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Alexey A Sokol

Publications and source records attributed to Alexey A Sokol.

6 recordsLinked to original sources
PubMed2006

A computational investigation of the different intermediates during organoalkoxysilane hydrolysis.

Using a combination of atomistic molecular dynamics (MD) simulations and density functional theory (DFT) calculations, the four steps of hydrolysis of aminopropyl-, thiolpropyl-, and butyltrimethoxysilane have been studied. Large box MD simulations at constant pressure and temperature yield appropriate pair distribution functions--which allows us to quantify the number of surrounding water molecules--as well as the density of the systems. These densities serve as input for small box DFT calculations, which allow further geometry optimization and calculation of the electronic structure of the systems. The periodic DFT calculations are compared with gas-phase simulations. In all cases, the first step of hydrolysis is exothermic with the extent depending on the type of silane as well as on the number of hydrogen bonds in the initial stage.

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PubMed2006

Adsorption of organosilanes at a Zn-terminated ZnO (0001) surface: molecular dynamics study.

Four different organosilanes (octyltrihydroxysilane, butyltrihydroxysilane, aminopropyltrihydroxysilane, and thiolpropyltrihydroxysilane) adsorbed at a reconstructed Zn-terminated polar ZnO (0001) surface are studied via constant temperature (298 K) molecular dynamics simulations. Both single adsorbed silane molecules as well as adsorbed silane layers are modeled, and the energy, distance, orientation, and alignment of these adsorbates are analyzed. The adsorbed silane molecules exhibit behavior depending on the chemical nature of their tail (nonpolar or polar) as well as on the silane concentration at the solid surface (single adsorption or silane layer). In contrast to the O-terminated ZnO surface studied previously, now adsorption can only occur at the vacancies of this reconstructed crystal surface, thus leading to an arched structure of the liquid phase near the crystal surface. Nevertheless, both nonpolar and polar single adsorbed silanes show a similar orientation and alignment at the surface (orthogonal in the former, parallel in the latter case) as for the O-terminated ZnO surface, although the interaction energy with the surface is considerably increased for nonpolar silanes while it is nearly unaffected for the polar ones. For adsorbed silanes within silane layers, the difference to single adsorbed silanes depends on the polarity of the tail: nonpolar silanes again show an orthogonal alignment, while polar silanes exhibit two different orientations at the solid surface-a head and a tail down configuration. This leads to two completely different but nevertheless stable orientations of these silanes at the Zn-terminated ZnO surface.

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PubMed2005

Hole localization in [AlO4]0 defects in silica materials.

First-principles calculations based on cluster models have been performed to investigate the ground state and the optically excited states of the [AlO(4)](0) hole in alpha-quartz and in the siliceous zeolite ZSM-5. The structure and spectroscopic properties of this defect have been studied using the recently developed Becke88-Becke95 one-parameter model for kinetics (BB1K) functional of Zhao et al., [J. Phys. Chem. A 108, 2715 (2004)]. Our results show that the BB1K method is significantly more reliable and more accurate than the standard density-functional theory (DFT) functionals at reproducing the localized spin density on one oxygen atom and the hyperfine coupling constants associated with the hole. Furthermore, we find that the BB1K results are in close agreement with experiments, and with the self-interaction-free unrestricted Hartree-Fock (UHF) and unrestricted second-order Møller-Plesset perturbation theory (UMP2) calculations. For the first time, we present results of the ground-state paramagnetic properties of the Al defect in ZSM-5. Similar to the theoretical work for defective alpha-quartz, we find that the BB1K, UHF, UHFLee-Yang-Parr, and UMP2 calculations show a localized hole on one oxygen neighboring the Al, while even the best to date thermochemically derived hybrid generalized gradient approximation density-functional, B97-2, predicts a different model where the hole is distributed over two oxygen. We have further considered the optical transitions of the [AlO(4)](0) center in alpha-quartz and ZSM-5. In both systems, our BB1K time-dependent density-functional theory (TDDFT) and configuration interaction singles (CIS) calculations predict that the most likely transition involves electron transfer from the hole-bearing oxygen to other neighboring oxygen ions. This reinforces the experimental conclusions obtained for defective alpha-quartz. Notably, the two lowest, most dominant excitation energies calculated by BB1K-TDDFT (1.99 and 3.03 eV) show excellent agreement with experiment (1.96 and 2.85 eV [B. K. Meyer, J.M. Spaeth, and J.A. Weil, J. Phys. C: Solid State Phys. 17, L31 (1987)]) clearly outperforming the CIS method and other DFT calculations available in the literature.

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PubMed2003

The initial stages of aminosilanol polymerisation.

For polysiloxanes to be used as a protective coating it is important that proton transfer, a trigger to polymerisation, is a facile process. Here we investigate the initial stages of polycondensation and compare different silanol tail groups and the effect of solvent (isopropanol). In the case of (3-aminopropyl)trihydroxysilane we see the potential for self catalysis as the tail group is a proton acceptor, while thiolpropyltrihydroxysilane and isopropanol do not promote proton transfer.

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PubMed2003

Understanding the interface between oxides and metals.

Computational methods based on DFT are applied to modelling structure and bonding in oxide supported metals. We examined three systems of contemporary interest: Cu/ZnO, Pd,Pt/ZrO2 and bimetallic transition metal clusters on silica supports.

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