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Partition coefficient

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Reference
Endpoint:
partition coefficient
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Principles of method if other than guideline:
This study used a custom-made glass apparatus which allowed the establishment of an octanol/air/water equilibrium for simultaneous determination of KAW, KOA and KOW. The apparatus consisted of two air-tight syringes with an air sampling port in one syringe and a water sampling port on the other. The left syringe contained a small volume of octanol-saturated water (~5 mL), 14C-D4 containing octanol phase (0.2~1.0 mL) on top of the water, and a gas phase with a volume around 70~80 cm3. The right syringe contained an octanol-saturated water phase (60~80 mL) and an air phase (40~40 cm3). The air phase in both syringes was connected through a valve during the equilibrium, but the valve was closed during sampling. Samples were collected from the water and octanol phases from 1 to 90 hours to establish attainment of equilibrium in the system. Water samples were extracted using a two-step procedure. Water and octanol samples were analysed using liquid scintillation counting for total 14C activity, as well as liquid chromatography with flow scintillation detection for speciation analysis of 14C-D4.
GLP compliance:
no
Type of method:
other: Syringe method
Partition coefficient type:
octanol-water
Type:
log Pow
Partition coefficient:
6.98
Temp.:
21.7 °C
pH:
7

 

 

 

Average Concentration (gL-1)

cVMS

Exp.

No Samples

Average Temp (oC)

Octanol

Air

Water

Log KAW

log KOA

log KOW

 D4

1

17

21.6

14.9

7.41 x 10-4

1.53 x 10-6

2.68 ± 0.12

4.30

6.98

2

20

21.7

8.0

4.17 x 10-4

8.34 x 10-6

2.70 ± 0.14

4.28

6.98

Average

 

21.7

 

 

 

2.69 ± 0.13

4.29

6.98

Conclusions:
A partition coefficient value of 6.98 was determined at 21.7°C in a reliable study conducted according to generally accepted scientific principles.

Description of key information

log Kow [octamethylcyclotetrasiloxane]: 6.98 at 21.7°C (syringe method)

log Koa [octamethylcyclotetrasiloxane]: 4.22 at 24°C

Key value for chemical safety assessment

Log Kow (Log Pow):
6.98
at the temperature of:
21.7 °C

Additional information

Two reliable studies are available for the n-octanol-water partition coefficient of D4; a syringe method gave a Log Kow of 6.98 at 21.7°C and a slow-stirring method gave a Log Kow of 6.49 at 25.1°C. The syringe method is selected as key study, since it allows for the establishment of octanol/air/water three-phase equilibrium and is considered to be the most appropriate method for this substance.

A log Kow value of 5.1 was also determined for the substance using the HPLC method, however, this method is considered unsuitable for silicon-type substances as such was disregarded. In a secondary source to which reliability could not be assigned, log Kow value of 5.1 was reported for the substance.

A log Koa value of 4.22 at 24°C was determined for the substance in a reliable study conducted according to scientifically sound method and in compliance with GLP.