The Unusual Surface Behavior of Dendrimer Siloxane Surfactants with Superior Wetting Performance

A homologous series of dendrimer tetrasiloxane (tris(trimethylsiloxy)silylpropyl) ammonium surfactants are synthesized and investigated for their aqueous surface tension, bulk phase behavior, and wetting behavior. Each surfactant homologue comprises two strongly surface-active components, a tetrasiloxane monoquat and a tetrasiloxane diquat, and the different homologues have different mono-to-diquat ratios. The two components appear to mix nonideally in the bulk solution as well as at the liquid-air interface, as evidenced by the formation at above a critical aggregation concentration of two coexisting association structures in the bulk liquid and two condensed surface states. While the lower surface tension state allows superspreading of water over a solid paraffin surface for all the homologue surfactant in the series, the maximum spreading area differs greatly for the different homologues. Surface tension measurements from this work also confirm a benchmark for the lower bound of aqueous surface tension enabled by the permethylated siloxane surfactant to be near 19.5 mN/m, which is still a significant gap from the lowest possible surface tension exhibited by neat permethylated siloxane oil (15.7 mN/m) and perfluorinated surfactant in aqueous solution.