Nanoscale and microscale structural changes alter the critical gelator concentration of self-assembled fibrillar networks

Abstract

It has been well established that self-assembled fibrillar networks require a meticulous balance between opposing molecular forces that control solubility and those intermolecular forces that direct epitaxial growth into axially symmetric elongated aggregates. The chemistry of the continuous phase (i.e., solvent) influences every level of structure in molecular gels. Solvent parameters induce low molecular weight gelators (LMOGs) to crystallize into different polymorphic forms, as well cause changes in the lamellar arrangement and domain size. These nanoscale alterations cause measureable differences in the microstructure, which induce physical macroscopic changes including the critical gelator concentration, melting temperature, melting enthalpy and opacity of the gel. Specifically, some solvents cause 12-hydroxyoctadecanoic acid (12HOA) to self-assemble into triclinic parallel polymorphic forms where the lamellar spacing indicates that 12HOA forms an interdigitated network (lamellar spacing <46 Å). The resulting molecular gels are opaque due to the presence of spherulitic crystals and have elevated critical gelator concentrations (i.e., greater than 1.5 wt%). Conversely, other solvents result in the formation hexagonal polymorphs and an extended bi-molecular length greater than 46 Å observed in the lamellar spacing. In these solvents, 12HOA forms translucent molecular gels, at concentrations less than 1.5 wt%, comprised of axially symmetric elongated crystals.