Spectroscopic and Electrochemical Study on Interfacial Structure and Interactions of Nonionic Surfactants and Self-assembled Monolayers for Various Applications

Abstract

Surface chemistry matters as surface has distinctive properties from the bulk and it is important to understand the surface reactions for applications in surfactants and self-assembled monolayers (SAMs). Interfacial study of nonionic surfactants can provide insights for cleaning performance of detergents on glassware and by using a surface specific technique sum frequency generation (SFG) vibrational spectroscopy, we were able to probe the surfactant solution/ silica interface noninvasively and nondestructively. Interfacial study of SAMs is important as SAMs on metal surfaces can be used to tether molecular catalysts for mechanistic study of electrocatalytic CO2 reductive reaction with their ordered structure and tunable electronic properties. However SAMs are not reductively stable on gold surface under such electrochemical conditions. This dissertation focuses on understanding interfacial structure and interactions of nonionic surfactants and SAMs at the interface using various surface-probing techniques and methods. Chapter 2 describes the techniques and methods used in this work, including SFG spectroscopy, X-ray absorption spectroscopy (XAS) with grazing angle, electrochemical methods of cyclic voltammetry (CV) and chronoamperometry (CA), a novel machine learning (ML) baseline correction method and polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). In Chapter 3, we studied the effect of nonionic surfactant solution bulk concentration and molecular composition on the interfacial water at a silica surface using combined techniques of SFG, quartz crystal microbalance with dissipation factor (QCM-D) and molecular dynamics (MD) simulation. This study shows that interfacial water ordering can be enhanced by increasing surfactant bulk concentration and increasing the number of ethylene oxide (EO) groups in surfactant composition. The effect of surfactant adsorption on silica surface on water orientation can provide insights for surfactant applications in cleaning and wetting. In Chapter 4, in situ XAS experiments were conducted for decanethiol SAMs on Au to understand the metal-sulfur bond. Based on d band model theory, the weakening of Au-S bond under reductive potentials is related to the filling of their antibonding states. In this study, we successfully assembled a custom-designed electrochemical flow cell and conducted in situ electrochemical experiments at grazing angle. The XAS results indicate that Au oxidation states changed under desorption potentials for SAMs on Au and Au-S bond appeared to be cleaved according to the fitting results of extended X-ray absorption fine structure (EXAFS) data. In Chapter 5, quantitative measurements of SAMs reductive desorption potential on different metals were successfully taken with hydroquinone (HQ) redox probe and the ML baseline correction method. The ML method uses LASSO regression with empirical background data was applied to solve the issues of nonideal background of CVs. This study shows that SAMs reductive desorption process is both potential and time dependent and the desorption potentials we calculated from the HQ coverage change confirm that HQ SAMs are most stable on Cu, then Ag, and least stable on Au. The understanding of interfacial behaviors of nonionic surfactants and SAMs will provide important insights for future study in various applications.