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Hydrate Formation at the Methane/Water Interface on the Molecular Scale Tadanori Koga,*,†,‡ Johnny Wong,† Maya K. Endoh,‡ Devinder Mahajan,†,‡,§ Christian Gutt, and Sushil K. Satija^ Chemical and Molecular Engineering Program, and ‡Department of Materials Science & Engineering, Stony Brook University, Stony Brook, New York 11794-2275, §Energy Sciences & Technology Department, Brookhaven National Laboratory, Upton, New York 11973-5000, HASYLAB at DESY, Notkestrasse 85, 22603 Hamburg, Germany, and ^Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 )
†
Received February 1, 2010. Revised Manuscript Received March 8, 2010 We report the nucleation process of methane hydrate on the molecular scale. A stationary planar interface separating methane gas and liquid water was studied by using in situ neutron reflectivity. We found that the angstrom-scale surface roughening is triggered as soon as the water phase contacts methane gas under the hydrate forming conditions. In addition, it was found that the microscopic surface structure remains unchanged until a macroscopic hydrate film is developed at the interface. We therefore postulate that the angstrom-scale surface roughening is attributed to the formation of microscopic hydrate “embryos” in a “dynamic equilibrium” manner.
Introduction Gas hydrates are inclusion compounds in which small hydrophobic guest molecules are trapped in cages formed by a network of hydrogen bonded water molecules.1 Of special interest is methane hydrate that contains a vast amount of methane, a clean energy source. In addition, methane hydrate has unique gas storage properties, as the hydrates can theoretically contain 180 volumes of methane gas (at standard pressure and temperature) per volume of a hydrate. This storage capability in a hydrate structure would be a promising technology, especially for remote locations that do not have a gas well for storage in close proximity. Furthermore, the use as methane hydrate may offer a potentially valuable alternative disposal or use of associated gases from offshore oil production sites. On the other hand, methane hydrate presents a potentially formidable environmental hazard. The unintentional release of methane, which is a potent greenhouse gas, would affect the global climate and the local marine environment. Moreover, hydrate formation within oil and gas transport pipelines under deep-sea conditions is a significant long-standing problem for the oil industry due to blockage of flow lines by hydrate plugs. Thus, further research is needed to clarify the mechanisms of the formation, decomposition, and inhibition in methane hydrate, which would benefit the national economy as well as the environment. It is believed that the surface of liquid water is crucial for the formation of gas hydrates, since gas concentrations dissolved are much higher at the interface so that the interfacial region would provide the environment for a hydrate initiation.2-4 According to several experimental studies on macroscopic gas hydrate formation, the initial film thickness of about 10 μm was formed on the water *To whom correspondence should be addressed. E-mail: tkoga@ notes.cc.sunysb.edu. (1) Sloan, E. D.; Koh, C. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press/Taylor & Francis Group: Boca Raton, FL, 2007. (2) Uchida, T.; Takeya, S.; Wilson, L. D.; Tulk, C. A.; Ripmeester, J. A.; Nagao, J.; Ebinuma, T.; Narita, H. Can. J. Phys. 2003, 81, 351–357. (3) Kuhs, W. F.; Staykova, D. K.; Salamatin, A. N. J. Phys. Chem. B 2006, 110, 13283–13295. (4) Lee, J. D.; Song, M.; Susilo, R.; Englezos, P. Cryst. Growth Des. 2006, 6, 1428–1439. (5) Sugaya, M.; Mori, Y. H. Chem. Eng. Sci. 1996, 51, 3505–3517.
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side of the interface5-7 along with the sharp system pressure drop.8 However, little is known about the microscopic understanding of the interface under the methane hydrate forming conditions due to a lack of methods capable of providing high resolution structural information in compressed gases. Very recently, Lehmk€ uhler et al. used X-ray reflectivity and X-ray diffraction techniques to study the structures of the CO2-water interface and showed the existence of small clusters of CO2 hydrate at the liquid CO2-water interface, while the formation of the adsorbed layer of CO2 molecules was observed at the gaseous CO2-water interface.9 In this Letter, we report in situ hydrate formation at the gaseous methane and liquid water interface on the molecular length scale. This would be more challenging than the CO2 hydrate experiments due to the much higher formation pressures required. In order to overcome this difficulty, we used a high pressure neutron reflectivity (NR) technique which enables us to characterize the molecular-level interfacial structures under compressed gases.10 We show that the angstrom-scale surface roughening is triggered by the formation of hydrate “embryos”, as soon as the water phase contacts methane gas under the hydrate forming conditions. In addition, we have clarified how the embryos grow into a macroscopic methane hydrate thin film at the interface by combining the pressure and temperature trace experiments and laser reflectivity experiments. We found that the embryos are metastable during the induction period and suddenly turn into the macroscopic hydrate film at the interface in conjunction with the pressure drop and temperature spike.
