Dansylated aminopropyl controlled pore glass: A model for understanding molecular-level interactions at surfaces
Page, Phillip M
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The interaction of molecules covalently bonded to solid surfaces play important roles in many chemical processes and reactions (e.g., catalysis, chemical sensing, separations, and solid-phase synthesis). With the growing emphasis on the environment, researchers in academia and industry have been exploring environmentally responsible (Green) solvent systems. In this context, ionic liquids (ILs) and supercritical CO 2 (scCO 2 ) have emerged as attractive alternatives to conventional liquid organic solvents in many chemical processes and reactions that involve solid surface chemistries. However, despite significant efforts, it remains challenging for researchers to identify an "optimal" set of experimental conditions using ILs or scCO 2 to effectively carry out a particular surface cleaning task, extraction, separation, and/or surface-based reaction. Thus, it is not surprising to learn that researchers often use a trial and error approach for optimization. Unfortunately, such an Edisonian approach, even if successful in the short term, wastes time and it becomes problematic to transfer what is learned in one system to a new challenge that may involve different solutes, cosolvents, compositions, stationary phase chemistries, interfaces, and/or solute-host matrices, and it can become challenging to compensate accurately for any local effects associated with the stationary phase. This dissertation centers on developing a fundamental, molecular-level understanding of solute-liquid and solute-fluid interactions at surfaces. Toward this end, we investigate aminopropyl controlled pore glass (CPG) that is covalently labeled with a solvent-sensitive fluorescent reporter group (dansyl) as our model for studying solvent-surface interactions. The dansyl group serves simultaneously as the model solute and an environmentally sensitive fluorescent probe. We assess the local microenvironment surrounding the surface-bound dansyl residues by recording the probe's emission spectrum. These experiments provide information on the physical and physicochemical properties at the solid-liquid/fluid interface, including the nature of the attachment sites, the accessibility of the surface-bound species to solvent/fluid, and the local surface site dipolarity. We also determine how the local microenvironment that surrounds the surface-bound dansyl groups are modulated by the probe's surface loading, solvent dipolarity, surface residue end capping, and fluid composition. Solvation at the silica surface is investigated first with conventional liquid organic solvents and then extended to ILs, scCO 2 , and cosolvent-modified scCO 2 . The results from this research suggest that at high dansyl loadings, the majority of the dansyl groups are solvated by other dansyl moieties and solvent does not significantly alter the local microenvironment surrounding the average dansyl moiety (i.e., the cybotactic region) to any significant level. At intermediate dansyl loadings, the average distance between the dansyl groups increases, and solvent is able to access/solvate/wet the dansyl groups and alter their cybotactic region to a greater extent. At the lowest dansyl loadings studied, the results suggest that these dansyl moieties are localized within solvent inaccessible/restrictive SiO 2 sites (e.g., small pores). The results also demonstrate that ILs solvate/wet the silica surface differently in comparison to molecular liquids (MLs). The cation component of ILs is the significant factor in how ILs solvate/wet silica surfaces. Solvation/wetting of surface-bound species at a silica surface depends on the cation size. The results from these studies also demonstrate the dramatic role that fluid density and composition play in tuning the local microenvironment surrounding dansyl groups immobilized on the CPG surface. Specifically, when comparing how binary mixtures of CO 2 and 3 mole % cosolvent (methanol, 2-propanol, 2,2,2-trifluoroethanol) solvate/wet CPG-bound dansyl moieties, the results suggest that that the cosolvent's pKa and molecular size determine how well these binary mixtures solvate/wet CPG surfaces.