When Like Destabilizes Like: Inverted Solvent Effects in Apolar Nanoparticle Dispersions

We report on the colloidal stability of nanoparticles with alkanethiol shells in apolar solvents. Small angle X-ray scattering and molecular dynamics simulations were used to characterize the interaction between nanoparticles in linear alkane solvents ranging from hexane to hexadecane, including \SI{4}{\nano\meter} gold cores with hexadecanethiol shells and \SI{6}{\nano\meter} cadmium selenide cores with octadecanethiol shells. We find that the agglomeration is enthalpically driven and that, contrary to what one would expect from classical colloid theory, the temperature at which the particles agglomerate increases with increasing solvent chain length. We demonstrate that the inverted trend correlates with the temperatures at which the ligands order in the different solvents, and show that the inversion is due to a combination of enthalpic and entropic effects that enhance the stability of the ordered ligand state as the solvent length increases. We also explain why cyclohexane is a better solvent than hexane, despite having very similar solvation parameters to hexadecane.Comment: SI + main manuscrip

Collagen breaks at weak sacrificial bonds taming its mechanoradicals

Collagen is a force-bearing, hierarchical structural protein important to all connective tissue. In tendon collagen, high load even below macroscopic failure level creates mechanoradicals by homolytic bond scission, similar to polymers. The location and type of initial rupture sites critically decide on both the mechanical and chemical impact of these micro-ruptures on the tissue, but are yet to be explored. We here use scale-bridging simulations supported by gel electrophoresis and mass spectrometry to determine breakage points in collagen. We find collagen crosslinks, as opposed to the backbone, to harbor the weakest bonds, with one particular bond in trivalent crosslinks as the most dominant rupture site. We identify this bond as sacrificial, rupturing prior to other bonds while maintaining the material’s integrity. Also, collagen’s weak bonds funnel ruptures such that the potentially harmful mechanoradicals are readily stabilized. Our results suggest this unique failure mode of collagen to be tailored towards combatting an early onset of macroscopic failure and material ageing

Ligand-Induced Incompatible Curvatures Control Ultrathin Nanoplatelet Polymorphism and Chirality

The ability of thin materials to shape-shift is a common occurrence that leads to dynamic pattern formation and function in natural and man-made structures. However, harnessing this concept to design inorganic structures at the nanoscale rationally has remained far from reach due to a lack of fundamental understanding of the essential physical components. Here, we show that the interaction between organic ligands and the nanocrystal surface is responsible for the full range of chiral shapes seen in colloidal nanoplatelets. The adsorption of ligands results in incompatible curvatures on the top and bottom surfaces of NPL, causing them to deform into helico\"ids, helical ribbons, or tubes depending on the lateral dimensions and crystallographic orientation of the NPL. We demonstrate that nanoplatelets belong to the broad class of geometrically frustrated assemblies and exhibit one of their hallmark features: a transition between helico\"ids and helical ribbons at a critical width. The effective curvature $\bar{\kappa}$ is the single aggregate parameter that encodes the details of the ligand/surface interaction, determining the nanoplatelets' geometry for a given width and crystallographic orientation. The conceptual framework described here will aid the rational design of dynamic, chiral nanostructures with high fundamental and practical relevance.Comment: 16 pages, 8 figure

Effect of Surface Ligands on Colloidal Stability, Shape and Sedimentation of Apolar Nanoparticles

Understanding how nanoparticles interact with one another and their surroundings is critical to controlling their colloidal stability and assembly behaviour, and surface ligands play a vital role in determining the inter-particle forces both during and after synthesis. How- ever, our ability to predict the effect of these molecules on how nanoparticles behave in solution is currently poor. This thesis presents a theoretical study of the effect of surface ligands on the colloidal stability, sedimentation and shape deformation of apolar nanoparticles. In particular, inspired by recent experimental results, we develop models of Au and CdSe nanoparticles coated with apolar ligands in apolar solvents and use molecular dynamics simulations to study the conformational and energetic state of the ligand shell and to characterise the interaction between nanoparticles in solution. This work is divided in two parts. In Part I, we characterise and explain the effect of surface ligands on the colloidal stability of apolar nanoparticles. We show that agglomeration in solution can be induced either by the van der Waals attraction between the cores or the attractive interaction between ordered ligand shells, depending on the particle size. We find that in the shell-dominated case, stability depends strongly on the difference in free energy between the ordered and disordered states of the ligands, being affected by even small changes in ligand and solvent structure. In Part II, we show that ligands can strongly affect other properties of nanoparticles in solution, which we do by studying the sedimentation and shape deformation of apolar CdSe nanoparticles. Overall, our results provide a microscopic description of the forces induced by surface ligands and explain why classical colloid theories often fail to explain the colloidal stability of apolar nanoparticles

Colloidal Stability of Apolar Nanoparticles: The Role of Particle Size and Ligand Shell Structure

Being able to predict and tune the colloidal stability of nanoparticles is essential for a wide range of applications, yet our ability to do so is currently poor due to a lack of understanding of how they interact with one another. Here, we show that the agglomeration of apolar particles is dominated by either the core or the ligand shell depending on the particle size and materials. We do this by using small-angle X-ray scattering and molecular dynamics simulations to characterize the interaction between hexadecanethiol passivated gold nanoparticles in decane solvent. For smaller particles, the agglomeration temperature and interparticle spacing are determined by ordering of the ligand shell into bundles of aligned ligands that attract one another and interlock. In contrast, the agglomeration of larger particles is driven by van der Waals attraction between the gold cores, which eventually becomes strong enough to compress the ligand shell. Our results provide a microscopic description of the forces that determine the colloidal stability of apolar nanoparticles and explain why classical colloid theory fails

Ligand-induced incompatible curvatures control ultrathin nanoplatelet polymorphism and chirality

International audienceThe ability of thin materials to shape-shift is a common occurrence that leads to dynamic pattern formation and function in natural and man-made structures. However, harnessing this concept to rationally design inorganic structures at the nanoscale has remained far from reach due to a lack of fundamental understanding of the essential physical components. Here, we show that the interaction between organic ligands and the nanocrystal surface is responsible for the full range of chiral shapes seen in colloidal nanoplatelets. The adsorption of ligands results in incompatible curvatures on the top and bottom surfaces of the NPL, causing them to deform into helicoïds, helical ribbons, or tubes depending on the lateral dimensions and crystallographic orientation of the NPL. We demonstrate that nanoplatelets belong to the broad class of geometrically frustrated assemblies and exhibit one of their hallmark features: a transition between helicoïds and helical ribbons at a critical width. The effective curvature κ ¯ is the single aggregate parameter that encodes the details of the ligand/surface interaction, determining the nanoplatelets’ geometry for a given width and crystallographic orientation. The conceptual framework described here will aid the rational design of dynamic, chiral nanostructures with high fundamental and practical relevance