3384
Energy & Fuels 2008, 22, 3384–3389
Toward a Direct Measurement of Paraffin Adhesion Forces Marta E. R. Dotto,† Cla´udio M. Ziglio,‡ and Se´rgio S. Camargo, Jr.*,† Engenharia Metalu´rgica e de Materiais, UniVersidade Federal do Rio de Janeiro, 21945-970, Ilha do Funda˜o, Rio de Janeiro, Brazil, and Petro´leo Brasileiro S.A., Petrobras, Q.7, 21941-598, Ilha do Funda˜o, Rio de Janeiro, Brazil ReceiVed April 3, 2008. ReVised Manuscript ReceiVed July 18, 2008
In the present work, the interaction forces between paraffin surfaces and solid substrates were studied using an atomic force microscope (AFM). Approximation and retraction curves, called force curves, were measured at room temperature between the AFM probe and stainless-steel (SS) substrates with either of them uncoated or coated with a paraffin film. A great influence of the surface topography on the force curves was observed. This influence results in a large variation of the interaction force values obtained at different spots of the sample surface. For some of the spots, which are associated with large paraffin crystals or agglomerates, the force curves change each time a measurement is performed, apparently because of the deformation of the paraffin crystal. For some other spots, which seem to be related to thin paraffin layers, a reproducible behavior is observed, allowing for the estimation of the interaction force values. A considerable dispersion of the values of adhesion forces and jump-in and jump-out distances was observed in all cases investigated. Nevertheless, the obtained average values indicate that the adhesion force between paraffin-paraffin is stronger than the adhesion force between paraffin and uncoated solid surfaces.
1. Introduction Paraffin films can be deposited from crude oils when exposed to cold temperatures. Crude oil is a complex mixture of hydrocarbons consisting of paraffins, asphaltenes, aromatics, naphthenes, and resins. Among these groups of hydrocarbons, high-molecular-weight paraffins (waxes) are responsible for some of the problems that are encountered during transportation and processing of crude oils. As in the reservoir, the temperatures and pressures are high, the solubility of the compounds of the crude oil is sufficiently high to keep them fully dissolved in the mixture with low viscosity. During transportation, as the crude oil leaves the reservoir and flows through the pipelines, its temperature begins to drop because of the cooler environment. Inasmuch as the solubility of high-molecular-weight paraffins decreases drastically with decreasing temperatures, stable paraffin crystals are formed at low temperatures. The crystallization of paraffin leads to the formation of gels with a complex morphology. The formation of paraffin gels on the walls of the pipelines plugs up the pipelines and restricts the flow. Mechanical techniques, such as pigging, are to be used to prevent the clogging of the pipelines.1-4 Many mechanisms have been proposed to elucidate wax deposition on pipeline walls, such as molecular diffusion, shear dispersion, Brownian diffusion, and gravity settling. An under* To whom correspondence should be addressed: PEMM/COPPE, Universidade Federal do Rio de Janeiro, Cx. Postal 68505, Rio de Janeiro, RJ, CEP 21941-972, Brazil. Telephone: (55-21)-2562-8506. Fax: (55-21)2290-1615. E-mail:
[email protected]. † Universidade Federal do Rio de Janeiro. ‡ Petrobras. (1) Singh, P.; Venhatesan, R.; Scott Floger, H. Mater., Interfaces, Electrochem. Phenom. 2000, 46 (5), 1059–1074. (2) Bott, T. R. Exp. Therm. Fluid Sci. 1997, 14, 356–360. (3) van Hoof, P. J. C. M.; Grimbergen, R. F. P.; Meekes, H.; van Enckevort, W. J. P.; Bennema, P. J. Cryst. Growth 1998, 191, 861–872. (4) Singh, P.; Youyen, A.; Floger, S. Thermodynamics 2001, 47 (9), 2111–2115.
Figure 1. Cantilever deflection as a function of the distance showing tip-sample interactions and the AFM scheme.
standing of the formation mechanisms and properties of the incipient paraffin gels is necessary.1 The mechanism of formation of paraffin crystals is generally divided in three stages: nucleation, growth, and finally, aggregation or coalescence of the paraffin crystals. The paraffin crystals formed adhere onto the solid surfaces or pipeline walls, resulting in the formation of well-adhered and dense deposits.5,6 In this way, an important issue that demands special attention and understanding of the mechanisms involved is the study of adhesion forces, which are the interaction forces that exist between paraffin particles and between these particles and solid surfaces.7,8 To understand the physical processes of the interaction forces between paraffin and substrates or paraffin-paraffin interaction, a systematic study of force curves obtained by an atomic force microscope (AFM) was performed. As shown in Figure 1, the (5) Hunt, A. Oil Gas J. 1996, 96–103. (6) Turner, W. R. Ind. Eng. Chem. Prod. Res. DeV. 1971, 10 (3), 238– 260. (7) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1991. (8) Brushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607–616.
