Stable Vesicles Formed by a Single Polyelectrolyte in Salt Solutions

Abstract

The conformation of polyelectrolyte (PE) affects a wealth of structural and dynamic properties of PE solutions and is also fundamental to the study of the folding and aggregation of proteins. Here, we theoretically investigate the classical problem of the conformation of a single homogeneous PE in salt solutions. We predict the formation of a stable vesicle structure at intermediate salt concentrations which is beyond the well-accepted framework of globule to pearl-necklace transition. The vesicle formed by a single PE is electrostatic in origin where salt ions play the role of surfactant. We demonstrate that the transitions from vesicle to both spherical globule and pearl-necklace are discontinuous; the latter involves a two-step nucleation with an intermediate vesicle-necklace state. We also calculate the full diagram of state and show the existence of a critical quadruple point that is uniquely determined for PE with a given chain length. The vesicle structure can be well-controlled by the external salt concentration, which facilitates the design of ionic-responsive single-chain nanoparticles.