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EC number: 473-390-7
CAS number: -
The dimensionless Henry’s Law Constant (HLC) was measured experimentally
by analytically quantifying the concentration of FC770 in both the
headspace (concentration low ppm) and the water phase (low ppb) of a
sealed vial after equilibration. The resulting measured HLC of 42,400
(dimensionless, expressed as Cg/Cw, 2.36e-5 expressed as Cw/Cg, or 1030
expressed as atm∙m³/mole) is rather high given the rather low vapor
pressure 50.6 mm Hg. However, this HLC is well within the range of many
super hydrophobic chemistries and halogenated aromatics.(1)
The HLC for a molecule is simply related to the free energy of hydration
(deltaGhyd)(1), which in turn can be considered as the sum of the energy
necessary to rearrange a molecule into the optimum conformation for
solvation (deltaGstructure) and the energy required to rearrange the
solvent in such a way that it will solvate the molecule (deltaGsolv).
The equation below provides the exact thermodynamic definitions:
deltaGhyd = deltaGstructure + deltaGsolvent = -RT∙ln(Cw/Cg)
where (Cw/Cg) is the dimensionless HLC and temperature is in Kelvin.
Solving the equation above for FC-770 gives a deltaGhyd = 26.1
KJ/mole. The experimentally determined deltaGstructure energy for the
optimized structure of
2,2,3,3,5,5,6,6-octafluro-4-(trifluoromethyl)morpholine in water is
-4.04 kJ/mole.(2) This structure differs from FC-770 by a
perfluoromethyl functionality as opposed to a perfluoropropyl
functionality in FC-770. As the two are structurally very similar, the
structural rearrangement energy is likely very close. Therefore, in
this argument, a deltaGsolv for FC-770 of 30 KJ/mole is used for
illustrative purposes. The sign of the energy of solvation is positive,
indicating that energy must be added to the system to get FC-770 in
solution. This is approximately the same amount of energy as is released
in the body through hydrolysis of the high-energy phosphate bond in ATP:
ATP4- + H2O → ADP3- + HPO42-
+ H+ (30.5 kJ/mol).(3)
Quite simply, this energy is in the range of bond cleavage and it is not
spontaneous. Solvation of FC-770 is not favored, just as spontaneous
formation of ATP from ADP and phosphate is not favored. Alternatively,
the solvation energy is roughly equivalent to the energy required to
break five hydrogen bonds while leaving the involved water molecules in
place in the bulk solution (ca. 6 kJ/mol).(4) This can be viewed as the
energy of forming a pocket in the bulk solution and optimizing it to fit
the FC-770 molecule. Upon desolvation of the FC-770 molecule the stored
solvation energy is released, forming a thermodynamic driver to remove
FC-770 from solution. This provides key thermodynamic evidence for the
net removal of FC-770 from open systems.
For environmental fate, FC-770 released to the air will predominantly
stay in the air as it is highly energetically unfavorable for FC-770 to
be in the water. This is true even for cold water as the temperature
dependence of Henry's Law can be regarded as constant over the
relatively narrow range from 22 °C to 0 °C. The value of deltaGhyd is
24.2 kJ/mol at the lower temperature. Release of FC-770 from the water
to the air is always favored.
There could be some potential for FC-770 to be trapped in ice,
particularly in high altitude cirrus clouds or in atmospheric gas
bubbles entrapped in ice. In general, one can classify ice - trace gas
interactions of three different kinds. One is the partitioning of gases
to the ice, leading to a (temporary) loss from the gas phase. This is
not an incision or entrapment mechanism. The second is a chemical
reaction with the ice (e.g. acids that deprotonate form a volatile
neutral molecule to a non-volatile anion and cation). The third is
chemical reaction on the ice while the species of interest is
partitioned to the ice surface. Temporary distribution to snow and is
not favored. Sorption to snow surfaces can be described by a Linear Free
Energy Relationship involving two hydrogen bonding terms and the
hexadecane-air distribution coefficient (L16) to account for
non-specific intermolecular forces.(5) Since FC-770 does not
participate in hydrogen bonding, the relationship is limited to log
Ksnow/air = 0.639*log L16 - 6.85 at -6.8 °C. The log L16 value is
expected to be similar to the log Koa of 1.5, therefore the log
Ksnow/air should be approximately -5. It is evident from this that
sorption to snow surfaces is not favored for FC-770. A similar
relationship describes sorption to liquid water surfaces at 15 °C,
Kws/air = 0.635 log L16 - 8.47 (5), indicating that liquid water
surfaces at higher temperatures are even less likely to retain FC-770.
FC-770 does not react with ice so that will not be a mechanism of
transport. In the unlikely event of photolytic transformation of FC-770
on ice surfaces, it would be the photodegradation products that would be
transported to the surface and not FC-770. We note that gas bubble
encapsulation may occur and result in transport of FC-770 to the
terrestrial compartment, this transport is as a gas in air and once the
entrapped gas is released, FC-770 will remain in the gaseous state.
Based on a high Log Koc, it could be postulated that FC-770 might adsorb
to suspended particulate matter and thereby be transport via atmospheric
settling or rain out to the terrestrial surface. It should be noted
that the suspended particulate matter is coated with water and is often
the source of nucleation for formation of aerosols and water droplets.
It is not a significant source of ice formation. Once the suspended
particulate begins to add additional water, FC-770 will be driven off by
the depositing water. Atmospheric deposition of FC-770 beyond simple
diffusion is not an important fate process.
1. Schuurmann, G. 2000. Prediction of Henry’s law constant of
benzene derivatives using chemical continuum-solvation models. J. Comp.
Chem., 21 (1): 17-34
2. Spartan Parallel Suit Database. (Available to users with a
license to use Spartan.) Spartan is a molecular modeling software which
uses theoretical quantum mechanical models to calculate physical
3. Lubert Stryer Biochemistry, 3rd edition, 1988. Chapter 13, p.
4. Martin Chaplin. 2019. Water Structure and Science: Hydrogen
Bonding in water (1).
5. Lei Y.D., Wania, F. 2004. Is rain or snow a more efficient
scavenger of organic chemicals? Atm. Environ., 38(22): 3557-3571.
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