Gas hydrates are crystalline compounds of hydrogen-bonded water molecules network acting as the host of water molecules network and the guest molecules placed in host cavities. Usually for the gas hydrates formation high pressure and low temperature are required. The most common hydrate structures in nature are: the cubic structure I (sI), the cubic structure II (sII), and the haxagonal structure H (sH). Depending on the size of the guest molecule one or another hydrate structure is formed [1].
Gas hydrate formation is an important process for a number of different applications: gas separation, energy storage, energy transport, cold energy storage, sequestration and desalination. Hydrate formation from liquid phase involves a very crucial step of hydrate nucleation process that is very important for developing efficient hydrate practical applications [2].
A typical time evolution of gas consumption during hydrate formation process can be divided into three main phases: dissolution phase, metastable supersaturated phase, and growth phase [2]. The process of dissolution phase starts with the increasing of the gas uptake due to the dissolution into the liquid phase.
Experimental molecular-level description of hydrate nucleation process in water phase is challenging not only due to the extremely small length/time scale involved but also due to the stochastic nature of the nucleation process, which is only possible with the state-of-the-art measurements. Also it should be noted that the anomalous behavior of different water properties is also important issue. Thus the density fluctuations of water with dimensions of ~ 0.6nm were observed [3] and it was confrmed later by theoretical investigations [4]. The properties of water are determined by its dynamic hydrogen bond network [5], which influences on the stochastic nature of the process of hydrate nucleation.
In the past few decades the numerous theoretical molecular dynamics (MD) simulations have been performed in which the authors unraveled many crucial aspects of crystal nucleation in liquids (see reviews [2, 6, 7]). These MD simulations have been used in order to obtain a detailed understanding of the nucleation process in different systems, where the time evolution of a liquid into a crystal is more or less accurately reproduced.
The hypotheses about hydrate formation [1] involve the hydrate precursor structures: prior to the hydrate formation process, the water molecules form individual clathrate cages (hydrate precursors) around the dissolved guest molecules, then if a favorable conditions are met, these cages will agglomerate and form solid clathrate hydrate.
Using MD simulations of hydration of hydrophobic molecules, the volumetric numerical data on this process details have been obtained. According obtained data, the nonpolar solutes change the structure of the water hydrogen bond network that realized in the creation of structured hydration shells around dissolved hydrophobic molecules [8–19]. Those clusters (often called as “postcursors”), preserve for a long period of time in liquid phase [20]. At the same time in works [21, 22] the authors didn’t show indication of enhanced structure around hydrophobic solutes and didn’t find clathrate cages in solution with long lifetime. In order to reveal the water structure around non-polar solutes, the experimental studies were carried out [23–27]. The results presented in these works are consistent with previous computer simulation data. There is loosely organized hydration cages around hydrophobic solutes, but without evidences of regular clathrate hydrate cages in solution is presented in water. The NMR investigation of tetrahydrofuran (THF) hydrate formation indicates that THF hydration shells with no evidence of hydrate precursor formation was found [28].
Until now there are many research studies of mechanism of gas hydrate nucleation [29–35]. Currently there are two main molecular mechanisms for hydrate nucleation. The first one is the labile cluster hypothesis (LCH) [29] and the second one is the local structure hypothesis (LSH) [30]. According to the LCH, prior to hydrate formation, water molecules form labile relatively long-lived unstable clusters (resemble the solid hydrate cavities). Then, under favorable conditions, these cages agglomerate and recombine into solid clathrate hydrate phase [30]. The LSH suggested that guest molecules arrange in a configuration similar to that of the crystal due to concentration fluctuations, and then the water molecules rearrange around the guest molecules to form a hydrate structure proposed by Radhakrishnan and Trout. The concept of LSH in which hydrate can appear with the ordering of guest molecules followed by water molecules, differs from LCH. Moreover, the authors of LSH argue against LCH by showing that it is thermodynamically favorable for the labile clusters to disintegrate rather than agglomerate to form a larger cluster.
In the hydrate phase the concentration of hydrophobic gas molecules such as methane is more than two orders of magnitude higher than the solubility of methane in liquid water [35]. In order to increase methane solubility it is possible to lower the temperature and/or increase the pressure of the mixture. In work [36] the authors examined metastable methane solubility using MD simulations. It was shown that for increasing methane concentration in the water phase in two orders in magnitude at fixed temperature, it is necessary to increase the pressure also in more than two orders. For example, at 258.5 K and P = 1.2 bar, the equilibrium concentration of methane in water is xM = 0.0005 molar fraction, and at P = 945 bar, xM = 0.05. It has been found so called the critical solubility, above which metastable solutions spontaneously form a hydrate phase is about 0.05 molar fraction, which is comparable with the concentration of methane in the hydrate (xM = 0.07) with half filling of all the hydrate cavities.
The very first stage during clathrate hydrate formation is characterized by gas consumption into the water phase with formation of metastable supersaturated water. This process is often called the induction time. Increasing of gas concentration is observed until formation of a critical nuclei of a new hydrate phase. A critical nucleus can be defined as the minimum amount of new phase that is capable of existing independently [37]. In water + gas systems during the induction time relaxation processes of the structural redistribution of dissolved guest molecules occur, leading to the initial stages of the growth phase formation. The metastable supersaturation phase remains a key step toward a complete understanding of hydrate nucleation [38].
