Light Matter Interactions: A Study of Soft Materials Using Linear and Nonlinear Spectroscopy

The adoption of complex fluids for various industrial applications is becoming normal. Complex fluids offer tunability, wide range solubility, and chemical and thermal stability which are the factors that conventional polar and non-polar solvents often lack. However, fundamental studies of these fluid systems are still lacking which is limiting the appropriate use of these complex fluids in many applications. The goal of this dissertation was to study and characterize complex fluids for application in electrolytes for redox flow batteries. Chapter 3 and chapter 4 feature the study of microemulsions and deep eutectic solvents (DES) by fluorescence techniques. Fluorescence studies of microemulsions revealed that the interfacial layer in the microemulsions, formed by the mix of surfactant and co-surfactant, can accommodate various small molecules diverse in structure. The hydration of the surfactant layer with the increase of water percentage beyond < 60% showed a sudden stiffening of the interfacial layer, which was reflected in the rotational time of the polar and ionic probes. The sudden stiffening of the surfactant region correlates to a morphological change in the microemulsion from bicontinuous to droplets. The fluorescence studies of DESs in chapter 4 revealed that the nonpolar probe molecule did not sense any environmental change with the increase of phenol content in the DES composition. The probe that senses the hydrogen bond network in their local environment revealed the existence of the hydrogen bond and also showed the weakening of its interaction at the higher phenol content in the DESs. Chapter 6 of this dissertation discusses the application of second harmonic generation spectroscopy to study the adsorption of molecules at the charged surface and shows that nonionic surfactants differed in structure adsorb at the charged surface differently. The SHG tool can be leveraged in the future to screen potential surfactant molecules for applications in electrolytes.