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Relative performance of several surfactants used for heavy crude oil emulsions as studied by AFM and force spectroscopy
Journal of Petroleum Science and Engineering 135 (2015) 652–659
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Relative performance of several surfactants used for heavy crude oil emulsions as studied by AFM and force spectroscopy James R. Karamath a,n, A. Adriana Vallejo-Cardona b, Ricardo Cerón-Camacho a, Icoquih N. Zapata-Peñasco a, Vicente Garibay-Febles a, Jorge Aburto a a
Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, San Bartolo Atepehuacan, 07730 Ciudad de México, Distrito Federal, Mexico Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco (CIATEJ), Normalistas 800, Colinas de la Normal, 44270 Guadalajara, Jalisco, Mexico b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 May 2015 Received in revised form 18 September 2015 Accepted 1 October 2015 Available online 22 October 2015
Through force spectroscopy, we use an atomic force microscope (AFM) to compare the forces between two extra heavy crude oil (EHCO) droplets. This is done in various aqueous environments in order to compare said forces in the presence of a variety of surfactants and ionic species. A crude oil droplet is attached to the AFM cantilever and pressed against other droplets placed on a substrate. Differences in the behavior of the forces as a function of surfactant chemistry, concentration and the velocity of the approach and retract of the droplets are observed for droplets with a range of diameters 10 o Ø (mm)o 70. The surfactant H4 (dodecyl-polyglucoside, designed at the Instituto Mexicano del Petroleo) is found to cause the highest forces between the droplets and greatest reduction of drop deformation when pressed together. Qualitative agreement is observed with previous theoretical works with regards to the addition of ionic compounds such as sea-water – the droplets approach more closely and coalescence may be favored in some cases. It is possible to observe structure on the surface of other oil drops by tapping mode AFM imaging. It is believed that this is the first time this has been done on extra heavy crude oil droplets. & 2015 Elsevier B.V. All rights reserved.
Keywords: Emulsion AFM AFM force spectroscopy Surfactants Extra heavy crude oil Crude oil transport
1. Introduction An ever growing fraction of the world's crude oil reserves, the basis of a large fraction of human energy needs, exists in the highly dense form of heavy or extra heavy crude. This property is usually defined in the petroleum industry by the oil's API gravity, arbitrarily defined as API gravity ¼(141.5/SG) 131.5 where SG is the oil's specific gravity at 60 °F (ASTM D287-12b, 2015). Heavy oil is an asphaltic, dense (low API gravity), and viscous oil that is chemically characterized by its content of asphaltenes (very large molecules incorporating most of the sulfur and perhaps 90 percent Abbreviations: (O/W), Oil-in-water (emulsion); AFM, Atomic Force Microscope; Ø, Oil drop diameter; (W/O), Water-in-oil (emulsion); DSC, Differential calorimetry scanning; EHCO, Extra heavy crude oil; SDS, Sodium dodecyl sulfate; TQA, Nonylphenol ethoxylate EO 15; BS1, Glucopon 425; BS5, Glucopon 625; GC10, Mixture of GC10:G2C14 at 1:1; H4, Dodecyl-polyglucoside; CMC, Critical micelle concentration; TM, (AFM) Tapping mode; CM, (AFM) Contact mode; vr, Ramp velocity (of AFM probe); Fmax, Force trigger (maximum force during a ramp); k, Force constant of cantilever; A, Approach curve of cantilever ramp (droplet approach one another); R, Retract curve of cantilever ramp (droplets retreat from one another); SSW, Synthetic sea water (ionic species); IMP, Instituto Mexicano del Petróleo n Corresponding author. E-mail address: [email protected] (J.R. Karamath). http://dx.doi.org/10.1016/j.petrol.2015.10.012 0920-4105/& 2015 Elsevier B.V. All rights reserved.
of the metals in the oil). Although variously defined, the upper limit for heavy oil has been set at 22° API gravity and a viscosity of 100 cP. Extra-heavy oil is that portion of heavy oil having an API gravity of less than 10° meaning that the crude is as dense or denser than water (Meyer and Attanasi, 2003; International Energy Agency, 2010). Difficulties exist in the extraction, transport and processing of such viscous crudes and understanding aspects such as surface energies, forces between droplets, effects of additives (viscosity reducers, emulsion (de)stabilizers) etc are vital to drive down economic costs and develop more environmentally friendly methods. Addressing the transport problem, several techniques exist to assist the flow of highly viscous crude oil from production platforms often thousands of kilometers to processing plants/refineries (Martinez-Palou et al., 2011). These include; heating the pipeline (clearly very energy inefficient and complicates pipeline design and cost), dissolution in lighter oils (Hénaut et al., 2007), addition of viscosity/drag reducing chemicals (which can be environmentally harmful and may need to be removed at further cost), use of pulsed electrical fields (Homayuni et al., 2011) (which cause a rather small viscosity reduction) and creation of oil-inwater (O/W) emulsions allowing the crude to essentially be
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transported like water. This latter option requires the addition of surface active agents (surfactants) to invert the emulsion from its naturally occurring water-in-oil (W/O) form to the desired O/W form. Surfactants can be chemical or biological in nature and act by altering the surface tension between the two liquids – the interfaces between the water and oil become populated with these agents due to their part hydrophobic part hydrophilic nature (e.g. a polar, hydrophilic head with a hydrophobic tail). There exist numerous methods to study the various effects of surfactants for crude oil on emulsion stability; these include (nonexhaustive list) fluorescence spectroscopy (Martinez-Palou et al., 2013; Vallejo-Cardona (in press)), optical microscopy, confocal microscopy (Hung and Castillo, 2006), static light scattering, dynamic light scattering, electrical pulse counting, sedimentation techniques, ultrasonic spectrometry (Smyth et al., 2004), nuclear magnetic resonance, neutron scattering (Verruto and Kilpatrick, 2008), dielectric spectroscopy (Sax et al., 1988), differential calorimetry scanning (DSC) (Clausse et al., 2005), analysis of particle size distribution, dynamic viscosity (Kodal, 2005) and commonly the ‘bottle test’ (ASTM D4007, 2011). However, a fundamental measurement that seems to have been overlooked is the forces between the oil droplets within the emulsion. Understanding these forces in a variety of ambient conditions (e.g. surfactant or salt concentration) allows for better optimization of the transport process. In atomic force microscopy (AFM) a tiny cantilever with a downwards facing needle (herein “tip”) at its end is moved with nanometer precision across or just above a surface. The end of the tip, commonly ca. 5 nm in radius, interacts with the surface through a variety of forces, van der Waals, electrostatic, capillary etc. For standard commercial systems in air, a triangular cantilever 100 mm long, 1 mm thick and 30 mm wide can have a force constant as low as 0.01 to 0.1 nN/nm, allowing a force sensitivity of just 10–100 pN by standard detection methods. As well as obtaining topographic images by scanning the probe in the XY plane across a surface as it is commonly used, one can also scan (or ramp) the cantilever up and down (z-direction) against a surface and study how the force between the tip and substrate varies as the tip and substrate approach/retract from one another. One can replace the tip of the cantilever with a tiny drop of (crude) oil. If the substrate has drops of similar oil on it, one can perform exactly the same kind of ramp and measure the forces between the two small oil droplets. This can be done in a (transparent) aqueous environment, one which contains distilled water, and defined concentrations of surfactants and salts, for example. These “impurities” will affect the interaction forces between the drops (surface charges and double layers, surface tension, liquid polarizability and space charges etc) which can be detected by the AFM optical system (force spectroscopy). Thus, in this article, measurements are presented of the forces of interaction between two approaching and retracting extraheavy crude oil (EHCO) droplets within an emulsion with various surfactant-doped aqueous solutions as the continuous phase.
2. Background Force spectroscopy by AFM between deformable colloidal (oil) droplets is itself not new, although the authors are not aware of any prior research with such heavy, viscous, chemically complex oils, nor with the surfactants used herein (those specifically designed for (heavy) crude oil based emulsions). Gunning et al. (2004) use tetradecane as their oil and study the effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the forces and chance of coalescence of the drops. The SDS is observed to electrostatically screen the electric double layer of the charged drops,
653
allowing them to approach closer and favor coalescence. In another tetradecane based article, Gromer and Gunning (2011) are impressively able to see the effect of depletion, that is the work of demixing a protein (sugar beet pectin) when the water is squeezed out/flows back between the two drops (thin film). This required very slow approach and retract speeds to avoid hydrodynamic forces from dominating – the demixing entropic forces were well below 100 pN. The effect of ionic additives, of the deformability of the drops and the velocity of the approach/retract are three of many parameters successfully modeled by group of Dagastine (Carnie et al., 2005; Dagastine et al., 2006) and verified by experiments between a solid particle and oil (Dagastine et al., 2004a, 2005) and between two oil drops (Dagastine et al., 2004b). Tetradecane is again the (very light) oil of choice throughout, although they do change the viscosity of the ambient solution by adding sucrose to the water (Dagastine et al., 2010). The success of their modeling allows several otherwise unobservable physical parameters to be deduced, including the physical distance (of the continuous phase) separating the drops, their shape, the velocity of their surfaces etc. each as a function of time. Heavy and extra heavy crude oils, however, are a complex mixture of various fractions; waxes, fatty acids, high and low boiling point oils, sedimentary (solid) material etc. The most important upshot of this is that there exists within its bulk selfemulsifying (surface active) agents that are not present in simple oils. The asphaltene and resin (polar) components of crudes especially are known to act as such and some of the remaining materials mentioned above are themselves surface active (Lee, 1999; Langevin et al., 2004). The forces these alone apply between the droplets in an emulsion or how they interact with additional surfactants are completely unknown.
