4.2 Interfacial Gaseous Layers
95
solubility of nitrogen gas in ethanol and in water increases with heating and hence
the supersaturation of nitrogen after the solvent exchange protocol is expected to
increase with the temperature of the solvents.
Even though “nanobubble-pancake composites” can metastably exist, the two
gaseous entities are not equivalent in that the driving force required to induce them
appeared different. The formation of micro gas pancakes required a higher supersaturation of the gas than the formation of interfacial nanobubbles. Other notable
characteristics of these systems were [1] (1) removal of the interfacial nanobubbles
that had been induced required a greater reduction of supersaturation of the gas
than that of induced micro gas pancakes (interfacial nanobubbles were more stable
than micro gas pancakes and could exist at a lower supersaturation of the gas), (2)
the two forms of the interfacial gaseous entities could coexist at a sufficiently high
supersaturation of the gas where one or more of interfacial nanobubbles sat on top
of a micro gas pancake, (3) a micro gas pancake could spontaneously coalesce with
another nearby micro gas pancake over time, (4) after the coalescence of two neighboring micro gas pancakes which each had an interfacial nanobubble on top, the
larger nanobubble grew at the expense of the smaller one, (5) the rate of the Ostwald
ripening between two such interfacial nanobubbles that sat on top of the same micro
gas pancake was greater than that between two interfacial nanobubbles that sat on
two separate micro gas pancakes.
An interesting question is whether a micro gas pancake and an interfacial
nanobubble above it could have been separated by a thin film of water or not. We note
that a thin water film between a micro gas pancake and an interfacial nanobubble
on top of it would be very unstable because (1) its thickness must be very thin
(below the spatial resolution of an AFM and must be thinner than the height of
the micro gas pancake itself) and (2) the van der Waals forces across a thin water
film between two gaseous phases would be attractive. The Hamaker constant for
air–water–air is 3.7 × 10
−20 J, which is even larger than the Hamaker constant for
water–hydrocarbon–water of about 0.4 × 10
−20 J [39].
In other words, an interfacial nanobubble that appeared to sit on top of a micro gas
pancake could have been a “bump” of a (deformed) micro gas pancake [2]. Just like
the gas inside a balloon can be deformed to a flat layer and a bump when the balloon
is pressed against a flat wall by non-uniform forces, interfacial gas could be deformed
by appropriate surface forces into a flat micro gas pancake and a bump that appeared
to be an interfacial nanobubble. Since the aqueous phase was supersaturated with
the gas and the solid wall was non-permeable to the gas, any gas trapped in between
the solid wall and the supersaturated aqueous phase was somewhat similar to the
gas trapped inside a deformed balloon. Then, perhaps some excess gas that could
not be accommodated inside a micro gas pancake could have become a bump that
looked like an interfacial nanobubble. However, interfacial nanobubbles were found
to form at a lower supersaturation of the gas than the micro gas pancakes. Then,
if this scenario were the case, the “bump” must have formed first and the flat layer
must have spread from the “bump” afterward as the supersaturation of the gas further
mounted [2].
95
solubility of nitrogen gas in ethanol and in water increases with heating and hence
the supersaturation of nitrogen after the solvent exchange protocol is expected to
increase with the temperature of the solvents.
Even though “nanobubble-pancake composites” can metastably exist, the two
gaseous entities are not equivalent in that the driving force required to induce them
appeared different. The formation of micro gas pancakes required a higher supersaturation of the gas than the formation of interfacial nanobubbles. Other notable
characteristics of these systems were [1] (1) removal of the interfacial nanobubbles
that had been induced required a greater reduction of supersaturation of the gas
than that of induced micro gas pancakes (interfacial nanobubbles were more stable
than micro gas pancakes and could exist at a lower supersaturation of the gas), (2)
the two forms of the interfacial gaseous entities could coexist at a sufficiently high
supersaturation of the gas where one or more of interfacial nanobubbles sat on top
of a micro gas pancake, (3) a micro gas pancake could spontaneously coalesce with
another nearby micro gas pancake over time, (4) after the coalescence of two neighboring micro gas pancakes which each had an interfacial nanobubble on top, the
larger nanobubble grew at the expense of the smaller one, (5) the rate of the Ostwald
ripening between two such interfacial nanobubbles that sat on top of the same micro
gas pancake was greater than that between two interfacial nanobubbles that sat on
two separate micro gas pancakes.
An interesting question is whether a micro gas pancake and an interfacial
nanobubble above it could have been separated by a thin film of water or not. We note
that a thin water film between a micro gas pancake and an interfacial nanobubble
on top of it would be very unstable because (1) its thickness must be very thin
(below the spatial resolution of an AFM and must be thinner than the height of
the micro gas pancake itself) and (2) the van der Waals forces across a thin water
film between two gaseous phases would be attractive. The Hamaker constant for
air–water–air is 3.7 × 10
−20 J, which is even larger than the Hamaker constant for
water–hydrocarbon–water of about 0.4 × 10
−20 J [39].
In other words, an interfacial nanobubble that appeared to sit on top of a micro gas
pancake could have been a “bump” of a (deformed) micro gas pancake [2]. Just like
the gas inside a balloon can be deformed to a flat layer and a bump when the balloon
is pressed against a flat wall by non-uniform forces, interfacial gas could be deformed
by appropriate surface forces into a flat micro gas pancake and a bump that appeared
to be an interfacial nanobubble. Since the aqueous phase was supersaturated with
the gas and the solid wall was non-permeable to the gas, any gas trapped in between
the solid wall and the supersaturated aqueous phase was somewhat similar to the
gas trapped inside a deformed balloon. Then, perhaps some excess gas that could
not be accommodated inside a micro gas pancake could have become a bump that
looked like an interfacial nanobubble. However, interfacial nanobubbles were found
to form at a lower supersaturation of the gas than the micro gas pancakes. Then,
if this scenario were the case, the “bump” must have formed first and the flat layer
must have spread from the “bump” afterward as the supersaturation of the gas further
mounted [2].