Nonsynchronous Change in the Head and Tail of Dioctadecyldimethylammonium Bromide Molecules during the Liquid Crystalline to Coagel Phase Transformation Process

Dioctadecyldimethylammonium bromide (DODAB) is known to self-assemble into several lamellar structures in water, existing as either liquid crystalline, gel, or coagel phases. In this work, by using differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray diffraction techniques, we have characterized the details of the phase transition mechanisms of the DODAB aqueous dispersions. It was found that the liquid crystalline converts to the coagel phase via a two-step mechanism: first to the gel phase upon cooling and then to the stable coagel phase. Although significant conformational changes in the hydrocarbon tails were observed in both steps, changes in the headgroups of DODAB were only detected in the second step. More interestingly, we found that the lipid tails change prior to the headgroups during the overall liquid crystalline to coagel phase transformation process. This is regarded as a nonsynchronicity phenomenon, which reflects the regional (head/tail) imbalance in molecular interactions. Such a nonsynchronicity phenomenon in the self-assembled aggregates composed of the medium-sized DODAB molecules will shed light on our understanding of the polymorphism and reversibility of amphiphiles including both surfactants and biomembrane phospholipids.