5.4 Effects of Electrolytes
141
hydrate would be limited to the thermodynamic inhibition effect of lowering the
activity of water. However, this is not the end of the story.
The surface tension of a salt solution typically goes down at low concentrations
(1 mM and below) from that of pure water whereas it goes back up at higher concentrations [104]. This rather peculiar phenomenon is known as the Jones–Ray effect
[104, 105]. The Jones–Ray effect appears ubiquitous and has been known for more
than 80 years, and yet the underlying physical mechanism remains unclear to this
day. According to the Gibbs adsorption isotherm, the Jones–Ray effect suggests that
ions positively adsorb at low concentrations and negatively adsorb at high concentrations. In other words, topmost layer of the aqueous phase where the guest gas
supersaturation is the greatest is rich in ions when the salt concentrations are low.
A related matter is that some large anions have greater affinity to an aqueous
surface than other ions in spite of their negative surface excesses. This individuality
or uniqueness of each ion species is called surface propensity or ion specificity [106–
114]. For example, iodide ions have a greater surface propensity than chloride ions.
Such ion specificity is expected to influence the nucleation of clathrate hydrates.
5.4.5 Salting-Out Effect of Ions
Another factor that impacts the nucleation of clathrate hydrates in electrolytes is the
salting-out effect of ions. The solubility of a hydrophobic, non-polar solute decreases
when salts are added to water and causes the non-polar solute to precipitate out of
the aqueous phase. This effect is called the salting-out effect and proportional to
the ionic strength. The salting-out effect has a broad range of implications, from
denaturing of proteins to the effectiveness of detergency. Since most guest gases are
non-polar and hydrophobic, the salting-out effect is expected to further reduce the
(already low) solubility of guest gases.
Lu et al. showed that a salt of a stronger ionic strength (and hence stronger saltingout effect), 1M MgSO 4 , resulted in much less thermodynamic inhibition of methane
hydrate than a salt of a weaker ionic strength (and hence salting-out effect), 2M
NaCl [115]. The chemical potential of the guest gas, and hence the driving force for
nucleation, does not change with the addition of a salt because it is identical to the
chemical potential in the gas phase (the chemical potential of water does change).
However, the kinetics is expected to be proportional to the absolute amount of the
guest gas molecules present in the salt solution especially in the topmost layer of
the aqueous phase, as expected from the principle of detailed balance we detailed
in Chap. 1. Since there are twice as many numbers of ions in 2M NaCl than in 1M
MgSO 4 , Lu et al.’s results suggest that the numbers of ions are a much more important
factor than the ionic strength of the salt solution in the inhibition of clathrate hydrates
(i.e., the colligative effect dominates).
141
hydrate would be limited to the thermodynamic inhibition effect of lowering the
activity of water. However, this is not the end of the story.
The surface tension of a salt solution typically goes down at low concentrations
(1 mM and below) from that of pure water whereas it goes back up at higher concentrations [104]. This rather peculiar phenomenon is known as the Jones–Ray effect
[104, 105]. The Jones–Ray effect appears ubiquitous and has been known for more
than 80 years, and yet the underlying physical mechanism remains unclear to this
day. According to the Gibbs adsorption isotherm, the Jones–Ray effect suggests that
ions positively adsorb at low concentrations and negatively adsorb at high concentrations. In other words, topmost layer of the aqueous phase where the guest gas
supersaturation is the greatest is rich in ions when the salt concentrations are low.
A related matter is that some large anions have greater affinity to an aqueous
surface than other ions in spite of their negative surface excesses. This individuality
or uniqueness of each ion species is called surface propensity or ion specificity [106–
114]. For example, iodide ions have a greater surface propensity than chloride ions.
Such ion specificity is expected to influence the nucleation of clathrate hydrates.
5.4.5 Salting-Out Effect of Ions
Another factor that impacts the nucleation of clathrate hydrates in electrolytes is the
salting-out effect of ions. The solubility of a hydrophobic, non-polar solute decreases
when salts are added to water and causes the non-polar solute to precipitate out of
the aqueous phase. This effect is called the salting-out effect and proportional to
the ionic strength. The salting-out effect has a broad range of implications, from
denaturing of proteins to the effectiveness of detergency. Since most guest gases are
non-polar and hydrophobic, the salting-out effect is expected to further reduce the
(already low) solubility of guest gases.
Lu et al. showed that a salt of a stronger ionic strength (and hence stronger saltingout effect), 1M MgSO 4 , resulted in much less thermodynamic inhibition of methane
hydrate than a salt of a weaker ionic strength (and hence salting-out effect), 2M
NaCl [115]. The chemical potential of the guest gas, and hence the driving force for
nucleation, does not change with the addition of a salt because it is identical to the
chemical potential in the gas phase (the chemical potential of water does change).
However, the kinetics is expected to be proportional to the absolute amount of the
guest gas molecules present in the salt solution especially in the topmost layer of
the aqueous phase, as expected from the principle of detailed balance we detailed
in Chap. 1. Since there are twice as many numbers of ions in 2M NaCl than in 1M
MgSO 4 , Lu et al.’s results suggest that the numbers of ions are a much more important
factor than the ionic strength of the salt solution in the inhibition of clathrate hydrates
(i.e., the colligative effect dominates).