pubs.acs.org/Langmuir © 2010 American Chemical Society
Direct Measurements of Contact Force between Clathrate Hydrates and Water Jung Hun (Kevin) Song, Alexander Couzis, and Jae W. Lee* Chemical Engineering Department, The City College of New York, Steinman Hall, 140th Street & Convent Avenue, New York, New York 10031 Received April 2, 2010. Revised Manuscript Received May 11, 2010 A method for precise and reproducible initial contact force measurements is introduced utilizing an apparatus fabricated with a microbalance and z-axis stage to study the interaction behavior between cyclopentane (CP) hydrate and water in a temperature controlled hydrocarbon environment. CP hydrate probes are prepared using hydrate slurries composed of 5 wt % CP and Wilhelmy rods. The CP hydrate probe is slowly brought into contact with water to determine the initial contact force. The effect of substrate morphology on the initial contact force is reported through employing aluminum substrates prepared using physical vapor deposition (PVD) and milling. Accurate and facile measurements are performed by applying a high-resolution microbalance with 0.1 μN resolution to provide repeatable and consistent results of initial contact force between hydrate and water.
Introduction Clathrate hydrates or gas hydrates are icelike crystalline compounds composed of small guest molecules (i.e., methane, carbon dioxide, cyclopentane) enclathrated within a network of hydrogen-bonded water cages. They are commonly found in petroleum and gas extraction processes in many offshore operations where the low-temperature and high-pressure conditions are favorable for hydrate formation.1-3 During the extraction process, aggregation and agglomeration of the hydrates leading to attachment onto a pipeline presents flow assurance complications that may lead to dramatic environmental and economical impacts.4-6 The current focus on hydrate complications has shifted from prevention of hydrate formation using thermodynamic inhibitors to risk management.5 In the risk management approach, the formation of hydrates is allowed during the extraction processes as long as appropriate conditions are preserved to minimize flow assurance issues. Although risk management presents to be a more cost-effective method, the probability for aggregation and agglomeration of hydrates leading to flow assurance issues remains to be addressed. Therefore, the characterization of the initial hydrate aggregation and agglomeration must be explored through interaction force studies. Of the many interaction forces associated with aggregation and agglomeration, adhesion force, defined as detachment or “pulloff” force, has been utilized to characterize the surface-interaction forces between various surfaces.7 Adhesion force consists of multicomponent forces, which include dispersion, interfacial, and *To whom correspondence should be addressed. Telephone: 212-650-6688. Fax: 212-650-6660. E-mail:
[email protected]. (1) Sloan, E. D. Nature 2003, 426, 353–359. (2) Sum, A. K.; Koh, C. A.; Sloan, E. D. Ind. Eng. Chem. Res. 2009, 48, 7457– 7465. (3) Sloan, E. D. Energy Fuels 1998, 12, 191–196. (4) Huo, Z.; Freer, E.; Lamar, M.; Sannigrahi, B.; Knauss, D. M.; Sloan, E. D. Chem. Eng. Sci. 2001, 56, 4979–4991. (5) Taylor, C. J.; Dieker, L. E.; Miller, K. T.; Koh, C. A.; Sloan, E. D. J. Colloid Interface Sci. 2007, 306, 255–261. (6) Dieker, L. E.; Aman, Z. M.; George, N. C.; Sum, A. K.; Sloan, E. D.; Koh, C. A. Energy Fuels 2009, 23, 5966–5971. (7) Israelachvili, J. Intermolecular & Surface Forces, 2nd ed.; Academic Press Limited: London, 1991; p 450. (8) de Lazzer, A.; Dreyer, M.; Rath, H. J. Langmuir 1999, 15, 4551–4559. (9) Anklam, M. R.; York, J. D.; Helmerich, L.; Firoozabadi, A. AIChE J. 2008, 54, 565–574.
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capillary forces.8-10 Many of the previous works limited the scope of the studies to only the capillary detachment force, as it is the dominating and readily measurable force.5,6,11,12 Although the conventional definition of adhesion force is important in characterizing the interaction behavior of two surfaces, it is also crucial to understand and characterize the establishment of the initial contact between two surfaces, which results in hydrate aggregation and agglomeration.13,14 This work defines the initial contact force as the establishment of the capillary bridge between two surfaces. The capillary bridge is of special interest, as it leads to the establishment of the largest interaction force between two surfaces.8,9 In this work, we report a novel method for the initial contact force measurements at the point of contact utilizing hydrates and liquid performed using an assembly of a high-resolution microbalance and a z-axis stage with micrometer movement resolution. We show that the methodology developed for the contact force measurements is highly precise and reproducible by conducting experiments between cyclopentane (CP) hydrates and deionized water on milled and physical vapor deposited metal surfaces inside of temperature controlled high-purity n-decane and various CP/n-decane mixture baths.
