The interaction of charged nanoparticles at interfaces

We study charged nanoparticles adsorbed onto surfactant bilayers using small-angle scattering of synchrotron radiation. The in-plane interaction of the particles is well described by a DLVO component (measured independently in solution) and a repulsive dipolar interaction due to the presence of the interface, with an amplitude close to the theoretical prediction. We prove that charged nanoparticles at soft interfaces are well described by the classical model of Poisson-Boltzmann and van der Waals terms; as a corollary, they do not experience the likecharge attraction reported in the literature for some systems of micron-sized spheres at interfaces. Introduction. – The distribution of ions close to soft interfaces (such as biological membranes) is a longstanding problem [1], with wide-ranging fundamental and practical implications. The variation in ion density as a function of the distance to the interface has been thoroughly studied, mostly by X-ray scattering techniques [2–4], but very little is known about the individual ionion interaction, although this interaction and the resulting ion correlation play an important role in biological processes [5]. Such information could also help validate and refine certain assumptions made in theoretical and numerical studies of charged systems at the nanoscale, e.g. in implicit solvent strategies [6, 7]. One often assumes that the interaction is accurately described by the DLVO model [8], which combines the electrostatic and van der Waals interactions, supplemented by steric repulsion. While this model has been very successful in explaining the interaction of macroscopic objects, its validity at the nanoscale is still in need of direct experimental verification. In this context, measuring the correlations between highly charged nanoparticles adsorbed at an interface could provide edifying results. However, due to their small size and often low scattering contrast, very little data exists for such systems. In the present study, we use small (1 nm in diameter) and multivalent inorganic anions ad(a)Current address: IESL, FORTH, Heraklion, Greece. (b)Corresponding author. Email: [email protected] sorbed onto bilayers formed by a nonionic surfactant (Figure 1) and determine their interaction in the plane of the layers. Due to the presence of heavy atoms, the structure factor of these particles can be measured (even at low concentration and in a disordered state) using the intense X-ray beams produced by third-generation synchrotron facilities. Using classical concepts from liquid state theory we then describe the structure factor in terms of the interaction potential and determine the relevant parameters of the latter. Fig. 1: Schematic representation of charged nanoparticles adsorbed onto a surfactant bilayer (only the top monolayer is represented).