Karl Freed of the University of Chicago is supported by an award from the Theoretical and Computational Chemistry program for research on the development of a systematic theoretical description of self-assembly. The work builds on the PI's prior NSF-supported work that focused on equilibrium association and contributed to the basic theoretical understanding of supramolecular assembly, complexity and emergence. Freed is now generalizing this approach to consider several non-equilibrium processes including the reversible formation of branched clusters, self-assembly of compact clusters and the coupling of surface adsorption and desorption with self-assembly.
The theoretical and computational work being carried out by Freed and his group is expected to have a broad impact through application in both biology and materials science and to be a key component in developing effective strategies for "bottom-up" manufacturing and fabrication of functional nanostructures.
A. Diversity of Equilibrium Self-assembly Processes The self-assembly of dynamic clusters is a ubiquitous phenomenon in which the constituent molecules or particles form and disintegrate clusters of various geometries in a dynamical equilibrium. This dynamic clustering process is prevalent in biopolymer, polyelectrolyte, and ionic solutions, solutions of amphiphiles and molecules exhibiting supramolecular assembly with highly directional (hydrogen bonding, pi-pi, polar, and multipole) interactions. Self-assembly is a basic element of bottom-up nanomanufacturing, and elucidating the factors controlling the structure and the stability of dynamical particle assemblies is key to successfully applying this fabrication approach. Many natural and synthetic self-assembly processes involve the mutual association of molecules or particles with complementary interactions (e.g., antigen-ligand binding of proteins), which in turn polymerize into larger scale structures. We develop a systematic Flory-Huggins type theory for this hierarchal assembly. The theory explains the observation of a peak in the shear viscosity of mutually associating fluid mixtures exhibiting polymerization at equilibrium. Molecular self-assembly often occurs in the presence of long chain polymers, and we develop a theory to describe the competition between self-assembly and phase separation that generally occurs in these complex fluid mixtures. We investigate a minimal equilibrium polymerization model for the general competition between self-assembly on a boundary and in solution that arises when an assembling system is in the presence of an adsorbing interface. As demonstrated by illustrative examples, the coupling between surface adsorption and self-assembly provides a powerful means of switching self-assembly processes on and off. Understanding and controlling this switching phenomenon will be useful in designing and directing self-assembly processes on surfaces for applications to nanomanufacturing and in developing treatments for diseases arising from pathological adsorption-induced assembly. Many living and nonliving structures in the natural world form by hierarchical organization, but physical theories that describe this type of organization are scarce. To address this problem, a model of equilibrium self-assembly is formulated in which dynamically associating species organize into hierarchical structures that preserve their shape at each stage of assembly. We develop a general virial expansion to describe the influence of molecular additives on the equilibrium self-assembly of proteins or other supermolecularly assembling species M in solution. B. The Descent to Glass-formation Our recent theory is the first molecular description that explains a wide variety of properties of glass-formation including the first explanation for the molecular origins of glass (thermal) fragility. As another first, we compare theory with experiment. The entropy theory of glass formation predicts systematic changes in fragility with chain stiffness, cohesive energy, polymerization index, and side chain length, and qualitative trends in these parameters are discussed. C. Electronic Structure Theory We have also been converting our electronic structure codes (based on work supported by previous NSF grants) into forms that can be distributed within the GAMESS suite of programs. Several programs are already being distributed, and efforts are continuing on others with Professor Rajat Chaudhuri, Indian Institute of Astronomy, Bangalore, India.
