Biodegradability and cytotoxicity of choline soaps on human cell lines: effects of chain length and the cation

Using choline as a counterion in fatty acid surfactants substantially increases their water solubility as compared to classical sodium and potassium soaps, and thereby enables the application of desirable longer-chain derivatives at ambient temperature. Since choline can be decomposed both physiologically and environmentally, corresponding fatty acid soaps are considered to be highly biocompatible. Recent toxicity and biodegradability studies of choline ionic liquids, including anions such as short- and middle-chain alkanoates, have verified the expected low toxic impact. However, according to the European Cosmetic Directive 76/768/EEC, all salts of choline are forbidden in cosmetic products, mainly just due to its classification as a quaternary ammonium ion. In order to facilitate their application in the future, we have investigated the biodegradability of choline soaps (ChCm) with alkyl chain lengths of m = 12-18 according to the OCDE 301F standard. Further, the cytotoxicity of ChCm surfactants with m = 8-16 was determined, both for odd-and even-numbered fatty acids. Studies were carried out using two different human cell lines, namely cervix carcinoma cells (HeLa) and keratinocytes (SK-Mel-28). For a better comparability to common soaps and to shed light on the influence of the cation, sodium and potassium homologues were also investigated. Results reveal an unexpected non-linear relationship between the hydrophobic chain length and the IC50 value. Most importantly, the presented data show that IC50 values of ChCm surfactants coincide with those of the widely applied sodium and potassium soaps. This demonstrates that choline carboxylate surfactants are harmless and thus strongly supports their applicability in customer end products.