Effects of ionic impurities on the crystal morphology of phosphoric acid hemihydrate

The adsorption of foreign molecules on the crystal surface is generally believed to contribute to modify crystal morphology and the different adsorption energies on different crystal faces may lead to different modifications of crystal morphologies. In order to obtain a deep understanding of the face growth intervening mechanism, phosphoric acid hemihydrate (H₃PO₄·0.5H₂O) is taken as an example to investigate the influence of several typical ionic impurities on its crystal morphology by applying a molecular simulation study using Cerius 2 software. The absolute values of the calculated binding energies for cations are larger than that for anions. Therefore, it is concluded from the simulation that cations inhibit the growth of some crystal faces, while anions has no significant inhibition to a particular crystal surface, which is in good agreement with the experimental observation. It is also noticed that significant differences of binding energies to the different crystal faces will lead to obvious modifications of crystal morphologies and higher charged cations have more obvious effects than lower charged cations on the crystal morphology modification.