engleski

Experimental study and molecular dynamics simulation of solvent vapour effects on solid state phase transition in dibenzoylmethane

Addition of a small amount of a liquid often has a catalytic effect on reactions in the solid state. Many solid state reactions have been observed to proceed simply by aging of a mechanical mixture of the reactants in an atmosphere of solvent vapours. The mechanism of the interaction between the solid reactants (and products) and the solvent vapour is therefore of considerable importance for the understanding of the catalytic effect of the added liquid on reactions in the solid state. We have performed a study of the effect of solvent vapour on the disappearance of one phase and the growth of the other. To simplify the approach, solid state phase transitions have been selected as ‘model reactions’. Phase transition was induced in a thin polycrystalline film of a metastable polymorph and the growth of the new phase is observed and measured microscopically. Solid state phase transition of 1, 3-diphenyl-1, 3-propanedione (dbm) has proven to be a particularly good model system as thin films of the metastable phase can be obtained almost perfectly reliably by fast evaporation of a chloroform solution of dbm. Also, the large difference in refraction indices between the two phases makes it simple to observe and measure the growth rate of the new phase, even the specific growth rates of various crystal faces. The results have shown that the growth proceeds usually by formation of large single crystal grains of the stable phase within a polycrystalline matrix of the metastable phase. The transformation rate increases approximately linearly with partial pressure of acetone or chloroform vapours, while water vapour does not have a measurable effect. However, when partial pressures of the solvent vapours are high (close to saturation at a given temperature), the transformation rate grows exponentially and a discernible layer of liquid phase can be noticed between the disappearing and the growing phases. The liquid is only present as an intermediate phase and disappears when the transformation is complete. Molecular dynamics simulations of the two phases in vacuo and in the presence of acetone, as well as of various interfaces between grains of the two phases have shown penetration of acetone into the surface layers which leads to formation of a thin lair of pseudo-solution on the crystal surface which facilitates the transfer of molecules from a crystal of the metastable phase onto a growing crystal of the stable phase.

solid state phase transitions ; crystal growth ; molecular dynamics

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