10.1021/ef800234d CCC: $40.75 2008 American Chemical Society Published on Web 08/16/2008
Direct Measurement of Paraffin Adhesion Forces
force curves plot the distance of the scanner movement (horizontal axis) versus the cantilever deflection (vertical axis) and are used to measure the vertical force of the tip (or AFM probe) on the surface.7-10 These curves can be used to examine the attractive, repulsive, and adhesive interactions between the tip and the sample as well as to set the imaging force. The curve begins with the tip not touching the sample (that is fixed on the top of the PZT scanner facing the tip, insert in Figure 1); therefore, the cantilever is undeflected and the scanner is fully retracted (point a). The scanner begins to extend (segment ab), and when the cantilever comes close enough to the sample to experience an attractive van der Waals force, the cantilever suddenly bends toward the surface (segment bc). Point b is known as the jump-in distance, i.e., the point of the contact that is originated by attractive forces between the tip and the sample causing the cantilever to bend. The cantilever deflects away from the surface until the scanner reaches full extension (segment cd) and then begins to retract (segment de). The slope of the curve mostly reflects the spring constant of the cantilever when the cantilever is softer than the sample. The cantilever deflection retraces the same curve (def), and then the scanner pulls the tip away from the surface (segment fg). In this segment, valuable information about the adhesion study may be obtained. The sharp break at point f is not a universal response but depends upon the type of adhesive interaction. The point g is known as the jump-out distance, i.e., the jump of the cantilever away from the surface. During the segment gh, the tip and sample are not in contact and are moving away from each other. In this paper, the interaction forces using an AFM were investigated. Force curves performed between an (uncoated or paraffin-coated) AFM probe and (uncoated or paraffin-coated) stainless-steel substrates will be presented and discussed. On the basis of these curves, an estimation of the adhesive interaction will be made. 2. Materials and Methods 2.1. Surface and Paraffin. The substrates used were crystalline Si and stainless-steel (SS) 316 L surfaces. Paraffin deposits were obtained by casting of paraffin-containing heptane solutions (1: 300, 5 µL) onto the substrates. The samples were prepared at the laboratory environment at room temperature. The paraffin used was supplied by CENPES/PETROBRAS and is a mixture of long-chain hydrocarbons, with carbon chain lengths ranging from C19 to C49 with linear, ramified, even, and odd chains. Heptane is a mixture of isomers. 2.2. Coating of the AFM Probe. The AFM probes were coated with paraffin thin films by thermal evaporation in high vacuum (∼10-6 torr). The same paraffin material described above was employed. Paraffin was placed in a tantalum crucible, while the AFM probe was fixed about 15 cm above the crucible with its tip pointing downward. The amount of paraffin (∼10-4 g) was calculated to obtain a thin film with thickness around several nanometers onto the AFM probe surface. The crucible was heated so that at the end of the evaporation process no paraffin was left in the crucible. The whole evaporation process lasts less than 1 min. 2.3. AFM Measurements. Force curves between the paraffincoated AFM probe and paraffin films on SS substrates were measured at room temperature using a Topometrix AFM. Forcecurve analysis is usually performed by recording deflection of the cantilever as the probe approaches the surface and then plotting the force as a function of the distance between the probe and the surface. The resulting force curves contain information about different interactions, such as elasticity, hardness, surface charge (9) Cappella, B.; Dietler, G. Surf. Sci. Rep. 1999, 34, 1–104. (10) Butt, H.-J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1– 152.
