Atomistic Simulation Study of Calcium,Phosphate and Fluoride Ion Association to the Teleopeptide‐Tails of Collagen – Initial Steps to Biomineral Formation

The attachment of single ions to putative adsorption sites in the tails of collagen fibers is investigated by means of molecular dynamics simulations and discussed with respect to the very early steps of apatite/collagen biomineral formation. Our studies clearly demonstrate an increased flexibility of the tails of the triple‐helical collagen protein. Apart from the termini of the backbone, several side chains were also observed to be freely accessible to ion attachment from aqueous solution. The teleopeptide was systematically scanned for suitable adsorption sites for calcium, phosphate and fluoride ions. Association of these ions was then explored from potential of mean force calculations. The resulting energy profiles reveal a variety of favorable protein‐ion bonds and hint at the suitability of the collagen tails to promote apatite aggregation.     

Shortcut method for evaluation and design of a hybrid process for enantioseparations

Hybrid processes for enantioseparations have a considerable potential for reducing investment and operational costs. An example is the combination of simulated moving bed (SMB) chromatography and selective crystallisation. However, the design of integrated processes is a difficult task. A shortcut method is presented that can serve as a tool for design and estimation of the potential of such processes. The approach requires only limited experimental data and thus allows for systematic parameter studies. The method is based on the determination of the purity-performance characteristic of the SMB process and rigorous application of mass balances. The use of relative mass fluxes allows derivation of simple algebraic expressions for essential process parameters. The significant potential of combining SMB and crystallisation is demonstrated for the example of the separation of mandelic acid enantiomers.     

Surface electronic properties of sulfur-treated GaAs determined by surface photovoltage measurement and its computer simulation

The Kelvin method together with the simulations of surface photovoltage has been used to determine the surface electronic properties, i.e. the surface band bending (qV_S), surface state density (N_SS0) and surface fixed charge (Q_Fx) of S₂Cl₂-treated GaAs (100) surfaces. The measured values of surface photovoltage (SPV) do not show saturation at high photon flux densities in contradiction to the simple theory of SPV. This behavior of SPV agrees very well with the rigorous computer simulations and can be explained in terms of the Dember effect. Moreover, the SPV values become insensitive to surface states at moderate photon flux densities. On this basis, the surface band bending of untreated (0.79 eV) and S₂Cl₂-treated (0.60 eV) GaAs surfaces was determined. The band diagrams summarizing the obtained results proved the influence on the potential variations not only from the ionized surface states and surface fixed charge but also from the surface dipole layer on the S₂Cl₂-treated GaAs surface. The dipole arises most probably due to the S-Ga bonding on the surface. The presented results offer an alternative explanation for increased PL commonly observed after the sulfidation in the absence of substantial reduction in the band bending.     

Atomistic Simulation Study of Cu0.327Ni0.673 Alloys: from Solid Solution to Phase Segregation

We present a combined Monte‐Carlo/molecular dynamics study of a Cu_0.327Ni_0.673 alloy system. On the basis of nearest‐neighbor coordination number analyses atomic clustering and phase segregation is explored. Along this line, free energy profiles are calculated and separated into entropic and energetic contributions. The competition of both terms was found in accordance to the experimental phase diagrams (phase separation of the solid solution below about 600 Kelvin). Two independent simulation runs were performed. At 1000 Kelvin the observed configurations correspond to solid solutions exhibiting a weak tendency to cluster atoms of identical species. At room temperature the energetic favoring of atomic separation is clearly dominant and leads to the formation of Ni‐rich and Cu‐rich domains. The latter are separated by interfacial regions whose width ranges from 0.5 to 1 nanometers.     

Motif Reconstruction in Clusters and Layers: Benchmarks for the Kawska–Zahn Approach to Model Crystal Formation

A recently developed atomistic simulation scheme for investigating ion aggregation from solution is transferred to the morphogenesis of metal clusters grown from the vapor and layers deposited on a substrate surface. Both systems are chosen as benchmark models for intense motif reorganization during aggregate/layer growth. The applied simulation method does not necessarily involve global energy minimization after each growth event, but instead describes crystal growth as a series of structurally related configurations which may also include local energy minima. Apart from the particularly favorable high‐symmetry configurations known from experiments and global energy minimization, we also demonstrate the investigation of transient structures. In the spirit of Ostwald’s step rule, a continuous evolution of the aggregate/layer structure during crystal growth is observed.