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Hydrophobic Effect on Gas Hydrate Formation in the Presence of Additives Ngoc Nguyen Nguyen, and Anh V. Nguyen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01467 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 16, 2017
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Hydrophobic Effect on Gas Hydrate Formation in the Presence of Additives Ngoc N. Nguyen, Anh V. Nguyen* School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia *
Corresponding author. Tel.: +61 7 336 53665; Fax: +61 7 336 54199; Email:
[email protected] Abstract Additives like surfactants, polymers, salts, and hydrophobic particles are well known (and used) to influence gas hydrate formation (GHF). This paper reviews and discusses the mechanisms of their effects. Apparently, the effects of additives on GHF vary greatly from an additive to another. Even a given additive can change from a promoter to an inhibitor and vice versa when the working condition is changed. The available literature cannot explain the diverse effects of additives. We argue that hydrophobic effect plays a critical role in gas hydrate formation. A dissolved hydrophobe organizes the surrounding water into a clathrate-like structure and thereby promotes hydrate formation. A hydrophile, however, disrupts the surrounding water structure and inhibits hydrate formation. Moreover, cooperative hydrophobic interactions create an increased gas concentration around a hydrophobe which also favors the hydrate formation. In contrast, a hydrophile competes with the gas for water and thereby hinders hydrate formation. Especially, when the additive is an amphiphile, the observed effect is a result of the competition between the hydrophobic moiety (a promoter) and hydrophilic moiety (an inhibitor). This hypothesis provides a universal explanation for the various effects of hydrate additives. Keywords: Gas hydrates, hydrophobic effect, inhibition, promotion, water structure
1
Introduction Gas hydrates (or gas clathrate hydrates) are ice-like crystalline solids comprising water and
suitable gases. The water molecules (the host) form a cage-like hydrogen bonded structure which
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encapsulates the gas molecules (the guest) inside, and the encaged gas molecules, in turn, exert a multi-directional force to prevent the cage-like structure from collapsing
1-2
. Such host-guest
cooperative interaction makes gas hydrates more thermodynamically stable than ice (i.e. at a given pressure, gas hydrates can form at a temperature well above the freezing point of water). Despite such astounding simplicity of their chemical composition, gas hydrates have been a topic of enduring interest. Since first discovered in 1810 by Sir Humphrey Davy 1, clathrate hydrates have increasingly attracted a widespread attention of researchers. Even though, over the subsequent century, gas hydrate research was merely for the satisfaction of scientific curiosity. Only from 1934 when Hammerschmidt
3
confirmed that clathrate hydrates cause subsea
pipelines plugging, these inclusion compounds were considered as a nuisance to flow insurances 2, 4-5
. Indeed, Hammerschmidt’s finding sparked a flourishing growth in hydrate research, with a
primary goal being for the prevention of hydrate formation inside transmission lines. From the late of 20th century, many promising applications of gas hydrates started to be conceptualized and demonstrated, including gas storage and transport anthropogenic CO2
14-19
and seawater desalination
6-9
20-21
, gas separation
10-13
, sequestration of
. Such new perspective on clathrate
hydrates has further drawn the attention of academia into this field. Also, the research started to focus on gas hydrate promotion, in parallel with the pre-existing hydrate inhibition direction. Nowadays, gas hydrates become an applied science with a dynamic growth in the number of scientific publications and form dedicated themes in many scientific journals. Here we review and discuss the molecular mechanisms of the effects of additives on gas hydrate formation. In particular, we discuss the critical role of hydrophobic effect in governing gas hydrate formation. We provide a universal explanation for the diverse effects of additives on gas hydrate formation.
2
The formation of gas hydrates
2.1 Macroscopic description and molecular concepts Gas hydrates form spontaneously when water contacts the gas under elevated pressure and low temperature
2, 5
. Figure 1 shows a typical T-P graph (T: temperature, P: pressure) recorded
from a stirred isochoric hydrate reactor 22. The huge drop in P coupled with a spike of T indicates the onset of the exothermic formation of gas hydrate inside the reactor. The waiting period 2 ACS Paragon Plus Environment
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between the point of time when T&P fall into the hydrate-forming condition and the onset of hydrate formation is described as the induction time. At the microscopic scale, induction time is the time needed for the gas to dissolve into water to produce a supersaturated solution and for the water structure to rearrange itself to accommodate the gas and form initial hydrate crystals.
Figure 1. An example of T-P graph of methane hydrate formation
22
. The sharp decreases in T
and P at the beginning of the experiment were induced by cooling. The continual decrease in P during induction period was induced by gas dissolution. A substantial drop in P coupling with a spike of T indicates the onset of gas hydrate formation in the reactor. Afterward, both T and P approached constant levels indicating that gas hydrate formation finished. Experiment was performed in a stirred isochoric reactor (0.45 L) at initial pressure of 7.5 MPa and target temperature set to 0.5°C. In nature, gas hydrates form naturally in the locations where the forming condition is met such as in permafrost regions or marine sediments with a water depth of between hundreds and thousands of meters 23. The temperature and hydrostatic pressure in those locations fall into gas hydrate stability zone which sustains the enclathration of natural gas by marine water. Interestingly, CO2 hydrate formed rapidly when liquid CO2 was disposed directly onto ocean floor at a water depth of 3650 m
24
. This experimental observation supports the idea of
sequestration of CO2 in subsea locations 15, 25. Despite such clear macroscopic observations, the molecular picture of gas hydrate formation remains poorly understood. Especially, the time-dependent (kinetic) properties are the most challenging domain in gas hydrate research enclathration remains controversial
1, 28-30
26-27
. To date, the molecular mechanism of gas
. Amongst few hypotheses available in the literature
1,
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28-30
, the labile clusters theory
1, 28
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probably provides the most insightful description of the
formation mechanism. This theory was developed by Sloan et al. based on a classical “iceberg” model originally proposed by Frank and Evans 1. It considers gas hydrate nucleation initiates from building locally structured water clusters around individually dissolved gas molecules (known as labile clusters or hydrophobic hydration shells). Under the hydrate-forming condition, the hydration shells transform into gas hydrate cages. Then, the cages agglomerate into hydrate nuclei via face-sharing. When the nucleus size exceeds a critical value, the nuclei grow rapidly into big hydrate masses (Figure 2) 1.
