Solubility of Synthetic Amorphous Silica (SAS)

WG on Nanomaterial in Cosmetic Products
SCCS members: U. Bernauer (Chairperson), L. Bodin, Q. Chaudhry, P.J. Coenraads, M. Dusinska, E. Gaffet (Rapporteur), E. Panteri, Ch. Rousselle, M. Stepnik, S. Wijnhoven
SCHEER members: W.H. de Jong
SCCS external experts: N. von Götz, A. Simonnard
Contact: SANTE-C2-SCCSec [dot] europa [dot] eu (SANTE-C2-SCCS[at]ec[dot]europa[dot]eu)
On request from: European Commission
SCCS Number: SCCS/1606/19
Adopted on: 20-21 June 2019

Conclusion of the opinion:

1. Does the SCCS consider that Synthetic Amorphous Silica (SAS) are soluble (100 mg/L or higher) or degradable/non-persistent in biological systems, in light of the nanomaterial definition of the Cosmetic Regulation?

Having considered the data provided in this dossier and that available in published literature, the SCCS concludes that:

i) the solubility values for hydrophilic SAS materials have been reported to range from 22 mg/L to 225 mg/L for the solubility tests performed in aqueous media following the OECD TG 105 protocol or the Enhanced OECD TG 105 protocol. The latter protocol has been noted to increase the solubility by a factor of 10 for some hydrophilic SAS materials.

ii) the solubility values of hydrophobic surface-treated SAS materials have been reported to range from 0.4 to 180 mg/L for solubility tests performed in aqueous media following the OECD TG 105 or following a modified Enhanced OECD TG 105 protocol (i.e. using 10% ethanol). The latter protocol has been noted to strongly increase the solubility of some hydrophobic SAS materials (by a factor up to 173).

The hydrophilic and hydrophobic SAS materials can therefore be regarded as “insoluble” (i.e. below 100 mg/L) to “very slightly soluble” (i.e. 100 mg/L to 1000 mg/L) by the SCCS based upon the terminology used in USP38 and USP 38 NF33 (Table 1 corrected by the SCCS).

In regard to the nanomaterial definition in the Cosmetic Regulation, none of the SAS materials (hydrophilic or hydrophobic) included in the dossier can be regarded as soluble. In fact, the Applicant had mistakenly interpreted the SAS materials as soluble on the basis of the solubility of some of the materials being 100 mg/L or higher. The threshold for regarding a material 'soluble' is 33.3 g/L under the USP38 and USP 38 NF33 categorisation (not 100 mg/L as claimed by the Applicant).

No data were provided to help establish whether the SAS materials could be regarded degradable/non-persistent in biological systems.

2. Can the SCCS indicate to which kind of Silica this solubility applies?

The solubility values reported in the dossier are applicable when SAS materials are subject to the following conditions:

- hydrophilic SAS: Silica and hydrated silica when solubilised in aqueous media,
- hydrophobic surface treated SAS: when solubilised in aqueous media containing up to 10% ethanol,
- at temperatures between 19.5 to 20.5°C,
- with a pH level of between 3 and 8,
- over a period between 3 days (hydrophilic SAS) up to 49 days (hydrophobic SAS).

3. Does the SCCS have any further scientific concerns with regard to solubility of Synthetic Amorphous Silica (SAS)?

- The solubility values considered by the SCCS in this Opinion may not be valid in situations where the SAS materials are formulated/used under conditions that are different from those used in the solubility tests - e.g. when used in a less/non aqueous formulation, or at a different temperature.

- In the context of the definition of nanomaterial under the Cosmetics Regulation, which relates to insoluble materials in conjunction with other size/particle related parameters, the question of solubility of a nano-structured material needs to be seen in perspective for use in cosmetics. For nano-structured materials, with the exception of the materials that are completely soluble, it is important to establish whether a proportion of these materials would still exist in undissolved form comprising nanoparticles, at the given use level in a cosmetic formulation.

- The SCC has noted that the protocols used for solubility tests have a strong influence on the solubility of SAS materials.

Keywords:

SCCS, scientific opinion, Synthetic Amorphous Silica (SAS), solubility, nanomaterials, Regulation 1223/2009

Opinion to be cited as:

SCCS (Scientific Committee on Consumer Safety), Opinion on solubility of Synthetic Amorphous Silica (SAS), 20-21 June 2019, SCCS/1606/2019. Corrigendum of 6 December 2019.