Mahesh K. Mahanthappa

Unusually Stable Aqueous Lyotropic Gyroid Phases from Gemini Dicarboxylate Surfactants

Aqueous lyotropic liquid crystal (LLC) assemblies with bicontinuous cubic morphologies (Q-phases) have shown promise in applications ranging from selective chemical separations to ion transporting media, yet universal design criteria for amphiphiles that adopt these unique structures remain elusive. Recent reports have demonstrated that cationic gemini surfactants exhibit a tendency to form bicontinuous cubic LLCs as compared to single-tail amphiphiles; however, the universality of this surfactant design motif in stabilizing Q-phases remains untested. Herein, we report the modular synthesis of a new class of anionic gemini surfactants derived from aliphatic carboxylic acids and demonstrate their unexpectedly strong propensity to form gyroid LLC phases with unprecedented stability between 25 and 100 °C over amphiphile concentration windows up to 20 wt % wide. By systematically varying the alkyl spacer length and surfactant counterions (Na(+), K(+), and (CH(3))(4)N(+)), we identify molecular motifs that favor formation of technologically useful bicontinuous cubic LLC morphologies.

Lithium transport through lithium-ion battery cathode coatings

The surface coating of cathodes using insulator films has proven to be a promising method for high-voltage cathode stabilization in Li-ion batteries, but there is still substantial uncertainty about how these films function. More specifically, there is limited knowledge of lithium solubility and transport through the films, which is important for coating design and development. This study uses first-principles calculations based on density functional theory to examine the diffusivity of interstitial lithium in the crystals of α-AlF3, α-Al2O3, m-ZrO2, c-MgO, and α-quartz SiO2, which provide benchmark cases for further understanding of insulator coatings in general. In addition, we propose an ohmic electrolyte model to predict resistivities and overpotential contributions under battery operating conditions. For the crystalline materials considered we predict that Li+ diffuses quite slowly, with a migration barrier larger than 0.9 eV in all crystalline materials except α-quartz SiO2, which is predicted to have a migration barrier of 0.276 eV along 〈001〉. These results suggest that the stable crystalline forms of these insulator materials, except for oriented α-quartz SiO2, are not practical for conformal cathode coatings. Amorphous Al2O3 and AlF3 have higher Li+ diffusivities than their crystalline counterparts. Our predicted amorphous Al2O3 resistivity (1789 MΩ m) is close to the top of the range of the fitted resistivities extracted from previous experiments on nominal Al2O3 coatings (7.8 to 913 MΩ m) while our predicted amorphous AlF3 resistivity (114 MΩ m) is very close to the middle of the range. These comparisons support our framework for modeling and understanding the impact on overpotential of conformal coatings in terms of their fundamental thermodynamic and kinetic properties, and support that these materials can provide practical conformal coatings in their amorphous form.

Self-assembly of gemini surfactants: A computer simulation study

The self-assembly behavior of gemini (dimeric or twin-tail) dicarboxylate disodium surfactants is studied using molecular dynamics simulations. A united atom model is employed for the surfactants with fully atomistic counterions and water. This gemini architecture, in which two single tailed surfactants are joined through a flexible hydrophobic linker, has been shown to exhibit concentration-dependent aqueous self-assembly into lyotropic phases including hexagonal, gyroid, and lamellar morphologies. Our simulations reproduce the experimentally observed phases at similar amphiphile concentrations in water, including the unusual ability of these surfactants to form gyroid phases over unprecedentedly large amphiphile concentration windows. We demonstrate quantitative agreement between the predicted and experimentally observed domain spacings of these nanostructured materials. Through careful conformation analyses of the surfactant molecules, we show that the gyroid phase is electrostatically stabilized related to the lamellar phase. By starting with a lamellar phase, we show that use of a bulkier N(CH(3))(4)(+) counterion in place of Na(+) drives the formation of a gyroid phase. Decreasing the charge on the surfactant headgroups by carboxylate protonation decreases the degree of order in the lamellar phase. Using our models, we show that the translational diffusion of water and the Na(+) counterions is decreased by several orders of magnitude over the studied concentration range, and we attribute these effects to strong correlations between the mobile species and the surfactant headgroups.

