Gas hydrate crystallization in thin glass capillaries: roles of

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Gas hydrate crystallization in thin glass capillaries: roles of supercooling and wettability Abdelhafid Touil, Daniel Broseta, and Arnaud Desmedt Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01146 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019

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Gas hydrate crystallization in thin glass capillaries: roles of supercooling and wettability Abdelhafid Touil,∗,†,‡ Daniel Broseta,∗,† and Arnaud Desmedt¶ †CNRS/TOTAL/UNIV PAU PAYS ADOUR E2S UPPA,Laboratoire des Fluides Complexes et de leurs Réservoirs, UMR 5150, 64000, Pau, France ‡Sonatrach - Direction Centrale de Recherche et Développement, 35000, Boumerdes, Algérie ¶Univ. Bordeaux, CNRS, ISM, UMR 5255, F33405 Talence France E-mail: [email protected]; [email protected] Phone: +33 (0)5 59 40 76 85

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Abstract We design and implement an experimental methodology to investigate gas hydrate formation and growth around a water – guest meniscus in a thin glass capillary, thus mimicking pore-scale processes in sediments. The glass capillary acts as a high-pressure optical cell in a range of supercooling conditions from 0.1 ◦ C, i.e. very close to hydrate dissociation conditions, to ∼ 40 ◦ C, very near the metastability limit. Liquid or gaseous CO2 is the guest phase in most of the experiments reported in this paper, and N2 in a few of them. The setup affords detailed microscopic observation of the roles of the key parameters on hydrate growth and interaction with the substrate: (i) supercooling below the dissociation temperature, (ii) substrate wettability to the host and guest phases. At low supercooling (less than 0.5 ◦ C), a novel hydrate growth process is discovered, which consists of a hollow crystal originating from the meniscus and advancing on the guest side along glass, fed by a thick water layer sandwiched between glass and this crystal.

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Introduction Gas hydrates are non-stoichiometric crystalline solids made up of cages of water molecules

trapping a different kind of (’guest’) molecules, such as CO2 , N2 , CH4 and other lowmolecular-weight alkanes. 1 Interest in gas hydrates is growing, motivated by their unique gas storage capacity, guest selectivity, heat of formation/melting (comparable to that of ice), and by their ubiquitous presence on Earth – in permafrost and in deep marine sediments. With both liquid water and guest phases present, gas hydrates usually nucleate at a water – guest interface, which then quickly covers itself up with a thin polycrystalline crust. The morphological features and growth rate of this crust along this interface strongly depend on the driving force or distance in temperature to the hydrate dissociation (equilibrium) conditions, i.e. Teq − T . 2–7 When the driving force increases, the morphology evolves from a tessellation of large crystallites to a smooth texture made up of very small crystallites, and the water - guest interface covers itself up more quickly. The thickening of this crust is extremely slow, 8,9 witnessing its near-impermeable character. Subsequent gas hydrate expansion from the crust into the bulk of the aqueous and/or guest-rich phases 10–13 appears to depend on the initial compositions of these phases. 14 When the hydrate crust growing at the water - guest interface encounters a solid substrate, its further growth is influenced not only by the driving force but also by substrate wettability. Further growth of the crust on the substrate is observed to occur under the guest phase for high-energy (e.g., polar), water-wet substrates such as glass or metal, but not on low-energy, intermediate-wet substrates. 15–18 This growth is mediated by a thin wetting water layer between the substrate and the crust, as hypothesized 1–2 decades ago by Tabe and coworkers 15 and by Beltran and Servio 16 and recently demonstrated experimentally in our laboratory from optical microscopy observations of cyclopentane hydrate crusts continuing their growth from the water–cyclopentane meniscus over the inner wall of thin glass capillaries. 19 In the latter study, the morphology and growth rate of the crust advancing on glass 3


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(the halo) were observed to be similar to those of free hydrate crusts, i.e., growing at the water – guest (cyclopentane) interface, at least when the wetting water layer was conductive enough to water. With guest-wet (silane-treated) substrates, the hydrate crust was observed to continue its growth over the substrate as well, but on the guest side of the water – guest interface, fed by a wetting layer of cyclopentane. 19 These experiments were conducted at atmospheric pressure, where cyclopentane hydrate is stable for temperatures below ∼ 7 ◦ C. This raises the question of whether the observed behavior is representative of that of common gas hydrates, which form at higher pressures. 20–22 We address this question in this paper, which reports and discusses observations of hydrate crystallization around the water – guest meniscus by means of an experimental setup and procedure adapted to a pressurized guest phase, here CO2 and N2 . These capillaries can be viewed as model (cylindrical) pores and therefore we expect our experiments to shed light on gas hydrate formation and growth processes in pores initially saturated with both water and free gas. These processes, which have recently been examined in sedimentary matrices by high-resolution (synchrotron) X-ray computerized tomography (CT), 23 differ from those encountered in fully water-saturated pores. Gas hydrate shells form over the water/gas interfaces (menisci) and the siliceous substrate, with a thin water layer present between the hydrate shell and the substrate. 23 Such features are consistent with those observed in simple substrate/fluid settings. 15,16,19 The outline of this paper is as follows. We present in next section the experimental setup and procedure, which allow optical microscopy observations of the region of the water – guest meniscus in a thin glass capillary under controlled temperature and pressure. The guest molecule is CO2 either gaseous or liquid, depending on pressure, or N2 . These observations (conducted at LFCR) are complemented by confocal Raman spectroscopy (conducted at ISM) to unambiguously identify the various phases present. The experimental results are then presented and discussed in the order they are obtained 4


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in the course of an experiment. We thus first present and discuss the observations obtained under the very strong supercooling (or very low temperature) conditions required for primary hydrate nucleation, which indeed occurs on the meniscus. Subsequent growth on and beyond the meniscus, i.e. in the bulk phases and over the glass substrate, are addressed in the first of the three sections reporting and discussing the experimental results (section 3). The second of these sections is concerned with conditions similar to those addressed in the previous section, except for substrate (glass) wettability: the inner wall of these capillaries is rendered non-water-wet by silane treatment. These experiments provide insights into the effects of substrate wettability on gas hydrate formation and growth (section 4). The third section presents results obtained by taking advantage of the very precise temperature control allowed by our experimental setup and procedure. It reports observations of hydrate growth under moderate supercooling, obtained by raising the temperature once primary hydrate formation and growth has occurred, as well as under very low supercooling, i.e. very close to the hydrate dissociation conditions, where unusual hydrate morphologies are observed. Conclusions and prospects are presented in the last section. Most of these experiments involve CO2 , and a few of them N2 , as the guest phase; the experiments with N2 are just mentioned briefly in the main text, more details being disclosed in the Supplementary Information.

