Gas Nanobubbles as Nucleation Acceleration in the Gas-Hydrate

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Gas Nanobubbles as Nucleation Acceleration in the Gas-Hydrate Memory Effect Tsutomu Uchida,* Kenji Yamazaki, and Kazutoshi Gohara Division of Applied Physics, Faculty of Engineering, Hokkaido University, N13 W8 Kita-ku, Sapporo 060-8628, Japan ABSTRACT: Gas-hydrate crystals have important roles in various energy and environmental issues and also have potential industrial applications. Yet their formation and dissociation mechanisms remain unclear. To accelerate their crystallization, one can use a thermal hysteresis process called the “memory effect”, which is recognized as a shortening of the induction time of gas hydrate nucleation. Although its mechanism is still under debate, submicron-sized bubbles, called “microand nanobubbles (MNBs)”, that are generated after the hydrate dissociation are thought to play an important role. In this study, we use a transmission electron microscope to observe freeze-fracture replicas of the ethane (C2H6)-hydrate dissociated solution to identify the existence of MNBs. We find that a significant number of MNBs remain dispersed in the dissociated solution for more than 1 day. We then measure the induction time of C2H6-hydrate formation under stirred conditions with and without such MNB solutions. Not only in the C2H6-hydrate dissociated water but also in the C2H6-bubbling solution we identify a significant and clear memory effect. We apply these findings to the mechanism of the memory effect in gas-hydrate crystallization, particularly the role of MNBs on the memory effect.

1. INTRODUCTION Gas hydrates are crystalline solid compounds consisting of hydrogen-bonded water molecules forming cages around gas molecules. These gas molecules include methane (CH4), ethane (C2H6), propane (C3H8), carbon dioxide (CO2), oxygen (O2), nitrogen (N2), hydrogen, and xenon (Xe). The abundance of natural gas hydrates occurring in deep-ocean continental margins and under the permafrost has attracted considerable attention as a new natural gas resource1−3 and as a source of CH4 in the global carbon cycle.4,5 They have also drawn attention as functional materials in industry, such as a gas-storage medium for natural gas6−8 and for hydrogen fuels.9−11 Compared to pure ice, the crystallization of gas hydrates requires a larger driving force (supersaturation or supercooling) and a longer induction time. The induction time is the time period taken for the formation of stable hydrate nuclei, and this time has been studied as a kinetic property.12−17 A long induction time reduces the practicality of hydrates, so hydrate formation and dissociation processes should be better understood to control hydrate crystallization. A way to accelerate gashydrate crystallization is through the thermal hysteresis process called the “memory effect”. In this effect, the induction time of gas hydrate nucleation becomes shorter for recrystallization than that for the initial crystallization. For example, CH4 hydrates12−14 have been shown to express the memory effect by using the dissociation solution of CH4 hydrate. The memory effect is also observed on clathrate hydrates, such as tetrahydrofran (THF) hydrate,18,19 hydrochlorofluorocarbon (HCFC-141b) hydrate,20 tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate,21 and cyclopentane hydrate.22 The memory effect has important implications not only for © 2016 American Chemical Society

industrial applications but also for the gas industry, in which the hydrates can plug up the gas pipeline,23 and for the safe extraction of natural gases from the deep-sea floor.3,24 Although evidence of the memory effect phenomenon is plentiful, its mechanism has been under debate. Sloan and Koh2 summarized two opposing hypotheses: (1) microscopic hydrate-like structures remain in solution,12−14,18,20−22 and (2) enough guest molecules remain in solution.26 More recently, Zeng et al.19,27 proposed another mechanism: (3) the impurity imprinting hypothesis,28,29 which is based on the inhibition property of antifreeze proteins on the homogeneous nucleation of THF hydrate19 and on the nucleation of C3H8 hydrate.27 Many studies have found that the recrystallization of the clathrate hydrate occurs at almost the same position that the last previous crystal dissociated.21 This finding has been taken as apparent support for the first hypothesis; however, there exists no direct evidence of such residual structures via neutron diffraction or via molecular dynamics (MD) simulations.30 MD simulations instead support the second hypothesis. Moreover, Rodger26 suggested that the memory effect of CH4 hydrate was due to the persistence of a highconcentration region and retarded diffusion of CH4 in the dissociated solution. As guest molecules are usually hydrophobic, the persistence of a high concentration of guest (especially of gas molecules) in water may appear unlikely. However, recent MD simulations find that nanobubbles (NBs) of the guest are generated after the gas hydrate dissociation.31−35 NBs dispersed in water (called “bulk NBs”36) have Received: August 8, 2016 Revised: November 2, 2016 Published: November 3, 2016 26620