3. Experimental The aims here are different to the above works. Though using similar methods, the objective was to try to differentiate between a number of surfactants through the forces they are responsible for in various emulsions. In other words, to see if there is any notable difference in such forces between individual oil drops in aqueous solutions comprised of surfactants and salts. Several of the surfactants are commercially available; the remaining two were developed at the Instituto Mexicano del Petróleo (see Section 3.4 below). In each case, the surfactants are specifically designed to improve the stability of O/W emulsions comprised of EHCOs. A summary of the surfactants is given in Table 1. All surfactants used are non-ionic in nature. The EHCO is designated C7; a summary of its properties is presented in Table 2. It was used in its raw, extracted state. Table 1 The surfactants used in the experiments in the present article along with their critical micellar concentration (CMC) and surface tension Y at their respective CMCs. The surface tension of pure water (against air) is Υ ¼71.97 dyn/cm at T ¼25 °C (Weast, 1981). Surfactant
CMC (ppm)
ΥCMC (dyn/cm)
TQA (Nonylphenol ethoxylate EO 15) BS1 (Glucopon 425) BS5 (Glucopon 625) GC10 (mixture of GC10:G2C14, 1:1) H4 (H4C12) (Dodecyl-polyglucoside)
87.6 200 100 21.6 100
43.3 32.5 31 45.4 54.9
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Table 2 Properties of the oil, C7, used throughout the experiments. Parameter
Unit
Value
API gravity at 25 °C Pour point temperature Kinematic viscosity at 50 °C Salt content Insoluble in n-heptane Insoluble in n-pentane Total sulfur Total Nitrogen Water and sediment Ashes SARA content: Saturated Aromatics Resins Asphaltenes
°API °C mm2/s lb/1000 bbl % % % w/w % w/w % w/w % w/w
9.18 6 2906.6 42.7 14.15 17.7 4.2 0.307 o 0.3 0.06
% % % %
w/w w/w w/w w/w
24.5 33.8 24.0 17.7
3.1. AFM equipment and cleaning A Vecco MultiMode AFM with a Nanoscope IV controller and standard liquid cell is employed throughout. The cell is cleaned by sonic agitation in tap water and detergent, followed by tap water, then distilled water and blown dry with clean compressed air. The same method applies to the o-ring and ports used to connect to the syringe which inject new solutions into the cell. The substrate is cleaved mica, although the duration between the time of cleaving and use in the experiment is not controlled. The cantilevers used were of type DNP as produced by Bruker. These are triangular in appearance and the nominal force constants k are in the range 0.06o k (N/m)o 0.35 for the long, thin cantilever to the wider, thicker cantilever respectively. The long, thin cantilever was used for force curve data, any of the 4 cantilevers were used for TM imaging purposes. The resonance frequency f in tapping mode (TM) was in the range 18 of (kHz) o30 in water. To more accurately determine the force constant of the cantilever the procedure of Gibson et al. (1996) otherwise known as the “reference cantilever” method was employed. The reference cantilever was of type CFLC as produced by Bruker and had a force constant kref ¼0.078 N/m. By measuring the angle between the cantilevers θ E10°, the sensitivity of the optical system against a hard surface S and against the reference cantilever Sref, the unknown cantilever force constant k was determined from
⎛s ⎞ k ref = ⎜⎜ − 1⎟⎟/cosθ 0 Kref ⎝ s ⎠ For some measurements the tip of the cantilever was removed using an ion beam of an electron microscope. In other cases, the cantilevers still had their topographic tips, but these were typically 2.5–6 mm high, whereas all drops had a height in excess of 10 mm. The cantilevers themselves were not specially treated as in Webber et al. (2008) or Lockie et al. (2011) in which chromium, then gold were sputtered onto the cantilever ends and then hydrophobized with decanethiol. In Dagastine et al. (2004b) the substrate is hydrophobic Melinex to better fix the oil drops. In the present data, it is observed that when a new cantilever is used, very unusual force curves (discussed further down) result in which the approach and retract curves only follow each other at high ramp speeds, not overlapping at all at slow speeds. The approach curve is seen to start to rise much more slowly, rising to the designated maximum (trigger) force over a longer distance without the distinctive increase in gradient as the piezo extends further. However, when the cantilever ends were left for an extended period (several days) with a covering of the same crude oil and
cleaned only in xylene, subsequent drops produced force curves that followed one another much more closely and had the expected form. The shapes of the curves (and variation of the substrate material) suggest that the cantilever drop was being displaced laterally when forced against the substrate drop. Upon optical inspection of the oil free cantilever end, it was clear that the surface was very rough, either due to chemical corrosion (holes) or crude oil impurities (strongly attached non-dissolvable particles). It is probable that this gave the surface either a much greater surface area for the drops to stick to and/or chemically changed it creating a surface with a higher affinity for the oil. In future work the more precise functionalization methods above will be employed. 3.2. Oil drop deposition The method of depositing oil drops in the aforementioned articles is by use of an atomizer. The viscosity of the oil here prevents this. Instead, the cleaned end of a stainless steel needle is dipped into the oil and touched down lightly by hand onto the mica surface. The first couple of touches create drops too large (deposited on a different substrate), but the next five or so touches create drops with diameters Ø in the range 10 oØ (mm)o 70. As such, very little oil is present in the experiment (each of the ca. 20 drops on the substrate has a volume of 10–100 pL). Note that the drops are sufficiently small that surface forces dominate over gravitational and buoyancy forces. Indeed, the AFM-TM images show how the peaks of the drops can be accurately modeled as spherical caps. 3.3. Adhering an oil drop onto the cantilever Essentially the same method is used as described by Gunning et al. (2004), which, once the system is ready with oil drops on the mica and pure water injected into the cell, simply involves pressing down the cantilever end onto a small substrate drop. The oil has a greater affinity for the cantilever than the mica and thus readily moves to the cantilever. 3.4. Biosurfactant synthesis All solvents available from commercial suppliers. Surfactants TQA (Nonylphenol ethoxylate EO 15), BS1 (Glucopon 425) and BS5 (Glucopon 625) were used without additional purification. The Mexican EHCO was provided by the Mexican petroleum company (PEMEX), again with no purification. The sugar-based biosurfactants GC10, G2C14 and their mixtures (Cerón-Camacho et al., 2013a) as well as H4C12 were prepared in house following a glycosylation procedure which is published elsewhere (Cerón-Camacho et al., 2013b). All surfactants used in this work were nonionic. All solutions were prepared freshly in distillated water and subsequently filtered using a 0.45 mm PTFE disc filter. 3.5. Data acquisition – force curves Nanoscope v614r1 software was used to control all data acquisition. A hard surface sensitivity calibration was first performed within pure water. The optical system subsequently went unaltered. Once a droplet was attached on a cantilever end and aligned with another drop on the substrate, the cantilever was engaged on the surface with a setpoint o0.5 V more than the photodiode V signal (typically this corresponded to less that 1 nN force). The system was then set to perform ramps, in which the substrate drop is raised to approach (A) the cantilever drop, then lowered to retract (R) from it. Ramp parameters were; ramp distance, z¼1.0 mm; ramp velocity, vr in the range 0.25ovr (mm/s)o17.4, maximum (trigger) force Fmax in the
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range 3oFmax (nN)o10, wait between ramps¼10 s. This wait time was necessary to allow the drops to reacquire their equilibrium positions and for liquid motions due to the previous ramp to disperse, as determined experimentally. Recently, Gunning et al. (2013) have studied the relaxation rheology and thin liquid film behavior between n-tetradecane drops by leaving a wait time when the maximum force occurs (i.e. between the approach and retract ramps). 3.6. Drop shape determination Optically, the diameters of the drops could be observed to better than 72 mm. By removing the liquid cell and placing it on its side in aqueous solution, its profile, height, diameter, radius of curvature etc. could also be estimated to similar accuracies. This was not possible for the substrate drops, where these parameters were determined by topographic imaging. Tapping mode (TM) images of the surface of the oil drops in aqueous solution were performed in two different ways. Firstly, with a standard, clean DNP cantilever and tip. This method produces clean, high resolution images of substrate drop surfaces whether small (10 nm) or large amplitude (100 nm) tapping vibrations were used. Again, very low engage forces (sub-nN) were utilized. Analysis of the cross section allows one to determine the radius from a least squares fit. In a second method, the substrate drop was imaged in contact mode with a droplet on the cantilever, similar to the experiment performed by (Gromer and Gunning, 2011). This is clearly a low resolution method, but has several distinct advantages. Firstly, one has the choice to use either a smaller droplet on the cantilever or on the substrate. The smaller (radius of curvature) droplet will image the larger. Thus the radius of curvature of both the cantilever droplet and substrate droplet can be measured. Note that every drop imaged has a height and diameter greater than the maximum extension of the piezoelectric scanner used (3.1 mm 12 mm 12 mm ¼ zmax ymax xmax). Thus only the peaks of the droplets are analyzed. Secondly, this provides a means to align the droplets nearly perfectly with one another – better than the optical 2 mm limit, although this problem should not affect the results significantly. Finally, it precludes the need to remove the liquid cell from the AFM and place it on its side in water to measure its radius. 3.7. Change of aqueous solution The aqueous environment was changed by simple injection using disposable syringes; pushing the fluid into one of the liquid cell ports thereby flushing the liquid in the cell volume out of the other port. The cell volume including ports is approximately 0.3 ml and each flush was performed in three 0.5 ml steps, 1.5 ml in total. For experiments involving many surfactants, after force curves were taken for one solution a flush with pure MilliQ (MQ) water was performed and further forces curves taken. These were checked with force curves in flushed MQ after every surfactant to ensure consistency – that the environment was sufficiently pure again ready for the next surfactant. This data acted effectively as a control by which one could have confidence that the changes observed were due to the surfactants and not variation of some other parameter.
