Registration Dossier

Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Additional information

1,1,1,3,3,3 -Hexamethyldisilazane (HMDZ) is a liquid at standard temperature and pressure, with a measured melting point of -76.2°C, and a measured boiling point of 125°C. It has a measured density of 0.77 g/cm³ 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 and 2400 Pa at 25°C.

The substance is classified for flammability as a highly flammable liquid Category 2 (H225: Highly Flammable liquid and vapour) in accordance with EC Regulation 1272/2008 on the basis of a measured flash point of 11.4°C and the boiling point of 125°C. 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, HMDZ 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 substance are waived on the basis of instability in water. The properties of the silanol hydrolysis products, trimethylsilanol and ammonia are assessed instead. The water solubility of the silanol hydrolysis product, trimethylsilanol is limited to around 1000 mg/l by condensation reactions. It has a measured log Kow of 1.19 at 20°C and is not surface active. The first dissociation constant of trimethysilanol has been reported to be around pKa= 11. Trimethylsilanol has a measured vapour pressure of 1300 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 as it is inorganic. 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 inequilibrium between NH3and NH4+, depending on the pH. Where the term Ammonia is used throughout this CSR, 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:

fraction unionised = 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 fresh water or salt water 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).