Heiko Briesen

Restructuring of colloidal aggregates in shear flow Coupling interparticle contact models with Stokesian dynamics

A method to couple interparticle contact models with Stokesian dynamics (SD) is introduced to simulate colloidal aggregates under flow conditions. The contact model mimics both the elastic and plastic behavior of the cohesive connections between particles within clusters. Owing to this, clusters can maintain their structures under low stress while restructuring or even breakage may occur under sufficiently high stress conditions. SD is an efficient method to deal with the long-ranged and many-body nature of hydrodynamic interactions for low Reynolds number flows. By using such a coupled model, the restructuring of colloidal aggregates under shear flows with stepwise increasing shear rates was studied. Irreversible compaction occurs due to the increase of hydrodynamic stress on clusters. Results show that the greater part of the fractal clusters are compacted to rod-shaped packed structures, while the others show isotropic compaction.Graphical abstract

Insights into Pharmaceutical Nanocrystal Dissolution: A Molecular Dynamics Simulation Study on Aspirin

The presented molecular dynamics simulations are the first simulations to reveal dynamic dissolution of a pharmaceutical crystal in its experimentally determined shape. Continuous dissolution at constant undersaturation of the surrounding medium is ensured by introducing a plane of sticky dummy atoms into the water slab. These atoms have a strong interaction potential with dissolved aspirin molecules, but interactions with water are excluded from the calculations. Thus, the number of aspirin molecules diffusing freely in solution is kept at a low value and continuous dissolution of the aspirin crystal is monitored. Further insight into face-specific dissolution is drawn. The dissolution mechanism of receding edges is found for the (001) plane. These findings are in good agreement with experimental results. While the proposed dissolution mechanism for the (100) plane is terrace sinking on a rough surface, no pronounced dissolution of the perfectly flat face is seen in the present work. Molecular simulations of pharmaceuticals in their experimentally obtained structure therefore have shown to be especially suited for the investigation of dissolving faces, where the edges have a pronounced effect. In contrast to previous studies a propagation of the dissolution front into the crystal face is reported, and the crystal bulk is stable over the whole simulation time of 150 ns.

Temperature- and pressure-dependent densities, self-diffusion coefficients, and phase behavior of monoacid saturated triacylglycerides: toward molecular-level insights into processing.

Using molecular-dynamics (MD) simulations the densities and self-diffusion coefficients of a range of liquid monoacid triacylglycerides (TAGs) have been studied as a function of temperature and, for the first time, pressure. While offset by their ambient properties, the response of the TAGs to temperature and pressure is qualitatively similar. Application of pressure was found to significantly increase densities and reduce diffusion of the TAG molecules, suggesting that it may have as much a role in processing and crystallizing TAGs as supercooling does. A solution of glycerol tripalmitate and glycerol trihexanoate was also studied, showing that application of pressure should lead to a significant decrease in the saturation point of the solution, which is an important consideration for processing TAGs. Different solid/liquid interfaces of glycerol tripalmitate have also been investigated. Although crystal growth could not be observed, dissolution of one interface was seen in the MD simulations. The results suggest that over moderate distances the melting of TAGs may be cooperative in nature, rather than involving dissolution of individual TAG molecules.

Simulating preferential sorption of tartrate on prismatic calcite surfaces

Understanding the influence of additives on crystal growth is required to engineer the crystal properties according to their functional applications. In this work, the sorption behavior of tartrate on calcite surfaces is investigated employing molecular dynamics simulations to understand additive-mediated crystal growth. The free energy landscapes for the sorption of tartrate are calculated using metadynamics. The adsorption binding energies of favorable conformations, orientations and positions of tartrate near the (104) and (1−10) calcite surfaces are determined. The obtained results provide a molecular-level explanation of the experimentally observed tartrate-stabilized exposure of prismatic {1−10} faces during calcite growth. The simulations show that tartrate preferentially adsorb directly to the (1−10) calcite surface, whereas tartrate is more loosely adsorbed on the (104) surface, mainly by solvent-mediated binding. The (1−10) geometry of calcite surface sites closely matches the structure of tartrate, with a specific role of carboxylate and hydroxyl groups in recognizing the calcium and carbonate ions, respectively. Two stable adsorption configurations are identified for the (1−10) face: (1) adsorbed tartrate with the effect of surface-induced conformational change and (2) incorporated tartrate into the surface by fitting one of the carboxylate groups into lattice position normally occupied by carbonate ions and additionally stabilized by binding of both hydroxyl groups to neighboring carbonate ions. The results indicate that surface energetics, structural matching and adsorbed water layer play a major role in the strength of the interactions and hence in the expression of calcite morphology. Preferential adsorption of tartrate on {1−10} surfaces could stabilize these otherwise fast-growing faces and thus inhibit crystal growth in {1−10} directions.

Data Filtering for Effective Analysis of Crystal–Solution Interface Molecular Dynamics Simulations

Analysis of processes occurring at the solid-solution interface during crystal growth and dissolution simulations requires an effective way to detect rare, uncorrelated transitions from the liquid to the solid state or vice versa. Because of the oscillatory behavior of molecules, this is not a trivial problem. Usually, to take the thermal vibration and rotational flexibility of the molecules into account, the data (e.g., orientation, center of mass position) needed to determine the molecular state are averaged over some time interval. Then they are evaluated using some order parameters to classify the individual molecules as being either crystalline or in solution. In this case, the results can be very sensitive to the time interval, which is mostly chosen in some heuristic way. To suppress the problem of fast non-Markovian dynamics and to make the identification of the molecular state more reliable and robust, the application of a Kalman filter, optionally combined with a hysteretic approach, is proposed in this contribution. A scheme to estimate the filter parameters is introduced. To demonstrate the approach, simple and widely used order parameters based on the structural features of molecules are taken. The obtained results are clearly superior to those based on the data averaging technique and are important for the effective transition rates calculation as well as for the general analysis of the time evolution of interfaces.

Deep eutectic solvent formation: a structural view using molecular dynamics simulations with classical force fields

ABSTRACT Deep eutectic solvents (DES) have gained a reputation as inexpensive and easy to handle ionic liquid analogues. This work employs molecular dynamics (MD) to simulate a variety of DES. The hydrogen bond acceptor (HBA) choline chloride was paired with the hydrogen bond donors (HBD) glycerol, 1,4-butanediol, and levulinic acid. Levulinic acid was also paired with the zwitterionic HBA betaine. In order to evaluate the reliability of data MD simulations can provide for DES, two force fields were compared: the Merck Molecular Force Field and the General Amber Force Field with two different sets of partial charges for the latter. The force fields were evaluated by comparing available experimental thermodynamic and transport properties against simulated values. Structural analysis was performed on the eutectic systems and compared to non-eutectic compositions. All force fields could be validated against certain experimental properties, but performance varied depending on the system and property in question. While extensive hydrogen bonding was found for all systems, details about the contribution of individual groups strongly varied among force fields. Interaction potentials revealed that HBA–HBA interactions weaken linearly with increasing HBD ratio, while HBD–HBD interactions grew disproportionally in magnitude, which might hint at the eutectic composition of a system.

Temperature profile optimization: potential for multi-enzymatic biopolymer depolymerization processes

Optimal control of temperature was applied to a population balance model of enzymatically catalyzed depolymerization of a soluble polymer coupled with denaturation of enzyme. The reaction time required to reach a desired yield was predicted to be reduced by more than 10

Dosage of Filter Aids in the Case of Pure Surface Filtration – An Optimal Control Approach

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