Article pubs.acs.org/JPCC
On the Spontaneous Formation of Clathrate Hydrates at Water− Guest Interfaces Lars Boewer,† Julia Nase,*,† Michael Paulus,† Felix Lehmkühler,†,§ Sebastian Tiemeyer,† Sebastian Holz,† Diego Pontoni,‡ and Metin Tolan† †
TU Dortmund, Fakultät Physik/DELTA, Maria-Goeppert-Mayer-Str. 2, 44227 Dortmund, Germany European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble Cedex 9, France
‡
S Supporting Information *
ABSTRACT: The formation of hydrates, cage-like water-gas structures, is of tremendous importance both in industries and research. Although of major significance, the formation process is not completely understood so far. We present a comprehensive study of hydrate formation at liquid−liquid interfaces between water and isobutane, propane, carbon dioxide, and at the liquid− gas interface between water and xenon. We investigated the structure of these interfaces under quiescent conditions in situ by means of X-ray reflectivity measurements both inside and outside the zone of hydrate stability. At the interfaces between water and liquid alkanes, no evidence for a structural change was found. In contrast, the accumulation of guest molecules inside nanothick interfacial layers was observed at the water−xenon and liquid− liquid water−CO2 interfaces. We show that only those systems initially exhibiting such guest-enriched interfacial layers developed into macroscopic gas hydrates within our observation times (∼12 h). Therefore, these layers act as triggers for the spontaneous formation of macroscopic hydrates.
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INTRODUCTION Gas hydrates, also known as clathrate hydrates, are cagelike structures where small guest molecules are enclosed in a hydrogen bond water network.1,2 Such materials played an important role in the aftermath of the exploded oil platform Deepwater Horizon in the Gulf of Mexico in 2010. Over a period of almost three months, 35 000−60 000 barrels of oil polluted the ocean every day. Many attempts to seal the leak at an ocean depth of 1500 m failed because steel caps installed to collect the oil were blocked by the formation of gas hydrates.3 Gas hydrates are typically formed at high pressure and low temperature, conditions that are found, for example, in deep sea regions. Natural gas hydrates mainly exist in two cubic lattices, structure I (sI) and structure II (sII), which differ in the number of water molecules per unit cell and in their lattice constant. sH, a hexagonal structure, is more rare. Huge amounts of natural methane hydrates exist at the ocean floor, representing an enormous reserve of fossil energy but also greenhouse gas.2,4,5 Especially, CO2 hydrate was considered as a promising candidate for gas storage, as CO2 molecules can be kept in solid hydrate structures and thus be prevented from contributing to the greenhouse effect.6 However, an in-depth understanding of the hydrate formation mechanisms is crucial to guarantee a safe and stable storage. Despite the major significance of hydrates in both industries and fundamental research, the formation process on molecular length scales is still not well understood and currently in the focus of scientific discussion. © 2012 American Chemical Society
It is the aim of this work to investigate experimentally hydrate formation at the water−guest interface, covering a large number of quiescent hydrate forming systems. So far, different formation models on the molecular length scale have been extensively investigated by molecular dynamics (MD) simulations from the theoretical point of view. Models that predict the nucleation of hydrate precursors or gas hydrate fragments at interfaces1,7,8 were discussed. In contrast, other authors did not observe interfacial structural modifications prior to hydrate formation.9−12 There is a clear need for experimental investigations aimed at discriminating the competing scenarios emerging from computer simulation work. However, because of the stochastic nature of hydrate formation, experimental studies of the very beginning of hydrate nucleation are for some systems hard to realize. The analysis of induction times for gas hydrate formation reveals time scales between minutes and several days, depending on the guest molecule. Recent studies on undisturbed liquid−gas interfaces like CO2−water or propane−water under conditions where gas hydrates are stable did not show any structural changes at these interfaces.13,14 A study of tetrahydrofuran (THF)−water mixtures at hydrate stability conditions excluded the existence of precursors in the supercooled THF−water mixture.15 In contrast, a neutron scattering study on methane hydrate formation proposed the formation of so-called hydrate embryos at the liquid−gas Received: December 7, 2011 Revised: March 2, 2012 Published: March 16, 2012 8548
dx.doi.org/10.1021/jp211784w | J. Phys. Chem. C 2012, 116, 8548−8553
The Journal of Physical Chemistry C
Article
interface.16 A time-resolved Raman spectroscopy study monitored the formation of structure sI hydrate from dissolved methane in water during cooling,17 while the growth of hydrate crystals from an ice powder was investigated by means of neutron diffraction18 and time-resolved NMR spectroscopy.19 Experimental studies also explored the diverse field of crystal growth and hydrate structures after the nucleation, the unexpected coexistence of sI and sII was found in methane hydrate crystals.20 The number of experimental investigations pointing to nonuniversal guest-specific mechanisms is, however, still too limited to allow for drawing definitive conclusions. We investigated different undisturbed hydrate forming systems with induction times varying from minutes and hours to several days. We performed X-ray reflectivity measurements in order to identify the fundamental mechanism of gas hydrate formation at interfaces. We show that spontaneous hydrate formation is systematically preceded by the appearance of an interfacial layer consisting of a highly supersaturated mixture of water and guest molecules. These layers cannot be identified as gas hydrate as will be detailed in the following. The systems lacking the guest-enriched interfacial layer did not lead to macroscopic hydrates within the longest observation periods of ∼12 h.
