Registration Dossier

Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

Releases of FC-770 are anticipated to be solely to the atmospheric compartment. This compound will not partition to terrestrial or aquatic compartments from the atmosphere based on partition coefficients (see below).

Henry’s Law constant (HLC) is used to describe the tendency for a substance to partition from water to air, the higher the value the greater the tendency for partitioning from water to air. For FC-770, an experimentally measured HLC of 1030 atm∙m³/mol was determined at 22 °C. This value provides a free energy for hydration from the gas phase of 26.1 kJ/mol. The free energy of hydration can be broken into terms for arranging the solute into the optimum conformation for hydration and for rearranging the structure of water in such a way that it will hydrate the molecule. This two terms have estimated energies of -4 kJ/mol and 30 kJ/mol, respectively. The positive sign of the total energy indicates an endothermic process, and the energy input is similar to that required for formation of high-energy biomolecules such as ATP. On a physical level, 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).  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 and, importantly, lack of distribution from air into the water column.  With regard to sorption to ice, 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 ice 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.  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 logarithm of the octanol air partition coefficient (log Koa), i.e. 1.5. Therefore the log Ksnow/air should be approximately -5.  It is evident from this value 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, indicating that liquid water surfaces at higher temperatures are even less likely to retain FC-770 than ice. FC-770 does not react with ice.  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.

The logarithm of the normalized organic carbon adsorption coefficient (log Koc) of FC-770 was calculated to be 4.71 based on the log Kow derived from experimental data. 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, though, that the Koc describes the organic carbon-normalized distribution of a substance to the soil phase *from the water phase,* rather than directly from the air. Any process that involves distribution of FC-770 from water is directly competitive with volatilization from the water phase, and any process of particle scavenging would be hindered by the requirement that FC-770 first pass through a superficial water layer. 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.  Further, distribution from air to entirely dry carbon would be better described by the Koa rather than the Koc, and general distribution to suspended particles by the Ksp (defined strictly in terms of vapor pressure). Distribution behaviour was further elucidated by modeling using the NewEQC Level III fugacity model, which is preferred to the fugacity model in EpiSuite (i.e., an adaptation of the previous EQC model with limited user inputtable parameters). NewEQC incorporates advances in the science of chemical partitioning and reactivity as compared to the original EQC fugacity model. The NewEQC model specifically includes improved treatment of input partitioning and reactivity data, temperature dependence, and sensitivity/uncertainty analysis, as well as providing full user control over a range of substance partitioning parameters. With 100% of a hypothetical 250 tonne/year FC-770 emission directed to the air compartment, as indicated by the registered uses, 99.996% of FC-770 was predicted to be in gas phase in the air compartment, whereas 0.0036% was predicted to be in the gas-filled pore spaces in the soil, and 0.00052% sorbed to soil solids. The half-time for transport from soil to air was predicted to be 1.59 hours, which is several orders of magnitude shorter than transport from air to soils (1610 days) or from air to water (461,000 years). Only negligible amounts of FC-770 are expected to be present in soils or aquatic systems. While all emissions of FC-770 are to the atmosphere, it should be pointed out that the volume released to the atmosphere is far in excess of that which is expected under conditions of use, since all registered uses require closed conditions to minimize release of FC-770. Nevertheless, under the condition of this model the soil compartment is predicted to contain 295 grams of FC-770 in total at steady state for a concentration of 4.4 femtograms/g bulk soil, while the aquatic compartment is predicted to contain 360 mg of FC-770 in total for a concentration of 0.7 femtograms/L bulk water. Once in the atmospheric compartment, this compound will not partition to terrestrial or aquatic compartments based on the same properties. Therefore, this compound will remain in the atmosphere when released from manufacturing and use. Owing to this, further testing of sorption in the terrestrial compartment would provide no useful information.

 

The logarithm of the octanol air partition (log Koa) coefficient is used to describe the tendency of a substance to partition from air into the lipid rich tissues of air breathing organisms. The logarithm of the octanol air partition coefficient (log Koa) of FC-770 was calculated to be 1.47 based on the experimentally measured water solubility of 66.2 ug/L and the experimentally determined vapor pressure of 50.6 mm Hg at 20 °C. This log Koa value indicates the FC-770 has a low potential to partition from air to the lipid rich tissues of air-breathing organisms.