2 2.1

Materials and methods Experimental setup The experimental setup is schematically depicted in Figure 1. Unless otherwise spec-

ified, the capillaries used in this study are round capillaries (VitrotubesT M ) made of fused silica (referred to as ’clear fused quartz’), with nominal internal and external diameters 200 5


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and 330 µm, respectively, and a length of 10 cm. The capillary is filled with the aqueous solution of interest (here, distilled water) over a length of ∼ 10 mm, flame-sealed at one of its end and then centrifuged in order to displace the aqueous phase towards the sealed end of the capillary. The capillary is then placed in a heating-cooling stage (Linkam CAP500). One important element in this stage is a silver block with a rectangular slot, where the capillary is inserted and which ensures a good thermal homogeneity: over a lateral distance of ±12.5 mm around the central observation hole, temperature T varies by less than 0.2 ◦ C for temperatures as high as 100 ◦ C. 24 T is measured by means of a platinum sensor and its stability is ensured to within ±0.1 ◦ C by means of a Linkam T95 controller and a PE95 nitrogen pump, and its accuracy is checked regularly against the melting points of ice and CO2 . An IR filter is positioned between the light source and the condenser of the microscope (an Olympus B50) to prevent any local heating of the observation zone. The open end of the capillary is inserted and glued (with cyanoacrylate or epoxy adhesives) inside a stainless-steel tube (outer diameter 1/16"), itself connected to a high-pressure pump (ISCO DM65) containing the guest phase (CO2 or N2 ). Pressure is checked with a digital manometer (Keller LEO2). The air initially present in the capillary is eliminated by moderate pressurizing with gas followed by purging through the 3-way valve nearest to the capillary (Figure 1). The pump, the camera (Ueye UI 3360) and the temperature controller are driven by a home-made software application developed using the Qt C++ language.

2.2

Experimental procedure The observations are carried out near the meniscus between the water and guest phases.

This meniscus is spherical, with water strongly wetting the silica capillary walls, i.e., the contact angle in water is low. Some experiments are conducted with silane-treated glass capillaries identical to those used in a recent study (at LFCR) on the contact angles of water – CO2 systems: the contact angle is ∼ 90 − 100 ◦ when CO2 is gaseous (at low pressure),

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Objective

Stainless steel tube

Gas reservoir

guest

water

Fused silica capillary

Heating/Cooling stage Lamp

Optical microscope with camera Raman spectrometer

Digital manometer

Valve High-pressure pump Temperature controller Liquid nitrogen dewar

Figure 1: Schematics of the experiment. Top: enlarged view of the capillary, one of its ends being sealed and the other inserted in a stainless-steel tubing connected to the gas vessel with pressure control, here a high-pressure pump.

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but rises to ∼ 160 ◦ when CO2 is liquid, i.e., at pressures higher than the CO2 saturation pressure (∼ 60 bar at 22 ◦ C). 25 The initial temperature, T = 15 to 20 ◦ C, is well outside the stability domain of the studied gas hydrates. In the case of previous hydrate presence in the capillary, the system is left at this temperature for a long enough time (a few tens of minutes) in order to avoid any subsequent ’memory effect’; i.e. the easier hydrate formation that occurs when the aqueous phase has contained ice or gas hydrate shortly before. This effect is exploited in some of the experiments to nucleate the hydrate phase under intermediate or low supercooling conditions. All experiments are carried out at constant pressure and start by chilling the capillary (loaded with water and the guest phase) at a constant rate (20 ◦ C/min), until the appearance of the solid (crystal) phase(s), ice and/or hydrate. The meniscus is observed to move during cooling, witnessing variations in water density with temperature. The temperature descent is stopped as soon as the solid phase (ice or hydrate) appears: due to the small sample volume and rapid cooling rate, this occurs for a temperature Tn well below the equilibrium melting temperature of the solid phase at the pressure of the experiment Teq , i.e. for strong supercooling ∆Tn = Teq − Tn . Tn is the temperature of primary nucleation, and Teq is determined experimentally once the solid phase has been formed, by very slowly increasing the temperature until the last crystal disappears. In most instances, the solid phase is only made up of hydrate, which starts forming on the meniscus as detected by an abrupt change in its appearance (Figure 2a) However, in a few instances, especially at low pressure (a few bar, typically), the bulk water freezes into ice, which forms on the inner wall and propagates rapidly (within less than a second) towards the center of the capillary. Ice has a grainy, polka-dotted texture due to the presence of gas inclusions, and its expansion causes the protrusion of ice into the guest side of the meniscus (Figure 2b). There is no occurrence of ice formation in the experiments reported below.

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Water

CO2 gas

Hydrate crust

Ice CO2 liquid (a)

(b)

Figure 2: Hydrate and ice are readily and unambiguously distinguishable.(a) CO2 hydrate formation on the meniscus between water and liquid CO2 in a capillary (internal diameter: 200 µm) at 30 bar and Tn ∼ −27 ◦ C (a). (b) Ice formation (6 bar and -32 ◦ C).

Table 1 lists the experimental conditions examined in this study with CO2 as the hydrateformer. Either bare (i.e. untreated) or silane-treated glass capillaries are used in the experiments, which are reported respectively in sections 3 (Strong supercooling conditions) and 5 (Moderate to low supercooling) and in section 4 (Effects of substrate wettability), respectively. Each line in this Table displays the (constant) pressure (from 10 to 47 bar) of the isobars investigated, together with the measured equilibrium temperature Teq and primary nucleation temperature Tn and maximum supercooling ∆Tn = Teq − Tn at that pressure. Despite being nucleation temperatures, the Tn ’s turn out to be reproducible to within 1 ◦ C, as presented in next section. The values we observe for Teq are in line with literature values. Figure 3 is a graphical version of this Table, with the investigated experimental isobars shown as horizontal lines in the T − p plane. These lines extend down to Tn and cross the equilibrium liquid three-phase water-hydrate-CO2 (Lw –H–VCO2 ) line (at Teq ).

