Adhesion Force between Cyclopentane Hydrate and Mineral Surfaces

Article pubs.acs.org/Langmuir

Adhesion Force between Cyclopentane Hydrate and Mineral Surfaces Zachary M. Aman,† William J. Leith,‡ Giovanny A. Grasso,‡ E. Dendy Sloan,‡ Amadeu K. Sum,‡ and Carolyn A. Koh*,‡ †

Centre for Energy, School of Mechanical and Chemical Engineering, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia ‡ Center for Hydrate Research, Department of Chemical & Biological Engineering, Colorado School of Mines, 1600 Illinois Street, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: Clathrate hydrate adhesion forces play a critical role in describing aggregation and deposition behavior in conventional energy production and transportation. This manuscript uses a unique micromechanical force apparatus to measure the adhesion force between cyclopentane hydrate and heterogeneous quartz and calcite substrates. The latter substrates represent models for coproduced sand and scale often present during conventional energy production and transportation. Micromechanical adhesion force data indicate that clathrate hydrate adhesive forces are 5−10× larger for calcite and quartz minerals than stainless steel. Adhesive forces further increased by 3−15× when increasing surface contact time from 10 to 30 s. In some cases, liquid water from within the hydrate shell contacted the mineral surface and rapidly converted to clathrate hydrate. Further measurements on mineral surfaces with physical control of surface roughness showed a nonlinear dependence of water wetting angle on surface roughness. Existing adhesive force theory correctly predicted the dependence of clathrate hydrate adhesive force on calcite wettability, but did not accurately capture the dependence on quartz wettability. This comparison suggests that the substrate surface may not be inert, and may contribute positively to the strength of the capillary bridge formed between hydrate particles and solid surfaces.

1. INTRODUCTION Gas hydrates are crystalline inclusion compounds, where molecular cages of water surround light hydrocarbons, such as methane.1 Hydrates are typically stable at high pressure and low temperature, where they are the thermodynamically preferred phase of water.1 In conventional energy (oil and gas) applications, hydrate particles may form in transportation lines, leading to significant operational and safety hazards.1 Methane hydrates may also form naturally in sediments beneath the seafloor and under the permafrost.1 The methane trapped in naturally occurring hydrate bearing sediments represents an alternative energy resource, potentially containing more energy than the sum of identified global crude oil, natural gas, and coal assets.1 In conventional applications, hydrate may form directly on steel pipeline walls (similar to a film),2 or preformed hydrate particles may deposit on existing water-coated or hydratecoated pipeline walls.2,3 Without remediation, hydrate deposition may lead to a positive feedback cycle, where additional hydrate is deposited as the flow path becomes obstructed. Sloan and Koh,1 in collaboration with J. Abrahamson, presented four primary stages of hydrate formation in oil-dominated systems, shown in Figure 1: (i) emulsification of water in oil; (ii) hydrate formation along the water−oil boundary; (iii) aggregation of © 2013 American Chemical Society

hydrate particles leading to higher effective oil viscosity; and (iv) jamming of large aggregates to form a hydrate plug. As with hydrate formation in unconventional applications, the system behavior may be governed by the properties of the available interfaces in the system. The flowline may also see the production of additional solids, such as sand, scale (e.g., calcium carbonate), precipitated asphaltene particles or aggregates, or precipitated wax particles.4−6 To our knowledge of the literature, hydrate−sand and hydrate−scale adhesive interactions have not been directly investigated. Multiple studies to-date have focused on hydrate−hydrate particle cohesion through capillary bridge cohesive mechanisms.7 Aman et al.7 presented a comprehensive model to estimate hydrate interparticle forces as a function of contact time, amount of unconverted water (available to form liquid bridges), temperature, and water−oil interfacial tension; this model includes a capillary bridge component8−10 that dominates the interparticle force for contact times below approximately 30 s, and a sintering component to account for fracture between hydrate bridges that form over time. The Received: September 10, 2013 Revised: November 12, 2013 Published: November 22, 2013 15551

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Figure 1. Conceptual mechanism for hydrate plug formation in oil-dominated systems, where hydrate particles may adhere or interact with other solids (e.g., the pipe wall).

