GEMINI SURFACTANTS - A NEW CLASS OF SELF-ASSEMBLING MOLECULES

''Gemini surfactant'' is a name assigned to a family of synthetic amphiphiles possessing, in sequence, a long hydrocarbon chain, an ionic group, a spacer, a second ionic group, and another hydrocarbon tail. Intramolecular chain/chain association was inhibited through the use of rigid spacers, thereby averting self-assembly into conventional micellar structures. Aggregation of the geminis was investigated by (a) surface tension, (b) film-balance methods, (c) dynamic light scattering, (d) H-1 and Na-23 NMR, and (e) spectral changes in an adsorbed dye. Among the more striking properties of geminis, one should cite the following: (a) a higher critical micelle concentration (according to surface tension and NMR) for geminis with two long chains of 16-20 carbons than that for shorter-chain analogs; (b) the lift-off areas in monomolecular films that are several times those of phospholipids, indicating that the geminis lie absolutely horizontally at the air/water interface; and (c) the formation of only small micelles, despite the potential to grow, polymer-like, into extended strands. It is argued that geminis, especially the longer-chain members, engage in self-coiling or submicellar aggregation when first exposed to water. Self-assembly into micelles and adsorption at the air/water interface then take place over hours or days at 23-degrees-C but much more rapidly at 50-degrees-C. Spectral data provide strong evidence for submicellar structures.