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Table 1: Conditions of the experimental isobars investigated: pressure and corresponding equilibrium temperature Teq , primary nucleation temperature Tn and maximum possible supercooling ∆Tn .

p, bar Bare glass 30 24 22 10 Silane-treated glass 47 24 14

Teq , ◦ C

Tn , ◦ C ∆Tn , ◦ C

7.0 5.3 4.7 -2.0

-27 -27.2 -27.5 -31.0

34.0 32.5 32.2 29.0

9.8 5.3 1.0

-22.0 -26.0 -28,5

31.8 31.3 29.5

50

w -V CO 2

45 40

H- L

Pressure (bar)

35 30 25 20 15 10 5 0 -32

-30

-28

-26

-24

-22

-20

-2

0

2

4

6

8

10

Temperature (°C)

Figure 3: Experimental isobars (horizontal dashed lines) extend down to the primary nucleation temperatures Tn (black triangles for silane-treated glass and black squares for bare glass) and cross at Teq the equilibrium liquid water-hydrate-CO2 three-phase line (Lw –H–VCO2 ).

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2.3

Phase assignment by Raman spectroscopy Complementary to optical microscopy, confocal Raman spectroscopy provides in-situ

information as to the nature and composition of the various phases. 26 Raman spectra acquisitions at different locations in the horizontal plane passing through the center of the capillary (focal plane) are performed with a HRevo microspectrometer (Horiba Jobin Yvon, Villeneuve d’Ascq, France) using a 532.18 nm wavelength laser as excitation source. The aperture of the confocal hole is at 100 µm to optimize the signal/noise ratio with respect to the focusing-collecting optics and to allow the focusing of the incident laser beam on µm-size parts of the sample with a 50X objective (Olympus). Thanks to the 800 mm focal distance and to the dispersion of scattered signal with a holographic grating of 600 lines/mm, the resulting spectral resolution is 1.5 cm−1 (half-width at half-maximum) with a Peltier-cooled CCD detector (Andor, Belfast, UK). The CAP500 Linkam stage is mounted on a motorized microscope stage (Marzhauser SCAN 75 X 50) that allows a precise control of the position along the two horizontal directions with sub-micrometric repeatability and a resolution of 0.01 µm. The objective of the microscope is also motorized, and its vertical displacement controlled to within 0.1 µm. As an example, Figure 4 displays the Raman spectra acquired on both sides of the water – CO2 meniscus, and on the meniscus itself with its hydrate crust, in an experiment conducted at 22 bar. The CO2 hydrate was formed at -27.5 ◦ C, and then the temperature raised and stabilized at -20 ◦ C: under these conditions, the CO2 is liquid. Three spectral regions are highlighted in Figure 4: the low-wave-number region (100 – 500 cm−1 , Figure 4b) witnessing intermolecular H 2 O external or "O–O" modes, the intermediate region (1200 – 1500 cm−1 , Figure 4a) probing the CO2 Fermi resonance bands, ν + and ν − , resulting from the coupling of the symmetrical stretching mode ν1 and of the harmonic of the bending mode 2ν2 , 27,28 and the high-wave-number region (2800-3800 cm−1 , Figure 4c) featuring water "O– H" stretching modes. The positions of the peaks in the intermediate region (Figure 4a) vary 11


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slightly with the environment of the CO2 molecules, whether enclathrated in the hydrate cages, dissolved in (supercooled) liquid water, in the CO2 -rich liquid or in the CO2 -rich gas (the latter obtained by heating to 6 ◦ C): in particular, the two Fermi resonance bands of the enclathrated CO2 are shifted towards lower values (1280 cm−1 and 1384 cm−1 ) in comparison to the CO2 molecules in the gas phase (1289 cm−1 and 1391 cm−1 ). It should be noted here that it is not possible to distinguish the CO2 enclathrated within the large cages from that in the small cages of the structure I hydrate, because the corresponding Raman signatures are too close to each other. 29 The difference between the CO2 in the hydrate and the CO2 in liquid water also resides in the width of the bands and in the relative intensities of the two Fermi resonance bands (Figure 4a). Another possibility of differentiating the aqueous phase from the hydrate consists in analyzing the intermolecular low-frequency region (Figure 4b), where the lattice modes indicate the existence of a solid phase, hydrate or ice. The lattice mode of a gas hydrate has a lower vibrational frequency (212 cm−1 ) than that of ice: the hydrogen bonds of a hydrate crystal are distorted and weakened compared to those of hexagonal ice (Ih), in which water molecules form perfect tetrahedral sites. The reverse behavior is expected at high wave numbers (O–H stretching), since the distortion of the hydrogen bonding network leads to the strengthening of the O–H bonds (Figure 4c). These spectral assignments are consistent with those determined by previous authors in dedicated sapphire cells 30 and in thin glass capillaries similar to ours. 31–34 Some Raman spectra of gaseous N2 and N2 hydrate on the water – N2 meniscus in the relevant pressure and temperature conditions can be found in the Supplementary Information, together with the procedure used to extract the intensity ratios of the different species.

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1200

(a)

1391 cm-1

1389 cm-1 1384 cm-1

CO2 hydrate CO2 liquid CO2 gas CO2 in water

1284 cm-1 1280 cm-1

1250

1300

212 cm-1

1350 Wavenumbers (cm-1)

1400

1450

1500

(c)

(b) 3164 cm-1

CO2 hydrate

intensity (a.u.)

intensity (a.u.)

1289 cm-1

intensity (a.u.)

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CO2 hydrate aqueous phase

aqueous phase 150

250 350 Wavenumbers (cm-1 )

450

2800

3000

3200 3400 3600 Wavenumbers (cm-1)

3800

Figure 4: Characterization of the various phases using Raman spectroscopy. The spectra of the CO2 hydrate are compared to those of liquid CO2 (a), CO2 -saturated liquid water at 22 bar and -20 ◦ C (a, b and c) and gaseous CO2 at 22 bar and 6 ◦ C (a).

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3

Strong supercooling conditions This section presents and discusses the optical microscopy observations of primary

CO2 hydrate formation on the water – CO2 meniscus and subsequent hydrate growth in bare (untreated) glass capillaries. All experiments are carried out under constant pressure (isobaric conditions) and start by chilling the capillary at the constant rate of 20 ◦ C/min, until some CO2 hydrate freezes on the meniscus. This freezing is detected from a sudden deviation of the meniscus from sphericity and a change in texture witnessing the presence of a polycrystalline crust (Figure 2a). It occurs at a temperature Tn of about -30 ◦ C identified here as the temperature of primary nucleation. The temperature descent is then stopped, and the ensuing hydrate growth processes are monitored on both sides of the meniscus. Consistent with the published observations (see the discussion below), at Tn the water does not freeze into ice (or it does so on very rare instances). Because of the oblique view on the meniscus and the relatively low speed of our camera (< 4 fps), the precise location of hydrate nucleation cannot be observed, and the advance of the hydrate polycrystalline crust front on the meniscus is too fast to be detected. Snapshots from two experiments are displayed in Figures 5 and 6, which show the nucleation and subsequent growth of the CO2 hydrate observed around the meniscus between liquid water and, respectively, liquid CO2 (p = 30 bar) and gaseous CO2 (p = 10 bar), just before (snapshot a) and after (snapshots b to d) CO2 hydrate nucleation has occurred on the meniscus, at Tn = −27 ◦ C (liquid CO2 ) and Tn = −31 ◦ C (gaseous CO2 ). At the same time (or immediately after, i.e., within 1/4 s) the meniscus has been covered with the crust, two remarkable features become apparent:

(i) CO2 hydrate "fibers" expand at a very high velocity (> 100 µm/s) from the hydratecapped meniscus into the bulk water. They are barely visible right after the hydrate crust has been formed on the meniscus, and become increasingly contrasted with time 14


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(compare snapshots c and d in Figures 5 and 6). Recall that the refractive indices of water and CO2 hydrate are very close 35 and the volume fraction occupied by these hydrate fibers is very small, in the order of a few percent, as they are grown from the CO2 initially dissolved in the supercooled liquid water (see section 5.1). (ii) A CO2 hydrate "halo" is seen advancing from the contact line over glass on the CO2 side of the meniscus. This halo is thicker near the contact line than at its leading edge (see snapshots 5c and 5d). Its initial velocity is ∼ 10 µm/s, and it slows down after having propagated over a few tens to hundreds of µms on glass, until coming to near arrest.

t=1s

t=0 Water

CO2 liquid

Hydrate crust

(a)

(b)

t = 16 s

t = 26 s Hydrate halo

(c)

(d)

Figure 5: CO2 hydrate formation and growth from the water – CO2 meniscus, here at 30 bar and Tn ∼ −27 ◦ C. Within less than 1 s (from a to b), the CO2 hydrate has nucleated on the meniscus and coated it with a thin polycrystalline crust. A CO2 hydrate halo then advances along the capillary wall. Fibers growing from the meniscus towards the bulk of the water are apparent in (d).

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t=0 water

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t = 0.25 s CO2 gas (a)

(b)

t=1s fibers

hydrate crust

t = 1.25 s halo (c)

(d)

Figure 6: Same as figure 5, but p = 10 bar, Tn ∼ −31 ◦ C. Within less than 1 second (from a to b), the CO2 hydrate has nucleated on the meniscus and covered it with a thin polycrystalline crust. The CO2 hydrate then propagates on both sides of the meniscus, as a halo spreading on glass on the CO2 side and as fast-growing fibers on the water side. Fiber velocity is estimated in the range of 200 µm/s..

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Similar features are observed with the water + N2 system, as is described in the Supplementary Information (SI). The primary nucleation of N2 hydrate takes place on the meniscus between water and gaseous N2 at very strong supercooling (Tn ∼ −37 ◦ C for p = 110 – 240 bar). Following a very rapid meniscus coverage by the polycrystalline crust, the N2 hydrate grows on both sides of the meniscus: on the water side, as dendrites or needles expanding rapidly in bulk water, and on the gas side as a halo advancing on glass. Compared to CO2 fibers, these dendrites are much more visible, presumably because of a higher contrast in the refractive indices of N2 hydrate and supercooled water. Further details can be found in the Supplementary Information (SI). This set of observations is worth being compared to those obtained using similar or related experimental settings, such as thin glass capillaries, gas-bearing aqueous fluid inclusions, large-size (centimetric) pressure cells equipped with see-through windows and pore networks in sedimentary matrices. The comparison of our findings to literature results is carried out in the Supplementary Information. The above experimental methodology lends itself to repeat measurements by cycling the temperature: following primary hydrate formation, the capillary is heated back to room temperature in order to melt the hydrate; after a waiting time long enough (a few minutes) to eliminate any memory effect, it is chilled again, etc. A systematic investigation has been undertaken into how the primary nucleation temperature Tn depends on pressure, cooling rate, etc., whose results will be reported elsewhere. The point worth being reported here is the reproducibility of nucleation temperatures Tn . When carried out under identical pressure and cooling rate, repeat tests provide values of Tn that fall in a narrow interval, typically ∼ 1 ◦ C. Even though nucleation is a stochastic process, the Tn s are determined quasideterministically because they are close to the limit separating the metastable region for the liquid water + CO2 system from the unstable region where spontaneous hydrate formation occurs; this limit is approached because the sample volumes are very small and cooling is

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very fast.

4

Effects of substrate wettability A few experiments have been carried out in glass capillaries rendered non-water-wet by

silane treatment, with CO2 as the guest phase only (not with N2 ). The treated capillaries are those recently used in one of our laboratories (LFCR) for studying cyclopentane hydrate growth 19 and contact angles of water – CO2 systems at room temperature (22 ◦ C). 25 In presence of cyclopentane and water, the silane-treated capillaries are wetted preferentially by cyclopentane, especially at low T (below 0 ◦ C). 19 In presence of CO2 and water, the silanetreated glass is intermediate-wet if CO2 is gaseous (i.e., at low pressure), with a contact angle (in water) θ in the range of 90 – 100 ◦ ; this angle increases with pressure and jumps to ∼ 160 ◦ above the CO2 saturation pressure (∼ 60 bar), i.e., the treated glass is CO2 -wet when CO2 is liquid. A similar wetting behavior is observed at the lower temperatures of interest , as we now report. Two experiments of CO2 hydrate nucleation and growth have been conducted in the treated capillaries : one at pressure p = 14 bar, where the CO2 remains gaseous all the way down to the hydrate nucleation temperature Tn , and the other at p = 24 bar, where the CO2 condenses before reaching Tn . Figure 7 shows snapshots of the two experiments. At 14 bar (Figure 7, left) θ remains in the range of 90 ◦ (intermediate-wet substrate) all over the temperature decent; whereas at 24 bar (Figure 7, right), θ rises to ∼ 160 ◦ when the CO2 condenses into a liquid phase (CO2 -wet substrate). In the experiment conducted at p = 14 bar (Figure 7, left), CO2 hydrate nucleation occurs on the water – CO2 meniscus at Tn = −28.5 ◦ C, where the CO2 is still gaseous and the contact angle lies in the range of ∼ 90 ◦ (the CO2 condensation temperature at 14 bar is ∼ −30 ◦ C). Images b and c in Figure 7 are taken 1 s (b) and 6 s (c) following CO2 18


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ϴ

water

CO2 liquid

CO2 gas

water

BF

(d)

(a)

t=1s

t=1s

fibers

halo (e)

(b)

t=6s

t=6s

halo

fibers (f)

(c)

Figure 7: Hydrate halos are controlled by the wettability of the substrate to the host and guest phases. Snapshots from two experiments conducted at 14 bar (left) and 24 bar (right) in a silane-treated glass capillary. Left: at 14 bar, CO2 is gaseous from room temperature (a) down to Tn = −28.5 ◦ C (b and c, taken 1 and 6 s following nucleation) and the substrate is intermediate-wet (θ slightly larger than 90 ◦ ). Right: at 24 bar, CO2 condenses into a liquid at T ∼ −15 ◦ C (d), and hydrate nucleation occurs at Tn = −26 ◦ C (d and e, taken 1 and 6 s following nucleation). A ’breath figure’ of water droplets (BF) appears at T ∼ −15 ◦ C on glass on the CO2 side of the meniscus (e), which disappears starting from the meniscus as soon the CO2 hydrate is present on the meniscus.

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hydrate nucleation. Similarly to what is observed in water-wet (untreated) capillaries (see previous section), hydrate fibers grow very rapidly from the meniscus deep into the bulk of the water phase (fiber growth velocity > 300 µm/s). No hydrate halo is seen advancing over this intermediate-wet glass substrate. In the experiment conducted at p = 24 bar (Figure 7, right), the CO2 , initially (at room temperature) gaseous, condenses when T ∼ −15 ◦ C, where the contact angle jumps to ∼ 160 ◦ , indicating a CO2 -wet substrate. A breath figure, corresponding to water droplets demixing from the CO2 -rich phase, appears on the inner capillary wall, as previously observed with water and cyclopentane. 36 CO2 hydrate nucleation occurs at Tn ∼ −26 ◦ C, slightly higher (by ∼ 2.5 ◦ C) than at 14 bar, a trend also observed with water-wet (untreated) capillaries (see previous section). A hydrate halo is seen advancing at Tn from the contact line, on the water side of the meniscus (Figure 7, e and f), with an initial velocity of ∼ 25 µm/s. It is interesting to note that these nucleation temperatures are higher by 1-2 ◦ C than those observed with untreated glass capillaries (see previous section), which suggests that nucleation occurs on the contact line. 37–41 Another experiment has been conducted at p = 47 bar, which shows features similar to those obtained at 24 bar (see Table 1). In summary, and similarly to what has been observed in our recent study with cyclopentane as the hydrate-former 19 and by other authors, 15,17,18 substrate wettability has an effect on gas hydrate nucleation and growth. In presence of an intermediate-wet or CO2 -wet glass, nucleation is promoted, i.e., it occurs for slightly smaller supercooling (by about 1 − 2.5 ◦ C), compared to a water-wet (hydrophilic) glass, which suggests that the contact line with glass is involved in the nucleation process. Substrate wettability orients further gas hydrate growth, which takes place as on the guest side of the contact line if the substrate is water-wet, and on the water side if it is guest-wet; over an intermediate-wet substrate there is no halo. The quick expansion of fibers or dendrites into bulk water is not influenced by substrate 20


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wettability.

5

Effects of supercooling on hydrate growth and morphology We describe in the following first how the gas hydrate evolves when raising T towards

Teq , then CO2 hydrate formation and growth at/from the meniscus under moderate and finally low supercooling conditions.

5.1

Raising T towards Teq By increasing the temperature from Tn to above 0 ◦ C, we observe first the breakage of

the hydrate crust or halo, which is dragged towards the water side of the meniscus due to the thermal contraction of water, then the recession towards the meniscus of the hydrate fibers. We focus here on the latter phenomenon. When T is slowly increased towards the hydrate dissociation temperature Teq , the hydrate fibers at the rear of the meniscus appear denser and shrink towards the meniscus, as is illustrated in Figure 8. The above sequence of hydrate rapidly growing (in the form of fibers) in bulk water from the water guest interface right after its coating with a polycrystalline hydrate crust at low T , followed by shrinkage of these fibers towards the interface when T is raised, has been reported many times (from observations through the see-through windows of conventional pressure cells, see the Supplementary Information), starting nearly two decades ago by Subramanian and Sloan 10 with methane and by Ohmura and coworkers 12 with CO2 hydrate as the guest phase. A qualitative explanation for the observed features is provided by Figure 9, which schematically depicts the temperature dependence under isobaric conditions (p constant) of 21


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fibers

halo (a)

water

(b) Figure 8: Hydrate fiber recession towards the meniscus when T is raised slowly to the equilibrium temperature. p = 30 bar and T = 5 ◦ C (a) and 6.6 ◦ C (b) below Teq . Time elapsed between (a) and (b): 45 seconds.

22


the two relevant guest concentrations in liquid water. One of these concentrations, which prevails very near the meniscus prior to hydrate formation, is described by curve a in Figure 9): this is the concentration (hereafter noted Ca ) of the guest (CO2 ) dissolved in liquid water in equilibrium with the guest-rich phase on the other side of the meniscus. The dotted part of this curve corresponds to metastable equilibrium, i.e. the hydrate is absent, for T below Teq . The second of these concentrations, Cb , defined for T below Teq , is the guest concentration in water in presence of (and equilibrium with) the hydrate: this is curve b. Whereas Ca increases with decreasing T , Cb varies in the opposite direction, and both meet at Teq corresponding to liquid water/hydrate/gas equilibrium. Hydrate stability region Cooling Heating Hydrate formation Water Water + hydrate

Concentration Cn Ci

(a) 2

eq 4 1 (a)

Cf

3 (b)

Tn

Teq

Ti

Temperature

Figure 9: Schematic evolution of guest concentrations in water during an isobaric hydrate formation/dissociation cycle (see text). The scales are arbitrary. Curve (a) corresponds to the liquid water/guest equilibrium and the dotted part is its metastable extension below Teq . Curve (b) describes the liquid water/hydrate two-phase equilibrium (T < Teq ). The two curves meet at the three-phase equilibrium conditions (eq).

At the start of the experiment when T is decreased from Ti (∼ 15 – 20 ◦ C) to Tn , the CO2 concentration in liquid water very near the meniscus first increases, from Ci to Cn (path 1 → 2 along curve a), which is reached at Tn , the temperature of primary nucleation, and 23

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then drops at Tn from Cn to Cf (path 2 → 3) when the hydrate fibers have been formed. A large part of the CO2 initially dissolved in water, namely that corresponding to the difference between Cn = Ca (Tn ) and Cf = Cb (Tn ), is in fact enclathrated in the hydrate fibers. The concentration difference ∆C(T ) = Ca (T ) − Cb (T ), which is proportional to the supercooling ∆T , can also be viewed as the driving force for hydrate formation. As Cn does not exceed 2 – 3 mol% for the pressure and temperatures of interest here, the volume fraction of the hydrate phase (which contains one molecule ofCO2 per 6 to 7 molecules of water) lies therefore in the percent range. The hydrate is expected to form and grow over a certain distance from the meniscus, where the local CO2 concentration in water is larger than Cf . When this system is heated but T remains below Teq , the CO2 concentration in the water equilibrated with the hydrate present follows the path 3 → 4 → eq (curve b), i.e., it increases: therefore, CO2 molecules have to be provided to the bulk water + hydrate system, which come from the dissolution of the tip of the fibers. It is important here to emphasize that the hydrate crust covering the meniscus is nearly impermeable and prevents any CO2 supply from the CO2 -rich phase to the other side of the meniscus. The dissolution of the hydrate fibers at their extremities to provide increased CO2 molecules to the water coexisting with the remaining fibers is responsible for the observed recession of the fiber front (Figure 8). Here, it is worth mentioning the observations by Roedder 42 and Collins 43 of CO2 -rich aqueous phase inclusions. These authors observed that upon warming the fibers or tiny dendrites of these hydrates transform into coarse, irregular, platy (platelike) crystals or, according to Roedder, 42 becoming "nebulous at first" and increasingly visible, "showing generally rounded shapes with a few obvious flat sides," which themselves transform on cooling again into rapidly growing cubo-octahedrons. 42 The results presented in next subsection also provide some illustrations of these changes.

24


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5.2

Moderate supercooling To form the hydrate under these conditions, the ’memory effect’ is exploited: the

CO2 hydrate is brought to complete dissociation by heating slightly (∼ 1 ◦ C) above Teq , and then the temperature is lowered below Teq shortly after the hydrate has been melted. Following a certain induction time (a few seconds, typically) at the latter temperature, the CO2 hydrate is observed to nucleate and then grow rapidly as a crust on the water – CO2 meniscus, and then to advance from the contact line over glass. This sequence of events is similar to that occurring for hydrate formation and growth under strong supercooling conditions and described above. It is illustrated in Figures 10a and 10b, which correspond to, respectively, a water – CO2 meniscus just following complete hydrate dissociation at 30 bar, 9 ◦ C (∼ Teq + 1 ◦ C) (a), and the same system in which T has been dropped (at a rate of 20 ◦ C/min) to 1.5 ◦ C, i.e. ∆T ∼ 6.5 ◦ C (b). In this experiment, a hydrate crystal with an apparent cubo-octahedron shape is observed to grow at 1.5 ◦ C on the water side of the meniscus (image c, Figure 10). Likewise, N2 hydrate cystals formed under moderate supercoolings have polygonal shapes: see the SI.

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hydrate crust water

CO2 gas

(a)

(b)

t=0

t =3s

hydrate monocrystal hydrate halo

(c)

t = 30 s

Figure 10: CO2 hydrate formation and growth at a water – CO2 meniscus at moderate supercooling conditions (p = 30 bar, ∆T ∼ 6.5 ◦ C, images b and c), in a capillary where the hydrate has been formed and dissociated shortly before (image a). Here, the capillary inner diameter is 250 µm.

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5.3

Low supercooling: evidence for a new crystal morphology In some experiments, we managed to melt the CO2 hydrate incompletely, in a manner

to leave one single small crystal in the bulk water near the water – gas meniscus (see, e.g., 11a), from which a new crystal could grow again by lowering temperature to slightly below Teq (11b to f). Upon melting the CO2 hydrate crystal at T ≳ Teq , the hydrate halo melts first and the resulting liquid water recedes towards the meniscus, leaving one hydrate crystal in bulk water, where it melts slowly if T is only slightly larger than Teq . Then, before complete melting, T is decreased to slightly below Teq in order to make this crystal grow. As illustrated in Figures 11a and 11b, taken from the Movie in the Supporting Information (SI), this hydrate crystal grows towards the meniscus, fed by the diffusive flux of CO2 in water coming from the gas side of the meniscus, which is free of any hydrate crust and therefore opposes no obstacle to CO2 transfer into bulk water. When the hydrate crystal reaches the meniscus (Figure 11b), a thin hydrate filament, with thickness of a few µms, appears at the point of contact and rapidly extends between the meniscus and the hydrate crystal, which is then repelled away from the meniscus (Figure 11, c and d). The filament then grows from its meniscus’ end (Figure 11, e and f) into a conical shape, which then evolves into a cylindrical hollow crystal concentric to the capillary (Figure 12). This cylindrical hollow crystal grows along the capillary wall, fed at its leading edge by the CO2 present and the water arriving through a thick water layer sandwiched between this crystal and the inner wall of the capillary (Figure 12). This water layer is itself drawn ahead of the leading edge of the crystal by the water-wet character of the capillary. The hollow crystal seems to move back towards the water phase as the water migrates from the rear of the meniscus to the edge of the growing hollow CO2 hydrate crystal. It is worth mentioning

27


t = 112 s

hydrate crystal water

CO2 gas t=0

(a)

(b) t = 132 s

hydrate filament

hydrate filament

t = 126 s

(c)

t = 138 s

t = 134 s

Birth of a hollow crystal

(d)

hollow crystal

(e)

(f)

Figure 11: A novel hollow crystal form of hydrate appears at low supercooling, p = 24 bar and T = 4.9 ◦ C (∆T = 0.4 ◦ C). First (a and b), there is a hydrate crystal in water growing slowly towards the meniscus until it touches it. Then (c and d) a hydrate filament grows rapidly between the meniscus and the hydrate crystal from the meniscus. At some point (e and f) the hydrate grows from the filament end along the meniscus itself and evolves into a hollow cylindrical crystal (see Figure 12).

