Charged Dendrimers with Finite-Size Counterions

Abstract

We report on the structure of dendritic polyelectrolytes accompanied by counterions in a good, salt-free, implicit solvent using Langevin dynamics simulations and a Flory-type approach. Our focus is on the modification of charged dendrimer conformations via the strength of electrostatic interactions and the counterion excluded volume. We study the effects caused by charges by varying the reduced Bjerrum length, λB *, between the extremes of weak and strong electrostatic interactions. The counterion excluded volume was controlled by the size of ions. We investigate counterions ranging from conventional ones, with the size comparable to the monomer size, to bulky ions. Our results indicate that, as compared to neutral dendrimers, dendritic polyelectrolytes exist in swollen conformations, and the degree of swelling changes non-monotonically with increasing λB *. For weak electrostatic couplings, counterion density within dendrimers is minor and their radius of gyration subtly exceeds the size of neutral dendrimers. For intermediate electrostatic couplings, Coulomb attraction between opposite charges promotes absorption of ions into dendrimers’ pervaded volume and counterion condensation on charged monomers. As a result, counterion density inside dendrimers abruptly increases and the ionic size starts to play a crucial role. In this regime, we observe that swelling of dendrimers reaches its maximum and is more pronounced for bulky counterions. For strong electrostatic couplings, complete condensation of conventional counterions proceeds, whereas for bulky ions condensation remains partial. In this regime, dendrimers deswell. In particular, in the presence of conventional ions, dendrimers collapse into globules, while, for bulky counterions, deswelling is suppressed.

Complexation between Dendritic Polyelectrolytes and Amphiphilic Surfactants: The Impact of Surfactant Concentration and Hydrophobicity

“…The treelike structure of the dendrimer consists of the core of two bonded monomers and trifunctional branching groups. The overall number of the monomers, N d , and that of the terminal groups, N t , are given by , N d = 2 + 4 S ( 2 G − 1 ) and N t = 2 G + 1 We assign monovalent positive charges to the terminal groups of the dendrimer so that the arrangement of charges corresponds to that of poly(amidoamine) (PAMAM) cationic dendrimers at neutral pH . The anionic surfactant molecules are modeled as chains including a negatively charged monovalent head bead and three hydrophobic tail monomers.…”

Size Effect of Multivalent Counterions on Polyelectrolyte Brushes in Different Polar Solvents

“…Specific ion effects are ubiquitous in chemistry and biology. Previous studies have demonstrated that ion-specific effects, reflected by factors such as sizes and valences of ions, have a great influence on the phase behaviors of different types of PE systems. − …”

“…Recently, several studies have made progresses in understanding the conformational behaviors of a single PE chain as a function of ionic size. − Gavrilov et al studied the conformation of a PE chain with small and bulky counterions by the dissipative dynamic simulation method . According to their findings, under conditions of strong electrostatic interactions, bulky asymmetrical counterions prevent ion pairs from forming multiplets, whereas small counterions result in the formation of quadrupoles and small ionic chains .…”

“…For large counterions, instead of forming a globule, the chain was elongated. Kłos and Paturej reported the effects of the counterionic size on the conformations of charged dendrimers . They found that charged dendrimers swell to a larger extent with bulky counterions.…”

“…Kłos and Paturej reported the effects of the counterionic size on the conformations of charged dendrimers. 14 They found that charged dendrimers swell to a larger extent with bulky counterions. End-tethered PEs coated onto planar surfaces or colloidal particles in solution, namely, PE brushes, constitute an interesting system of PEs.…”