Professor Mahesh Mahanthappa

CSENG Chemical Eng & Mat Sci
College of Science & Engineering
Twin Cities
Project Title: 
Soft Materials Self-Assembly

Lyotropic liquid crystals (LLCs), which exhibit structurally periodic aqueous and hydrophobic nanodomains, thermodynamically arise from the supramolecular self-assembly of minimally hydrated amphiphilic molecules ("soaps"). The Mahanthappa research group is specifically interested in amphiphile self-assembly into LLCs with both 3D bicontinuous network phases and low-symmetry discontinuous micellar morphologies. By virtue of their tunable chemistries that stem from the underlying amphiphile structures and their fascinating mesoscopic structures, these two classes of LLCs find wide-ranging applications as molecular separation membranes, soft templates for mesoporous inorganic materials synthesis for catalysis and optics applications, and as drug delivery vehicles.

In the area of discontinuous micellar LLCs, these researchers have recently discovered that a wide variety of amphiphiles form a previously unrecognized "zoo" of low-symmetry packings of quasispherical micelles that are much more complex than more common, high-symmetry body-centered cubic (BCC), face-centered cubic (FCC), and hexagonally close-packed (HCP) structures. Remarkably, these new tetrahedrally-close packed aqueous LLC Frank-Kasper (FK) phases are micellar mimics of the well-known structures of various heavy elements and transition metal alloys. These FK phases are characterized by large, low-symmetry unit cells comprising ≥ 7 micelles that exhibit unusual X-ray diffraction patterns with high degrees of long-range translational order, and they are periodic approximants to both dodecagonal and icosahedral quasicrystals (QCs). This research seeks to elucidate the mechanisms by which this new and other related FK phases form, by systematically examining how surfactant structure and sample processing history dictate lyotropic sphere packing symmetry selection and the thermodynamic stabilities of these complex supramolecular assemblies.

Since real-spacing imaging of these aqueous LLC phases using electron microscopy is extraordinarily difficult, the researchers typically use variable temperature synchrotron small-angle X-ray scattering (VT-SAXS) analyses to characterize these unusual phases. This group was the first to recognize in 2014 that a Rietveld refinement of these data could be performed to obtain the structure factor ("Fourier") amplitudes for the structure in reciprocal space, which can then be used as inputs for the charge-flipping algorithm implemented within the SUPERFLIP software package to reconstruct electron density maps for the structures formed by self-assembled soft materials to understand how they form. These electron density map reconstructions require large amounts of computing power, incompatible with traditional desktop computers and other computing resources.

Calculations that depend on MSI resources have the potential to expand the applications of LLC materials, by understanding the mechanisms by which they enable access to new and more complex structures with unusual and useful properties. The notion that frustration of at least two competing length-scales can drive complex pattern formation is a relatively new and poorly explored idea. Thus, this work may ultimately inform new methods for harnessing frustration in self-assembled systems, as a means of rationally accessing complex periodic and aperiodic structures derived from myriad constituent building blocks. Work published in 2019 describes the reconstruction of electron density maps for Frank-Kasper sigma and A15 that the group discovered in lyotropic liquid crystals derived from the commercially available surfactant Brij 58. Without MSI resources, the computations required to conduct these detailed structural analyses would have been impossible.

 

Project Investigators

Ashish Jayaraman
Aaron Lindsay
Professor Mahesh Mahanthappa
Andreas Mueller
 
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