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Fuel 156 (2015) 87–95

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Fuel
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The dual effect of sodium halides on the formation of methane gas
hydrate
Ngoc N. Nguyen, Anh V. Nguyen ⇑
School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Sodium halides are methane hydrate

promoters and inhibitors.
 Promoter capability decreases with

decreasing the ion size.
 Our findings are explained by

hydrophobic hydration.
 Gas hydrate promotion by halides is

owing to their hydrophobic nature.
 Salt recovery into gas hydrates is

significant.

a r t i c l e

i n f o

Article history:
Received 15 September 2014
Received in revised form 10 April 2015
Accepted 14 April 2015
Available online 23 April 2015
Keywords:
Methane
Gas hydrate
Salt effect
Hydrophobic hydration

a b s t r a c t
Inorganic salts are known to inhibit the formation of gas hydrates. Here we show the duality of sodium
halides of submolar concentration in affecting the formation of methane gas hydrates. Sodium halides,
especially NaI, at low concentration effectively promote methane hydrate formation while they all turn
to be an inhibitor at high concentration. Maximum gas consumption, growth rate and induction time
were experimentally determined as a function of salt type and concentration. We explain the dual effect
of salts by the hydrophobic hydration. The promoting effect of dilute sodium halides is due to the fact
that large and polarizable anions (e.g. iodide) behave as hydrophobic entities and interact with surrounding water molecules to form hydrophobic hydration shells whose water structure is similar to that of
hydrophobic hydration shells of methane. Since hydrophobic hydration of methane in neat water is thermodynamically unfavourable because it associates with a negative entropy change and a partial loss in
the hydrogen-bonded network, the structurally similar shells of halide ions facilitate the process of
entropy change and, therefore, facilitate gas hydrate nucleation. Our proposal also explains the decrease
in the promoting capability of salts in the order from iodide to fluoride because of the decrease in
hydrophobicity of the halide ions. The inhibition effect of salts at high concentration is explained by
the advantageous competition of the halide ions with methane gas molecules to gain water for hydration

as well as their radical effect on distorting the water structure. Our hypothesis is experimentally supported by the difference in the salt recovery into hydrates and the hydrophobicity (measured by contact
angle) of halide ions. Further research is required to obtain a fuller insight of the influence of salts and
additives on gas hydrate formation.
Ó 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +61 7 336 53665; fax: +61 7 336 54199.
E-mail address: (A.V. Nguyen).
/>0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.


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N.N. Nguyen, A.V. Nguyen / Fuel 156 (2015) 87–95

1. Introduction
Gas clathrate hydrates, commonly referred to as gas hydrates,
are of enduring interest because of their fascinating science and
huge potential applications [1]. They are ice-like crystalline solids
comprising water (the host molecules) and a suitable gas (the
guest molecules). The water molecules form a cage-like structure
which traps the gas molecules inside and the gas molecules, in
turn, stabilize the water solid-like structure [2], explaining why
gas hydrates can form at temperature fairly beyond the freezing
point. Since their first discovery in 1810 by Sir Humphrey Davy,
gas hydrates have still been a hot topic of research owing to their
scientific mystery. Although the intensive research activities have
revealed interesting insights of this simply composed but sophisticatedly behaving material, many important questions still remain a
big challenge to scientists and engineers for the future application
of methane hydrates [3].
Regarding applications, gas hydrates have enormous potentials