Polydispersity-driven shift in the lamellar mesophase composition window of PEO-PB-PEO triblock copolymers

The influence of broad and continuous center block polydispersity on the melt-phase self-assembly of OBO triblock copolymers (O = poly(ethylene oxide) and B = poly(1,4-butadiene)) is reported for a series of samples derived from tandem chain transfer ring-opening metathesis polymerization (ROMP-CT) and anionic ring-opening polymerization (AROP). By virtue of the polymerization techniques employed in these syntheses, the midblocks exhibit polydispersity indices Mw/Mn = 1.75, whereas the end blocks have relatively narrow dispersities Mw/Mn ≤ 1.25. Using a combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) to characterize these materials and their narrow dispersity homologues derived from anionic polymerization, we demonstrate that the combination of both chain length and composition polydispersity inherent in these polydisperse triblocks shifts the composition window of stability for the lamellar mesophase. Furthermore, these studies reveal that polydispersity in the center B segments of these OBO triblocks results in large lamellar domain spacing increases, enables stable coexistence of two morphologies in a single sample, and frustrates lattice ordering in a composition-dependent manner.

High-resolution structures of a heterochiral coiled coil.

Significance d polypeptides represent an attractive platform for biomedical applications because of their resistance to proteolytic degradation. However, the structural principles that underlie associations between L- and D-protein partners remain poorly understood because there has been very little atomic-resolution structural characterization of such heterochiral assemblies. Here we report two X-ray crystal structures of the racemic form of an α-helical peptide derived from the influenza M2 protein. Both structures contain large heterochiral coiled–coil interfaces. The ubiquity and regularity of coiled coils has inspired extensive design effort directed toward homochiral tertiary and quaternary structures, and we anticipate that the insights from these crystal structures will facilitate the design of an analogous rich set of heterochiral proteins and assemblies. Interactions between polypeptide chains containing amino acid residues with opposite absolute configurations have long been a source of interest and speculation, but there is very little structural information for such heterochiral associations. The need to address this lacuna has grown in recent years because of increasing interest in the use of peptides generated from d amino acids (d peptides) as specific ligands for natural proteins, e.g., to inhibit deleterious protein–protein interactions. Coiled–coil interactions, between or among α-helices, represent the most common tertiary and quaternary packing motif in proteins. Heterochiral coiled–coil interactions were predicted over 50 years ago by Crick, and limited experimental data obtained in solution suggest that such interactions can indeed occur. To address the dearth of atomic-level structural characterization of heterochiral helix pairings, we report two independent crystal structures that elucidate coiled-coil packing between l- and d-peptide helices. Both structures resulted from racemic crystallization of a peptide corresponding to the transmembrane segment of the influenza M2 protein. Networks of canonical knobs-into-holes side-chain packing interactions are observed at each helical interface. However, the underlying patterns for these heterochiral coiled coils seem to deviate from the heptad sequence repeat that is characteristic of most homochiral analogs, with an apparent preference for a hendecad repeat pattern.

Inverse Pmn cubic micellar lyotropic phases from zwitterionic triazolium gemini surfactants

Inverse lyotropic liquid crystal phases comprising spatially-limited aqueous nanodomains have potentially wide-ranging applications, including the encapsulation and selective delivery of small molecules and as aqueous nanoreactors for molecular catalysis. We report the aqueous self-assembly of two homologous, zwitterionic bis(alkyltriazolium sulfobetaine) gemini surfactants into previously unknown inverse cubic micellar phases with Pmn symmetry (space group #223). Using temperature-dependent X-ray scattering, we demonstrate that these highly ordered, inverse discontinuous micellar phases are stable over remarkably wide amphiphile concentration windows between T = 30–90 °C at ambient pressure. These observations suggest that the packing arrangements adopted by structurally anisotropic gemini surfactants enable ready access to only the third known inverse micellar lyotropic phase, and the first one to exhibit a tetrahedral close-packing of inverse micelles.

Optimizing AlF3 atomic layer deposition using trimethylaluminum and TaF5: Application to high voltage Li-ion battery cathodes

Atomic layer deposition (ALD) of conformal AlF3 coatings onto both flat silicon substrates and high-voltage LiNi0.5Mn0.3Co0.2O2 (NMC) Li-ion battery cathode powders was investigated using a Al(CH3)3/TaF5 precursor combination. This optimized approach employs easily handled ALD precursors, while also obviating the use of highly toxic HF(g). In studies conducted on planar Si wafers, the films growth mode was dictated by a competition between the desorption and decomposition of Ta reaction byproducts. At T ≥ 200 °C, a rapid decomposition of the Ta reaction byproducts to TaC led to continuous deposition and high concentrations of TaC in the films. A self-limited ALD growth mode was found to occur when the deposition temperature was reduced to 125 °C, and the TaF5 exposures were followed by an extended purge. The lower temperature process suppressed conversion of TaFx(CH3)5−x to nonvolatile TaC, and the long purges enabled nearly complete TaFx(CH3)5−x desorption, leaving behind the AlF3 thin films. NMC cathod...