4. Results 4.1. Ramp velocity dependence For the sake of brevity, preliminary data is only summarized here. The core of this section will concentrate on the comparison
Fig. 1. Force curves as a function of ramp velocity. (A) In a pure water environment. Retract [R] curves only shown for the slowest and fastest ramps. (B) In BS5 (400% CMC) and synthetic sea water, all approach [A] and retract [R] curves shown. (C) Quantifying the graphs. The interaction range (distance over which force grows from 0.25 nN to 3.25 nN on the approach curves) as a function of ramp velocity shown for the two aqueous solutions.
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between surfactants. The effect of the velocity of the ramp was experimentally checked both in pure water and in an aqueous solution of BS5 significantly in excess of its CMC (400%) and synthetic sea water. The results of these two separate experiments are shown in Fig. 1. The velocity data was taken in a random order in both cases. The results of the aqueous solution agree qualitatively with those of Dagastine et al. (2006). The drops are produced with a charge and repel one another upon approach in pure water. The changes to the space charge due to the addition of the impurities acts to shield the drop's charge allowing them to approach more closely. In a viscous environment this results in a narrower thin film layer of the continuous medium clamped between the drops. It is thus more difficult for liquid to flow into or out of this region, increasing the force between the drops. Slower ramps give the ambient solution time to enter and leave; faster ramps do not, effectively creating a vacuum force of attraction upon retract, or a high pressure force of extra repulsion upon approach. In pure water the drops evidently do not approach each other closely and this effect is not visible. Without further comment, it is mentioned how the gradient of the approach curve has an inverse relationship with the velocity of the ramp in the case of pure water, but a proportional relationship in the SSW-BS5 case; these are shown in Fig. 1C. 4.2. Concentration of surfactant and presence of sea-water Briefly, the effect of changing only the solution concentration was studied. This was performed with BS1 and later with GC10. In each case an aqueous environment of MQ water, þ75% CMC surfactant, þ400% CMC surfactant, and þ200% CMC surfactant and SSW was used. (Note that, although heavily salted water is used here, the CMC value is taken from distilled water value. It is well known that ionic species drastically reduce the CMC and surface tensions of many surfactants (Miyagishi et al., 2001). Also, the exact concentration of SSW was not measured but it was of order 0.1 M in terms of salt content.) In Fig. 2A the force curves for the BS1 test are shown (similar results exist for GC10, although the SSW data was not obtainable due to droplet sticking). In Fig. 2B the force between the drops when pushed together 50 nm closer than when the force between them is 1.0 nN is displayed for each curve. Whilst too little data exists (in terms of the concentration variable) to determine a trend, the important data points are the final two – that, after sufficient flushing, the system can be returned to its initial state – the pure water force curve can be recovered. Additionally, it is observed that a surfactant solution with SSW shows very little “adhesion upon retraction” as observed above with BS1 and SSW, presumably because the surfactant is non-ionic and thus itself has little effect on the space charge around the oil drops. As stated, very similar data is observed for GC10 solutions. 4.3. Droplet imaging The cantilever droplets could be imaged optically from the side and their size and shape determined (contact angle, radius of curvature etc). The substrate drops could only be optically viewed from above. By AFM, however, the droplets were imaged both in tapping mode (TM) using a standard tip on a low-k cantilever, and in contact mode (CM) using the same type of DNP cantilever but with another droplet affixed to the end. Discussing the TM images first, it was found that either low or high (drive) amplitudes could used to successfully image without any risk of the drop sticking to the cantilever. High amplitudes tended to be better in terms of lower noise images and better phase contrast. Fig. 3 shows a 3D representation of an oil drop on mica imaged this way; a cross-
Fig. 2. Force curves taken with various aqueous solutions based on BS1. All curves were obtained with a ramp velocity of vr ¼ 8720 nm/s and a 1 mm ramp. Curves shown in chronological order. (A) In BS1 solutions of varying strength and with/ without synthetic sea water (SSW). Much smaller attractive forces seen with surfactant only added. (B) Force between the drops in above solutions when the piezo is displaced 50 nm more than when the force was 1.0 nN.
sectional analysis provided the radius of curvature of the drop, R¼12.5 mm. Note that the corners do not represent the flat mica substrate, rather this is where the drop height has fallen below the reach of the z-piezo scanner (which only has a range of 3.1 mm) and thus the tip is not in contact with the drop. Likewise, only the peaks of the drops used for force experiments were observable due to the x–y piezo limit of the scanner; approximately 12 12 mm2. It is interesting to note that no changes to the drop shapes were observed upon changing the environment from pure water to water with 400% CMC GC10 added, even after several hours, despite the significant change in surface tension the surfactant caused. In separate tests in air with different oil, it was even possible to observe various structures on the EHCO drop surface. An example image is shown, the insets of which show the TM phase and amplitude error signals which better highlight the structures seen. These are possibly the formation of paraffin crystals on the surface. It should be noted that no surface structure was ever observed for the drops used for the force curve experiments. Imaging with an oil drop in contact mode produced similar results albeit with rather more noise. Fig. 3B shows an example image taken in this way. An engage force of approximately F¼0.3 nN was used. 4.4. Comparing the surfactants In three final experiments, force curve data was taken in MQ
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Fig. 3. Topographic images of EHCO droplets in aqueous solution (each 13 13 μm2). (A) TM scan of the spherical cap peak of a drop on mica. (B) CM scan with a small droplet on the cantilever. (C) TM (topographic) image of a different oil with structure observed on the surface, possibly due to the creation of paraffin crystals (2nd order correction applied). (D) TM phase image of same. (E) Error signal image of same.
