Article pubs.acs.org/EF
Bitumen−Silica Interactions in a Deep Eutectic Ionic Liquid Analogue Nuerxida Pulati, Timothy Tighe, and Paul Painter*
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The EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: In previous work, it has been shown that a deep eutectic mixture (DEM) of choline chloride and urea facilitates the detachment of bitumen from oil sands. To probe the mechanism underlying this observation, adhesive forces at the surface of the bitumen samples in deionized water and this DEM are compared using atomic force microscopy. PeakForce Tapping mode was used for force−distance measurement. Silicon nitride tips with dimensions of the order of 20 nm and silica colloidal particle attached probes with diameters of the order of 40 μm were used. For both AFM probes, the adhesion force between the probe tip and the bitumen surface decreased significantly with increasing DEM concentration, as did the dissipation energy.
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INTRODUCTION The use of a hot or warm water process to extract bitumen from oil sands has always raised environmental concerns. However, there now appears to be a renewed interest in alternative technologies in an effort to mitigate various problems, notably those associated with the generation of contaminated wastewater streams presently collected in very large tailing ponds. Solvent extraction has been a particular focus of much recent work,1−9 but this approach also presents problems. These include the presence of mineral fines in the extracted bitumen7 and the recovery of solvent from extracted sand. Work in this laboratory has focused on a related approach. It is essentially a solvent extraction process but uses ionic liquids (ILs) as an extraction aid and separating fluid.10−12 Using atomic forces microscopy (AFM), it was shown that, in an imidazolium ionic liquid, the energy of adhesion between silica and bitumen is close to one-tenth that found in water,13 facilitating detachment. These ILs also have a higher density than the solvent diluted bitumen and were chosen to be essentially immiscible with hydrocarbons. As a result, they formed a separating layer between the extracted bitumen and sand. Solids and liquids were separated using centrifugation. Recovery of residual hydrophilic IL from the extracted sand was readily accomplished using a water wash. IL can be recovered from this wash stream using vacuum distillation, because of the negligible vapor pressure of most ILs. With regard to the liquid phase, ILs and hydrocarbons are also easily separated and recovered using centrifugation, because of their significant density difference. In addition, because mineral fines have a surface charge, they have a thermodynamic preference for the IL phase, where repulsive forces between mineral particles are also “screened”. This facilitates aggregation and settling of these fines. A significant problem with this approach is that most ILs are expensive and to various degrees toxic. However, in recent work we demonstrated that bitumen can be separated from both Alberta and Utah oil sands using a so-called analogue IL based on a deep eutectic mixture (DEM) of choline chloride and urea (ChCl/U) and a diluent such as naphtha.14 (This DEM was first described by Abbott et al.15) Unlike conventional ILs, these eutectics are relatively cheap, commodity © 2015 American Chemical Society
chemicals and environmentally friendly. (ChCl is an animal feed supplement and a component of baby formula.) It was shown that a two-step extraction of an Alberta oil sand sample gave yields in excess of 90%. Infrared spectra indicated that only trace amounts of minerals could be found in the extracts and only small amounts of extractable hydrocarbons remain on the minerals (about 6% of the original bitumen content), for the most part attached to clay fines.14 The ChCl/U DEM used in this work clearly facilitated extraction, but an interesting question remains. How good are these eutectic mixtures relative to conventional ILs, such as the imidazoliums used in previous work? Here we will present the results of an AFM study of bitumen/silica interactions in DEM solutions to address this question. This work involved a colloidal probe. However, as we will discuss in the body of this report, bitumen surfaces are heterogeneous, consisting of micrometer-scale domains with different mechanical properties.16−30 The use of micrometer-sized colloidal particles (∼40 μm) can therefore mask some of this variation. In addition, the preparation of colloidal silica AFM probes is not trivial, as we will describe, so that results for only a limited range of DEM solutions were successfully obtained. Accordingly, adhesion experiments using a silicon nitride tip were also performed. Like silica, these have a negative surface charge over the pH range of interest, but their much smaller tip radius (∼20 nm) revealed an interesting dependence of properties on bitumen surface morphology.
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EXPERIMENTAL SECTION
Materials. A Utah oil sand sample was used for most of the experiments in this study. This material was provided by Oil Sands Technologies and was obtained from a stored pile of mined oil sands kept in the open that as a result had weathered for at least 3 years. For some experiments, an Alberta oil sands sample was used. This was obtained from the Alberta Research Council. Laboratory samples of choline chloride (98+%) were purchased from Alfa Aesar, and urea was obtained from Sigma. The preparation of the ChCl/U deep eutectic based mixture was carried out according to Abbott et al.15 The ChCl was mixed with U in Received: October 16, 2015 Revised: December 17, 2015 Published: December 17, 2015 249
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Energy & Fuels a 1:2 mol ratio, heated to 74 °C, and stirred until a homogeneous liquid was obtained. The liquid remains a homogeneous mixture upon cooling to room temperature. Bitumen was extracted from the oil sands using the DEM/naphtha procedure described in a recent publication.14 Instrumentation. A Dimension Icon AFM (Bruker, Billerica, MA, USA) was used to measure the forces between bitumen and probe particles. Nanomechanical PeakForce Tapping mode was used for force−distance curve measurements. Force measurements were performed in a fluid cell. The liquids used in this study included deionized water with different concentrations of DEM (ChCl/U). After a test liquid was injected into the liquid cell, the system was left to stabilize for a half-hour. Loading force is an important factor in the experiments. The larger the loading force, the more the tip penetrates the sample, which can lead to surface damage. However, too small a loading force results in an unstable or wavy force−distance curve, so it is important to find an appropriate range. In previous work,13 a loading force of 18−20 nN was used and initial work in this study used the same parameters. This gave inconsistent results using silicon nitride probes, and the loading force was subsequently reduced to 9−11 nN to prevent damage to the sample, as well as to obtain a stable force−distance curve. The experimental data reported here all used a 9−11 nN loading force. Bitumen Surface Preparation. Mechanical-grade, single-sidepolished silicon wafers were purchased from University Wafer and used as the substrate for supporting the bitumen films, prepared by spin-coating from naphtha solutions. The silicon wafers (2 cm × 2 cm) were first washed with THF to remove organic contaminants, rinsed with deionized water, blow-dried, and kept dry in an oven. The concentration of bitumen in naphtha was 25% by weight. The bitumen solution was centrifuged at 1200 rpm for 30 min. Although this may be inadequate to remove very fine particles, infrared spectra of the bitumen films, published previously,14 indicated that any absorption bands due to clays or fine silica particles were at the detection limit (∼0.1%). About 0.1 mL of the prepared bitumen solution was dropped slowly onto a silica wafer spinning on the spin coater at 2000 rpm for 20 s and 5000 rpm for 1 min. The spin-coated bitumen surface was then dried under vacuum (∼25 in. Hg) for 48 h at room temperature. Under optimal spin-coating conditions, the bitumen-coated wafer had a black, mirror-like appearance. Silica Colloidal Probe Preparation. Colloidal silica particles of known radius were attached to the AFM probe and used to obtain force−distance curves in deionized water or DEM solutions. Silica microspheres (average diameter, 40 μm) purchased from Silicycle were used as models of sand grains for colloidal force measurements. Triangular-shaped AFM cantilevers were purchased from Bruker. Scanasyst-Air probes were used, which give stable force−distance curves. The cantilever’s properties are listed in Table 1. The actual spring constant of the cantilevers was calibrated with the Bruker Dimension Icon AFM instrument using the thermal tune method.
(3) The glue edge was found, and using the AFM contact mode, some of the glue was picked up on the AFM probe. Great care was taken with the AFM probe/glue position. (4) Once a drop of glue was picked up, a silica colloidal particle was engaged and attached using the contact mode. (5) The glue was allowed to set at room temperature for 24 h before using the probe. This procedure is illustrated in Figure 1.