Figure 2. Hydrate particle adhesion to a substrate through a capillary liquid water bridge in the presence of liquid hydrocarbon (oil), where the capillary interaction is dominated by three interfacial tensions (inset): water−oil, solid−oil, and solid−water.

Equation 1 does not account for particle−surface sintering (e.g., where the water phase in Figure 2 may solidify to form hydrate), because the hydrate growth rate and induction time (i.e., time required to observe growth) are poorly understood for particle−surface contact. The sintered bridge strength may be represented by either cohesive failure at the neck or adhesive failure along the hydrate−surface interface. Jung and Santamarina11 observed cohesive failure of CO2 and CH4 hydrate with a calcite substrate, and adhesive failure with a mica substrate. Previous work by Aspenes et al.2 measured hydrate− substrate adhesive forces at low contact times (i.e., in the capillary dominated regime represented by eq 1). Aspenes et al.2 observed an increase in cyclopentane hydrate adhesive force with substrate surface free energy (SFE) above approximately 55 mJ/m2, and no dependence on substrate SFE below this value. Recent cohesive force studies by Aman et al.7 have suggested a hydrate−cyclopentane liquid interfacial tension

capillary bridge model may be extended geometrically to account for particle contact with a surface:9 FA =

4πRγ cos θ 1+

H d

⎞ ⎛ H ⎟ ⎜ + 2πRγ sin(θ + α)⎜sin α − Ha a + d ⎟⎠ ⎝ (1)

where FA is the particle-surface adhesive force, γ is the water− oil interfacial tension, H is the separation distance between the particle and surface, d is the immersion depth of the particle in the bridge, θ is the mean wetting angle of water on the hydrate surface, and α is the embracing angle (or functional width of the liquid bridge). These geometric terms are illustrated in Figure 2, where the water wetting angle on the substrate is a function of water−oil, solid−water, and solid−-oil interfacial tensions (γwater−oil, γsolid−water, and γsolid−oil, respectively). 15552

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Figure 3. Top-down schematic of the experimental cell setup for hydrate−surface adhesive force tests. In each experiment, the mean of 40 pull-off adhesive force tests is reported, where the error bounds represent a 95% confidence interval for a normal distribution.7 Traditionally, the reported force and error bounds are normalized by the radius of the hydrate particle (resulting in units of mN/m), which is supported by the direct relationship in eq 1. 2.2. Contact Angle Measurements. Equation 1 also suggests a strong dependence on the mean water wetting angle (θ), where water-, intermediate-, and oil-wet surfaces are, respectively, identified through water wetting angles of 0−75°, 75−115°, and 115−180°.2 In flow assurance applications, strongly water-wetting surfaces may lead to strong hydrate adhesive forces (per eq 1), while oil-wet surfaces may minimize or prevent hydrate deposition.2,13,15 For quartz and calcite substrates, water droplet contact angles were measured directly, using a sessile drop technique with a CAM200 apparatus (KSV Instruments). 2.3. Mineral Roughness and Composition. Each mineral sample was characterized with an EDAX JSM-7000F (JEOL) field emission scanning electron microscope (SEM). The SEM was combined with energy dispersive X-ray spectroscopy (EDS) measurements to determine the approximate atomic composition of the crystals (provided in the Supporting Information). SEM images were recorded at 65×, 650×, and 6500× magnification. The surface roughness features are compared to multiple average roughness feature values collected with a Dektak IIA two-dimensional profilometer (Sloan Inc.). 2.4. Solid Surfaces. Quartz and calcite samples were cleaved to expose raw surfaces, and were stored in water-saturated liquid cyclopentane (99% Acros Organics) at ambient temperature. The mineral surfaces were washed briefly with Citranox or Sodosil cleaning agents (Sigma-Aldrich), followed by sequential 10 s rinses with deionized water, ethanol, and acetone.2 All mineral samples were rinsed with ethanol and acetone between adhesive force experiments, and were stored in the same water-saturated liquid cyclopentane container. Adhesion force tests were highly distributed (i.e., over a large range of force) if the minerals were stored in dry cyclopentane, which may be the result of capillary bridge water distributing unevenly over the mineral surface after each pull-off trial, hence the storage in water-saturated cyclopentane. In addition to a suite of tests on each raw mineral surface (cleaved by hand), each mineral surface roughness was modified manually with 100, 220, and 400-grit finish sandpaper (where the abrasive surface was applied at the same angle and orientation). The modified mineral surfaces were cleaned and stored according to the procedure described above.