in energy supply to climate change and industrial processes. For
example, occurring massively in numerous oceanic sediments
worldwide and containing an energy estimated to double the
energy of total fossil fuels available [4], methane (natural gas)
hydrate poses both an excellent opportunity for future energy supply and an unignorable risk to the environment if released to the
atmosphere. Hence, establishing an environment-friendly approach
to exploit natural gas hydrate is not only important for energy security but also crucial for environment protection. In another fashion,
artificial gas hydrate is considered as a promising approach for oceanic sequestration of carbon dioxide [5–7], gas separation [8–13] and
desalination [14,15]. For example, carbon dioxide gas hydrate was
reported to form in the marine environment at the depth of
3700 m [7], supporting the idea of storage of carbon dioxide in form
of gas hydrate in ocean sediments.
Gas hydrate formation is a sensitive process which is influenced
by the aqueous solution. Both thermodynamic and kinetic properties are radically affected by additives or impurities such as surfactants [8,16–18], salts and fine solid particles [19,20]. Depending on
the type and the concentration of additives, the influence can
either promote or inhibit the hydrate formation. This fact has given
rise to a tireless effort to establish the influence of additives, with
the aim to control the formation and dissociation of gas hydrates.
Of the influencing factors, the presence of salts is critical
because of its relevance to the potential application of gas hydrate
processes. As a result, a great deal of research has been focused on
examining the influence of salts on gas hydrate formation and dissociation. The previous research, however, mostly focused on concentrated saline solutions, resulting in the conclusion that salts are
a thermodynamic inhibitor [21–25]. It was not until the very
recent work by Faezeh et al. who investigated the influence of
sodium halides, of low concentrations of between 0 and 500 mM,
on the formation of carbon dioxide gas hydrate, showing that
sodium halides at low concentrations promote the formation of
carbon dioxide gas hydrate [26]. Therefore, salts are not only an
inhibitor as widely known but can also be a promoter as in case
of low concentrations.

Besides carbon dioxide, methane is also the most common gas
hydrate former. However, methane is different from carbon dioxide in that methane does not dissociate in water whereas carbon
dioxide can partially dissociate in water, lowering the solution
pH slightly. Hence, it is important to know whether the gas
hydrates of these two guests share the same salt-dependent patterns. More importantly, previous research mostly focused on
experimental measurement of macroscopic kinetic parameters
and left the microscopic mechanism unanswered [3]. While the
inhibition of gas hydrate formation by concentrated saline solutions is attributed to the reduction in gas solubility, increase in

viscosity, water-gaining competition between ions and guests as
well as the perturbation of water structure by ions, the promotion
by dilute sodium halides solutions has not been satisfactorily
explained.
This work aims to investigate the influence of sodium halides, of
submolar concentrations (0–1000 mM), on the formation of
methane gas hydrate and provide an explanation for the experimental observation. Indeed, the research outcome helps to draw
a more comprehensible conclusion about the influence of sodium
halides on methane gas hydrate formation.
2. Experimental procedure
2.1. Materials
Methane used in this work was of 99.995% purity and bottled in
a G-size cylinder, supplied by Coregas (Brisbane, Australia). Salts
used were sodium iodide (99.99% purity, Merck), sodium bromide
(99% purity, Sigma Aldrich), sodium chloride (99.9% purity, Ajax
FineChem) and sodium fluoride (99% purity, Mallinckrodt). Water
used was deionized (DI) water produced by a Milli-Q system
(Milipore, USA). Saline solutions of different concentrations were
prepared by dissolving an accurate amount (weighted using a
Mettler Toledo digital balance with 0.0001 g sensitivity) of the
desired salt in an accurate volume of DI water using a volumetric