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here that we have tried but failed to prepare and observe a hollow N2 hydrate crystal by using this procedure. Figure 12 displays snapshots of two experiments conducted at p = 22 bar and different low supercoolings ∆T . In the first experiment (images a and b) T and ∆T are equal to 4.6 ◦ C and ∼ 0.2 ◦ C, respectively; the thickness of the water layer is ∼ 20 µm, and the hollow crystal grows along the capillary at a rate ∼ 4.1 µm/s. In the second experiment (images c and d), the temperature T and the supercooling ∆T are first (image c) equal to 4.7 ◦ C and ∼ 0.1 ◦ C: the thickness of the water layer is ∼ 30 µm, and the hollow crystal grows at a rate ∼ 0.7 µm/s. Later in this second experiment (image d), T is decreased to 4.6 ◦

C (∆T ∼ 0.2 ◦ C), and the diameter and growth rate of the hollow crystal progressively

increase to values similar to those observed in the first experiment conducted at the same ∆T ∼ 0.2 ◦ C. A novel CO2 hydrate morphology and growth process has thus been discovered, consisting of a cylindrical hollow crystal advancing concentrically along the capillary, fed by a thick water layer between this crystal and the capillary inner wall. This hollow crystal is transparent, indicating that is a monocrystal. The brilliant line on this hollow crystal is likely to be a grain boundary. The shadowing on the edges of this crystal are refraction effects: due to the much lower index of refraction of the CO2 phase, the nearly parallel light rays are deviated from the vertical direction when passing the interface between water+hydrate and CO2 , and therefore do not fall on the cameras detector. That gaseous CO2 is present in the center of the capillary is confirmed by the confocal micro-Raman spectroscopy measurements reported hereafter. The formation process requires that a hydrate crystal, e.g., that resulting from the incomplete dissociation at T > Teq of a hydrate formed previously, be initially present in bulk water near the meniscus. It thus differs from the process described recently by Ou and coworkers, 34 in which a CO2 hydrate crystal was formed in the bulk water far from the

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water CO2 gas

a

c After 45 sec

After 73 sec

b

d

glass capillary water layer gas hollow hydrate crystal

Figure 12: Hollow CO2 hydrate crystal growth in a capillary at 22 bar and low supercooling. The temperature in the first experiment (images a and b) is 4.6 ◦ C (∆T = 0.2 ◦ C). The temperature in the second experiment is 4.7 ◦ C (∆T = 0.1 ◦ C) at the beginning (image c) and then (image d) is decreased to 4.6 ◦ C (∆T = 0.2 ◦ C). The black arrows indicate water arriving from the rear of the meniscus to the edge of the hollow crystal, and white arrows show the growth direction of the hollow crystal. A schematic cross-sectional view and a representation of the leading (and growing) edge of the hollow hydrate crystal is also shown.

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meniscus in a much thinner glass capillary (R = 25 µm): this crystal occupied the whole section of the capillary, forming a meniscus with water, and was observed to grow in the direction of the water–CO2 meniscus as well. Clearly, the growth rate of the hollow CO2 hydrate crystal and its morphological features, including its diameter and the thickness of its accompanying water layer, are very sensitive to ∆T : the hollow crystal advances at a lower rate and the water layer is thicker when supercooling decreases. These trends are similar to those observed for hydrate halos growing on glass at higher supercoolings, which however differ from the hollow hydrate crystal of interest here in that they are polycrystalline. 19 The growth mechanism of the cylindrical hollow crystal has similarities with that of hydrate halos on glass and polycrystalline crusts at the interface between a water and guest phases. 19 In short, the growth of the hydrate hollow crystal at its leading edge is ensured by the underlying water layer, which wets the glass substrate (see Figure 12) and therefore forms an interface with the CO2 phase; this water becomes saturated with CO2 , and there is a flux of CO2 molecules in water towards the edge of the crystal (where CO2 concentration in water is lower, see Figure 9). Comparing the growth rates of the hollow crystal (∼ 0.7 and 4.1 µm/s for ∆T ∼ 0.1 and 0.2 ◦ C, respectively) to those determined at higher ∆T (> 1.5 ◦ C) for polycrystalline CO2 hydrate layers growing along water – CO2 interfaces, 44 we observe that both sets of values are consistent, in the sense that the growth rates of the hollow crystal are in the range expected by extending the power law (a power 5/2 of ∆T ) observed for ∆T (> 1.5 ◦ C) 44 to the ∆T of interest here, namely ∆T ∼ 0.1 and 0.2 ◦ C. It thus can be stated with confidence that the hollow hydrate crystal is the limit (for low ∆T ) of the polycristalline hydrate crusts commonly observed at water – guest interfaces or riding over a water-wet substrate. Different growth rates would probably have been observed if the crystal habits were different. 45 This experiment has been repeated at ISM, where a Raman mapping has been carried out along the segment (labeled with x in Figure 13b) joining the center to the inner wall of

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the capillary in the region of the hollow hydrate crystal (see Figures 13 a and b). To spatially map the various phases in the capillary, the following Raman signatures of each phase have been selected: the CO2 stretching mode in the gas state at 1390 cm−1 , the one corresponding to CO2 encapsulated in the hydrate at 1384 cm−1 , the water O–H stretching broad profile centered at ca. 3300 cm−1 (see subsection Phase assignement by Raman spectroscopy) and the characteristic silica band centered at ca. 500 cm−1 . Each point of the curves presented in Figure 13c is then obtained by considering the intensities (integrated area of the peak) of these 4 characteristic bands normalized to the total intensity (see the Supplementary Information for details). These intensity profiles reflect the radial distributions of the different phases in the region of the hollow crystal (Figure 13c). In the center of the capillary (x, the distance from the center of the capillary, varying from 0 to ∼ 40 µm) the Raman signature of gaseous CO2 is predominant and that of the CO2 hydrate is absent. In the region closer to the inner wall (x from 50 to 100 µm), the signature of CO2 stretching together with that of O–H stretching indicate the occurrence of both CO2 hydrate and water phases. Moreover, one can observe a shift between the intensity maxima for the O–H stretching profile and that of the encapsulated CO2 profile: the maxima are observed at x = 95 µm and x = 82 µm, respectively. In other words, the H2 O maximum intensity is observed between the hydrate phase and the capillary wall (identified with the help of the increase of the silica profile at x ∼ 100 µm). Such observations clearly indicate that (i) the hydrate exhibits a hollow shape and (ii) CO2 is depleted near the inner capillary wall, where the water phase is present.

