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). ...

In previous work, we found that the presence of a sulfonyl or alkyl group adjacent to the ester bond of sugar ester surfactants is associated with a dramatic reduction in the rate of biodegradation relative to that of unsubstituted esters. In this study, we investigated the pathways followed during the biodegradation of sucrose laurate, sucrose α-sulfonyl laurate, and sucrose α-ethyl laurate to determine the reasons for their different biodegradation rates. Through the use of high-performance liquid chromatography and proton nuclear magnetic resonance spectroscopy, the nature of the intermediates formed during the biodegradation of these three key sugar esters was determined. It was found that sucrose laurate biodegradation occurs via initial ester hydrolysis. In contrast, sucrose α-sulfonyl laurate degrades by initial alkyl chain oxidation. This indicates that the ester hydrolysis pathway is blocked by the sulfonyl group adjacent to the ester bond so that biodegradation is forced to proceed via the slower alkyl chain oxidation pathway. Sucrose α-ethyl laurate was degraded at least in part by alkyl chain oxidation, indicating that ester hydrolysis was also inhibited by the presence of an ethyl group. It is therefore concluded that previously observed relationships between structure and biodegradability arise because of the influence that different structural elements have on the pathways followed during biodegradation.Paper no. S1141 in JSD 3, 13-27 (January 2000).KEY WORDS: Biodegradation, ester, fatty acid, high-performance liquid chromatography, laurate, nuclear magnetic resonance, pathway, sucrose, sulfonyl, surfactant.As discussed in our previous paper (1), sucrose fatty acid esters are known to be rapidly biodegradable (2-8). However, relationships between biodegradability and the chemical structure of these surfactants have not been studied. Understanding these relationships would facilitate a targeted design of sugar ester surfactants for use in consumer products, while retaining a high degree of biodegradability.In our previous paper, we reported the ultimate biodegradabilities of an array of sugar ester surfactants whose structures were systematically varied. This revealed that a sulfonyl or alkyl group adjacent to the ester bond is associated with a reduction in the biodegradation rate. The biodegradation of sucrose laurate was complete within 12 h, while that of sucrose α-sulfonyl laurate occurred more slowly, reaching about 85% in 25 d. Biodegradation of sucrose α-ethyl laurate occurred at a rate that was between those of sucrose laurate and sucrose α-sulfonyl laurate, reaching completion in 4 d. In contrast, other structural modifications, including variations in the sugar head group size and the length and number of alkyl chains, did not have a significant effect on biodegradability. In order to establish reasons behind these relationships between structure and biodegradability, the chemical pathways followed during biodegradation of three key sugar esters were investigated. The three surfactants c...

Rate constants for the base-catalyzed hydrolysis of sucrose laurate, sucrose α-sulfonyl laurate, and sucrose α-ethyl laurate were measured at several temperatures in pH 11 buffer. Activation energies and Arrhenius factors for the hydrolysis reactions were determined. At 27°C, sucrose laurate hydrolyzed fastest and sucrose α-ethyl laurate slowest. Activation energies and Arrhenius factors showed that both steric and electronic factors affect the rates of ester hydrolysis. Other work has shown that bacterial hydrolysis of sugar fatty acid esters is inhibited in the presence of either α-sulfonyl or α-alkyl groups. A kinetic study of base-catalyzed ester hydrolysis has revealed reasons for the inhibition of bacterial hydrolysis and provided information regarding ester stability at elevated pH.Biodegradation of sugar fatty acid esters by initial hydrolysis was found to be inhibited by the presence of α-alkyl or α-sulfonyl substituents adjacent to the ester bond (1,2). Lipases catalyze bacterial hydrolysis by a nucleophilic process involving a negatively charged tetrahedral intermediate (3-6). The essence of this enzymatic process is similar to the base-catalyzed hydrolysis of esters by hydroxide. This occurs almost universally by a bimolecular nucleophilic mechanism that also involves the formation of a negatively charged tetrahedral intermediate. Both processes are therefore expected to be subject to some of the same steric and electronic influences.Although there is a negatively charged residue in the active site of lipases, it is unlikely that electrostatic repulsion with sulfonyl groups inhibits hydrolysis since this is not observed in the case of other negatively charged substrates (6). On the other hand, the presence of alkyl groups adjacent to the carbonyl carbon of esters is known to reduce the rate of hydrolysis considerably (7). By determining the importance of steric and electronic factors in controlling the kinetics of base-catalyzed hydrolysis of sucrose esters, deductions can be made regarding factors affecting the corresponding enzymatic process.In addition, many cleaning formulations in which these surfactants may be applied have an elevated pH. However, stability of sugar esters to hydrolysis under these conditions is not well understood. It is therefore useful to investigate the factors influencing base hydrolysis of sugar esters from the point of view of formulation stability.In the current study, high-performance liquid chromatography (HPLC) was used to measure base hydrolysis rates of three key sugar esters: sucrose laurate, sucrose α-ethyl laurate, and sucrose α-sulfonyl laurate. The general structure of these surfactants is shown in Scheme 1; R=H in the case of sucrose laurate, SO 3 − Na + in the case of sucrose α-sulfonyl laurate, and CH 2 CH 3 in the case of sucrose α-ethyl laurate. Second-order rate constants for hydrolysis at elevated pH were determined at several different temperatures. This allowed energies of activation and Arrhenius factors to be determined in each case. The relative inf...

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