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Environmental fate & pathways

Adsorption / desorption

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Link to relevant study record(s)

adsorption / desorption: screening
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the physicochemical properties of the substance indicate that it can be expected to have a low potential for adsorption
Reason / purpose for cross-reference:
data waiving: supporting information
Reason / purpose for cross-reference:
data waiving: supporting information

Description of key information


log Koc [ 1,1,1,3,3,3-hexamethyldisilazane]: not relevant due to very rapid hydrolysis in contact with water

log Koc [trimethylsilanol]: 1.6

log Koc [ammonia]: 1.1

Key value for chemical safety assessment

Additional information

The substance hydrolyses very rapidly to form trimethylsilanol and ammonia. Trimethylsilanol has a low log Kow value (<3) and thus has low potential for adsorption, and the requirement for testing is waived.

Based on its solubility, ammonia (unionised) is not expected to adsorb to soil particulate matter, suspended solids or sediment. (Environment Agency, 2007).

The ammonia ion is bound in soil by the attraction of the positive charge on the ammonium ion to the negatively charged soil micelles. In soil, ammonium is adsorbed primarily by four mechanisms: chemical (exchangeable), fixation (non-exchangeable), reaction with organic matter and physical attractive forces.

For environmental exposure assessment, adsorption is predicted for the hydrolysis products, trimethylsilanol and ammonia, using the ‘non-hydrophobics’ log Kow prediction method for log Koc developed Sabljić and Güsten (1995) for the European Commission, and recommended in EU Guidance.

The relevant equation is:

Log Koc = 0.52 log Kow + 1.02


Therefore, log Koc for the silanol hydrolysis product, trimethylsilanol is 1.6.

A log Koc of 1.1 is estimated for ammonia.


Environment Agency, 2007. Proposed EQS for Water Framework Directive Annex VIII substances: ammonia (un-ionised). Science Report: SC040038/SR2. SNIFFER Report: WFD52(ii)


Sabljić A and Güsten H (1995) QSARs for soil sorption. in: overview of structure-activity relationships for environmental endpoints. Hermens JLM (ed), report prepared within the framework of the project "QSAR for prediction of fate and effects of chemicals in the environment", an international project of the Environmental Technologies RTD programme (DG XII/D-1) of the European Commission under contract number EV5V-CT92-0211.