Synergic effect in gelation by two-component mixture of chiral gelators

Abstract

Observation of the synergic gelation effect (SGE) in two-component gels is reported. An equimolar mixture of (S,S)-bis(LeuOH) oxalamide [(S,S)-1] and (S,S)-bis(leucinol) oxalamide [(S,S)-2] is able to gel up to 7 times larger a volume of p-xylene than an equal mass of each component and up to 5 times larger a volume than an equal mass of the (S,S)-1 + (R,R)-2 or (S,S)-1 + rac-2 equimolar mixtures. The homochiral (S,S)-1 + (S,S)-2 combination of gelators is capable of hardening a volumes up to 5 times larger of certain solvents than the heterochiral (S,S)-1 + (R,R)-2 combination. Experimental evidence provided by determination of gelation efficiency, by ¹H NMR, TEM and XRD studies, by determination of phase transition diagrams and calculations of thermodynamic parameters for gel melting processes shows that synergism depends on the chirality of the components, on the solvent properties and on the gel morphology. It was found that each component tends to form reversed bilayers in lipophilic solvents which then interact and organize into gel unit fibers. The unit fibers formed by interaction of the (S,S)-1 and (S,S)-2 bilayers and of the (S,S)-1 and (R,R)-2 bilayers are distereoisomeric. In certain solvents, such diastereomeric unit fibers give different gel morphologies which in turn results by different thermal stabilities of the gels, and in some cases by dramatically different gelling efficiencies of gelator mixtures. The latter observation is denoted as the synergic gelation effect (SGE).