As a method development focused thesis, the goal of this dissertation is to develop new methods to study nanoparticle and counterion transport at the polyelectrolyte-solution interface that researchers can use to characterize material performance in separations and ion transport applications. We start by describing the fundamental interactions that drive adsorption and desorption and thus impact the transport of nanoparticles in solution near solid interfaces. Next, in a model study of negatively charged gold nanoparticles (AuNPs, 20 nm, zeta potential = -58.3 mV) adsorbing to a positively charged weak polyelectrolyte layer-by-layer (LbL) film (85 nm, zeta potential = +14.5 mV), we show that a new label-free microscopy technique, interferometric scattering microscopy (iSCAT), gives single nanoparticle insight into the adsorption process by directly measuring adsorption events. We enhance the applicability of iSCAT by improving particle detection capabilities in imaging adsorption events at rough interfaces where stage movements complicate image analysis with machine learning. We complement the single particle iSCAT studies with ensemble measurements of AuNP adsorption to LbL films with quartz crystal microbalance with dissipation (QCM-D) and unite the results of the single particle iSCAT and ensemble QCM-D measurements by accounting for transport differences using finite element analysis (FEA). The union of a label-free single particle technique like iSCAT and an ensemble method like QCM-D through FEA simulations provides a novel workflow other researchers can use to characterize the performance of coating materials for nanoparticle separations technologies. In the last chapter, we develop a coarse-grained model that retains the chemical specificity of a strong polyelectrolyte in poly[(2-(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) using the MARTINI forcefield. We then build simulations of salt-free (only enough counterions to balance charge on the brush) 150 monomer MARTINI PMETAC brushes at experimentally relevant grafting densities of σ = 0.05, 0.10, 0.20, and 0.40 chains/nm2. Using 5 microsecond simulations, we investigate the effects of σ on brush and counterion structure, ion dissociation dynamics, polymer mobility, and counterion diffusivity. Results show that counterion diffusivity is controlled by three factors: electrostatic interactions, polymer mobility, and steric hindrance. The interplay of these factors leads to diffusivity that depends non-monotonically on σ in both the lateral and axial directions with the counterion diffusivity peaking at σ = 0.10 chains/nm2. The PE brush simulations developed here are easily modifiable and we propose other variables for future studies for all chapters as we conclude in Chapter 6.