flask. The aqueous solutions were kept for 6 h to be stable and
homogenized before applied to experiments. In the cases of sensibly oxidisable salts such as sodium iodide and sodium bromide, the
solutions were stored in a cold and dark environment in a fridge.
2.2. Experimental setup and procedure for methane gas hydrate
formation
The system used to study the influence of sodium halides on
kinetics of methane gas hydrate formation is schematically
depicted in Fig. 1. The main component is a stainless steel reactor
(8) (Parr Instruments, USA) which has a volume of 450 mL and can
withstand for a pressure up to 2900 psi (20 MPa). It is assembled
with a stirrer (M) the speed of which is adjusted and controlled
by a speed controller (9). A set of pipes, valves and data acquisition
system are also assembled to the reactor. Of these components,
valve (2) and valve (4) are used for controlling the pressurization
of reactor, valve (3) is for depressurization and ventilation, and
valve (5) is a relief valve which can automatically activate to
release gas if the vessel is over-pressurized.
The pressure and temperature inside reactor are simultaneously
recorded by a Wika S-11 pressure transducer (PT) (Wika, Germany)
and a T-type thermocouple (TT) (Parr Instruments, USA), respectively. The time-dependent readouts are processed by a National
Instruments NI-DAQ 9174 data acquisition device before being displayed on the screen and stored in a PC by a Labview VI developed
by our team. This data acquisition system records the instantaneous pressure and temperature every second and produces average values for every 30 s. The outputs are shown and saved in both
graphical and numerical forms.
Each experiment for methane gas hydrate formation was performed in the following procedure. Reactor was initially cleaned
three times with DI water and dried by compressed air, then partially filled with 80 mL of saline solution of desired concentration
before being properly sealed. In order to eliminate contamination
to gas hydrate system, the air initially inside the reactor was discharged before starting the measurement. It was achieved by recharging the reactor with methane gas to 500 psi and then completely venting it three times. Subsequently, the reactor was pressurized to 1465 psi (10 MPa) by compressing methane from the


N.N. Nguyen, A.V. Nguyen / Fuel 156 (2015) 87–95

89

Fig. 1. Schematic diagram of the experimental setup.

gas cylinder. This high pressure, well above equilibrium pressure of
2.7 MPa for CH4 hydrate formation at 1.5 °C, suitably generated in
high driving force and, therefore, better kinetics of gas hydrate formation for our studies. It assisted in reducing the experiment time
and mitigating the stochasticity of gas hydrate formation (gas
hydrate formation is less stochastic at high driving force [1]). Once
the target pressure was reached, reactor was disconnected from
the gas cylinder. In the meantime, data acquisition system was
assembled. The reactor was then submerged into cooling bath with
temperature being pre-set to 0.5 °C. The stirrer was switched on to
rotate at a speed of 120 rpm. Then the data acquisition system
started recording and displaying the change of temperature and
pressure versus time.
Once gas hydrate formation had finished, indicated by the stabilisation of both temperature and pressure, data acquisition system and stirrer were stopped and taken off, the reactor was then
quickly released and opened. Gas hydrate crystal was quickly separated and weighted using a Mettler Toledo digital balance with
0.0001 g sensitivity. The mass of the residual solution in the reactor was also determined. The entire duration of one experiment
lasted for about 3 h.
2.3. Calculation of gas uptake and hydrate formation rate
As the reactor was enclosed and no chemical reaction occurred,
the total amount of methane inside the reactor remained constant
during the experiments and was equal to the amount of gaseous
methane at the beginning of each experiment which is described
by n0 (moles). During an experiment, methane transferred from
the gas phase into the hydrate phase and was consumed by gas
hydrate formation. Mass transfer led to a decrease in the amount
of gaseous methane in the reactor, and visually indicated by a

decrease in pressure. Gas consumption at time t is calculated using
equation of state of real gas as follows [26]:

DnðtÞ ¼ n0 À nt ¼



PV
ZRT




À

t¼0

PV
ZRT



VR. Thus, for a relative comparison the effect of salts, the change
in V (less than 1% as per our calculation) can be neglected. Z is
the methane compressibility factor which is calculated as a function
of T and P using the Brill–Beggs equation [27]. The growth rate of
hydrate, r(t), was calculated using the gas consumption versus time
as follows:

rðtÞ ¼

dDn Dnðt þ DtÞ À DnðtÞ
%
dt
Dt

ð2Þ

where the typical time step is D(t) = 0.5 min. Moving averaging
with 5 time steps was successfully applied to the numerical calculation of derivatives remove the physically unreal pikes occurring
by the numerical errors.
2.4. Determination of salt concentration and hydrophobicity
The concentrations of sodium halides in the feed solutions, gas
hydrates (product) and residual solutions (waste) were determined
by inductively coupled plasma (ICP) technique.
The hydrophobicity of halide salts was determined by measuring the contact angle between a droplet of the saturated salt solution placed on its crystal surface. The saturated solutions were
used instead of pure water to avoid the dissolution of crystal surfaces. The large salt crystals were prepared by crystallization from
their saturated solution under controlled humidity and temperature. The volume of each droplet was 1 lL. The contact angle was
measured using a camera to capture the images of the droplet on
the crystal surface. The images were then digitized and processed
to calculate the contact angle using a Matlab code developed by
our team. The images were taken at the frequency of 15 frames
per second.
3. Experimental results
3.1. Gas pressure and temperature versus time: P–T graphs

