Interactions and Stress Relaxation in Monolayers of Soft Nanoparticles at Fluid-Fluid Interfaces

Abstract

Nanoparticles with grafted layers of ligand molecules behave as soft colloids when they adsorb at fluidfluid interfaces. The ligand brush can deform and reconfigure, adopting a lens-shaped configuration at the interface. This behavior strongly affects the interactions between soft nanoparticles at fluid-fluid interfaces, which have proven challenging to probe experimentally. We measure the surface pressure for a stable 2D interfacial suspension of nanoparticles grafted with ligands, and extract the interaction potential from these data by comparison to Brownian dynamics simulations. A soft repulsive potential with an exponential form accurately reproduces the measured surface pressure data. A more realistic interaction potential model is also fitted to the data to provide insights into the ligand configuration at the interface. The stress of the 2D interfacial suspension upon step compression exhibits a single relaxation time scale, which is also attributable to ligand reconfiguration.

Collapse mechanisms and extreme deformation of particle-laden interfaces

“…This is consistent also with the observation that monolayers of dodecanethiol-capped gold nanoparticles on polar subphases (high θ) undergo buckling [53]. In contrast, nanoparticles capped with amphiphilic ligands at the interface of water with a fluorinated oil can exhibit short-range, soft repulsion [66] that enable expulsion of the particles upon interface compression [67]. Simultaneous pendant drop tensiometry and grazing-incidence small-angle X-ray scattering (GISAXS) on the same system showed that the collapse behavior of the monolayer depends on the rate of compression [68].…”

The entropic bond in colloidal crystals

“…Nevertheless, we found that the, ostensibly, nonbonded platelet systems we studied exhibit a form of bonding that could be extracted from considering effective interactions arising between platelets. We consider the form of bonding discussed here different not only from chemical bonds but also from nonbonded interactions considered in, e.g., proteins and clusters (37,(45)(46)(47)(48)(49)(50)(51). There, supramolecular objects reconfigure among different conformations via transition states that can also be described with disconnectivity graphs.…”

Proteins at air–water and oil–water interfaces in an all-atom model

“…These issues could perhaps be sorted out by considering artificial systems in which proteins are replaced by ligand-grafted nanoparticles as considered by Garbin et al 33 . The ligand used in this case was amphiphilic mercapto-undecyl-tetra (ethylene glycol) and the experiment involved measurements of the surface pressure.…”