Experimental Section Materials. Deuterated methane and deuterium oxide (CD4, and D2O, both 99.9% from Cambridge Isotope Laboratories Inc.) were chosen to enhance neutron scattering contrast as well as (6) Uchida, T.; Kawabata, J. Energy 1997, 22, 357–361. (7) Freer, E. M.; Selim, M. S.; Sloan, E. D. Fluid Phase Equilib. 2001, 185, 65–75. (8) Taylor, C. J.; Miller, K. T.; Koh, C. A.; Sloan, E. D. Chem. Eng. Sci. 2007, 62, 6524–6533. (9) Lehmk€uhler, F.; Paulus, M.; Sternemann, C.; Lietz, D.; Venturini, F.; Gutt, C.; Tolan, M. J. Am. Chem. Soc. 2009, 131, 585–589. (10) Koga, T.; Seo, Y. S.; Shin, K.; Zhang, Y.; Rafailovich, M. H.; Sokolov, J. C.; Chu, B.; Satija, S. K. Macromolecules 2003, 36, 5236–5243.
Published on Web 03/15/2010
DOI: 10.1021/la1004853
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Figure 1. Schematic geometry for the NR experiments. The same experimental configuration was used for the LR experiments with a photodiode detector. to reduce incoherent neutron scattering generated by H atoms. The phase diagram for the D2O/CD4 system was obtained by shifting the equilibrium temperature in the phase diagram for the H2O/CH4 system by 2.5 C to account for the deuterium isotope effect of water,11 whereas the deuterium effect of CD4 was assumed to be negligible. High-Pressure Cell. In order to mimic natural conditions of the methane hydrate stability, we built a stainless steel highpressure cell designed for laboratory studies. The cell design is exactly same as the steel high-pressure cell reported previously.10 The high-pressure cell was fitted with two cylindrical sapphire optical windows to allow adequate transparency for neutron and laser reflectivity experiments. Deuterated water (10 mL) was loaded on a Teflon trough. CD4 gas was loaded into the highpressure cell by means of a hand operated syringe pump (High Pressure Equip. Co.) to the desired pressure. Prior to pressurization, the air space was purged with the gas at low pressure. Methane pressure inside the cell was monitored by an OMEGADYNE pressure transducer (TH-1) and a pressure gauge meter (INFS-0001-DC1). The temperature of the cell was controlled via an external circulating bath (Thermo Scientific). The cell temperature was held within (0.015 C, and the pressure variation was limited to (0.1%. The temperature and pressure changes during the formation of macroscopic methane hydrate were monitored through a LabVIEW (National Instruments Co.) based data acquisition program. Neutron Reflectivity (NR) Studies. The NR measurements were performed on the NG7 neutron reflection spectrometer at National Institute of Standards and Technology (NIST). The experimental configuration is schematically shown in Figure 1. A position sensitive detector (PSD) was used for the experiments to probe the specular and off-specular (i.e., diffuse scattering intensity) components from the interface simultaneously. As shown in Figure 1, the specular reflection arises when incident and outgoing neutron beams located in the same scattering plane satisfy the law of reflection (Ri = Rf) at the interface. The diffuse scattering defined here is the integrated scattering intensity along the qx (or qy) axis except for the specular component. The acquisition time for each NR measurement over the qz-range used in this study was about 2 h. The two-dimensional scattering patterns were decomposed into specular and diffuse components using the “reflpak” program developed for NR analysis.12 The specular intensity (R) was then analyzed by comparing the observed reflectivities with the calculated ones by using the following equation: Z 2 dFðzÞ -iqz z Rðqz Þ ¼ RF ðqz Þ e dz dz
ð1Þ
(11) Chun, M.-K.; Yoon, J.-H.; Lee, H. J. J. Chem. Eng. Data 1996, 41, 1114– 1116. (12) Kienzle, P. A.; O’Donovan, K. V.; Ankner, J. F.; Berk, N. F.; Majkrzak, C. F. http://www.ncnr.nist.gov/reflpak, 2000-2006.
