Magnesium bis(dinonylnaphthalenesulphonate)

Water solubility

Description of key information

In a studies according to the column elution method (Laus 2014)  the water solubility of the substance was reported to be  21 mg/L . A QSAR according to Waternt v1.01 (EPIWIN) showed a much lower water solubility of 0.00094 µg/L. These values, when considered together, indicate that the water solubility of the test substance is similar to those of its structural analogues DNNSA and BaDNNSA.

Key value for chemical safety assessment

Water solubility:

0.229 mg/L

at the temperature of:

20 °C

Additional information

In the water solubility tests that are available for all three compounds, difficulties to assess an actual value were seen. This is not surprising in view of the complex structure of the substances. The ECHA guidance on Information Requirements [1] indicates for UVCB substances: “solubility of a single substance in water is not applicable to substances which are multi-component, such as multi-constituent or UVCB substances, i.e. complex substances. The usually accepted meaning of ‘solubility’ in such cases is ‘the composition of the aqueous solution formed at equilibrium under a defined set of conditions’. Temperature and the amount of substance added per unit volume of water (i.e. the ‘loading’) are the main factors to consider. It may not always be possible to establish that equilibrium of all components has been achieved; in these cases, time and type of agitation of the test vessels must also be described”. Different components of the UVCB may exhibit different water solubility, so that in practice water solubility is not a single point, but likely to a range. It is noted that for complex substances water solubility may also be dependent on the loading rate. In addition, the analytical method (in all available studies based on a single peak in the chromatogram) may influence the assessed value strongly.

The column elution method as used for MgDNNSA showed a rising tendency of solubility, but sufficient analyzable fractions were found to allow assessment of the solubility. The differences found between the results on source and target substances could be explained by the prolonged (1 hour) mixing time used to incubate the column for the MgDNNSA study compared to a shorter mixing period for BaDNNSA and DNNSA. This may have led to a larger load on the MgDNNSA column compared to the columns with both other compounds and a concomitantly higher potential for elution as reflected in the higher concentrations analyzed. Problems associated with turbidity of the solutions were not reported in the studies on the source and target substances, but were seen clearly in the water solubility studies on other structural analogues e.g. Zinc bis(di C8-C10, branched, C9 rich, alkylnaphthalene sulphonate) and Lithium (di C8-C10, branched, C9 rich, alkylnaphthalene sulphonate)[1]. Nevertheless all studies claim that no particles were present in the analyzed samples.

In view of the uncertainties described above, QSARs were run using the Epiwin model for MgDNNSA, although the Epiwin model in principle is not suitable for salts (only specific cations can be modelled). The structures of MgDNNSA as shown in table 1 can be modelled but do not reflect the actual substance accurately, as this substance is an ionizable salt. Therefore the most reliable estimation for water solubility will be based on the test with the acid, which shows a water solubility of 0.229 mg/L. This value is considered to represent a worst case estimate to be used in the risk assessment on the target substance.

[1]Laus, 2014 Determination of the solubility in water of Zinc bis(di C8-C10, branched, C9 rich, alkylnaphthalene sulphonic acid) according to OECD 105 resp. EU A.6

Laus, 2014 Determination of the solubility in water of Lithium (di C8-C10, branched, C9 rich, alkylnaphthalene sulphonic acid) according to OECD 105 resp. EU A.6