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4 Interfacial Gaseous States
Ethanol is a solvent that is commonly used for cleaning of glassware and metal
vessels in a laboratory. The residual ethanol is then typically rinsed with water,
which is in effect an unintended “solvent exchange” process on the surface of the
cleaned glassware or metal vessels. In addition, unintended and therefore uncontrolled temperature gradients are common in daily life. We thus suspect that thermodynamically metastable interfacial gaseous states are far more commonplace than
people realize. Such unintended and uncontrolled supersaturation of air and the
surprising ease with which interfacial gaseous states can form have broad implications. For example, even though ethanol and water are mutually miscible at all
proportions, rinsing ethanol on a solid wall with water may not result in the same
surface as rinsing a dry solid wall with water after completely getting rid of ethanol
in a vacuum oven. Likewise, temperature gradients induced by a detection instrument could become another source of generation of interfacial gaseous states. For
example, Atomic Force Microscopy (AFM) has been a common instrument used for
the study of interfacial gaseous states on smooth solid substrates. The proximity of
the electronics of an AFM and the heating therefrom could be a matter of significance.
One of the most surprising aspects of the interfacial gaseous states is that more than
one form of gaseous states can coexist on an atomically smooth (within a cleavage step
of) hydrophobic Highly Ordered Pyrolytic Graphite (HOPG) [1]. Flat, quasi-twodimensional gaseous layers (so-called “micro gas pancakes”) apparently coexisted
with more common spherical-cap-shaped interfacial nanobubbles at a sufficiently
high supersaturation of the gas [1]. An AFM detected images of “nanobubble–micro
gas pancake composites” where some of such interfacial nanobubbles apparently sat
on top of micro gas pancakes [1]. The height of the micro gas pancakes was limited to
the maximum of about 5 nm (and the average of less than 2 nm) whereas their lateral
dimensions extended from several hundred nanometers to tens of micro meters [1].
So their aspect ratio was about three orders of magnitude. The contact angle formed
by an interfacial nanobubble on a micro gas pancake (in a composite) was estimated
to be about 155° (that was measured through the aqueous phase). This value is close
to the contact angle of an interfacial nanobubble that was directly sitting on a bare
HOPG surface [34, 36].
The formation of micro gas pancakes was influenced by several factors (type of
the solvents, the flow rates of the fluids, the temperature of the system, the level
of degassing, and the concentration of the ethanol used for the “solvent exchange”
procedure) [1]. There was a trend that the surface coverage of the micro gas pancakes
increased with the solubility of the gas in the solvents used for the solvent exchange
protocol. The temperature of the solvents used in the solvent exchange protocol also
significantly influenced the surface coverage of the micro gas pancakes. The surface
coverage of the micro gas pancakes and the size of the interfacial nanobubbles both
increased with increasing temperature of the solvent. These results are expected
from the mechanism behind the solvent exchange protocol. For example, under a
partial pressure of 700 mmHg, the solubility of nitrogen in ethanol is 3.611 × 10
−5
in mole fraction, and the solubility of nitrogen in water is 0.1274 × 10
−5 in mole
fraction, both at 20 °C. The solubility of nitrogen increases to 3.639 × 10
−5 in
ethanol and decreases to 0.1047 × 10
−5 in water at 40 °C [37, 38]. So the gap in the
4 Interfacial Gaseous States
Ethanol is a solvent that is commonly used for cleaning of glassware and metal
vessels in a laboratory. The residual ethanol is then typically rinsed with water,
which is in effect an unintended “solvent exchange” process on the surface of the
cleaned glassware or metal vessels. In addition, unintended and therefore uncontrolled temperature gradients are common in daily life. We thus suspect that thermodynamically metastable interfacial gaseous states are far more commonplace than
people realize. Such unintended and uncontrolled supersaturation of air and the
surprising ease with which interfacial gaseous states can form have broad implications. For example, even though ethanol and water are mutually miscible at all
proportions, rinsing ethanol on a solid wall with water may not result in the same
surface as rinsing a dry solid wall with water after completely getting rid of ethanol
in a vacuum oven. Likewise, temperature gradients induced by a detection instrument could become another source of generation of interfacial gaseous states. For
example, Atomic Force Microscopy (AFM) has been a common instrument used for
the study of interfacial gaseous states on smooth solid substrates. The proximity of
the electronics of an AFM and the heating therefrom could be a matter of significance.
One of the most surprising aspects of the interfacial gaseous states is that more than
one form of gaseous states can coexist on an atomically smooth (within a cleavage step
of) hydrophobic Highly Ordered Pyrolytic Graphite (HOPG) [1]. Flat, quasi-twodimensional gaseous layers (so-called “micro gas pancakes”) apparently coexisted
with more common spherical-cap-shaped interfacial nanobubbles at a sufficiently
high supersaturation of the gas [1]. An AFM detected images of “nanobubble–micro
gas pancake composites” where some of such interfacial nanobubbles apparently sat
on top of micro gas pancakes [1]. The height of the micro gas pancakes was limited to
the maximum of about 5 nm (and the average of less than 2 nm) whereas their lateral
dimensions extended from several hundred nanometers to tens of micro meters [1].
So their aspect ratio was about three orders of magnitude. The contact angle formed
by an interfacial nanobubble on a micro gas pancake (in a composite) was estimated
to be about 155° (that was measured through the aqueous phase). This value is close
to the contact angle of an interfacial nanobubble that was directly sitting on a bare
HOPG surface [34, 36].
The formation of micro gas pancakes was influenced by several factors (type of
the solvents, the flow rates of the fluids, the temperature of the system, the level
of degassing, and the concentration of the ethanol used for the “solvent exchange”
procedure) [1]. There was a trend that the surface coverage of the micro gas pancakes
increased with the solubility of the gas in the solvents used for the solvent exchange
protocol. The temperature of the solvents used in the solvent exchange protocol also
significantly influenced the surface coverage of the micro gas pancakes. The surface
coverage of the micro gas pancakes and the size of the interfacial nanobubbles both
increased with increasing temperature of the solvent. These results are expected
from the mechanism behind the solvent exchange protocol. For example, under a
partial pressure of 700 mmHg, the solubility of nitrogen in ethanol is 3.611 × 10
−5
in mole fraction, and the solubility of nitrogen in water is 0.1274 × 10
−5 in mole
fraction, both at 20 °C. The solubility of nitrogen increases to 3.639 × 10
−5 in
ethanol and decreases to 0.1047 × 10
−5 in water at 40 °C [37, 38]. So the gap in the