Design Rules for Binary Bisamide Gelators: toward Gels with Tailor-Made Structures and Properties

This study intends to develop design rules for binary mixture of gelators that govern their assembly behavior and subsequently explore the impact of their supramolecular assembly patterns on the gels' rheological properties. To achieve these goals, nBA gelators with odd and even parities [n-methylene spacers between the amide groups (n = 5-10) and 17 carbons at each end] were blended at different ratios. Such bisamides with simple structures were selected to study because their different spacer lengths offer the possibility to have matching or non-matching hydrogen bonds. The results show that the assembly behavior of binary mixtures of bisamide gelators is the same in the solid and gel states. Binary mixtures of gelators, which only differ two methylene moieties in the spacer length, form compounds and co-assemble into fibers and sheets observed for (5BA)₁(7BA)₁ and (6BA)₁(8BA)₁ mixtures, respectively. Binary gelator mixtures of the same parity and a larger spacer length difference still lead to mixing for the odd parity couple (5BA)₁(9BA)₁₎, but to partial phase separation for the even parity mixture (6BA)₁(10BA)₁. Binary mixtures of gelators of different parities gave complete phase separation in the solid state, and self-sorted gels consisting of discrete fibers and sheets in the gels of (5BA)₃(6BA)₁ and (5BA)₃(10BA)₁. The even-even binary gels (20 wt %) consisting of co-assembled sheets show higher G' than odd-odd binary gels (20 wt %) consisting of co-assembled fibers. In general, the self-sorting of odd and even molecules into the separate primary structures results in a dramatic decrease of G' compared to the co-assembled gels (20 wt %), except for (5BA)₁(9BA)₁ gel (20 wt %). It might be due to larger woven spheres in (5BA)₁(9BA)₁ gel (20 wt %), which probably have a less entangled gel network.