| The life activities of the human body need to rely on different structures, and the weak interaction between molecules is the key to regulating these processes. When compared to chemical bond (covalent bond, ionic bond, etc.), weak intermolecular interactions can only account for one percent for bond energy. This makes the measurement of weak molecular interaction becomes, if not probable, very difficult.In this study, we propose a novel method to deduce the interfacial interaction forces by using surface-sensitive second-order Fermi resonant signals, generated in sum frequency generation vibrational spectroscopy (SFG-VS).We apply this method to study the influence of phospholipid alkyl chain length and the molecular distance on the interfacial interactions. Besides, we explore the specific ion effect dynamics and how salt ions influence and regulate the interfacial interaction forces. We apply DMPG as a model molecule to study phospholipid Langmuir monolayer assembly structure and dynamics during phase transition. We clarify the disassembly process of phospholipid vesicles with different sizes and alkyl chain lengths.The dissertation contains seven chapters.In Chapter 1, we introduce the intermolecular weak interaction and the present research situation of phospholipid self-assembly. We also briefly introduce the research ideas and contents of this thesis.In Chapter 2, we present the theoretical basis of detection of weak intermolecular interaction, including the introduction of sum frequency generation vibrational spectroscopy, Fermi resonance and the calculation method of intermolecular interactions.In Chapter 3, we employ the new method to study the influence of phospholipid alkyl chain length and the molecular distance on the interfacial interactions. The result further determines the feasibility of our method.In Chapter 4, we study the nature of specific ion effect and the effect of ions on the intermolecular weak interaction by the method of Fermi resonance. Through the Hofmeister series of salt ions and phospholipid DLPC, we found that the Hofmeister effect is not simply controlled by the van der Waals interactions, in which electrostatic interaction and hydration interaction also play an important role.In Chapter 5, we take SCN" as an example to study the effects of salt ions on Langmuir films with different phospholipid molecules. We also explore the interactions between SCN" and different chain length, different charge of phospholipids. The study indicates that van der Waals interaction, closely related with phospholipid intermolecular distance, is a key factor to specific ion effect.In Chapter 6, we discuss phospholipid monolayer assembly in the process of liquid-expanded (LE) to liquid-condensed (LC) phase transition using Fermi resonance method. The phase transition kinetics and non-synchronous change of the tail and head groups of the phospholipid were clarified.In Chapter 7, we introduce the intermolecular weak interaction in another important phospholipids self-assembly system:Vesicles. Study on the phospholipid alkyl chain lengths and the sizes of vesicles in the process of assembling shows that the long chain alkyl phospholipid molecules DSPC and DMPC cannot proceed disassembly spontaneously at room temperature. The DLPC can be disassembled spontaneously in aqueous solution and the smaller the size, the faster disassembly is. |
