Modeling and Analysis of the X-ray Powder Diffraction Structure of gamma-Zirconium Phosphates Pillared with Butyl Chains through Molecular Dynamics Simulations

Detailed insights into the structural features of gamma-zirconium phosphate pillared with butyl chains were obtained by molecular dynamics simulations (NPT and NVT type) of models. The available X-ray powder diffraction structure provided reasonable initial conditions (interlayer distances and unit cell parameters) to generate the models submitted to the simulations (timing 1 ns). Using the results of the molecular dynamics trajectories, the X-ray structure of the compound was adequately reproduced by an average of the structural models corresponding to the four accessible orientations of the butyl chains toward the a and b cell axes. This result showed that, in the crystallite, the chains were arranged along the four possible orientations. The predominant structural motif of the molecular assembly in the bulk of pillared gamma-zirconium phosphate butyldiphosphonate was found to be composed of layers laid one upon the other with the pillars of each layer pointing in the same direction, with rare interruptions of this sequence by opposite positioning of the butyl chains of two interfaced layers. The molecular dynamics technique was thus found to be a valid approach to solving otherwise intractable issues concerning structural features of gamma-zirconium phosphate derivatives pillared with aliphatic chains, and it is likely that this promising technique can be extended to a variety of similar compounds.