Core–shell nanoparticle monolayers at planar liquid–liquid interfaces: effects of polymer architecture on the interface microstructure

Lucio Isa,Diego Pontoni,Erik Reimhult,Antoni Sánchez-Ferrer,Torben Gillich,Ronald Zirbs,Adrienne Nelson,Raffaele Mezzenga,Davide C E Calzolari

doi:10.1039/c3sm27367a

Abstract

Self-assembly of core–shell nanoparticles (NPs) at liquid–liquid interfaces is rapidly emerging as a strategy for the production of novel nano-materials bearing vast potential for applications, including membrane fabrication, drug delivery and emulsion stabilization. The development of such nanoparticle-based materials is facilitated by structural characterization techniques that are able to monitor in situ the self-assembly process during its evolution. Here, we present an in situ high-energy X-ray reflectivity study of the evolution of the vertical position (contact angle) and inter-particle spacing of core–shell iron oxide–poly(ethylene glycol) (PEG) nanoparticles adsorbing at flat, horizontal buried water–n-decane interfaces. The results are compared with time-resolved interfacial tension data acquired with the conventional pendant drop method. We investigate in particular the effect of varying polymer molecular weights (2–5 kDa) and architectures (linear vs. dendritic) on the self-assembly process and the final structure of the interfacially adsorbed NP monolayers. Linear PEG particles adsorb more rapidly than dendritic PEG ones and reach full interface coverage and stable NP monolayer structure, while dendritic PEG particles undergo a slower adsorption process, which is not completed within the experimental time window of ∼6 hours. All NPs are highly hydrophilic with effective contact angles that depend weakly on PEG molecular weight and architecture. Conversely, the in-plane NP separation depends strongly on PEG molecular weight. The measured inter-particle separation at full interface coverage yields low iron oxide core content, indicating a strong deformation and flattening of the linear PEG shell at the interface. This finding is supported by modeling and has direct implications for materials fabrication, e.g. for the realization of core–shell NP membranes by in situ cross-linking of the polymer shells.

Highlights

Self-assembly of nanoparticles (NPs) has recently seen an upsurge as a strategy to obtain novel materials due to the extraordinary physical and chemical properties of the nanoscale building blocks and the potential for parallel fabrication of complex hierarchical structures.[1,2,3,4] Despite the great promise, the design of functional materials that can be precisely structured via ne-tuning of the building block properties still remains an open challenge.A promising approach for achieving the desired structural and functional control involves the assembly of core–shell NPs at liquid–liquid interfaces.[5]
In this article we study a series of NPs with iron oxide cores stabilized by shells of irreversibly gra ed poly(ethylene glycol) (PEG) of different molecular weights and architectures
The effective thickness of the PEG shell was estimated by measuring the hydrodynamic diameter of the particles in MilliQ water by Dynamic Light Scattering (DLS) and subtracting the core size measured by small-angle X-ray scattering (SAXS)

Summary

Introduction

A promising approach for achieving the desired structural and functional control involves the assembly of core–shell NPs at liquid–liquid interfaces.[5] In these interfacial systems, the inorganic cores can act as sensors or actuator elements, while solvated polymer shells can provide NP stabilization, guide the assembly, and enable responsive functions.[6,7] Independent tuning of the core and shell properties enables control over inter-particle separation, microstructure, and mechanical properties of the nal assemblies These aspects are relevant for NP self-assembly at liquid interfaces (SALI).[8,9] Adsorption of composite, surface-active nanoparticles at liquid interfaces can be used to produce controlled quasi-twodimensional (2D) assemblies such as macroscopic planar membranes,[10] or microscopic responsive capsules and vesicles,[11,12,13,14,15] which are suitable for delivering

Methods

Results

Conclusion