Chemical vs enzymatic synthesis of polyglycerol-based biosurfactants for cosmetic applications

Surplus accumulation of glycerol represents a major burden both in environmental and economic terms.[1] Among other large-volume transformations of glycerol, polymerization is extremely promising because it yields polyglycerols (PGs), which are widely used as intermediates in the production of polyglycerol fatty acid esters (PGFAEs). These products are gaining prominence as non-ionic surfactants in several fields, in particular in cosmetic industry, because they display a broad range of hydrophilic-lipophilic balance (HLB) values.[2] Nowadays, PGFAEs are industrially produced batchwise by direct esterification of PGs with an unsaturated fatty acid, under harsh reaction conditions. However, the high temperatures (160-230 °C) and the use of a non-selective homogeneous acid catalyst result in polycondensation reactions that tend to broaden the molecular weight distribution of products.[2] Alternatively, PGFAEs can be produced by enzymatic esterification in the presence of a lipase.[3] The reaction occurs under milder conditions (80 °C) and it is characterized by higher selectivity, leading to less a complex mixture of compounds. Moreover, if a supported enzyme is used, it can be easily separated from products and recycled. The use of enzyme-catalyzed reactions is frequently not affordable for many small and medium-sized enterprises, especially in the cosmetic field, which still relies on cheaper homogeneous catalysis for the production of PGFAEs. Nevertheless, during the last few years, sustainability is increasingly perceived as added value by the final consumer. The reaction conditions of the enzymatic esterification of diglycerol with stearic acid in the presence of the immobilized lipase Novozym® 435 were optimized, as well as the optimal number of enzyme recycles was investigated. The products obtained through the enzymatic reaction were characterized and compared with those yielded through the method based on homogeneous catalysis. Finally, interfacial features, emulsifying properties and relative stability over time of the polyglycerol-based surfactants obtained with the two approaches (chemical vs enzymatic) were deeply investigated. References 1. Z. Gholami, A.Zu. Abdullah, K.-T. Lee, Renewable and Sustainable Energy Reviews 2014, 39, 327. 2. V. Norn, Emulsifiers in Food Technology, Second Edition, John Wiley & Sons, Ltd. 2015, 8, 181. 3. B. Peng, C.-Y. Xiong, Y. Huang, J.-N. Hu, X.-M. Zhu, Z.-Y. Deng, J. Agric. Food Chem. 2018, 66, 8104.