Rheology and thermotropic properties of bis-urea-based organogels in various primary alcohols

The thermotropic and viscoelastic properties and the structure of thermoreversible organogels composed of enantiomerically pure trans-1,2-bis(3-dodccylureido)cyclohexane (1) and primary alcohols, i.e., propanol, butanol, hexanol, and octanol, have been investigated by melting-point measurements, differential scanning calorimetry (DSC), rheology, and electron microscopy. The electron microscopic studies revealed that gelation of those solvents occurred through the formation of an entangled network by 1. In all solvents, the gels behave as viscoelastic solids, with G' >> G" at frequencies >0.01 Hz (time scales < 100 s) and values of G' up to 500 kPa (50 mM in propanol). The thixotropic character of the gels, however, indicates that the network structure is dynamic at larger time scales. Although the structure of the gels in these solvents is not significantly different, both the thermal stability and the strength of the gels depend on the solvent, and they increase in the order octanol < hexanol < butanol < propanol. It was concluded that solvophobic effects become important for the stabilization of gels of 1 in more polar solvents.