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Figure 2. Changes in the laser intensity and system pressure versus time at T = 6.5 C and P = 6.8 MPa. The specular laser intensity from the D2O/CD4 interface showed a discontinuity at the end of the induction period determined from the pressure drop. The intensity drop was complete within 40 s. Inset: Expanded temperature trace near the end of the induction period (indicated by the dashed line), showing the temperature spike that is consistent with the exothermic hydrate formation phenomenon. with RF being the Fresnel reflectivity of a specimen with the average scattering length density F¥ with an infinitely sharp interface and F(z) being the scattering length density profile in the direction normal to the surface. Laser Reflectivity (LR) Studies. The LR experiments were conducted with a He-Ne laser beam (λ = 632 nm). The same experimental configuration as the NR experiments was used with a photodiode detector with a slit to capture only the specular component of the reflected laser beam. The incident (Ri) and exit (Rf) angles of the laser were fixed at 15 with respect to the sample surface plane, providing structure information with a spatial resolution of about 700 nm. As will be discussed, this experimental setup enables us to identify the initial formation of macroscopic methane hydrate at the interface. All the experiments were performed at T = 6.5 C to prevent formation of D2O ice and condensation of D2O at the optical (sapphire) windows for the NR and LR techniques. During all the experiments, the samples were not stirred to favor hydrate formation.
Results and Discussion Macroscopic Hydrate Formation. Before the NR experiments, the induction period,13 which is the time for methane hydrate to be detected macroscopically, was independently determined from the pressure and temperature traces under the hydrate forming conditions. Figure 2 shows the representative pressure change as a function of time at the initial conditions of T = 6.5 C and P = 6.8 MPa. The hydrate equilibrium pressure (Pb) at T = 6.5 C, above which stable hydrates form, is predicted to be Pb = 3.7 MPa. In the figure, the pressure slope changes distinctly at t = 183 min after a slow and steady pressure drop of the methane pressure, which is defined as the induction period of time.13 The induction phenomenon was instantaneously followed by the exothermic macroscopic hydrate formation indicated by a distinct temperature spike (corresponding to the predicted energy release of 2.9 109 J/m3),14 as shown in the inset of Figure 2. As summarized in Figure 3, the induction time determined by the trace experiments increased (up to several days) as the hydrate equilibrium pressure was approached, which can be explained by the classical nucleation theory.15 Figure 2 also shows the specularly reflected intensity from the methane/water interface. From the figure, we can clearly (13) Lekvam, K.; Ruoff, P. J. Am. Chem. Soc. 1993, 115, 8565–8569. (14) Ballard, L.; Sloan, E. D. Fluid Phase Equilib. 2004, 216(2), 257–270. (15) Kashchiev, D.; Firoozabadi, A. J. Cryst. Growth 2002, 243, 476–489.
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Figure 3. Pressure dependence of the induction period of time at T = 6.5 C. Pb corresponds to the hydrate equilibrium pressure at T = 6.5 C, above which stable hydrates form.
see the discontinuity in the intensity at t = 183 min, indicating the formation of a macroscopic (micrometer-scale) rough surface. Interestingly, the time at which the micrometer-scale roughness forms is in good agreement with that when the environmental quantities identify the emergence of the bulk methane hydrate. Based on the previous experimental fact that the sharp pressure drop occurs just after the initiation of the macroscopic hydrate film growth,8 we conclude that the discontinuity corresponds to the development of a macroscopic hydrate thin film at the interface. In other words, the emergence of the micrometer-scale surface roughness, which can be detected by the LR technique, is an indication of the bulk hydrate formation at the interface. Microscopic Hydrate Formation. The initiation of the hydrate thin film on the molecular scale was investigated by the NR technique. Figure 4a shows the representative pressure dependence of the specular components as a function of the wave vector transfer normal to the surface, qz = 4π sin Ri/λN, where λN and Ri are wavelength of neutrons (λN = 4.1 A˚) and the incident angle of the neutron beams, respectively. We found that the deviations from the calculated Fresnel reflectivities (indicated by the solid lines) become more pronounced at the high qz-region when D2O was in contact with methane gas in the hydrate stable conditions (P > Pb). The best-fit results using the scattering length density profile F(z) based on the Parratt algorithm16 yielded the root-mean-square surface roughness (σ) of 10 ( 3 A˚ at P > Pb, while the σ value was 3 A˚ in air, which is in good agreement with the capillary wave theory,17 and slightly increased to 5 A˚ at P = 2 MPa. It should be noted that we did not observe any Kissing fringes within the qz-range under the pressure conditions used (0.1 e P e 8.3 MPa), suggesting no formation of an additional layer (i.e., an adsorbed layer of methane gas or a methane hydrate layer) with thickness of more than 10 A˚ on the D2O phase or underneath the CD4/D2O interface. In addition, as shown in Figure 4b, we found that the diffuse scattering intensity, which is almost constant regardless of the observed qz-values, increases by a factor of about 3 at P = 2.0 MPa relative to the air and leads to the further significant increment at and above P = 3.8 MPa. Figure 4c summarizes the pressure dependence of the diffuse scattering intensity at qz = 0.15 A˚-1, showing the abrupt increase at P = 3.8 MPa. Hence, the NR results give clear evidence that the CD4/D2O interface undergoes a transition from a liquidlike smooth surface to a more roughened surface (16) Parratt, I. G. Phys. Rev. Lett. 1954, 95, 359–369. (17) Braslau, A.; Pershan, P.; Swislow, G.; Ocko, B.; Als-Nielson, J. Phys. Rev. A 1988, 38, 2457–2470.