Figure 1. Top: AFM probe (A), probe approaching and touching glue (B), and looking for a silica colloidal particle (C). Bottom: Approaching a silica colloidal particle (D), picking up colloidal particle (E), and attached particle (F). Silicon Nitride Probes. In order to measure a stable force− distance curve, the choice of cantilever and tip is important. Properties such as tip material, cantilever stiffness and resonance frequency, and tip shape and sharpness all need to be considered and adjusted for the system to be studied. Several different types of probes were tested in order to obtain a stable force−distance curve. The results presented here used a Scanasyst-Fluid silicon nitride tip with 20 nm (dull) tip radius. The cantilever parameters are listed in Table 1. Each tip was calibrated prior to the experiments. The calibration was performed as follows: (1) The silicon wafer was approached on a noncompliant part of the (hard) sample. A representative force curve was captured, and the deflection sensitivity in the linear part of the curve was updated. (2) The tip was withdrawn and the cantilever thermally tuned to calculate the spring constant. To measure the adhesion force between a probe and the surface, a force−distance curve is acquired. The force curve analysis was performed via the multiple curve analysis function of Nanoscope Analysis (Bruker). To ensure a representative force profile, force measurements were carried out at several locations on the bitumen substrate for a given pair of bitumen film substrate and tip probes. An average value or a distribution of adhesion force and energy was obtained.
Table 1. Characteristics of Bruker Scanasyst-Air and Scanasyst-Fluid Probes AFM cantilevers
Scanasyst-Air
Scanasyst-Fluid
resonant frequency (kHz) spring constant (N/m) length (μm) tip radius (nm) tip materials
70 (45−90) 0.4 (0.2−0.8) 115 (100−130) 2 (12 max) silicon
150 (100−200) 0.7 (0.35−1.4) 70 (65−75) 20 (60 max) silicon nitride
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ADDITIONAL BACKGROUND Bitumen Surface Heterogeneity. In order to put the results presented here into context, we will first summarize recent AFM studies, which have provided considerable insight into the surface microstructure of bitumen.16−30 Although it is usual to present background information such as this in an introductory section, we think a discussion of our results flows more naturally by including it immediately prior to a presentation of the data. Essentially, three surface structures have been observed in AFM work. One of these structures consists of elliptical domains called the peri-phase embedded in a second, continuous so-called para- or perpetua-phase. However, the peri-phase shows an additional structure (catana-phase), wonderfully labeled “bees”, because of the pattern of pale and
It was found that attaching a colloidal tip to the AFM probe is not trivial, but the following procedure was developed: (1) A small amount of 30 min set Permatex 84107 Permapoxy extra strength epoxy was spread on a glass slide. (2) Some silica colloidal particles were spread on a second glass slide. 250
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Energy & Fuels dark lines they display.16 This structure is now considered to be part of the peri-phase. The appearance of bee structures is associated with the presence of a crystallizable wax fraction, together with a wrinkling or buckling of the surface.21−23,30 In the samples studied by Lyne et al.,25 the bees disappeared above a temperature of 57 °C. Nevertheless, even in a sample with no crystallizable wax, a microstructure could still be observed by AFM,27 although one that is less rich, consisting of small clusters of phase-separated material that were attributed to flocculated asphaltenes. There are two additional points from the AFM work that need to be considered. First, the continuous- or para-phase is softer and more liquid-like (viscous) than the peri-phase.22,23,26 Second, the size of the elliptical peri-phase domains increases with the asphaltene content of the samples.24 Both these observations suggest that the peri-phase has a higher concentration of asphaltenes. If the peri-phase and its bees are associated with wax content, they also have a higher concentration of crystallizable alkanes. Given the thermodynamic incompatibility of alkanes and asphaltenes, this observation is at first sight counterintuitive. However, Alcazar-Vara et al.31,32 point to various publications that suggest asphaltene aggregates can act as nucleation sites for wax precipitation. Be that as it may, asphaltenes and waxes are not the principal components in many bituminous materials, while the observed peri-phrase elliptical domains appear to be the major surface component. In this regard, the recent work