ð1Þ
t

where nt are the molar amount of methane at time t, T and P are the

instantaneous absolute temperature and pressure, R is the universal
gas constant, V is the gas volume in the reactor. The volume V is
equal to the total volume of reactor VR less the volume of fluid Vf.
As specific volume of methane gas hydrate is slightly larger than
that of water, Vf increases slightly during gas hydrate formation,
causing a small decrease in V. However, the volume of the resulting
gas hydrate is very small, compared to the total volume of reactor

Fig. 2 shows the typical results obtained with NaI solutions for
the change in gas pressure and temperature versus time. For the
first ten minutes, there was a sudden drop in both the temperature
and pressure due to the cooling of the system being immerged into
the cooling bath. The drop in pressure was induced by the gas contraction and dissolution into the liquid phase. The induction period
was then followed. During this period, the temperature of system
remained fairly constant at approximately 1.5 °C while the gas
pressure continued decreasing, indicating that methane continued


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N.N. Nguyen, A.V. Nguyen / Fuel 156 (2015) 87–95

concentrations over it and another group of curves for high salt
concentrations below it. The transition salt concentration was
50–75 mM. The dilute salt solutions promote hydrate formation
while concentration higher than the transition concentration inhibits hydrate formation. For example, 50 mM NaI solution increased
the gas consumption by about 20% compared to neat water.
However, concentrated salt solutions such as 250 mM NaF solution
and 1000 mM solutions of NaI, NaBr or NaCl significantly reduced
the gas consumption. Furthermore, no significant rise in temperature was observed for these cases because the associating slow

kinetics produced no significant amount of heat exceeding the heat
transfer efficiency of the system. It noted that as a 250 mM NaF
solution was found to virtually inhibit gas enclathration and, therefore, no higher concentrations of NaF was investigated.
3.3. Growth rate and induction time

Fig. 2. Pressure (solid lines) and temperature (dashed lines) versus time during
methane gas hydrate formation in sodium iodide solutions of different concentrations. The pressure and temperature at zero time were 1465 psi (10 MPa) and 24–
25 °C, respectively.

dissolving into liquid phase to create a supersaturated solution
needed for the initiation of gas hydrate formation. The induction
period lasted for a few minutes to hours, depending on the salt
concentration. Subsequently, the pressure still continued decreasing but the temperature started rising rapidly to about 5–7 °C and
stayed at this temperature for about one hour even though the
cooling bath kept cooling the system at the required temperature.
This increase in temperature is assigned to the exothermal formation of gas hydrates and indicates the onset of gas hydrate formation. After some time, the gas hydrate formation diminished and
completed as evidenced by the sudden drop in temperature to a
constant value and the stabilization of pressure at a constant level.
The changes in gas pressure and temperature with NaBr, NaCl and
NaF solutions share the same trend. Therefore, we only show here
the T–P graphs of the experiments with NaI solutions.
3.2. Methane gas consumption versus time
The change in gas pressure and temperature versus time was
converted into the gas consumption using Eq. (1). Fig. 3 shows
the gas consumption versus time and the corresponding temperature of methane hydrate formation in sodium halides solutions of
different concentrations. For the concentrations examined the
change in gas consumption with time follows a similar increasing
trend. The rapid increase for the first ten minutes is due to the dissolution of gas because of cooling as shown in Fig. 2. This increase
in gas consumption for this period is independent of salt type, possibly because of the low salt concentrations used in the experiments, creating no discernible effect in gas solubility [28,29].
After the first ten minutes, the gas consumption started increasing due to gas hydrate formation and became salt-dependent. The

curve for neat water (blue) divides the gas consumption curves
into two groups, one group of curves for very dilute salt