DOI: 10.1021/acs.jpcc.6b07995 J. Phys. Chem. C 2016, 120, 26620−26629

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The Journal of Physical Chemistry C unique properties such as a negligible rise speed in water,37 a higher inner pressure derived from the Young−Laplace equation, and a surface electric charge (ζ-potential).38 Although no direct microscopic measurements have yet confirmed the existence of NBs, several experimental approaches support a significant contribution from NBs on the memory effect.30,39 Recently, Uchida et al.40 identified experimentally the existence of submicron-sized bubbles, called “micro- and nanobubbles (MNBs)”, dispersed in water after CH4 hydrate dissociation. Using Raman spectroscopy, they confirmed them as the bulk CH4 MNBs and concluded that they were generated by CH4hydrate dissociation. This finding indicated that the guest-gas concentration in the solution was kept above the equilibrium solubility, while MNBs survived in the solution. In the present study, we investigate whether or not the bulk MNBs actually contribute to the memory effect of gas-hydrate crystallization. C2H6 hydrate is chosen here because we would like to check the generality of the MNB generation after the hydrate dissociation. C2H6 hydrate has the same crystalline structure with CH4 hydrate (structure I), but its hydration number is larger due to the vacant 12-hedral cages. The dissociated pressure of C2H6 hydrate is lower than that of CH4 hydrate at the same temperature, so the hydrostatic pressure effect (such as the guest concentration in water due to the Henry’s law) would be smaller. In general, MNBs form not only in bulk water but also on solid surfaces such as the wall of the reactor and the stirrer. These surface MNBs, as well as other interfacial gaseous states, have properties distinct from bulk MNBs.36,41−44 As a solid surface in general affects the heterogeneous nucleation of clathrate hydrates, these surface MNBs may contribute to the memory effect.28 We acknowledge this possibility but consider it unlikely in the case of MNB-induced nucleation because surface MNBs have an interior pressure smaller than that of bulk MNBs.44,45 In our experiments, we first observe the generation of bulk MNBs in water after the dissociation of a C2H6 hydrate sample via a similar experimental procedure as our previous work.40 After that, to identify the memory effect, we run experiments of C2H6-hydrate crystallization under a stirring condition using various source solutions: pure water, C2H6-hydrate dissociated water, and C2H6-bubbling solutions that have not experienced prior hydrate crystallization. Finally, we examine experimental evidence here for the effect of different gas MNBs (Xe and N2) on C2H6-hydrate crystallization and thus investigate the role of MNBs on the nucleation acceleration effect.

solution decreases from room temperature to 274.2 K under the constant gas pressure of 2.5 MPa. When the C2H6 hydrate nucleates, the temperature of the vessel increases suddenly due to the exothermic reaction. After sufficient C2H6 hydrate forms in the vessel, the vessel is cooled below 173.2 K by immersing in the liquid N2 bath; the remaining C2H6 gas in the vessel is released; and the solid sample is recovered. The details of the replica sample preparation are mentioned elsewhere.40,47−49 Concerning the replica sample analyses, several pieces of C2H6-hydrate crystals (approximately 1.4 g) are immersed into 5 mL pure water in the silica-glass cell used for the previous study40 to dissociate C2H6 hydrates at room temperature. During or after C2H6 hydrate dissociation, a small amount of the hydrate-dissociated solution (less than 3 μL) is quickly put into a liquid N2 bath to freeze in place any inclusions that exist in the solution. The prepared frozen sample is then fractured under vacuum (10−4 to 10−5 Pa) and low temperature (approximately 100 K) to reduce the formation of artifacts on the fractured surface. The bubbles that were in the original bulk solution become pits in the fractured, flat ice surface. The replica film of the fractured surface is therefore a snapshot cross-section of the bulk solution, thus providing data on the morphologies and number concentrations of bulk MNBs in the solution. We make a replica film of this fractured surface by evaporating platinum and carbon (JEOL type JFD-9010) and then recover it by melting the ice body in water. We used a high-resolution TEM (JEOL, JEM-2010) at a 200 kV accelerating voltage to observe the replica film transferred on a Cu grid having 43 μm × 43 μm openings. An imaging plate (Fujifilm, FDL-UR-V) is used for acquiring the resulting image. As observed previously,40,47−49 the MNBs are found here as circular or oval holes in the replica film. Based on the sample preparation procedures mentioned above, these MNBs were originally bulk MNBs in the solution. To estimate their sizes, we measure both the major and minor axes of each hole and calculate the spherical diameter having the same cross-section area. The diameter distribution is obtained, and the average diameter (D) is calculated as the arithmetic average (n ranges between 80 and 150). For the measurement of number density of MNBs (N), we count all MNBs observed in an opening of the grid. After averaging the number of MNBs in a sample from 10−23 openings, we estimate N by assuming a uniform distribution of MNBs in a frozen droplet. To reduce the observation bias, D and N are obtained from at least three replicas per solution. The changes of D and N during the storage period are examined by frequently sampling from a water sample stored at room temperature (approximately 293 K) after the C2H6-hydrate dissociation. To reduce the evaporation of dissolved gases from the solution during storage, the solution is stored in a sealed glass bottle. The acceleration effect on C2H6-hydrate recrystallization is investigated by measuring the induction time. The system used for the crystallization of C2H6 hydrate was the same hydrateformation system mentioned above. Thus, all experiments are done under a similarly stirred condition. The induction time is defined here as the time difference from the time when the temperature of the system reached the equilibrium condition for C2H6 hydrate (approximately 286 K at 2.5 MPa)2 to the time when the temperature of the vessel started to increase due to the hydrate nucleation. The average induction time and its standard error are used to compare the experimental results.

