Crystal engineering of pharmaceutical solid solutions

Solid solutions enable the fine tuning of crystalline materials’ properties by adjusting the composition of their components. They can be applied to several classes of materials with a wide range of applications but are difficult to prepare. A general guidelines for the preparation of solid solutions were summarised by Hume-Rothery (1926) and Kitaigorodsky (1973, 1984). As the use of solid solution in the pharmaceutical field is still understudied, the present work describes several research themes which consist in the formation of novel solid solutions of APIs. They are lithium in salts and in coordination polymers, organic compound (steroids), and metal complexes. One of the aims of the present work is to verify the aforementioned guidelines of solid solution formations. From this perspective, they are chosen compounds that present challenges in their structure to be made as solid solution, according to the conventional rules. From the pharmaceutical aspect, solid solutions can aid in the preparation of pharmaceutical materials with desirable properties, by varying the ratios of components in continuum or at least to a certain extent. The successful syntheses of Li/Na mixed salts, Li coordination polymers, and steroid solid solutions demonstrate that solid solutions can form amongst non-isostructural molecules and pseudo-isomorphous pairs. It is also observed that the existence of more stable forms may hinder solid solution formation despite the isomorphous nature of the parent structures.