Synthetic amorphous magnesium silicate, with molar ratio (SiO2:MgO) range of 1.4-4

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

It is considered appropriate to address the data requirements for synthetic amorphous magnesium silicate by read-across to the available studies on structurally similar substances.
A hydrolysis study is available on synthetic amorphous silica, which due to sharing similar structural, physical and chemical properties to synthetic amorphous magnesium silicate, is considered appropriate for read-across purposes.

Reason / purpose for cross-reference:

read-across source

Specific details on test material used for the study:

Synthetic amorphous silica is being used as a read across for synthetic amorphous magnesium silicate.

Transformation products:

yes

No.:

#1

Remarks on result:

other: see 'Remark'

Remarks:

Hydrolysis to a minor, not yet quantifiable degree is limited to a gel-layer surrounding the silica/silicate particle in aqueous medium. Thus, hydrolysis of silica was shown to be largely independent of the pH between 1.1 and 8.9, which contributes to the limited dissolution of silica/silicates in water. Therefore, hydrolysis of silica is assumed to play no substantial role in the dissolution process.

Conclusions:

Based on a read-across study on synthetic amorphous silica, it is concluded that hydrolysis is assumed to play no substantial role in the dissolution process of magnesium silicate.

Executive summary:

A hydrolysis study is available for synthetic amorphous silica from a well documented scientific journal publication by Löbbus et al (1998). Due to sharing similar structural, physical and chemical properties to synthetic amorphous magnesium silicate, synthetic amorphous silica is considered appropriate for read-across purposes.  

It was shown that hydrolysis to a minor, not yet quantifiable degree is limited to a gel-layer surrounding the silica/silicate particle in aqueous medium. Thus, hydrolysis of silica was shown to be largely independent of the pH between 1.1 and 8.9, which contributes to the limited dissolution of silica/silicates in water. Therefore, similar to silica, hydrolysis is assumed to play no substantial role in the dissolution process of magnesium silicate.

Reference 2

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

Accepted, well documented publication

Reason / purpose for cross-reference:

read-across source

Qualifier:

no guideline available

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

Radiolabelling:

no

Analytical monitoring:

yes

Transformation products:

yes

No.:

#1

Details on hydrolysis and appearance of transformation product(s):

see Report, Table 1 and Fig. 4.

Remarks on result:

other: see 'Remark'

Remarks:

Hydrolysis to a minor, not yet quantifiable degree is limited to a gel-layer surrounding the silica/silicate particle in aqueous medium. Thus, hydrolysis of silica was shown to be largely independent of the pH between 1.1 and 8.9, which contributes to the limited dissolution of silica/silicates in water. Therefore, hydrolysis of silica is assumed to play no substantial role in the dissolution process.

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 was about 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.

Description of key information

Hydrolysis to a minor, not yet quantifiable degree is limited to a gel-layer surrounding the silica/silicate particle in aqueous medium. Thus, hydrolysis of silica was shown to be largely independent of the pH between 1.1 and 8.9, which contributes to the limited dissolution of silica/silicates in water. Therefore, similar to silica, hydrolysis is assumed to play no substantial role in the dissolution process of magnesium silicate.