Octamethyltrisiloxane

Partition coefficient

Description of key information

Log Kow: 6.6 at 25.3°C (OECD 123)

Log Koa = 3.79± 0.01 at 20.8±0.4°C

Log Kaw = 3.04± 0.20 at 20.8±0.4°C

Key value for chemical safety assessment

Log Kow (Log Pow):

6.6

at the temperature of:

25.3 °C

Additional information

Three reliable measured n-octanol/water partition coefficient data are available for octamethyltrisiloxane. An n-octanol/water partition coefficient (log K_ow) of 6.6 was determined using OECD 123 and in compliance with GLP. The result is considered to be reliable and selected as key study. Other reliable data reported log K_ow values of 6.87 at 20.8°C and 6.6 at 24.1°C. In other secondary sources to which no reliability could be assigned, log K_ow values of 4.8 at 25°C and 4.8 - 5.4 were reported for the substance. The study with the highest reliability was assigned as key study.

A disregarded log K_ow of 4.6 at 22.3°C (Dow Corning Corporation, 2006) was also determined for the substance because the method used is reportedly associated with a possible under estimation of values of log K_ow bearing in mind susceptibility to hydrolysis. The method used was the slow-stirring method with the analytical determination based on ¹⁴C-radiolabelled test substance. Liquid scintillation counting was used for the analysis of total radioactivity in each phase. The accuracy of the reported value rested on the assumption that the ¹⁴C radioactivity in the aqueous phase represented only ¹⁴C-test substance, which was not confirmed during the study. The much greater value of log K_ow for the substance determined in the key study (Dow Corning Corporation, 2008) suggests that this assumption was invalid. This is most likely due to the presence of trace level impurities or degradation (radiolysis) products in the radiolabelled test material having lower K_ow values than the parent compound.