intuitive representation is the inverted profile z(ρ), see, for example, Figure 3, bottom. As the profile is laterally averaged, the reflectivity technique cannot directly resolve a possible lateral structure at the interface. Assuming a liquid−gas or liquid−liquid interface with capillary wave roughness σ, eq 1 leads to 2
R(qz) = RFe−σ qz
EXPERIMENTAL METHODS X-ray reflectivity is a technique that is sensitive to the electron distribution at interfaces. X-rays hitting the sample at a given incident angle α are reflected at interfaces between materials of different electron density, that is, different number of electrons per unit volume. The detector is set to the incident angle, see a schematic setup in Figure 1. The reflected intensity is measured
Table 1. Characteristics of the Different Sample Systemsa
a
as a function of α. This technique provides laterally averaged profiles of the electron density with sub Ångström resolution.21 In the first Born approximation, the reflectivity of an interface is given by22
∫
⎛ dρ(z) ⎞ iq z ⎜ ⎟e z d z ⎝ dz ⎠
system
phase
induction time
hydrate structure
water−isobutane water−propane water−CO2 water−xenon
liquid−liquid liquid−liquid liquid−liquid liquid−gas
long long short short
sII sII sI sI
Qualitative induction times are from ref 1.
the structure of the interface between water and guest phase when setting temperature and pressure to conditions of hydrate stability. We classified the investigated systems depending on whether we observed spontaneous macroscopic hydrate growth during the experimental time (∼ hours) (short induction time) or not (long induction time). We discuss in the following the electron density relative to the gas saturated water subphase. The sample cell used for all experiments was made of stainless steel following the design specifications presented in ref 26. The inner cell, made of aluminum with a wall thickness of 2 mm, is able to sustain gas pressures of up to 50 bar. The aluminum walls are traversed by the X-ray beam. The inner cell has a diameter of 120 mm and allows for the preparation of liquid surfaces and liquid−liquid interfaces. The large diameter ensured that the interface was perfectly flat as required for our measurements, except for a curvature next to the cell wall. We injected into the cell ∼40 mL of water such that its surface was ∼2 mm above the cell bottom. Ultrapure deionized water from an ELGA purification system (resistivity 18.2 MΩ cm) was used. In the different experimental runs, liquid isobutane (Messer, purity 99.95%), liquid propane (Air Products, purity 99.99%), or liquid CO2 (Air Products, purity 99.9995%) phases with a height of >2 mm each were produced by condensation of the pure gases. In the case of the water−CO2 system, a ring made of Teflon in the aluminum cell generated a very well controlled water meniscus. The isobutane and CO2 systems
Figure 1. Molecular scale representation of a typical interface under investigation with schematic representation of the X-ray scattering geometry.
1 ρs
(2)
where σ is on the order of several Ångströms. The formation of thin layers manifests itself via deviations of the measured reflectivity curves from the shape described by eq 2 (see Figure S1, Supporting Information). Thus, interfacial structural changes on molecular length scales can be studied by X-ray reflectivity, in particular changes of roughness or layer formation at liquid−gas and liquid−liquid interfaces. System perturbations, such as stirring, are known to accelerate hydrate nucleation.23 We investigated all systems under quiescent conditions, that is, avoiding all external interventions on the system. In that way, one can observe spontaneous hydrate formation, by which we mean formation in the absence of any external trigger. Because of the strong X-ray absorption of the cell and the thick liquid samples, we used the high energy (69.9 keV) liquid surface diffractometer24,25 of the ID15A beamline at the ESRF. We investigated the interfaces of various hydrate forming systems, as summarized in Table 1, with regard to changes in
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R(qz) = RF(qz)
2
2
(1)
where RF indicates the reflectivity of an ideal sharp interface (Fresnel reflectivity). ρs is the electron density of the substrate. qz = (4π/λ) sin(α) is the momentum transfer depending on the X-ray wavelength λ (0.177 Å in our experiments) and on the Xray incidence/reflection angle α. This technique is sensitive to changes in the laterally averaged electron density profile ρ(z) along the interface-normal spatial coordinate z.21 A more 8549
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K) the hydrate stability temperature show no significant differences. The water−propane interface measured at conditions where hydrate formation is possible (T = 275 K, p ≈ 5 bar = 2phyd) shows a similar qz dependence. A fit to the data yields interfacial roughnesses of σibut = (4.1 ± 0.2) Å and σprop = (4.35 ± 0.2) Å. A calculation from capillary wave theory with nominal interfacial tensions and diffractometer resolution and an estimation of the intrinsic roughness from the radius of gyration gives roughnesses of σtheo ibut = 4.6 Å 27,28 The small deviation between experiment and σtheo prop = 4.3 Å. and theoretical calculation of 0.3 Å for isobutane can be attributed to an error in the estimation of the intrinsic roughness by the radius of gyration. From these results, no indication for a structural rearrangement at the interface, e.g., the formation of gas hydrate precursors or an accumulation of the dissolved gas, was detected. A similar result was found at the water−propane liquid−gas interface in a previous study,14 where the interfacial roughness of a water surface at a propane pressure of 1 bar was determined to 3.65 Å, in agreement with capillary wave theory. For isobutane and propane, we did not observe macroscopic hydrate formation during the time of the experimental run (∼10 h). To our knowledge, in both systems, no macroscopic formation of gas hydrate was observed in the absence of external perturbations as mechanical movement or stirring. Tanaka et al.29 studied the growth of propane hydrate, however, after introducing a memory to the system by first freezing it and then heating and dissolving the ice and hydrate crystals. Water−Carbon Dioxide. To obtain a broad overview, we investigated likewise the water−liquid carbon dioxide interface. This system is known to form hydrate structure I spontaneously at the liquid−liquid interface without stirring within minutes, in contrast to the liquid−gas water−CO2 system.13 Consequently, a temperature dependent series of Xray reflectivities of the water−liquid carbon dioxide interface was recorded. Starting at a temperature of 285 K, the temperature was reduced, and reflectivity curves were recorded until the formation of macroscopic amounts of CO2 hydrate was detected. The formation of gas hydrate at an interface causes a temperature jump as well as a complete loss of the reflected intensity. Decreasing the temperature along the condensation curve, the region of CO2 hydrate stability is reached at T ≈ 283.4 K and p ≈ 45.3 bar. As visible in Figure 3, the reflectivities above T = 277 K can be refined by a simple model of an interface roughened by capillary waves. This situation changes at a temperature of 277 K and a pressure of p ≈ 38.5 bar ≈ 1.2phyd, where the system is strongly supercooled. The decrease of intensity at low qz indicates the formation of a thin layer with a lower electron density with respect to water. In the corresponding electron density profiles in Figure 3 (bottom), the formation of a layer with a thickness of about 80 Å is clearly visible. Macroscopic formation of CO2 hydrate set in just after this last reflectivity measurement. Water−Xenon. The unexpected interfacial layer formation was further investigated by measurements of the system water− gaseous xenon. Xenon forms hydrate structure I at experimentally easily accessible conditions of phyd = 4.1 bar at T = 283 K.1 Figure 4, top, shows reflectivities of the water− xenon interface at 283 K inside and outside the hydrate stability zone (1 and 6.4 bar). A simple water−nitrogen interface serves as a reference. Even at 1 bar xenon pressure, it is impossible to model the water interface with a simple error function of roughness σ. Instead, the curves are well described taking into
were investigated at temperatures below and above the gas hydrate stability point in order to detect changes in the interfacial roughness. The water−propane interface was studied at hydrate formation conditions. For the measurements at the water−xenon (Air Liquide, purity 99.996%) interface, water was gently poured into the inner aluminum cell. The cell was then flushed with helium, and a reference reflectivity measurement of the water−helium interface was recorded. Afterward, the cell was flushed with xenon in order to achieve a pure phase of gaseous xenon in the cell. The first reflectivity measurement was performed at a xenon pressure of 1 bar. Subsequently, the gas pressure was raised stepwise, and several reflectivity measurements were repeated at constant pressure after each pressure increase. In order to assess the reproducibility of the results and the sample preparation methods, several identical experimental runs were repeated with a freshly prepared sample for each run. The technical requirements for these experiments are rather challenging, as low viscosities and interfacial tensions make the interface sensitive to vibrations and as the critical angle, in the case of the liquid−liquid water−CO2 system, is particularly small (
