Acetonitrile as a probe of the structure and dynamics in important analytical and interfacial systems

Acetonitrile (CH₃CN) is a simple, amphiphilic molecule that is widely used in a variety of analytical applications. This dissertation describes Raman and NMR spectroscopic studies of CH₃CN to probe structure and dynamics in CH₃CN-H₂O mixtures, reversed-phase chromatographic systems, and reverse micellar solutions. These techniques provide a wealth of information concerning structure and dynamics, and represent different experimental timescales. They also offer an alternative to fluorescence probe techniques that can be complex when examining interfacial or confined systems. Although bulk CH₃CN-H₂O mixtures are widely studied, controversy exists regarding the solution structure as a function of composition. Earlier reports describe the structure as a series of discrete regions (“microheterogeneity”), however, recent investigations suggest a continuous microheterogeneous environment with no drastic changes in structure with composition. Present studies corroborate this model and highlight the validity of a two-state approach for studying the bulk mixture. Additionally, the dynamics of CH₃CN-H₂O mixtures can be described by a microscopic hydrodynamic model, allowing the determination of solution viscosity from a microscopic measurable quantity. More complex systems involving interfaces or solutes in nanoscopic domains present additional difficulties when studying structure and dynamics due to interfacial interactions and geometric confinement. This dissertation aims to elucidate information about CH₃CN in these complex systems. First, present studies show that CH₃CN is motionally restricted at a C₁₈-bonded chromatographic interface compared to in bulk solution due to stationary phase interactions, but a quantitative viscosity determination of the surface-affected CH₃CN was not possible due to difficulties in determining the amount of CH₃CN at the interface. Second, the location and extent of partitioning of CH₃CN in Aerosol-OT (AOT)-isooctane solutions have been examined. Contrary to previous reports, these results indicate that CH₃CN is not located in the micelle interior, but adds to the continuous phase and interacts with the surfactant tails. Partitioning of CH₃CN into the micelle interior only occurs when sufficient amounts of H₂O are present to fully hydrate the high concentrations of ions in the interior, and then increases proportionally with increasing H₂O content. These studies highlight the necessity of a systematic characterization to accurately describe structure and dynamics in complex liquid systems.