120
5 Nucleation of Gas Hydrates
depend on the solubility of the guest gas in the liquid water. If the heterogeneous
nucleation of clathrate hydrate were to take place in a thin wetting film of aqueous
guest gas solution on the foreign solid substrate (Case 3), then the nucleation rate
would be a complex function of both the solubility of the guest gas in liquid water and
the thickness of the wetting film that is dominated by a complex structural component of the disjoining pressure [25] (see Sect. 4.1 for a general description of the
disjoining pressure and Sect. 1.4 for a tangible example of the structural component
of the disjoining pressure). It is thus not even clear what the equivalent of Eq. (5.2.1)
is or how θ should be defined for clathrate hydrate nucleation [22].
5.2.2 Historical Perspective of Gas Hydrate Nucleation
The early studies on the kinetics of clathrate hydrates were mainly about the rate of
crystal growth that can be measured from the rate of guest gas consumption than the
nucleation process itself [26–33]. In the rare occasions when the nucleation was of
interest, induction times at constant subcoolings were reported as the representative
measure of the nucleation of clathrate hydrate [26–33]. A large number of induction
time measurements are required to reliably determine its average for each subcooling
of interest, as we saw in Chap. 2, which was hard to do prior to the recent advent of
automated measurements. Induction times were often very long, especially at small
subcoolings, which rendered the measurements extremely time consuming to an
extent they become impractical. As we saw in Chap. 2, a maximum waiting time was
often set for induction time measurements at a constant temperature. A consequence
of doing so was that only the lower limit of the induction time could be known, which
made it impossible to determine the average induction time. Perhaps unsurprisingly,
none of these early studies advanced far enough to reach a stage that could determine
the nucleation rates of clathrate hydrates.
Collection of a larger number of data points within a given timeframe can be
facilitated by carrying out multiple measurements in parallel, but the heterogeneous
nucleation rate usually depends on specific solid walls (surface roughness, surface
defects, etc.) involved in a system. A notable effort from this approach is the recent
investigations of clathrate hydrate formation in water-in-oil (W/O) emulsions using
a high-pressure micro differential scanning calorimeter (HP–µDSC). If each water
droplet in a W/O emulsion could act as a separate “reactor”, then the use of a
W/O emulsion could astronomically increase the number of data points that can
be measured in a limited timeframe and at the same time avoid solid wall contacts
[34, 35]. However, these studies could not determine the nucleation rate because
the total interfacial area involved in an emulsion sample could not be reliably determined. Manakov and coworkers recently advanced this line of study and estimated
the total water–oil interface area per unit mass of an emulsion from ex situ droplet
size distribution data and used this information to determine the nucleation rate of
5 Nucleation of Gas Hydrates
depend on the solubility of the guest gas in the liquid water. If the heterogeneous
nucleation of clathrate hydrate were to take place in a thin wetting film of aqueous
guest gas solution on the foreign solid substrate (Case 3), then the nucleation rate
would be a complex function of both the solubility of the guest gas in liquid water and
the thickness of the wetting film that is dominated by a complex structural component of the disjoining pressure [25] (see Sect. 4.1 for a general description of the
disjoining pressure and Sect. 1.4 for a tangible example of the structural component
of the disjoining pressure). It is thus not even clear what the equivalent of Eq. (5.2.1)
is or how θ should be defined for clathrate hydrate nucleation [22].
5.2.2 Historical Perspective of Gas Hydrate Nucleation
The early studies on the kinetics of clathrate hydrates were mainly about the rate of
crystal growth that can be measured from the rate of guest gas consumption than the
nucleation process itself [26–33]. In the rare occasions when the nucleation was of
interest, induction times at constant subcoolings were reported as the representative
measure of the nucleation of clathrate hydrate [26–33]. A large number of induction
time measurements are required to reliably determine its average for each subcooling
of interest, as we saw in Chap. 2, which was hard to do prior to the recent advent of
automated measurements. Induction times were often very long, especially at small
subcoolings, which rendered the measurements extremely time consuming to an
extent they become impractical. As we saw in Chap. 2, a maximum waiting time was
often set for induction time measurements at a constant temperature. A consequence
of doing so was that only the lower limit of the induction time could be known, which
made it impossible to determine the average induction time. Perhaps unsurprisingly,
none of these early studies advanced far enough to reach a stage that could determine
the nucleation rates of clathrate hydrates.
Collection of a larger number of data points within a given timeframe can be
facilitated by carrying out multiple measurements in parallel, but the heterogeneous
nucleation rate usually depends on specific solid walls (surface roughness, surface
defects, etc.) involved in a system. A notable effort from this approach is the recent
investigations of clathrate hydrate formation in water-in-oil (W/O) emulsions using
a high-pressure micro differential scanning calorimeter (HP–µDSC). If each water
droplet in a W/O emulsion could act as a separate “reactor”, then the use of a
W/O emulsion could astronomically increase the number of data points that can
be measured in a limited timeframe and at the same time avoid solid wall contacts
[34, 35]. However, these studies could not determine the nucleation rate because
the total interfacial area involved in an emulsion sample could not be reliably determined. Manakov and coworkers recently advanced this line of study and estimated
the total water–oil interface area per unit mass of an emulsion from ex situ droplet
size distribution data and used this information to determine the nucleation rate of