2. EXPERIMENTAL SECTION Here we briefly explain the experimental system. Further details for preparing C2H6 hydrate crystal are in our previous papers.40,46 Approximately 50 mL of pure water (15 MΩ cm in resistivity of distilled, deionized water) and about 2.5 MPa pure C2H6 gas (99.95 vol % in purity, Hokkaido Air Water, Sapporo, Japan) are introduced in the high-pressure vessel (223 cm3 in internal volume) equipped with a mechanical stirrer, a sheathed thermocouple (type T), and a pressure gauge (Nagano Keiki Seisakusho, type KH15). The main body of the vessel is made from 316 stainless steel. (As the vessel and stirrer have been used for a long time, the surface condition should be sufficiently rough to readily form surface MNBs.) Then the vessel is set in a temperature-controlled bath at 274.2 ± 0.2 K to form C2H6-hydrate crystals while agitating at approximately 300 rpm. Initially, the temperature of the 26621

DOI: 10.1021/acs.jpcc.6b07995 J. Phys. Chem. C 2016, 120, 26620−26629

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The Journal of Physical Chemistry C Statistical differences are evaluated with the student t test following one-way ANOVA. For the initial solutions, we use pure water (the same material as used for the initial hydrate formation), the C2H6hydrate dissociated solution (immersed in approximately 10 g of C2H6 hydrate in 50 cm3 pure water), hereafter the “dissociated solution”, and the C2H6 MNB solution that was prepared via a commercially supplied microbubble generator (Aura Tech, type OM4-GP-040), hereafter the “C2H6-bubbling solution”. The C2H6-bubbling solution is prepared by the following procedures: approximately 1 L of distilled water and 0.25 MPa pure C2H6 gas (both of them were the same sources as that supplied for the C2H6-hydrate crystallization) are mixed in the generator for 1 h at 293.2 K. The experiment on C2H6hydrate recrystallization from the sample solution stored in a sealed bottle is done at various time periods after the generation. In addition to using C2H6-bubbling solution, we also prepared N2-bubbling solution and Xe-bubbling solution for investigating the effect from the type of gas MNBs on the memory effect. N2 or Xe gases (99.999 or 99.995 vol % in purity, respectively, Hokkaido Air−Water) were supplied to the microbubble generator instead of C2H6 gas to prepare each bubbling solution.

Figure 2. Typical histogram of MNB distributions after C2H6 hydrate dissociation. Samples obtained at 0 h (open bar, n = 91), 3 h (slashed bar, n = 135), and 29 h (solid bar, n = 141).

Moreover, the distribution slightly changed after C2H6hydrate dissociation, depending on the storage period. The initial solution is multipeaked. Then, at 3 h, both the smaller and the larger MNBs increase in number as the middle-size MNBs decrease. Finally, at 29 h, the MNB size distribution has a single maximum at 200−300 nm. The measured bubble sizes are likely underestimates because the fractured position of the frozen sample is not controlled and is unlikely to lie at the maximum bubble cross-section. Also, as the opening of the grid is 43 μm square, this method cannot obtain the size distribution of MNBs exceeding several tens of micrometers. However, our previous work48 shows that the size distribution is almost the same as that obtained by the light-scattering method. Moreover, our focus here is a qualitative comparison between replica samples prepared by the same procedure in different periods after the C2H6-hydrate dissociation. Thus, we adopt the estimation procedures for the MNB distribution via a replica method in the present study. Now consider the change of bubble distribution during the storage period of the solution. Figures 3 and 4 show the storage-time dependence of D and N, respectively, which are obtained from replica samples up to about 1 week after dissociation. Each error bar shows the average standard error of

3. RESULTS 3.1. Distribution of MNBs in C2H6-Hydrate-Dissociated Water. TEM images of the freeze-fracture replica samples indicate that a large amount of MNBs existed in the dissociated solution. Figure 1 shows typical MNBs observed on the replicas

Figure 1. TEM images of MNBs on replica samples. Time durations after C2H6 hydrate dissociation are (a) 0.5 h, (b) 1 h, (c) 3 h, and (d) 6 h. Each scale bar shows 500 nm.

prepared from the dissociated solutions stored from 0.5 to 6 h at room temperature (approximately 293 K). Most of them are circular or oval-shaped holes of diameter below 5 μm. By measuring the size and number of the holes included in a replica film, we obtained the MNB size distribution. Figure 2 shows three typical distributions: (1) just after the solid hydrate completely dissociated (0 h: open bar, n = 91), (2) three hours after dissociation (slashed bar, n = 135), and (3) 29 h after dissociation (solid bar, n = 141). This figure shows that the dissociated solutions include MNBs from below 100 nm to over 1 μm in diameter.

