Nonisothermal order-disorder phase transition of alkylammonium ions in nanoconfined space

摘要

To understand the effect of confined space (interlayer space in montmorillonite (MMT)) on the nonisothermal ordering transition (chain packing) kinetics and disorder transition (chain melting) behavior including the conformational changes of the chain segment of the cationic surfactants, we have characterized MMT modified with dioctadecyl dimethylammonium (DC_(18)DM) ions (MMT-DC_(18)DM) using temperature-modulated differential scanning calorimeter (TMDSC), wide-angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR) techniques. For MMT-DC_(18)DM, the chain conformational disorder-order phase transition took place during the cooling process. The transition peak was much broader and it appeared at lower temperature (T_c) when compared to the crystallized dioctadecyl dimethylammonium bromide (DC_(18)DM-Br), as a reference.rnIn MMT-DC_(18)DM, the formation of gauche conformers was enhanced and the chains were not as densely packed as in crystalline DC_(18)DM-Br. The normal crystallization took place in the bulk during the nonisothermal crystallization of DC_(18)DM-Br. The confined ions (DC_(18)DMs) in one or two dimensional order contributed to the nonisothermal chain packing for a higher cooling rate of 5.0-20.0 ℃/min. The observed chain packing in confined space at different T_c ranges (cooling rate >5.0 ℃/min) could be explained by much lower energy barrier.