Thermal and Rheological Properties of Physical Gels Formed from Benzylidene-d-sorbitol Derivatives

The thermal and rheological properties of gels formed from 2,4-(mono)-O-benzylidene-d-sorbitol (MBS), 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 from DBS in glycerol (GL) were investigated. In all the gels studied, the values of the enthalpy of melting for the gel obtained from the Eldridge–Ferry plot agreed well with those obtained from the peak area of the DSC curves. This agreement has not been demonstrated clearly for biopolymeric systems thus far but can now be successfully shown by first using small molecule systems. In conclusion, the transitions of the gel formation and gel melting proceed via a second order or higher order process. The experimental results show that the ease by which formation and stabilization of gels takes place for the sorbitol derivatives in EG is in the following order: DBS > TBS > MBS. Accordingly, the main reason for the stabilization of sorbitol gels in alcoholic solvents is concluded to be not simply due to hydrogen bonding or hydrophobic interactions among benzylidene groups but rather it is due to a delicate balance of chemical structure that facilitates the formation of crystals. In particular, the symmetric structure of a DBS molecule, which has a rigid and chiral ten-member ring symmetrically connected with two benzylidene groups in equatorial positions, is considered to be the most crucial reason to form most stable gels studied in this work.