Effect of temperature on self-assembly/disassembly transition of dialkylurea supramolecular gels at high pressure

Abstract

Using 1-methyl-2,4-bis(N′-octadecaneureido)benzene (MBB-18) as a gelator and chlorobenzene (CB) as the solvent, supramolecular gels were prepared. Their phase behaviors were investigated under high pressure using a diamond anvil cell with in situ Raman spectroscopy and optical microscopy at a range of temperatures. As a reference, the phase transition of CB was also examined under corresponding conditions. Unlike the single-component system, the phase behavior of the MBB-18/CB gels, being a dual-components system, involves both phase transition of CB and transition of the supramolecular self-assembly/disassembly. The results indicated that the observed phase transition of the gels at 23 °C and 1.07 GPa can be attributed to a transition from liquid CB to solid CB. Similar results were found at 60 °C and 1.44 GPa. At 150 °C and 0.85 GPa, the phase transition of the gels should be attributed to self-assembly transition of MBB-18 because CB remains in the liquid state in this case. This is a significant result. MBB-18 cannot self-assemble at 150 °C and at normal pressure because the temperature is too high. However, an increase of the pressure to 0.85 GPa enables the self-assembly to occur at the same temperature.