Registration Dossier

Administrative data

Endpoint:
hydrolysis
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Accepted, well documented publication
Cross-referenceopen allclose all
Reason / purpose for cross-reference:
reference to same study
Reason / purpose for cross-reference:
reference to other study

Data source

Reference
Reference Type:
publication
Title:
Current considerations for the dissolution kinetics of solid oxides with silica
Author:
Löbbus M, Vogelsberger W, Sonnefeld J, Seidel A
Year:
1998
Bibliographic source:
Langmuir 14(16), 4386-4396 (American Chem. Society)

Materials and methods

Test guideline
Qualifier:
equivalent or similar to guideline
Guideline:
other: OECD 105 (water solubility)
Principles of method if other than guideline:
Development of a dissolution model for silicon oxides, taking into account Si-O bond cleavage and formation as well as protolysis of silanol groups. Three different models, pH dependent surface potential, diffuse double layer, and gel layer, are investigated.
Experiments carried out with a well-defined amorphous silica (Monospher 250 (Merck Darmstadt) included dependence on pH and NaCl concentration at 40 °C.
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
Synthetic amorphous silicas:
CAS-No. 112945-52-5, CAS-Name: Silica, amorphous, fumed, crystalline-free;
CAS-No. 112926-00-8, CAS-Name: Silica gel, precipitated, crystalline-free.

Test material: Monospher 250 (Merck Darmstadt)
Radiolabelling:
no

Study design

Analytical monitoring:
yes
Details on test conditions:
Dissolution experimentsThe dissolution kinetics of Monospher has been investigated with respect to its dependence on pH and NaCl concentrations of 0.1, 0.01, and 0.001 mol/L at 40 °C. Mass/volume ratio: 0.2 g silica/100 mL electrolyte solution, not bufferedConfirming experiments, starting from saturated and supersaturated solutions

Results and discussion

Preliminary study:
not relevant
Transformation products:
yes
Identity of transformation products
No.:
#1
Reference
Reference substance name:
Unnamed
IUPAC name:
orthosilicic acid / silicate ion
Identifier:
common name
Identity:
orthosilicic acid / silicate ion
Details on hydrolysis and appearance of transformation product(s):
see Report, Table 1 and Fig. 4 (see below: AD under attached background material)
Dissipation DT50 of parent compound
pH:
6
Temp.:
40 °C
Hydrolysis rate constant:
0 d-1
Type:
zero order
Remarks on result:
other: Other kinetic parameters: In Report: Overall rate constant of dissolution k+ = 10^-7.43*mol/(L*sec) at lim[H+]-->0 / based on a gel and diffuse layer model (p. 9, fig. 9) / Re-calculation on per-day basis using 86400 sec/day: k+ = 0.0032 mol/(L*d).

Any other information on results incl. tables

The surface of silica may be covered by a partial hydrolysed gel layer when in contact with water (p. 4389). This layer is in equilibrium with the outer aqueous phase and constitutes a diffusion barrier for ions and water.

It is known and generally recognised that proton and hydroxide ion promoted dissolution plays a more prevailing role in the dissolution process than a simple hydrolysis of siloxane bridges (p. 4390).

The experimental part showed that there was a distinct pH dependence in the rate of dissolution, increasing with the pH increasing. However, the free dissolved SiO2 reached a maximum independent of the pH (Report, Table 1, Fig.. 4). It is concluded that the total amount of silica dissolved is relatively constant in a broad range of pH (1.1 < pH < 8.9) (p. 4393).

The level of maximum solubility ranged betwee 2.3 and 2.7 mmol SiO2/L (here: Monosphere, see also 4.8 Water solubility).

The degree of hydrolysis that may be involved in the dissolution process could not yet be solved.

Applicant's summary and conclusion