Figure 3. Average diameter of MNBs (D) in the solution versus storage period (solid circles). Each error bar shows the average standard deviation. Open square shows D for the C2H6-bubbling solution prepared by a commercially supplied generator (sampling: 2 h after generation). Open triangle shows the control value obtained by the replica film of pure water. 26622

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Figure 4. Average number density of MNBs (N) in the solution versus storage period (solid circles). Each error bar shows the average standard deviation. Open square shows N for the C2H6-bubbling solution prepared by a commercially supplied generator (sampling: 2 h after generation). Open triangle shows the control value obtained from pure water by using the replica film.

Figure 5. Temperature profiles in the high-pressure vessel at 2.5 MPa C2H6 pressure with pure water (dotted line), with the dissociated solution (thin solid line), and with C2H6-bubbling solution (thick solid line). The origin of the time axis is the time when the temperature reached 286 K, the equilibrium temperature of C2H6 hydrate at 2.5 MPa.2

the TEM image measurements from at least three samples. These figures confirm that in an earlier stage (from during the dissociation to approximately 3 h after dissociation) a larger number of larger sized MNBs existed in the solutions. In this stage, N is ∼2−5 × 109 mL−1, and D is around 1 μm. After further storage of the solution sample at room temperature, however, N gradually decreases by about a half, to ∼1−3 × 109 mL−1, and D also decreases to approximately 450 nm. These values remain for at least 30 h in the solution. The obtained N and D values are consistent to those in the C2H6-bubbling solution (shown as the open square in these figures). After 1 week of storage, N decreases less than the control (shown as the open triangle in these figures). The disappearance of MNBs formed by the hydrate dissociation indicates that the solution becomes undersaturated with guest molecules because dissolved guest molecules had left the dissociated solution during this storage period under the present storage condition. These changes on MNB distributions are qualitatively similar to those observed in CH4 hydrate-dissociated solutions,40 although the initial number concentration is slightly larger in the C2H6 case here. We conclude that dissociating gas hydrates produce a significant number of MNBs in water and that such MNBs maintain their number and size at least for more than 1 day at room temperature if stored in a sealed glass bottle. 3.2. Induction Time Measurements of C2H6 Hydrates and Memory Effect Observations. To investigate the memory effect on the C2H6-hydrate recrystallization, we measured the induction time of C2H6-hydrate crystallization with various sources of water including MNBs. These sources are pure water, the dissociated solution, and the C2H6-bubbling solution. To align the solution conditions in the outside of the pressure vessel, each source-water sample is prepared in a beaker prior to putting it into the vessel. For the C2H6-bubbling solution, we also checked the effect of storage time (from 0.25 to 48 h) and temperature (either room temperature (293 K) or in the refrigerator (277 K)). Figure 5 shows the typical temperature profiles in the C2H6hydrate crystallization with three source-water samples. The system of C2H6-hydrate crystallization is the same as that used for the original hydrate crystallization; that is, the nucleation time is recognized by a sudden increase of temperature. The crystallization occurs during the temperature decrease from room temperature to the bath temperature (274 K) and

involves two steps: (1) an initial small temperature increase, followed by (2) a large temperature increase. During the initial small temperature rise, the stirrer continued to work, indicating that only a fraction of the sample had crystallized. We determined the nucleation time (the end of the induction time) as the starting point of this initial small peak. During the subsequent, and larger, temperature increase, the stirrer stopped, indicating that C2H6 hydrate formed catastrophically, transforming most of the source water to C2H6 hydrates. Although the two-step crystallization process is interesting, we focus here on the induction time. The crystallization process of C2H6 hydrate, according to the temperature profile, is almost the same whether we used the dissociated solution or the C2H6-bubbling solution as the source water (Figure 5). Moreover, both the dissociation solution and C2H6-bubbling solutions have shorter induction times than that with pure water. Therefore, we confirmed experimentally that both cases with solutions including C2H6 MNBs expressed the memory effect of C 2 H 6 -hydrate crystallization, and this memory effect occurred under a stirred condition. We repeated the induction-time measurement of the C2H6 hydrates with these source samples several times. The results show that the induction times in MNB-containing source waters are significantly shorter than that in pure water (Figure 6) although the values show scatter (Table 1). When the C2H6bubbling solution was prepared, no gas-hydrate crystals had formed in the system. This finding argues against the hypothesis of the memory effect mechanism that a remnant of the hydrate-like structure remains in the water and promotes crystallization. We argue instead that C2H6 MNBs in the source water play an important role in the memory effect of C2H6hydrate crystallization. The various storage conditions of C2H6-bubbling solutions seem to have little effect on the induction time (see Table 1). This result indicates that, within these storage conditions, enough MNBs were stored in the solution to promote nucleation and is consistent with the TEM findings shown in Figures 3 and 4. We now ask whether or not guest molecules are required for expressing the memory effect. The high concentration of bulk MNBs indicates a large-area gas−liquid interface, which may 26623

