4.3 Is the Surface of Gas Hydrates Dry?
99
is coated with a quasi-liquid layer or not is an important question that influences the
adhesion and cohesion forces of clathrate hydrate crystals. Practically, these factors
determine the agglomeration behavior of clathrate hydrate slurries in flow lines and
its transportability. They also impact the efficacy of anti-agglomerates we covered
in Chap. 3. We posed this question in a 2015 review article [18], and its key points,
together with supplementary information, will be produced in this section.
Very recently, there has been a new development in the pre-melting of ice that
the quasi-liquid layer is not a continuous “layer” that suggests complete wetting of
liquid water on ice, as earlier thought, but rather consists of patches of droplets of
liquid water, which is suggestive of partial-wetting nature [45]. This rather surprising
new finding that liquid water does not completely wet ice will shed new light to the
topic, as we will see below.
Pre-melting of ice and the similarities between clathrate hydrates and ice suggest
that the surface of a clathrate hydrate crystal is also likely coated with a quasi-liquid
layer. However, there is an important a priori difference between pre-melting of ice
and pre-melting of clathrate hydrate, if it indeed occurs. For pre-melting of ice, the
quasi-liquid layer is pure liquid water and thus is the same chemical compound as
ice: H 2 O. In contrast, dissociation of clathrate hydrate forms liquid water and a guest
gas. Since the solubility of a guest gas in liquid water is much lower than that can
be accommodated in the clathrate form, most of the dissociated guest gas will phase
separate from the aqueous phase and form a separate gaseous phase. Then, the quasiliquid layer that coats the surface of a clathrate hydrate crystal, if existed, would be
almost pure water and have a different chemical composition than the underlying
clathrate hydrate crystal.
Such difference might not sound significant at first, but could be of significance
especially with respect to adhesion and cohesion between clathrate polycrystals and
particles. Yet another thought experiment may be in order here. When two blocks
of ice cubes are brought together below the melting point, the pre-melting layers
on each ice cube merge when contacted and form a single continuous liquid film.
Such liquid film will fill any gap between the two ice cubes that may have existed
due to the surface roughness. Then, after the contact, the merged water film will no
longer be exposed to a gas phase. The salient point here is that a second phase, be
it a gaseous phase or a foreign solid wall, is essential for pre-melting of ice to take
place. Without one, the water film will freeze and form one large merged block of
ice cube.
For clathrate hydrate such as methane hydrate, pre-melting, if occurred, would
not “melt” the top layer to form a thin film of liquid methane aqueous solution. The
solubility of methane in water is simply too low to accommodate all the methane
gas that has formed by the pre-melting. The net result of pre-melting of methane
hydrate would then be the formation of a thin film of almost pure water that has a
different composition than the solid phase from which it originated, and methane gas
will readily escape from the pre-melting layer to an open space. Then, unlike ice,
two blocks of methane hydrate would not merge to form a single block of methane
hydrate when brought together because the liquid water trapped between the two
blocks of clathrate hydrate lacks the amount of methane that is required to re-form
99
is coated with a quasi-liquid layer or not is an important question that influences the
adhesion and cohesion forces of clathrate hydrate crystals. Practically, these factors
determine the agglomeration behavior of clathrate hydrate slurries in flow lines and
its transportability. They also impact the efficacy of anti-agglomerates we covered
in Chap. 3. We posed this question in a 2015 review article [18], and its key points,
together with supplementary information, will be produced in this section.
Very recently, there has been a new development in the pre-melting of ice that
the quasi-liquid layer is not a continuous “layer” that suggests complete wetting of
liquid water on ice, as earlier thought, but rather consists of patches of droplets of
liquid water, which is suggestive of partial-wetting nature [45]. This rather surprising
new finding that liquid water does not completely wet ice will shed new light to the
topic, as we will see below.
Pre-melting of ice and the similarities between clathrate hydrates and ice suggest
that the surface of a clathrate hydrate crystal is also likely coated with a quasi-liquid
layer. However, there is an important a priori difference between pre-melting of ice
and pre-melting of clathrate hydrate, if it indeed occurs. For pre-melting of ice, the
quasi-liquid layer is pure liquid water and thus is the same chemical compound as
ice: H 2 O. In contrast, dissociation of clathrate hydrate forms liquid water and a guest
gas. Since the solubility of a guest gas in liquid water is much lower than that can
be accommodated in the clathrate form, most of the dissociated guest gas will phase
separate from the aqueous phase and form a separate gaseous phase. Then, the quasiliquid layer that coats the surface of a clathrate hydrate crystal, if existed, would be
almost pure water and have a different chemical composition than the underlying
clathrate hydrate crystal.
Such difference might not sound significant at first, but could be of significance
especially with respect to adhesion and cohesion between clathrate polycrystals and
particles. Yet another thought experiment may be in order here. When two blocks
of ice cubes are brought together below the melting point, the pre-melting layers
on each ice cube merge when contacted and form a single continuous liquid film.
Such liquid film will fill any gap between the two ice cubes that may have existed
due to the surface roughness. Then, after the contact, the merged water film will no
longer be exposed to a gas phase. The salient point here is that a second phase, be
it a gaseous phase or a foreign solid wall, is essential for pre-melting of ice to take
place. Without one, the water film will freeze and form one large merged block of
ice cube.
For clathrate hydrate such as methane hydrate, pre-melting, if occurred, would
not “melt” the top layer to form a thin film of liquid methane aqueous solution. The
solubility of methane in water is simply too low to accommodate all the methane
gas that has formed by the pre-melting. The net result of pre-melting of methane
hydrate would then be the formation of a thin film of almost pure water that has a
different composition than the solid phase from which it originated, and methane gas
will readily escape from the pre-melting layer to an open space. Then, unlike ice,
two blocks of methane hydrate would not merge to form a single block of methane
hydrate when brought together because the liquid water trapped between the two
blocks of clathrate hydrate lacks the amount of methane that is required to re-form