An Effective Volume Fraction Controls Both Dynamics and Thermodynamics in Vesicle Suspensions with Tunable Electrostatic Interactions

Surfactant vesicle suspensions are colloidal systems of great interest in industrial and biomedical applications. In the presence of charged vesicles and electrolytes, electrostatic interactions are crucial to determine their stability and dynamics. To elucidate how volume fraction and electrolyte concentration affect the microscopic structure and dynamics of these systems, here we employ Brownian dynamics simulations of charged spherical vesiscles in sodium-bromide solutions, using interaction parameters measured in previous experiments. Our results identify a colloidal state diagram, where both the dilute-to-dense and the fluid-to-arrested state crossovers shift toward lower volume fractions as electrolyte concentration decreases. We find how interactions augment the first-neighbor distance and shape vesicles' radial distribution. Based on the microscopic structure, we define an effective volume fraction that collapses onto salt-independent master curves for both dynamic and thermodynamic indicators, effectively making the state diagram one-dimensional. These findings improve the understanding of charged vesicles, opening new ways for predicting and designing the properties of novel formulations.