may be on the order of water−cyclopentane liquid interfacial tension (51 mN/m), which may explain the transition in substrate SFE dependence. This work focuses on hydrate adhesion force for two solid substrates (quartz and calcite) that may play a critical role in both traditional and unconventional energy systems; adhesive forces to these substrates have not been reported in the literature, to date. Sand (quartz) may be found as a coproduced solid in oil production lines, while calcite may precipitate out of solution as scale, leading to additional flow assurance challenges.12,13

2. MATERIALS AND METHODS 2.1. Micromechanical Force Method. Hydrate−substrate adhesive force data were collected with a micromechanical force (MMF) apparatus, the development of which has been extensively discussed in the literature.2,7 Briefly, the MMF apparatus consisted of a Zeiss S100 inverted light microscope, placed atop a vibration isolation table and encased in a drybox filled with cyclopentane-saturated nitrogen (75% Omnisolv). The microscope stage featured an aluminum cooling jacket connected to an external ethylene glycol/ water mixture bath. The jacket surrounded an aluminum cell, with a circular cover glass on the bottom of the cell to allow for visualization. The experimental cell was filled with liquid cyclopentane and housed two cantilevers. The left-hand cantilever was connected to a manual micromanipulator, while the right-hand cantilever was connected to a remotely operated micromanipulator (Eppendorf Patchman 5173). In adhesive force trials, a deionized water droplet was placed on the tip of the left-hand cantilever (Figure 3) and quenched in liquid nitrogen to form ice; the ice particle was placed immediately in the cyclopentane bath (maintained at 268 K). The right-hand cantilever was attached to a substrate surface. The cell temperature was slowly raised (0.1 K/s) to approximately 276 K (the cyclopentane hydrate melt temperature is 281 K).7 The ice particle immediately melts, leading to the conversion of a cyclopentane hydrate shell (30−50 μm in thickness) at the ice point.14 The hydrate shell was allowed to anneal for 30−90 min after initial conversion.14 Pull-off measurements were performed according to a four-step procedure (illustrated in Figure 3): (i) the substrate was slowly brought into contact with the hydrate particle, creating a preload force on the hydrate particle due to bending of the left-hand cantilever; (ii) the substrate and particle were allowed to rest under constant preload force for 10 s; (iii) the substrate surface was then raised slowly (approximately 30 μm per second) until the hydrate particle detached; and (iv) the displacement between the substrate and hydrate particle was captured visually. The displacement was multiplied by the cantilever spring constant (i.e., Hooke’s law) to estimate the adhesive force for the pull-off trial.

3. RESULTS AND DISCUSSION 3.1. Mineral Composition and Roughness. The quartz used in this study was visibly opaque. Simple interpretation of the EDS results (provided in the Supporting Information), 15553

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Figure 4. SEM images of calcite (left column) and quartz (right column) at three magnifications: 6500× (row A); 650× (row B); and 65× (row C).

