Incorporation mechanism of guest molecules in crystals: solid solution or inclusion?

Guest molecules (impurities or additives), together with some crystallization solvent, are often incorporated into the host crystals during crystallization from solution. The guest molecules may be incorporated either in solid solution or in liquid inclusions, or by both mechanisms. The mechanism of guest incorporation has been examined by a simple calculation method which is based on the equality of the guest/solvent mole ratio in the initial crystallization medium and in the putative inclusions. Application of this calculation method to eight guest+host systems described in the literature has shown that a negligible amount (at most 0.2%) of the guest molecules is incorporated into the crystal lattice in liquid inclusions. Therefore, it is concluded that the vast majority of the guest molecules are incorporated into the crystals in solid solution, as previously suggested, but hitherto unproven, for these guest-host systems.