Energy & Fuels, Vol. 22, No. 5, 2008 3385
Figure 2. Plot of the interaction force versus separation distance between the AFM probe and a crystalline silicon substrate performed at different spots of the sample surface. The approximation and retraction curves obtained correspond to closed and open symbols, respectively.
density, and adhesion. Cantilevers with an ultra sharp tip (NSC14/ AlBS from Micromash) of about 20 nm in diameter, 15-20 µm of height, spring constant k of 5 N/m, and resonant frequency f of 160 kHz were used. Before the force curves were acquired, the cantilever spring constant was calibrated using a pure crystalline silicon sample and the program supplied by the manufacturer of the AFM equipment. All measurements were performed using the same probe coated with or without paraffin.11 Force measurements were performed at different spots on the surface. A total of 10 force curves were obtained at each spot. The first curve obtained usually presented a different behavior from the subsequent curves, as will be shown and discussed later in this work. In certain cases, the nine subsequent curves presented a reproducible behavior, allowing us to obtain average values of the measurements taken on the same spot. More details about the acquisition method of force curves can be found elsewhere.11
3. Results and Discussion In this section, the results of force curves, which reflect the interactions forces between the AFM probe and the substrate will be presented and discussed in four parts: (i) uncoated AFM probe against uncoated stainless-steel substrate, (ii) AFM probe coated with paraffin film against uncoated stainless-steel substrate, (iii) uncoated AFM probe against stainless-steel substrates coated with paraffin film, and (iv) paraffin-coated AFM probe against paraffin-coated stainless-steel substrate. Before the measurements on stainless-steel substrates were performed, force curves were obtained with the uncoated AFM probe against a clean crystalline silicon surface to calibrate the system with respect to the cantilever spring constant. Figure 2 shows a set of approximation and retraction curves obtained at several different spots of the sample surface. As one can observe, this set of curves show basically the same qualitative behavior. The jump-in, jump-out, and adhesion force values presented standard deviations of about 10, 10, and 15%, respectively, confirming the good reproducibility of the measuring system. 3.1. Force Curves between the Uncoated AFM Probe and Uncoated Stainless-Steel Substrate. Figure 3 shows the results obtained for force curves between the uncoated AFM probe and uncoated stainless-steel substrate. Each plot (a-d) corresponds to a different spot on the substrate surface. Although 10 different measurements were performed at each spot, for the sake of clarity, only one measurement (approximation-retraction) is shown. The observed curves present an abrupt behavior (in (11) Dotto, M. E. R.; Camargo, S. S., Jr.; Ziglio, C. M. Energy Fuels 2007, 21, 1296–1300.
3386 Energy & Fuels, Vol. 22, No. 5, 2008
Dotto et al.
Figure 3. Plot of the force as a function of the distance between the uncoated AFM probe and uncoated SS surface. Curves were acquired at different spots.
a similar way to those obtained on crystalline silicon; see Figure 2), which is typical of an elastic interaction. The adhesion force values presented in the parts a-d of Figure 3 correspond to the mean value of each set of curves performed at the same spot. Considering the set of curves acquired at one and the same spot, the observed values of interaction forces and jump-in/out distances are reasonably reproducible, resulting in a small dispersion from the average values. However, when the results obtained for different spots of the sample surface are compared, a large variation of the interaction forces from one point to another can be observed, although the behaviors of the curves are qualitatively the same. For example, in the case of Figure 3b (∼58 nN), the value obtained is more than 3 times larger than the one measured in Figure 3d (∼16 nN). A similar reasoning applies to the values of jump-in and jump-out distances. 3.2. Force Curves between the Paraffin-Coated AFM Probe and Uncoated Stainless-Steel Substrate. Figure 4 presents two sets of curves obtained at different spots on SS surfaces using an AFM probe coated with an evaporated thin film of paraffin. The observed curves are qualitatively quite similar to the previous case. This is evident from the comparison of the results presented in Figures 3 and 4. Also, in this case, there is considerable variation of the force values with the spot where the curves were performed. This result seems to be the same regardless of whether or not the AFM probe is coated with paraffin and, therefore, is attributed to SS surface effects. It is well-known that the interaction forces have a great dependence on the morphology and properties of the surfaces involved. Stainless steel is a polycrystalline material with its particular microstructure (in this case, austenitic), grain boundaries, and topography/morphology (roughness and imperfections) that result from its preparation process. Therefore, the resulting surface is irregular and roughness is high when
Figure 4. Plot of the force as a function of the distance between the paraffin-coated AFM probe and uncoated SS surface. Curves were acquired at different spots.
compared to that of a crystalline silicon sample, which is very flat and smooth. This may be the cause of the observed differences on the force curves obtained at different spots. The very similar results obtained with the AFM probe both uncoated
Direct Measurement of Paraffin Adhesion Forces
Energy & Fuels, Vol. 22, No. 5, 2008 3387
Figure 5. Force curves between the uncoated AFM probe and paraffin-coated SS substrates performed at different spots (square symbols and solid lines correspond to approximation curves, and circle symbols and dot lines correspond to retraction curves).
