Surface characterization of self-assembled monolayers for applications of selective ionic transport through nanoporous membranes

Electrokinetics is the study of ionized particles or molecules and their interactions under an applied electric field. Transport processes include those of colloids and other charged particles in electrolytes, whose motion under applied external electric fields is described in part by fluid mechanics. In particular, there is growing interest for the application of electromigration through nanoporous membranes to water desalination. For thin double layers, the volumetric flow ratio (VFR) between electroosmosis and pressure driven flow is inversely proportional to 1⁄r^2 , indicating the advantages of utilizing electrokinetics for systems with small characteristics length scales. Bare solid surfaces of metal and metal oxides tend to adsorb organic contaminants in order to lower the free energy between the metal and the environment. The adsorbed matter can alter the interfacial properties of microfluidic and nanofluidic systems. The adsorbed matter does not have any specific functional properties; therefore, it is hard to reproduce any physical properties (e.g., thermoconductivity, electroconductivity, hydrophobicity). Self-assembled monolayers (SAMs) provide a unique way to control and functionalize the interfacial properties of metal, metal oxide, and semiconductors for nanoscale devices. The control in functionality can improve the performance of nanoscale devices by improving process precision. In this dissertation, a focus of interfacial transport phenomena is proposed in order to achieve improved-charge selective nanofluidic systems. There have been numerous studies on the quality of organic SAMs as a blocking mechanism for prevention of ion adsorption with applications ranging from biosensors to chemical sensing through nanoporous membranes. The applications of these devices are often limited by the quality of the SAM. For transport studies utilizing a SAM on a gold-coated nanoporous membrane, electrochemical impedance spectroscopy (EIS) can be used as a probe to characterize insulative surface properties. Insulation is especially crucial for short length scales found in microchannels and nanopores. A well-grown monolayer can help reduce adsorption of ions on walls of nanopores/nanochannels, which can lead to lower irreversibilities for charge selective systems. There have been extensive studies, which demonstrate that by making a polymer membrane conductive mostly through a process called electroless gold plating, that charge selectivity can be accomplished by controlling the surface charge of the conductive membrane by applying a range of cathodic and anodic potentials. The membranes are functionalized with SAMs in order to prevent adsorption of ions. Past work has shown that selectivity of ion adsorption can vary, depending on whether anodic or cathodic potentials are applied across the membrane. A second motivation of this work has been to study and characterize the quality of SAMs with the intent to minimize adsorption ions on a membrane surface in order to maximize charge selectivity. By minimizing adsorption, smaller over-potentials can be applied to achieve charge selectivity. Lastly, the fabrication of a membrane permeate flow cell is described which was then utilized to study the transport of organic analytes through a conductive nano-capillary array membrane (NCAM) by UV absorption spectroscopy. The goals of the transport studies are to demonstrate improved charge selectivity when well-grown SAMs are used over a wide range of potentials applied across a membrane. In addition, the studies are to further implement chemical separations by applying potentials within the millivolt range (≤±400 mV). This work improves on previous studies of applying potentials across conductive NCAMs by determining the critical voltage range at which potentials can be applied with minimum ion conduction through the SAM. Future work will also be addressed where it is suggested to explore the idea of competing effects between the contributions of the diffuse layer potential at the membrane surface and nanopore wall