Study of the initial steps of hydrate formation [39] showed that a dodecahedral water cage (DWC) immersed in bulk water phase can efficiently adsorb dissolved methane molecules and in top of that the DWC lifetime increases exponentially with the number of methane molecules adsorbed. At the same time adsorbed methane molecules reduce the number of hydrogen bonds that tend to break the DWC, and change the hydrogen bond network of the DWC surroundings from the liquid water structure toward the cagelike structure of hydrate. Such behavior argues in favor of considering hydrate nucleation as a cooperative process, where the all guest molecules can stabilize hydrate cages and hydrate structure itself [15, 39]. Another approach to describe initial evolution of water + gas systems toward hydrate formation is done by Guo et al. [40]. They proposed the cage adsorption hypothesis (CAH). This theory predicts a critical concentration of ͠ 0.04 molar fraction for methane aqueous solution, above which methane hydrate begins to nucleate [37].
To obtain a deep insight into the mechanisms behind initial stages of the metastable supersaturation phase and growth of the hydrate phase under a high driving force (high excess pressure and low temperatures of supercoiling) the different MD techniques and methods have been developed. The key parameters found are order parameters [41, 42]. Nucleation pathways associated with both the host or guest ordering and mutual influence of the guest and host ordering are being explored. These parameters are used for describing the dynamics and the mechanisms of hydrate formation and other key properties.
The number of different simulation works [43–47] revealed that nano-bubbles of a gas phase trapped inside water phase could act as centers of hydrate nucleation on the liquid-gas interface that functions as the heterogeneous nucleation center. Also in works of Uchida group [48, 49] it was shown that induction time of C2H6 and Xe hydrates is significantly lower if the water solution was made using a hydrate with nano-/micro- bubbles formation that acts as nucleation centers.
In the MD simulations applied thermostat dissipates the released energy from the hydrate formation process and restores selected thermodynamic properties of the system. Under these conditions, the formation of hydrate cages and the assembly of amorphous hydrate-like clusters which relax over time to more stable uniform or mixtures of hydrate phases has been observed [50].
To produce hydrate structures formation with the presence of the gas-water interface, the MD simulation studies are carried out at high pressures (from 500 to 2000 bar) and/or low temperature [14–19, 22, 31–33, 36, 37, 51–54]. In the mentioned works, the nucleation time varied from several hundreds of nanoseconds up to several microseconds depending on the exact thermodynamic (P-T) conditions, selection of guest type, and used model of water molecules.
The different approach was applied for studying supersaturated homogeneous aqueous methane solution by Sarupria and Debeneditti [55]. The authors simulated a methane solution with 0.07 mol fraction of methane at T = 240 K and P = 200 bar and reported several hundred nanoseconds nucleation time for 0.07 molar concentration of methane, and found structures similar to structures reported in the case of gas-water interface at similar temperature [14–19, 31–33, 36, 37]. At higher methane concentration (0.08) the nucleation time is drastically reduced [56]. Moreover, increasing subcooling degree leads to formation more ordered hydrate structures.
Later simulations showed four phases of nucleation [57]. There are increase of solvated methane concentration in the aqueous domain via diffusion through the methane − water interface, the formation of unstructured cluster of methane molecules solvated in water, the decreasing of water content of the nucleus to a value compatible with the type II methane clathrate hydrate composition and final reordering of solvated methane and water as is ‘blob’ hypothesis [33].
The structures of methane, propane, methane + propane hydrates formed from water mixtures with high structural order and low computational time has been obtained using MD simulations [56, 58]. In these works a moderate subcooling level has been applied and overall thermodynamic conditions considered were within P and T range of practical interest. Another important steps to reveal hydrate formation mechanism were made in the following works [34, 51, 52, 55, 59]. where the initial stages of the metastable supersaturation phase and growth of the hydrate phase at high excess pressure and low temperatures of supercooling with an aqueous phase, prepared by melting the hydrate phase has been studied. The study of hydrate nucleation in “methane + water + THF” system [60, 61] showed the dependence of induction time, growth rate and cage structure on P-T conditions and THF concentration.
Later we studied dynamic hydrogen bonds network of liquid water phase [5, 62]; based on this basis we developed efficient methods for deep analysis of hydrogen bonds network and method for determining hydrate nucleus. In previous work [63] we showed the possibility of methane hydrate growth from bulk water + methane solution or in the presence of sea salt and hydrate seed at excess pressure without notable induction time. However the nucleation mechanism was not studied.
In the presented work we have focused on examination of methane hydrate growth outside its thermodynamic stability region (without excess pressure and/or supercooling is applied) using MD simulations methods from supersaturated methane solution. All initial solutions have been made by uniform distribution of gas molecules inside bulk water phase, which is resulted in ideally stirred solution. The aim is to study the dynamics of H-bond network and influence of fluctuation hydrate nuclei on hydrate growth process in supersaturated methane solution with an emphasis on studying the behavior of a water molecule with a high degree of local order.