Physical and Chemical Properties

Water solubility is a key property affecting PFOA and its salts in the environment. The free acid and the ammonium salt are solid at 20°C. The free acid readily dissociates to perfluorooctanoate (PFO), the conjugate base that is commonly measured in environmental media and biological samples. The conjugate base of the PFOA acid and its salts is highly soluble in water. It has been noted that PFOA salts self-associate at the surface but disperse with agitation and form micelles at higher concentrations (US EPA 2003).

Solubility is dependent on the acid dissociation constant (pKa) of the acid form, and the pKa value commonly reported and used is approximately 2.5 (Kissa 1994). However, Burns et al. (2008) determined a pKa of 3.8 ± 0.1, suggesting that the neutral species can exist in the environment. Jasinski et al. (2009) determined a pKa of 3.3 ± 0.4. Goss (2008), using analogues and molecular models, suggested that the pKa is more likely to be close to -0.5. Goss (2008) suggested that PFOA is expected to have a low pKa, such that 99% of the compound will occur in its anionic form (i.e., PFO) under most environmental conditions suggesting that the environmental partitioning of PFOA will be dominated by the anionic form. As the free acid readily dissociates to perfluorooctanoate (PFO), models for global fate, transport/partitioning and bioaccumulation may not have considered the pH of the environmental compartment in conjunction with the pKa values of the compound. Thus, the distribution ratios among air, water and other compartments may not have been calculated correctly, and the speciation of PFOA/PFO may not have been considered appropriately.

The physical and chemical properties of APFO are summarized in Table 3.

Table 3. Physical/chemical Properties of PFOA Ammonium Salt (APFO)

Property Value Type Reference
Molecular mass (g/mol) 414.0639
Melting point (°C) 130 (sublimation) Experimental Gilliland 1992
157–165 (water) Lines and Sutcliffe 1984
Vapour pressure (Pa) 0.0013 at 25°C
(1 × 10-5 mmHg at 25°C)1
Experimental 3M Environmental Laboratory 1993
Henry’s law constant (Pa·m3/mol) Not available
log Kow (dimensionless) 5 ± 0.5 Modelled Jasinski et al. 2009
3.62–6.30 Modelled Arp and Goss 2009
log Koa (dimensionless) 5.73–6.80 Modelled Arp and Goss 2009
log Koc (dimensionless) 2.06 Experimental Higgins and Luthy 2006
Water solubility 0.1 g/ml Experimental Sigma-Aldrich Canada Ltd. 2008
> 10 % Experimental 3M Environmental Laboratory (1993)
pKa (dimensionless) 2.5 Experimental Kissa (1994)
3.8 +/- 0.1 Experimental Burns et al. (2008)
-0.5 Modelled and analogues Goss (2008)
Abbreviations: Koa, octanol–air partition coefficient; Kow, octanol–water partition coefficient; Koc, sediment organic carbon coefficient; pKa, acid dissociation constant.
1 Value in parentheses provided in the original reference.

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