanwhile keeping the droplet from being captured by the stationary drop during the collision. In the case of a double-layer force (cf. curve 3), depending on the height of the force barrier, the mobile droplet is either being pushed exclusively by a repulsive force and repelled far from its original position, or it is being pulled strongly to the stationary droplet by a large attractive force in the deep minimum of curve 3, and eventually being captured. Therefore, the most important message from the current experiments is that any mechanism which allows the force profiles to resemble curves 1 and 2 would explain our experimental data. In order to take the shape of curve 1, the repulsive part of the force must be a fast decaying one since the attractive part (the van der Waals force) is well defined and is proportional to h−2 without retardation or h−3 with retardation. A steric force obviously meets this
Results of droplet–droplet interaction force measurements indicate that emulsified water droplets in toluene-diluted bitumen are probably stabilized by steric repulsion between adsorbed layers on the droplet surfaces. It has been further found that these adsorption layers are not uniform. The layer thickness varies from 7.5 to 40 nm on a single droplet.
Acknowledgments The authors wish to thank Dr Tadek Dabros and Dr Tony Yeung for their helpful discussions. This work was supported by NSERC in the form of Industrial Research Grant #194390-1996 to X. Wu.
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