water with 400% CMC surfactant for each surfactant. The experiments each started in pure MQ water, then the system was flushed with a randomly chosen surfactant solution, then flushed again with MQ water (at least 1.5 ml – until the force curves returned to their initial shape) and this full process repeated with another random surfactant. By the end of a full experiment all surfactant solutions were experimented on typically twice, with force curves in pure water taken between each one, acting as a quasi-continuous control. These data are shown in Fig. 4, again using the force 50 nm of piezo movement closer than 1.0 nN force to quantify the system. The error bars represent the full range of variation of data from 3 to 6 force curves. The parameters used for the first experiment are presented in Table 3; those for the second and third are similar, but using a cantilever with k ¼ (0.053570.0082) N/m. Immediately it is clear that the surfactants perform very similarly; the differences between them are small, only just discernible over the variation between experiments. For instance, the full range of variation in experiment 2 (Fig. 4C) is 3.01 (H4) – 2.85 (BS5) ¼ 0.16 nN. The error bars, which represent the full range of the three curves taken for each data point, are typically 0.01– 0.05 nN. What is further evident is that the differences between the force curves in pure water and in surfactant solution and
between the surfactants themselves are not the same in the three experiments. The differences are much clearer in the first experiment than in the last two. The force curves for experiment 1 are all shown in Fig. 4A. Note that a weak solution of TQA was used, 75% CMC instead of 400% (this was corrected in subsequent experiments). The x-axis (zpiezo displacement) has been adjusted for each curve such that they coincide at 0.5 nN. In this way the difference between the gradients is clearer, as is the relative deformation of the drops. Similar curves exist for experiments 2 and 3 but the differences between the surfactants are less clear. From Fig. 4A it is seen that the interaction between the drops is effectively much stiffer when the surfactants are present, most notably for H4. The gradient of the curve for H4 quickly approaches that of the sensitivity calibration (bare cantilever against mica surface), but for the range of forces used, this level of stiffness is never reached in pure water. This also means that the deformation of the drops when surfactant is present is less than in pure water; again, in the case of H4 the deformation is the least and quickly reaches a maximum (curve parallel to the sensitivity calibration curve). It is obvious from the discrepancy between the latter two and the first experiment that there are still parameters in the system that are not fully understood. However, in each experiment the
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Fig. 4. Results of the three experiments studying the differences in the force curve data with different surfactants. (A) Experiment 1 force curves, adjusted such that all pass through zero piezo displacement at 0.5 nN force between the drops. Second y-axis shows the deflection of the cantilever. Only one of each surfactant's force curve is plotted, for clarity. (B) Force between the drops 50 nm closer than 1.0 nN data from experiment 1. (Note that surfactant TQA was only used at 75% CMC erroneously). (C) The same for experiment 2. (D) The same for experiment 3. Note that sets of data points were taken for H4 and only one for BS1.
Table 3 System parameters used in the first of three experiments. A similar setup used for the second and third experiments, although with a different cantilever and smaller drops. Parameter
Value (exp. 1)
Units
Cantilever spring constant, k z-Piezo ramp velocity, vr Temperature, T Retract delay between ramps Surfactant concentration Cantilever drop diameter, ØCL Mica droplet diameter, Ømica
0.0657 0.015 8750 237 2 10 400% CMC 58.5 7 1.5 62.5 7 2
N/m, nN/nm nm/s °C s mm mm
surfactants that present the strongest and weakest forces between the drops are the same; H4 followed by GC10 or TQA and then BS5 with BS1. This order matches perfectly that of the surface tensions of the surfactants as were displayed in Table 1, even down to how relatively close the GC10 mixture is to TQA and BS1 to BS5. This data is summarized in Table 4. In the case of non-ionic surfactants, it is not surprising that the dominant effect is from the surface tension due to the lack of their ability to significantly affect the space charge around the drops.
Table 4 Summary of the results across the three experiments (all surfactants at 400% of their CMC in MQ water). Columns 2, 3 and 4 show the average of the force 50 nm closer than 1.0 nN in each experiment in nN. The bracketed numbers show the order of these forces (same order number given when results are within the errors). The final column repeats the surface tensions of the aqueous environments (at their CMC). Surfactant
1st exp.
2nd exp.
3rd exp.
ΥCMC (dyn/cm)
MQ water TQA GC10 BS5 BS1 H4
2.17 – 3.62 3.52 3.50 4.05
2.34–2.45 2.98 [2] 2.98 [2] 2.88 [3] 2.89 [3] 3.06 [1]
2.42–2.53 3.06 [2] 3.08 [2] 2.85 [3] 2.88 [3] 3.18 [1]
71.2 43.3 [2] 45.4 [2] 31 [3] 32.5 [3] 54.9 [1]
[2] [3] [3] [1]
5. Conclusions Of the surfactants it is observed that the H4 is the most potent in increasing the force between the EHCO droplets, followed by GC10 and TQA, then BS1 and BS5. This order matches very closely the surface tensions of these surfactants (at their CMC values). Implicit in this result is that the deformation of the droplets is less when H4 surfactant is present than with the remaining surfactants. The difference between the surfactants is small at 400% CMC in pure water. As such, if one assumes a similar production cost for
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each one, it may be that GC10 is the most efficient, since it has a very low CMC, thus requiring very little product to act as an EHCO emulsion stabilizer. One of our primary findings is that AFM force spectroscopy is a viable method for comparing surface tensions and other mechanical/surface effects between different surfactants. The complex nature of the crude oils used here are again emphasized, especially compared to the light and pure oils used in the past literature e.g. Gunning et al. (2004) and Dagastine et al. (2006). As stated, such crude oils are complex mixtures of compounds which themselves contain surface active agents, greatly complicating any analysis. Despite the difficulties with surface preparation and the complex oils, we show that the forces in such dense and viscous crude oil based emulsions acts broadly in the same manner as with the aforementioned lighter oils. Thus we extend the range of droplet viscosities over which such experiments can be performed, as well as prove its viability with newly designed surfactants for EHCO emulsions, which are already becoming an important method to transport such heavy crude oils. The forms of the approach and retract curves in flushed (pure water) samples followed the forms shown by Gromer and Gunning (2011) whilst the curves in the presence of simple ionic species (in the present article, synthetic sea water) follow the forms modeled by Dagastine et al. (2006) and Carnie et al. (2005) (with their addition of anionic surfactants). In the former, the forces simply increased between the drops as the approached and fell as they receded. With a space charge due to salts/anionic species present, the approach curves were steeper and retract curves showed a strong attractive term, due to the electrostatic shielding of the droplets' inherent charge. This is an important consideration in the case of (the breaking of) crude oil emulsions, since for transport they will be mixed with sea-water, not pure water. Clearly further development to the procedure is required to produce more self-consistent results equivalent to those observed by the groups of Dagastine and Gunning. It may be possible to achieve this by replicating their methods for drop fixation on the cantilever and substrate. Likewise there is plenty of parameterspace yet to investigate; it is very possible that greater difference between the surfactants might be observed if data was taken at fixed concentration below the CMC instead of at 400%, or with seawater present. A robust monitoring system may be necessary to ensure the crude is mixed and sealed well before experiments in case settling and/or lighter fraction evaporation occurs. Future work should also replicate the final set of experiments with the addition of an ionic species, since this would be one step closer to the real-life situation. With such development and parameter space exploration, it may be possible to probe the surfactant's effect on emulsion stability by taking data across an extended time frame and offer an alternative method to determine CMCs for surfactants by more data points as a function of concentration. The behavior of crude oil emulsion with time and the optimum concentrations of surfactant needed are becoming increasingly important as the use of these heavy crudes increases worldwide. Finally, it was possible to accurately model/observe the drop sizes optically and through AFM imaging, as attempted by Gunning et al. (2004). With standard-tip-on-drop imaging it is possible
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to see detailed structure (resolutiono20 nm) on the surfaces of some oil drops in TM, allowing the visualization of what were probably paraffin crystals in the oil surface. To our knowledge, this is the first time such detailed structures have been observed by AFM on the surface of liquid droplets. Using drop-on-drop imaging, one can measure both the drops' radii of curvature and better center the drops for force curves if required. This can then be used to normalize results across many experiments. In future work, we may try to observe any changes in the contact angles (or shapes) of the droplet with time; none were observed despite changes to the continuous phase here (over relatively short time frames).
Acknowledgments This project was funded by the Instituto Mexicano del Petróleo, Grant number D.01225. The authors would like to thank Prof. Raymond Dagastine of The University of Melbourne (Australia) and Dr. Patrick Gunning of the Institute of Food Research (Norwich, UK) for enlightening discussions during the course of the project.