Figure 2. Mechanism of gas hydrate formation through labile clusters hypothesis 1. Four stages of gas enclathration process are described as A, B, C and D. Significant research efforts based on experimental measurements simulations
34-36
31-33
and computer
have sought the evidence of labile clusters. The majority of the outcomes
support the existence of a local water structure (hydration shell) around a dissolved guest, in particular, a methane molecule. Even though, the results indicate that the number of water molecules on a hydration shell of methane (the coordination number) is around 19 33, 37, which is smaller than the desired values of 20 for small sI (512) and 24 for large sI (51262) cages. Hence, the hydration shells must undergo a structural transformation, by adopting water molecules from the bulk, through various intermediate states before accomplishing a correct conformation of clathrate hydrate. The time needed for such structural rearrangement and transformation is apparently manifested by the induction period in gas hydrate experiments.
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2.2 The formation of gas hydrate at the interface Gas hydrate formation normally initiates at the gas-water interface rather than in the bulk liquid
27
. This characteristic is highly important. It suggests that interface-active agents may
affect hydrate formation more effectively than bulk-active ones do. It also infers that the interfacial water needs to be accounted for when explaining gas hydrate phenomena. The interfacial formation of gas hydrates is evidenced by the occurrence of a hydrate thin film at the guest-water interface (in quiescent condition) 38-40. The film stops growing in its thickness when it becomes diffusion-resistant and thereby hinders the mass transfer across the interface
38
. For
example, the final thickness of methane hydrate film was reported to be in between 20-100 ߤm depending on experimental condition 38. The formation of hydrate film was more observable in the system containing a small water droplet placed in a guest phase or a small guest bubble placed in an aqueous phase 39-41. Figure 3 shows the occurrence of a methane hydrate film at the surface of a water droplet in methane environment 41.
Figure 3. Sequential video graphs of methane hydrate thin film formation at a methane-water interface
41
. A water droplet was exposed to methane environment at T = 3.5°C and P = 5.66
MPa. The initiation and propagation of a hydrate film on the droplet surface is identifiable. Figure 4 explains the mechanism of hydrate formation at a methane-water interface
42
.
First, CH4 molecules transfer across the interface into water. Then the interfacial water forms hydrate cages around dissolved CH4 molecules right under the interfacial plane. Finally, the cages agglomerate into a hydrate crystal. The resulting hydrate crystal then obstructs the dissolution of methane into water. Consequently, no further hydrate growth can take place under the interface. Therefore, only a hydrate film is formed. In contrast to the quiescent system, in an agitated system, the turbulence breaks the film and the convection flow conveys the hydrate nuclei into the bulk where they grow into big hydrate masses 1. 5 ACS Paragon Plus Environment
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Figure 4. A theoretical description of methane hydrate formation at methane-water interface 42. This process involves the diffusion of methane molecules through the interface (methane dissolution), the formation of hydrate cages around dissolved methane molecules right under the interface and the subsequent incorporation of cages to form a hydrate layer.
3
The effects of additives on gas hydrate formation The formation of gas hydrates is very sensitive to the impurity of the medium. The
presence of any foreign entities other than water and the guest can cause substantial effects on thermodynamic and kinetic properties of gas hydrate systems. The foreign entities (additives) may be intentionally added to hydrate systems to control the formation kinetics, or they may present in hydrate systems naturally such as dissolved salts, bio-surfactants and colloids. In this section, we review the effects of important additives on gas hydrate formation and discuss the existing explanations for their effects. The reviewed literature in this section is helpful for our general discussion in Section 4.
3.1 The effects of thermodynamic inhibitors Thermodynamic hydrate inhibitors (THIs) are the additives that shift the phase equilibrium of a hydrating system to lower temperature and higher pressure, thereby, inhibiting the formation of gas hydrates in a prevailing condition. Systematic investigations into hydrate inhibitors were triggered in 1934 when Hammerschmidt
3
confirmed that clathrate hydrates are responsible for
subsea pipeline plugging. The best known THIs are alcoholic compounds (e.g. methanol, ethylene glycol, etc.) and inorganic salts
4, 43-44
. Since THIs affect the bulk phase of hydrate
system, they need high dosages (30-50 mass %) to give a satisfactory inhibition performance 4446
. Such high dosages lead to serious concerns about the operational and environmental costs
46
.
44-
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Fundamentally, it is widely agreed that THIs inhibit gas hydrate formation by decreasing the activity of water. THIs are highly polar molecules (or ions in the case of salts) which bind water strongly by hydrogen bonds (or by electrostatic forces for ions). The strong binding leads to increased competition with the hydrate former for water. Consequently, there is less water available for forming a hydrate with the gas 1. The effectiveness of a THI depends upon its capability of binding water
43
. For example, ethanol and ethylene glycol have similar molar
volumes, but ethylene glycol has two hydroxyl groups available for hydrogen bonds while ethanol has only one. Consequently, ethylene glycol gives a better hydrate inhibition than ethanol does 43. Similarly, multivalent ions bind water more strongly than large-size monovalent ions do 47. As a result, multivalent ions are better THIs compared to large-size monovalent ions. However, a number of THIs also exhibit as hydrate promoters when they are used at low concentration. For example, experiments have reported a promoted methane hydrate formation in dilute alcoholic solutions 1, 48-49. A similar promotion effect was also observed for CH4 and CO2 hydrate formation in dilute sodium halide solutions
50-52
. The fundamentals underpinning such
promoted hydrate formation in the dilute solutions of THIs cannot be explained by the available literature. In Section 4, we present an explanation for this peculiar effect.
