Centrifugal and Asymmetrical Field-Flow Fractionation for the analysis of ZnO particles in cosmetic products

Abstract

Zinc oxide (ZnO) is a versatile material used in many productive sectors, of which the rubber application accounted for the largest market share, followed by ceramics, chemicals, pharmaceuticals, cosmetics, personal care and agricultural products. In the cosmetic sector, ZnO is used as a colorant or bulking agent, but mainly as UV-blocker, soothing and protective coating against skin irritation and abrasion, analogously to its use in the pharmaceutical products, mostly as ointments and creams, where it carries out antibacterial, antimicrobial, disinfecting and drying actions. The recent increase of nano-materials to enhance the quality of many products has determined the revision of many existing international regulations, either in the cosmetic and food sectors. The European Union (EU) Commission’s Working Group on Cosmetics, in 2016, for example, stated that ZnO-NPs cannot be used in spray products, or if it used in dermal application, ZnO-NPs must have a median diameter of the particle number size distribution D50 (50 % of the number below this diameter) > 30 nm and D1 (1 % below this size) > 20 nm (1). The size analysis of ZnO particles dispersed in cosmetic or pharmaceutical formulations is consequently now an important task that analytical methods should face (2), since they could be used to discriminate the presence of particles in the nano or/and micro form. This implies that the analytical methods should be able possibly to determine the particle size distribution or at least to contribute, for example through a separation, to its determination. There are a number of analytical methods that might be used to size the particles (3,4). In many cases, the determination of the average particle sizes and the size distribution of particles incorporated in cosmetic products, requires a preliminary extraction step. This work has consequently two main aims, to study whether and how the extraction procedures should differ depending on the cosmetic matrices and to compare the separation performances of two FFF techniques, Centrifugal FFF (CF3) and Asymmetrical Flow FFF (AF4), both used for the particle size characterization. The separation performances of CF3 and AF4 are evaluated by analysing (i) two different types of pristine ZnO powdered particles, bare and superficially modified with triethoxycaprylyl silane, available on the market to formulate cosmetic products, (ii) four cosmetic creams formulated in-house, and (iii) three cosmetic commercial products, sold in Italy and Germany for the baby skin care