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Water at α-alumina surfaces
(2018)
The (0001) surface of α-Al₂O₃ is the most stable surface cut under UHV conditions and was studied by many groups both theoretically and experimentally. Reaction barriers computed with GGA functionals are known to be underestimated. Based on an example reaction at the (0001) surface, this work seeks to improve this rate by applying a hybrid functional method and perturbation theory (LMP2) with an atomic orbital basis, rather than a plane wave basis. In addition to activation barriers, we calculate the stability and vibrational frequencies of water on the surface. Adsorption energies were compared to PW calculations and confirmed PBE+D2/PW stability results. Especially the vibrational frequencies with the B3LYP hybrid functional that have been calculated for the (0001) surface are in good agreement with experimental findings. Concerning the barriers and the reaction rate constant, the expectations are fully met. It could be shown that recalculation of the transition state leads to an increased barrier, and a decreased rate constant when hybrid functionals or LMP2 are applied.
Furthermore, the molecular beam scattering of water on (0001) surface was studied. In a previous work by Hass the dissociation was studied by AIMD of molecularly adsorbed water, referring to an equilibrium situation. The experimental method to obtaining this is pinhole dosing. In contrast to this earlier work, the dissociation process of heavy water that is brought onto the surface from a molecular beam source was modeled in this work by periodic ab initio molecular dynamics simulations. This experimental method results in a non-equilibrium situation. The calculations with different surface and beam models allow us to understand the results of the non-equilibrium situation better. In contrast to a more equilibrium situation with pinhole dosing, this gives an increase in the dissociation probability, which could be explained and also understood mechanistically by those calculations.
In this work good progress was made in understanding the (1120) surface of α-Al₂O₃ in contact with water in the low-coverage regime. This surface cut is the third most stable one under UHV conditions and has not been studied to a great extent yet. After optimization of the clean, defect free surface, the stability of different adsorbed species could be classified. One molecular minimum and several dissociated species could be detected. Starting from these, reaction rates for various surface reactions were evaluated. A dissociation reaction was shown to be very fast because the molecular minimum is relatively unstable, whereas diffusion reactions cover a wider range from fast to slow. In general, the (112‾0) surface appears to be much more reactive against water than the (0001) surface. In addition to reactivity, harmonic vibrational frequencies were determined for comparison with the findings of the experimental “Interfacial Molecular Spectroscopy” group from Fritz-Haber institute in Berlin. Especially the vibrational frequencies of OD species could be assigned to vibrations from experimental SFG spectra with very good agreement. Also, lattice vibrations were studied in close collaboration with the experimental partners. They perform SFG spectra at very low frequencies to get deep into the lattice vibration region. Correspondingly, a bigger slab model with greater expansion perpendicular to the surface was applied, considering more layers in the bulk. Also with the lattice vibrations we could obtain reasonably good agreement in terms of energy differences between the peaks.
Time-dependent correlation function based methods to study optical spectroscopy involving electronic transitions can be traced back to the work of Heller and coworkers. This intuitive methodology can be expected to be computationally efficient and is applied in the current work to study the vibronic absorption, emission, and resonance Raman spectra of selected organic molecules. Besides, the "non-standard" application of this approach to photoionization processes is also explored. The application section consists of four chapters as described below.
In Chapter 4, the molar absorptivities and vibronic absorption/emission spectra of perylene and several of its N-substituted derivatives are investigated. By systematically varying the number and position of N atoms, it is shown that the presence of nitrogen heteroatoms has a negligible effect on the molecular structure and geometric distortions upon electronic transitions, while spectral properties are more sensitive: In particular the number of N atoms is important while their position is less decisive. