Thermal and Structural Insights into Temperature-Induced Phase Behavior and Hydration-Dependent Self-Assembly of...

N,N'-Diacyl-l-lysines (DALs) with decanoyl to hexadecanoyl acyl chains were synthesized and characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), fluorescence spectroscopy, and field-emission scanning electron microscopy (FESEM). DTA thermograms exhibited two overlapping transitions during the first heating scan. While the first heating was terminated ∼10 °C above the end-set temperature of the initial transitions (case I), in the subsequent heating scan, two transitions were observed at the original transition temperature and at a temperature ∼20-25 °C higher. In contrast, a single higher-temperature transition (∼20-25 °C above) appeared when the sample was heated significantly beyond this range (case II), and a single transition occurred if the scan ended immediately after the first transition (case III). PXRD data suggested that DALs adopt a tilted bilayer packing arrangement. FTIR and PXRD studies on C11- and C15-DALs revealed changes in molecular interactions, diffraction patterns, and crystallinity across cases I-III. However, neither acyl-chain length nor the thermal protocol affected their thermal stability, while the solvent used for recrystallization/dissolution altered phase behavior and crystallinity without compromising stability. Laurdan generalized polarization (GP) in DALs depended on the hydration method. Samples obtained after freeze-thaw cycles followed by a short sonication exhibited lower GP than those prepared by direct sonication. Temperature-dependent studies revealed biphasic changes in GP. The morphology of DALs depended on the chain length and hydration method. These findings provide insights into the structural, thermal, and morphological behavior of double-chain amphiphiles, with implications for their potential applications.