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Koc values of the category members of fatty acids were calculated by established calculation interfaces in order to indicate the adsorption potential of the substances and for the needs of the CSA.

Koc values were calculated with the US EPA EPISuite interface using QSAR/QSPR predictions (KOCWIN v. 2.00) according to the MCI method. Based on the neutral chemical form of fatty acids, the estimated Koc values increase with increasing chain length from 28.95 for octanoic acid (C8) up to 129,000 for docosanoic acid (C22).

However, since fatty acids (C8 - C22) are weak acids with pKa between 4.75 – 5.7 and occur in their ionisable form in the environment, the Koc values were corrected based on the soil-water partitioning coefficient calculation method for ionizable substances developed by Franco, Fu & Trapp (2009, ChemProp v6.4). The Koc values were predicted to be between 111.6 for octanoic acid (C8) and 66,696 for C22, which reflects a slight decrease of the adsorption potential of fatty acids when considering the speciation of the molecules in the calculation. 

 

Table 1: Koc and Log Koc values calculated for the fatty acids with Franco, Fu & Trapp model for ionizable substances at neutral pH.

Substance

 

 

 

 

Franco, Fu & Trapp

model

 

Log Koc

Franco, Fu & Trapp

model

 

Koc

Log Pow used

 

 

 

pKa used

 

 

 

 

 124-07-2, C8

O=C(O)CCCCCCC

 

2.05

111.6

3.05

5.3

112-05-0, C9

O=C(O)CCCCCCCC

2.07

116.8

3.52

4.95

334-48-5, C10

CCCCCCCCCC(=O)O

2.64

432.40

4.1

5.7

143-07-7, C12

O=C(O)CCCCCCCCCCC

2.77

589.09

5

5.3

544-63-8, C14

O=C(O)CCCCCCCCCCCCC

2.92

822.24

5.98

4.9

57-10-3, C16

O=C(O)CCCCCCCCCCCCCCC

3.36

2308.32

7.17

4.75

57-11-4, C18

O=C(O)CCCCCCCCCCCCCCCCC

3.91

8117.55

8.23

4.75

112-85-6, C22

O=C(O)CCCCCCCCCCCCCCCCCCCCC

4.82

66696.03

9.91

4.77

Furthermore, transport and distribution of fatty acids through the atmospheric compartment is not expected due to their low vapour pressure values ranging from < 0.0001 Pa to < 5.8 Pa at 25 °C.