Oral Presentation Abstracts

Modeling the Effect of Silicon to Aluminum Ratio on the Adsorption of Water in Zeolites

Micah L. Welsch and Brian B. Laird

Department of Chemistry, University of Kansas

Zeolites, microporous aluminosilicates, are highly sought after adsorbents for their low cost, tunability, and reusability. Most commonly they are utilized for the selective separation of gases such as CH₄, N₂, and CO₂ in industrial scale processes. Efficient separation of adsorbates in the aqueous phase is also of great interest particularly in the removal of per- and polyfluoroalkyl substances (PFAS) from water, a contaminate that has been linked to several adverse health effects in humans. Understanding the adsorption of each component in a given separation process is vital in the identification and development of effective zeolites. Many experimental and computational studies have been performed to characterize the adsorption of water in zeolites for various applications. What is lacking is a detailed examination of water adsorption into zeolites of varying Silicon to Aluminum Ratio (SAR) from both the vapor and liquid phase. This study considers the BEA, FAU, MOR, MFI, CHA, and MEL frameworks as silica and with SAR’s ranging from 5 to 300. Grand Canonical Monte Carlo (GCMC) was performed to calculate the adsorption of water from the liquid and vapor phase at several pressures to characterize it’s dependence on zeolite SAR.

Atomic Force Microscopy Study of the Assembly Kinetics of a Model Gold Binding Peptide

Chris Johnson¹, Taylor Bader², Trisha Nair¹, Candan Tamerler², and Cindy L. Berrie¹

¹Department of Chemistry and ²Department of Mechanical Engineering, University of Kansas

Material-binding peptides have been selected to have high selectivity and specificity for particular materials.  Such peptides are being explored for use in the immobilization of enzymes for applications including biosensing, and biocatalysis.  They show promise for the creation of nanoscale arrays multi-enzyme systems, allowing for controlled organization and orientation of the attached enzymes. In this work, we investigate the assembly and kinetics of the peptides with the goal of elucidating the factors responsible for the specificity and selectivity of these peptides. AFM measurements of surface bound peptide films as a function of both time and concentration of one gold specific peptide, AuBP1 (WAGAKRLVLRRE), have been carried out and their analysis suggests a complex assembly process involving significant reorganization of the film is occurring during the assembly process.  The observations are consistent with a competition between peptide-peptide and peptide surface interactions resulting significant restructuring of the peptide film as the coverage of surface bound peptide changes.  Further experiments involving the modification of the peptide interactions through changes in pH and ionic strength are currently being carried to further elucidate what the role of specific interactions in the assembly.

Single-Molecule Fluorescence Investigation of Dye Adsorption onto the Surface of Anodic Aluminum Oxide Nanopores

Akash Nathani, Daniel A. Higgins and Takashi Ito

Department of Chemistry, Kansas State University

Nanoporous anodic aluminum oxide (AAO) membranes are envisioned as promising media for chemical separations and sensing. It is imperative to have a comprehensive understanding of the chemical properties of their nanopore surfaces to take full advantage of their uniform cylindrical nanoporous structures in these applications. AAOs are fabricated by electrochemical oxidization in an acid solution. The fabrication process leads to intrinsic incorporation of acid anions into the aluminum oxide matrix, which has been shown in the literature by energy dispersive X-ray spectroscopy and other techniques. A number of experimental parameters, including the type of acid electrolytes (e.g., H₂SO₄, H₂C₂O₄, and H₃PO₄) and their concentration, anodization potential, current density, reaction time, and temperature, have been carefully selected to prepare AAO membranes of desired morphology (e.g., pore size, thickness). Simultaneously, these parameters affect the extent of the anion incorporation, which will affect the surface properties of these membranes. Spectroscopic studies have shown pH-dependent adsorption of dye molecules on the AAO membranes based on electrostatic interaction: Zeta potential studies have shown the influence of pore size on the surface charge, in turn affecting the cell adhesion and protein adsorption properties of the membrane. However, there is a limited understanding of the effects of anion incorporation on the interactions of solute with the pore wall.  

