Trends in mica–mica adhesion reflect the influence of molecular details on long-range dispersion forces underlying aggregation and coalignment

Abstract

Oriented attachment of nanocrystalline subunits is recognized as a common crystallization pathway that is closely related to formation of nanoparticle superlattices, mesocrystals, and other kinetically stabilized structures. Approaching particles have been observed to rotate to achieve coalignment while separated by nanometer-scale solvent layers. Little is known about the forces that drive coalignment, particularly in this "solvent-separated" regime. To obtain a mechanistic understanding of this process, we used atomic-forcemicroscopy-based dynamic force spectroscopy with tips fabricated from oriented mica to measure the adhesion forces between mica (001) surfaces in electrolyte solutions as a function of orientation, temperature, electrolyte type, and electrolyte concentration. The results reveal an ∼60°periodicity as well as a complex dependence on electrolyte concentration and temperature. A continuum model that considers the competition between electrostatic repulsion and van der Waals attraction, augmented by microscopic details that include surface separation, water structure, ion hydration, and charge regulation at the interface, qualitatively reproduces the observed trends and implies that dispersion forces are responsible for establishing coalignment in the solvent-separated state.orientation-dependent interparticle forces | dynamic force spectroscopy | atomic force microscopy | solvent structure | DLVO theory C rystallization by particle attachment (CPA) is a common mechanism by which single crystals form in solutions (1). In contrast to classical growth processes of monomer-by-monomer addition and Ostwald ripening, CPA occurs through assembly of higher-order species ranging from multi-ion complexes (2) and polymeric clusters (3) to fully formed nanocrystals. Among the numerous styles of CPA, none has garnered more attention than oriented attachment (OA) by which crystalline nanoparticles assemble into larger single-crystal structures through attachment on coaligned crystal faces (4, 5). OA often leads to formation of hierarchical structures, such as highly branched nanowires (6), tetrapods (7), and nanoparticle superlattices (8), endowed with unique properties (9) that are inexorably tied to this nonclassical process of crystallization.OA has been inferred for metals (10), semiconductors (11), and insulating oxides (6); it has been directly observed via liquid phase transmission electron microscopy (LP-TEM) (5), and the evolution of particle distributions during OA has been captured with cryogenic TEM (12). OA is highly dependent on solution conditions, including pH, ionic strength, and temperature, and is marked by two important stages. In the first stage, particles approach one another but do not make contact; rather they remain separated by an intervening solvent layer of O(1) nm in thickness (5). This solvent-separated state can occur on such an extensive scale that a kinetically stabilized particle array-sometimes referred to as a mesocrystal-is formed in which particles are crystallographically...

Connecting energetics to dynamics in particle growth by oriented attachment using real-time observations

“…The MD results also indicate that a possible reason for the discrepancy in the barriers is that the repulsive hydration force depends strongly on the surface chemistry 47 , as well as curvature in the case of small particles 48 . Importantly, molecular simulations under the low salt concentration solution conditions relevant to this study would require prohibitively large and long simulations to provide insight into the long range solution-mediated interactions and standard molecular potentials are not parametrized to capture long-range dispersion interactions between nanoparticles 32 . Thus, to understand the interactions responsible for both the negligible barrier below 1 nm and the attractive interactions and torques beyond 5 nm, it is necessary to employ theoretical frameworks capable of probing both short-and long-range interactions that are mediated by the solution.…”

“…9). Because these long-range oscillations are directly related to solution structuring and are difficult to test and validate with either molecular simulations or experiments that probe forces between crystal faces indirectly, direct measurements, such as those made with AFM-based force spectroscopy using single-crystal tips 7,12,32,64 , can provide an important test.…”

“…Important physical insights into OA events have been recently obtained from real-time observations using in situ microscopy techniques. For example, previous studies used liquid phase (LP)-TEM to investigate the dynamics of OA events in the iron oxide 9 , Pt-Fe 27 , and Au 28,29 systems, while others employed AFM-based techniques to measure the forces between crystal surfaces, as well as their dependence on solution composition and structure for TiO 2 7,30,31 , ZnO 12 and muscovite mica 32 . In the case of iron oxide, two nanoparticles approach each other and rotate continuously until their lattices are perfectly aligned.…”

Natural, incidental, and engineered nanomaterials and their impacts on the Earth system

“…Aggregation is sensitive to interparticle van der Waals forces, hydration forces, and surface electrostatic forces that are subject to specific environmental variables (such as hydration state, pH, and electrolyte type and concentration), as well as hydrodynamic forces of flow. Classical surface charge-force models in a liquid medium [such as Derjaguin-Landau-Verwey-Overbeek (DLVO)] need to be extended to encompass nonspherical particle shapes and the molecular-level details that give rise to steric, hydration, and ion correlation forces between particles (111,112). Direct measurements of interparticle forces in controlled environments with very high precision are now possible (113,114).…”

Interplay between Short‐ and Long‐Ranged Forces Leading to the Formation of Ag Nanoparticle Superlattice

“…Accomplishing this goal requires an in‐depth understanding of the dynamic behavior of NPs in solutions over various time and length scales, its correlation to interparticle forces, and resultant superlattice structures . The NP self‐assembly processes involve various short‐ and long‐ranged forces, including Brownian force ( F Br ), van der Waals attractive force ( F vdW ), hydrodynamic force ( F D ), electrostatic force ( F elec ), hydration force ( F hyd ), and steric hindrance force ( F Sh ) . Recent advances in liquid cell transmission electron microscopy (LC‐TEM) enable us to understand the dynamics of NPs, behaving either individually or in cluster form, to delineate the competition of interparticle forces in the nonequilibrium state.…”