Experimental Details CP hydrate slurry is formulated using 5 wt % mixture of CP (Sigma-Aldrich) and deionized (DI) water (Millipore) in a scintillation vial at 1 °C. A total of 18 μL of CP slurry is placed at the tip of a platinum probe (KSV Instrument) that has been chilled in liquid nitrogen (Figure 1A). The rod along with the slurry is immersed into liquid nitrogen to obtain a frozen CP hydrate slurry sphere with an approximate diameter of 3.0 ( 0.2 mm. The probe, consisting of the frozen slurry sphere along with the rod, is placed into a -5 °C CP bath. After 15 min, the bath temperature is (10) Freitas, A. M.; Sharma, M. M. J. Colloid Interface Sci. 2001, 233, 73–82. (11) Fan, X.; Ten, P.; Clarke, C.; Bramley, A.; Zhang, Z. Powder Technol. 2003, 131, 105–110. (12) Taylor, C. J.; Miller, K. T.; Koh, C. A.; Sloan, E. D. Chem. Eng. Sci. 2007, 62, 6524–6533. (13) Nespolo, S. A.; Chan, D. Y. C.; Grieser, F.; Hartley, P. G.; Stevens, G. W. Langmuir 2003, 19, 2124–2133. (14) Gillies, G.; Kappl, M.; Butt, H. J. Adv. Colloid Interface Sci. 2005, 114, 165– 172.
Published on Web 05/18/2010
DOI: 10.1021/la101309j
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Figure 1. Schematics of the apparatus and measurement process. (A) Formation of frozen slurry. (B) Conversion of frozen slurry to CP hydrate probe in CP bath. (C) Immersion of CP hydrate probe in the liquid medium. (D) Establishment of contact between CP hydrate probe and water. slowly raised to 1.5 °C and kept at the temperature for a desired period of time (i.e., 24-72 h) for the slurry to convert to hydrates (Figure 1B). Once the hydrate conversion is confirmed, the bath temperature is lowered to -5 °C for 15 min. During the hydrate conversion period, smooth aluminum substrates are prepared by depositing metal using physical vapor deposition (PVD) on a silicon wafer.15,16 Additional substrates including Al disks and beakers are prepared using a high-speed milling machine. A chosen substrate is submerged in a temperature controlled n-decane (Acros Chemical) or CP/n-decane bath of 25:75 or 50:50 volumetric ratio at 2.8 ( 0.2 °C. Then, DI water is placed on the substrate for the contact force measurement. The converted probe is slowly removed from the CP bath into liquid nitrogen to preserve the shape and the composition. The frozen probe is attached onto 0.1 μN resolution microbalance (KSV Film Balance 2000) that has been fastened to a programmable motorized z-axis stage (Velmex) with micrometer resolution movements as illustrated in Figure 1. The probe is lowered into the hydrocarbon bath and positioned approximately 25-200 μm (ΔZ) above the water droplet, allowing the probe to equilibrate (10-15 s) prior to the measurement (Figure 1C). Subsequently, the stage speed is lowered to 1.6 μm/s until the contact is established (Figure 1D). The contact force measurements are repeated, and the data are obtained by KSV software. The measurement procedure is repeated with ice probes. Detailed experimental procedures are provided in the Supporting Information.
The experimental setup incorporates a widely utilized Wilhelmy platinum plate/rod technique to measure the initial contact force. The technique utilizes direct measurements of capillary contact force or weight exerted on a hydrate probe by a liquid medium at the point of contact, which has been a standard methodology incorporated in many surface tensions studies at different interfaces.17-21
By adapting and incorporating the Wilhelmy rod technique, novel and precise contact force measurements are achieved for the hydrate systems. Cyclopentane (CP) is chosen as a hydrate former to be supported at the tip of the platinum rod to synthesize CP hydrate probes. CP, unlike many of its gas hydrate formers, is widely employed in many studies, as it does not impose strenuous environmental restrictions to achieve stable hydrates and has been shown to readily form in ambient pressure.6,22-24 Utilization of the slurry has shown to be important as it allows consistent control over the desired shape and size of the hydrate probes as well as the subsequent promotion of remaining ice to hydrates. Over the course of 72 h in an excess CP environment, the frozen slurry is allowed to convert to hydrate exhibiting similar formation behavior with previously reported findings.12,25,26 Once the conversion has been completed, the probe is attached to the 0.1 μN resolution microbalance and lowered into the n-decane or CP/n-decane bath. The movement of the probe is precisely controlled and repeated with the programmable z-axis stage retaining micrometer resolution. With a baseline calibration performed, the microbalance is able to resolve up to 0.01 μg mass changes. Prior to the measurements, the microbalance is zeroed at ΔZ to eliminate the buoyancy effect introduced by the Wilhelmy rod. In addition, the buoyancy is determined to be negligible (Fbuoyancy = 0.015 μN/μm) during the course of the studies. All shapes and weights of the hydrate probes remain constant for the duration of up to 100 and 500 s of immersion in 2.8 °C high-purity n-decane and CP/n-decane baths, respectively. Images of typical contact force measurements are shown in Figure 2A and B. Figure 2A depicts a hydrate above a 18 μL water droplet (approximately 150 μm separation) on PVD Al substrate prior to the contact, whereas Figure 2B depicts the postcontact image taken approximately 15 s after the contact. No apparent
(15) Song, J. H.; Kretzschmar, I. ACS Appl. Mater. Interfaces 2009, 1, 1747–1754. (16) Sproul, W. D. Surf. Coat. Technol. 1996, 81, 1–7. (17) Miyama, M.; Yang, Y. X.; Yasuda, T.; Okuno, T.; Yasuda, H. K. Langmuir 1997, 13, 5494–5503. (18) Tretinnikov, O. N.; Ikada, Y. Langmuir 1994, 10, 1606–1614. (19) Wang, J. H.; Claesson, P. M.; Parker, J. L.; Yasuda, H. Langmuir 1994, 10, 3887–3897. (20) Shin, J. Y.; Abbott, N. L. Langmuir 1999, 15, 4404–4410. (21) Vogler, E. A. Langmuir 1992, 8, 2013–2020.
(22) Zhang, J. S.; Lee, J. W. J. Chem. Eng. Data 2009, 54, 659–661. (23) Lo, C.; Zhang, J. S.; Somasundaran, P.; Lu, S.; Couzis, A.; Lee, J. W. Langmuir 2008, 24, 12723–12726. (24) Whitman, C. A.; Mysyk, R.; White, M. A. J. Chem. Phys. 2008, 129, 174502. (25) Uchida, T.; Ebinuma, T.; Kawabata, J.; Narita, H. J. Cryst. Growth 1999, 204, 348–356. (26) Skovborg, P.; Ng, H. J.; Rasmussen, P.; Mohn, U. Chem. Eng. Sci. 1993, 48, 445–453.
Results and Discussion
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Letter
Figure 2. Initial contact force measurements. (A) Setup of CP hydrate probe above a water droplet on aluminum substrate prepared by PVD. Scale bar 1 mm. (B) Image of contact measurement after CP hydrate probe is in contact with the water droplet. Scale bar 1 mm. (C) Typical initial contact force in the microbalance with two repeated contact measurements. (D) Average contact force measurements between CP hydrate-water and ice-water.
physical changes are observed in the probe after the contact has been initiated, indicating limited or no dissociation of the CP from the hydrate (refer to Experimental Details section in the Supporting Information). Figure 2C shows a typical contact measurement curve consisting two contact force measurements utilizing a single probe in a 2.8 °C n-decane bath. The first initial contact force requires approximately 4 s (lead time) to reach equilibrium for the system. The first data point of the plateau is 0.2142 mN and is determined to be the initial contact force. The probe remains in contact with the water droplet for approximately 15 s with an average contact force of 0.2139 ( 0.0005 mN for the duration of the measurement. As observed in the first measurement, approximately 4 s of lead time is required to reach the initial contact force. The second initial and average contact force in Figure 2C is measured to be 0.2130 and 0.2114 ( 0.0008 mN, respectively. A small variation is observed in preceding and following measurements as the change in the acceleration and speed as well as the inherent vibration from the stage is translated to the microbalance. The combined duration for the two contact force measurements lasts approximately 85 s. To further understand the accuracy of the initial contact force measurements and study the effect of interfacial environment, experiments between CP hydrate-water in n-decane and CP/ n-decane baths, and ice-water in n-decane are performed on an Al coated Si substrate prepared by PVD (Figure 2D). Ten measurements employing duplicate conditions are performed (measurements shown in the Supporting Information). Consistent contact force measurements are obtained for all of the measurements with variations ranging from 4 to 7%. Average contact force values are shown in Table 1. As the ratio of CP increases in the bath, the contact force decreases due to decrease in the water-oil interfacial (27) Zeppieri, S.; Rodriguez, J.; de Ramos, A. L. L. J. Chem. Eng. Data 2001, 46, 1086–1088.
