Lateral Segregation within Vesicles and Micelles Due to Calcium Cross-Bridging of Polyanionic Amphiphiles

Mixtures of neutral and charged amphiphiles are ubiquitous and assemble into various morphologies, including vesicles and cylinder micelles, but segregation within such assemblies of amphiphilic polymers has not been observed. Here, with mixtures of anionic and neutral polymer amphiphiles, calcium ions are shown to induce meso-scale domains within both morphologies. Calcium mediates attractions, or crossbridges, between the anionic amphiphiles that not only rigidify the charged membranes across a fluid-gel transition, but also lead to lateral phase separation without destroying the assemblies. A systematic phase diagram for these robust polymer assemblies shows that long-lived domains occur in an unexpectedly small region near the polyanion's pK's for protonation and cation association. The phase behavior appears well described by a model in which – among other electrostatic and entropic contributions – counterion entropy outcompetes crossbridging to drive remixing of the polyacid at high pH. Initial observations extend from polymers to the polyanionic phosphatidylinositol (4,5)-bisphosphate, highlighting the generality of crossbridge-induced domain formation.