Effective Electrostatic Interactions in Solutions of Polyelectrolyte Stars with Rigid Rodlike Arms

摘要

In solutions of star-branched polyelectrolytes, electrostatic interactionsbetween charged arms on neighboring stars can compete with intra-starinteractions and rotational entropy to induce anisotropy in the orientationaldistribution of arms. For model stars comprising rigid rodlike arms with evenlyspaced charged monomers interacting via an effective screened-Coulomb (Yukawa)potential, we explore the influence of arm orientational anisotropy oneffective star-star interactions. Monte Carlo simulation and density-functionaltheory are used to compute arm orientational distributions and effective pairpotentials between weakly charged stars. The degree of anisotropy is found toincrease with the strength of electrostatic interactions and proximity of thestars. As two stars begin to overlap, the forward arms are pushed back byinter-star arm-arm repulsion, but partially interdigitate due to rotationalentropy. At center-center separations approaching complete overlap, the armsrelax to an isotropic distribution. For nonoverlapping stars,anisotropy-induced changes in intra- and inter-star arm-arm interactionslargely cancel and the effective pair interactions are then well approximatedby a simple Yukawa potential, as predicted by linear response theory for acontinuum model of isotropic stars [A. R. Denton, Phys. Rev. E 67, 11804(2003)]. For overlapping stars, the effective pair interactions in the simplerigid-arm-Yukawa model agree closely with simulations of a molecular model thatincludes flexible arms and explicit counterions [A. Jusufi, C. N. Likos, and H.Lowen, Phys. Rev. Lett. 88, 018301 (2002); J. Chem. Phys. 116, 11011 (2002)].