Tetrakis(2-butoxyethyl) orthosilicate

Hydrolysis

Description of key information

Hydrolysis half-life: 1.7 h (99.6 min) at pH 4, 4 h at pH 7 and 1.7 h (103.0 min) at pH 9 and 25°C (EU C.7 )

Key value for chemical safety assessment

Half-life for hydrolysis:

4 h

at the temperature of:

25 °C

Additional information

Half-life values of 99.6 minutes, 240 minutes, and 103.0 minutes at pH 4, 7, and 9 respectively and 25°C

were determined in a reliable study conducted according to an appropriate test protocol with restrictions. The study was not conducted according to GLP. The amount of co-solvent used in the sample preparation exceeded the recommendation in the test guideline (<1%), and the poor water solubility of the test substance resulted in phase separation and incomplete dissolution. The calculated half-lives should therefore be regarded as approximate values.

As the hydrolysis reaction may be acid or base-catalysed, the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalysed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

k_obs= k₀+ k_H3O+[H3O⁺] + k_OH^-[OH^-] + k_a[acid] + k_b[base]

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

Therefore, at low pH:

k_obs≈k_H3O+[H3O⁺]

At pH 4 [H₃O⁺] = 10^-4mol dm^-3and at pH 2 [H₃O⁺] = 10^-2mol dm^-3; therefore, k_obsat pH 2 should be approximately 100 times greater than k_obsat pH 4.

The half-life of a substance at pH 2 is calculated based on:

t_1/2(pH 2) = t_1/2(pH 4) / 100

The calculated half-life of the substance at pH 2 is therefore approximately 1 minute at 25°C.

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT₅₀(XºC) = DT₅₀(T°C) * e^(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Thus, for tetrakis(2-butoxyethyl) orthosilicate, the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 88 minutes. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half -life is approximately 30 seconds.

The hydrolysis products in this case are monosilicic acid and 2 -butoxyethanol. At concentrations >100-150 mg/l as SiO₂ according to published sources, monosilicic acid condenses rapidly into a highly cross-linked network (gel) or colloidal particles of polysilicic acid. The condensation rate is dependent on temperature, concentration, and pH of the system. A dynamic equilibrium is established between monomer, oligomers and insoluble amorphous polysilicic acid. Further details are given in a supporting report (PFA 2015ao) attached in Section 13 of the IUCLID dataset.

Hydrolysis of the read-across substance Tetraethyl orthosilicate (CAS Number 78-10-4)

Data for the substance tetraethyl orthosilicate (CAS 78-10-4) are read-across to the submission substance tetrakis(2 -butoxyethyl) orthosilicate for appropriate endpoints (see Section 1.5 of the Chemical Safety Report).The silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

 

For tetraethyl orthosilicate, hydrolysis half-lives at 25°C of 0.1 h at pH 4.4 h at pH 7, and 0.22 h at pH 9 were determined in accordance with OECD 111 (DCC 2003).