Molecular Arrangement and Thermal Properties of Bisamide Organogelators in the Solid State

The crystal structure and phase behavior of bisamide gelators are investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy, Xray diffraction (XRD), and molecular modeling, aiming at a better understanding of bisamide gel systems. A homologous series of bisamide model compounds (nBAs) were prepared with the (CH2)n spacer between the two amide groups, where n varies from 5 to 10, and with two symmetric C17 alkyl tails. With increasing spacer length, the thermal properties show a clear odd-even effect, which was characterized using our newly developed analytical model DSCN(T). Using XRD, all studied nBA compounds turn out to have a layer-like structure. The XRD patterns of the odd BA series are very similar but show marked differences compared to the XRD patterns of the even series, which in turn are very similar. The odd-membered 5BA molecules are nearly perpendicular to the stacked layers, as described by a pseudo-orthorhombic unit cell, whereas the even-membered 6BA molecules are tilted at an angle with respect to the layer normal, as described by a triclinic unit cell. In both the odd and even series, the inter-layer interaction is the van der Waals interaction. The 6BA hydrogen bonding scheme is very similar to that of Nylon 6,10 alpha, unlike the 5BA H bonding scheme. The packing of the C17 alkyl tails in the 5BA layers is similar to polyethylene, and unlike 6BA. The slightly higher crystalline density of 6BA (1.038 g cm-3) as compared to 5BA (1.018 g cm-3) explains the higher melting point, higher enthalpy of fusion, and the observed shift of N-H stretch bands to higher wave numbers. The structural differences observed between the odd and even BA series reflect the different structure-directing effect of parallel versus antiparallel amide hydrogen bonding motifs. These differences underlie the observed odd-even effect in the thermal properties of nBA compounds.

Automated summary

The crystal structure and phase behavior of bisamide gelators were studied using various scientific methods, such as DSC, FTIR spectroscopy, XRD, and molecular modeling. The results show that the thermal properties and structure of the bisamide gelators exhibit an odd-even effect. The differences in the hydrogen bonding motifs and alkyl tail packing between the odd and even series of compounds contribute to the observed odd-even effect.