Bridging the gap between molecular dynamics and hydrodynamics in nanoscale Brownian motions

Drag on nanoparticles in a liquid: from slip to stick boundary conditions

Stokes' law with stick boundary conditions has been widely accepted for the transport of microscale particles in a liquid. For nanoparticles, however, the hydrodynamic boundary conditions become unclear. In this work, the drag force acting on nanoparticles suspended in a liquid and the hydrodynamic boundary coefficient were calculated by using molecular dynamics simulations. For weak interfacial couplings, slip boundary conditions can be used to describe the particle transport, whereas at strong interfacial couplings, the hydrodynamic boundary coefficient converges to a value greater than the prediction by Stokes' law. In the present paper, we propose a density accumulation length to determine the effective particle size, which makes Stokes' law valid for nanoparticles. For a copper nanoparticle suspended in an argon liquid, the density accumulation length increases to 0.32 nm with increasing solid-liquid coupling strength. Furthermore, it is found that there exists a transition from slip to stick boundary conditions as the solid-liquid intermolecular coupling strength increases. The results presented in this work provide guidance for the prediction and manipulation of the transport properties of nanoparticles in a liquid.

Predicting the Geometry of Core-Shell Structures: How a Shape Changes with Constant Added Thickness

The core-shell assembly motif is ubiquitous in chemistry. While the most obvious examples are core/shell-type nanoparticles, many other examples exist. The shape of the core/shell constructs is poorly understood, making it impossible to separate chemical effects from geometric effects. Here, we create a model for the core/shell construct and develop proof for how the eccentricity is expected to change as a function of the shell. We find that the addition of a constant thickness shell always creates a relatively more spherical shape for all shapes covered by our model unless the shape is already spherical or has some underlying radial symmetry. We apply this work to simulated AOT reverse micelles and demonstrate that it is remarkably successful at explaining the observed shapes of the chemical systems. We identify the three specific cases where the model breaks down and how this impacts eccentricity.

Elucidating the Hydrotropic Mechanism of the Antagonistic Salt PPh4Cl

PPh₄Cl is an antagonistic salt that recently showed promise as a hydrotropic agent. Here, we give mechanistic insights into the PPh₄Cl-assisted solubility of a dye molecule using molecular dynamics simulations. Our findings reveal that dye molecules aggregate into a cluster which leads to an accumulation of PPh₄⁺ ions in its vicinity and subsequent exclusion of water molecules from the region. The structural organization is attributed to the preferential interaction of dye molecules and PPh₄Cl. The origin of such preference arises from the difference in π-π and CH-π interaction among the pairs. The hydrodynamic radius of PPh₄Cl indicates a low propensity for cluster formation, which enhances its hydrotropic behavior. The process of dye dissolution is thermodynamically favored and occurs through a cooperative mechanism. Our studies provide molecular insight into experimental observations crucial for the design of novel hydrotropes with enhanced solubilizing properties.

Short-time dynamics of a tracer in an ideal gas

A small tagged particle immersed in a fluid exhibits Brownian motion and diffuses on a long timescale. Meanwhile, on a short timescale, the dynamics of the tagged particle cannot be simply described by the usual generalized Langevin equation with Gaussian noise, since the number of collisions between the tagged particle and fluid particles is rather small. On such a timescale, we should explicitly consider individual collision events between the tagged particle and the surrounding fluid particles. In this study we analyze the short-time dynamics of a tagged particle in an ideal gas, where we do not have static or hydrodynamic correlations between fluid particles. We perform event-driven hard-sphere simulations and show that the short-time dynamics of the tagged particle is correlated even under such an idealized situation. Namely, the velocity autocorrelation function becomes negative when the tagged particle is relatively light and the fluid density is relatively high. This result can be attributed to the dynamical correlation between collision events. To investigate the physical mechanism which causes the dynamical correlation, we analyze the correlation between successive collision events. We find that the tagged particle can collide with the same ideal-gas particle several times and such collisions cause a strong dynamical correlation for the velocity.

Multivalent Polymer-Peptide Conjugates-A General Platform for Inhibiting Amyloid Beta Peptide Aggregation

Protein aggregation is implicated in multiple deposition diseases including Alzheimer's Disease, which features the formation of toxic aggregates of amyloid beta (Aβ) peptides. Many inhibitors have been developed to impede or reverse Aβ aggregation. Multivalent inhibitors, however, have been largely overlooked despite the promise of high inhibition efficiency endowed by the multivalent nature of Aβ aggregates. In this work, we report the success of multivalent polymer-peptide conjugates (mPPCs) as a general class of inhibitors of the aggregation of Aβ40. Significantly delayed onset of fibril formation was realized using mPPCs prepared from three peptide/peptoid ligands covering a range of polymer molecular weights (MWs) and ligand loadings. Dose dependence studies showed that the nature of the ligands is a key factor in mPPC inhibition potency. The negatively charged ligand LPFFD (LD) leads to more efficient mPPCs compared to the neutral ligands, and is most effective at 7% ligand loading across different MWs. Molecular dynamics simulations along with dynamic light scattering experiments suggest that mPPCs form globular structures in solution due to ligand-ligand interactions. Such interactions are key to the spatial proximity of ligands and thus to the multivalency effect of mPPC inhibition. Excess ligand-ligand interactions, however, reduce the accessibility of mPPC ligands to Aβ peptides, and impair the overall inhibition potency.