OECD | Draft Test Guideline: Determination of the Hydrophobicity Index of Nanomaterials Through an Affinity Measurement

INTRODUCTION

1. This Test Guideline (TG) describes a method to determine the hydrophobicity index (Hy) of nanomaterials (NMs) as dispersed in an aqueous solution, through an affinity measurement. By measuring their binding rate to different engineered surfaces (collectors), Hy expressesthe tendency of the NM to favour the binding to a non-polar surface because of its low affinity for water.

2. Hydrophobicity is defined as "the association of non-polar groups or molecules in an aqueous environment which arises from the tendency of water to exclude non-polar molecules" (IUPAC Gold Book: https://goldbook.iupac.org/html/H/HT06964.html). It then represents the tendency of a substance to repel water. The degree of hydrophobicity of a macroscopic flat surface can be determined by measuring the contact angle (CA) between water droplets at the surface of a solid and the surface of the solid. In general, a hydrophobic surface is defined by a CA higher than 90°, a cut-off value used to distinguish hydrophobic from hydrophilic surfaces (Law, 2014). Nanomaterials are covered by the definition of a "substance" in REACH so the same definition applies. In technical applications, hydrophobic NMs are often described to “swim on water”. A few methods are described for characterising the hydrophobicity of NMs, for example dye adsorption assays, contact angle, and hydrophobic interaction chromatography. However, these methods do not enable a quantification of the hydrophobicity of NMs, pose issues of reproducibility and technical difficulties or they require a large amount of material. Extensive reviews of these methods are available in the literature (e.g. Zuo et al., 2010).