4.1 Disjoining Pressure
91
the strength of the van der Waals forces, because the sign determines whether the
Van der Waals forces in a particular setting is attractive or repulsive. The situation
is somewhat analogous to the base vectors of an orthogonal series in a Hilbert space
in which we are more interested in the “direction” of each vector than its length (see
Chap. 2). If one is only required to know the sign of a Hamaker constant, then a
very simple relationship has been established for the van der Waals forces between
two semi-infinite media across a thin film: the Van der Waals forces between 1 and
2 across 3 in Fig. 4.4 is repulsive when n 1 > n 3 > n 2 or n 1 < n 3 < n 2 and attractive
otherwise [10, 11].
A thin film is thus thermodynamically stable when the Van der Waals force is
repulsive across the thin film. This is to say that the Van der Waals component of
the disjoining pressure is positive; Π = Π (h) > 0. Conversely, the film is thermodynamically unstable when the Van der Waals force is attractive across a thin film.
This is to say that the Van der Waals component of the disjoining pressure is negative; Π = Π (h) < 0. These considerations are important not only in the wetting
properties of materials, including the pre-melting of ice and quasi-liquid layers on
clathrate hydrates [18], which dictate the adhesion, cohesion, and agglomeration of
clathrate hydrate particles, but also in more practical matters of sample preparations
in nucleation studies [19].
4.2 Interfacial Gaseous Layers
4.2.1 Introduction
The availability of the guest gases and their physical states (i.e., whether the guests
are in a dissolved state or form a separate gaseous phase) next to a solid substrate
have direct impacts on the heterogeneous nucleation of clathrate hydrates. Therefore,
the physical properties of interfacial gaseous states on a solid surface in water are of
great interest to the nucleation of clathrate hydrates. It turned out that a variety of
interfacial gaseous states may exist or coexist in water under suitable physicochemical conditions. Given that a distinct attribute of a gas phase is that all types of gases
are miscible with each other at all proportions, it is surprising that multiple gaseous
states can coexist in an aqueous media, which presents a surprisingly rich domain
[20].
It has long been known that formation of a gaseous layer can be thermodynamically favorable for sufficiently rough surfaces as shown in Fig. 4.5, left panel. The
interfacial free energy of the system can decrease by the formation of an interfacial
gaseous layer when (1) the water–gas interfacial area is sufficiently smaller than
the gas–solid interfacial area and (2) the gas–solid specific interfacial free energy
is much lower than the water–solid specific interfacial free energy. Wetting on such
rough and/or heterogeneous surfaces has a long history since the days of Wenzel [21]
and Cassie and Baxter [22].
91
the strength of the van der Waals forces, because the sign determines whether the
Van der Waals forces in a particular setting is attractive or repulsive. The situation
is somewhat analogous to the base vectors of an orthogonal series in a Hilbert space
in which we are more interested in the “direction” of each vector than its length (see
Chap. 2). If one is only required to know the sign of a Hamaker constant, then a
very simple relationship has been established for the van der Waals forces between
two semi-infinite media across a thin film: the Van der Waals forces between 1 and
2 across 3 in Fig. 4.4 is repulsive when n 1 > n 3 > n 2 or n 1 < n 3 < n 2 and attractive
otherwise [10, 11].
A thin film is thus thermodynamically stable when the Van der Waals force is
repulsive across the thin film. This is to say that the Van der Waals component of
the disjoining pressure is positive; Π = Π (h) > 0. Conversely, the film is thermodynamically unstable when the Van der Waals force is attractive across a thin film.
This is to say that the Van der Waals component of the disjoining pressure is negative; Π = Π (h) < 0. These considerations are important not only in the wetting
properties of materials, including the pre-melting of ice and quasi-liquid layers on
clathrate hydrates [18], which dictate the adhesion, cohesion, and agglomeration of
clathrate hydrate particles, but also in more practical matters of sample preparations
in nucleation studies [19].
4.2 Interfacial Gaseous Layers
4.2.1 Introduction
The availability of the guest gases and their physical states (i.e., whether the guests
are in a dissolved state or form a separate gaseous phase) next to a solid substrate
have direct impacts on the heterogeneous nucleation of clathrate hydrates. Therefore,
the physical properties of interfacial gaseous states on a solid surface in water are of
great interest to the nucleation of clathrate hydrates. It turned out that a variety of
interfacial gaseous states may exist or coexist in water under suitable physicochemical conditions. Given that a distinct attribute of a gas phase is that all types of gases
are miscible with each other at all proportions, it is surprising that multiple gaseous
states can coexist in an aqueous media, which presents a surprisingly rich domain
[20].
It has long been known that formation of a gaseous layer can be thermodynamically favorable for sufficiently rough surfaces as shown in Fig. 4.5, left panel. The
interfacial free energy of the system can decrease by the formation of an interfacial
gaseous layer when (1) the water–gas interfacial area is sufficiently smaller than
the gas–solid interfacial area and (2) the gas–solid specific interfacial free energy
is much lower than the water–solid specific interfacial free energy. Wetting on such
rough and/or heterogeneous surfaces has a long history since the days of Wenzel [21]
and Cassie and Baxter [22].