Using ion-pair interactions to control molecular and colloidal self-assembly

Control over the assembly of molecules at a solid-liquid interface provides a way of controlling the properties of bulk materials and devices. For example, manipulating the interactions between thiols on gold has proven to be a powerful tool for tailoring the insulating and wetting properties of a solid surface. The results presented in this dissertation are the first to employ ion-pair interactions to control molecular self-assembly at a solid-liquid interface. The formation of a stable ion-pair in solution, between two oppositely charged organic ions, is used to tune the properties of self-assembled monolayers (SAMs) on gold. The ion-pairs are shown to create steric hindrance during the self-assembly process, thereby limiting the packing density of the thiol on gold. The packing density can be systematically controlled through variation in the steric size of the ion-pair. The reversible nature of the non-covalent ion-pair interaction enables the ion-pair SAMs to be easily converted into low-density monolayers (LD-SAMs). The LD-SAMs synthesized in this work are found to exhibit reversibly switchable interfacial properties when voltage is applied to the gold electrode. The dynamic response of the LD-SAM is shown to arise from a reversible conformational change within the film, driven by polarization of the surface-bound thiols in the presence of an electric field. Ion-pair interactions, incorporated onto the surface of gold nanoparticles, can also be used to direct the self-assembly of gold nanoparticles at an oil-water interface. The liquid-liquid interface is an attractive platform for colloidal assembly due to the ability of light and chemical reagents to access both sides of the interface. In contrast to micron-sized particles, the driving force for attachment of nanoparticles at an oil-water interface depends strongly on particle surface chemistry. This study demonstrates that 10 nm diameter gold nanoparticles, functionalized with ion-pairs, spontaneously assemble into a metallic sheet at the oil-water interface. The sheet formation is shown to be tunable with pH and correlated with the solubility of the ion-pair nanoparticles in the bulk aqueous phase. The results of these experiments show that stoichiometric ion-pair interactions are important for building tunable two-dimensional arrays from gold nanoparticles at the oil-water interface.