Void-Bearing Hierarchical Coacervates in the Decylammonium Bromide/Sodium Salicylate Vesicle-to-Coacervate Pathway

A longstanding challenge in surfactant assembly is accessing vesicle and condensed states in low-molecular-weight systems without precipitation or severe kinetic trapping while obtaining structural insights into the resulting mesostructures. Here, we show that a two-component mixture of decylammonium bromide (DeAB) and sodium salicylate (NaSal) exhibits concentration-dependent vesicle and coacervate formation at low surfactant concentrations. At a fixed [DeAB] = 10 mM, well below the critical micelle concentration, equimolar NaSal (10 mM) lowers the aggregation threshold and produces precipitation-free vesicles, consistent with a multilamellar organization. By contrast, increasing [NaSal] to 50 mM induced liquid-liquid phase separation, forming an amphiphile-rich condensed phase. Freeze-fracture transmission electron microscopy (TEM) and two- and three-dimensional cryo-TEM further revealed that the condensed phase was structurally heterogeneous, containing persistent internal voids and coexisting nanoscale architectures. Together, these results identify the DeAB/NaSal system as a molecularly simple platform in which vesicle and coacervate states can be accessed through counterion concentration. This system provides an experimentally tractable basis for mechanistic studies of structured surfactant condensates, including protocell-relevant models.