Structural study on aggregation behavior of star-type trimeric surfactant in the presence of sodium salicylate

Abstract

We investigated the aggregation behavior of star-type trimeric surfactants (3C12trisQ) with a hydrocarbon chain length (n) of 12 in sodium salicylate (NaSal) aqueous solution at various surfactant (CD) and salt concentrations (CS) using small-angle X-ray scattering (SAXS) and rheological measurements. At a low surfactant concentration (CD = 7 mM), 3C12trisQ aggregates in solution evolved from ellipsoidal micelles, to rodlike or wormlike micelles and finally to multi-lamellar vesicles (MLVs) with increasing CS. In contrast, at higher surfactant concentrations (CD = 14 and 28 mM), we did not observe vesicle formations of 3C12trisQ because 3C12trisQ were insoluble in solution at higher NaSal concentrations. The structural transitions of the 3C12trisQ aggregates strongly depended on both CS and CD. We also compared the MLV structure formed by 3C12trisQ with that formed by gemini-type surfactants (12-2-12). The repeat distance of the 3C12trisQ MLV (33.95 Å) was remarkably larger than that of the 12-2-12 (22.36 Å). We found that introducing spacer chains, i.e., from dimeric to trimeric surfactants, caused a decrease in NaSal concentrations at which micelle-to-vesicle transitions were observed.