Simulations of Structural Assembly in Charged Dendrimers with Surfactants

Using Langevin dynamics simulations, we studied the formation of complexes among two cationic dendrimers, anionic surfactants, and counterions. The system includes free dendrimers, surfactant micelles, and mixed clusters, where micelles bind to dendrimers. These micelles are categorized into bridge micelles, which connect two dendrimers, and corona micelles, which are bound to individual dendrimers. We observe that more hydrophobic surfactants exhibit a stronger tendency to assemble with each other and dendrimers, reducing the number of free micelles, which predominantly exist as unimers while enlarging both bridge and corona micelles. Most counterions remain uncondensed and diffuse in solution. On this basis, we conclude that aggregate formation resembles an ion exchange process, where large micelles act as multivalent ions, replacing counterions in their assembly with dendrimers. Additionally, we introduce aggregate indexing, represented by pairs of integers, specifying the number of dendrimers and surfactants in aggregates. A histogram of aggregate counts for each index is used to analyze the aggregates in detail. We find that various mixed clusters occur with specific probabilities, including statistically favored clusters defined by their most probable composition. We also observe that, for weakly hydrophobic surfactants, one-dendrimer clusters dominate, while strongly hydrophobic surfactants and high-generation dendrimers form two-dendrimer clusters. Furthermore, the aggregates are characterized by the mean number and mass of the bridge and corona micelles they contain as well as their mean effective charge. The favored mixed clusters typically contain a small number of micelles per dendrimer, with the micelle mass increasing as the surfactant hydrophobicity increases. The effective charge of the aggregates primarily depends on the combined charge of dendrimers and surfactants with minor contributions from counterions. For high-generation dendrimers, the charge remains positive, while for lower-generation dendrimers, it shifts from positive to negative at a specific number of complexed surfactants. In particular, charge inversion is also observed in the favored mixed clusters.