Conformational properties and interaction of polyelectrolyte pH-sensitive stars

Changes in the conformations of two polyelectrolyte stars with amphiphilic segments during their convergence are investigated by the Scheutjens-Fleer method (a numerical method of solving self-consistent field equations). The stars are placed in an aqueous-salt solvent at a pH close to the pK of the segments of polyelectrolyte arms. Individual stars have a two-phase quasi-micellar conformation at a sufficiently strong hydrophobicity of segments in the uncharged state and at a sufficiently low salt concentration. This conformation is formed via separation of star arms into two groups: One consists of uncharged arms and forms a dense core in the star center, while the other consists of arms coming through and forming a corona with charged arms. When stars in this conformation converge, the transition of arms from the corona on the side of the oncoming star to the core occurs inside both stars. When stars touch each other with their cores, the stars merge into a united quasi-micelle. The free energy of interaction of stars is a nonmonotonic function of distance D between their centers. This value grows with a decrease in D until the cores come into contact. Upon the contact and merging of the cores, the free-energy values are characterized by a local minimum corresponding to an ellipsoid-shaped united quasi-micelle. With an increase in the concentration of salt in the solution, individual stars adopt extended conformations. In this case, the interaction of two stars is repulsive at all D values. However, if the salt concentration very slightly exceeds the threshold value of formation of a core in an isolated star, then the initially extended stars during their approach toward each other adopt (at first, each of them separately) a quasi-micellar conformation, while their cores grow as they converge and merge upon contact.