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

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 selfassembly 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 oxidepoly(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.

Adsorption Energies of Poly(ethylene oxide)-Based Surfactants and Nanoparticles on an Air–Water Surface

“…15 This large lattice spacing indicates PEOfunctionalized NPs interact repulsively over distances longer than the equilibrium brush thickness in bulk solution, revealing instead the PEO corona around each interfacially adsorbed NP to be strongly deformed and highly flattened (e.g., Figure 9). Similar morphologies have been directly observed for larger soft particles.…”

“…The separation distance of the nanoparticles measured at air− water surfaces (calculated using eq 3 to be 39.7 and 37.3 nm for L5 and L.25, respectively) and at the decane−water interface (measured using X-ray reflectivity 15 at saturation to be 54.4 and 39.8 nm for L5 and L.25, respectively) are both significantly larger than the diameter of nanoparticles in bulk aqueous solution (measured to be 30 and 23 nm for L5 and L2.5, respectively). 15 This large lattice spacing indicates PEOfunctionalized NPs interact repulsively over distances longer than the equilibrium brush thickness in bulk solution, revealing instead the PEO corona around each interfacially adsorbed NP to be strongly deformed and highly flattened (e.g., Figure 9).…”

“…14,15 Additionally, such assumptions may break down if nanoparticles aggregate at the interface. In this case, the binding energy of a single particle is no longer sufficient to describe binding within a particle raft; instead, interactions between particles in the aggregates become relevant.…”

Controlling the dynamics of a bidimensional gel above and below its percolation transition

“…Structural investigations by grazing-incidence x-ray scattering have been employed to elucidate the structure of deposited Langmuir-Blodgett [9] and unsupported Langmuir monolayers [10]. In favorable cases, the contact angle of individual particles could be estimated by accurate modeling of the x-ray reflectivity curve [11,12].…”

Contact angle and adsorption energies of nanoparticles at the air–liquid interface determined by neutron reflectivity and molecular dynamics

“…Isa et al expanded this work to address the influence of the NPs shell architecture in determining the monolayer interfacial microstructure. 29 In situ high-energy X-ray reflectivity was used to quantify the vertical position and inter-particle spacing of core-shell iron oxide poly(ethylene glycol) (PEG) NPs adsorbed at water-n-decane interfaces. These studies, however, could not be used to directly extract information of the ligand shell covering metallic NPs, as the contribution of the metallic core to the X-ray scattering is overwhelming making impossible to detect the weak scattering of the shell.…”