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Releases of FC-72 are anticipated to be primarily to the atmospheric compartment. This compound is unlikely to partition to terrestrial or aquatic compartments from the atmosphere based on partition coefficients (see below).

The experimental vapor pressure (178.3 mm Hg) and Henry’s Law constant (42,000 dimensionless, or 1021 atm∙m³/mol, at 22.9 °C) suggests that FC-72 is expected to partition from wet or dry terrestrial surfaces into the atmosphere. The category member perfluoro-N-methylmorpholine has an experimental soil-atmosphere partitioning half life of 66.1 ± 16.0 minutes, which is substantially shorter than that predicted by EUSES but in reasonable agreement with the value predicted using a USEPA calculation method. Soil volatilization half-lives for FC-72 are estimated to be 9.0 hours for agricultural soils and 34 minutes for grassland and industrial soils as calculated using the USEPA method. The volatilization half-life indicates that FC-72 will be removed from soil and sediments quickly. This half life may underestimate the atmospheric partitioning from natural soils due to the layer of water on soils particulates (volatilization driven by water-atmosphere partitioning rather than soil-atmosphere partitioning).

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-72, the HLC of 1021 atm∙m³/mol at 22.9 °C indicates that it is likely to volatilize from the aquatic compartment into the atmosphere. In distribution modeling using a Level 3 fugacity model, with 100% of FC-72 emission directed to the atmospheric compartment, 99.996% of FC-72 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 0.00014% sorbed to soil solids. The half-time for transport from soil to air was predicted to be 1.45 hours, which is several orders of magnitude shorter than transport from air to soils or from air to water. Only negligible amounts of FC-72 are expected to be present in aquatic systems or soils.

The logarithm of the organic carbon normalized adsorption coefficient (log Koc) is used to describe the tendency for a substance to partition from water to soils or suspended solids. The experimental water solubility of FC-72 is slight (84.1 µg/L) and the log Koc was calculated(1) to be 4.15 based on the measured log Kow value (i.e., 5.0 at 23.3 °C). Based on these data FC-72 is anticipated to partition from water to soils and suspended solids for a short period of time. Given the rapid partitioning to the atmosphere from soils and from water, the overall net partitioning will be to the atmosphere.

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 calculated log Koa of FC-72 is 0.39, based on the dimensionless Henry’s Law constant value at 22.9 °C and the average experimental Kow value (that is, the untransformed average of concentration ratios) of 103,700 at 23.3 °C. This log Koa value indicates that FC-72 has a low potential to partition from air to the lipid rich tissues of air-breathing organisms.

All of the members of this category stem from the same manufacturing process, have similar physicochemical properties including high vapor pressure and low water solubility relative to the hydrocarbon analogs (e. g., hexanes v. perfluorohexanes), and also lack any chemically reactive groups, which forms the technical basis for the category. Members of this category are fully fluorinated, meaning that fluorine, rather than hydrogen, is bonded to all carbon atoms in the molecule. Fluorine is the most electronegative of the elements (fluorine has an electronegativity of 3.98 on the Pauling scale, as compared to 2.55 for carbon or 2.20 for hydrogen). This electronegativity is expected to dominate over all other aspects of substance chemistry and is the underlying basis for similarity of substances in this category. Because these substances exhibit similarity in their physicochemical properties and toxicological properties in mammals, and because available data indicates that parent molecules are not reactive toward biological molecules and cannot undergo bioactivation or indeed any reaction by normal enzymatic processes, they are considered to constitute a chemical category. Data gaps for transport and distribution can therefore be addressed by read-across between category members. Please see IUCLID section 13 for the category justification and a matrix of physicochemical and distribution data for members of the Perfluorinated Organic Chemicals C5-C18 category.

(1) European Chemicals Bureau: Technical Guidance Document of Risk Assessment, Chapter 4.