3.2 The effects of low-dosage hydrate inhibitors on gas hydrate formation Unlike THIs which affect the bulk phase of hydrate-forming systems, low-dosage hydrate inhibitors (LDHIs) are interface-active agents which influence the interfacial properties of gas hydrates. The use of LDHIs is of increasing interest owing to their low operational dosages, typically less than 1 mass %
45-46
. LDHIs are conventionally categorized into kinetic hydrate
inhibitors (KHIs) and anti-agglomerants (AAs) which are different from each other by the acting mechanisms. While KHIs act primarily as gas hydrate anti-nucleators, AAs allow gas hydrates to form but prevent hydrate nuclei from further agglomerating into big hydrate masses
45
classical
(PVP),
KHIs
are
vinylic
polymers,
of
which,
polyvinylpyrrolidone
. The
polyvinylcaprolactam (PVCap) and antifreeze proteins (AFPs) are the most popular 4, 53. AAs are surface-active compounds such as alkyl aromatic sulfonates or alkylphenyethoxylates 46. Beside these traditional LDHIs, a vast number of new LDHIs are being introduced. Some of them are listed in Table 1.
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Table 1. New types of LDHIs reported in the recent literature LDHIs
Predominant effects
Pectin
66% reduction of working dosage and 10-fold increase in effective time in comparison with typical KHIs 54
Amino acids Poly-carbonyl amides Ionic liquids
Good inhibition capacity and environmental friendliness 55-57 Effective LDHIs 58-59. Much better inhibition achieved when combined with nonylphenol ethoxylates or poly ethylene glycol 60 Good inhibition capacity 61-62
Natural products (NPs)
NPs such as chitosan 63 and starches 64 are promising green LDHIs.
Synergists
The addition of a synergist (such as L-tyrosine 65, poly ethylene oxide 66, NaCl 67, ionic liquids 62 and so on) enhances the inhibition performance of the used LDHIs
It is proposed that LDHIs inhibit gas hydrate formation via binding to the surface of an ensuing hydrate nuclei 68. The adsorption of LDHIs molecules on the surfaces of hydrate nuclei disrupts the growth of hydrate crystals
68
. However, recent studies have shown that an
accelerated hydrate growth can occur in the presence of LDHIs
69-71
. For example, it was found
that while 0.5 and 1.5 mass% of PVCap could suppress the nucleation and growth rate of gas hydrate efficiently, 3.0 mass % of this agent induced a catastrophic hydrate growth 70. In general, the presence of LDHIs can retard the nucleation and growth of gas hydrate within a certain period after which a fast growth takes place steadily hydrate growth is not clear
69
69-70, 72
. The reason for such catastrophic
. Therefore, further fundamental investigations into this
phenomenon are needed.
3.3 Surfactants effects on gas hydrate formation 3.3.1 Physicochemical characteristics of surfactants Surfactants are amphiphilic compounds. Each surfactant molecule consists of a hydrophilic (water-liking) head and a hydrophobic (water-hating) hydrocarbon tail (as shown in Figure 5a for sodium dodecyl sulfate). Due to such special molecular structure, surfactants have a tendency of 9 ACS Paragon Plus Environment
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minimizing the exposure of the hydrophobic tails to aqueous phase 73. As such, surfactants prefer adsorbing on the hydrophobic-aqueous interface with hydrophilic heads remaining in the solution and hydrophobic tails spreading into the hydrophobic phase (Figure 5b). Otherwise, when the interfacial adsorption is saturated, they aggregate to form micelles in solutions (Figure 5b). The micellization takes place only when the concentration of the surfactant is higher than a certain value called critical micellar concentration (CMC), and the solution temperature is above a certain value called Krafft point (Figure 5c). Both CMC and Krafft point are specific to each surfactant 74.
Figure 5. The amphiphilic structure of SDS surfactant (a); dodecyl sulfate (DS‾) adsorption on solution surface and DS‾ micelle formation in bulk (b); the Krafft point of SDS surfactant (c). On Figure (b), the gas phase and the counter ions (Na+) are omitted for clarity. The presence of surfactants in a solution induces substantial changes in physicochemical properties of the system. Interfacial adsorption of surfactants decreases the interfacial tension between the solution and the adjacent phase
75
which may affect the mass transfer across the
interface. It may also carry charges to the interface and subsequently influences the orientation and mobility of interfacial water gases in aqueous solutions
78-80
22, 76-77
. The presence of surfactants increases the solubility of
. Moreover, due to its amphiphilic nature, a surfactant molecule
produces two opposite effects on the local water structure. The hydrophobic tail organizes surrounding water molecules into a clathrate-like cluster, as postulated by Frank and Evans in their well-known work in 1945
81
. The hydrophilic head (together with the counter ion in the
case of ionic surfactant) binds water strongly and disrupts the local water structure, similar to the 10 ACS Paragon Plus Environment
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effect of inorganic salts
82
. These special properties are of great relevance to gas hydrate
formation.