Growth rate provides explicit information on gas hydrate kinetics and, therefore, is indicative to the influence of salts. The rate of
hydrate growth in different salt solutions at different concentrations was calculated using Eq. (2). Fig. 4 shows the calculated rate
versus time. The growth rate for the first 10–20 min is not related
to gas hydrate formation and is not shown in Fig. 4.
As shown in the left graph of Fig. 4, the rate of hydrate growth
in individual solutions of low salt concentration (75 mM) reaches a
peak at a specific time which corresponds to the sudden rise in
temperature as exemplified in Fig. 2 for NaI. Evidently, this significant hydrate growth indicates the onset of gas hydrate formation.
Approximately, the next one hour is the gas hydrate formation period as evidenced by both the temperature rise and moderate
growth rate. Then, the growth rate eventually decreased to zero
(and the temperature approached a constant value – see Fig. 2),
indicating that gas hydrate formation finished. The peak of the
growth in water did not occur sooner than the peaks of the growth
in the dilute salt solutions.
The right graph of Fig. 4 shows the hydrate growth rate in high
salt concentration solutions. Evidently, the growth rate significantly decreased in the concentrated salt solutions. The peaks of
the growth rate in all solutions are below the peak of the growth
rate in water. The peaks of the growth rate in the 1000 mM NaI
and NaBr solutions are shifted to the end, indicating not only slow
growth rate but also long induction time as discussed in the next
paragraph. In the 1000 mM NaCl and 250 mM NaF solutions, no
apparent peaks on the growth rate (and temperature, not shown)
curves are observed, explicitly indicating that methane gas hydrate
formation in these two solutions was virtually inhibited. A reduction in gas consumption by approximately 50% in comparison with
water was also identified with the two solutions (Fig. 3).
We define the induction time as the time interval between the
beginning of each experiment and the point of time at which

growth rate peaked. Theoretically, induction time is the period of
time between the creation of supersaturated solution and the
occurrence of first crystals of gas hydrate. It is, however, practically
difficult to quantify induction time defined in this way. On contrary, our definition is practically applicable and provides a good
approximation for comparing the effect of salts on the kinetics of
gas hydrate formation. As shown in Fig. 4, the induction time of
methane gas hydrate formation in 75 mM salt solutions was
shorter than that in water and 75 mM NaI solution significantly
reduced the induction time of methane hydrate formation.
Fig. 5 shows the experimental results for the maximum gas consumption and induction time. Evidently, with increasing salt concentration, the gas consumption by the hydrate formation first
increased, reaching a maximum at 50–75 mM and then decreased.
The effect of the dilute NaI solution on the gas consumption was
the most significant, while the effect of NaF solutions on increasing


N.N. Nguyen, A.V. Nguyen / Fuel 156 (2015) 87–95

91

Fig. 3. Effect on sodium halides and their concentration on methane consumption versus time. The water line divides the curves into two groups, showing the dual effect of
sodium halides on the formation of methane gas hydrate: sodium halides promote gas hydrate formation at low concentration whereas they become an inhibitor at higher
concentration.

Fig. 4. Effect of sodium halides and concentration on growth rate of methane hydrate versus time. The blue line for water is shown as a reference. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)

the gas consumption was the weakest. The trend of the change in
induction time with increasing salt concentration was reverse to
the trend of the gas consumption. In case of no peak appearing
on the growth rate curve, the corresponding induction time was

extremely long and, therefore, was described as infinity.

4. Discussion about the effect of sodium halides
4.1. The dual effect of sodium halides on gas hydrate formation
Our experimental results for both the growth rate and induction
time show that the halide salts display the dual effect on methane
hydrate formation. At low concentration, the halides promote the
hydrate formation, while at high concentration they act as inhibitors. The transition concentration is around 50–75 mM. The results
also show that not only salt concentration is critical to the hydrate
formation but also the type of halide anions can significantly
impact the hydrate formation. The promoting capability decreases
in the order NaI, NaCl % NaBr, NaF. Thus, the halide radius, polarizability and charge density are the determining factors of promoting
capability of sodium halides. The larger and more polarizable
halide ions are the greater promoting capability they can display.