References ASTM D287-12b, 2015. Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method). ASTM D4007, 2011. Standard Test Method for Water and Sediment in Crude Oil. Carnie, S.L., et al., 2005. Langmuir 21, 2912–2922. Cerón-Camacho, R., et al., 2013a. Fuel 110, 310–317. Cerón-Camacho, R., et al., 2013b. Chimie 16, 427–432. Clausse, D., et al., 2005. Adv. Colloid Interface Sci. 117, 59–74. Dagastine, R.R., et al., 2004a. J. Colloid Interface Sci. 269, 84–96. Dagastine, R.R., et al., 2004b. J. Colloid Interface Sci. 273, 339–342. Dagastine, R.R., et al., 2005. Faraday Discuss. 129, 111–124. Dagastine, R.R., et al., 2006. Science 313, 210–213. Dagastine, R.R., et al., 2010. Langmuir 26 (14), 11921–11927. Gibson, C.T., et al., 1996. Nanotechnology 7 (3), 259–262. Gromer, A., Gunning, A.P., 2011. Microsc. Anal. 25 (1), 9–12. Weast, R.C. (Ed.), 1981. Handbook of Chemistry and Physics, 61st ed. CRC Press, Boca Raton, pp F-45. Gunning, A.P., et al., 2004. Langmuir 20, 116–122. Gunning, A.P., et al., 2013. Soft Matter 9, 11473–11479. Hénaut, I., et al., 2007. Method of Optimizing Heavy Crude Transportation by Incorporation under Pressure of Dimethyl Ether. U.S. Patent 20070295642, December 27. Homayuni, F., et al., 2011. Pet. Sci. Technol. 29 (19), 2052–2060. Hung, J., Castillo, J., 2006. Advanced environmental, chemical and biological sensing technologies IV. J. SPIE 6377IV, 17. International Energy Agency, 2010. World Energy Outlook 2010. IEA Publications, Paris, p. 145. Kodal, S.L., 2005. Crude-oil emulsions. In: Fanchi, J.R. (Ed.), SPE Petroleum Engineering Handbook 1. Richardson, Texas, pp. 533–570. Langevin, D., et al., 2004. Oil Gas Sci. Technol. 59 (5), 511–521. Lee, R.F., 1999. Spill Sci. Technol. Bull. 5, 117–126. Lockie, H.J., et al., 2011. Langmuir 27, 2676–2685. Martinez-Palou, R., et al., 2011. J. Pet. Sci. Eng. 75, 274–282. Martinez-Palou, R., et al., 2013. Fuel 113, 407–414. Meyer, R.F., Attanasi, E.D., 2003. Heavy Oil and Natural Bitumen—Strategic Petroleum Resources, USGS Fact Sheet PS-070-03. Miyagishi, S., et al., 2001. J. Colloid Interface Sci. 238, 91–95. Sax, B.-M., et al., 1988. Prog. Colloid Polym. Sci. 77, 109–114. Smyth, C., et al., 2004. J. Lab. Autom. 9 (2), 87–90. Vallejo-Cardona, A.A., et al., 2015. Fuel, in press. Verruto, V.J., Kilpatrick, P.K., 2008. Langmuir 24 (22), 12807–12822. Webber, G.B., et al., 2008. Soft Matter 4, 1270–1278.
Contents lists available at ScienceDirect
Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
Relative performance of several surfactants used for heavy crude oil emulsions as studied by AFM and force spectroscopy James R. Karamath a,n, A. Adriana Vallejo-Cardona b, Ricardo Cerón-Camacho a, Icoquih N. Zapata-Peñasco a, Vicente Garibay-Febles a, Jorge Aburto a a
Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, San Bartolo Atepehuacan, 07730 Ciudad de México, Distrito Federal, Mexico Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco (CIATEJ), Normalistas 800, Colinas de la Normal, 44270 Guadalajara, Jalisco, Mexico b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 May 2015 Received in revised form 18 September 2015 Accepted 1 October 2015 Available online 22 October 2015
Through force spectroscopy, we use an atomic force microscope (AFM) to compare the forces between two extra heavy crude oil (EHCO) droplets. This is done in various aqueous environments in order to compare said forces in the presence of a variety of surfactants and ionic species. A crude oil droplet is attached to the AFM cantilever and pressed against other droplets placed on a substrate. Differences in the behavior of the forces as a function of surfactant chemistry, concentration and the velocity of the approach and retract of the droplets are observed for droplets with a range of diameters 10 o Ø (mm)o 70. The surfactant H4 (dodecyl-polyglucoside, designed at the Instituto Mexicano del Petroleo) is found to cause the highest forces between the droplets and greatest reduction of drop deformation when pressed together. Qualitative agreement is observed with previous theoretical works with regards to the addition of ionic compounds such as sea-water – the droplets approach more closely and coalescence may be favored in some cases. It is possible to observe structure on the surface of other oil drops by tapping mode AFM imaging. It is believed that this is the first time this has been done on extra heavy crude oil droplets. & 2015 Elsevier B.V. All rights reserved.
Keywords: Emulsion AFM AFM force spectroscopy Surfactants Extra heavy crude oil Crude oil transport
1. Introduction An ever growing fraction of the world's crude oil reserves, the basis of a large fraction of human energy needs, exists in the highly dense form of heavy or extra heavy crude. This property is usually defined in the petroleum industry by the oil's API gravity, arbitrarily defined as API gravity ¼(141.5/SG) 131.5 where SG is the oil's specific gravity at 60 °F (ASTM D287-12b, 2015). Heavy oil is an asphaltic, dense (low API gravity), and viscous oil that is chemically characterized by its content of asphaltenes (very large molecules incorporating most of the sulfur and perhaps 90 percent Abbreviations: (O/W), Oil-in-water (emulsion); AFM, Atomic Force Microscope; Ø, Oil drop diameter; (W/O), Water-in-oil (emulsion); DSC, Differential calorimetry scanning; EHCO, Extra heavy crude oil; SDS, Sodium dodecyl sulfate; TQA, Nonylphenol ethoxylate EO 15; BS1, Glucopon 425; BS5, Glucopon 625; GC10, Mixture of GC10:G2C14 at 1:1; H4, Dodecyl-polyglucoside; CMC, Critical micelle concentration; TM, (AFM) Tapping mode; CM, (AFM) Contact mode; vr, Ramp velocity (of AFM probe); Fmax, Force trigger (maximum force during a ramp); k, Force constant of cantilever; A, Approach curve of cantilever ramp (droplet approach one another); R, Retract curve of cantilever ramp (droplets retreat from one another); SSW, Synthetic sea water (ionic species); IMP, Instituto Mexicano del Petróleo n Corresponding author. E-mail address: [email protected] (J.R. Karamath). http://dx.doi.org/10.1016/j.petrol.2015.10.012 0920-4105/& 2015 Elsevier B.V. All rights reserved.
of the metals in the oil). Although variously defined, the upper limit for heavy oil has been set at 22° API gravity and a viscosity of 100 cP. Extra-heavy oil is that portion of heavy oil having an API gravity of less than 10° meaning that the crude is as dense or denser than water (Meyer and Attanasi, 2003; International Energy Agency, 2010). Difficulties exist in the extraction, transport and processing of such viscous crudes and understanding aspects such as surface energies, forces between droplets, effects of additives (viscosity reducers, emulsion (de)stabilizers) etc are vital to drive down economic costs and develop more environmentally friendly methods. Addressing the transport problem, several techniques exist to assist the flow of highly viscous crude oil from production platforms often thousands of kilometers to processing plants/refineries (Martinez-Palou et al., 2011). These include; heating the pipeline (clearly very energy inefficient and complicates pipeline design and cost), dissolution in lighter oils (Hénaut et al., 2007), addition of viscosity/drag reducing chemicals (which can be environmentally harmful and may need to be removed at further cost), use of pulsed electrical fields (Homayuni et al., 2011) (which cause a rather small viscosity reduction) and creation of oil-inwater (O/W) emulsions allowing the crude to essentially be
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transported like water. This latter option requires the addition of surface active agents (surfactants) to invert the emulsion from its naturally occurring water-in-oil (W/O) form to the desired O/W form. Surfactants can be chemical or biological in nature and act by altering the surface tension between the two liquids – the interfaces between the water and oil become populated with these agents due to their part hydrophobic part hydrophilic nature (e.g. a polar, hydrophilic head with a hydrophobic tail). There exist numerous methods to study the various effects of surfactants for crude oil on emulsion stability; these include (nonexhaustive list) fluorescence spectroscopy (Martinez-Palou et al., 2013; Vallejo-Cardona (in press)), optical microscopy, confocal microscopy (Hung and Castillo, 2006), static light scattering, dynamic light scattering, electrical pulse counting, sedimentation techniques, ultrasonic spectrometry (Smyth et al., 2004), nuclear magnetic resonance, neutron scattering (Verruto and Kilpatrick, 2008), dielectric spectroscopy (Sax et al., 1988), differential calorimetry scanning (DSC) (Clausse et al., 2005), analysis of particle size distribution, dynamic viscosity (Kodal, 2005) and commonly the ‘bottle test’ (ASTM D4007, 2011). However, a fundamental measurement that seems to have been overlooked is the forces between the oil droplets within the emulsion. Understanding these forces in a variety of ambient conditions (e.g. surfactant or salt concentration) allows for better optimization of the transport process. In atomic force microscopy (AFM) a tiny cantilever with a downwards facing needle (herein “tip”) at its end is moved with nanometer precision across or just above a surface. The end of the tip, commonly ca. 5 nm in radius, interacts with the surface through a variety of forces, van der Waals, electrostatic, capillary etc. For standard commercial systems in air, a triangular cantilever 100 mm long, 1 mm thick and 30 mm wide can have a force constant as low as 0.01 to 0.1 nN/nm, allowing a force sensitivity of just 10–100 pN by standard detection methods. As well as obtaining topographic images by scanning the probe in the XY plane across a surface as it is commonly used, one can also scan (or ramp) the cantilever up and down (z-direction) against a surface and study how the force between the tip and substrate varies as the tip and substrate approach/retract from one another. One can replace the tip of the cantilever with a tiny drop of (crude) oil. If the substrate has drops of similar oil on it, one can perform exactly the same kind of ramp and measure the forces between the two small oil droplets. This can be done in a (transparent) aqueous environment, one which contains distilled water, and defined concentrations of surfactants and salts, for example. These “impurities” will affect the interaction forces between the drops (surface charges and double layers, surface tension, liquid polarizability and space charges etc) which can be detected by the AFM optical system (force spectroscopy). Thus, in this article, measurements are presented of the forces of interaction between two approaching and retracting extraheavy crude oil (EHCO) droplets within an emulsion with various surfactant-doped aqueous solutions as the continuous phase.
