140
5 Nucleation of Gas Hydrates
near a gas–aqueous interface are lower than those in the bulk solutions, and the
topmost layer of the aqueous phase is almost totally devoid of ions.
5.4.4 Concentration Gradient of Guest Gases Near a Salt
Solution–Guest Gas Interface
It has been reported that some THIs like methanol [95], ethylene glycol [96]
and propanol [97], could enhance the rate of clathrate hydrate formation when doped
at low concentrations. Farhang et al. reported a similar promotion effect of dilute
sodium halide solutions for CO 2 clathrate hydrate [98]. They reported that a variation in the concentration from 10 to 100 mM of the sodium halide solutions induced
a transition from promotion to inhibition of CO 2 hydrate formation. A promotion
effect of CO 2 hydrate formation was observed most acutely for sodium iodide, while
sodium chloride was found to be an inhibitor at all concentrations studied [98].
These promotion effects cannot arise from the thermodynamic effect of lowering of
the activity of water by the solutes and thus must be kinetic in nature.
Ions could impact the nucleation of clathrate hydrates through the local structuring
of water molecules around each ion (hydration). The hydrophobic hydration of nonpolar entities like alkanes impacts the local water structure differently than the hydration shells of hydrophilic solutes like ions. Hydrophilic ions generally put stresses
onto the tetrahedral network of hydrogen-bonded water molecules [99]. The higher
the salt concentrations the weaker the tetrahedral structure and the lower the hydration number [100]. In contrast, hydrophobic (non-polar) solutes like alkanes enhance
the tetrahedral network with larger numbers of hydrogen bonds [101–103]. It is thus
reasonable to expect that such different tetrahedral networks of hydrogen-bonded
water would influence the nucleation rate of clathrate hydrates.
As we saw in the previous sections, clathrate hydrates nucleate at an aqueous–
guest gas interface. For ions to influence the nucleation of clathrate hydrates, they
need to be present in the vicinity of an aqueous–guest gas interface. Even though
the guest gas diffusion penetrates many micrometers from the topmost surface, the
guest concentration is the highest at the topmost surface and gradually diminishes
deeper into the bulk water. The presence of turbulence or rigorous stirring may
facilitate mixing of guest gases into the aqueous phase and reduce the steepness
of the concentration gradient of the guest gases. Nevertheless, the concentration
gradient cannot be inverted.
The driving force for nucleation is related to the nucleation rate by the Arrhenius
law for a given supersaturation of the guest gas, as we saw in Chap. 1. The exponential
dependence of the Arrhenius law means that the nucleation probability will diminish
rapidly as the supersaturation falls. And yet the topmost layer of the aqueous phase
where the guest gas supersaturation is the greatest could almost be totally devoid of
ions due to the negative adsorption, as described by the Gibbs adsorption isotherm. If
this were the whole story, then the influence of salts on the nucleation rate of clathrate
5 Nucleation of Gas Hydrates
near a gas–aqueous interface are lower than those in the bulk solutions, and the
topmost layer of the aqueous phase is almost totally devoid of ions.
5.4.4 Concentration Gradient of Guest Gases Near a Salt
Solution–Guest Gas Interface
It has been reported that some THIs like methanol [95], ethylene glycol [96]
and propanol [97], could enhance the rate of clathrate hydrate formation when doped
at low concentrations. Farhang et al. reported a similar promotion effect of dilute
sodium halide solutions for CO 2 clathrate hydrate [98]. They reported that a variation in the concentration from 10 to 100 mM of the sodium halide solutions induced
a transition from promotion to inhibition of CO 2 hydrate formation. A promotion
effect of CO 2 hydrate formation was observed most acutely for sodium iodide, while
sodium chloride was found to be an inhibitor at all concentrations studied [98].
These promotion effects cannot arise from the thermodynamic effect of lowering of
the activity of water by the solutes and thus must be kinetic in nature.
Ions could impact the nucleation of clathrate hydrates through the local structuring
of water molecules around each ion (hydration). The hydrophobic hydration of nonpolar entities like alkanes impacts the local water structure differently than the hydration shells of hydrophilic solutes like ions. Hydrophilic ions generally put stresses
onto the tetrahedral network of hydrogen-bonded water molecules [99]. The higher
the salt concentrations the weaker the tetrahedral structure and the lower the hydration number [100]. In contrast, hydrophobic (non-polar) solutes like alkanes enhance
the tetrahedral network with larger numbers of hydrogen bonds [101–103]. It is thus
reasonable to expect that such different tetrahedral networks of hydrogen-bonded
water would influence the nucleation rate of clathrate hydrates.
As we saw in the previous sections, clathrate hydrates nucleate at an aqueous–
guest gas interface. For ions to influence the nucleation of clathrate hydrates, they
need to be present in the vicinity of an aqueous–guest gas interface. Even though
the guest gas diffusion penetrates many micrometers from the topmost surface, the
guest concentration is the highest at the topmost surface and gradually diminishes
deeper into the bulk water. The presence of turbulence or rigorous stirring may
facilitate mixing of guest gases into the aqueous phase and reduce the steepness
of the concentration gradient of the guest gases. Nevertheless, the concentration
gradient cannot be inverted.
The driving force for nucleation is related to the nucleation rate by the Arrhenius
law for a given supersaturation of the guest gas, as we saw in Chap. 1. The exponential
dependence of the Arrhenius law means that the nucleation probability will diminish
rapidly as the supersaturation falls. And yet the topmost layer of the aqueous phase
where the guest gas supersaturation is the greatest could almost be totally devoid of
ions due to the negative adsorption, as described by the Gibbs adsorption isotherm. If
this were the whole story, then the influence of salts on the nucleation rate of clathrate