Abstract
Using coarse-grained dissipative particle dynamics (DPD) with explicit electrostatics, we performed (i) an extensive series of simulations of the electrostatic co-assembly of asymmetric oppositely charged copolymers composed of one (either positively or negatively charged) polyelectrolyte (PE) block A and one water-soluble block B and (ii) studied the solubilization of positively charged porphyrin derivatives (P+) in the interpolyelectrolyte complex (IPEC) cores of co-assembled nanoparticles. We studied the stoichiometric mixtures of 137 A(10)(+)B(25) and 137 A(10)(-)B(25) chains with moderately hydrophobic A blocks (DPD interaction parameter a(AS)=35) and hydrophilic B blocks (a(BS)=25) with 10 to 120 P+ added (a(PS)=39).
The P+ interactions with other components were set to match literature information on their limited solubility and aggregation behavior. The study shows that the moderately soluble P+ molecules easily solubilize in IPEC cores, where they partly replace PE+ and electrostatically crosslink PE- blocks.
As the large P+ rings are apt to aggregate, P+ molecules aggregate in IPEC cores. The aggregation, which starts at very low loadings, is promoted by increasing the number of P+ in the mixture.
The positively charged copolymers repelled from the central part of IPEC core partially concentrate at the core-shell interface and partially escape into bulk solvent depending on the amount of P+ in the mixture and on their association number, A(S). If A(S) is lower than the ensemble average (n), the copolymer chains released from IPEC preferentially concentrate at the core-shell interface, thus increasing A(S), which approaches (n).
If A(S) > (n), they escape into the bulk solvent.