100
4 Interfacial Gaseous States
methane hydrate. Methane gas that has escaped can no longer easily diffuse back
into the thin film of pure water in the narrow gap between the two blocks after they
have been brought together. Consequently, the cohesion force between two methane
hydrate blocks would be many orders of magnitude smaller than that between two
ice blocks.
It is thus not immediately clear if pre-melting of clathrate hydrate is thermodynamically favorable or, if it were, if its physical properties are similar to that on ice. In the
literature, reports on pre-melting of clathrate hydrates are scarce. Aman et al. experimentally inferred the presence of a quasi-liquid layer on the surface of cyclopentane
hydrate [46]. Yan et al. [47] and Ding et al. [48] reported the presence of quasi-liquid
layers on the surface of clathrate hydrates from molecular dynamics simulations. In
particular, Ding et al. specifically reported an occurrence of pre-melting on the surface
of methane hydrate from molecular dynamics simulation [48]. Maeda et al. found
that the cohesion hysteresis between two cyclopentane hydrate particles just below
the thermodynamic dissociation temperature was vanishingly small [49]. Adhesion
hysteresis or cohesion hysteresis, not adhesion force or cohesion force itself, is highly
correlated to friction forces [50]. Thus, Maeda et al.’s finding suggest that the friction
force between two cyclopentane hydrate particles is likely very small. Such small
friction forces are indicative of a surface coated with a quasi-liquid layer that may act
as an excellent lubricant (like what occurs to ice). In short, circumstantial evidence
points to the existence of a quasi-liquid layer on the surface of clathrate hydrate, at
least for temperatures just below the thermodynamic dissociation temperature of the
clathrate hydrate. Below we examine the thermodynamic basis of pre-melting.
4.3.2 Thermodynamic Basis of Pre-melting
An important conclusion of Sect. 1.4 is that the specific interfacial free energy of
either side of a thin film may be adequately approximated by the specific interfacial
free energy of two corresponding semi-infinite media. This approximation corresponds to neglecting the disjoining pressure of the thin film, as we saw in Sect. 4.1,
in particular, Eq. (4.1.5). We continue to rely on this approximation for now.
We start from the basics. The internal energy, U, arises from the bonding between
atoms and molecules and it can be lowered by a formation of a crystalline structure. In
contrast, the entropy, S, favors disorder and hence formation of an ordered crystalline
structure increases the entropic component of the free energy. Since the entropy
component of the Gibbs free energy, G, is −TS, the impact of the entropy component
on G increases with temperature, T. Thus, there is a point in T above which the
disorderly liquid phase becomes favorable and below which the ordered crystalline
phase becomes favorable. This point defines the melting point, T m .
On a surface of a crystalline material or a disordered liquid, the reduced numbers
of the bonds of atoms or molecules result in a higher U compared to that in the bulk of
the same material or the liquid bed at a given T. This reduced average bond numbers
of atoms or molecules at the surface compared to that in the bulk, and the consequent
4 Interfacial Gaseous States
methane hydrate. Methane gas that has escaped can no longer easily diffuse back
into the thin film of pure water in the narrow gap between the two blocks after they
have been brought together. Consequently, the cohesion force between two methane
hydrate blocks would be many orders of magnitude smaller than that between two
ice blocks.
It is thus not immediately clear if pre-melting of clathrate hydrate is thermodynamically favorable or, if it were, if its physical properties are similar to that on ice. In the
literature, reports on pre-melting of clathrate hydrates are scarce. Aman et al. experimentally inferred the presence of a quasi-liquid layer on the surface of cyclopentane
hydrate [46]. Yan et al. [47] and Ding et al. [48] reported the presence of quasi-liquid
layers on the surface of clathrate hydrates from molecular dynamics simulations. In
particular, Ding et al. specifically reported an occurrence of pre-melting on the surface
of methane hydrate from molecular dynamics simulation [48]. Maeda et al. found
that the cohesion hysteresis between two cyclopentane hydrate particles just below
the thermodynamic dissociation temperature was vanishingly small [49]. Adhesion
hysteresis or cohesion hysteresis, not adhesion force or cohesion force itself, is highly
correlated to friction forces [50]. Thus, Maeda et al.’s finding suggest that the friction
force between two cyclopentane hydrate particles is likely very small. Such small
friction forces are indicative of a surface coated with a quasi-liquid layer that may act
as an excellent lubricant (like what occurs to ice). In short, circumstantial evidence
points to the existence of a quasi-liquid layer on the surface of clathrate hydrate, at
least for temperatures just below the thermodynamic dissociation temperature of the
clathrate hydrate. Below we examine the thermodynamic basis of pre-melting.
4.3.2 Thermodynamic Basis of Pre-melting
An important conclusion of Sect. 1.4 is that the specific interfacial free energy of
either side of a thin film may be adequately approximated by the specific interfacial
free energy of two corresponding semi-infinite media. This approximation corresponds to neglecting the disjoining pressure of the thin film, as we saw in Sect. 4.1,
in particular, Eq. (4.1.5). We continue to rely on this approximation for now.
We start from the basics. The internal energy, U, arises from the bonding between
atoms and molecules and it can be lowered by a formation of a crystalline structure. In
contrast, the entropy, S, favors disorder and hence formation of an ordered crystalline
structure increases the entropic component of the free energy. Since the entropy
component of the Gibbs free energy, G, is −TS, the impact of the entropy component
on G increases with temperature, T. Thus, there is a point in T above which the
disorderly liquid phase becomes favorable and below which the ordered crystalline
phase becomes favorable. This point defines the melting point, T m .
On a surface of a crystalline material or a disordered liquid, the reduced numbers
of the bonds of atoms or molecules result in a higher U compared to that in the bulk of
the same material or the liquid bed at a given T. This reduced average bond numbers
of atoms or molecules at the surface compared to that in the bulk, and the consequent