Ligand-induced order in Spotted vesicles and Striped micelles

Mixtures of amphiphiles are ubiquitous and assemble into various morphologies, including giant vesicles and cylinder micelles that raise the possibility of mesoscopic segregation within the assemblies -- perhaps even in response to binding of small ligands. Here, with anionic and neutral polymer amphiphiles mixed within vesicle and cylinder morphologies, divalent cations are shown to induce meso-scale domains and thus generate `\textit{responsive} \textit{Janus }structures'. Whereas past reports with lipid systems appear conflicted, calcium forms definitive crossbridges between the anionic polymer amphiphiles, rigidifying the charged membranes across a fluid-gel transition and also leading to lateral phase separation without disrupting the assemblies. A systematic phase diagram for these robust assemblies shows that long-lived domains occur in an unexpectedly narrow region near the polyanion's p$K$'s for protonation and cation association. The phase behavior appears well described by a relatively simple model in which -- among electrostatic and entropic contributions -- counterion entropy outcompetes attractive crossbridging to drive remixing of the highly charged polyacid at high pH, contrary to intuition.