The size effect on molecular assembly and crystallization has emerged as a critical issue in a wide range of industrial sectors including pharmaceutical and healthcare products, food and nutritional materials, petrochemicals, and organic fine chemicals. For example, drug capsules are known to affect a drugs solid-state form and therefore its circulating half-life and bioavailability. Surface patterns and defects approaching the critical nucleus size will certainly influence the crystallinity and charge transport properties of thin film microelectronic devices. As the integrated fluidic patterns in high-throughput crystallization screening become increasingly small and sophisticated, it is essential to understand how fluid channel geometry and size at nanoscale affect the nucleation and crystallization processes. The nanoconfinement effect must be addressed before these technologies can come to fruition.
This project focuses on the crystallization behavior of organic molecules on patterns and in structured media when the confinement size is near the critical dimension. Thecentral hypotheses are 1) when the confinement size is greater than the critical nucleus diameter, site-specific crystallization will occur whose shape, size, and distribution are dictated by the pattern; and 2) when the confinement size is less than the critical size, the pattern will either prohibit nucleation or trap precritical clusters. The research will begin with simple lipophilic benzoic acids as the nucleating agents and lipid/alkane bilayers as the confining media. Encapsulation studies will be conducted in collaboration with the Max Planck Institute of Colloids and Interfaces using layer-by-layer (LbL) capsules with chemical heterogeneity mimicking the lipid bilayer. Structural analysis will be conducted using Atomic Force Microscopy, Confocal Raman Microspectroscopy, and Selected Area Electron Diffraction. The aims of the research are 1) to monitor nucleation and early stages of crystallization, and 2) to control crystal morphology using confinement at the critical dimension.
INTELLECTUAL MERIT. Experimental studies of crystallization steps in confinement are necessary for a better understanding of structural and morphological transitions undergone by discrete molecular clusters en route to the critical nucleus and the final crystal structure. The self assembled patterning method enables the investigation of the relationship between the consitituent molecular structure and the pattern structure. The molecularly smooth pattern allows a separate study of confinement imposed by chemical heterogeneity rather than by topography. The project addresses the largely unresolved issue concerning the stability and order of the precritical clusters despite recent evidence that such clusters can exist in nanoconfinement and may even be highly ordered. The other significant conceptual deviation of this approach from the established ones is to achieve site-specific nucleation and oriented crystal growth, i.e., azimuthal crystallization, by nanopatterns. Epitaxy crystallization requires the dimensional and stereochemical match of the guest/host crystalline planes. The project intends to show that azimuthal crystallization can be achieved without such stringent constraints and with no prior knowledge of the guest crystal structure.
BROAD IMPACTS. 1) Technology. Confinement of molecular assemblies is applicable to drug encapsulation, molecular electronic circuitry, and high-throughput crystallization screening. 2) International cooperation. The Ph.D. students will benefit from working at a leading international research institution in colloids and surface science. 3) Integration of research and education. The PI is leading the efforts to update Wayne States Basic Materials Engineering curriculum and to create a Nanotechnology Track for a Chemical Engineering B.Sc. degree.