3.3.2 Evidence of surfactants affecting gas hydrate formation Table 2. Common surfactants as gas hydrate promoters Type of surfactants
Name of surfactants Sodium dodecyl sulfate (SDS) 83-88 Sodium tetradecyl sulfate (STS) 88
Anionic surfactants
Sodium hexadecyl sulfate (SHS) 88 Sodium oleate 88 Tetra-n-butyl ammonium bromide (TBAB) 11, 13, 89-92 Dodecyl trimethyl ammonium chloride (DTAC) 86, 89
Cationic surfactants
Dodecyl amine hydrochloride (DAH) 93 N-dodecylpropane-1,3-diamine hydrochloride (DN2Cl) 93 Tetrahydrofuran (THF) 10, 83, 85-86 Ethoxilated nonylphenol 94
Non-ionic surfactants
Tween 80 95 Cyclopentane 96-97
Surfactants probably provide the best acceleration of gas hydrate kinetics. It was reported that sodium dodecyl sulfate (SDS) solution of 284 ppm increased the rate of methane enclathration by 700 times works
83-87
98
. This surfactant was also investigated intensively in many other
. Besides SDS, a huge number of other surfactants were investigated. The most
common of them are listed in Table 2. In general, it was found that the addition of surfactants could increase hydrate kinetics by the orders of magnitude. Regarding the effect of surfactant types, it was sometimes noted that anionic surfactants were more effective than non-ionic surfactants yet cationic surfactants were effective only at low concentrations
99
. However, this 11
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conclusion might be overgeneralized as dodecyl amine hydrochloride (DAH) and Ndodecylpropane-1,3-diamine hydrochloride (DN2Cl), both are cationic, exhibited a strong promotion of methane hydrate formation at a concentration as high as 3000 ppm 93. Interestingly, some surfactants display better promotion performances when are mixed. For example, a more efficient promotion was achieved when SDS was mixed with tetrahydrofuran (THF)
83, 85
.
Likewise, tetra-butyl ammonium bromide (TBAB) and dodecyl trimethyl ammonium chloride (DTAC) displayed a better promotion under a shared effect than the individual effect of each surfactant 89.
3.3.3 Governing mechanism of surfactants for gas hydrate formation The actual mechanism by which surfactants affect gas hydrate formation is poorly understood. There are a number of hypotheses for explaining the accelerated formation of gas hydrates in surfactant solutions, but they are constructed based on assumptions rather than experimental evidence. For example, it is widely believed that surfactant aggregates (micelles) are responsible for the promoted formation of gas hydrates in surfactant solutions
98
. The
micelles are deemed to act as minute gas reservoirs which facilitate the gas dissolution and provide nucleation sites for gas hydrate formation
98
. However, this hypothesis has been under
criticism that surfactants, more likely, cannot form micelles in the condition of gas hydrate experiments since the working temperature goes lower than the Krafft point
87, 100
. Even though
micelles are still used as a theoretical framework for interpreting the accelerated formation of gas hydrates in surfactant solutions on these days 101. However, given if micelles do present in hydrate-forming systems, how do they promote hydrate formation still being an open question. The micelles are assumed to act as minute gas reservoirs and provide nucleation sites for gas hydrates
98
. Indeed, a hydrophobic guest (i.e.
ethane as on Figure 6) can be favorably confined in the interior (core) of the micelle thanks to hydrophobic interactions. Therefore, surfactant micelles may act as minute gas reservoirs in the solution. However, the nucleation of hydrate in the confined space within a micelle may not be possible, due to the exclusion of water. Similarly, the nucleation of gas hydrate on or around a micelle is possibly unfavorable as well, due to the high density of ions in such location (as seen in Figure 6). The ions cause a radical disruption of local water structure, similar to in concentrated saline solution
82
. Hence, more convincing evidence is needed to confirm or rule 12 ACS Paragon Plus Environment
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out such role of surfactant micelles in gas hydrate formation. In another hypothesis, it was proposed that the accelerated formation of gas hydrates in surfactant solutions arises from the adsorption of surfactants on hydrate-liquid interfaces 102. The observed promotion was attributed to the decrease in interfacial tension due to the surfactant adsorption 87, 102.
Figure 6. Illustration of a micelle SDS surfactants in ethane hydrate-forming system
98
. The
surfactant micelle is thought to act as a minute gas reservoir, thereby, facilitates the gas hydrate formation. The circular grey area presents a possible micellar interior space (bounded by the dotted line) with high density of ethane. The straight lines represent the hydrophobic part of SDS molecules. Nonetheless, the available hypotheses cannot provide a satisfactory explanation for the dual effects (both promotion and inhibition) of surfactants
22, 103
. It is also hard to explain the
difference in the effectiveness between surfactants. Table 3 shows that the effectiveness of different surfactants is various. For example, Du et al. compared the effects of four surfactants on methane hydrate formation in the same experimental condition
93
. The results showed that
sodium dodecyl sulfate has the strongest effect (in term of decreasing induction time and increasing methane uptake), followed by dodecyl amine hydrochloride and then Ndodecylpropane-1,3-diamine hydrochloride. The fourth surfactant, dodecyl trimethyl ammonium chloride, however, has no discernible effect on methane hydrate kinetics 93. Similarly, Okutani et al. showed that sodium dodecyl sulfate and sodium tetradecyl sulfate enhance gas hydrate kinetics strongly while the effect of sodium hexadecyl sulfate is lesser oleate has no discernible effect
88
88
. In contrast, sodium
. The available literature is insufficient for explaining these
experimental data. In Section 4, we discuss the possible explanation for these data.