3.2. Adhesion Force Measurements. Cyclopentane hydrate−substrate adhesive forces were measured for each mineral and surface preparation, with particles that were annealed for 25 min and a particle−surface contact time of 10 s. The results (Figure 5) show that hydrate adhesion forces to both minerals are 5−10× higher than those for grade 309 stainless steel (approximately 0.83 ± 0.1 mN/m reported by Aspenes et al.).2 Mineral surfaces refinished with both 100- and 400-grit sandpaper resulted in average adhesive forces below 1 mN/m, while adhesive forces increased for the 220-grit sandpaper to approximately 25% the magnitude of the force for the unmodified surfaces. Adhesion force measurements with the unmodified (raw) quartz and calcite surfaces resulted, for some experiments, in rapid conversion of the hydrate particle (i.e., beyond the initial hydrate shell) upon contacting the substrate. Free water in the

combined with visual inspection, suggested a heterogeneous mineral composition that may include calcite (CaCO3). Calcite was not visibly opaque, and EDS results indicated an oxygen− calcium ratio (8.7:1) that was significantly larger than the expected ratio (1.2:1); the calcite sample was also believed to have a heterogeneous composition. Scanning electron microscopy images were taken for each sample at 65×, 650×, and 6500× magnifications (Figure 4). The quartz images show surface roughness at multiple length scales, with roughness features extending 1−100+ micrometers. While the calcite surface also shows a distribution of roughness features, most are constrained to the 10 μm length scale. The roughness values for raw calcite and quartz were estimated to be approximately 0.7 and 9.0 μm, respectively, from 2D profilometry; these values agree qualitatively with the SEM images. 15554

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quartz adhesive force for 25 min annealing time is larger than that for hydrate−calcite, which may be due to hydrate sintering behavior with quartz. Bai et al.14 studied CO2 hydrate nucleation in systems with silica using molecular dynamics, and concluded that the silica−CO2−water contact line preferentially led to hydrate growth when compared to the CO2−water interface. Additional studies are required to further probe the hydrate sintering and growth behavior on calcite and quartz; the difference in contact time and annealing time dependence may be the result of unique mechanisms for hydrate growth at each mineral surface. Adhesive force dependence on annealing time may indicate that shell porosity is a controlling factor for hydrate sintering, instead of the growth rate along the water capillary bridge. This hypothesis could be tested with visual measurement of the hydrate film growth rate along the substrate interface. An example of visible hydrate growth at the quartz surface is provided in Figure 7. The hydrate particle began sintering to the quartz surface after 20 s of contact time, and converted available water after 5 min of contact. Sintering was observed to begin after approximately 15 s of contact, when the quartz surface was raised during a pull-off trial (thereby exerting stress on the hydrate particle). The amount of growth in Figure 7D suggests liquid water from the particle core migrated across the initial hydrate shell in contact with quartz. To test this hypothesis, dodecyl benzene sulfonic acid (DDBSA) was injected after the significant hydrate growth pictured in Figure 7D; no change in the hydrate morphology was observed after injection. DDBSA was previously observed by Aman et al.7 to result in rapid cyclopentane hydrate growth from unconverted water inside the hydrate shell, resulting in a similar physical appearance to Figure 7D. The absence of additional hydrate growth with DDBSA injection therefore suggests minimal free water remained inside the hydrate particle, after contact with the quartz surface in Figure 7. To further test this hypothesis, a small cyclopentane hydrate particle (100 μm diameter), where the initial shell growth may exceed the total particle diameter,15 was contacted under high preloading conditions with a quartz surface; no hydrate growth was observed in this trial. 3.3. Wettability and Adhesion Force Measurements. To better understand the effect of surface roughness modification (Figure 5), the contact angle of water on each mineral sample16 was measured as a function of modified surface roughness (i.e., with 100-, 220-, and 400-grit sandpaper). Theoretical approaches, such as Young’s equation,8 suggest a monotonic relationship between wetting angle and surface roughness, based simply on the effective interfacial area between the droplet and substrate. Miller et al.17 measured an increase in receding water contact angle on PTFE surfaces from 43° to 60° with an increase in average surface feature size from 8.3 to 83.6 nm; Fecht and Verner17 similarly reported an increase in static water contact angle from 112° to 146° for PTFE surfaces with feature sizes from 0 to 450 nm. Miller et al.18 also relate the extent of droplet wetting to the slope change introduced by each surface asperity, which may provide an important consideration for future mineral adhesion studies. These investigations with PTFE suggest that water wetting angle, and hydrate adhesive force through eq 1, should increase with increasing surface roughness. A more recent investigation by Jopp et al.19 clarified that the dependence of wetting angle on surface roughness is nonlinear for features on the order of 100 μm with both extruding and pore-type structure. The present investigation measured contact angles from 15° to 35°

