In this work, four nonionic surfactants based on sorbitan monolaurate (Span 20) were synthesized by introducing ethylene oxide gas (n = 20, 40, 60, 80 ethylene oxide units) into Span 20 to give four new surfactants with different hydrophilic-lipophilic balance (HLB), namely, E(20), E(40), E(60), and E(80). The structures of the prepared nonionic surfactants were elucidated using Fourier-transform infrared (FT-IR) and (1)H-nuclear magnetic resonance (NMR) spectroscopy. The surface-tension measurements were recorded. The effects of the prepared nonionic surfactants on the simultaneous saccharification and fermentation (SSF) of microwave/alkali-pretreated rice straw to produce ethanol were investigated. From the obtained data, it was found that the addition of the nonionic surfactants at 2.5 g/L had a positive effect on SSF. The maximum ethanol yield (76 and 55%) was obtained after 72 hr for rice straw using Kluyveromyces marxianus and Saccharomyces cerevisiae, respectively. Also, it was found that the ethanol yield increases with increasing HLB of the prepared nonionic surfactants by increasing ethylene oxide units. The adsorption of nonionic surfactants on lignocelluloses is proposed to be due to hydrophobic and hydrogen bonding interactions between nonionic surfactants and the lignin part in the lignocelulose. It can be concluded that additions of surface-active compounds, such as nonionic surfactants, increase enzymatic conversion of rice straw for bioethanol purposes.
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