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Foams Stabilized by Tricationic Amphiphilic Surfactants

The unique surface properties of amphiphilic molecules have made them widely used in applications where foaming, emulsifying or coating processes are needed. Novel surfactant architectures with multi-cephalic and multi-tailed molecules have reportedly enhanced their anti-bacterial activity in connection with tail length and the nature of the head group, but their ability to produce and stabilize foam is mostly unknown. Here we report on experiments with tris-cationic, triple-headed, double- and single-tailed amphiphiles and their foamability and foam stability with respect to head group, tail number and tail length. The amphiphiles are composed of an aromatic mesitylene core and three benzylic amonium bromide groups, with alkyl chains attached to one or two of the head groups. Whereas shorter (14 carbons in length) double-tailed molecules are found to produce very stable foams, foams made with single tail molecules of the same length show poor foamability and stability, and foams with longer (16 carbons in length) double-tail molecules do not foam with the methods used. By contrast, the structure of the non-tail-bearing head group (trimethylammonium vs. pyridinium) has no impact on foamability. Furthermore, observations of the coarsening rate at nearly constant liquid content indicate that the enhanced foam stability is a result of lower gas permeability through the surfactant monolayer. Finally, the critical aggregation concentration (CAC) of the surfactants demonstrates to be a good predictor of foamability and foam stability for these small molecule surfactants. This results inform how surfactant architecture can be tailored to produce stable foams.