Hydroxyl-terminated carbosilane dendrimer films at water-toluene and water-air interfaces

We report a fully atomistic molecular dynamics study of fourth-generation amphiphilic carbosilane dendrimers adsorbed at water-toluene and water-air interfaces across a wide range of surface densities. The dendrimers, featuring a tetrafunctional silicon core, trifunctional =Si(CH3)- branching units, and terminal hydroxyl groups, adopt a characteristic "umbrella" conformation at low coverage, with hydrophilic termini anchored in water and the hydrophobic core aligned with the nonpolar phase. Interfacial chemistry fundamentally modulates this architecture: favorable toluene-core interactions promote core swelling and homogeneous lateral dispersion, whereas the sharp hydrophobic mismatch at the water-air interface drives dendrimer aggregation. Lateral compression induces a systematic transition from flattened, anisotropic conformations to more isotropic, near-spherical geometries, accompanied by redistribution of hydrogen bonds from dendrimer-water to intra- and interdendrimer contacts. Under strong confinement, steric crowding and transient interdendrimer hydrogen bonding lead to dynamic arrest, with lateral diffusion suppressed by orders of magnitude. These findings establish a unified molecular picture of how surface density and solvent compatibility jointly control conformation, interactions, and mobility in amphiphilic dendrimer monolayers, providing design principles for tunable interfacial materials with tailored mechanical and transport properties.