2. Background Force spectroscopy by AFM between deformable colloidal (oil) droplets is itself not new, although the authors are not aware of any prior research with such heavy, viscous, chemically complex oils, nor with the surfactants used herein (those specifically designed for (heavy) crude oil based emulsions). Gunning et al. (2004) use tetradecane as their oil and study the effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the forces and chance of coalescence of the drops. The SDS is observed to electrostatically screen the electric double layer of the charged drops,
653
allowing them to approach closer and favor coalescence. In another tetradecane based article, Gromer and Gunning (2011) are impressively able to see the effect of depletion, that is the work of demixing a protein (sugar beet pectin) when the water is squeezed out/flows back between the two drops (thin film). This required very slow approach and retract speeds to avoid hydrodynamic forces from dominating – the demixing entropic forces were well below 100 pN. The effect of ionic additives, of the deformability of the drops and the velocity of the approach/retract are three of many parameters successfully modeled by group of Dagastine (Carnie et al., 2005; Dagastine et al., 2006) and verified by experiments between a solid particle and oil (Dagastine et al., 2004a, 2005) and between two oil drops (Dagastine et al., 2004b). Tetradecane is again the (very light) oil of choice throughout, although they do change the viscosity of the ambient solution by adding sucrose to the water (Dagastine et al., 2010). The success of their modeling allows several otherwise unobservable physical parameters to be deduced, including the physical distance (of the continuous phase) separating the drops, their shape, the velocity of their surfaces etc. each as a function of time. Heavy and extra heavy crude oils, however, are a complex mixture of various fractions; waxes, fatty acids, high and low boiling point oils, sedimentary (solid) material etc. The most important upshot of this is that there exists within its bulk selfemulsifying (surface active) agents that are not present in simple oils. The asphaltene and resin (polar) components of crudes especially are known to act as such and some of the remaining materials mentioned above are themselves surface active (Lee, 1999; Langevin et al., 2004). The forces these alone apply between the droplets in an emulsion or how they interact with additional surfactants are completely unknown.
3. Experimental The aims here are different to the above works. Though using similar methods, the objective was to try to differentiate between a number of surfactants through the forces they are responsible for in various emulsions. In other words, to see if there is any notable difference in such forces between individual oil drops in aqueous solutions comprised of surfactants and salts. Several of the surfactants are commercially available; the remaining two were developed at the Instituto Mexicano del Petróleo (see Section 3.4 below). In each case, the surfactants are specifically designed to improve the stability of O/W emulsions comprised of EHCOs. A summary of the surfactants is given in Table 1. All surfactants used are non-ionic in nature. The EHCO is designated C7; a summary of its properties is presented in Table 2. It was used in its raw, extracted state. Table 1 The surfactants used in the experiments in the present article along with their critical micellar concentration (CMC) and surface tension Y at their respective CMCs. The surface tension of pure water (against air) is Υ ¼71.97 dyn/cm at T ¼25 °C (Weast, 1981). Surfactant
CMC (ppm)
ΥCMC (dyn/cm)
TQA (Nonylphenol ethoxylate EO 15) BS1 (Glucopon 425) BS5 (Glucopon 625) GC10 (mixture of GC10:G2C14, 1:1) H4 (H4C12) (Dodecyl-polyglucoside)
87.6 200 100 21.6 100
43.3 32.5 31 45.4 54.9
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Table 2 Properties of the oil, C7, used throughout the experiments. Parameter
Unit
Value
API gravity at 25 °C Pour point temperature Kinematic viscosity at 50 °C Salt content Insoluble in n-heptane Insoluble in n-pentane Total sulfur Total Nitrogen Water and sediment Ashes SARA content: Saturated Aromatics Resins Asphaltenes
°API °C mm2/s lb/1000 bbl % % % w/w % w/w % w/w % w/w
9.18 6 2906.6 42.7 14.15 17.7 4.2 0.307 o 0.3 0.06
% % % %
w/w w/w w/w w/w
24.5 33.8 24.0 17.7
3.1. AFM equipment and cleaning A Vecco MultiMode AFM with a Nanoscope IV controller and standard liquid cell is employed throughout. The cell is cleaned by sonic agitation in tap water and detergent, followed by tap water, then distilled water and blown dry with clean compressed air. The same method applies to the o-ring and ports used to connect to the syringe which inject new solutions into the cell. The substrate is cleaved mica, although the duration between the time of cleaving and use in the experiment is not controlled. The cantilevers used were of type DNP as produced by Bruker. These are triangular in appearance and the nominal force constants k are in the range 0.06o k (N/m)o 0.35 for the long, thin cantilever to the wider, thicker cantilever respectively. The long, thin cantilever was used for force curve data, any of the 4 cantilevers were used for TM imaging purposes. The resonance frequency f in tapping mode (TM) was in the range 18 of (kHz) o30 in water. To more accurately determine the force constant of the cantilever the procedure of Gibson et al. (1996) otherwise known as the “reference cantilever” method was employed. The reference cantilever was of type CFLC as produced by Bruker and had a force constant kref ¼0.078 N/m. By measuring the angle between the cantilevers θ E10°, the sensitivity of the optical system against a hard surface S and against the reference cantilever Sref, the unknown cantilever force constant k was determined from
⎛s ⎞ k ref = ⎜⎜ − 1⎟⎟/cosθ 0 Kref ⎝ s ⎠ For some measurements the tip of the cantilever was removed using an ion beam of an electron microscope. In other cases, the cantilevers still had their topographic tips, but these were typically 2.5–6 mm high, whereas all drops had a height in excess of 10 mm. The cantilevers themselves were not specially treated as in Webber et al. (2008) or Lockie et al. (2011) in which chromium, then gold were sputtered onto the cantilever ends and then hydrophobized with decanethiol. In Dagastine et al. (2004b) the substrate is hydrophobic Melinex to better fix the oil drops. In the present data, it is observed that when a new cantilever is used, very unusual force curves (discussed further down) result in which the approach and retract curves only follow each other at high ramp speeds, not overlapping at all at slow speeds. The approach curve is seen to start to rise much more slowly, rising to the designated maximum (trigger) force over a longer distance without the distinctive increase in gradient as the piezo extends further. However, when the cantilever ends were left for an extended period (several days) with a covering of the same crude oil and
cleaned only in xylene, subsequent drops produced force curves that followed one another much more closely and had the expected form. The shapes of the curves (and variation of the substrate material) suggest that the cantilever drop was being displaced laterally when forced against the substrate drop. Upon optical inspection of the oil free cantilever end, it was clear that the surface was very rough, either due to chemical corrosion (holes) or crude oil impurities (strongly attached non-dissolvable particles). It is probable that this gave the surface either a much greater surface area for the drops to stick to and/or chemically changed it creating a surface with a higher affinity for the oil. In future work the more precise functionalization methods above will be employed. 3.2. Oil drop deposition The method of depositing oil drops in the aforementioned articles is by use of an atomizer. The viscosity of the oil here prevents this. Instead, the cleaned end of a stainless steel needle is dipped into the oil and touched down lightly by hand onto the mica surface. The first couple of touches create drops too large (deposited on a different substrate), but the next five or so touches create drops with diameters Ø in the range 10 oØ (mm)o 70. As such, very little oil is present in the experiment (each of the ca. 20 drops on the substrate has a volume of 10–100 pL). Note that the drops are sufficiently small that surface forces dominate over gravitational and buoyancy forces. Indeed, the AFM-TM images show how the peaks of the drops can be accurately modeled as spherical caps. 3.3. Adhering an oil drop onto the cantilever Essentially the same method is used as described by Gunning et al. (2004), which, once the system is ready with oil drops on the mica and pure water injected into the cell, simply involves pressing down the cantilever end onto a small substrate drop. The oil has a greater affinity for the cantilever than the mica and thus readily moves to the cantilever. 3.4. Biosurfactant synthesis All solvents available from commercial suppliers. Surfactants TQA (Nonylphenol ethoxylate EO 15), BS1 (Glucopon 425) and BS5 (Glucopon 625) were used without additional purification. The Mexican EHCO was provided by the Mexican petroleum company (PEMEX), again with no purification. The sugar-based biosurfactants GC10, G2C14 and their mixtures (Cerón-Camacho et al., 2013a) as well as H4C12 were prepared in house following a glycosylation procedure which is published elsewhere (Cerón-Camacho et al., 2013b). All surfactants used in this work were nonionic. All solutions were prepared freshly in distillated water and subsequently filtered using a 0.45 mm PTFE disc filter. 3.5. Data acquisition – force curves Nanoscope v614r1 software was used to control all data acquisition. A hard surface sensitivity calibration was first performed within pure water. The optical system subsequently went unaltered. Once a droplet was attached on a cantilever end and aligned with another drop on the substrate, the cantilever was engaged on the surface with a setpoint o0.5 V more than the photodiode V signal (typically this corresponded to less that 1 nN force). The system was then set to perform ramps, in which the substrate drop is raised to approach (A) the cantilever drop, then lowered to retract (R) from it. Ramp parameters were; ramp distance, z¼1.0 mm; ramp velocity, vr in the range 0.25ovr (mm/s)o17.4, maximum (trigger) force Fmax in the