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Table 3. The effectiveness of surfactants in promoting gas hydrate formation Surfactants
Promotion effectiveness
Sodium dodecyl sulfate (SDS)
Strong 88
Sodium tetradecyl sulfate (STS)
Strong 88
Sodium hexadecyl sulfate (SHS)
Moderate 88
Sodium oleate (SO)
Indiscernible 88
Sodium dodecyl sulfate (SDS)
Strong 93
Dodecyl trimethyl ammonium chloride (DTAC)
Indiscernible 93
Dodecyl amine hydrochloride (DAH)
Strong 93
N-dodecylpropane-1,3-diamine hydrochloride (DN2Cl)
Moderate 93
3.4 The effects of hydrophobic particles on gas hydrate formation 3.4.1 The hydrophobicity of solid surfaces The term hydrophobicity describes the water-hating characteristics of material. It is macroscopically measured by the non-wettability of a surface. The opposite is hydrophilicity which indicates the wettability of a surface. Quantitatively, the contact angle of a water droplet on a surface is a measure of surface hydrophobicity. In gas hydrate formation, the involvement of solid surfaces is inevitable. There always present the surface of equipment. Likewise, the formation of natural gas hydrates in geological sediments is always affected by the surfaces of minerals, rocks and solid masses. In many cases, solid particles are intentionally added to hydrate-forming systems as hydrate additives, amongst them hydrophobic fumed silica powder is the best known. This powder is of ultrafine size with the surface being coated by alkyl chains 104. In gas hydrate experiments, it is mixed with water to a proportion of 1-5 mass % by using a highspeed blender to disperse water phase into minuscule droplets whose surfaces are fully covered
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by the powder
6-7, 105
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. The resulting mixture is apparently dry, free-flowing and usually referred
to as dry water 6-7.
3.4.2 Evidence of hydrophobic particles affecting gas hydrate formation Previous studies showed that hydrophobic particles (or more strictly saying, hydrophobic solid surfaces) could promote the formation of gas hydrates effectively. Especially, dry water produces an excellent promotion for both carbon dioxide
106
and methane
6-7, 107
hydrates. For
example, the methane uptake was increased from 3 (v/v, gas volume in standard condition per hydrate volume) in ordinary water to 175 (v/v) in dry water, under the same experimental condition 6. The induction time was also shortened to 5-10 minutes in dry water system 6. In another work, the effect of surface hydrophobicity of sands on hydrate formation was studied 108. The results showed a decrease in the induction time, from 576 s for untreated (hydrophilic) sands to 71 s for treated (hydrophobic) sands
108
. Furthermore, hydrate formation was more
deterministic (less stochastic) when hydrophobic sands were used 108. Table 4. Effects of hydrophobic solid surfaces on gas hydrate formation Solid surfaces
Predominant effects
Hydrophobic fumed silica (dry water)
Gas uptake increased from 3 (v/v) in ordinary water to 175 (v/v) in dry water with short induction time 6. No agitation needed 6-7
Hydrophobized silica sands
Induction time decreased from 576 (s) in original (hydrophilic) sand beds to 71 (s) in treated (hydrophobic) sand bed; Induction time was more reproducible in hydrophobic sand beds 108
Hydrophobized glass beads
Shortened induction time and increased hydrate formation rate. No agitation needed 109
Metallic surfaces
Shortened induction time and enhanced final water conversion 110
Wet nanopore activated carbon
Unprecedented high gas uptake (24 g methane per 100 g dry activated carbon) with ultrafast hydrate formation kinetics 8
Carbon nanotubes
The gas uptake increased by 300% with shorter induction time compared with pure water 111
Graphene oxide
The induction time decreased by 96% compared to the case of pure water 112
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Carbon-based materials also attracted a widespread attention of researchers. For example, methane hydrate formation in confined spaces of activated carbons was studied 8. The results showed an ultrafast hydrate formation with an unprecedented high methane uptake, namely 24 g CH4 per 100 g dry activated carbon, achieved under mild condition (2ºC and 3.5 MPa) 8. Similarly, graphene oxide particles
112
and multi-walled carbon nanotubes
111
were found to
provide a good hydrate promotion. Table 4 presents a number of common hydrophobic solid surfaces that were used as hydrate promoters. From these data, it suggests that the hydrate formation is possibly governed by the surface hydrophobicity rather than by the chemical nature of the substrates.
3.4.3 Governing mechanism of hydrophobic particles for gas hydrate formation The promoted hydrate formation in the presence of hydrophobic particles is conventionally attributed to the increased interfacial area 6, 8. For example, the average size of dry water (water droplets covered by hydrophobic silica powder) was found to be 20 µm 6. Dry water, therefore, is deemed to provide an adequate gas-water contact for a fast hydrate formation 6. Similarly, the ultrafast methane hydrate formation in activated carbons was attributed to the facilitated methane-water contact in the interior pore surface 8. In another saying, hydrophobic particles are assumed to promote gas hydrate formation through a kinetic route. However, it was also found that dry water shifted the equilibrium of methane hydrate to a higher temperature and a lower pressure
105
. This effect indicated that dry water acted as a
thermodynamic promoter rather than a phase-contact facilitator. Moreover, hydrate kinetics was improved by increasing the surface hydrophobicity of the solids
108, 113
. These findings are very
interesting, but they cannot be explained the available literature. In Section 4, we present a universal explanation for the effect of hydrophobic particles on gas hydrate formation.
4
General discussions Our literature review and discussion in previous sections have indicated a significant
deficiency of the current literature for explaining the effects of additives on gas hydrate formation. Previous works were overwhelmed on macroscopic measurements of kinetic and thermodynamic properties of hydrate systems under the presence of hydrate additives
114-116
yet 17
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insufficient attention was paid to the fundamental understanding. The macroscopic measurements produce experiment-specific results, the results that depend on the conditions adopted (e.g. type and concentration of additives, type of equipment, etc.) 26. Consequently, it is difficult to generalize and conceptualize the findings. In fact, water structure plays a vital role in gas hydrates
1, 117
. Especially, the dynamics of
hydrogen bonds is of paramount importance. The labile cluster, the memory effect and the metastability of hydrate systems are all played by hydrogen bonds. However, hydrogen bonds remain to be a long-standing mystery 118-121. This knowledge gap has created many difficulties in elucidating hydrate phenomena at the microscopic level, which is apparently reflected by slow progress in understanding time-dependent properties of gas hydrates (while the structural properties and thermodynamics have been well documented)
26-27
. It also leads to difficulties in
understanding the effects of additives at the molecular level. The existing explanations for the promoted or inhibited hydrate formation in the presence of additives are additive-specific and experiment-specific, meaning that each of the explanations is only valid for a specific experimental condition. In following sections, we discuss that hydrophobic effect serves as a universal mechanism of the effects of hydrate additives.
