116
5 Nucleation of Gas Hydrates
It appears that homogeneous nucleation of I h does not occur around −42 °C
when the droplet is smaller than the critical nucleus size. For 5 nm diameter droplets,
for example, water could be subcooled to −70 °C [9]. This deep subcooling is of
different nature from the viscous slowdown we detailed above.
For heterogeneous nucleation of ice from water vapor in the presence of a solid
wall, wedges become preferred nucleation sites because condensation of water vapor
at the contacts of the wedges does not require nucleation, as we saw in Chap. 1.
Condensation onto a narrow wedge is effectively a one-dimensional nucleation
along the wedge that does not require any surmounting of an activation barrier [18].
However, after water vapor has condensed to such a wedge, freezing of the confined
liquid water in the narrow wedge requires surmounting of a large activation barrier
of the order of −35 °C of subcooling [19, 20]. Condensation of water vapor into such
a wedge would deplete moisture from the surrounding vapor, and consequently the
activity of the water vapor would fall that prevents ice from forming elsewhere. The
end result is that only ice at the mouth of the wedges can be observed, after the fact
[19, 20].
At small subcoolings, in contrast, capillary melting prevents freezing of liquid
water in wedges. Instead, ice preferentially forms by condensation and freezing on
flat, open surfaces [19, 20]. This latter effect is relevant to icing on aircraft wings.
5.2 Nucleation Rate of Gas Hydrates
The previous section showed that nucleation of ice, despite being studied for much
longer than nucleation of clathrate hydrates, is still full of surprises and mysteries.
Then, is there any wonder that nucleation of clathrate hydrates, that involves more
components than ice, is hardly understood? Given the similarities between ice and
clathrate hydrates, many, if not all, of the complications and mysteries concerning
the nucleation of ice detailed in the previous section are expected to be present
in nucleation of clathrate hydrate as well. However, so little is understood about
nucleation of clathrate hydrate that we have not yet reached a stage to even start
addressing such outstanding complications and mysteries.
We saw in Chap. 1 that nucleation rate is central to nucleation phenomena of
a given system and so is the case for clathrate hydrates. In fact, a primary difficulty in the investigations of nucleation of clathrate hydrate has been the inability of
researchers to determine and compare nucleation rates of clathrate hydrates across
various systems of different scales and complexity, which in turn has been limiting
the ability of researchers to study the nucleation process itself. Our ability to reliably determine the nucleation rate of a given clathrate hydrate must thus precede,
not follow, investigations of any of the outstanding complications and mysteries
surrounding the nucleation of ice that are also expected to be present for clathrate
hydrate. It is conceivable that insights gained from nucleation of clathrate hydrate
may in turn shed new light to our understanding of nucleation of ice. The central
theme of this section is how the theoretical framework detailed in Chap. 1, and the
5 Nucleation of Gas Hydrates
It appears that homogeneous nucleation of I h does not occur around −42 °C
when the droplet is smaller than the critical nucleus size. For 5 nm diameter droplets,
for example, water could be subcooled to −70 °C [9]. This deep subcooling is of
different nature from the viscous slowdown we detailed above.
For heterogeneous nucleation of ice from water vapor in the presence of a solid
wall, wedges become preferred nucleation sites because condensation of water vapor
at the contacts of the wedges does not require nucleation, as we saw in Chap. 1.
Condensation onto a narrow wedge is effectively a one-dimensional nucleation
along the wedge that does not require any surmounting of an activation barrier [18].
However, after water vapor has condensed to such a wedge, freezing of the confined
liquid water in the narrow wedge requires surmounting of a large activation barrier
of the order of −35 °C of subcooling [19, 20]. Condensation of water vapor into such
a wedge would deplete moisture from the surrounding vapor, and consequently the
activity of the water vapor would fall that prevents ice from forming elsewhere. The
end result is that only ice at the mouth of the wedges can be observed, after the fact
[19, 20].
At small subcoolings, in contrast, capillary melting prevents freezing of liquid
water in wedges. Instead, ice preferentially forms by condensation and freezing on
flat, open surfaces [19, 20]. This latter effect is relevant to icing on aircraft wings.
5.2 Nucleation Rate of Gas Hydrates
The previous section showed that nucleation of ice, despite being studied for much
longer than nucleation of clathrate hydrates, is still full of surprises and mysteries.
Then, is there any wonder that nucleation of clathrate hydrates, that involves more
components than ice, is hardly understood? Given the similarities between ice and
clathrate hydrates, many, if not all, of the complications and mysteries concerning
the nucleation of ice detailed in the previous section are expected to be present
in nucleation of clathrate hydrate as well. However, so little is understood about
nucleation of clathrate hydrate that we have not yet reached a stage to even start
addressing such outstanding complications and mysteries.
We saw in Chap. 1 that nucleation rate is central to nucleation phenomena of
a given system and so is the case for clathrate hydrates. In fact, a primary difficulty in the investigations of nucleation of clathrate hydrate has been the inability of
researchers to determine and compare nucleation rates of clathrate hydrates across
various systems of different scales and complexity, which in turn has been limiting
the ability of researchers to study the nucleation process itself. Our ability to reliably determine the nucleation rate of a given clathrate hydrate must thus precede,
not follow, investigations of any of the outstanding complications and mysteries
surrounding the nucleation of ice that are also expected to be present for clathrate
hydrate. It is conceivable that insights gained from nucleation of clathrate hydrate
may in turn shed new light to our understanding of nucleation of ice. The central
theme of this section is how the theoretical framework detailed in Chap. 1, and the