132
5 Nucleation of Gas Hydrates
sample cells) despite the same thermal history of the samples. The mechanism of
the memory imprinting is not known, nor is the mechanism as to how the imprinted
memory can be erased by moderate heating of the sample.
The nanobubbles hypothesis postulates that the very high level of supersaturation
of the guest gas after the dissociation of clathrate hydrate would lead to formation of
nanobubbles in the bulk of the supersaturated solution [76, 77]. Since nanobubbles
are small, they would be invisible to naked eye and persist for a long time because
they would not float quickly due to buoyancy.
The interfacial gaseous states hypothesis postulates that the high level of supersaturation of the guest gas after the dissociation of clathrate hydrate would lead
to formation of interfacial gaseous states on solid walls of a container [78, 79]
because heterogeneous nucleation of bubbles on a solid surface is always energetically favorable to homogeneous nucleation of bubbles in the bulk of a solution
[80]. Of course, such heterogeneous nucleation or cavitation could also occur on
small, sub-micrometer sized contaminant particles in the liquid which would be hard
to distinguish from homogeneous nucleation experimentally. Homogeneous nucleation of nanobubbles or cavitation has been known to become virtually irrelevant
in water at normal temperatures that are far below the critical point [80], however,
given the very high levels of supersaturation of the guest gases after the dissociation
of clathrate hydrates, we cannot totally discount the nanobubble hypothesis.
Among these hypotheses put forward over the years, the structural memory
hypothesis is the least likely hypothesis of all for two reasons. First, clathrate hydrate
formation is a first-order phase transition that is accompanied by the latent heat of
fusion and the memory effect was observed after the dissociated water had been raised
to a temperature that was several Kelvins higher than the thermodynamic equilibrium dissociation temperature (superheating temperature). If the memory effect had
indeed been caused by some sort of residual structure in the dissociated water, it
would be difficult to comprehend how it was possible to raise the temperature of
the dissociated water to above the thermodynamic equilibrium dissociation temperature while preserving the residual structure. Second, evidence is mounting that there
are significant variations in the level of memory preservation among an ensemble
of samples that had the same thermal history. Here, either the surface history of
container walls was different or different sample cells were used. One would expect
the level of memory preservation to be independent of the solid container walls if
the memory effect had indeed originated from some form of residual structure in the
aqueous phase.
We note at this stage that the finding that transfer of a small amount of dissociated
water to a sample of fresh water still induced the memory effect that was comparable
to the memory effect found in a 100% dissociated water [70] does not necessarily
support the structural memory hypothesis. Other entities like imprinted solid impurities, supersaturated domains of the dissociated solution or nanobubbles could be
transferred together with the structure of the dissociated water, regardless of whether
the structure of the dissociated water differs from that of fresh water or not.
5 Nucleation of Gas Hydrates
sample cells) despite the same thermal history of the samples. The mechanism of
the memory imprinting is not known, nor is the mechanism as to how the imprinted
memory can be erased by moderate heating of the sample.
The nanobubbles hypothesis postulates that the very high level of supersaturation
of the guest gas after the dissociation of clathrate hydrate would lead to formation of
nanobubbles in the bulk of the supersaturated solution [76, 77]. Since nanobubbles
are small, they would be invisible to naked eye and persist for a long time because
they would not float quickly due to buoyancy.
The interfacial gaseous states hypothesis postulates that the high level of supersaturation of the guest gas after the dissociation of clathrate hydrate would lead
to formation of interfacial gaseous states on solid walls of a container [78, 79]
because heterogeneous nucleation of bubbles on a solid surface is always energetically favorable to homogeneous nucleation of bubbles in the bulk of a solution
[80]. Of course, such heterogeneous nucleation or cavitation could also occur on
small, sub-micrometer sized contaminant particles in the liquid which would be hard
to distinguish from homogeneous nucleation experimentally. Homogeneous nucleation of nanobubbles or cavitation has been known to become virtually irrelevant
in water at normal temperatures that are far below the critical point [80], however,
given the very high levels of supersaturation of the guest gases after the dissociation
of clathrate hydrates, we cannot totally discount the nanobubble hypothesis.
Among these hypotheses put forward over the years, the structural memory
hypothesis is the least likely hypothesis of all for two reasons. First, clathrate hydrate
formation is a first-order phase transition that is accompanied by the latent heat of
fusion and the memory effect was observed after the dissociated water had been raised
to a temperature that was several Kelvins higher than the thermodynamic equilibrium dissociation temperature (superheating temperature). If the memory effect had
indeed been caused by some sort of residual structure in the dissociated water, it
would be difficult to comprehend how it was possible to raise the temperature of
the dissociated water to above the thermodynamic equilibrium dissociation temperature while preserving the residual structure. Second, evidence is mounting that there
are significant variations in the level of memory preservation among an ensemble
of samples that had the same thermal history. Here, either the surface history of
container walls was different or different sample cells were used. One would expect
the level of memory preservation to be independent of the solid container walls if
the memory effect had indeed originated from some form of residual structure in the
aqueous phase.
We note at this stage that the finding that transfer of a small amount of dissociated
water to a sample of fresh water still induced the memory effect that was comparable
to the memory effect found in a 100% dissociated water [70] does not necessarily
support the structural memory hypothesis. Other entities like imprinted solid impurities, supersaturated domains of the dissociated solution or nanobubbles could be
transferred together with the structure of the dissociated water, regardless of whether
the structure of the dissociated water differs from that of fresh water or not.