From Molecular Simplicity to Supramolecular Complexity: Low Symmetry Packings of Ionic Spherical Micelles

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Surfactants are small molecules in which a water-soluble headgroup is attached to an insoluble tail. Common examples of useful surfactants are soaps and detergents. As a result of the different solubilities of the headgroup and the tail, surfactants spontaneously form ordered assemblies in water. The assemblies can range from very simple spheres dispersed in water to much more sophisticated geometries. However, predicting the geometry of a complex assembly from the chemical structure of the surfactant remains a significant challenge. Motivated by the desire to control surfactant assembly and access complex structures with unusual and useful properties, Prof. Mahesh Mahanthappa and his research team at the University of Minnesota-Twin Cities seek to understand how surfactant structure dictates the packing geometry and the stability of an assembly. Their discoveries could have broad implications for the future design and development of surfactants for applications such as enhanced oil recovery, personal care product formulation, drug encapsulation, and therapeutic delivery. The project also contributes to the interdisciplinary research training of undergraduate and graduate students, including members of underrepresented groups. Public outreach demonstrations using visually appealing soap bubble froths showcase the properties of surfactants and help to arouse the interest of students in science.A delicate balance of non-covalent interactions drives the supramolecular assembly of minimally hydrated amphiphiles into different lyotrophic liquid-crystalline phases. Of particular interest is the complex Frank-Kasper (FK) phases, which apparently arise from a frustrated non-covalent force balance that minimizes local variations in amphiphile solvation, while maximizing global micelle cohesion across the supramolecular assembly. Specific experiments of this project aim to 1) understand how the ionic amphiphile tail structure, counterion symmetry, and chirality impact micellar aggregation and consequent packing of the micelles into high coordination number, low symmetry lattices; 2) evaluate whether these complex packings are long-lived metastable states or thermodynamic equilibrium phases; and 3) assess whether non-ionic amphiphiles might form these complex phases. An important goal is to uncover new molecular design criteria that drive the formation of lyotropic liquid crystals with exceptional translational order, at length scales far exceeding those of their constituent molecular and supramolecular micelle building blocks.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.