The dual effect of salts on hydrate formation is of both scientific
and practical importance. It contributes to a comprehensive understanding of the effect of salts on the formation of gas hydrates since
the conventional standpoint considers salts as a gas hydrate inhibitor only. A misunderstanding of the promoting behaviour of dilute
salt solutions may be serious since we may thus underestimate the
correspondingly potential risks. For example, fluids inside submarine pipelines often contain small content of salts (sodium chloride and magnesium chloride) and underestimating the effect of
their presence may cause catastrophic problems. For example,
the explosion of BP’s rig in the Gulf of Mexico in 2010 was attributed to the blockage of pipelines caused by gas hydrate formation
[30].
Understanding the microscopic conceptual picture of the salt
effect is a challenging but fascinating task. The extraordinary properties of sub-molar concentration salt solutions have increasingly
attracted research interests in several fields and have been intensively investigated thanks to the advancement in instrumental
techniques and theoretical approaches. Many ongoing efforts
reveal anomalous behaviours of dilute sodium halide solutions
on microscopic scale and, thereafter, linking them to many macroscopic observations is rewarding. However, there still remain many
mysteries about sub-molar solutions and motivates researchers in

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accomplishing the structural conformation of clathrate. We visually simplify this process as depicted on Fig. 6.
As the transformation progresses, the size and the structural
conformation of hydration shells change by establishing and
breaking hydrogen bonds, and water clusters undergo through different high-energy states such activation barriers [4].
Consequently, hydrophobic hydration shells can only form and
then transform to gas clathrate cavities if the thermodynamic driving force of formation and transformation surpasses activation barriers. This argument explains why gas hydrate can nucleate at a
certain region of temperature and pressure. Once clathrate cavities
have formed in solution, they develop to construct unit cells by
vertex-linking or face-sharing and then further grow into gas
hydrate crystals.
4.3. Possible effects of halide ions on gas hydrate nucleation

Fig. 5. Dependence of maximum gas consumption (top) and induction time
(bottom) on sodium halide salts and their concentration. The induction times at
high concentrations of NaCl and NaF are extremely (infinitely) long.

many disciplines. For instance, as the salts influence gas hydrate
kinetics in many aspects such as changing the induction time,
the growth rate and the gas consumption, they must change the
fashion of both gas hydrate nucleation and growth. In the following
sections, we attempt to provide and argue possible explanations
for our result via linking some extraordinary properties of halide
ions, water structure and microscopic picture of gas hydrate
formation.

4.2. Microscopic conceptual picture of gas hydrate formation
To discuss the possible mechanisms of the dual effect of sodium
halides on gas hydrate kinetics, we should first briefly describe the
pathway of gas hydrate formation. So far, several hypotheses of gas
hydrate formation have been proposed but all of them have been
under controversy and criticism [4]. According to a widely
accepted hypothesis developed by Sloan et al. [4] (as originally
suggested by Frank and Evans in their ‘‘iceberg’’ model), gas
hydrate nucleation originates from the formation of water clusters
around dissolved guest molecules in such process known as
hydrophobic hydration. The water at hydrophobic hydration shells
is proposed to be in ‘‘pre-hydrate’’ structure. This theory has been
intensively tested by both experimental measurements (e.g. [31–33])
and computer simulations (e.g. [34–36]) to determine water
structure on hydration shells and the coordination number. Most
of the outcomes appeared to support the hypothesis since they
proved the existence of hydration shells around hydrophobic
molecules, in particular, methane molecules. However, both computer simulation and experimental measurements have shown
that the number of water molecules in the hydration shell of
methane in aqueous solution (so-called coordination number) is
around 19 [33,37] which is smaller than the expected values of
20 and 24 for sI small (512) and large (51262) cages, respectively.
Hence, the hydration shells have been proposed to undergo
transformations, through various intermediate states, before

