Large PAMAM Dendron Induces Formation of Unusual P4₃32 Mesophase in Monoolein/Water Systems

Compact macromolecular dendrons have previously been shown to induce the formation of discontinuous inverse micellar assemblies with Fd3 m symmetry in monoolein/water systems. Here, we demonstrate that a large PAMAM dendron (G5: fifth generation) induces the formation of a very unusual mesophase with P4₃32 symmetry. This mesophase had previously been observed in monoolein/water systems only on addition of cytochrome c. The P4₃32 mesophase can be considered an intermediate phase between the bicontinuous Ia3 d and discontinuous micellar mesophases. We present a detailed investigation of the phase behavior of monoolein/water as a function of G5 concentration and temperature. Addition of 1% G5 in 85/15 monoolein/water system induces a transition from the L_α to Ia3 d phase. Further increase in G5 concentration to above 2% induces the formation of the P4₃32 phase. In contrast to this, incorporation of lower generation PAMAM dendrons (G2-G4) in monoolein/water yields a qualitatively different phase diagram with the formation of the reverse micellar Fd3 m phase. PAMAM dendrons of all generations, G2-G5, bear terminal amine groups that interact with the monoolein headgroup. The compact molecular architecture of the dendrons and these attractive interactions induce bending of the monoolein bilayer structure. For smaller dendrons, G2-G4, this results in the formation of the Fd3 m phase. However, the large size of the G5 dendron precludes this and a rare intermediate phase between the Ia3 d and discontinuous micellar phase, and the P4₃32 mesophase forms instead.