Abstract:
Lyotropic liquid crystals (LLCs), a class of ‘soft’ materials, are self-assembled nanostructures of water and lipids, exhibiting properties intermediate between isotropic liquids and solid crystals. LLCs exhibit rich polymorphism as a function of water content and temperature. These materials have gained substantial popularity in the last decade owing to their promising applications in material science and engineering, biology, nanochemistry, pharmaceutical industry etc. The diverse functionalities of LLCs are predicted to be closely related to the inherent characteristics of the different nanoscopic domains inside the mesophases, specifically the precise nature of the aqueous nanochannels. A molecular-level understanding of the aqueous LLC channels is therefore essential to bridge the gap with the macroscopic functions of the materials to enhance their performance.
In this thesis, various excited state phenomena have been utilized to study the behaviour of the LLC water molecules and the different factors governing their molecular properties. The first study in the thesis mainly focuses on the impact of topology on the micropolarity, microviscosity, and the hydration dynamics of the water molecules in the two cubic mesophases, gyroid (Ia3d) and diamond (Pn3m). Subsequently, the hydrogen bond accepting and donating parameters in the aqueous LLC domains of the inverse hexagonal (HII) and the cubic phases (Pn3m and Ia3d) were estimated by the phenomenon of excited state intramolecular proton transfer. In the following study, the simultaneous interplay between the effects of micropolarity and hydrogen bonding abilities of the LLC water molecules was evaluated by the unique multi-parametric sensitivity of the excited state proton coupled electron transfer phenomenon. All the studies indicate that the different physical aspects of the water molecules exhibit a location-dependent behaviour inside the aqueous channels, pointing towards the presence of multiple discrete water networks. The relationship between the topology of the various mesophases and the nature of the aqueous channels has also been explored. The specific properties exhibited by the water molecules have been linked to the structural features of each mesophase, including the connectivity of the water channels, the negative curvature of the lipid-water interface, the hydrophobic packing stress and the curvature elastic energy.
