Molecular Simulations of Hydrotalcite Intercalated with Pyrenetetrasulfonate
Marek Veteška, Miroslav Pospíšil
Ke Karlovu 3, 121 16
In this work hydrotalcite-like structures intercalated by pyrenetetrasulfonate anions were calculated by methods of molecular simulations (i.e. the molecular mechanics and the classical molecular dynamics). The hydrotalcites are layered materials with anions and water molecules which are held between layers only by nonbond interactions. Molecular simulations are based on empirical force fields intended for calculations of optimized structures with minimal potential energy.
Based on experimental data (X-ray diffraction and thermogravimetry) rough initial models were build for 4 different distances between the hydrotalcite layers with 2 pyrenetetrasulfonate anions in the first interlayer. About 4*10 different suitable initial models destined for the molecular mechanics were sequentially selected from roughly 4*12 billion possible models by systematic scanning an entire state space (algorithm Supramol). The experimentally determined amount of water molecules was randomly inserted into relevant models. After various techniques of calculations (molecular mechanics with free cell parameters, using a pressure, various techniques of charge calculation, systematic bending of sulfonates, various initial position of water molecules) it appeared as optimal to calculate with rigid hydrotalcite layers and cell parameters and with charges calculated separately for every molecule and layer. Arrangement of water was refined by subsequent molecular dynamics.
The sample with interlayer distance 9,83 Å and 0 water molecules. The resulting energy of models with anions positioned in one layer parallel with hydrotalcite layers or with anions slightly tilted were very similar. It can be assumed that all these calculated models could appear in the real sample or arrangement of molecules in the interlayer is slightly variable.
The sample with interlayer distance 13,63 Å and 36 water molecules. The resulting energy of models with anions situated in 2 horizontal layers approximately in the thirds of the interlayer, variously turned, shifted and tilted were very similar, too. Thus arrangement of molecules in the interlayer is again slightly variable. The water molecules form three layers, first one in the middle, two ones in the quarters of the interlayer.
The sample with two interlayer distances 11,74 Å and 12,81 Å and 25 water molecules. On the base of calculations it seems to be the most probable that the measured sample were not stabilized and therefore it appears as two-phase.
This work was supported from Ministry of Education of the Czech Republic MSM 0021620835 and GAČR 203/05/2306.