In situ structure and force characterization of 2D nano-colloids at the air/water interface

Abstract

We have developed a novel experimental method, which enables quantitative determination of interaction forces between interfacial nanoparticles as a function of the inter-particle distance at liquid interfaces.

Direct Measurement of Surfactant-Mediated Picoforces among Nanoparticles in a Quasi-Two-Dimensional Environment

“…However, in our systems, the lowest lateral separations between adjacent NPs range between ∼8 and ∼20 nm, well above the expected bilayer thickness . Therefore, we argue that, in the systems investigated in this work, sparse NP monolayers always form, and above 4.4 × 10 –6 M, both surfactant-decorated NPs and free surfactants adsorb at the air/water interface . This effect could also justify the shorter interparticle distance measured for shorter-tailed surfactants at bulk concentrations higher than 10 –6 M. In particular, given their higher surface activity, longer-tailed surfactants might more effectively adsorb at the air/water interface and, in turn, they might more effectively hinder the NP adsorption.…”

“…We believe that this is due to the increasing surface activity with tail length which prompts the adsorption of more NPs. Previous studies conducted on monolayers formed upon addition of C 16 TAB to silica Sicastar NPs, which were not synthesized and provided by the same supplier of Ludox NPs investigated here, showed significant variations of the interparticle distance with surfactant concentration . The difference may arise either from the different NP size or from other differences between silica NPs provided by different suppliers.…”

“…In other words, the compression-induced surface-pressure increase is due to the gradual approach of NPs, "modulated" by the repulsive interparticle forces opposing this approach. 16,17 In this context, the monolayer compression modulus could be evaluated from the compression isotherms, showing a clear dependence upon both the surfactant tail length and concentration (Figure 5). In particular, monolayers formed by surfactants with longer hydrophobic tails show both higher moduli and a higher sensitivity to surfactant concentration.…”

“…Indeed, owing to the availability of various cationic surfactants having the same charged head but different hydrophobic tail lengths, this system might allow us to investigate the effect of the surfactant tails on the strength and distance range of interparticle repulsions. In particular, when these experiments are performed on uniform sparse silica NP monolayers, obtained by adding minute amounts of cationic surfactants, 17 these monolayers are expected to be more sensitive to subtle variations in the interparticle forces induced by different environmental changes. Finally, the comprehension of forces acting between nanoparticles trapped at liquid interfaces is of paramount importance for the development of tailored lowdimensional structures, as liquid surfaces and interfaces provide a promising environment for the spontaneous assembly of molecules and nanomaterials into 2D or quasi-2D systems.…”

“…It is possible to perform qualitative investigations of interparticle forces between NPs irreversibly adsorbed at liquid interfaces by compressing the so-obtained NP monolayer . Moreover, by simultaneously recording the compression work and the associated interparticle distance reduction via synchrotron radiation grazing incidence X-ray small-angle scattering (GISAXS), it is possible to measure, with subnanometer resolution, the interparticle forces as an average over a large number of NPs. In this framework, negatively charged silica nanoparticles assembled at liquid interfaces upon decoration of oppositely charged surfactants − are a particularly suitable model system for the investigation of ligand-mediated interparticle forces.…”