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Table 1. Average Contact Force Measurements between CP Hydrate/Ice and Water Using 10 Measurements Performed on PVD Al Substrate probe
liquid media
contact force (mN)
CP hydrate CP hydrate CP hydrate ice
decane 25:75 CP/decane 50:50 CP/decane decane
0.2203 0.1853 0.1301 0.3239
tensions (from 52 mN/m of decane27 to 42 mN/m of CP obtained by using a pendant drop28). However, experiments utilizing a higher ratio of CP (75%þ) result in induced formation of hydrate on the water droplet leading to a dramatic increase in the adhesion force. As a comparison, ice-water measurements are performed and have shown higher contact force resulting from increased availability of water to form a capillary bridge between surfaces.29 Similar to CP hydrates, the surface morphology and transparency of ice probes are monitored during the measurements for the integrity of the probes. One contact force measurement can be performed with minimal changes. Further, an increased accuracy of contact force measurements is achieved by applying PVD to obtain uniform and microscopically smooth Al substrate.30 The smooth Al substrate utilized to support water droplets eliminates the potential effects due to irregular substrate surface morphology.31 In addition to the Al substrate prepared by the PVD method (Figure 3A), the other two Al substrates are prepared via machine milling and machine burrowing of an Al rod to form a beaker. These substrates are applied in studying the effect of the substrate surface morphology (28) Lin, S. Y.; McKeigue, K.; Maldarelli, C. AIChE J. 1990, 36, 1785–1795. (29) Petrenko, V. F. J. Phys. Chem. B 1997, 101, 6276–6281. (30) Aspenes, G.; Dieker, L. E.; Aman, Z. M.; Hoiland, S.; Sum, A. K.; Koh, C. A.; Sloan, E. D. J. Colloid Interface Sci. 2010, 343, 529–536. (31) Pawar, A. B.; Kretzschmar, I. Langmuir 2008, 24, 355–358.
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Figure 3. Surface morphology effect. (A) SEM image of aluminum substrate prepared using PVD. Scale bar 2 μm. (B) SEM image of aluminum disks prepared by milling with microscopic surface variations. Scale bar 2 μm. (C) Average contact force measurements made using CP hydrates and various surfaces.
on CP hydrate-water contact force measurements in n-decane bath. Until now, the effect of the substrate surface morphology on the contact force between hydrate and water has not yet been studied. It is crucial to understand this effect, as the interaction may vary as the result of the surface morphology variation affecting the contact force at the walls of the extraction pipeline.7,30,32-36 For milled Al substrates, SEM studies confirm the uneven surface morphology with multiple grooves measuring 2-3 μm in separation and 2-10 μm in height (Figure 3B). Although the average initial contact force measurement (0.2097 ( 0.0367 mN) is similar in comparison to the PVD substrate (0.2203 ( 0.0093 mN), the samples measured on milled substrate exhibit approximately 4 times greater variation in the measured initial contact force (Figure 3C). The increased variation may be attributed to the pinning or trapping of the contact point resulting in different contact angles between water and the substrate from one sample to the next.37-39 Subsequently, the discrepancy in contact angle results in a different interfacial tension leading to different initial contact force.8 An increase in the lead time (from 4 to 7 s) is also exhibited for the milled surface. As the initial contact between the probe and water is established, changes to the interfacial dynamics result in potential shifting of the pinned contact point.37-39 The potential changes to the contact point can result in the system requiring longer lead-time. Initial contact force measurements between CP hydrate and water bath are also performed to understand the interfacial (32) Nicholas, J. W.; Dieker, L. E.; Sloan, E. D.; Koh, C. A. J. Colloid Interface Sci. 2009, 331, 322–328. (33) Ata, A.; Rabinovich, Y. I.; Singh, R. K. J. Adhes. Sci. Technol. 2002, 16, 337–346. (34) Rabinovich, Y. I.; Adler, J. J.; Esayanur, M. S.; Ata, A.; Singh, R. K.; Moudgil, B. M. Adv. Colloid Interface Sci. 2002, 96, 213–230. (35) Rabinovich, Y. I.; Adler, J. J.; Ata, A.; Singh, R. K.; Moudgil, B. M. J. Colloid Interface Sci. 2000, 232, 10–16. (36) Rabinovich, Y. I.; Adler, J. J.; Ata, A.; Singh, R. K.; Moudgil, B. M. J. Colloid Interface Sci. 2000, 232, 17–24. (37) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (38) Deegan, R. D. Phys. Rev. E 2000, 61, 475–485. (39) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22, 2966–2967.
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dynamics where the surface effects due to the substrate are minimal.40,41 CP hydrate probes are brought into contact with a larger volume of water (cylindrical bath: 11.4 cm2 area 4 cm depth) immersed in n-decane. The initial contact force measurement is similar to ice-water measurement on a PVD Al surface with a similar 4 s lead time, indicating the importance of the substrate surface morphology with different volumes of water (Figure 3C). Although this Letter focuses on the initial contact force, the detachment force between CP hydrate and water bath is also observed. The detachment force obtained (39.42 ( 2.02 mN/m using our wetting perimeter) shows similar values to previously reported findings confirming the accuracy of the applied apparatus.29
Conclusions The described methodology allows facile and precise contact force measurements between two surfaces immersed in a liquid media. The accuracy of the contact force measurements is within an error of
