Zinc bis(dinonylnaphthalenesulphonate)

Water solubility

Description of key information

In two studies according to the column elution method (Laus 2014) and the flask method (Laus 2014), water solubilities of 17.2 and 15.1 mg/L respectively were reported. A QSAR according to Waternt v1.01 (EPIWIN) showed a much lower water solubility of 0.00098 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 ZnDNNSA, BaDNNSA and DNNSA, 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 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 methods to assess the water solubility included both column elution and slow stirring approaches. The slow stirring approach for ZnDNNSA showed an increase of solubility when stirring time was prolonged. This might be attributed to adsorbed substance becoming available due to the long stirring process. In the column elution tests, the differences found could be explained by the prolonged (1 hour) mixing time used to incubate the column for the ZnDNNSA study compared to a shorter mixing period for BaDNNSA and DNNSA. This may have led to a larger load on the ZnDNNSA column compared to the columns with both other compounds and a concomitantly higher potential for elution as reflected in the higher concentrations analyzed. The preparation of an analyzable solution without any dissolved particles was also a difficult process, which is not described in detail in all of the available reports. Problems associated with turbidity of the eluted solutions were not reported in the studies with BaDNNSA and the acid, but was seen clearly in the studies on ZnDNNSA. Nevertheless all studies on the three substances claim that no particles were present in the analyzed samples.

In view of the uncertainties described above, QSARs were run using the Epiwin model on representative structures of ZnDNNSA, BaDNNSA and DNNSA. The the Epiwin model in principle is not suitable for salts (only specific cations can be modelled). In absence of reliable and accurate data, the most reliable value for water solubility is considered to be the result from 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 for the water solubility of ZnDNNSA.