Simulations of Dendrimers with Flexible Spacer Chains and Explicit Counterions under Low and Neutral pH Conditions

Abstract

We study the properties of weak dendritic polyelectrolytes of generation G = 5 with flexible spacers of various lengths and explicit counterions in an athermal solvent using Monte Carlo simulations based on the bond fluctuation model. The calculations are performed for molecules under neutral and low pH conditions. At neutral pH we assume that only the terminal groups of the dendrimers bear positive charges, while at low pH both the terminal groups and the branching units are charged. In our study, the full Coulomb potential and the excluded volume interactions are taken into account explicitly with the reduced temperature τ as the main simulation parameter. We observe an interplay of condensation of counterions, trapping of counterions inside the dendrimer’s volume and evaporation of counterions into the surrounding solution giving rise to a nonmonotonous electrostatic swelling of the dendrimer with temperature. Decreasing pH leads to higher swelling and stronger spacer-length dependence. At low pH spacer length-scaling cannot be applied and longer spacers shift the maximum of the swelling to lower temperature. To explain the swelling effect we apply a Flory-type argument where both trapped but noncondensed counterions and uncompensated charges due to evaporation of counterions are taken into account. This model properly reflects the swelling behavior with respect to temperature, pH and spacer-length variation, but quantitatively underestimates the swelling effect. We further investigate the pH-effects on density and charge profiles of the dendrimer.