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(b)

(a)

100 µm

50 µm x

2R

Gas phase

Hydrate

0.9

Glass

CO2 hydrate CO2 gas O-H stretch Silica

0.8 0.7 Intensity ratio [a.u.]

Water

0.6

(c)

0.5 0.4 0.3 0.2 0.1 0 0.0

20.0

40.0

60.0 X (µm)

80.0

100.0

120.0

Figure 13: Raman mapping along a segment from the center to the inner wall of the capillary at 22 bar and 4.3 ◦ C (0.4 ◦ C of supercooling). (a) and (b) show the images by reflection microscopy of the hollow hydrate with 10x and 50x objectives, respectively. See the SI for details.

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6

Conclusions and prospects The experimental setting and procedure devised in this research open the way to investi-

gations into gas hydrate crystallization processes in presence of a substrate at scales poorly examined so far - below the mm and above a few tens of nanometers - under an unprecedented control of temperature, pressure, confinement and substrate wettability. CO2 and N2 are the guest molecules used in this study. Upon cooling under isobaric conditions the water and guest phases, the hydrate nucleates on the water – guest meniscus and then grows over the meniscus and finally on its both sides, in a manner that depends strongly on supercooling and substrate wettability. Primary nucleation is detected easily from the observation of the water – guest meniscus, which abruptly changes aspect at the nucleation temperature Tn . Owing to the very small sample volumes and the high cooling rates, the observed Tn ’s are much lower - by more than 30 ◦ C – than the three-phase (liquid water – hydrate – guest-rich phase) equilibrium temperature. This strong metastability is typical of small systems undergoing a crystallization transition. To the best of our knowledge, this is the first experimental investigation of gas hydrate nucleation and growth under such high supercooling conditions, which are likely to be very close to those of spontaneous hydrate nucleation corresponding to the limit of water + gas metastability: this subject will be addressed more thoroughly in a forthcoming publication. Following primary nucleation, a polycrystalline hydrate crust covers the meniscus almost instantaneously (within less than a second), and then hydrate growth proceeds on the water side of the meniscus as rapidly growing fibers. The crust continues its growth over the glass substrate, on the guest side of the meniscus if the substrate is water-wet (untreated glass), on the water side if the substrate is guest-wet (silane-treated glass and liquid CO2 ), and there is no growth if the substrate is intermediate-wet (silane-treated glass and gaseous CO2 ). The experiments with N2 have been conducted in water-wet (untreated) glass only; the N2 hydrate crust grows over glass on the guest side of the meniscus (see the Supplementary 34


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Information). The growth of this crust (or halo) is fed by a layer of the wetting phase present between the glass wall and the halo. These features, which have also been observed in previous work with cyclopentane hydrate in open capillaries (i.e. at atmospheric pressure), thus seem to hold quite generally. Hydrate growth has also been investigated under moderate and low supercoolings. Under moderate supercooling (∆T ∼ a few ◦ C), macroscopic monocrystals are observed to grow on the water side of the meniscus. Under low supercooling (∆T < 0.5 ◦ C), a novel CO2 hydrate morphology is discovered, consisting of a hollow hydrate originating from the meniscus and growing as a cylindrical mono-crystal advancing parallel to the glass wall, with a thick water layer feeding the growth of the hollow crystal. The growth rate of the hollow hydrate and the thickness of the water layer, which both can be measured precisely, depend in a sensitive manner on ∆T . Further work is needed to better characterize and understand this dependence.

7

Acknowledgements

The authors would like to express their gratitude to the French National Research Agency for partly funding the present study, which is part of the project HYDRE "Mechanical behavior of gas-hydrate-bearing sediments" ANR-15-CE06-0008 and to Communauté dAgglomération Pau Béarn Pyrénées (project Laboratoire en capillaire). We thank Ross BROWN, Patrick BOURIAT, Manuel ILDEFONSO and Claire PETUYAPOUBLAN for their support and insightful comments.

Supporting Information Available • Movie.avi shows how a CO2 hydrate hollow crystal forms from the water – CO2 35


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meniscus at low supercooling conditions: p = 24 bar and T = 4.9 ◦ C (∆T = 0.4 ◦ C). First, a hydrate crystal in water grows slowly towards the meniscus until it touches it. Then a hydrate filament grows rapidly between the meniscus and the hydrate crystal, which is repelled from the meniscus. Seconds later, the filament’s edge on the meniscus evolves into a hollow cylindrical crystal advancing under the CO2 vapor with no change of speed or texture. The water layer between the inner wall of the capillary and the hydrate hollow crystal ensures growth at its leading tip. This water comes from the rear of the initial water-CO2 meniscus. • SIArticleGasHydrate.pdf contains three sections. The first section reports the insitu Raman spectroscopy and optical microscopy analysis of N2 hydrate growth in a glass capillary at various thermodynamic conditions. The second section is a literature review of the nucleation and growth features of CO2 and N2 hydrates in thin glass capillaries, fluid inclusions in minerals and rocks, conventional high pressure cells with see-through windows, and . The third section describes the procedure for inferring intensity ratios of the various species from Raman spectra for the hollow crystal, with an example for three locations in the capillary.

This material is available free of charge via the Internet at http://pubs.acs.org/.

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8

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Graphical TOC Entry

water CO2 gas

glass capillary water layer CO2 gas hollow hydrate crystal

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Gas hydrate crystallization in thin glass capillaries: roles of

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