of Fischer and Dillingh is important.27 These authors showed that the surface region probed by AFM is significantly enriched in the peri-phase and that in the bulk there is a larger fraction of the softer para-phrase, which is presumably mostly maltenes. Previously, Lyne et al.25 had also concluded that the peri-/ catana-phase, which they called the bee laminate phase, separates from the bulk of the bitumen and blooms to the surface. Naphtha Solutions. In the separation work we reported recently14 we used naphtha as a diluent. Although this no doubt resulted in the phase separation of an asphaltene rich component within the organic phase, both this component and the maltene rich component are immiscible with the ChCl/ U deep eutectic mixture. Both remain part of a no doubt complex organic phase. Part of the purpose of this work is to examine how the DEM facilitates separation in the DEM/ naphtha process we have developed,14 so here we maintain the use of naphtha as a diluent. However, it is apparent that in samples prepared for AFM work by other authors, reviewed earlier, the surface is made up of various phase-separated domains, with the harder peri-phase predominating.
sands. The force−distance curve is shown in Figure 2. Each force plot represents a complete cycle of the tip−substrate
Figure 2. Representative force−distance approach (red) and retraction (black) curves obtained in deionized water using the Si3N4 probe on Utah bitumen samples using AFM PeakForce QNM. The approach/ withdrawal cycle was collected in DI water at 20 ± 2 °C.
approach and retraction. When the tip is far away from the surface, there is no interaction and no measured force. At a distance of about 20 nm from the bitumen surface, a jump-tocontact attractive force of about 3 nN was observed. This behavior indicates that a fairly long range or strong tip/sample attraction exists in DI water. After contact, the deflection of the cantilever increased until it reached a maximum load (peakforce). The tip then retracts from the sample. The adhesion between tip and sample resulted in a peak in the retraction force of about 17.5 nN, as measured by the minimum in the retraction plot shown in Figure 3. There is a large hysteresis between the approach and retraction parts of the force curve. The tip relaxes back to the
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RESULTS AND DISCUSSION AFM Silicon Nitride (Si3N4) Probe Measurements. The surface microstructures or phases described in the preceding paragraphs have dimensions of the order of a few micrometers so that measurements using a colloidal probe (∼40 μm diameter) are likely to sample more than one phase. We therefore first consider results obtained using Si3N4 probes that have tip diameters of about 20 nm. Because both silica and silicon nitride have negatively charged surface groups in the pH range of interest here (the latter as a result of surface oxidation33,34), both give insight into the interactions that are important in extracting bitumen from oil sands. We first consider force−distance curves obtained in deionized (DI) water using bitumen obtained from Utah oil
Figure 3. Representative force−distance approach and retraction curves obtained in deionized water using the Si3N4 probe at two different locations on a Utah bitumen sample. The approach/ withdrawal cycle was collected in DI water at 20 ± 2 °C. 251
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Multiple measurements (∼10, sometimes more) at sites around a given position on the film were made. Normal curves for the most part involved a pull-off or retraction force of the order of 14−20 nN, with the retraction force returning to the point of zero force at a surface/film distance in the range 40− 90 nm. We will call this the final pull-off distance. Fat curves had a smaller pull-off force, in the range 6−10 nN, with a very large tip−sample pull-off regime, which usually ends more than 100 nm from the surface. For both fat and normal curves, heterogeneity in surface structure and properties results in a significant variation in the data around a given position. Nevertheless, the adhesion force measurement gives results in a consistent range for each position on the film surface. This is shown in Figure 4, where a set of force−distance data for two different tips is shown. It can be seen that most of the pull-off force data points lie in the range 18−20 nN. Turning now to measurements in the DEM and its solutions, reproducible force/distance curves were difficult to obtain in nondiluted mixtures, because the high viscosity of the pure ChCl/U eutectic resulted in an unstable laser signal and drift during measurements. The DEM was therefore diluted with measured amounts of deionized water, and interactions in a series of different DES aqueous solutions were studied. Typical approach and retraction force−distance curves for 97% and 90% DEM aqueous solutions are shown in Figure 5, where they
