Gemini surfactants and their antibacterial properties

This paper focuses on the antibacterial mechanism of Gemini surfactants, and it is expected that Gemini surfactants can effectively kill bacteria and provide some help to slow down the spread of novel coronavirus. The word Surfactant comes from the English Surfactant, which is a condensation of the phrases Surface, Active and Agent [1]. Surfactants are substances that are active on surfaces and interfaces, have a very high ability and efficiency to reduce surface (boundary) tension, and form molecular ordered assemblages in solutions above a certain concentration, thus having a series of application functions. Surfactants have good dispersibility, wettability, emulsification ability and antistatic properties, and have become the key materials for the development of many fields, including fine chemical industry, and have a significant promoting effect on improving process, reducing energy consumption and improving production efficiency [2]. With the development of society and the continuous progress of the world's industrial level, the application of surfactants has gradually spread from daily chemicals to various fields of the national economy, such as antibacterial agents, food additives, new energy fields, pollutant treatment and biopharmaceuticals.

Traditional surfactants are "amphiphilic" compounds composed of polar hydrophilic groups and non-polar hydrophobic groups, whose molecular structure is shown in Figure 1(a) [7]. At present, with the refinement and systematic development of the manufacturing industry, the requirements for the performance of surfactants in the production process are gradually increasing, so it is of great significance to find and develop surfactants with higher surface properties and special structures. The discovery of Gemini surfactants fills the gap mentioned above and meets the requirements of industrial production. A common Gemini surfactant is a compound with two hydrophilic groups (usually ions or non-ions with hydrophilicity) and two hydrophobic alkyl chains [8]. As shown in Figure 1(b), in contrast to conventional single-chain surfactants, Gemini surfactants bind two hydrophilic groups together by a binding group (spacer). In short, the structure of Gemini surfactants can be understood as the formation of the hydrophilic head groups of two conventional surfactants by cleverly bonding together with binding groups. The special structure of the Gemini surfactant leads to its high surface activity. The main reasons are: (1) the hydrophobic effect of the Gemini surfactant is enhanced by the two hydrophobic tail chains, and the tendency of the surfactant to leave the aqueous solution is increased; (2) The tendency of separation between hydrophilic head groups, especially ionic head groups due to electrostatic repulsion, is greatly weakened by the influence of binding groups; (3) The special structure of the Gemini surfactant affects its aggregation behavior in aqueous solution, making it have a more complex and varied aggregation form. Compared with traditional surfactants, Gemini surfactants have higher surface (interface) activity, lower critical micelle concentration, better wettability, emulsification ability and antibacterial ability. Therefore, the development and utilization of Gemini surfactants is of great significance for the development and application of surfactants.

The "amphiphilic structure" of traditional surfactants gives them unique surface properties. As shown in Figure 1(c), when conventional surfactants are added to water, the hydrophilic head groups tend to dissolve inside the aqueous solution, while the hydrophobic groups inhibit the dissolution of the surfactant molecules in water. Under the combined action of these two trends, the surfactant molecules enrich at the gas-liquid interface and arrange in order to reduce the surface tension of water. Unlike conventional surfactants, Gemini surfactants are "dimers" that bind traditional surfactants together by spacer groups, which can more effectively reduce water surface tension and oil/water interfacial tension [9]. In addition, Gemini surfactants also have lower critical micelle concentration, better water solubility, emulsification, foaming, wetting and antibacterial properties.