5.3 The Memory Effect
133
The hypotheses (2), (3), and (4) are all plausible to some extent but still have some
trouble in explaining some of the known attributes of the memory effect by themselves. On the one hand, the guest supersaturation hypothesis and the nanobubble
hypothesis do not account for a major attribute of the memory effect that it varied
among an ensemble of samples that had the same thermal history. Since both of these
hypotheses place the origin of the memory effect to the bulk of the dissociated water,
there would be little reason to expect that the walls of the container that contain the
dissociated water would make a difference. On the other hand, the impurity imprinting
hypothesis does not account for the primary attribute of the memory effect that the
memory effect vanishes (the memory is erased) after the dissociated water is heated
10–15 K above the thermodynamic phase boundary and/or for prolong period. It is
not at all clear how the perceived memory, once imprinted to a solid wall, could be
erased by simply heating the sample by modest 10 or 15 K above the superheating
temperatures that have failed to erase the memory effect.
Maeda recently found that the memory effect was absent in a quasi-free water
droplet suspended in squalene and concluded that a solid wall was required for the
manifestation of the memory effect [78]. This result alone would support the impurity imprinting hypothesis. However, given the difficulty of the impurity imprinting
hypothesis detailed above, Maeda proposed that interfacial gaseous states we detailed
in Chap. 4 were the source of the memory effect (the interfacial gaseous states
hypothesis) [78]. Guo et al. then directly detected interfacial gaseous states on the
solid walls in dissociated water [79], which further supported the interfacial gaseous
states hypothesis.
Importantly, the interfacial gaseous states hypothesis could explain why the
memory effect has been so elusive (hard to positively affirm its existence) in the
past studies, let alone quantify its magnitude. Formation of a bubble or cavitation
is a heterogeneous nucleation process that generally depends on the surface chemistry and the surface roughness of the solid wall present [80]. Then, the formation of
interfacial gaseous states, after the dissociation of clathrate hydrates, is also itself a
heterogeneous nucleation process, as we detailed in Sect. 4.2. Thus the probability
of formation of interfacial gaseous states after the dissociation of clathrate hydrates
likely varies for different sample cells or vessels.
In a static system, clathrate hydrate forms at the surface of water and its dissociation will render the top layer of water supersaturated with the guest gas. Then,
a section of the solid wall from the top of the meniscus of the dissociated water
down to the thickness of the clathrate hydrate layer before it has dissociated would
become susceptible to the formation of interfacial gaseous states. It is not difficult
to envision that the water level would change for different measurements that use
the same sample cell or container, and consequently expose different stripe sections
of the solid wall of the container to the “interfacial nanobubble-susceptible regions”
(Fig. 5.3).
Even for a given measurement in a given sample cell or container, the water
level might vary with time due to evaporation or other causes (e.g., stirring action in
some experimental systems). The possibility that at least some sections of the solid
walls in the system become susceptible to the formation of interfacial gaseous states
133
The hypotheses (2), (3), and (4) are all plausible to some extent but still have some
trouble in explaining some of the known attributes of the memory effect by themselves. On the one hand, the guest supersaturation hypothesis and the nanobubble
hypothesis do not account for a major attribute of the memory effect that it varied
among an ensemble of samples that had the same thermal history. Since both of these
hypotheses place the origin of the memory effect to the bulk of the dissociated water,
there would be little reason to expect that the walls of the container that contain the
dissociated water would make a difference. On the other hand, the impurity imprinting
hypothesis does not account for the primary attribute of the memory effect that the
memory effect vanishes (the memory is erased) after the dissociated water is heated
10–15 K above the thermodynamic phase boundary and/or for prolong period. It is
not at all clear how the perceived memory, once imprinted to a solid wall, could be
erased by simply heating the sample by modest 10 or 15 K above the superheating
temperatures that have failed to erase the memory effect.
Maeda recently found that the memory effect was absent in a quasi-free water
droplet suspended in squalene and concluded that a solid wall was required for the
manifestation of the memory effect [78]. This result alone would support the impurity imprinting hypothesis. However, given the difficulty of the impurity imprinting
hypothesis detailed above, Maeda proposed that interfacial gaseous states we detailed
in Chap. 4 were the source of the memory effect (the interfacial gaseous states
hypothesis) [78]. Guo et al. then directly detected interfacial gaseous states on the
solid walls in dissociated water [79], which further supported the interfacial gaseous
states hypothesis.
Importantly, the interfacial gaseous states hypothesis could explain why the
memory effect has been so elusive (hard to positively affirm its existence) in the
past studies, let alone quantify its magnitude. Formation of a bubble or cavitation
is a heterogeneous nucleation process that generally depends on the surface chemistry and the surface roughness of the solid wall present [80]. Then, the formation of
interfacial gaseous states, after the dissociation of clathrate hydrates, is also itself a
heterogeneous nucleation process, as we detailed in Sect. 4.2. Thus the probability
of formation of interfacial gaseous states after the dissociation of clathrate hydrates
likely varies for different sample cells or vessels.
In a static system, clathrate hydrate forms at the surface of water and its dissociation will render the top layer of water supersaturated with the guest gas. Then,
a section of the solid wall from the top of the meniscus of the dissociated water
down to the thickness of the clathrate hydrate layer before it has dissociated would
become susceptible to the formation of interfacial gaseous states. It is not difficult
to envision that the water level would change for different measurements that use
the same sample cell or container, and consequently expose different stripe sections
of the solid wall of the container to the “interfacial nanobubble-susceptible regions”
(Fig. 5.3).
Even for a given measurement in a given sample cell or container, the water
level might vary with time due to evaporation or other causes (e.g., stirring action in
some experimental systems). The possibility that at least some sections of the solid
walls in the system become susceptible to the formation of interfacial gaseous states