(235f) Nanoparticle Gels in the Molecular Limit

Anisotropic or patchy interactions between nanoparticles are expected to result in low volume fraction stiff gels. Here we explore gel formation in suspensions of ~1.5 nm particles which form at extremely low concentrations (<0.001 wt%). One specific type involves hydrophobic peptide based molecules – Fmoc-diphenylalanine (Fmoc-FF). We find that when water is added to a DMSO/Fmoc-FF solution, gels are formed along a line of increasing Fmoc-FF concentration with decreasing water concentration. Rigid (G’ > 10⁵ Pa) space filling gels are observed at Fmoc-FF concentrations as low as 0.1 wt%. Gelation in these systems is associated with formation of fibrils that branch to fill space.  Here we report on the kinetics of gelation and demonstrate that these gels are reversible in the sense that they can be disrupted mechanically and rebuild strength over time. We attempt to understand the gelation process as arising from increasing strength of attraction between Fmoc-FF molecules with increasing water concentration. We find no indication that the system is approaching a spinodal for fluid-fluid phase separation as the gel point is reached or that the average diffusivity of species in solution decreases steadily at the gel point is approached, suggesting that gelation is associated with crossing a solubility boundary as opposed to the slow growth of structures as the strength of interparticle attraction grows. We present studies of gelation kinetics, mechanics of the resulting gels and changes in kinetics of relaxation of density fluctuations as the gel point is approached.