4.1 Hydrophobic effect in gas hydrate formation The hydrophobic effect in the context of this paper describes a hydrophobicity-specific effect that an additive introduces to the hydrate system. In another saying, it refers to the effect that depends on the hydrophobicity rather than the chemistry of the additive. We argue that the hydrophobic effect plays a key factor governing the effects of additives on gas hydrate formation. We justify this argument as follows. According to a concept postulated by Frank and Evan experimental evidence
122
81
and confirmed by recent
, a dissolved hydrophobe organizes the surrounding water into a
clathrate-like structure (known as hydrophobic hydration shell). Moreover, the cooperative hydrophobic interaction between the hydrophobe and the gas also create an increased local gas concentration around the hydrophobe
123-124
. Since the formation of first gas hydrate cages in
pure water is thermodynamically difficult (due to a negative change of entropy), the presence of the clathrate-like hydration shells can act as the seeds (nucleation sites) in the solution. The 18 ACS Paragon Plus Environment
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nearby gas molecules then incorporate to the seeds to build up gas hydrate cages. The increased local density of gas around the hydrophobe makes this cooperative nucleation occur faster. This molecular picture is schematically depicted in Figure 7. It describes the hypothesis of hydrophobic effect on gas hydrate formation. In fact, the formation of semi-clathrate hydrates of tetra-butyl ammonium bromide (TBAB), tetrahydrofuran (THF) or cyclo-pentane (CP) provides concrete evidence confirming this hypothesis. The solutes (TBA+ cation, THF or CP molecule) form their clathrate-like hydration shells first; then the dissolved gas molecules incorporate to the shells to build up gas hydrate structures 92, 125-127.
Figure 7. Hypothesis of hydrophobic effect on gas hydrate formation in the presence of a hydrophobic additive In the light of this hypothesis, the effect of the hydrophobicity of an additive can be discussed. Purely hydrophobic additives (e.g. CP) produce a greater hydrophobic effect per one dissolved molecule. However, these additives have a poor solubility in water. In contrast, partial hydrophobic additives (e.g. TBA+ and THF) produce a lesser hydrophobic effect, but they have good solubility in water. Therefore, it may be difficult to compare the collective effects of additives based their hydrophobicity. However, it can be guaranteed that for a single dissolved molecule, a better hydrophobicity results in a better hydrate promotion performance. This argument is supported by the experimental observations that tetra-butyl ammonium bromide (TBAB) and tetra-pentyl ammonium bromide (TPeAB) are hydrate promoters, but tetra-propyl ammonium bromide (TPrAB), tetra-ethyl ammonium bromide (TEAB) and tetra-methyl ammonium bromide (TMAB) are hydrate inhibitors
128
. Obviously, TBAB and TPeAB have
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longer hydrocarbon chains and therefore are more hydrophobic in comparison with TPrAB, TEAB, and TMAB (Figure 8). When an additive is hydrophilic, it binds water strongly and disrupts the local water structure. A hydrophile competes with the gas for water. These effects are opposite to the effects of a hydrophobe. Consequently, a hydrophile inhibits gas hydrate formation. Moreover, it is noted that this hypothesis deals with the effects of additives on the nucleation stage of gas hydrate formation. Therefore, it may not be applicable to the additives which affect the growth stage (e.g. anti-agglomerants). In the following sections, we discuss the application of this hypothesis in explaining the effects of various additives on gas hydrate formation.
Figure 8. The effects of hydrocarbon chain length (hydrophobicity) of tetra-alkyl ammonium bromide on gas hydrate formation. The single sphere represents the Br‾ counter ion
4.2 Hydrophobic effect of amphiphilic compounds on gas hydrate formation In the light of hydrophobic effect, it is inferred that an amphiphilic molecule creates two oppositely competitive effects on gas hydrate formation. While the hydrophobic part acts as a hydrate promoter, the hydrophilic part (and the counter ion in the case of ionic surfactants) acts as a hydrate inhibitor. The macroscopic observation of promoted or inhibited hydrate formation in the solution of an amphiphilic compound is the reflection of the competition between these two component effects. Therefore, a given additive may have both promotion and inhibition effects as reported experimentally 1, 48-49, 103, 129-130, depending on which of the component effects 20 ACS Paragon Plus Environment
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being dominant in the working condition. This molecular picture explains the dual effects (both promotion and inhibition) of many amphiphilic compounds on gas hydrate formation 129-130
1, 48-49, 103,
. In a quantitative way, the latest spectroscopic findings have made it possible to quantify
the hydrophobic effect on gas hydrate formation. The local water structure around amino acids has been probed and correlated with gas hydrate formation
55, 131-132
. Amino acids with lower
hydrophobicity (having a smaller hydrocarbon group or stronger hydrophilic group) were found to disrupt local water structure strongly and inhibit gas hydrate formation effectively. In contrast, amino acids with higher hydrophobicity enhanced the local water structure and acted as hydrate promoters
55, 133
. These experimental findings provide convincing evidence of the competition
between the hydrophobic and hydrophilic moieties of an amphiphilic molecule in affecting gas hydrate formation. The findings thereby support the hydrophobic effect hypothesis.
