Phosphoric acid, isononyl ester

Partition coefficient

Description of key information

Walker (2012) determined a measured log10 Pow in the range 1.09 to 4.28 for the multicomponent substance.  However, it was determined that an average log Kow of 1.86 was the appropriate value for the whole substance. 

Key value for chemical safety assessment

Log Kow (Log Pow):

1.86

Additional information

A reliable OECD 117 HPLC determination of logK_OWwas performed by Walker (2012) . The study report shows a chromatogram with 4 peaks. The first peak was tentatively identified as the monoester. The three peak complex was ascribed to the diester, although it was not established why the diester would be resolved into a three-broad-peak complex. The HPLC results rule out the presence of (a significant amount of) the triester; this would have a much higher logK_OWthan any of the observed peaks.

It is likely that neither the monoester nor the diester will show significant fractions of neutral species at relevant pH values. According to SPARC, estimated pKa values for the monoester are 1.11 and 6.38, whereas the estimated pKa value for the diester is 0.59. At a pH of 5, at which the HPLC determination was performed, therefore, both the monoester and the diester will be primarily present in their monodissociated form. Using the Henderson-Hasselbalch equation as a satisfactory approximation, at a pH of 5 and a pKa of 1.1 (monoester), the ratio of A^–to HA is 7760, and at a pKa of 0.59 (diester), the ratio is 25700.

According to the KOWWIN algorithm, the estimated logK_OWfor the monoester is 3.14, and the estimated logK_OWfor the diester is 7.05. This applies to the neutral HA form only, as KOWWIN cannot account for charged species or even phase-separated speciation.

The OECD 117 report presents a logK_OWof 1.09 for the monoester and a logK_OWranging from 3.83 to 4.28 for the diester. A simple zeroth order speciation calculation, disregarding any shifts in equilibrium, for the monoester suggests that at a logK_OWof 3.14 (estimated value) for the neutral monoester and a pKa of 1.1, the observed partitioning of the monoester over water and octanol phases would result in an observedK_OWof 0.18 (logK_OW= -0.75); for the diester (pKa 0.59, neutral, estimated logK_OW7.05) the observedK_OWwould be 424 (logK_OW= 2.63). In both cases the HPLC-determined value is around 1.7 log units higher than the speciation-based calculation. This difference is most likely due to differences in the effect dissociation and speciation has on the distribution behaviour of phosphates in octanol/water systems vs HPLC systems. Regardless, it can be seen that the HPLC results are unlikely to underestimate logK_OWvalues under environmental or physiological conditions, where these phosphate esters are (fully) ionized.

Since substance is a multicomponent substance, it is not useful to express its partitioning behaviour in terms of its individual components, much less in terms of its most hydrophobic component. Approaches for deriving a compound hydrophobicity parameter for mixtures, including complex mixtures of unknown composition have been forwarded; such as the hydrocarbon block method referred to in Ch R.11: PBT assessment of the Guidance on information requirements and chemical safety assessment. We refer specifically to the following papers:

Verhaar, H.J.M., van Loon, W.M.G.M., Busser, F.J.M., Smit, E., and Hermens, J.L.M. (1994). Development of Parameters for Evaluating the Effects of Mixtures of Organic Micropollutants on the Environment.

Verbruggen, E.M.J., van Loon, W.M.G.M., and Hermens, J.L.M. (1996). A hydrophobicity parameter for compex organic mixtures. Environmental Science and Pollution Research 3. pp. 163-168.

The approach described by Verbruggen et al. represents a mole fraction weighted average logK_OWas an overall hydrophobicity parameter for a mixture. For RL612/11, this would result in an overall (weighted average) logK_OWof 3.56. This value is derived as follows:

 

Assumptions:

•  For the test sample components, the response of the UV detector is directly correlated with molecular weight; M for the monoester is 224.2 and for the diester M is 350.5.

•  The ‘triplet’ identified as such in the OECD 117 report represents only diester.

As such, the following table represents our mole fraction weighted average

 

Peak	Substance	M	Area fraction	Mole fraction	logKOW
1	Isononyl phosphate	224.2	0.655	0.748	1.09
2	Diisononyl phosphate	350.5	0.062	0.045	3.83
3	Diisononyl phosphate	350.5	0.19	0.139	4.17
4	Diisononyl phosphate	350.5	0.093	0.068	4.28
	Average		1.00	1.00	1.86	average (logKOW)
					3.56	log (averageKOW)

 

 

Average (logK_OW) represents the average of the mole fraction weighted logK_OWvalues. Log (averageK_OW) represents the log of the average of the mole fraction weightedK_OWvalues. The latter approach is a worst case approach that emphasizes the more hydrophobic fraction. Both approaches however result in an average logK_OWfor the multicomponent substance that is below 4.