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Figure 4. NR results for the D2O/CD4 system at T = 6.5 C. (a) Specular and (b) diffuse components at the three different pressures. The solid lines in (a) correspond to the calculated Fresnel reflectivities. (c) Diffuse scattering intensity at qz = 0.15 A˚-1 as a function of pressure. The red and blue dashed lines correspond to Pb and the critical pressure (Pc = 4.6 MPa), respectively.
at the angstrom-scale when the methane gas contacts with the water phase in the hydrate stable region of the phase diagram. In order to further study the time evolution of the angstromscale surface structure during the induction period, we independently measured the diffuse scattering intensity at the one fixed qz value (qz = 0.15 A˚-1) by a 1 min interval recording. As a result, we found that the increase in the diffuse scattering was complete within 1 min (after we set the pressures) even near Pb, in contrast to the induction time for the macroscopic hydrate formation (Figure 3). The experimental fact that the diffuse scattering intensity remains unchanged during the induction period would lead to the important conclusion that the angstrom-scale surface roughening is attributed to the formation of hydrate “embryos” at the interface. This supports the proposed mechanism of hydrate formation:1,13 oligomeric “precursors” are formed in the induction period, but flicker until the end of the induction period when such metastable structures turn into polymeric (macroscopic) hydrates. We postulate that the formation of the critical nuclei (the predicted size of about 30 A˚18) is accomplished at the end of the induction period. The critical nuclei then form the macroscopic hydrate film with the micrometer-scale surface roughness at the interface immediately in conjunction with the pressure drop and temperature spike. Interestingly, the hydrate formation observed in the present study is different from that at the CO2/water interface reported by (18) Larson, M. A.; Garside, J. Chem. Eng. Sci. 1986, 41, 1285–1289.
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Lehmk€uhler et al.9 When the pressures were set below the condensation pressure of CO2, the adsorbed layer of CO2 gas molecules was formed on the water surface and grew up to 40 A˚ thickness as the condensation pressure of CO2 was approached; the hydrate formation was triggered only at the liquid CO2-water interface. As shown in Figure 4c, the methane hydrate embryos are formed even at the gas-water interface as far as the pressure is set above Pb. This difference suggests that the molecular level gas hydrate formation is not unique although both hydrates form similar clathrate structures (the cubic structure I hydrate1) under geologic temperature and pressure conditions. Another factor to be considered would be the difference in the nature of the compressed gases: methane gas used in this study is rather close to a supercritical fluid (Tc = -82.7 C and Pc = 4.6 MPa) that has both gas and liquid properties. Further studies on the mechanisms of other hydrates at the gas-water interfaces are in progress.
Conclusions In situ surface sensitive techniques and pressure-temperature traces were used to investigate the mechanism of the nucleation
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and growth of methane hydrate from microscopic to macroscopic scales. We found that the angstrom-scale surface roughening is triggered at the methane-water interface as soon as the water phase contacts methane gas under the hydrate forming conditions. The microscopic surface structure remains unchanged during the induction period and suddenly turns into the macroscopic hydrate film at the interface in conjunction with the pressure drop and temperature spike. Hence, our results indicate that the gas-water interface plays a crucial role to control and develop the formation of methane hydrate. We are currently studying the effect of surfactants, which significantly improve the formation rate of macroscopic hydrates,19 on the growth of the embryos into macroscopic hydrates, facilitating development of a technology for methane extraction from natural hydrate deposits. Acknowledgment. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund (47110-G10) for support of this research. The authors thank D. Forman, M. Eaton, J. Jerome, and M. Rafailovich for helping with the setup for the LR experiments. (19) Zhong, Y.; Rogers, R. E. Chem. Eng. Sci. 2000, 55, 4175–4187.
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