On the Origin of the Formation and Stability of Physical Gels of Di-O-benzylidene-d-sorbitol

The thermal and fluorescence properties of gels formed from 1,3:2,4-di-O-benzylidene-d-sorbitol (DBS) and 1,3:2,4:5,6-tri-O-benzylidene-d-sorbitol (TBS) in ethylene glycol (EG) and other alcoholic solvents were investigated. The fluorescence of the benzylidene ground state dimer was observed even in the solution state, indicating that DBS and TBS tend to aggregate in EG. The concentration dependence of the DBS−EG and TBS−EG fluorescence spectra shows that the main structures of DBS and TBS fibers are identical with one another and that each ten-member bicyclo ring overlaps with other rings: the 1,3-O-benzylidene and 2,4-O-benzylidene groups overlap with each of the other units. The fluorescence results show that some 5,6-O-benzylidene groups of TBS interacted with 1,3- and 2,4-benzylidene groups, which is the reason that TBS−EG gels are less stable than DBS−EG gels. The very slow heating of DBS−EG gels showed that needlelike crystals formed at around 370 K for DBS concentrations of 1, 5, and 10%. This indicates that the hydrogen bonding of EG with DBS fibers is cut off at around 370 K. The solvent dependence of enthalpies for melting, ΔH m, and setting, ΔH s, of DBS gels was measured. Since (i) enthalpies were not dependent on each intrinsic property of the solvent and (ii) plots of both ΔH s versus the density of hydroxy groups of the solvent and ΔH m versus the density of all the groups that can form hydrogen bonds were found to give straight lines, DBS fibers are concluded to mainly consist of DBS molecules and to be supported by solvent molecules on their surfaces.