interaction-free (zero-force) position through the liquid, which exerts a viscous drag. This viscous force counteracts any abrupt snap-off and produces a gradual decrease of cantilever bending with increasing separation.22 However, there is a large tip− sample separation regime in the retraction curve, indicating that there is a significant degree of nonelastic deformation. Accordingly, the dissipation energy (the area between the approach and retraction curves) measures not only the adhesion energy between the tip and the surface but also the work of nonelastic sample deformation. Force−distance curves do not all have same characteristics but can be separated into two types. Representative plots of each type are shown in Figure 3. Most of the observed force− distance curves (∼70%) were similar to the blue curve in Figure 4. We call these “normal” curves. The second type of curve
Figure 4. Measured pull-off or adhesion force and retraction force curve end position relative to the mean of water (each data point refers to one single force curve measurement).
corresponds to the black curve in the same figure. For these curves, in the approach part of the cycle there is a repulsive force at long distances (∼100 nm) and there is no jump-tocontact attractive force. The plots are also characterized by a very large tip−sample pull-off regime, which usually ends at distances in excess of 100 nm. We call these “fat” curves. One possibility is that this large pull-off regime is because a soft surface phase is being probed, presumably the para-phase described previously. The tip penetrates this softer region to a greater extent than “harder” regions (presumably the periphase), and because of the viscous nature of the material, detachment occurs after considerable deformation of the surface bitumen has occurred. Because the harder peri-phase predominates at the surface, most of the curves we observed were “normal”. Another possibility is that the “fat” curves are a result of tip contamination with a viscous bitumen component. However, as we will show later, moving a given tip to another region of the sample after obtaining a fat curve resulted in data that appeared as a normal curve. We would emphasize that the purpose of this study is to compare adhesion forces in water and DEM solutions, so in what follows we will compare results for both normal and fat curves.
Figure 5. Representative force−distance curves collected using the Scanasyst-Fluid probe on a Utah bitumen sample. The approach/ withdrawal cycle was collected in water at 20 ± 2 °C. Black data points were obtained in DI water. Blue and red data points were obtained in 97% and 90% ChCl/U aqueous solutions, respectively.
are compared to the curve obtained in DI water. (These curves are all normal.) The jump-in force of about 3 nN in DI water does not appear in DEM solutions. In addition, upon approach there appears to be a longer range repulsive force between the tip and the surface in the DEM than in DI water. This is possibly because electrostatic repulsions are screened to some degree in the DEM, allowing attractive van der Waals forces to predominate. The pull-off force in DI water is ∼20 nN. The pull-off force is significantly smaller in the DES solutions, dropping to a value of around 1 nN in the 97% solution and 3 nN in 90% DES solution. The dissipation energy (the area between the approach and retraction curves) decreases by a 252
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Energy & Fuels factor of more than 80% in both DEM solutions relative to that in DI water. As with the measurements in DI water, the measured pull-off force (and dissipation energy) falls in a range around a given film location with a given tip. This is illustrated by two sets of data, this time obtained in 80% DEM aqueous solutions with two different AFM tips, labeled tip 3 (blue data points) and tip 10 (red data points) in Figure 6. First the data obtained with tip
Figure 7. Measured adhesion force and retraction force curve end position relative to the mean for 80% DEM solutions in DI water (each data point refers to one single force curve measurement).
energy are still significantly less than the values determined in DI water. The results of a large number of experiments obtained by systematically varying DEM concentrations between 10% and 90% are summarized in Figure 8, where the distance to the end
Figure 6. Representative force−distance approach/retraction curves for a Utah bitumen sample. Black data points were obtained in DI water. Blue and red data points were obtained in 80% ChCl-U aqueous solutions with AFM tips labeled 3 and 10, respectively.