DOI: 10.1021/acs.jpcc.6b07995 J. Phys. Chem. C 2016, 120, 26620−26629

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the same temperature. Of the three gases, the induction time of C2H6-hydrate crystallization with the N2-bubbling solution is the longest (Figure 7). Although the average induction time is

Figure 6. Average induction times of C2H6-hydrate crystallization in various source-water samples. Error bars show the standard deviation. * indicates P < 0.05 by the student t test. Figure 7. Average induction times of C2H6-hydrate crystallization in various gas-bubbling solutions. Error bars show the standard error. * indicates P < 0.05 by the student t test.

Table 1. Induction Time of C2H6-Hydrate Crystallization in Various Source-Water Samples source pure water

C2H6 hydrate dissociated solution (dissociated solution)

C2H6-bubbling solution

induction time [min] 24.5 34.5 2.5 5.0 67.0 40.3 7.5 45.0 6.5 6.7 13.0 2.5 21.5 15.0 4.4 7.5 4.3 6.0 5.0 5.5 0.5 4.0 4.6 3.6 5.8 6.5 5.4 1.5 4.4 4.1 2.5 2.0

slightly shorter with the N2-bubbling solution than that with pure water, statistical analysis shows that the N2-bubbling solution expresses no acceleration effect. On the other hand, the induction time with the Xe-bubbling solution is similar to that with the C2H6-bubbling solution, expressing a significant acceleration effect (Figure 7). The resulting data are listed in Table 2. It is very interesting that the C2H6-hydrate

remarks distilled & deionized water

Table 2. Induction Time of C2H6-Hydrate Crystallization in Source-Water Samples That Include Nonguest Molecules used after complete dissociation at room temperature (about 293 K)

source

induction time [min]

N2-bubbling solution

17.0 45.2 56.5 9.0 7.5 5.8 6.8 6.7 6.6 6.2 5.5 10.3 5.8 8.2 5.5 5.8 7.0 4.7 6.0

Xe-bubbling solution stored stored stored stored stored stored stored stored stored stored stored stored

at at at at at at at at at at at at

293 293 293 293 293 293 277 277 277 277 293 277

K K K K K K K K K K K K

for for for for for for for for for for for for

0.25 h 0.5 h 2h 4h 9.5 h 14.5 h 24 h 24.5 h 25.5 h 28 h 46 h 48 h

remarks stored stored stored stored stored stored stored stored stored stored stored stored stored stored stored stored stored stored stored

at at at at at at at at at at at at at at at at at at at

293 293 293 277 293 293 293 293 293 293 293 293 277 293 293 277 293 293 293

K K K K K K K K K K K K K K K K K K K

for for for for for for for for for for for for for for for for for for for

0.25 h 0.25 h 4h 6.5 h 16 h 18 h 21 h 25 h 40 h 0.25 h 1h 2h 3h 4h 5h 7h 26 h 47 h 122.5 h

crystallization under a stirred condition can be accelerated by MNBs with nonguest gas, but the acceleration magnitude depends on the kind of gas.

accelerate the heterogeneous nucleation of gas hydrates. So we compare the induction times of C2H6-hydrate crystallization with MNB-containing solutions from different gas sources, specifically C2H6, N2, and Xe. The N2 and Xe are selected because, compared to C2H6, N2 forms gas hydrate at a higher pressure, whereas Xe forms gas hydrate at a lower pressure at

4. DISCUSSION TEM observations on the freeze-fracture replica samples of the C2H6-hydrate dissociated water show that submicron-sized C2H6 MNBs are generated during hydrate dissociation, and their size and number distributions are comparable with those 26624

