The molecule 1,3:2,4-Di-O-Benzylidene sorbitol (DBS) is a common gelator that forms thermally reversible gels in diverse organic solvents. Solid state 13C and proton NMR techniques, along with electron microscopy, are utilized in an exploratory study of DBS in the gelled state where we consider both in situ and dried gels. The gels were formed in either acetone or benzene, the former being a better solvent for DBS. We find the in situ or dried DBS gels to be composed of rigid twisted nanofibrils ( 15 nm to 21 nm in diameter). The fibrils show order, but not crystalline order, they contain no trapped solvent, the molecular mobility at the fibril surface is modestly enhanced, and all the free hydroxyl groups of the sorbitol moiety are involved in strong hydrogen bonding. We also attempt to find a truly crystalline form of DBS whose structure, as judged by similarity of 13C spectra, is close to that of the fibrils. We partially succeeded in this quest, employing melt crystallization followed by slow cooling. However, this sample was a mixed-crystal having small domains, where only one type of domain had structural similarity with the fibrils. We also investigate the long time evolution of the in situ DBS gel network. Specifically, high-resolution NMR kinetic studies were performed over periods of days where the residual concentration of DBS in acetone solution was monitored during and after gel formation. The DBS concentration over these long time scales evolved slowly and we identified a simple mathematical equation to describe this phenomenon.
Pub Type: Journals
Di-O-benzylidene sorbitol, NMR, gel, nanofiber, fiber helicity, self-assembly, aging, thermally reversible gelation, hydrogen bonding