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range 3oFmax (nN)o10, wait between ramps¼10 s. This wait time was necessary to allow the drops to reacquire their equilibrium positions and for liquid motions due to the previous ramp to disperse, as determined experimentally. Recently, Gunning et al. (2013) have studied the relaxation rheology and thin liquid film behavior between n-tetradecane drops by leaving a wait time when the maximum force occurs (i.e. between the approach and retract ramps). 3.6. Drop shape determination Optically, the diameters of the drops could be observed to better than 72 mm. By removing the liquid cell and placing it on its side in aqueous solution, its profile, height, diameter, radius of curvature etc. could also be estimated to similar accuracies. This was not possible for the substrate drops, where these parameters were determined by topographic imaging. Tapping mode (TM) images of the surface of the oil drops in aqueous solution were performed in two different ways. Firstly, with a standard, clean DNP cantilever and tip. This method produces clean, high resolution images of substrate drop surfaces whether small (10 nm) or large amplitude (100 nm) tapping vibrations were used. Again, very low engage forces (sub-nN) were utilized. Analysis of the cross section allows one to determine the radius from a least squares fit. In a second method, the substrate drop was imaged in contact mode with a droplet on the cantilever, similar to the experiment performed by (Gromer and Gunning, 2011). This is clearly a low resolution method, but has several distinct advantages. Firstly, one has the choice to use either a smaller droplet on the cantilever or on the substrate. The smaller (radius of curvature) droplet will image the larger. Thus the radius of curvature of both the cantilever droplet and substrate droplet can be measured. Note that every drop imaged has a height and diameter greater than the maximum extension of the piezoelectric scanner used (3.1 mm 12 mm 12 mm ¼ zmax ymax xmax). Thus only the peaks of the droplets are analyzed. Secondly, this provides a means to align the droplets nearly perfectly with one another – better than the optical 2 mm limit, although this problem should not affect the results significantly. Finally, it precludes the need to remove the liquid cell from the AFM and place it on its side in water to measure its radius. 3.7. Change of aqueous solution The aqueous environment was changed by simple injection using disposable syringes; pushing the fluid into one of the liquid cell ports thereby flushing the liquid in the cell volume out of the other port. The cell volume including ports is approximately 0.3 ml and each flush was performed in three 0.5 ml steps, 1.5 ml in total. For experiments involving many surfactants, after force curves were taken for one solution a flush with pure MilliQ (MQ) water was performed and further forces curves taken. These were checked with force curves in flushed MQ after every surfactant to ensure consistency – that the environment was sufficiently pure again ready for the next surfactant. This data acted effectively as a control by which one could have confidence that the changes observed were due to the surfactants and not variation of some other parameter.
4. Results 4.1. Ramp velocity dependence For the sake of brevity, preliminary data is only summarized here. The core of this section will concentrate on the comparison
Fig. 1. Force curves as a function of ramp velocity. (A) In a pure water environment. Retract [R] curves only shown for the slowest and fastest ramps. (B) In BS5 (400% CMC) and synthetic sea water, all approach [A] and retract [R] curves shown. (C) Quantifying the graphs. The interaction range (distance over which force grows from 0.25 nN to 3.25 nN on the approach curves) as a function of ramp velocity shown for the two aqueous solutions.
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between surfactants. The effect of the velocity of the ramp was experimentally checked both in pure water and in an aqueous solution of BS5 significantly in excess of its CMC (400%) and synthetic sea water. The results of these two separate experiments are shown in Fig. 1. The velocity data was taken in a random order in both cases. The results of the aqueous solution agree qualitatively with those of Dagastine et al. (2006). The drops are produced with a charge and repel one another upon approach in pure water. The changes to the space charge due to the addition of the impurities acts to shield the drop's charge allowing them to approach more closely. In a viscous environment this results in a narrower thin film layer of the continuous medium clamped between the drops. It is thus more difficult for liquid to flow into or out of this region, increasing the force between the drops. Slower ramps give the ambient solution time to enter and leave; faster ramps do not, effectively creating a vacuum force of attraction upon retract, or a high pressure force of extra repulsion upon approach. In pure water the drops evidently do not approach each other closely and this effect is not visible. Without further comment, it is mentioned how the gradient of the approach curve has an inverse relationship with the velocity of the ramp in the case of pure water, but a proportional relationship in the SSW-BS5 case; these are shown in Fig. 1C. 4.2. Concentration of surfactant and presence of sea-water Briefly, the effect of changing only the solution concentration was studied. This was performed with BS1 and later with GC10. In each case an aqueous environment of MQ water, þ75% CMC surfactant, þ400% CMC surfactant, and þ200% CMC surfactant and SSW was used. (Note that, although heavily salted water is used here, the CMC value is taken from distilled water value. It is well known that ionic species drastically reduce the CMC and surface tensions of many surfactants (Miyagishi et al., 2001). Also, the exact concentration of SSW was not measured but it was of order 0.1 M in terms of salt content.) In Fig. 2A the force curves for the BS1 test are shown (similar results exist for GC10, although the SSW data was not obtainable due to droplet sticking). In Fig. 2B the force between the drops when pushed together 50 nm closer than when the force between them is 1.0 nN is displayed for each curve. Whilst too little data exists (in terms of the concentration variable) to determine a trend, the important data points are the final two – that, after sufficient flushing, the system can be returned to its initial state – the pure water force curve can be recovered. Additionally, it is observed that a surfactant solution with SSW shows very little “adhesion upon retraction” as observed above with BS1 and SSW, presumably because the surfactant is non-ionic and thus itself has little effect on the space charge around the oil drops. As stated, very similar data is observed for GC10 solutions. 4.3. Droplet imaging The cantilever droplets could be imaged optically from the side and their size and shape determined (contact angle, radius of curvature etc). The substrate drops could only be optically viewed from above. By AFM, however, the droplets were imaged both in tapping mode (TM) using a standard tip on a low-k cantilever, and in contact mode (CM) using the same type of DNP cantilever but with another droplet affixed to the end. Discussing the TM images first, it was found that either low or high (drive) amplitudes could used to successfully image without any risk of the drop sticking to the cantilever. High amplitudes tended to be better in terms of lower noise images and better phase contrast. Fig. 3 shows a 3D representation of an oil drop on mica imaged this way; a cross-
Fig. 2. Force curves taken with various aqueous solutions based on BS1. All curves were obtained with a ramp velocity of vr ¼ 8720 nm/s and a 1 mm ramp. Curves shown in chronological order. (A) In BS1 solutions of varying strength and with/ without synthetic sea water (SSW). Much smaller attractive forces seen with surfactant only added. (B) Force between the drops in above solutions when the piezo is displaced 50 nm more than when the force was 1.0 nN.
sectional analysis provided the radius of curvature of the drop, R¼12.5 mm. Note that the corners do not represent the flat mica substrate, rather this is where the drop height has fallen below the reach of the z-piezo scanner (which only has a range of 3.1 mm) and thus the tip is not in contact with the drop. Likewise, only the peaks of the drops used for force experiments were observable due to the x–y piezo limit of the scanner; approximately 12 12 mm2. It is interesting to note that no changes to the drop shapes were observed upon changing the environment from pure water to water with 400% CMC GC10 added, even after several hours, despite the significant change in surface tension the surfactant caused. In separate tests in air with different oil, it was even possible to observe various structures on the EHCO drop surface. An example image is shown, the insets of which show the TM phase and amplitude error signals which better highlight the structures seen. These are possibly the formation of paraffin crystals on the surface. It should be noted that no surface structure was ever observed for the drops used for the force curve experiments. Imaging with an oil drop in contact mode produced similar results albeit with rather more noise. Fig. 3B shows an example image taken in this way. An engage force of approximately F¼0.3 nN was used. 4.4. Comparing the surfactants In three final experiments, force curve data was taken in MQ
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Fig. 3. Topographic images of EHCO droplets in aqueous solution (each 13 13 μm2). (A) TM scan of the spherical cap peak of a drop on mica. (B) CM scan with a small droplet on the cantilever. (C) TM (topographic) image of a different oil with structure observed on the surface, possibly due to the creation of paraffin crystals (2nd order correction applied). (D) TM phase image of same. (E) Error signal image of same.