Imprint control of BaTiO3 thin films via chemically induced surface polarization pinning

Surface-adsorbed polar molecules can significantly alter the ferroelectric properties of oxide thin films. Thus, fundamental understanding and controlling the effect of surface adsorbates are crucial for the implementation of ferroelectric thin film devices, such as ferroelectric tunnel junctions. Herein, we report an imprint control of BaTiO3 (BTO) thin films by chemically induced surface polarization pinning in the top few atomic layers of the water-exposed BTO films. Our studies based on synchrotron X-ray scattering and coherent Bragg rod analysis demonstrate that the chemically induced surface polarization is not switchable but reduces the polarization imprint and improves the bistability of ferroelectric phase in BTO tunnel junctions. We conclude that the chemical treatment of ferroelectric thin films with polar molecules may serve as a simple yet powerful strategy to enhance functional properties of ferroelectric tunnel junctions for their practical applications.

Synthetic Mimics of Bacterial Lipid A Trigger Optical Transitions in Liquid Crystal Microdroplets at Ultralow Picogram-per-Milliliter Concentrations

We report synthetic six-tailed mimics of the bacterial glycolipid Lipid A that trigger changes in the internal ordering of water-dispersed liquid crystal (LC) microdroplets at ultralow (picogram-per-milliliter) concentrations. These molecules represent the first class of synthetic amphiphiles to mimic the ability of Lipid A and bacterial endotoxins to trigger optical responses in LC droplets at these ultralow concentrations. This behavior stands in contrast to all previously reported synthetic surfactants and lipids, which require near-complete monolayer coverage at the LC droplet surface to trigger ordering transitions. Surface-pressure measurements and SAXS experiments reveal these six-tailed synthetic amphiphiles to mimic key aspects of the self-assembly of Lipid A at aqueous interfaces and in solution. These and other results suggest that these amphiphiles trigger orientational transitions at ultralow concentrations through a unique mechanism that is similar to that of Lipid A and involves formation of inverted self-associated nanostructures at topological defects in the LC droplets.

Low-symmetry sphere packings of simple surfactant micelles induced by ionic sphericity

Significance Surfactants (“soaps”) spontaneously self-assemble into spherical micelles in water, which pack into ordered crystalline states. Such soft particles have long been assumed to adopt the same closest-packed configurations observed with hard spheres (e.g., billiard balls). Here, we show that surfactant micelles also form complex, tetrahedrally closest-packed Frank–Kasper (FK) phases. Surprisingly, the low-symmetry unit cells of these structures comprise multiple particle types with discrete size distributions. We demonstrate that these unexpected structures arise from simultaneous optimization of interparticle electrostatic interactions and the spherical symmetry of the charged ion clouds around each micelle. This discovery bridges previous reports of FK phases in neutral soft materials such as block polymers, dendrimers, and giant shape amphiphiles and in metal alloys. Supramolecular self-assembly enables access to designer soft materials that typically exhibit high-symmetry packing arrangements, which optimize the interactions between their mesoscopic constituents over multiple length scales. We report the discovery of an ionic small molecule surfactant that undergoes water-induced self-assembly into spherical micelles, which pack into a previously unknown, low-symmetry lyotropic liquid crystalline Frank–Kasper σ phase. Small-angle X-ray scattering studies reveal that this complex phase is characterized by a gigantic tetragonal unit cell, in which 30 sub-2-nm quasispherical micelles of five discrete sizes are arranged into a tetrahedral close packing, with exceptional translational order over length scales exceeding 100 nm. Varying the relative concentrations of water and surfactant in these lyotropic phases also triggers formation of the related Frank–Kasper A15 sphere packing as well as a common body-centered cubic structure. Molecular dynamics simulations reveal that the symmetry breaking that drives the formation of the σ and A15 phases arises from minimization of local deviations in surfactant headgroup and counterion solvation to maintain a nearly spherical counterion atmosphere around each micelle, while maximizing counterion-mediated electrostatic cohesion among the ensemble of charged particles.