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Partition coefficient

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Description of key information

Depending on pH:
HMD: range of log kow varied from -4.04 to 0.44
Terephthalic acid: range of log kow varied from -11.059 to 2
Based on its two components, the substance terephthalic acid, compound with hexane-1,6-hexanediamine (1:1) is considered to have a low potential of bioaccumulation.

Key value for chemical safety assessment

Additional information

According to ECHA guidance R7a, the relative extent to which ionisable substances (as salts) are likely to be dissociated in the environment (with pH usually in the range 5 - 9) can have a marked effect on physico-chemical properties such as n-octanol/water partition coefficient (Kow). For substances which dissociate, values for Kow shall be derived for the forms (i.e. neutral and/or dissociated) which are relevant at environnementally relevant pH range.

When dealing with salts of organic compounds, as it is the case here for Terephtalic acid compound with hexane-1,6-diamine (1:1), ECHA guidance R7a also indicates that QSAR methods will be valuable in assessing the properties of each half of an organic salt. Therefore, a weight-of evidence approach was used here based on QSAR data for each half of the salt:

- Hexamethylenediamine (HMD, CAS 124-09-4): Four data are available and were used in a weight-of-evidence approach. Two data were scored as Klimisch 2. Among them, that obtained through the recommended SPARC model provides results in function of pH. The calculated log D varies from -4.04 to 0.44 at 25°C over the whole pH range, due to ionisation of the substance. At environmentally relevant pH (i.e .from 5 to 9), the calculated log D varies from -4.04 to -2.04. The other Klimisch 2 data from Meylan (1995) used a QSAR method to calculate a Log P equal to 0.35; keeping in mind that the model does not take into account temperature and pH (fragments are considered unionised). The two Klimisch 4 data report Log Kow of 0.28 and 0.35. Considering the previous data, it is thus suspected these values are for the unionised form. On the basis of all these data, in particular those at environmentally relevant pH, the substance hexamethylenediamine (or hexane-1,6-diamine, HMD) should have low potential for bioaccumulation.

- Terephtalic acid (CAS 100 -21 -0): Three data are available and were used in a weight-of-evidence approach.They were scored as Klimisch 2.

Among them, that obtained through the recommended SPARC model provides results in function of pH. The calculated log D varies from -11.059 to 0.913 at 25°C over the whole pH range, due to ionisation of the substance. At environmentally relevant pH (i.e. from 5 to 9), the calculated log D varies from -9.095 to -1.396. Another value from QSAR using the EPIsuite calculation gave a log kow of 1.76. Finally, an experimental value is also available and gives a log Kow of 2 (Hansch et al. 1995). On the basis of all these data, in particular those at environmentally relevant pH, the substance terephthalic acid should have low potential for bioaccumulation.