and coated with paraffin indicate that the effect of the substrate is of greater importance and masks a possible effect of the paraffin coating on the AFM probe. Quantitatively, however, despite the considerable dispersion of the results, the interaction force and jump-in/out distances values obtained in this case seem to be slightly larger than in the previous one, as will be shown later. 3.3. Force Curves between the Uncoated AFM Probe and Paraffin-Coated Stainless-Steel Substrate. Figure 5 shows the force curves obtained with the uncoated AFM probe and the SS substrate coated with a paraffin thin film as a function of the distance. Six sets of curves (a-f) are presented corresponding to six different spots chosen at the sample surface. The curves that appear in symbols correspond to the first measurement performed in the corresponding spot (square and circle symbols are the approximation and retraction curves, respectively), while subsequent measurements appear as lines. In some cases, the first curve performed is substantially different from the subsequent curves acquired in the same spot. This is
the case of Figure 5f for instance. For this reason, in such cases, the first curve was ignored for the estimation of the interaction force and jump-in/out values. Although the information obtained from the first curve is valuable to understand the system, it cannot be employed to obtain a quantitative estimation of the interaction between the probe and the substrate surface. In some other spots (see Figure 5d), no substantial change of the behavior of first curve performed comparing to the subsequent curves is observed. In general, the curves observed in the present case show a much more gradual behavior than in the previous ones (Figures 3 and 4). This is an indication that the interaction between the AFM tip and the deposited paraffin film is of a viscous nature and is in accordance with the results observed for paraffin deposited onto crystalline silicon substrates.11 Besides that, a great discrepancy of the behavior of the force curves from spot to spot was also observed. Indeed, in some cases, the curves present a complex behavior because all of the curves acquired at the same spot are observed to be
3388 Energy & Fuels, Vol. 22, No. 5, 2008
Figure 6. AFM images of paraffin deposits on SS substrates. Dimensions: (a) 50.12 µm × 50.12 µm × 59 nm and (b) 5 µm × 5 µm × 14 nm.
considerably different from each other (see, for example, parts a and b of Figure 5). In some other cases, the curves are “well-behaved” and, except for the first pair of approximation-retraction curves, present a reproducible behavior, permitting the estimation of the interaction forces and jumpin/out values (see parts c-f of Figure 5). The observation of different force versus distance behaviors for the different spots where the curves were performed may be explained by the microstructure presented by the paraffin thin films on SS substrates. It is important to mention that in the case of force curves obtained against paraffin films deposited on silicon substrates this behavior does not occur.11 Figure 6 shows AFM images obtained for the paraffin deposits on SS substrates. It can be clearly observed in this figure the presence of large paraffin crystals and agglomerates and also the scratches and defects because of SS substrate preparation. The paraffin crystal typical dimensions are up to a few tens of micrometers wide and several tens of nanometers high.
Dotto et al.
In a previous work, the influence of substrates in the morphology of paraffin films was studied.12 AFM images show the presence of different structures, such as monomolecular layers, thin monocrystals, large and thick crystals, and large agglomerates. The observed microstructure was found to be dependent upon the substrates used.12,13 In the AFM images of Figure 6, one can observe regions that present large paraffin agglomerates and some other regions that are covered by some monolayers of paraffin. Because the diameter of the AFM probe (about 20 nm) is much smaller than the paraffin crystals, it is reasonable that the measurements present different behaviors at different spots of the sample surface. Therefore, one may attribute results such as those shown in parts a and b of Figure 5 to the force curves obtained on large paraffin crystals or agglomerates, which would be progressively deformed, resulting in a different force curve each time a measurement is performed. Curves such as those of parts c-f of Figure 5, on the other hand, seem to be related to the measurements taken on regions were the substrate is covered by thin layers of paraffin, resulting in reproducible force curves in a similar way to paraffin films deposited on Si substrates.11 The influence of paraffin layers on all obtained force curves in this case can be clearly concluded from the comparison of curves from Figure 5 to those of the previous cases (Figures 3 and 4). Nevertheless, some influence of the SS substrate is also expected in the present case because its microstructure is still apparent, even though the substrate is covered with paraffin. 