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applied gas pressure, only the bulk Xe-MNB is in a stable condition of its hydrate, and thus it would be more likely to nucleate hydrate compared to bulk N2-MNBs. This is one possible reason for the observed induction-time difference depending on the type of gas. The obtained result (Figure 7) agrees with this predicted trend, so we argue that bulk MNBs accelerate hydrate nucleation more than surface MNBs; thus the bulk MNBs produce the memory effect. How might the stirrer affect the observed hydrate formation? In general, agitation has little effect on the generation and stability of surface MNBs,44 but the effect is unclear for bulk MNBs. Bulk MNBs are formed using high-speed circulation (see, e.g., ref 55); thus, our stirrer could generate some bulk MNBs, but the amount is likely insignificant. The stirring should also not reduce the number of bulk MNBs by promoting their coagulation because each MNB is predicted to have the same sign of surface charge and thus mutually repel.36,55 Therefore, we conclude that our stirrer likely did not significantly change the bulk MNB distributions. We now discuss how bulk MNBs may have led to the observed memory effect. At first, we observed a significant shortening of the induction time of C2H6-hydrate crystallization with 300 rpm agitation. This shortening occurred both in the dissociated solutions and in the C2H6-bubbling solutions. As the latter solution cannot contain any “residual structure” of hydrates, we conclude that the relic structure of water itself is not needed to express the memory effect. This is consistent with the experimental investigation of Xe-hydrate crystallization observed with the synchrotron X-ray computed tomographic microscopy31 and with the recent review of Ripmeester and Alavi.29 The X-ray computed tomographic study indicated that the memory effect implies an enrichment of guest molecules in the water phase as nanometer-sized gas aggregation in the liquid (NBs). It is interesting that the acceleration effect on the C2H6hydrate crystallization was observed with water having MNBs containing nonguest molecules and that the effect depends on the type of gas. If the existence of MNBs merely provides a large gas−liquid interface area, then MNBs having a similar size and number distribution should express a similar acceleration effect on the gas-hydrate crystallization, independent of the gas type. We also would expect a similar effect if some impurities other than MNBs affect the nucleation acceleration. The present experiments show, however, that Xe MNBs have a significant acceleration effect similar to C2H6 MNBs but that N2 MNBs show only a small effect. Therefore, we consider that the impurity imprinting mechanism on the memory effects is not important under the present conditions. Conversely, it is also possible that some kinds of MNBs themselves may work as the nucleation site of gas hydrates. The effect of the storage conditions of C2H6-bubbling solution (storage temperature and storage duration) on the memory effect was not significant. Nevertheless, the storage temperature has a critical value for maintaining the memory effect. For example, Sloan and Koh2 summarized that the memory effect is destroyed at temperatures greater than 301 K or after several hours of heating. Takeya et al.25 also showed that the memory effect vanished when the solution was kept at 298 K for 1 h. Since the storage temperatures in the present study were 293 and 277 K, both below 298 K, our conclusion is consistent with previous studies. The critical time period beyond which the memory effect vanishes, on the other hand, has not been precisely determined, but long storage duration

of the C2H6-bubbling solution. The consistency of this result with our previous results40 and with the prediction by MD simulations31−35 supports the general result that gas-hydrate dissociation in water produces MNB-including solution. Now we consider how these MNBs formed after gas-hydrate dissociation. Based on the crystal structure of gas hydrates, a guest molecule is surrounded by several H2O molecules. When the structure is dissociated, the guest molecules are released one by one into the liquid solution. Thus, a guest molecule dissolves into liquid water instead of coagulating at the dissociating hydrate boundary. The buildup of solute molecules immediately produces a supersaturation condition for the gas− liquid interface. From this condition, guest gas bubbles can nucleate and grow. As dissociation is not static, the bubble nucleation can occur not only on the solid surface but also in the bulk solution by the heterogeneities of thermal and density conditions. Indeed, due to the absorption of latent heat, the region near the dissociating hydrate should be cooler and thus more likely to nucleate bulk MNBs. These processes have been predicted by the MD simulations31−35,39 and by experiments.50 Our replicas show that we have bulk MNBs, but surface MNBs should also exist on the solid surfaces of our pressure vessel and stirrer.36,41−44 So we next consider the effect of surface MNBs on the hydrate formation in this study. During the setting-up of the experiment, we inject 50 mL of source solutions that include bulk MNBs through a beaker to align the experimental conditions in the outside of the pressure vessel. When surface MNBs are generated in the system, they will deplete some of the gas in the existing bulk MNBs, thus modifying their distribution. However, the system is sealed soon after the injection of the solution, and subsequently the C2H6 gas is pressurized. The solution becomes supersaturated again, making the bulk MNB distribution become stabilized, whereas the same surface MNBs remain.43,51,52 The obtained results show that the induction times of C2H6 hydrate formation with the solutions including C2H6 MNBs are significantly shorter than that with pure water. Thus, this shortening of the time is clearly caused by the existence of MNBs in the system. We conclude that MNBs, whether bulk or surface, accelerate the hydrate formation process. We now offer a possible argument against the role of surface MNBs on nucleation. As the reaction vessel is the same in all experiments, the generation condition of surface MNBs should be the same. Also, both the size distribution and the internal pressure of surface MNBs (which is close to atmospheric pressure) are not sensitive to the kind of gas.36,45 Under an applied hydrostatic pressure, surface MNBs will decrease only their thickness due to their being “pinned” to the solid surface.52 Thus, surface-MNB size is less sensitive to applied pressure (and thus gas type) than bulk MNBs. Also, since their internal pressure is small, the change of morphology of surface MNBs would have only a small effect on the surrounding conditions such as the amount of gas molecules dissolved in the solution. Therefore, if the nucleation came mainly from surface MNBs, then we should not have observed the induction time depending on the type of gas in the MNBs. On the other hand, bulk MNBs are expected to accelerate hydrate nucleation because they have internal pressures of several MPa.40,53 Even under an applied 2.5 MPa pressure, the size of bulk MNBs would not be different, but the conditions surrounding the bubble are changed. At a temperature of 274.2 K, the dissociation pressures of Xe hydrate and N2 hydrate are 0.16 MPa54 and 17.73 MPa,2 respectively. So, at 2.5 MPa 26625