3 are considered. The adhesion force measured in water for this tip averaged 19.3 nN. Upon exchanging water for an 80% DEM solution, the adhesion force values dropped to an average of 2.3 nN, about 12% of the value in water. This experiment was then repeated with a different tip (tip 10), and the first set of results is also shown in Figure 6. The adhesion force now averages 6.1 nN, relative to a value of 18.7 nN, in DI water, still 32% smaller, but larger than that measured with tip 3. It can also be seen that the dissipation energy (area between the approach and retract curves) is larger, with the pull-off regime ending at a significantly greater distance from the surface. This corresponds to the difference between normal and fat curves observed in DI water experiments, but the pull-off force and pull-off end distance are both smaller. The position of this tip on the bitumen film was then changed, and a second set of results was obtained. The new average value of the adhesion force was then determined to be 1.7 nN, about 10% of the value in water and in the same range as the values obtained with tip 3. To emphasize, multiple measurements at sites around a given position on the film using these tips were made and the results are summarized in Figure 7. This indicates that the fat curves are not a result of tip contamination but that there are different regions of the sample that give different values of the adhesion force that lie outside the scatter in the data. This is consistent with the heterogeneous nature of the surface found in the work referenced earlier. Nevertheless, in the presence of DEM there is still a large decrease in the adhesion force relative to the values found in water. As a result, even when soft regions of the surface are being probed, the pull-off force and dissipation
Figure 8. Plot of the distance to the end of the pull-off regime vs the adhesion or pull-off force. Red data points represent values obtained in DEM solutions, while the blue data points represent data obtained in DI water.
of the pull-off regime is plotted against the adhesion or pull-off force. These are average values for a set of measurements obtained with a given tip around a given location. The results shown in Figures 5 and 7 indicate that the measured values lie in a range of roughly ±1 nN around these mean values. Red data points represent values obtained in DEM solutions, while 253
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Energy & Fuels the blue data points represent data obtained in DI water. As would be expected given the heterogeneity of bitumen surfaces, there is considerable scatter in the results, but clear trends can be identified. To aid the eye, the figure is divided into four zones, first delineated by data points that are characterized by adhesion or pull-off forces that are either greater or less than 9 nN (vertical dashed line in Figure 8). A second divide is defined by a diagonal line stretching from a pull-off distance of 50 nm at an adhesion force of zero to a value of 90 nm at an adhesion force of 20 nN. We will consider the data lying below this diagonal first. Most of the data points for DI water lie in the bottom right-hand part of the plot, having a pull-off force in excess of 9 nN, with most having a pull-off force in excess of 15 nN. They are also characterized (on the whole) by pull-off end distances in excess of 40 nN, with most having a pull-off distance in excess of 60 nN. On the other hand, the data obtained in DEM solutions are characterized by an adhesion force that is less than 9 nN and a pull-off end distance that is less than 40 nN. Within this data set, there is also a trend to smaller pull-off forces at higher DEM concentrations, although there is considerable overlap, particularly at intermediate concentrations, as might be expected given the scatter shown in Figures 5 and 7. The force/distance plots that gave the data lying below the diagonal in Figure 6 all had the appearance of normal curves. As might be anticipated, fat curves resulted in those data lying above the diagonal in Figure 7. In general, these had smaller pull-off forces and larger pull-off regimes, but the trend remains the same, with smaller pull-off forces in DEM solutions relative to DI water. AFM Silica Colloidal Probe Measurements. As mentioned previously, measurements with silica colloidal tips were frustrating. In part, this was associated with the difficulty in attaching colloidal particles to the probe. There were experiments where we suddenly got a zero pull-off force, indicating colloidal probe detachment. In addition, even when successfully attached, not all particles turned out to have the necessary shape and mechanical properties suitable for the experiments we wished to perform. Accordingly, the