Andreas Erlebach

Kinetics of Decelerated Melting

Abstract Melting presents one of the most prominent phenomena in condensed matter science. Its microscopic understanding, however, is still fragmented, ranging from simplistic theory to the observation of melting point depressions. Here, a multimethod experimental approach is combined with computational simulation to study the microscopic mechanism of melting between these two extremes. Crystalline structures are exploited in which melting occurs into a metastable liquid close to its glass transition temperature. The associated sluggish dynamics concur with real‐time observation of homogeneous melting. In‐depth information on the structural signature is obtained from various independent spectroscopic and scattering methods, revealing a step‐wise nature of the transition before reaching the liquid state. A kinetic model is derived in which the first reaction step is promoted by local instability events, and the second is driven by diffusive mobility. Computational simulation provides further confirmation for the sequential reaction steps and for the details of the associated structural dynamics. The successful quantitative modeling of the low‐temperature decelerated melting of zeolite crystals, reconciling homogeneous with heterogeneous processes, should serve as a platform for understanding the inherent instability of other zeolitic structures, as well as the prolific and more complex nanoporous metal–organic frameworks.

Ab Initio energetics of SiO bond cleavage

A multilevel approach that combines high‐level ab initio quantum chemical methods applied to a molecular model of a single, strain‐free SiOSi bridge has been used to derive accurate energetics for SiO bond cleavage. The calculated SiO bond dissociation energy and the activation energy for water‐assisted SiO bond cleavage of 624 and 163 kJ mol−1, respectively, are in excellent agreement with values derived recently from experimental data. In addition, the activation energy for H2O‐assisted SiO bond cleavage is found virtually independent of the amount of water molecules in the vicinity of the reaction site. The estimated reaction energy for this process including zero‐point vibrational contribution is in the range of −5 to 19 kJ mol−1.

Thermodynamic compatibility of actives encapsulated into PEG‐PLA nanoparticles: In Silico predictions and experimental verification

Achieving optimal solubility of active substances in polymeric carriers is of fundamental importance for a number of industrial applications, including targeted drug delivery within the growing field of nanomedicine. However, its experimental optimization using a trial‐and‐error approach is cumbersome and time‐consuming. Here, an approach based on molecular dynamics (MD) simulations and the Flory–Huggins theory is proposed for rapid prediction of thermodynamic compatibility between active species and copolymers comprising hydrophilic and hydrophobic segments. In contrast to similar methods, our approach offers high computational efficiency by employing MD simulations that avoid explicit consideration of the actual copolymer chains. The accuracy of the method is demonstrated for compatibility predictions between pyrene and nile red as model dyes as well as indomethacin as model drug and copolymers containing blocks of poly(ethylene glycol) and poly(lactic acid) in different ratios. The results of the simulations are directly verified by comparison with the observed encapsulation efficiency of nanoparticles prepared by nanoprecipitation.

Parametrization in Models of Subcritical Glass Fracture: Activation Offset and Concerted Activation

There are two established but fundamentally different empirical approaches to parametrize the rate of subcritical fracture in brittle materials. While both are relying on a thermally activated reaction of bond rupture, the difference lies in the way as to how the externally applied stresses affect the local energy landscape. In the consideration of inorganic glasses, the strain energy is typically taken as an off-set on the activation barrier. As an alternative interpretation, the system’s volumetric strain-energy is added to its thermal energy. Such an interpretation is consistent with the democratic fiber bundle model. Here, we test this approach of concerted activation against macroscopic data of bond cleavage activation energy, and also against ab initio quantum chemical simulation of the energy barrier for cracking in silica. The fact that both models are able to reproduce experimental observation to a remarkable degree highlights the importance of a holistic consideration towards non-empirical understanding.

Different Reactivity of As4 towards Disilenes and Silylenes

The activation of yellow arsenic is possible with the silylene [PhC(NtBu)2 SiN(SiMe3 )2 ] (1) and the disilene [(Me3 Si)2 N(η1 -Me5 C5 )Si=Si(η1 -Me5 C5 )N(SiMe3 )2 ] (3). The reaction of As4 with 1 leads to the unprecedented As10 cage compound [(LSiN(SiMe3 )2 )3 As10 ] (2; L=PhC(NtBu)2 ) with an As7 nortricyclane core stabilized by arsasilene moieties containing silicon(II)bis(trimethylsilyl)amide substituents. In contrast, the compound [Cp*{(SiMe3 )2 N}SiAs]2 (4) containing a butterfly-like diarsadisilabicyclo[1.1.0]butane unit is formed by the reaction of As4 with the disilene 3. Both compounds were characterized by single-crystal X-ray diffraction analysis, NMR spectroscopy, and mass spectrometry. The reaction outcomes demonstrate the different reaction behavior of yellow arsenic (As4 ) compared to white phosphorus (P4 ) in the reactions with the corresponding silylenes and disilenes.

Structure Prediction of Rare Earth Doped BaO and MgO Containing Aluminosilicate Glasses–the Model Case of Gd2O3

The medium-range atomic structure of magnesium and barium aluminosilicate glasses doped with Gd2O3 as a model rare earth oxide is elucidated using molecular dynamics simulations. Our structure models rationalize the strong dependence of the luminescence properties of the glasses on their chemical composition. The simulation procedure used samples’ atomic configurations, the so-called inherent structures, characterizing configurations of the liquid state slightly above the glass transition temperature. This yields medium-range atomic structures of network former and modifier ions in good agreement with structure predictions using standard simulated annealing procedures. However, the generation of a large set of inherent structures allows a statistical sampling of the medium-range order of Gd3+ ions with less computational effort compared to the simulated annealing approach. It is found that the number of Si-bound non-bridging oxygen in the vicinity of Gd3+ considerably increases with growing ionic radius and concentration of network-modifier ions. In addition, structure predictions indicate a low driving force for clustering of Gd3+, yet no precise correlation between the atomic structure and luminescence lifetimes can be conclusively established. However, the structure models provided in this study can serve as a starting point for future quantum mechanical simulations to shed a light on the relation between the atomic structure and optical properties of rare earth doped aluminosilicate glasses.

WO3 as a nucleating agent for BaO/SrO/ZnO/SiO2 glasses – experiments and simulations

Recently, it has been shown that the Ba1−xSrxZn2Si2O7 crystal phase has a negative coefficient of thermal expansion. However, technological applications of this material as low thermal expansion glass ceramics are limited by the undesired surface crystallization of the corresponding glass. Surface nucleation, however, can be turned into bulk nucleation by adding nucleating agents. Here, glasses in the base system BaO–SrO–ZnO–SiO2 with small additions of ZrO2 and WO3 were synthesized and their crystallization behavior was investigated using thermal analysis, X-ray diffraction, electron microscopy, and density functional theory simulations. The addition of WO3 leads to the formation of volume crystals with a scheelite crystal structure (Ba1−xSrxWO4) in high number density. The limited incorporation of Si4+ ions into these crystals is discussed. Possible crystal lattice sites for Si4+ were located by density functional theory simulations. In a much lower number density, crystals with a crystal structure similar to the high-temperature polymorph of BaZn2Si2O7 and the Ba1−xSrxZn2Si2O7 composition crystallize as well. These crystals can reach a larger size than the scheelite crystals that occur in parallel. The overall microstructure is thus formed by small dendritic crystals with a scheelite structure and huge Ba1−xSrxZn2Si2O7 crystals. Both of them seem to grow independently of each other. In spite of the high anisotropy of both phases, the microstructure is revealed to be free of cracks.