Separation of Enantiomers through Local Vorticity: A Screw Model Mechanism

We present a model to explain the mechanism behind enantiomeric separation under either shear flow or local rotational motion in a fluid. Local vorticity of the fluid imparts molecular rotation that couples to translational motion, sending enantiomers in opposite directions. Translation-rotation coupling of enantiomers is explored using the molecular hydrodynamic resistance tensor, and a molecular equivalent of the pitch of a screw is introduced to describe the degree of translation-rotation coupling. Molecular pitch is a structural feature of the molecules and can be easily computed, allowing rapid estimation of the pitch of 85 drug-like molecules. Simulations of model enantiomers in a range of fluids such as $\Lambda$- and $\Delta$-Ru(bpy)_3]Cl_2 in water and (R,R)- and (S,S)-atorvastatin in methanol support predictions made using molecular pitch values.A competition model and continuum drift diffusion equations are developed to predict separation of realistic racemic mixtures. We find that enantiomeric separation on a centimeter length scale can be achieved in hours, using experimentally-achievable vorticities. Additionally, we find that certain achiral objects can also exhibit a non-zero molecular pitch

Thermal Transport in Citrate-Capped Gold Interfaces using a Polarizable Force Field

The interfacial thermal conductance from solvated gold nanostructures capped with sodium citrate was determined using reverse nonequilibrium molecular dynamics (RNEMD) methods. The surfaces of spherical nanoparticles and the (111) surfaces of fcc gold slabs were modeled using the density readjusting embedded atom method (DR-EAM) as well as with the standard embedded atom method (EAM), and the effects of polarizability on the binding preferences of citrate were determined. We find that the binding configurations of citrate depend significantly on gold surface curvature and are not strongly influenced by surface polarizability. The interfacial thermal conductance was also determined for the spherical nanoparticles and (111) surfaces, and we find that applying DR-EAM increases the interfacial thermal conductance for systems with spherical nanoparticles much more sharply than for systems with (111) surfaces. Through analysis of excess charge density near the interface, we find that inclusion of polarizability has a larger impact on image charge creation in nanospheres than it does for the planar (111) interfaces. This effectively increases the interaction strength to polar species in the solvent, yielding larger interfacial thermal conductance estimates for the nanospheres

Thermal Transport is Influenced by Nanoparticle Morphology: A Molecular Dynamics Study

Molecular dynamics simulations were performed to model the interfacial thermal conductance (<i>G</i>) from bare gold nanoparticles (icosahedral, cuboctahedral, and spherical) to a hexane solvent. The computed conductance was found to depend not only on particle shape, but also on the size of the nanoparticles, particularly for nanospheres. These results are compared with conductance out of the planar facets: (111), (100), and (110); all commonly exhibited in small patches on the spherical particles. Undercoordination of the surface atoms and the vibrational density of states in the icosahedra explain some of these observations. The exposed surfaces of icosahedral particles are dominated by (111) facets with 9-coordinated gold atoms. Cuboctahedral particles are dominated by the (100) and (111) facets with 8- and 9-coordinated surface atoms, respectively. The nanospheres approach a constant surface density of 6–9 coordinated sites at large particle sizes, and these surface atoms play a large role in the conductance to the solvent. The surface-normal vibrational densities of states were used to explain a simple surface undercoordination model, which recovers most of the contributions to the size-dependent conductance

Why is Ice Slippery? Simulations of Shear Viscosity of the Quasi-Liquid Layer on Ice

The temperature and depth dependence of the shear viscosity (η) of the quasi-liquid layer (QLL) of water on ice-I_h crystals was determined using simulations of the TIP4P/Ice model. The crystals display either the basal {0001} or prismatic {101̅0} facets, and we find that the QLL viscosity depends on the presented facet, the distance from the solid/liquid interface, and the undercooling temperature. Structural order parameters provide two distinct estimates of the QLL widths, which are found to range from 6.0 to 7.8 Å, and depend on the facet and undercooling temperature. Above 260 K, the viscosity of the vapor-adjacent water layer is significantly less viscous than the solid-adjacent layer and is also lower than the viscosity of liquid water

Friction at Ice‑I_h/Water Interfaces Is Governed by Solid/Liquid Hydrogen-Bonding

Nonequilibrium molecular dynamics simulations of solid/liquid friction at ice/water interfaces suggest that the surface density of solid to liquid hydrogen bonds directly correlates with interfacial friction. The basal {0001}, prismatic {101̅0}, pyramidal {202̅1}, and secondary prism {112̅0} facets of ice-I_h were drawn through liquid water with a momentum flux between the solid and liquid phases. Solid to liquid hydrogen bonds were identified using local tetrahedral ordering of the water molecules. An expression for friction coefficients appropriate for negative slip boundary conditions is presented, and the computed friction of these interfaces is found to be invariant to the shear rate and direction of shear relative to the surface features. Structural and dynamic interfacial widths for all four facets were found to be similar (6.6–9.5 Å structural and 9–15 Å dynamic) and are largely independent of the shear rate and direction. Differences in the solid to liquid hydrogen bond density are explained in terms of surface features of the four facets. Lastly, we present a simple momentum transmission model using the density of solid/liquid hydrogen bonds, the shear viscosity of the liquid, and the structural width of the interface