Figure 5. Cyclopentane hydrate adhesion force (with a 10 s contact time and particle annealing time of 25 min) on stainless steel, and mineral surfaces (quartz and calcite) with surface roughness modification (corresponding to grit of sandpaper used and compared to the unmodified surface). Error bounds represent 95% confidence interval.

hydrate particle core may substantiate the large volume of observed hydrate growth, which may be the result of fracturing the hydrate shell upon contact between the hydrate particle and substrate. This phenomenon was observed in half of the calcite trials and all of the quartz trials, and was observed less frequently for particle annealing times of 90 min. In trials where no visible growth was observed, hydrate−calcite and hydrate− quartz adhesive forces increased by 15× and 3×, respectively, as contact time was increased from 10 to 30 s (Figure 6). This

Figure 6. Cyclopentane hydrate adhesion force on unmodified calcite and quartz minerals with different particle annealing times and hydrate−mineral contact times. Experiments are compared to a contact time dependence model (“Model”) from Aman et al.7 Error bounds represent 95% confidence interval.

result indicates that hydrate−mineral sintering is likely a dominant adhesive force mechanism, even in cases where no changes were observed in hydrate morphology. The results were compared to the sintering model presented by Aman et al.7 for hydrate−steel adhesion force, illustrating hydrate− calcite adhesive force may be more sensitive to contact time than quartz or steel. Decreasing the annealing time from 90 to 25 min significantly increases the hydrate adhesive forces (Figure 6). This may be the result of a more porous hydrate shell at 25 min of annealing time,7 where liquid water may migrate more easily from the particle core to the hydrate− mineral contact point. The measured increase in hydrate− 15555

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Figure 7. Growth of cyclopentane hydrate in contact with a raw quartz surface: (A) prior to contact; (B) 15 s of contact; (C) 20 s of contact; and (D) 5 min of contact.

accurately predicts the magnitude of contact angle dependence for calcite, but does not agree with the experimental data for quartz. The quartz slope (adhesion force versus wetting angle) is approximately 50× larger than the slope predicted by eq 1 (over the range 15−30°). As hydrate−substrate sintering may not contribute to the adhesive mechanism at 10 s contact times, the observed discrepancy between the quartz data and the capillary force model suggests that the assumed dependence of adhesive force on water wetting angle (θ) may require further consideration. As shown in Figure 2, the measured water wetting angle contains energetic contributions from both the substrate−water and substrate−oil interfaces. For the case of hydrate particle adhesion to a mineral surface (Figure 2), both the hydrate and mineral solids contribute energy to the overall capillary bridge tension. The capillary force model, as shown in eq 1, assumes that these energetic contributions are equal for both the hydrate and mineral surfaces. The assumption of an inert substrate may not be valid in hydrate−quartz systems, and requires further investigation to compare solid−fluid interfacial tension values for both hydrate and quartz surfaces. Aman et al.20 presented an experimental method to decouple interfacial tension from cohesive force data; further wettability and adhesive force studies may allow the extension of this approach to the hydrate−mineral systems. Future work may focus on the incorporation of a new term in eq 1, accounting for the substrate contribution to the observed hydrate adhesive force. There is insufficient evidence at this point, however, to infer physiochemical mechanisms behind the differences in adhesion sensitivity to wetting angle for quartz and calcite. Systematic hydrate adhesion force studies with a large diversity of samples are required, to better isolate the mineral properties responsible for enhanced hydrate growth.

