(607d) Molecular Gel Formation As a First Order Phase Transition

Selfâ??assembling systems based on dipeptide molecules that form ordered fibrous structures are seen as promising new materials for applications in tissue engineering, drug delivery, biosensors, photoelectronics, etc. However, the gelation mechanism in these types of systems has been largely overlooked and remains poorly understood, limiting the development of comprehensive design guidelines for specific applications. We explore the gel transition of the aromatic dipeptide molecule fluorenylmethoxycarbonylâ??diphenylalanine (Fmocâ??FF). The addition of water to a solution of Fmocâ??FF in dimethyl sulfoxide (DMSO) results in the selfâ??assembly of Fmocâ??FF molecules into a spaceâ??filling fibrous network. We provide evidence that gel formation is associated with a first order phase transition resulting in nucleation and growth of strongly anisotropic crystals with high aspect ratios. The strength of attraction between Fmoc-FF molecules as a function of water concentration is estimated from longâ??time self-diffusion measurements using ¹H NMR diffusionâ??ordered spectroscopy (DOSY). The resulting phase behavior follows that observed for a wide range of other crystallizing nanoparticles and small molecules, consistent with the short-range nature of the intermolecular attractions. Furthermore, a lag time in the formation of the new phase is observed, indicating the existence of a free energy barrier to the formation of a crystal nucleus of critical size. The application of classical nucleation theory for a cylindrical nucleus indicates that one-dimensional crystal growth is driven by an imbalance of the surface energies of the ends and sides of the fiber. We believe that the discussed concepts can be extended and applied to other similar molecular gel systems, thereby providing guidelines that would facilitate the design of novel molecular gelators.