Phosphorodithioic acid, O,O-di-dodecyl-esters, zinc salts, neutral and basic

Endpoint summary

Additional information

Structural similarities are observed between target substance Phosphorodithioic acid, O,O-di-dodecyl-esters, zinc salts (CAS 4563-56-8) and source substance Zinc bis[O,O-bis(2-ethylhexyl)] bis(dithiophosphate) (CAS 4259-15-8) substance. They contain the same functional groups with difference in alkyl ester chain, where the target is composed of linear dodecyl chains and source of branched ethylhexyl chains. Comparing the fatty alcohols of the raw substances involved in the reaction, 2-ethylhexanol is much more soluble in water than dodecanol, correspondingly the Pow values of 2-ethylhexanol is lower the that of dodecanol. However, target and source substances reveals the different patterns of physicochemical parameters as set out in the Table below, implying that properties of the target and source substances are dominantly determined by the chelated molecular structure of dialkyldithiophosphoric acid and Zn than the form and length of alkyl chains. Therefore the target and source substances may have the similar behaviour in the toxicological aspects.

Due to complex molecular structures and conversable forms in the aqueous solution, determination of a pH-dependent hydrolysis is technically difficult and unreliable. 

The source substance is not readily biodegradable. Less than 5% biodegradation was observed for the source substances in screening tests for ready biodegradation. Hence the substance can be considered as not biodegradable in water. Based on these results the conclusion might be drawn that both substances are poorly biodegradable. Both substances belong to the group of zinc dialkyldithiophosphates, which consist of a phosphorodithioic acid structure with alkyl or alkaryl ester substituent groups. Thus, target and source substance contain the same functional groups and only have difference on alkyl chain length (C12 in target substance and C8 in source substance). These structural similarities suggest the similar bioavailability. Therefore, poor biodegradability should apply also to the target substance.

Furthermore, the target and source substances have a moderate water solubility (150 mg/L and 9.1 mg/L respectively) and a low vapour pressure (<4.0 x 10-2 Pa at 20 °C) and therefore are non-volatile. The water solubility of 150 mg/L was the mean value of 0.16 g/L at 23 °C and pH 5.75, based on phosphorus and 0.14 g/L at 23 °C and pH 5.75, based on sulphur).

The partition coefficient of target substances was determined obtained to be between 3.4 and 5.6 at 20 °C, in two by means of HPLC determinations. In the study of determination of partition coefficient, two peaks show in the HPLC chromatogram during the measurement of logPow, which implies the complex structures of ZDDP: The monomeric or dimeric neutral forms together with basic form are present in the substance. The basic form might less water soluble than the neutral forms, therefore might have a higher logPow than the neutral forms. Based on this assumption it can be concluded that the logPow value of 5.6 determined by HPLC belongs to the basic form of ZDDP, whereas the logPow value of 3.4 is obtained because of neutral forms.

The log Pow values of target and source substance are considered to be comparable at the same level and deviations reflect the method variance. The neutral forms and basic form of the molecule could be probably not distinguished by GC-MS, whereas can be separated by HPLC. Due to this reason, it is expected that if the log Pow of target substance measured by means of derivatisation followed by GC-MS, only the value of 3.4 will be obtained.

 

Endpoint	Target: CAS 4563-56-8	Source: CAS 4259-15-8
Biodegradability	RA CAS 4259-15-8	Experimental result: not readily biodegradable
Bioaccumulation	-	--
Adsorption/desorption	Waiver	--