2,2,4,4,6,6-hexamethylcyclotrisilazane

Endpoint summary

Description of key information

The substance, 2,2,4,4,6,6-hexamethylcyclotrisilazane 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.

2,2,4,4,6,6-Hexamethylcyclotrisilazane is a liquid at standard temperature and pressure, with a measured melting point of -18.3±0.5°C and a measured boiling point of 187.7±0.5°C. It has a predicted density of 0.93 g/cm³ at 20°C and a predicted vapour pressure of 35 Pa at 25°C.

The substance is classified as a flammable liquid in accordance with Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 59°C and a measured boiling point of 187.7±0.5°C. It has a measured auto-ignition temperature of 369±3°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, 2,2,4,4,6,6-hexamethylcyclotrisilazane reacts very rapidly (half-life < 1 minute at 25°C and pH 4, 7 and 9) to produce dimethylsilanediol (3 moles) and ammonia/ammonium (3 moles) according to the following equation:

C₆H₂₁N₃Si₃ + 6H₂O → 3(CH₃)₂Si(OH)₂ + 3NH₃

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 product, dimethylsilanediol and ammonia are assessed instead.

The silanol hydrolysis product, dimethylsilanediol, may undergo condensation reactions in solution to give siloxane dimers, and linear and cyclic oligomers and a dynamic equilibrium is established .The overall rate and extent of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution. It may also change over time.

The condensation reactions of silanediols may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. The reactions are reversible unless oligomers such as the cyclic tetramer concentration exceeds their solubility; in this case, the cyclic tetramer forms a separate phase, driving the equilibrium towards the tetramer. For dimethylsilanediol, a solution at 100 mg/l (often the top loading used in ecotox tests) is predicted to contain >99% monomer, with small amounts of dimer, trimer, and cyclic tetramer. At loadings above about 1000 mg/l the concentration of the cyclic tetramer of the silanol hydrolysis product is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the cyclic tetramer 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 saturation concentration in water of the silanol hydrolysis product, dimethylsilanediol, is therefore limited by condensation reactions to approximately 1000 mg/l. However, it is very hydrophilic (calculated solubility is 1E+06 mg/l at 20°C using a QSAR method) with measured low log K_ow of -0.38 at 20°C. It is not surface active and it is less volatile than the parent substance (predicted vapour pressure = 7 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 K_ow 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 in equilibrium between NH₃ and 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:

fraction unionised= 1/(10^pKa-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 pK_a is reciprocally related to temperature. For every 9°C increase in temperature, the proportion of unionised ammonia approximately doubles (EA 2007).

Reference:

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