Impact of Substrate Properties on the Formation of Spherulitic Films: A Case Study of Salbutamol Sulfate

Spherulitic assemblies have applications as carriers for drug delivery and as targeting vectors. Presently, the driving force behind the formation of spherulites comprising small drug molecules is not fully understood. Herein, the impact of different substrate types on spherulitic crystallization of salbutamol sulfate (SS) was investigated. Freshly cleaved mica, silicon (111) wafer (SiW), uncoated borosilicate glass (UG), silane coated glass (CG), and stainless steel (MS) were used as substrates. It was demonstrated that the spherulite growth can be controlled via the substrate selection. Spherulite formation was inhibited on hydrophilic substrates, such as mica, possibly due to stronger intermolecular interactions between SS and the substrate than SS-SS interactions. Contact angle measurements established that mica possessed the lowest contact angle (15.1 +/- 0.5 degrees), and the values of this parameter increased in the order of UC < SiW < CG < MS. Substrate roughness also played a key role in controlling spherulite formation. SiW, UC, and CG had isotropic surfaces with low average roughness, facilitating spherulite formation. Overall, this work demonstrates that it is possible to successfully produce SS spherulites using a single step process at room temperature. Furthermore, the formation of SS spherulites can be tuned by the hydrophobicity of the substrate, an approach that could be applied to assembling spherulites of other small organic molecules.