In this study, we have employed single-molecule fluorescence imaging for the quantitative characterization of Rhodamine B (RhB) adsorption to the cross-section surface of the 20 nm pore-size AAO membrane. Adsorption of single RhB molecules from aqueous solution onto a sectioned membrane surface was directly measured using highly inclined and laminated optical sheet microscopy. RhB adsorption was enhanced by increasing the dye concentration in aqueous solution. The resulting relationship between the surface dye density and dye concentration could be successfully used to quantitatively assess the affinity of the dye onto the AAO nanopore surface using the Langmuir model. We are currently investigating RhB adsorption onto AAO nanopores with diameters of 10, 40, 80, and 120 nm to understand the influence of anion dopants on dye adsorption.

Energy Uphill Charge Separation Observed in Non fullerene Acceptor/Polymer Bulk Heterojunction.

Kushal Rijal¹, Neno Fuller¹, Fatimah Rudayni^1,2, Wai-Lun Chan¹

¹Department of Physics and Astronomy, University of Kansas and ²Department of Physics, Jazan University, Jazan, Saudi Arabia

Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved groundbreaking power conversion efficiencies of around 20%. These NFA OPVs can efficiently generate free carriers at the donor/acceptor (D/A) interface despite a very small energy level offset. However, fundamental mechanisms governing this efficient charge separation are still not well understood. We will present our recent time-resolved two-photon photoemission (TR-TPPE) spectroscopy measurements on the temporal dynamics of excitons in a representative NFA/polymer blend, the Y6/PM6 bulk heterojunction (BHJ). We find that charge transfer (CT) excitons rapidly emerge from singlet (S1) excitons of Y6 within a few picoseconds of photoexcitation, which indicates a rapid charge transfer in the BHJ. Subsequently, the CT exciton population bifurcates into a higher energy charge-separated (CS) state and a lower energy trapped state. We propose that the energy uphill process, the conversion from CT excitons to the CS state, is driven by entropy. The entropy-driven charge separation would be promoted by the specific nanostructure of NFA BHJs. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award No DE-SC0024525.

Evaluating Effects of Approximations in Electronic Structure Methods for Optical Rotation Calculations

Taylor Parsons¹, Ty Balduf¹, James R. Cheeseman², and Marco Caricato¹

¹Department of Chemistry, University of Kansas and ²Gaussian, Inc.

The basis set dependence of optical rotation (OR) calculations is examined for various choices of gauge/level of theory. The OR is calculated for a set of 50 molecules using B3LYP and CAM-B3LYP and 17 molecules using coupled cluster with single and double excitations (CCSD). Correlation consistent basis sets are used. An inverse power extrapolation formula is then utilized to obtain OR values at the complete basis set (CBS) limit. We investigate the basis set convergence for these methods and three choices of gauge: length gauge (with gauge-including atomic orbitals LG(GIAOs), for DFT), the origin-invariant length gauge [LG(OI)], and the modified velocity gauge (MVG). The results show that all methods converge smoothly to the CBS limit and that the LG(OI) approach has a slightly faster convergence rate than the other choices of gauge. While the DFT methods reach gauge invariance at the CBS limit, CCSD does not. The significant difference between the MVG and LG(OI) results at the CBS limit indicates that CCSD is not quite at convergence in the description of electron correlation for this property. On the other hand, gauge invariance at the CBS limit for DFT does not lead to the same OR values for the two density functionals, which is also due to electron correlation incompleteness. A limited comparison to gas-phase experimental OR values for the DFT methods shows that CAM-B3LYP seems more accurate than B3LYP. Overall, this study shows that the LG(OI) approach with the aug-cc-pVTZ basis set for DFT, and with the CBS(DT) extrapolation for CCSD, provides a good cost/accuracy balance.

Ion Mobility Separations and Structural Isotopic Shifts in Novel Buffer Gas Compositions

