Self-assembly of a dialkylurea gelator in organic solvents in the presence of centrifugal and shearing forces

A novel dialkylurea gelator, 1-methyl-2,4-bis(N′N′-octadecaneureido)benzene (designated as MBOB) was synthesized, which can turn some organic solvents into organogels at extremely low concentrations (<2 wt%). The 1H NMR spectra of MBOB in solution (110 °C) and in the gel state (30 °C) indicate that intermolecular hydrogen bonding is the driving force for the self-assembly of MBOB. In the process of the self-assembly of MBOB, orientation of MBOB aggregates occurs under the influence of external fields, such as a centrifugal force and shearing force fields. The minimum gelation concentrations of MBOB in organic solvents under a centrifugal force field were significantly higher than those in the absence of a centrifugal force field, indicating a significant effect of the external field on the self-assembly of MBOB. Field emission scanning electron microscopy (FE-SEM) provided evidence for a significantly phase transition of the MBOB aggregates from an amorphous state in the absence of the external field to an oriented state under conditions of a centrifugal or shearing force during the gelation process. A self-assembled structure of MBOB is proposed based upon an X-ray diffraction (XRD) analysis and a molecular simulation. DSC analysis of the organogels indicates that the phase transition temperature increased from 58.5 °C in the absence of the external field to 63.3 °C under a centrifugal force field and 62.2 °C under a shearing force field. The enthalpy of the phase transition decreased from 3.1 J/g in the absence of an external field to 2.6 J/g under a centrifugal force field and 2.7 J/g under a shearing force field.

A possible mechanism for a novel dialkylurea gelator assembly is proposed as above. In the absence of a force field, the fiber bundles of the gelator aggregates become spontaneously organized into entangled random networks. These fiber bundles show oriented alignments along the direction of force-induced.Figure optionsDownload as PowerPoint slide