The understanding and control of crystallographic polymorphism and crystal habit of organic as well as inorganic compounds is scientifically and technologically important to a number of industries. To date, however, the experimental control of polymorphs (crystalline solids with different arrangements of the same constituents) is difficult. Since a polymorph is determined at the nucleation of a crystal, methods that lead to an advanced understanding of early crystal formation pathways and mechanisms are highly desirable. Towards this aim, in this project we employ arrays of self-assembled monolayers (SAMs).
Self-assembled monolayers (SAMs) are well-defined surfaces that can be used to study the relationship between the nucleation event and the final polymorph selection. Furthermore, by tuning the substrate-crystal interface energy, potentially crystalline order of SAMs can promote the nucleation of polymorphs not accessible via solution methods. It is these two advantages, i.e. the establishment of scientific correlations between nucleation and observed polymorph and access to polymorphs not accessible via solution methods, that have led us in this project to choose heterogeneous surface nucleation via SAMs as the primary means to study polymorph selection.
In the second-year of work, we examined 1-undecanethiol (UDT) and 11-mercapto- 1-undecanol (MUOH) SAM chemistries on gold, and trichloro(octadecyl)silane (OTS) and trichloro(phenyl)silane (PTS) on oxide bearing silicon substrates in the presence of various solvent systems to investigate their ability to influence the nucleation, polymorph selection and crystal growth of acetaminophen (ACM). We found that for evaporating solvent from a single droplet on SAMs, both solvent(s) and SAM substrate work together to control crystal polymorph selection. On hydrophobic surfaces (UDT, OTS, PTS), use of pure solvents resulted in ACM form I (monoclinic), while a mixture of water and dioxane produced form II (orthorhombic). In addition to polymorph selection, under these conditions we found that for form II different SAM surface chemistries influence crystal orientation. Finally, by introducing a doctor-blading process, for first experiments of PTS SAMs on a silicon waver, polymorph selectivity could be achieved varying the solvent from 1,4-dioxane to ethanol. This opens the door to similar experiments at the Cornell High Energy Synchrotron Source (CHESS) in the third-year period. By studying at CHESS the known relationships between the structure of the crystal on one side, and the nucleating surface and conditions (quiescent versus shear) on the other, we hope to gain insights into the early formation pathways of crystallographic polymorphs.