4.3 Hydrophobic effect of solid particles on gas hydrate formation The analyses of the solution-solid interface by using molecular dynamics simulation and Raman spectroscopy have shown two important findings
113, 134
. First, water structure was
enhanced near a hydrophobic surface but disrupted near a hydrophilic surface
113, 134
. These
effects respectively share the same physicochemical principle with the effects of a hydrophobe and a hydrophile on local water structure as discussed previously. For example, Raman spectroscopy was recently used to study water structure in the vicinity of glass surface of different hydrophobicity
113
. Three cases investigated were clean glass surface (hydrophilic),
glass surface coated with N,N-dimethyl-Noctadecyl-3-aminopropyl trimethoxysilyl chloride (partially hydrophobic) and coated with octadecyltrichlorosilane (hydrophobic). The spectral results showed that water structure gets more ice-like ordering in the vicinity of hydrophobic surface than in the bulk phase or near either hydrophilic or partially hydrophobic 113. This finding agrees well with the results from recent simulation work
134
. In the meantime, the formation of
THF and CP hydrates in the presence of each of these glass surfaces (particles) was also investigated
113
. The kinetics data showed a correlation between hydrate kinetics and particle’s
surface hydrophobicity hydrates
108, 134
113
. This finding is consistent with recent results on both CO2 and CH4
. Moreover, simulation data also showed that gas concentration was increased
near a hydrophobic surface while decreased near a hydrophilic surface (Figure 9) 134. Both of the 21 ACS Paragon Plus Environment
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effects (on water structure and gas density) were governed by the hydrophobicity of the solid surface, which agreed with the experimental observations that gas hydrate formation was increasingly promoted by increasing the hydrophobicity of the solid surfaces
108, 113, 134
. These
findings indicate that the hydrophobic effect hypothesis is also valid for macroscopic hydrophobe and hydrophile (particles instead of molecules). More importantly, it is inferred that the hydrate formation is governed by surface hydrophobicity rather than the chemistry of solid surface. This is the reason why various solid surfaces with different chemistries can exhibit comparable promotion effects (see Table 4).
Figure 9. Simulation snapshots show an increased methane concentration near a hydrophobic surface (a) in contrast with a decreased concentration near a hydrophilic surface (b)
134
. Such
interfacial gas enrichment (a) contributes to the origin of the promotion of gas hydrate formation under the presence of hydrophobic solid surfaces (as listed in Table 4).
4.4 Hydrophobic effect of inorganic ions on gas hydrate formation Inorganic salts are well-known thermodynamic hydrate inhibitors. However, recent experiments have shown a promoted formation of CH4 and CO2 hydrates in dilute NaX solutions (X = I‾ and Br‾), along with an inhibited formation in the concentrated solutions
50-52
. Here we
use the hydrophobic effect hypothesis to explain such a peculiar promotion effect of the NaX. First, it is argued that the peculiar effect of sodium halides on gas hydrate formation arises from the ions-specificity of the halides ions
50
. The ions-specificity states that the effect of ions on
water structure depends on their charges and sizes. Inorganic ions like halides (I‾, Br‾, Cl‾ and F‾) form their solvation shells in aqueous solutions. However, the structure of their solvation shells changes from I‾ to F‾, depending upon their ion size. I‾ has the largest radius and thus lowest charge density. Therefore, the electron shell of I‾ interacts weakly with its nucleus and 22 ACS Paragon Plus Environment
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can be distorted by an external field (i.e., by the nearby charge or water dipoles) 135-136. For this reason, I‾ is known as a soft, polarizable ion
135
. The softness and polarizability of halide ions
decrease in the order: I‾, Br‾, Cl‾ and F‾. Indeed, F‾ has a small radius and, hence, high charge density and are referred to as a hard ion
135-136
. Soft ions (I‾) bind water molecules weaker than
the water molecules bind themselves. The consequence is that I‾ does not disrupt the surrounding water network strongly and, thus, the solvation shell of I‾ is a cage-like structure similar to the hydration of nonpolar solutes (see Figure 10)
136
. In contrast with soft ions, hard ions (like F‾)
bind water strongly and disrupt the intrinsic water network radically (i.e., all hydrogen bonds between surrounding water molecules are broken, see Figure 10)
136
. Moreover, I‾ has a strong
surface propensity (tendency of staying at the solution surface, similar to a hydrophobe) while F‾ prefer staying in the bulk phase spectroscopic experiment review
135
138
135, 137
. This phenomenon has been confirmed by both
and computer simulation
139-140
and discussed in details in another
. Such anomalous properties of I‾ imply that a large, soft inorganic ion can behaves
like a hydrophobe (non-polar solute) in aqueous solutions. Thus, I‾ is deemed to bear hydrophobic nature. In contrast, F‾ is very hydrophilic (a water-loving ion). Br‾ and Cl‾ are intermediate. The hydrophobicity of a soft ion (i.e. I‾) is evaluated by its surface propensity which can be quantified by using spectroscopic experiment
138
or molecular dynamic simulation
technique 139.
Figure 10. Solvation shell of (a): a hard cation (i.e. Na+); (b): a hard anion (i.e. F‾); and (c): a soft anion (i.e. I‾). The intrinsic water network collapses around a hard ion (hydrogen bonds are broken) due to the strong binding between the ion and water molecules. In contrast, water still maintains its intrinsic network around a soft ion as the water-water binding is stronger than ionwater binding, similar to the cage-like hydration of non-polar solutes. Figure reproduced from 136
.