for calcite and 15° to 40° for quartz, with average feature sizes (height) of 0.2−2.5 μm. A nonlinear dependence of wetting angle on average feature size was observed. This behavior may be related to the competition between extrusion-type features (controlling overall surface area) and porosity (controlling the extent of water intrusion), although further studies are required to fundamentally understand this nature with heterogeneous mineral samples. Adhesive force is observed to decrease with wetting angle (Figure 8) over a range of approximately 15−45°. The

Figure 8. Cyclopentane hydrate adhesion force as a function of wetting angle for calcite (+) and quartz (○); error bounds for both axes represent 95% confidence intervals. The dashed curve represents the contact angle dependence from eq 1.

magnitude of this dependence (i.e., slope within each data set) is stronger for quartz than for calcite. The data presented in Figure 8 were collected with a 10 s contact time, where Aman et al.7 demonstrated this time scale would involve only capillary forces contributing to the force (i.e., cyclopentane hydrate may not begin sintering within this contact time). The capillary force dependence on water wetting angle is summarized in eq 1, which is provided in Figure 8 (dashed curve) for comparison with the experimental data. The capillary force model (eq 1)

4. CONCLUSIONS Quantitative knowledge of hydrate−substrate adhesion forces is critical to hydrate aggregation and deposition models in conventional oil and gas transportation. This study deploys a 15556

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Colorado School of Mines, and fruitful discussions with Profs. Manika Prasad and Wendy Wempe.

unique micromechanical force apparatus to draw insight into how coproduced solids (e.g., sand and scale) affect gas hydrate adhesion force behavior. Quartz and calcite were employed as model minerals for sand and scale, respectively, and the composition of each crystal was estimated by energy dispersive spectroscopy. Direct micromechanical experiments show cyclopentane hydrate adhesion forces to quartz and calcite minerals are 5−10× higher than was reported for hydrate−steel. In some instances, the contact between raw (unmodified) quartz and a cyclopentane hydrate particle led to rapid growth of the hydrate on the substrate (exhausting the unconverted water core within 5 min); morphological changes of this magnitude have been previously reported only in systems containing strong surfactants. Rapid conversion behavior was not observed in cases where the hydrate diameter was below 100 μm (i.e., where the particle did not have an unconverted water core). In cases where hydrate did not visibly grow into the mineral surface, hydrate adhesive forces increased 3−15× when increasing surface contact time from 10 to 30 s. Hydrate− calcite adhesion force measurements for each surface roughness were found to agree with a capillary cohesion model; however, hydrate−quartz adhesion forces were found to be more sensitive to contact angle than suggested by the existing model. This work illustrates that hydrate−mineral contact may not be well-represented by inert model materials, such as stainless steel. An accurate representation of hydrate shell growth and annealing time is required to estimate the availability of free water able to reach the mineral surface, which may increase the rate of hydrate growth and affect pipeline transportability.

