Time-resolved study of nanoparticle adsorption at liquid surfaces: structure, thermodynamics and kinetics

Hypothesis: The real time monitoring at the nanoscale of the irreversible adsorption of nanoparticles at liquid surfaces has not been possible so far, given the inherent limitations of the main techniques employed in the dynamic characterization of these systems. However, a deep understanding of the nanoparticle adsorption dynamics might allow the designing of novel interfaces responding to external stimuli with controlled rates. Grazing incidence X-ray small angle scattering (GISAXS) might enable following the adsorption of nanoparticles at liquid surfaces with high temporal and spatial resolution. However, this necessitates to carefully design the experimental protocol, allowing to monitor the very first minutes of adsorption. Experiments: We developed a time-resolved GISAXS methodology to follow in real time the surfactant-induced adsorption of nanoparticles at the air/water interface. The approach is based on the remotely-controlled spreading of few microlitres of concentrated surfactant solutions on pre-aligned nanoparticle dispersions. The time-resolved GISAXS structural data, revealing the formation of a progressively denser nanoparticle monolayer, were also coupled to the dynamic surface tension measurements. Findings: Analysing the time evolution of the nanoparticle surface excess allowed us to determine both thermodynamic and kinetic parameters. Our results demonstrate that both the non-equilibrium surface excess and the adsorption kinetics significantly changed with the ionic strength, while the surfactant concentration played a minor role. This paves the way to the design of stimuli responsive nanoparticle-decorated surfaces with tailored nanostructures and adsorption rates.