data we obtained using a silica colloidal probe is more limited than the results obtained using a Si3N4 tip, but nevertheless show the same trends. Figure 9 is an example of force−distance curves obtained using an Alberta bitumen sample in DI water and 75% DEM aqueous solution. The blue data points represent an experiment performed in DI water. In contrast to what was observed with the Si3N4 tips, a jump-to-contact attractive force was not observed using silica colloidal probes. In addition, significant repulsive forces were observed at a distance of about 150 nm from the surface. The measured adhesion or pull-off force was similar to that obtained using Si3N4 probes, however, with an average value of about 18 nN and a standard deviation of about 3 nN. In addition a much larger pull-off region was observed, as illustrated in Figure 9, with a final pull-off distance in excess of 400 nm. This is to be expected because of the larger area of the colloidal probe in contact with the bitumen surface. In a 75% DEM solution, the repulsive forces on approach take effect at a greater distance, close to 200 nm. The pull-off force for the experiment shown in Figure 9 is reduced to about 2.5 nN, while the pull-off distance is just over 200 nm. Nine different spots on the bitumen surface were tested in the 75% DEM solutions, and the data are summarized in Figure 10. The pull-off forces, shown on the left hand part of the plot,
Figure 9. Representative force−distance curves collected using a silica colloidal probe. The approach/withdrawal cycle was collected at 20 ± 2 °C (blue data points, DI water; red data points, 75% DES aqueous solution).
Figure 10. Measurement of the adhesive (pull-off force) and the dissipation energy for an Alberta bitumen sample. Left hand scale and data, pull-off force in DI water and a 75% DEM solution. Right hand scale and data, measurements of the dissipation energy.
were distributed between 1.5 and ∼4.3 nN, with an average pull-off force of about 2.9 nN, about 85% smaller than the value observed in DI water. The dissipation energy is shown on the right hand part of the plot and is more than 90% less in the DEM solution than in DI water. The results obtained using the Utah oil sands bitumen are summarized in Figure 11. For this sample results for both 75% and 50% DEM solutions were successfully obtained. As in Figure 10, pull-off force data are shown on the left hand part of the plot, while dissipation energy is plotted on the right hand part of the plot. There is a significant reduction in both parameters in the DEM solution relative to DI water.
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CONCLUSIONS The pull-off forces measured for a silicon nitride AFM tip in solutions of a deep eutectic mixture (DEM) of choline chloride and urea were significantly smaller than in water. Two types of retraction curves were observed, one assigned to interactions 254
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Figure 11. Measurement of the adhesive (pull-off force) and the dissipation energy for a Utah bitumen sample. Left hand scale and data, pull-off force in DI water, a 75% DEM solution, and a 50% DEM solution. Right hand scale and data, measurements of the dissipation energy.
between the probe tip and a harder part of the surface, while the second type of curve was characteristic of interactions with a softer, more viscous phase. At high DEM concentrations the pull-off force for the harder phase was reduced by a factor of about 10 relative to water. A significant reduction in the pull-off force and dissipation energy was also observed in the data from the more viscous phase, but the reduction was smaller, of the order of 30−40% for concentrated DEM solutions. Experiments using a silica colloidal probe proved to be more difficult, and only one type of retraction curve was observed. Nevertheless, a significant reduction in both the pull-off (adhesion) force and dissipation energy where observed, with the values of these quantities in 75% DEM solutions being approximately 25% of the values in water. These results are not quite as good as those obtained in previous work using imidazolium ILs, where a reduction by a factor of about 10 was obtained.13 However, the use of a DEM clearly significantly lowers the pull-off force and facilitates detachment.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare the following competing financial interest(s): We have patents on a technology involving the extraction of bitumen from oil sands using an ionic liquid based technology and we are seeking to commercialize this technology.
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REFERENCES
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DOI: 10.1021/acs.energyfuels.5b02375 Energy Fuels 2016, 30, 249−255