3.4. Force Curves between the Paraffin-Coated AFM Probe and Paraffin-Coated Stainless-Steel Substrate. Figure 7 shows the sets of force curves obtained with paraffin-coated AFM probe on four different spots of a paraffin-coated stainlesssteel substrate. Again, the variation of the behavior of the first curve performed (square symbol for approximation curve and circle symbol for retraction curve) in comparison to the subsequent curves can be observed. Also, as in the previous case, it is clear from Figure 7 that the variation of the behavior of curves from spot to spot. Parts a and c of Figure 7 present behavior with good reproducibility, permitting us to estimate of the interaction force values, while in parts b and d of Figure 7, the poor reproducibility prevents any quantitative estimation. These results are qualitatively quite similar to those of the previous section. In Figure 7a, the value obtained for the adhesion force is approximately 140 nN, while in Figure 7c, the force adhesion value is only 32 nN. This large variation seems to be due to the properties of the surface, as discussed above. However, although the number of the measurements performed is not very high, the interaction force values in this case present a tendency to be larger than in the previous case. High values, such as 140 nN, were not observed in the case where the force curves were performed with uncoated AFM tips, suggesting that the interaction force between paraffin particles is stronger than between paraffin and a solid surface. 3.5. Estimation of the Adhesion Interaction. Figure 8 shows the average values and standard deviations for adhesion forces and jump-in and jump-out distances for the four different conditions that were studied in this work, as described above. These values were obtained from a set of about 50 well-behaved curves performed at several different spots for each condition (12) Dotto, M. E. R.; Martins, R. N.; Ferreira, M., Jr. Surf. Coat. Technol. 2006, 200, 6479–6483. (13) Plomp, M.; van Enckevort, W. J. P.; van Hoof, P. J. C. M.; van de Streek, C. J. J. Cryst. Growth 2003, 249 (34), 600–613. (14) Ohring, M. The Materials Science of Thin Films; Academic Press: New York, 1992.
Direct Measurement of Paraffin Adhesion Forces
Energy & Fuels, Vol. 22, No. 5, 2008 3389
Figure 8. Average values and standard deviations for the adhesion force and jump-in and jump-out distances for the four different conditions studied in this work: (1) uncoated AFM probe versus uncoated SS surface, (2) paraffin-coated AFM probe versus uncoated SS surface, (3) uncoated AFM probe against paraffin-coated SS surface, and (4) paraffin-coated AFM probe against paraffin-coated SS surface.
the measured interaction forces. The increase in the jump-in distance simply indicates an increase of thickness; i.e., the thicker the layers, the sooner they become in contact. From the jump-out distance, on the other hand, one could in principle obtain information about adhesive interaction between the two surfaces that are in contact. The similar increase observed for the jump-in and jump-out distances would indicate a simple effect of thickness. However, this increase is accompanied by a corresponding increase of the adhesion force values, which suggests that a stronger interaction occurs when one goes from conditions 1 to 4, indicating a greater adhesive interaction between paraffin layers in comparison to the interaction between paraffin and uncoated solid surfaces. 4. Conclusions
Figure 7. Force curves as a function of the distance between the paraffin-coated AFM probe and the paraffin-coated SS substrates. Curves were acquired at different spots.
studied in the present work. One can observe that the obtained values are dependent upon the condition under analysis, indicating the influence of the surfaces that are in contact on
In this paper, the interaction forces between the AFM probe and stainless-steel substrates with either of them uncoated or coated with a paraffin film were studied and lead to the following conclusions: (1) There is a great influence of the SS surface topography on the force curves. This influence results in a variation of the interaction force values obtained at different spots of the sample surface. This effect is apparently more important and masks a possible effect of the presence of an evaporated paraffin film on the AFM probe. (2) The force curves are extremely dependent upon the microstructure of the paraffin thin film. For some of the spots, which are associated with large paraffin crystals or agglomerates, the force curves change each time a measurement is performed apparently because of the deformation of the paraffin agglomerate. For some other spots, which seem to be related to thin paraffin layers, a reproducible behavior is observed, allowing for the estimation of the interaction force values. (3) A considerable dispersion of the values of adhesion force and jump-in and jump-out distances was observed in all cases investigated. Nevertheless, the obtained average values indicate that the adhesion force between paraffin and paraffin seems to be stronger than the adhesion force between paraffin and uncoated solid surfaces. Acknowledgment. We are grateful to Renata A. Sima˜o (PEMM/ COPPE/UFRJ) for helpful discussions about AFM measurements. The authors acknowledge the financial support from Cenpes/Petrobras. EF800234D