The enclathration of gas in neat water, as described in
Section 4.2, is thermodynamically unfavourable due to the negative
entropy change and partial loss in the number of hydrogen bonds
associating with the process of enclathration. Consequently, the

formation of first hydrophobic hydration shells of methane in neat
water is thermodynamically difficult. Apparently, this thermodynamic barrier is observed as the existence of metastability of gas
hydrate system.
In sodium halide solutions, an ion-specific effect causing the
extraordinary promotion of gas hydrate kinetics is possibly the
hydrophobic nature of halide ions. It has been suggested via both
experimental (e.g. [38–40]) and simulation (e.g. [41]) studies that
large and polarizable halide ions such as iodides display a
hydrophobic nature and, therefore, function as hydrophobic entities whereas small and charge-dense ions such as fluorides cannot
play the same role. Discussion on the physics behind hydrophobic
nature of ions is complicated and beyond the scope of this paper.
This extraordinary concept, however, is useful for approaching a
reasonable explanation of the promoting behaviour of sodium
halides.
As an ion, iodide is easily hydrated forming a solvation shell,
compared to methane. Also considered as a hydrophobic entity,
the hydration of iodide is hydrophobic hydration and its solvation
shell, therefore, is a hydrophobic hydration shell. In contrast to the
case of multivalent ions or small and charge-dense ions, iodide–
water interaction is weaker than water–water interaction [42].
We, therefore, propose that even being too weak to fully collapse
local water structure, the weak iodide–water interaction is still
sufficiently strong to structurally rearrange adjacent water molecules into a fashion similar to water structure on the hydration
shell of methane. Therefore, the existence of these similar
hydrophobic hydration shells is proposed to facilitate the occurrence of the hydrophobic hydration of methane. In the other
words, the hydrophobic hydration shells of iodide ions play the
important role of the seeding for gas enclathration.
Obviously, the hydrophobic nature of halides is the central basis
for this hypothesis. The idea that polarizable ions have their
hydrophobic behaviours has been suggested via computer simulation [41] and the inference from several spectra interpretations

[38–40]. The experimental results shown in Table 1 further prove
the hydrophobic nature of halides. The contact angle between
the saturated salt solution and the salt crystal surface is a measure
of the hydrophobicity of the ions. For example, the zero contact
angle for NaF shows that fluoride likes water and is strong
hydrated by water, whereas the contact angle of 12.7° for NaI
shows that iodide does like water as fluoride and is not hydrated
by water as much as fluoride; indeed, of the halides investigated,
iodide is the most hydrophobic halide. The results for the contact
angles in Table 1 evidently show NaI is the most hydrophobic,


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N.N. Nguyen, A.V. Nguyen / Fuel 156 (2015) 87–95

Fig. 6. Conceptual diagram of the formation of clathrate cages via hydrophobic hydration.

Table 1
Contact angle (CA) between salt crystal surfaces and their saturated solutions.
Example image
Surface

CA (°)
0s

1/15 s

5/15 s

Equilibrium (after 1 s)

NaF

0.0 ± 0.0

NaCl

0.0 ± 0.0

NaBr

6.7 ± 0.2

NaI

followed by NaBr whereas NaCl and NaF are hydrophilic. As these
salts share the same sodium cation, the difference in hydrophobicity is logically due to the halide anions.
It is evident from these results for contact angle that as fluoride
is very hydrophilic [39] it cannot play the function as effectively as
iodide does. On contrary, fluoride ion strongly interacts with adjacent water molecules electrostatically and can collapse the local
water structure radically, a phenomenon known as the perturbation of water structure by ions which is reported in the literature
[43,44]. Consequently, gas hydrate formation is hindered since
the initiation of gas hydrate nucleation associating with tetrahedrally ordered water structure is hindered.
Furthermore, the existence of transition concentrations is possible because at these concentrations, the number of ions and, therefore, their hydration shells is adequate for the seeding for
nucleation. If ion density is higher than transition concentration,
there occurs the competition between ions and gas molecules to
gain water for hydration. As ions bind water more strongly than
gas molecules do, gas molecules lose their ability to constrain
water to establish hydrophobic hydration shells (which are

