Abstract
Methodology was developed to expand the range of benign alkyl glycoside surfactants to include also anionic types. This was demonstrated possible through conversion of the glycoside to its carboxyl derivative. Specifically, octyl β-D-glucopyranoside (OG) was oxidised to the corresponding uronic acid (octyl β-D-glucopyranoside uronic acid, OG-COOH) using the catalyst system T. versicolor laccase/2,2,6,6-tetramethylpiperidinyloxy (TEMPO) and oxygen from air as oxidant. The effects of oxygen supply methodology, concentrations of laccase, TEMPO and OG as well as reaction temperature were evaluated. At 10 mM substrate concentration, the substrate was almost quantitatively converted into product, and even at a substrate concentration of 60 mM, 85% conversion was reached within 24 h. The surfactant properties of OG-COOH were markedly dependent on pH. Foaming was only observed at low pH, while no foam was formed at pH values above 5.0. Thus, OG-COOH can be an attractive low-foaming surfactant, for example for cleaning applications and emulsification, in a wide pH range (pH 1.5–10.0).
Highlights
Surfactants are used in a wide range of applications, but historically, they have caused severe environmental problems due to poor biodegradability, toxicity, and other undesirable properties
Mass spectrometry and NMR spectroscopy confirmed that the product was octyl β-D-glucopyranoside uronic acid
Compared with chemical methods, using protection and deprotection of other hydroxyl groups (Ferlin et al 2008) or chemoselective catalysts (Boelrijk et al 1996), the laccase/TEMPO mediated oxidation was shown to be a promising green alternative. This is a chemoenzymatic reaction in which TEMPO is oxidised by laccase to oxoammonium TEMPO, which in turn oxidises the primary alcohol of octyl β-D-glucopyranoside (OG) to a carbonyl group, and subsequently to a carboxyl group
Summary
Surfactants are used in a wide range of applications, but historically, they have caused severe environmental problems due to poor biodegradability, toxicity, and other undesirable properties. Alkyl glycosides constitute a group of benign surfactants with attractive functional properties (Hill 2010; von Rybinsky and Hill 1998). They can be produced from renewable raw materials and they are nontoxic and biodegradable. The introduction of ionisable groups can provide additional advantages, such as the possibility to modulate the surfactant properties by variation of pH. This has been studied in the case of acidic sophorolipids, surfactants which naturally contain carboxyl groups. The method has been used extensively for polysaccharide oxidation, giving products used for emulsion stabilization, thickening, controlled delivery systems, hydrogels, microspheres, etc. (Pierre et al 2017)
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