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The Journal of Physical Chemistry C tends to reduce the memory effect. The present study shows experimentally that the memory effect of C2H6-hydrate recrystallization occurs with the MNB containing solutions stored for at most 2 days (Table 1). This duration is consistent with the period that a large amount of bulk MNBs can remain in the solution (Figure 4) and is roughly consistent with other MNB studies.40,44 Therefore, we consider that the memory effect on gas-hydrate crystallization is expressed as long as the MNBs survive in the solution. Based on our previous experiments on O2-bubbling solutions,49 for the MNBs to survive, the solution must be kept at a supersaturation condition with the O2 gas. Also, the lifetime of the bulk MNBs depends on the shielding condition of the solution during its storage. (The shielding is done to reduce the release rate of dissolved gas.) Therefore, the critical time period for the memory effect may depend on the shielding condition of the solution during its storage. About the memory-effect mechanism, we conclude that neither the residual structure nor the impurity imprinting is dominant in the present study conditions. Instead, the existence (or generation) of bulk MNBs in the solution is important for expressing the memory effect. These MNBs act as a guest molecule buffer to maintain the solution in a supersaturation condition. They also increase the gas−liquid interfacial area and are expected to work as a heterogeneous nucleation site for gas hydrates. This phenomenon is usually observed in various crystallization experiments56,57 and simulations.33 These arguments suggest that the guest supersaturation mechanism is dominant for the memory effect. Finally, another possible role of the existence of bulk MNBs on the acceleration of the hydrate nucleation is as follows. If some bulk MNBs collapse due to agitation or the shrinkage of the bubble, they would produce small shock waves that trigger nucleation. Indeed, the collapse of bulk MNBs occurs even under a static condition in which it has been found to release energy high enough to generate reactive oxygen species.58 But we do not consider this mechanism further. We now discuss how MNBs increase the nucleation probability based on the analysis used for the static condition experiments. The cumulative probability distribution function at time t, Pi(t), can be expressed by a nucleation rate Ji in a given volume of liquid as25 P i(t ) = 1 − exp[−J i (t − to i)]

Figure 8. Nucleation probabilities versus induction time for different source solutions: pure water (open circles), C2H6-hydrate dissociated water (solid squares), C2H6-bubbling solution (solid circles), N2bubbling solution (solid diamonds), and Xe-bubbling solution (solid triangles). Each dashed or dotted line is the fitting curve from eq 1.

Table 3. Nucleation Rate J Estimated by Fitting to Equation 1 and the Expected Induction Time ⟨Δtind⟩ for Various Source-Water Samplesa source water pure water C2H6 hydrate dissociated water C2H6-bubbling solution N2-bubbling solution Xe-bubbling solution a

τ0 [min]

J [× 103 min−1]

⟨Δtind⟩ [min]

13.6 (4.0) 4.6 (0.2)

4.6 (13) 509 (117)

― 12.3

3.1 (0.2) 6.7 (0.2) 5.7 (0.1)

463 (48) 728 (215) 1169 (233)

12.6 5.7 14.5

The number in parentheses is the standard error for fitting.

the solution includes bulk MNBs but that the difference in different kinds of gases is not clear. To estimate the magnitude of the memory effect, we apply the quantification method proposed by May et al.59 in Figure 8. We estimate the expected induction time for each source i, ⟨Δtind⟩, as the area enclosed by the two cumulative probability curves between Pi(t) and Pw(t) and the horizontal lines of P(t) = 0 and 1. The values estimated by the method of finite difference are listed in Table 3 (right column). They indicate that the magnitude of the memory effect of the dissociated solution is equivalent to that of C2H6-bubbling solution. The Xe-bubbling solution has a memory effect similar to or slightly larger than that of C2H6-MNB solution, but the N2-bubbleing solution has a smaller effect. Thus, this analysis provides quantitative support for the experimental results in Figures 6 and 7. Now consider why the memory effects of the C2H6-hydrate crystallization depend on the type of gas. If MNBs increase the gas−liquid interface area, which provide the nucleation sites, C2H6 MNBs would have the largest acceleration effect on the C2H6-hydrate crystallization because they would act not only as the nucleation sites but also as the source of the guest molecules in the solution. So the Xe MNBs would play a particular role on the gas-hydrate nucleation. The solubility of C2H6 molecules in water at 101.325 kPa and 298.2 K is 3.40 × 10−5 mol/mol.60 For Xe, the value is about 2.3 times higher, but for N2, the solubility is a factor of only 0.35.60 In addition, the interior pressure of bulk MNBs is as high as several MPa.40,54 Such a high pressure suggests that a large amount of dissolved Xe atoms and C2H6 molecules should surround the MNBs. The