12.7 ± 1.5

pre-hydrate cages). Another consequence is also a reduction in
gas solubility in concentrated salt solutions. This argument
explains the inhibiting effect of salts at high concentrations.
The final point worth discussing is the salt collection by gas
hydrates. Gas hydrate is conventionally believed to be salt-free
and, therefore, expected to be a novel method for desalination.
However, our hypothesis of considering hydration shells of polarizable ions as seeds for gas hydrate nucleation should lead to the
consequence that gas hydrate crystals must contain ions, i.e. these
ions must be encapsulated in initial seeds of gas hydrate. Indeed,
the results of the chemical assaying of the products and mass balance, as shown in Table 2, support our hypothesis. Evidently, the
synthesized methane gas hydrate was not salt-free. These results
are consistent with our results from an experimental study on
CO2 gas hydrate [26] and computer simulation by Qi et al. [45].
Furthermore, the recovery of NaI with hydrate is also higher than
recovery of NaF. A possible reason is that as NaI is more hydrophobic, it involves more actively in the nucleation of gas hydrate, as
per our hypothesis. Consequently, a larger number of iodide ions
are encapsulated in gas hydrate crystals, as the seeds for gas


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Table 2
Mass balance showing the salt distribution in methane hydrates.
Salt

a

Mass (g)

Salt recoverya (%)

Sodium concentration (g/g)

Feed

Hydrate

Waste

Feed

Hydrate

Waste

NaF

80.018
80.087
80.061

32.540
34.762
37.999

47.372
45.282
42.024

0.001024
0.002351
0.002048

0.000799
0.001787
0.001575

0.001160
0.003235
0.002463

33.74 ± 1.42

NaI

80.188
80.228
80.212
80.227

41.150
40.142
45.440
39.522

38.786
40.015
34.772
40.402

0.001058
0.001188
0.001188
0.001587

0.000842
0.001090
0.001082
0.001784

0.001352
0.001500
0.001481
0.001400

48.43 ± 3.18

Mass of salt in the hydrate phase divided by the total mass of salt in the feed.

hydrate nucleation. However, it is not known whether or not the
ions are within the crystal lattice or they are just trapped between
the hydrate crystals. Further studies are required to answer this
question.
5. Conclusion
A series of experiments were successfully conducted to investigate hydration of methane in water and sodium halide solutions.

The changes in pressure and temperature during the methane
hydrate formation were measured as a function of time and concentration of NaF, NaCl, NaBr and NaI. The gas consumption, growth
rate, induction time and maximum gas consumption were determined. The experimental results show the dual effect of the salts
on methane hydrate formation. While it has been widely reported
that salts are an inhibitor of gas hydrate formation, the outcome
of our research evidently proves that sodium halides can be either
a promoter at low concentration or an inhibitor at high concentration. Furthermore, large and soft (polarizable) ions like iodide were
shown to be more effective promoters whereas small and hard
(high-charge density) ions like fluoride were observed to be an
effective inhibitor. We propose that the difference in hydrophobicity (as measured by contact angle) of halides gives rise to this experimental observation. Our hypothesis is supported by the results of
the contact angle measurements and the salt recovery by the
methane hydrate. Further research is needed to better understand
the role of hydrophobic hydration and hydrophobic entities in gas
hydrate formation, and establish a fuller understanding of the salt
effect on the formation of gas hydrates.
Acknowledgements
The authors would like to thank to Mr. Tuan D. Nguyen for his
technical assistance in the data acquisition system and Ms. Faezeh
Farhang for her technical assistance in the experimental setup.
Ngoc N. Nguyen acknowledges the Australian Government for
the Australian Awards Scholarships (AAS).
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