Sugar fatty acid ester surfactants: Structure and ultimate aerobic biodegradability

Abstract

Ultimate aerobic biodegradabilities of an array of sugar ester surfactants were determined by International Standards Organisation method 7827, "Water Quality-Evaluation in an Aqueous Medium of the Aerobic Biodegradability of Organic Compounds, Method by Dissolved Organic Carbon" (1984). The surfactants were nonionic sugar esters with different-sized sugar head groups (formed from glucose, sucrose, or raffinose) and different lengths and numbers of alkyl chains [formed from lauric (C 12 ) or palmitic (C 16 ) acid]. Analogous anionic sugar ester surfactants, formed by attaching an α-sulfonyl group adjacent to the ester bond, and sugar esters with α-alkyl substituents were also studied. It was found that variations in sugar head group size or in alkyl chain length and number do not significantly affect biodegradability. In contrast, the biodegradation rate of sugar esters with α-sulfonyl or α-alkyl groups, although sufficient for them to be classified as readily biodegradable, was dramatically reduced compared to that of the unsubstituted sugar esters. An understanding of the relationship between structure and biodegradability provided by the results of this study will aid the targeted design of readily biodegradable sugar ester surfactants for use in consumer products.Surfactant biodegradability is a crucial factor in determining whether their concentrations in the environment remain below detrimental levels. Surfactants derived from sugar fatty acid esters are attractive because of their ready biodegradability, low toxicity, low irritation to eyes and skin, and the renewable nature of the sugar and fatty acid starting materials. They are widely used in food, cosmetic, and pharmaceutical formulations (1-3). Physicochemical properties of these surfactants can be tailored to suit potential applications by varying the sugar head group size and the length and number of alkyl chains. As well as nonionic surfactants, analogous anionic sugar ester surfactants can be produced by incorporation of a sulfonate group. These anionic sugar esters are more water soluble than their nonionic counterparts and may more easily replace conventional anionic surfactants in product formulations. Many effects of structural variations on the physicochemical properties of sugar ester surfactants have been reported (4-6).Sucrose fatty acid esters are rapidly biodegradable (7-13). However, the relationship between biodegradability and chemical structure of sugar ester surfactants has not been comprehensively studied. The aim of the current research was to investigate the ultimate aerobic biodegradation of surfactants derived from sugar fatty acid esters so as to develop an understanding of the relationships between surfactant structure and biodegradability. The biodegradabilities of an array of sugar ester surfactants in which the structure was systematically varied were determined. Structures of the surfactants studied are indicated in Scheme 1. Sugar head group size was varied from a monosaccharide (glucose) to a trisaccharide (raffinose). ...

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