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The registered substance, 1,1,1,3,3,3-hexamethyldisilazane (CAS 999-97-3; EC No. 213-668-5), is not stable in water, which affects the approach to the determination of physicochemical properties. The significance of this for read-across is discussed in relevant sections of the dossier.

1,1,1,3,3,3-Hexamethyldisilazane is a liquid at standard temperature and pressure, with a measured melting point of -76.2°C at 1013 hPa, and a boiling point of 125°C at 1013 hPa. It has a relative density of 0.7742 at 20°C, a measured kinematic viscosity of 0.9 mm²/s at 20°C and a measured vapour pressure of 1900 Pa at 20°C, 2400 Pa at 25°C and 7400 Pa at 50°C.

The substance is classified for flammability in accordance with Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 11.4°C at 1013 hPa and the boiling point of 125°C at 1013 hPa. It has a measured auto-ignition temperature of 331°C and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, the submission substance reacts very rapidly (half-life <<1 minute at 25°C and pH 4, 7 and 9) to produce trimethylsilanol and ammonia according to the following equation:

(CH3)3SiNHSi(CH3)3 + 2H2O → 2(CH3)3SiOH + NH3

Therefore, requirements for testing of water-based physicochemical properties for the submission substance are waived on the basis of instability in water. The properties of the silanol hydrolysis products, trimethylsilanol and ammonia are assessed instead.

The silanol hydrolysis product, trimethylsilanol may undergo condensation reactions in solution to give the siloxane dimer (hexamethyldisiloxane; CAS 107-46-0) and a dynamic equilibrium is established. The overall rate of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in solution.

The condensation reactions of monosilanols may be modelled as an equilibrium between monomer and dimer. The reaction is reversible unless the dimer concentration exceeds its solubility; in this case, the dimer forms a separate phase, driving the equilibrium towards the dimer. For trimethylsilanol, a solution at 100 mg/L (the highest concentration often used in ecotoxicity tests) is predicted to contain >99.9% monomer. At loadings above about 500 - 1000 mg/L the concentration of the dimer is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the dimer is expected to have a high volatility from water and this may cause losses from water under some conditions. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The concentration of trimethylsilanol in water may be determined by this condensation reaction rather than by the solubility of trimethylsilanol itself. It has a measured solubility of 995 mg/L at 20°C and a measured log Kow of 1.19 at 25°C and is not surface active. Trimethylsilanol has a measured vapour pressure of 1290 Pa at 20°C and 1900 Pa at 25°C.

Ammonia is very soluble in water (510-530 g/L) and will ionise to form NH4+ under most environmental conditions. It has a high vapour pressure (861 kPa at 20°C); log Kow is not relevant because ammonia is inorganic, but an indicative value of 0.23 is predicted. It has a pKa of 9.25 at 25°C. Under ambient environmental conditions, ammonia is a stable substance that shows normal acid/base chemical activity with the following equilibria:

NH4+ + H2O ↔ NH3 + H3O+

NH3 + H2O ↔ NH4+ OH-

The ammonia/ammonium ion in aqueous solution exists in equilibrium betweenNH3and NH4+, depending on the pH. Where the term ammonia is used throughout this dossier, it refers to the equilibrium mixture of ammonia/ammonium unless otherwise stated. In general, as pH increases, the fraction of the total ammonia which is unionised increases. The fraction of unionised ammonia can be calculated using the following equation:

fractionunionised = 1/(10pKa-pH+1)

At pH 8.5, the proportion of unionised ammonia is approximately 10 times that at pH 7.5. The concentration of unionised ammonia will be lower at higher ionic strengths of very hard freshwater or saltwater environments. This effect can be significant in estuarine and marine waters. Moreover, the pKa is reciprocally related to temperature. For every 9°C increase in temperature, the proportion of unionised ammonia approximately doubles (EA 2007).


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

PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003

Additional information