(1)

where i is the type of source water (w: pure water, H: dissociated water, C2: C2H6-bubbling solution, Xe: Xe-bubbling solution, N2: N2-bubbling solution) and τoi is an onset time of nucleation. The nucleation probability at t = tm is given by Pi(tm) = m/n, where m is the number of times nucleation occurred on or before time tm and n is the total number of experiments with the same kind of water source. The variation of Pi(t) with different source waters is shown in Figure 8. The parameters J and τo are obtained by fitting eq 1 to our data (OriginLab, OriginPro 9.0J) and listed in Table 3 (third column from the left). The relative nucleation rate Ji/Jw in various source water i to Jw in pure water shows that every Ji/Jw increases by a factor of 102. We expect that JH and JC2 have similar values. The largest value of J is observed in Xe-bubbling solutions. This suggests that Xe molecules would provide the most effective nucleation site for C2H6 hydrate. The reason for the larger value of JN2 is not clear but may arise from the large uncertainty in the fitting process. We conclude that the nucleation rate of C2H6 hydrate accelerates significantly when 26626

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The Journal of Physical Chemistry C

storage material of guest molecules in the water but also a heterogeneous nucleation site for gas-hydrate crystallization.

resulting structure in the solution should increase the viscosity.61 If so, we can speculate that the water molecules surrounding the Xe and C2H6 MNBs may have a highly hydrogen-bonded local network, as was proposed previously.62 These possibilities are supported by Xe-hydrate experiments. Takahashi et al.55 measured the Xe-hydrate formation in the Xe-MB solution. They cooled a solution with Xe MBs having sizes broadly distributed about 25 μm at room temperature down to 274.6 K and obtained Xe-hydrate particles of about 15 μm diameter. This result suggests a high potential of Xe MNBs as heterogeneous nucleation sites for gas hydrates. Another example of an anomalous Xe role on nucleation acceleration was reported in a study of hyperpolarized 129Xe NMR spectroscopy observations.63 Prior to the Xe hydrate (structure I) crystallization, Xe encaged into both cages of structure II THF hydrate, even though it is a metastable state. Thus, in this case, the dissolved and absorbed Xe would interact with surrounding H2O molecules to construct a hydrogen-bonded framework. Even though no Xe-hydrate crystals formed, at about 283.2 K and 0.5 MPa of Xe pressure, Raman spectroscopic analysis detected water structurization near the plasma membrane of a neuron.64 Therefore, the water molecules with a high concentration of Xe, such as in the region surrounding a Xe MNB, tend to form a highly hydrogenbonded network, and this network would have a higher probability than a less hydrogen-bonded network to serve as a heterogeneous nucleation site for gas hydrates. Conversely, N2 MNBs would merely provide a large area of gas−liquid interface. So they may accelerate the heterogeneous nucleation a little but less than that with Xe or C2H6. For the gas-bubbling solution, another possible acceleration mechanism for hydrate nucleation is the mixed-gas effect. If some dissolved C2H6 molecules pass into bulk MNBs of Xe or N2, the MNBs become Xe+C2H6 mixed-gas MNBs or N2+C2H6 mixed-gas MNBs. As Xe+C2H6 mixed-gas hydrate has a lower dissociation pressure than that of C2H6 hydrate, the Xe+C2H6 mixed-gas MNBs may help nucleate gas hydrates due to the larger driving force. However, N2+C2H6 mixed-gas MNBs have the opposite effect. Therefore, Xe MNBs can accelerate the nucleation of C2H6 hydrate, but N2 MNBs cannot. Thus, if this mechanism was active in our experiments, it must have had relatively little influence on the results.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-11-706-6635. ORCID

Tsutomu Uchida: 0000-0003-2304-376X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partly supported financially by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Grant No. 25600035 and 26105009) and cross-ministerial Strategic Innovation Promotion Program (SIP). TEM observations were financially supported by the Nanotechnology Platform program and technically supported by Drs. N. Sakaguchi, K. Ohkubo and T. Tanioka (Hokkaido Univ.). The authors acknowledge to the anonymous reviewers for their useful suggestions.

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REFERENCES

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5. CONCLUSIONS The formation of micro- and nanobubbles (MNB) after C2H6hydrate dissociation in water was identified using transmission electron microscopic observations of freeze-fracture replicas. The bulk MNB size distribution stabilized within 30 h of dissociation, reaching an average diameter of about 450 nm and a number concentration of the order 109 mL−1. These values are consistent with those observed in CH4 hydrate-dissociated solution,40 which suggests that gas hydrates in general will generate large amount of MNBs when they are dissociated in water. The induction time of C2H6-hydrate crystallization was measured under stirring conditions with various sources of water to investigate the effect of MNBs on the memory effect. Compared to pure water, the C2H6 hydrate-dissociated solution, the C2H6-bubbling solution, and the Xe-bubbling solution all expressed significantly shorter induction times, whereas the N2-bubbling solution did not. These results confirm that MNBs play important roles in the memory effect of gas-hydrate crystallization. Thus, MNBs are not only a 26627

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