Amino acid Gemini surfactants: a review on their synthesis, properties and applications

The modern surfactant industry demands more efficient and environmentally friendly products as environmental regulations become more stringent. Amino acid Gemini surfactants (AAGSs), as the emerging green surfactants of amino acid surfactants, together with the Geminis, which consist of two or more amphiphilic moieties chemically linked by a spacer group, are the most reliable of these efficient green chemicals. With the diversity of structure and topological configuration of the spacer group, AAGSs have stronger aggregation ability and multifarious molecular configuration and aggregation morphology. They are used in various fields such as pharmaceuticals, biomedicine, daily chemicals, oil recovery and environmental treatment. This review contains detailed discussions on the synthesis of AAGSs from natural and non-natural sources using reactions such as Hofmann alkylation and Schotten-Baumann condensation. The physicochemical properties such as low CMC, good emulsification, different micelle forms, high biodegradability, antibacterial ability and biocompatibility are presented. Future research directions of AAGSs are also suggested, including their optimized synthesis, clarification of structure-performance relationships, and exploration of new applications in a wider range of fields.