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Based on the hydrophobic nature of halide ions, the dual effects (both inhibition and promotion) the NaX on gas hydrate formation can be elucidated. At high concentration of NaX, the increased competition between ions and the gas for water results in a shortage of water for gas hydrate formation. Consequently, gas hydrate formation is inhibited. In low concentration regime, however, the competition does not cause a significant effect in the availability of water, yet the presence of cage-like hydration shells of halide ions is proposed to act as seeding for gas hydrate formation (Figure 11) 50.
Figure 11. The explanation of the dual effect of halide ions on gas hydrate formation 50. Cagelike hydration shells of iodide seed the gas enclathration (middle image). The increased completion for water in concentrated salt solutions inhibits gas enclathration (right image)
4.5 Water structure at the interface in the presence of surface adsorption of amphiphilic compounds The interfacial water structure is of paramount importance given the fact that gas hydrate nucleation takes place at the interfaces
38-42, 141
. Driven by the hydrophobic assembly,
hydrophobic or amphiphilic compounds tend to adsorb on the interface for minimizing the exposure of the hydrocarbon moiety to the aqueous solution
123
. The interfaces involved in gas
hydrate formation include gas-solution interfaces, hydrate-solution interfaces and solid-solution interfaces (also including equipment wall-solution interface). The interfacial adsorption leads to an increased presence of the additive at the interface. It is, therefore, critical to know the additive-induced changes in the interfacial water first, before being able to elucidate how such changes in the interfacial water affect the subsequent nucleation and growth of the hydrate. Consider the gas-solution interface for example. Recent experiments have reported an anomalous inhibition of gas hydrate formation in dilute solutions of sodium dodecyl sulfate 24 ACS Paragon Plus Environment
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(SDS)
22, 103
. The results showed that, at a ultralow concentration (i.e. 0.3 mM), SDS had a
negligible effect on the bulk solution, but the interfacial adsorption was already significant 22. As a result, the effect of the SDS in the bulk solution could be neglected. In contrast, the effect of the SDS at the interface was found to be important. At the interface, the hydrophobic tails of the dodecyl sulfate (DS‾) anions were inserted into the gas phase (see simulation snapshot on Figure 12a). Therefore, they were deemed to have no considerable effect on the hydrate formation. The effect of the hydrophilic heads at the interface was quantified by using interface-specific techniques (Sum frequency generation vibrational spectroscopy with the aid of MD simulations). The results showed a strong water alignment underneath the surface charge (adsorbed DS‾ anions)
22, 142
. Such aligned configuration of water has been proven to be the origin of the
anomalous hydrate inhibition in dilute surfactant solutions
22, 142
. Figures 12b and 12c indicate
the enclathration of methane in ordinary water (b) and in the aligned water underneath surface charge (c) 22. The enclathration of methane is inhibited in aligned water because the driving force of methane enclathration tends to organize the water into a hydrate cage while the interfacial field inclines the water into aligned.
Figure 12. The adsorption of DS‾ anions on the gas-solution interface in ultralow concentration regime (a); the methane enclathration in ordinary water (b) and in the aligned water underneath surface adsorption of surfactants (c)
22
. The negative charges in (c) represent the hydrophilic
heads of DS‾.
5
Conclusion and remarks We have reviewed the effects of important additives on gas hydrate formations. The
experimental observations on effects of these additives are extremely diverse. The available 25 ACS Paragon Plus Environment
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literature faces deficiencies to explain the experimental data. We have presented a hypothesis based on hydrophobic effect for understanding gas hydrate formation in the presence of additives. The hypothesis deals with the effects of an additive on the local water structure and local gas concentration, as dependences upon its hydrophobicity. This hypothesis provides a satisfactory reasoning for various effects of hydrate additives. It, therefore, can serve as a universal mechanism of the effects of additives on gas hydrate formation. Future research is needed to further validate the proposed hypothesis and includes the followings. - A quantified correlation between the hydrophobicity of additives, local water structure and gas hydrate formation would be needed. For “molecular” additives, their hydrophobicity can be changed by altering the hydrocarbon tails and their function groups. Their effects on local water structure can be quantified by spectral means (e.g., Raman spectroscopy, sum frequency generation vibrational spectroscopy). The effects of hydrophobicity of the additives on clathrate hydrate formation can be determined by kinetics measurement. Indeed, this research direction has been recently undertaken with amino acids
55, 131-132
. The local water structure
around amino acids was probed and correlated with gas hydrate formation
55, 131-132
. The
research outcomes should be expanded to surfactants and other compounds. - Quantifying the hydrophobicity of a solid surface, the water structure in the vicinity of the surface and the kinetics of gas hydrate formation in the presence of the surface would be needed. The hydrophobicity of a solid surface can be changed by surface coating. Water structure in the vicinity of the surface can be quantified by spectral means (e.g., Raman spectroscopy, sum frequency generation vibrational spectroscopy) and computer simulations. The effect of the solid surface (solid particles) on clathrate hydrate formation can be determined by kinetics measurement. From the obtained results, the correlation between surface hydrophobicity, local water structure and hydrate kinetics can be constructed. This research direction has been successfully performed by different research groups
108, 113, 134
.
However, we would need further systematic investigations. The outcome from these research activities will address gas hydrate promotion and inhibition through the changes in local water structure. Hence, it can provide the molecular and
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universal understanding of gas hydrate promotion and inhibition. It, therefore, further validates the proposed mechanism.
Acknowledgements Ngoc N. Nguyen gratefully acknowledges Australian Government for the Australian Awards Scholarship (AusAID Scholarship).
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GA
Highlights
Available literature cannot explain the diverse effects of hydrate additives
Hydrophobic effect is the key factor governing the effects of hydrate additives
Hydrophobe forms a cage-like hydration shell while hydrophile disrupts water structure
Gas density increases around a hydrophobe but decreases around a hydrophile
A universal theory for the effects of additives on hydrate formation is proposed
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