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Partition coefficient

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partition coefficient
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
key study
Study period:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable method for pigments with low solubilities
Justification for type of information:
This summary justification relates to organic pigments that (a) do not contain metals and do not represent salts, (b) are poorly soluble in water and n-octanol, (c) have a molecular weight > 200 g/mol and (d) have no surface modification that may change the properties of the substance (poorly soluble organic pigments (PSOPs) hereafter). The full justification, the sections referred to as well as the references are provided in the justification document attached in section 13.2.
1. The criteria of the nanomaterial definition have no scientific basis.
With respect to the upper limit of 100 nm, Commission Recommendation 2011/696/EU states that ‘there is no scientific evidence to support the appropriateness of this value’. With respect to the 50% threshold, a review of this Commission Recommendation states that ‘(s)ince the EC definition of nanomaterial should not be related to hazard or risk considerations, the selection of a threshold is essentially a policy choice and should be justified as such’ (Rauscher et al., 2015). Given the lack of a scientific basis of the criteria for the nanomaterial definition, it is inconceivable why dispersion/dispersion stability and related issues such as agglomeration/aggregation are considered relevant for nanoforms, but not bulk forms of organic pigments (see section 2.1 for more details).
2. Bulk and nanoforms of PSOPs are not expected to differ substantially with respect to dispersibility/dispersion stability.
PSOPs in bulk and in nanoform have a strong tendency to agglomerate and aggregate, and this tendency increases with decreasing particle size. Even if dispersion is achieved, particles will tend to reagglomerate and – in fact – commercial pigment preparations need to be stabilised in the medium/vehicle system to prevent reagglomeration. This is also evident in several measurements performed on dispersions of organic pigments. The difference between bulk forms and nanoforms of PSOPs with respect to agglomeration/aggregation and dispersibility/dispersion stability is therefore expected to be small (see sections 1.1 and 2.2 for more details).
3. The determination of the dispersion and agglomeration behaviour is problematic.
(2a) OECD GD 318 (OECD, 2020) recommends using dispersing agents for nanomaterials that are not miscible with aqueous media to increase the dispersibility, while it is also recognised that such an approach would not generate data on the dispersion stability of the ‘pristine material‘. In contrast, ECHA (2017c) states that ‘(u)se of synthetic dispersants is not recommended to prepare the stock dispersion or solution for aquatic toxicity testing, unless they are constituents of the registered substance (product formulation), in which case the bioassay should be conducted with the as-produced material’. PSOPs are manufactured as powders that do not contain dispersants as constituents and dispersions therefore need to be prepared by other means.
(2b) Both OECD TG 318 and the draft TG on agglomeration behaviour apply sonication of the stock dispersion to disperse particles. ECHA (2017c) states: ‘The dispersion method should not change the characteristics of the test material’. It is likely to be impossible to demonstrate the lack of such changes following sonication of the stock dispersion, because sonication-induced modifications and the creation of artefacts are well known (Hartmann et al., 2015). Furthermore, sonication may result in different dispersion patterns that do not adequately reflect the dispersion of PSOPs in pigment preparations or environmental media and the test period of OECD TG 318 may be too short to reflect dynamic processes such as reagglomeration or Ostwald ripening taking place over time under real-world conditions (see section 2.2 for more details).
(2c) OECD TG 318 assumes dispersibility to be possible. Accordingly, the test item is assumed to be dispersed at the start of the test. Dispersion stability is only indirectly assessed via sedimentation, which is a function of density. The agglomeration behaviour of PSOPs – in contrast to inorganic nanomaterials of much higher densities – may be underestimated by this method but is not measured directly (also see point 2d below). Consequently, demonstrating low dispersibility of PSOPs may be impossible using this method. Tests in physiological media demonstrated low dispersibility (AAN > 3) for all nano organic pigments tested (Arts et al., 2016; Hofmann et al., 2016).
(2d) Much of the test conditions in OECD TG 318 and guidance documents has been derived from inorganic nanomaterials and their impact on organic pigments is uncertain. Importantly, OECD TG 318 has been validated using only inorganic nanomaterials, i.e. titanium dioxide and silver (OECD, 2017a). This fact makes the applicability to organic pigments questionable, e.g. due to the lower density difference to water compared to these inorganic nanomaterials (also see point 2c above).
4. No adopted test method exists for the determination of the most meaningful parameter.
Given the problems discussed above, testing the dispersion stability in water instead of log Kow (REACH Annex VII) may only be of little value for the assessment of the environmental fate of PSOPs. OECD TG 318 as well as the draft TG on agglomeration behaviour address only homoagglomeration, while the heteroagglomeration attachment efficiency ‘has been found to be the most suitable parameter to inform fate modelling and risk assessment’ (OECD, 2020). Therefore, testing heteroagglomeration would be a more useful indicator of environmental fate. However, ‘methods are not yet progressed enough to develop a fully validated TG for heteroagglomeration testing’ (OECD, 2020). Therefore, testing of the most adequate endpoint is not feasible due to a lack of an adopted OECD TG or other EU Test Method in Regulation EC No 440/2008.
5. Relevant conclusions can be reliably assumed.
OECD TG 318 test results are assigned to (a) low, (b) intermediate or (c) high dispersion stability. OECD (2020) only differentiates the conclusions to inform further testing between dispersions that are ‘fully stable’ and those that are not. Based on the considerations above, dispersions of PSOPs can reliably be assumed to be not ‘fully stable’, a conclusion that is supported by initial measurements of dispersion stability according to OECD TG 318 (see section 2.2 for more details).
Overall, testing the dispersion stability is not justified based on the above considerations. The log KOW is an adequate parameter for the truly dissolved fraction of the submission substance while the undissolved fraction of dispersed particles of the submission substance can reliably be assumed to be not fully stable.

Data source

Reference Type:
study report
Report date:

Materials and methods

Principles of method if other than guideline:
Calculation of log Pow from the solubility in octanol (see section 4.9) and the solubility in water (see section 4.8 ).
Type of method:
other: calculation
Partition coefficient type:

Test material

Constituent 1
Chemical structure
Reference substance name:
EC Number:
EC Name:
Cas Number:
Molecular formula:
Test material form:
solid: nanoform, no surface treatment

Study design

Analytical method:
other: The analytical methods for the underlying data are given in section 4.8 and 4.9

Results and discussion

Partition coefficientopen allclose all
Key result
Partition coefficient:
25 °C
ca. 7
Key result
log Pow
Partition coefficient:
25 °C
ca. 7
Details on results:
See below

Any other information on results incl. tables

At ambient temperature (24 -25°C for solubility in water and 25 -26°C for solubility in octanol), the solubility in octanol was determined to be 5960 µg/L (section 4.9) and the solubility in water was determined to be 7.5 µg/L (section 4.8). An average temperature of 25°C is given here.

With the equation given above, the follwoing values were calculated:

Pow = 5960 µg/L / 7.5 µg/L

Pow = 795

log Pow = 2.9

Applicant's summary and conclusion

The test substance has a log Pow of 2.9.
Executive summary:

The log Pow of the test substance was calculated from the solubilities in water and in octanol. The log Pow at 25 °C was 2.9.