Citrate esters in sunscreen formulations

In sunscreen formulations, several components act synergistically to provide effective protection to the exposed skin. Alongside the UV filters themselves, emollients are particularly relevant to both effectiveness and consumer acceptance of the product. This is due to their influence on the solubility of solid organic UV filters, dispersibility of inorganic UV filters and skin feel of sunscreen products. 

With the specific challenges posed by sunscreen formulation in mind, we will take a closer look at the use of citrate esters as emollients in this context. Citrate esters are clear, odourless, oily liquids and have a long history of safe use as emollients in various personal care products. Jungbunzlauer’s CITROFOL® citrate esters are obtained by acidic esterification of citric acid, which is produced by fermentation of carbohydrate raw materials like corn. Among Jungbunzlauer’s portfolio of citrate esters, Triethyl Citrate (CITROFOL® AI) and Tributyl Citrate (CITROFOL® BI) are the most relevant derivatives for use in sunscreen formulation. Both come with a guaranteed non-GMO status. As shown in Table 1, both are regarded as medium spreading emollients, but differ with regard to other physico-chemical properties.

The fundamental parameter for our investigations was the solvent power of Triethyl Citrate and Tributyl Citrate in relation to selected standard organic UV filters. We also assessed the influence of both citrate esters on sun protection factor (SPF) and UVA protection factor (UVAPF) in vitro. Their sensory profile was evaluated and compared to a benchmark emollient in a sunscreen matrix. The dispersibility of TiO2 and ZnO in Triethyl Citrate was investigated with a specific focus on certified natural formulations. Based on the results, we outline the benefits of using citrate esters as emollients with regard to cost-in-use and formulation flexibility.