Electrolyte interactions with colloidal nanoparticles (NPs) in aqueous solutions have been implicated in a wide range of research and applications. Existing studies on electrolyte interactions with NPs are primarily based on the electrical double layer (EDL) theory. However, the EDL model provides very limited information on how electrolytes directly bind to NPs, electrolyte impact on charge distribution on NPs, and NP morphological modification upon electrolyte binding. Furthermore, the previous reports have mainly focused on either cations or anions binding onto NPs, while the potential cation and anion coadsorption onto NPs and NPacilitated cation-anion interactions remain largely uncharted. Filling these knowledge gaps are critical to enhance the fundamental understanding of interfacial interactions of electrolytes with NPs. Experimental characterization of cations and anions at the solid/liquid interface is a challenging analytical task. In the first study, we demonstrated the first direct experimental evidence of ion pairing on gold nanoparticles (AuNPs) in water by using surface enhanced Raman spectroscopy (SERS) in combination with electrolyte washing. Unlike ion pairing in aqueous solutions where the oppositely charged ions are either in direct contact or separated by a solvation shell, the ion pairing on AuNPs refers to cation and anion coadsorption onto the same NP surface regardless of separation distance. Ion pairing reduces the electrolyte threshold concentration in inducing AuNP aggregation and enhances the competitiveness of electrolyte over neutral molecules in binding to AuNPs. In the second study, we demonstrated that binding, structure, and properties of an ionic species on AuNPs are significantly dependent on the counterion adsorbed on AuNPs. These counterion effects include electrolyte-induced AuNP aggregation and fusion, quantitative cation and anion coadsorption on AuNPs, and SERS spectral distortion induced by the ionic species on AuNP surfaces. In the final study, we proposed that ion pairing as the main mechanism for reducing electrostatic repulsion among organothiolates self-assembled on AuNPs in water by using a series of experimental and computational studies. The work described in this dissertation provides a series of new insights into electrolyte interfacial interactions with AuNPs
