Fourier Transform Infrared Study on the Phase Transitions of a Water-Dioctadecyldimethylammonium Chloride System

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Abstract
Fourier transform infrared spectroscopy have been applied to study thermotropic phase transitions of the 21 g% water-dioctadecyldimethylammonium chloride (DODAC) system, in which three successive phases of coagel, gel, and liquid crystal appear with increasing temperature. In the coagel phase, the methylene chains take the (rara-zigzag conformation packed in parallel with each other. The hydrophilic part of DODAC in this phase is in a fixed state, and the presence of the bound water is evident. In the gel phase, the rotational motion of the methylene chain around the chain axis occurs in the hexagonal lattice. There exists more bound water in this phase than in the coagel phase, and the hydrophilic part in a rather fixed state. These findings are in conflict with the generally accepted concept of the gel state by x-ray studies. It is found molecular-spectroscopically that there exists an intermediate state between the coagel and gel phases, which is associated with the cooperative rearrangement of the hydrophilic part of DODAC and water. In the liquid crystalline phase, both hydrophobic and hydrophilic parts of DODAC are in fused states and the water spectrum becomes similar to that of liquid water. These structural aspects in each phase were compared with the previous results for the water-octadecyltrimethylammonium chloride system.

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