Morphology and wettability tunable two-dimensional superstructure assembled by hydrogen bonds and hydrophobic interactions

A novel tripodal gelator, functionalized by three urea and three azobenzene moieties grafted with three long alkyl chains, was designed and synthesized. The morphologies and surface properties of the xerogels prepared from this gelator strongly depend on the polarity of the gelling solvent. Cabbage-like topography and superhydrophobicity were observed in the xerogel formed from a low polar aromatic solvent such as xylene. The wettability of a xerogel could be turned from hydrophobicity to hydrophilicity by applying a sol-gel process with different solvents. Spectral and structural analysis of the xerogels revealed a basic bilayer arrangement of molecules with polarity changing on going from the inner hydrophilic regions toward the outer region (edge) of the layer. The cooperation and relative competition of hydrogen bonds, hydrophobic interactions, and azobenzene-azobenzene interactions are suggested to be the main contribution for the bilayer structure self-assembly. This two-dimensional self-assembly and the growth of nanostructures are remarkable in view of the usual fibrous aggregates given by organogels.