Comparative study of conformational behavior of hydroxyl-terminated carbosilane dendrimers at water–toluene and water–air interfaces

In this study, we investigate how molecular density-governed by dendrimer generation and branching functionality-influences the conformational behavior and hydrogen bonding of OH-terminated carbosilane dendrimers in water, air, toluene, and at water-air and water-toluene interfaces by atomistic molecular dynamics simulations. We focus on the 4-3 series (G2-G4), featuring a tetrafunctional core and trifunctional branching, and compare it with the denser, more rigid 4-4G3 dendrimer of the third generation (tetrafunctional at both core and branching points). In hydrophobic environments, terminal OH groups form linear intramolecular aggregates; the 4-4G3 exhibits markedly reduced toluene uptake (10% vs 40% volume change for 4-3G4) and severely restricted intramolecular dynamics, with some OH groups remaining kinetically trapped near the core-a phenomenon requiring microsecond-scale simulations for proper characterization. In aqueous solution, 4-3 dendrimers expose OH groups at their periphery to form hydrogen bonds with water, whereas 4-4G3 retains a significant fraction of OH groups internally, forming intramolecular H-bonds instead. At interfaces, 4-3 dendrimers adopt flattened "umbrella" conformations to maximize interfacial H-bonding with water, swelling slightly into toluene to form biconvex shapes, while 4-4G3 remains nearly spherical due to steric constraints, forming over four times more intramolecular H-bonds and fewer with water than the more flexible 4-3G4. These findings establish molecular density as a key determinant of solvation, dynamics, and interfacial adaptability, providing a foundation for understanding structure-composition-property relationships in dendrimer monolayers under lateral confinement.