Phase Transitions And Their Regional Cooperativity In Self-Assembled Aggregates Consisting Of Amphiphiles

Amphiphiles are important building blocks for supramolecular self-assembly. Thespontaneous formation process of ordered aggregates through various interactionsbetween amphiphiles is an important method to construct new structures, create newmaterials, and obtain new functions. Since the structural changes in the self-assembledaggregates consisting of amphiphiles are usually accompanied by phase state changes,the studies on the phase states and their transformation mechanisms can deepen ourunderstanding on the nature and principle of the self-assembly processes of amphiphiles.In this thesis, calorimetry, transmission electron microscopy, synchrotron X-rayscattering, and infrared spectroscopy (including various spectral analysis methods suchas two-dimensional correlation analysis and image analysis) have been applied to studythe self-assembly and phase transition mechanisms of several typical examples ofamphiphiles (mainly lipids) dispersed in water. Particular efforts have been devoted tothe investigation of the submolecular-level details during the structural/morphologicaland phase state changes of these amphiphile aggregates.We investigated the rearrangement paces of the different regions of a series oftypical lipids (i.e., the "regional cooperativity") during the phase transitions (includingthe "lamellar–lamellar"(DLPE, DPPC, and DODAB),"lamellar–nonlamellar"(SLPC),and "nonlamellar–nonlamellar"(DSPE-PEG2000) transitions), and proposed anddemonstrated for the first time that these medium-sized molecules can havenonsynchronous change paces in their different parts. These studies provide newinsights into the triggering mechanisms of phase transitions. The regional cooperativityissue opens a broad window for us to seek the complete answer of a basic question:How do the phase transitions of amphiphiles occur and proceed?During the phase behavior studies of these amphiphile aggregates, we also payspecial attention to their crystallization behaviors: The crystallization behavior of theamphiphilic ionic liquid, C16mimCl, depends strongly on its concentration whendispersed in water, and we analyzed the submolecular details of the morphological andphase state changes of the C16mimCl–water system. For the anionic and cationicsurfactant mixtures (SDSO3–DTAB and SDSO4–DTAB), we found that the slight difference in the chemical structure of the head part of the anionic component largelyaffects the thermal stability and structure of the two crystal phases. For DLPE, byemploying a novel H/D exchange method, we found that water plays a role like"catalyst" during the physical transition process from the metastable crystal phase to thestable crystal phase. For the DPPC–DODAB binary system, we found that whenDPPC/DODAB=1/2, DODAB can crystallize in the binary system, and we carefullyanalyzed the formation and transformation mechanisms of the crystal phase of DODABin the binary mixture. For the isothermal crystallization of DLPE, we found that thedifferent parts (the head, interface, and tail) of the DLPE molecules changesynchronously during the crystallization process and an analysis using Avrami equationshows that the crystallization process is controlled by both the interfacial and interlayerdiffusions. For the two single lipid systems, DPPC and DODAB, they both exhibit thenonsynchronicity phenomenon during the crystallization processes, and we unveil thenucleation and growth mechanisms of the crystal phases from the submolecularviewpoint.