Nonadditivity of nanoparticle interactions

Abstract

Understanding interactions between inorganic nanoparticles (NPs) is central to comprehension of self-organization processes and a wide spectrum of physical, chemical, and biological phenomena. However, quantitative description of the interparticle forces is complicated by many obstacles that are not present, or not as severe, for microsize particles (μPs). Here we analyze the sources of these difficulties and chart a course for future research. Such difficulties can be traced to the increased importance of discreteness and fluctuations around NPs (relative to μPs) and to multiscale collective effects. Although these problems can be partially overcome by modifying classical theories for colloidal interactions, such an approach fails to manage the nonadditivity of electrostatic, van der Waals, hydrophobic, and other interactions at the nanoscale. Several heuristic rules identified here can be helpful for discriminating between additive and nonadditive nanoscale systems. Further work on NP interactions would benefit from embracing NPs as strongly correlated reconfigurable systems with diverse physical elements and multiscale coupling processes, which will require new experimental and theoretical tools. Meanwhile, the similarity between the size of medium constituents and NPs makes atomic simulations of their interactions increasingly practical. Evolving experimental tools can stimulate improvement of existing force fields. New scientific opportunities for a better understanding of the electronic origin of classical interactions are converging at the scale of NPs.

Physical adsorption at the nanoscale: Towards controllable scaling of the substrate-adsorbate van der Waals interaction

“…With an influence ranging from protein-drug binding to the double helix in DNA [5], the peculiar pedal adhesion in the gecko [6,7], and even cohesion in regolith and rubble-pile asteroids [8,9], these nonbonded forces are quantum mechanical in origin and arise from electrodynamic interactions between the constantly fluctuating electron clouds that characterize molecules and materials [10]. While our understanding of vdW interactions is rather complete at the smallest atomistic and the largest macroscopic scales, these pervasive forces exhibit a range of surprising and poorly understood effects at the nanoscale [10][11][12][13][14][15][16].This lack of understanding is best exemplified by recent puzzling experimental observations, which include (i) ultralong-range vdW interactions extending up to tens of nanometers into heterogeneous Si/SiO 2 dielectric interfaces [17,18], and influencing the delamination of extended graphene layers from silicon substrate [19]; (ii) complete screening of the vdW interaction between an atomic force microscope (AFM) tip and a SiO 2 surface by the presence of a single layer of graphene adsorbed on the surface [20]; (iii) superlinear sticking power laws for the physical adsorption of metallic clusters on carbon nanotubes with increasing surface area [21]; and (iv) nonlinear increases in the vdW attraction between homologous molecules and an Au(111) surface as a function of the molecular size [22]. Recently, theoretical evidence was found for exceptionally long-ranged vdW attraction between coupled low-dimensional nanomaterials with metallic character [11] or small band gap [10,14].…”

Nanoparticle Assembly:A Perspective and some Unanswered Questions

“…Additionally, the effects of charge, of solvation forces, any interactions emerging from the nonsphericity (e.g. arising from the faceting of the NPs, or of any asymmetry in the grafting of chains or ligands to the NP), and the interactions between any bound ligands play a large role but the methods for their proper account remain unclear at this time 28 . Many of our current constructs to model these different elements use conventional macroscopic ideas -for example, the polymer-induced inter-NP interaction in the presence of unbound polymer chains is assumed to be modelled by the venerable ideas of Asakura and Oosawa 47,48 .…”

“…In a different vein, the spontaneous assemblies of many thousands of anisotropic NPs modified driven by often subtle differences in their interaction reached a level of sophistication comparable to biological systems in respect to geometrical complexities and functions 26,27 . These strides can be made when we partially relax the requirements for structural determinism and rely on dynamic collective interactions of NPs that we have only begun to understand 28 . One could also note that in all cases the development of models that elucidate the rules for NP structure formation is happening largely independent of experiment.…”

“…How do these colloidal ideas get modified when we consider situations where the constituent NPs and polymers are comparable in size? Similarly, while we acknowledge the limitations of the DLVO formalism 28 , we continue to employ it to calculate interparticle interactions in the absence of alternate methods. It is clear that development of more reliable methods to estimate inter-particle interactions in the presence of solvents and other matrices will be critical for theory/ simulations to provide predictive inputs.…”

“…Such forward models, which are implemented multiple times in an inverse design protocol, remain to be developed in contexts beyond DNA-driven assembly. For NP assemblies driven by the colloid interactions, much better understanding of the non-addivity of the van der Waals, electrostatic, and other forces is needed 28 . In the majority of the realistic assemblies in solution, the system energetics largely drive the assembly.…”