water with 400% CMC surfactant for each surfactant. The experiments each started in pure MQ water, then the system was flushed with a randomly chosen surfactant solution, then flushed again with MQ water (at least 1.5 ml – until the force curves returned to their initial shape) and this full process repeated with another random surfactant. By the end of a full experiment all surfactant solutions were experimented on typically twice, with force curves in pure water taken between each one, acting as a quasi-continuous control. These data are shown in Fig. 4, again using the force 50 nm of piezo movement closer than 1.0 nN force to quantify the system. The error bars represent the full range of variation of data from 3 to 6 force curves. The parameters used for the first experiment are presented in Table 3; those for the second and third are similar, but using a cantilever with k ¼ (0.053570.0082) N/m. Immediately it is clear that the surfactants perform very similarly; the differences between them are small, only just discernible over the variation between experiments. For instance, the full range of variation in experiment 2 (Fig. 4C) is 3.01 (H4) – 2.85 (BS5) ¼ 0.16 nN. The error bars, which represent the full range of the three curves taken for each data point, are typically 0.01– 0.05 nN. What is further evident is that the differences between the force curves in pure water and in surfactant solution and
between the surfactants themselves are not the same in the three experiments. The differences are much clearer in the first experiment than in the last two. The force curves for experiment 1 are all shown in Fig. 4A. Note that a weak solution of TQA was used, 75% CMC instead of 400% (this was corrected in subsequent experiments). The x-axis (zpiezo displacement) has been adjusted for each curve such that they coincide at 0.5 nN. In this way the difference between the gradients is clearer, as is the relative deformation of the drops. Similar curves exist for experiments 2 and 3 but the differences between the surfactants are less clear. From Fig. 4A it is seen that the interaction between the drops is effectively much stiffer when the surfactants are present, most notably for H4. The gradient of the curve for H4 quickly approaches that of the sensitivity calibration (bare cantilever against mica surface), but for the range of forces used, this level of stiffness is never reached in pure water. This also means that the deformation of the drops when surfactant is present is less than in pure water; again, in the case of H4 the deformation is the least and quickly reaches a maximum (curve parallel to the sensitivity calibration curve). It is obvious from the discrepancy between the latter two and the first experiment that there are still parameters in the system that are not fully understood. However, in each experiment the
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Fig. 4. Results of the three experiments studying the differences in the force curve data with different surfactants. (A) Experiment 1 force curves, adjusted such that all pass through zero piezo displacement at 0.5 nN force between the drops. Second y-axis shows the deflection of the cantilever. Only one of each surfactant's force curve is plotted, for clarity. (B) Force between the drops 50 nm closer than 1.0 nN data from experiment 1. (Note that surfactant TQA was only used at 75% CMC erroneously). (C) The same for experiment 2. (D) The same for experiment 3. Note that sets of data points were taken for H4 and only one for BS1.
Table 3 System parameters used in the first of three experiments. A similar setup used for the second and third experiments, although with a different cantilever and smaller drops. Parameter
Value (exp. 1)
Units
Cantilever spring constant, k z-Piezo ramp velocity, vr Temperature, T Retract delay between ramps Surfactant concentration Cantilever drop diameter, ØCL Mica droplet diameter, Ømica
0.0657 0.015 8750 237 2 10 400% CMC 58.5 7 1.5 62.5 7 2
N/m, nN/nm nm/s °C s mm mm
surfactants that present the strongest and weakest forces between the drops are the same; H4 followed by GC10 or TQA and then BS5 with BS1. This order matches perfectly that of the surface tensions of the surfactants as were displayed in Table 1, even down to how relatively close the GC10 mixture is to TQA and BS1 to BS5. This data is summarized in Table 4. In the case of non-ionic surfactants, it is not surprising that the dominant effect is from the surface tension due to the lack of their ability to significantly affect the space charge around the drops.
Table 4 Summary of the results across the three experiments (all surfactants at 400% of their CMC in MQ water). Columns 2, 3 and 4 show the average of the force 50 nm closer than 1.0 nN in each experiment in nN. The bracketed numbers show the order of these forces (same order number given when results are within the errors). The final column repeats the surface tensions of the aqueous environments (at their CMC). Surfactant
1st exp.
2nd exp.
3rd exp.
ΥCMC (dyn/cm)
MQ water TQA GC10 BS5 BS1 H4
2.17 – 3.62 3.52 3.50 4.05
2.34–2.45 2.98 [2] 2.98 [2] 2.88 [3] 2.89 [3] 3.06 [1]
2.42–2.53 3.06 [2] 3.08 [2] 2.85 [3] 2.88 [3] 3.18 [1]
71.2 43.3 [2] 45.4 [2] 31 [3] 32.5 [3] 54.9 [1]
[2] [3] [3] [1]
5. Conclusions Of the surfactants it is observed that the H4 is the most potent in increasing the force between the EHCO droplets, followed by GC10 and TQA, then BS1 and BS5. This order matches very closely the surface tensions of these surfactants (at their CMC values). Implicit in this result is that the deformation of the droplets is less when H4 surfactant is present than with the remaining surfactants. The difference between the surfactants is small at 400% CMC in pure water. As such, if one assumes a similar production cost for
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each one, it may be that GC10 is the most efficient, since it has a very low CMC, thus requiring very little product to act as an EHCO emulsion stabilizer. One of our primary findings is that AFM force spectroscopy is a viable method for comparing surface tensions and other mechanical/surface effects between different surfactants. The complex nature of the crude oils used here are again emphasized, especially compared to the light and pure oils used in the past literature e.g. Gunning et al. (2004) and Dagastine et al. (2006). As stated, such crude oils are complex mixtures of compounds which themselves contain surface active agents, greatly complicating any analysis. Despite the difficulties with surface preparation and the complex oils, we show that the forces in such dense and viscous crude oil based emulsions acts broadly in the same manner as with the aforementioned lighter oils. Thus we extend the range of droplet viscosities over which such experiments can be performed, as well as prove its viability with newly designed surfactants for EHCO emulsions, which are already becoming an important method to transport such heavy crude oils. The forms of the approach and retract curves in flushed (pure water) samples followed the forms shown by Gromer and Gunning (2011) whilst the curves in the presence of simple ionic species (in the present article, synthetic sea water) follow the forms modeled by Dagastine et al. (2006) and Carnie et al. (2005) (with their addition of anionic surfactants). In the former, the forces simply increased between the drops as the approached and fell as they receded. With a space charge due to salts/anionic species present, the approach curves were steeper and retract curves showed a strong attractive term, due to the electrostatic shielding of the droplets' inherent charge. This is an important consideration in the case of (the breaking of) crude oil emulsions, since for transport they will be mixed with sea-water, not pure water. Clearly further development to the procedure is required to produce more self-consistent results equivalent to those observed by the groups of Dagastine and Gunning. It may be possible to achieve this by replicating their methods for drop fixation on the cantilever and substrate. Likewise there is plenty of parameterspace yet to investigate; it is very possible that greater difference between the surfactants might be observed if data was taken at fixed concentration below the CMC instead of at 400%, or with seawater present. A robust monitoring system may be necessary to ensure the crude is mixed and sealed well before experiments in case settling and/or lighter fraction evaporation occurs. Future work should also replicate the final set of experiments with the addition of an ionic species, since this would be one step closer to the real-life situation. With such development and parameter space exploration, it may be possible to probe the surfactant's effect on emulsion stability by taking data across an extended time frame and offer an alternative method to determine CMCs for surfactants by more data points as a function of concentration. The behavior of crude oil emulsion with time and the optimum concentrations of surfactant needed are becoming increasingly important as the use of these heavy crudes increases worldwide. Finally, it was possible to accurately model/observe the drop sizes optically and through AFM imaging, as attempted by Gunning et al. (2004). With standard-tip-on-drop imaging it is possible
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to see detailed structure (resolutiono20 nm) on the surfaces of some oil drops in TM, allowing the visualization of what were probably paraffin crystals in the oil surface. To our knowledge, this is the first time such detailed structures have been observed by AFM on the surface of liquid droplets. Using drop-on-drop imaging, one can measure both the drops' radii of curvature and better center the drops for force curves if required. This can then be used to normalize results across many experiments. In future work, we may try to observe any changes in the contact angles (or shapes) of the droplet with time; none were observed despite changes to the continuous phase here (over relatively short time frames).
Acknowledgments This project was funded by the Instituto Mexicano del Petróleo, Grant number D.01225. The authors would like to thank Prof. Raymond Dagastine of The University of Melbourne (Australia) and Dr. Patrick Gunning of the Institute of Food Research (Norwich, UK) for enlightening discussions during the course of the project.
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