98
4 Interfacial Gaseous States
disjoining pressure are expected to suppress the growth of a gaseous film and limit
its growth to a very small thickness. In fact, we saw that the same mechanism is
likely in play in surface freezing where the thickness of a frozen normal alkane film
on top of its liquid bed remains monomolecular over a few Kelvins above the bulk
freezing point (Sect. 1.4).
The experimental results in a sufficiently supersaturated system showed that interfacial nanobubbles often sat on top of the micro gas pancakes whereas interfacial
nanobubbles rarely sat directly on a bare HOPG substrate. It is conceivable that interfacial nanobubbles (the “bumps”) formed on a bare HOPG substrate first while the
supersaturation was still building up, which in turn then provided the nucleation sites
for subsequent formations of micro gas pancakes as the supersaturation mounted still
higher. This is to say that the micro gas pancakes could have nucleated from existing
interfacial nanobubbles that had formed first. Unfortunately, due to the typically poor
time resolution of an AFM, we could only observe the end result of the interfacial
nanobubbles sitting on top of the micro gas pancakes.
That interfacial gaseous states can form with ease, not only on a rough surface
but also on a molecularly smooth surface under suitable conditions, and the energy
barrier to such nucleation appears surprisingly low, has important implications to
heterogeneous nucleation of clathrate hydrates. Given that typical surfaces involved
in a clathrate hydrate study are neither molecularly smooth nor carefully controlled
to avoid generation of interfacial gaseous states, their presence is almost certain.
In particular, interfacial gaseous states hold the key to the understanding of the
long-standing mystery of the so-called memory effect in the nucleation of clathrate
hydrates, as we will see in Chap. 5.
4.3 Is the Surface of Gas Hydrates Dry?
4.3.1 Introduction
The wetting properties of clathrate hydrate surfaces are not a part of interfacial
gaseous states, however, the underlying physical concepts are very similar and we
cover the subject at this stage. A pre-melting layer, also known as a surface melting
layer, refers to a quasi-liquid layer that exists at the surface of a solid below its
melting point, T m [40, 41]. An important point here is that such quasi-liquid layer is
thermodynamically stable. The phenomenon itself has been known since the days of
Faraday and has since been found for many solids [40–44]. Large amounts of efforts
have been specifically directed to the study of pre-melting of ice because of its wideranging environmental implications. For example, the reason that ice is slippery and
one can skate on ice is because a thin pre-melting layer of liquid water acts as an
effective lubricant.
The central question in this section is whether the same phenomenon occurs to the
surface of clathrate hydrate or not. Whether the surface of a clathrate hydrate crystal
4 Interfacial Gaseous States
disjoining pressure are expected to suppress the growth of a gaseous film and limit
its growth to a very small thickness. In fact, we saw that the same mechanism is
likely in play in surface freezing where the thickness of a frozen normal alkane film
on top of its liquid bed remains monomolecular over a few Kelvins above the bulk
freezing point (Sect. 1.4).
The experimental results in a sufficiently supersaturated system showed that interfacial nanobubbles often sat on top of the micro gas pancakes whereas interfacial
nanobubbles rarely sat directly on a bare HOPG substrate. It is conceivable that interfacial nanobubbles (the “bumps”) formed on a bare HOPG substrate first while the
supersaturation was still building up, which in turn then provided the nucleation sites
for subsequent formations of micro gas pancakes as the supersaturation mounted still
higher. This is to say that the micro gas pancakes could have nucleated from existing
interfacial nanobubbles that had formed first. Unfortunately, due to the typically poor
time resolution of an AFM, we could only observe the end result of the interfacial
nanobubbles sitting on top of the micro gas pancakes.
That interfacial gaseous states can form with ease, not only on a rough surface
but also on a molecularly smooth surface under suitable conditions, and the energy
barrier to such nucleation appears surprisingly low, has important implications to
heterogeneous nucleation of clathrate hydrates. Given that typical surfaces involved
in a clathrate hydrate study are neither molecularly smooth nor carefully controlled
to avoid generation of interfacial gaseous states, their presence is almost certain.
In particular, interfacial gaseous states hold the key to the understanding of the
long-standing mystery of the so-called memory effect in the nucleation of clathrate
hydrates, as we will see in Chap. 5.
4.3 Is the Surface of Gas Hydrates Dry?
4.3.1 Introduction
The wetting properties of clathrate hydrate surfaces are not a part of interfacial
gaseous states, however, the underlying physical concepts are very similar and we
cover the subject at this stage. A pre-melting layer, also known as a surface melting
layer, refers to a quasi-liquid layer that exists at the surface of a solid below its
melting point, T m [40, 41]. An important point here is that such quasi-liquid layer is
thermodynamically stable. The phenomenon itself has been known since the days of
Faraday and has since been found for many solids [40–44]. Large amounts of efforts
have been specifically directed to the study of pre-melting of ice because of its wideranging environmental implications. For example, the reason that ice is slippery and
one can skate on ice is because a thin pre-melting layer of liquid water acts as an
effective lubricant.
The central question in this section is whether the same phenomenon occurs to the
surface of clathrate hydrate or not. Whether the surface of a clathrate hydrate crystal