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(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2008. (2) Aspenes, G.; Dieker, L. E.; Aman, Z. M.; Høiland, S.; Sum, A. K.; Koh, C. A.; Sloan, E. D. Adhesion Force Between Cyclopentane Hydrates and Solid Surface Materials. J. Colloid Interface Sci. 2010, 343, 529−536. (3) Song, J. H. K.; Couzis, A.; Lee, J. W. Investigation of Macroscopic Interfacial Dynamics between Clathrate Hydrates and Surfactant Solutions. Langmuir 2010, 26, 18119−18124. (4) Rousseau, G.; Zhou, H.; Hurtevent, C. Calcium Carbonate and Naphthenate Mixed Scale in Deep-Offshore Fields. SPE 3rd International Symposium on Oilfield Scale, Aberdeen, U.K.; SPE: 2001; 68307. (5) Khadim, M.; Sarbar, M. Role of Asphaltene and Resin in Oil Field Simulations. J. Pet. Sci. Eng. 1999, 23, 213−221. (6) Kelland, M. History of the Development of Low Dosage Hydrate Inhibitors. Energy Fuels 2006, 20, 825−847. (7) Aman, Z. M.; Brown, E. P.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Interfacial Mechanisms Governing Cyclopentane Clathrate Hydrate Adhesion/Cohesion. Phys. Chem. Chem. Phys. 2011, 13, 19796−19806. (8) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1991. (9) Rabinovich, Y.; Esayanur, M.; Moudgil, B. Capillary Forces between Two Spheres with a Fixed Volume Liquid Bridge: Theory and Experiment. Langmuir 2005, 21, 10992−10997. (10) Rabinovich, Y. I.; Esayanur, M. S.; Johanson, K. D.; Adler, J. J.; Moudgil, B. M. Measurement of Oil-Mediated Particle Adhesion to a Silica Substrate by Atomic Force Microscopy. J. Adhes. Sci. Technol. 2002, 16. (11) Jung, J. W.; Santamarina, J. C. Hydrate Adhesive and Tensile Strengths. Geochem., Geophys., Geosyst. 2011, 12 (8), Q08003. (12) Kelland, M. A. Production Chemicals for the Oil and Gas Industry; CRC Press: Boca Raton, FL, 2009. (13) Aspenes, G. The Influence of Pipeline Wettability and Crude Oil Composition on Deposition of Gas Hydrates During Petroleum Production. Ph.D. Thesis, University of Bergen, 2009. (14) Bai, D. S.; Chen, G. J.; Zhang, X. R.; Wang, W. Nucleation of the CO2 Hydrate from Three-Phase Contact Lines. Langmuir 2012, 28, 7730−7736. (15) Taylor, C. J.; Miller, K. T.; Koh, C. A.; Sloan, E. D. Macroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface. Chem. Eng. Sci. 2007, 62, 6524−6533. (16) Aspenes, G.; Høiland, S.; Barth, T.; Askvik, K. M.; Kini, R. A.; Larsen, R. Petroleum Hydrate Deposition Mechanisms: The Influence of Pipeline Wettability. Proc. 6th International Conference on Gas Hydrates, Vancouver, 2008. (17) Fecht, H.-J.; Verner, M. The Nano-Micro Interface; John Wiley & Sons: New York, 2006. (18) Miller, J. D.; Veeramasuneni, S.; Drelich, J.; Yalamanchili, M. R. Effect of Roughness as Determined by Atomic Force Microscopy on the Wetting Properties of PTFE Thin Films. Polym. Eng. Sci. 1996, 36, 1849−55. (19) Jopp, J.; Grull, H.; Yerushalmi-Rozen, R. Wetting Behavior of Water Droplets on Hydrophobic Microtextures of Comparable Size. Langmuir 2004, 20, 10015−10019. (20) Aman, Z. M.; Olcott, K.; Pfeiffer, K.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Surfactant Adsorption and Interfacial Tension Investigations on Cyclopentane Hydrate. Langmuir 2013, 29, 2676−2682.

ASSOCIATED CONTENT

S Supporting Information *

Energy dispersive spectroscopy was performed on the quartz and calcite mineral samples used in the experimental work described above, to estimate compositional heterogeneity. These results are included in Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: 303-273-3237. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.A.K. acknowledges partial support for this work by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (DOE-BES award DE-FG02-05ER46242). The authors acknowledge support for W.J.L. from the Renewable Energy Materials Research Science and Engineering Center’s (REMRSEC) Research Experience for Undergraduates (NSF-REU) program at the Colorado School of Mines. The authors also acknowledge the support for materials from the Colorado School of Mines Hydrate Consortium (current and past members): BP, Chevron, ConocoPhillips, ExxonMobil, Halliburton, MultiChem, Nalco, Petrobras, Schlumberger, Shell, SPT Group, Statoil, and Total. The authors acknowledge laboratory support (scanning electron microscopy and 2-D profilometry) from the Department of Metallurgical and Materials Engineering at the 15557

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