Investigation of the efficiency of gelation of melamine with the positional isomers of aminobenzoic acid

Novel thermoreversible gelling systems consisting of the low-molecular-weight gelators, melamine (M) and three positional isomers of aminobenzoic acid (AB) were investigated. According to the rheological measurements, the gel strength at lower temperatures was in the order of pAB/M>mAB/M>oAB/M. This sequence was consistent with a computational analysis based on the stronger hydrogen bonding and the lower energy of each cluster. When heated, the sequence of the degelation temperature was in the order of pAB/M>oAB/M>mAB/M. This degelation temperature agreed with the DSC analyses which also demonstrated the thermoreversibility of these gel systems. It was surprising that the mAB/M which is a stronger gel than the oAB/M could be more easily converted to a sol than oAB/M. FTIR revealed that during heating, the mAB from the mAB/M could change from the uncharged form to the zwitterionic form (mABz). In addition, the computational results indicated that the hydrogen bonding between the mABz and M was much weaker than between the mAB and M. This may result in the lower degelation temperature of the mAB/M system. PXRD analyses showed different patterns of the mixtures compared to the individual compounds and indicated different interactions between M and the different ABs.