Atena Tajaddodi, Hayden Thurman, Alexandre A. Shvartsburg

Department of Chemistry and Biochemistry, Wichita State University

Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) separates ions by the increment of mobility at high electric fields. The ion mass is more orthogonal to that increment than absolute mobility underlying the linear IMS, enabling superior FAIMS resolution of structural isomers including the modified peptides, lipids, and aromatic compounds. The recently discovered structural specificity of isotopic envelopes due to natural or artificial stable isotope labels in the FAIMS dimension offers an innovative approach to isomer identification operationally resembling NMR. These isotopic shifts and their informative inter-isomer spreads qualitatively depend on the buffer gas identity and are largely orthogonal to the underlying isomer separations. Here we explore high-resolution FAIMS utilizing the dispersion voltage up to 7.0 kV provided by next-generation electronics and novel buffers comprising H₂, O₂, and SF₆ components. The SF₆ mixtures with extreme non-Blanc effects and resistance to electrical breakdown allow compensation voltages up to 100 V for halogenated anilines - double the values for same species in the CO₂ mixtures and the highest ever reported for polyatomic ions. We now expand the isotopic shift studies beyond aromatics - to the isomeric quaternary ammonium salts and carbohydrates. Different isotopic shifts for these species are additive and mutually orthogonal as for aromatics, showing the generality of those key properties. Advancing to practical applications, we explore the separations and isotopic shifts for illicit designer drugs where the isomeric differentiation holds major forensic significance. Small isotopic shifts for fluorinated compared to chlorinated isomers support the origination of effect from the center-of-mass transposition.

A Kirkwood-Buff Derived Force Field for Aqueous Alkali and Alkaline Earth Metal Nitrates

Farhat Khurshid and Paul E. Smith

Department of Chemistry, Kansas State University

A non-polarizable force field is developed for the simulation of aqueous Alkali Metal (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and Alkaline Earth Metal (Mg²⁺, Ca²⁺, Sr²⁺) nitrate salts using classical molecular dynamics with the Simple Point Charge/Extended (SPC/E) water model. Experimental activities and densities of the salt solutions are used to derive Kirkwood Buff Integrals (KBI) which are then used as target data to validate the accuracy of the force field parameters. Partial charges on oxygen and nitrogen atoms in the nitrate anion are taken from existing quantum calculations, with the Lennard-Jones (LJ) parameter for nitrate oxygen adjusted to obtain the best description of the nitrate anion that reproduce the experimental values of KBIs. The LJ parameters for the cation to nitrate oxygen interaction were also adjusted to reduce the degree of ion aggregation and to reproduce the experimental KBIs. The final force field parameters were then further tested by calculating diffusion coefficients, dielectric decrements, and surface tension values, and comparing with experimental values.

Quantifying Crystal Growth Rates in Calcite

Cade M. Ifland¹, Christine A. Orme², Bret N. Flanders¹

¹Department of Physics, Kansas State University and ²Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory

The ability to measure and control the kinetics of crystal formation is crucial to many industries and applications. While the rate at which the individual molecules, or growth units, attach to the crystal can be measured microscopically with atomic force microscopy (AFM), there is a need to relate these microscopic growth rates to those determined macroscopically. Our approach, which we demonstrate using CaCO3, is to grow crystals on a quartz crystal microbalance (QCM) and to measure their increasing mass as a function of time. The time-derivative of the mass-profile then gives an absolute growth-rate. Assuming each CaCO₃ seed grows at a uniform rate in each spatial dimension, this absolute rate can be normalized by the active surface area of the CaCO₃ seeds, which we find by imaging them while measuring the mass. This analysis yielded normalized growth-rates (NGR, in particles/ms2) that show good agreement with AFM-derived growth-rates. Furthermore, we can use our analysis to investigate different crystal growth conditions, some of which grow too fast to measure with AFM. Specifically, we grow crystals on electrode interfaces using electro-precipitation (EP). By varying the voltage that drives EP-based growth, we observe that the (NGR) scale with the driving voltage amplitude; furthermore, we show that exceptionally strong electro-precipitation can be induced by the galvanic coupling of the electrode to a dissimilar metal. We have measured the NGRs that multiple different metals cause. These observations hold relevance to the use of galvanic couples between metals in soil to sequester CO₂ into crystallized carbonates.

Exploring the Unusual Reactivity of the Hydrated Electron with CO₂

Pauf Neupane¹, David M. Bartels², Ward H. Thompson¹

¹Department of Chemistry, University of Kansas and ²Radiation Laboratory, University of Notre Dame, Notre Dame

The hydrated electron is a powerful reducing agent and is known to exhibit unusual reactivity, with similar activation energies for several kinetically different reactions. In this work, we explore the reaction mechanism of the hydrated electron with carbon dioxide using ab initio molecular dynamics simulations.  In its reduced form, the CO₂ molecule excites into the vibrational states of bending and symmetric stretch. For the first time, the rate constant for the hydrated electron-CO₂ reaction complex to react to form CO₂- is estimated from simulations. Results are obtained at 298 and 373 K, and it is found that the rate constant is insensitive to temperature, consistent with the low measured activation energy for the reaction.