Thiol ligands bound to the metallic core of nanoparticles determine their interactions with the environment and self-assembly. Recent studies suggest that equilibrium between bound and free thiols alters the ligand coverage of the core. Here, X-ray scattering and MD simulations investigate water-supported monolayers of gold-core nanoparticles as a function of the core-ligand coverage that is varied in experiments by adjusting the concentration of total thiols (sum of free and bound thiols). Simulations demonstrate that the presence of free thiols produces a nearly symmetrical coating of ligands on the core. X-ray measurements show that above a critical value of core-ligand coverage the nanoparticle core rises above the water surface, the edgeto-edge distance between neighboring nanoparticles increases, and the nanoparticle coverage of the surface decreases. These results demonstrate the important role of free thiols: they regulate the organization of bound thiols on the core and the interactions of nanoparticles with their surroundings.

We report the results of molecular dynamics simulation studies that explore two features of the phase diagrams of two two-dimensional systems composed of particles with everywhere repulsive isotropic pair potentials, one proposed by Piñeros, Baldea, and Truskett, and the other by Zhang, Stillinger, and Torquato, each of which supports a high-density Kagome lattice phase.These features are (i) the sequences of phases and the phase transitions characteristic of each system as the density is increased along an isotherm, and (ii) the character of transient structured fluctuations in the phases adjacent to a high-density Kagome lattice phase. As to (i), comparison of the sequences of phases supported by the pair potentials used in our simulations and those supported by other pair potentials provides information vis a vis a relationship between the shape of the pair potential and the density dependence of the sequence of phases. The commonalities in the phase diagrams of the several 2D systems suggests the existence of a universal mechanism driving all to favor a similar series of packing arrangements as the density is increased.However, the collection of simulations considered shows that satisfying the only such general rule proposed, namely the Süto theorem relating the character of the Fourier transform of the pair potential to the existence of multiple ground states of a system, is not a necessary condition for the support of multiple distinct lattice structures by a particular pair potential. As to (ii), the "open" structure of a Kagome lattice requires an unusual rearrangement of the particle packing in the phase from which it emerges. We find that on an isotherm in the liquid phase, close to the liquid-to-Kagome phase transition, the transient structured fluctuations in the liquid have Kagome symmetry whereas deeper in the liquid phase the transient structured fluctuations have hexagonal symmetry. As the deviation of the liquid density from the transition density decreases transient fluctuations with hexagonal symmetry are replaced with those with Kagome symmetry with a coexistence domain of a few percent. When the transition is string phase-to-Kagome 2 phase the transient structured density fluctuations in the string phase near the transition do nothave Kagome character; there are both configurations with six-fold and other than six-fold symmetries, with stronger preference for six-fold symmetry in the Truskett system than in the Torquato system. The path of the string-to-Kagome transition in the Truskett system involves intermediate particle configurations with honeycomb symmetry that subsequently buckle to form a Kagome lattice. The path of the string-to-Kagome transition in the Torquato system suggests that the Kagome phase is formed by coiled strings merging together at three-particle joining sites. The increasing concentration of these joining sites as the density is increased generates a Kagome phase with imperfections such as 8-particle rings.

We report the structure of transient fluctuations in the liquid phase of a two-dimensional system that exhibits several ordered phases with different symmetries. The density-temperature phase diagram of the system studied, composed of particles with a repulsive shouldered soft-core pair interaction, has regions with stable liquid and hexatic phases, a square solid phase, two separate hexagonal solid phases, and a quasi-crystalline phase with 12-fold symmetry. We have examined the character of the structured fluctuations by computing the same-time aperture cross correlation function of particle configurations in several fluid regions near to and far from phase transition lines. The two primary goals of our study are (1) determination if the spectrum of structures of the fluctuations in the liquid is broader than or limited to the motifs exhibited by the ordered phases supported by the system and (2) determination of the density domains in the liquid that support particular transient structured fluctuations. In the system studied, along a low-temperature isotherm in the temperature-density plane that intersects all the ordered phases we find that the liquid phase exhibits structured fluctuations with hexagonal symmetry near both liquid-hexatic transition lines. Along the same isotherm and in the stable liquid between the lower density hexatic-to-liquid and the higher density liquid-to-square solid transitions, we find that transient hexagonal ordered fluctuations dominate the liquid region near the hexatic-to-liquid transition and square ordered fluctuations dominate the liquid region near the liquid-to square solid transition, but both of these structured fluctuations occur at all densities between these transition lines. At a higher temperature, at phase points in the liquid above, but close to the density maximum of an underlying transition, there are ordered fluctuations that can be correlated with the structure of the lower temperature phase. Although it is expected that very close to a liquid-ordered phase boundary a structured fluctuation in the liquid will have the same symmetry as the ordered phase, it is not obvious that structured fluctuations in thermodynamic states deep in the liquid phase will be similarly restricted. The most striking result of our calculations is that no evidence is found in the liquid phase for structured fluctuations with other symmetries than those of the ordered phases of the system.

The interaction between two ligated nanoparticles depends on whether they are isolated or immersed in a liquid solvent. However, very little is known about the influence of solvent vapor on the interaction between two ligated nanoparticles. Recent experiments yield the surprising result that the cyclic exposure of solvent free suspended monolayers of dodecane thiol ligated gold nanoparticles (AuNPs) to water vapor and dry nitrogen generates reversible cyclic decreases and increases in Young’s modulus of the monolayer, implying corresponding cyclic changes in the AuNP–AuNP interaction. We examine how water vapor interacts with an isolated dodecane thiol dressed AuNP and how water vapor affects the interaction between a pair of nanoparticles, using all-atom molecular-dynamics simulations. We find that there is condensation of water molecules onto the ligand shell of an AuNP in the form of clusters of 100–2000 molecules that partially cover the shell, with most of the water in a few large clusters. A water cluster bridges the AuNPs, with a sensibly constant number of water molecules for AuNP–AuNP separations from the edge-to-edge contact up to center-to-center separations of 100 Å. The wet AuNP–AuNP interaction has a slightly deeper and wider asymmetric well than does the dry interaction, a change that is qualitatively consistent with that implied by the observed water vapor induced change in Young’s modulus of a monolayer of these AuNPs. We find that macroscopic analyses of water drop–deformable surface interactions and dynamics provide both guidance to understanding and qualitatively correct predictions of the phenomena observed in our simulations.

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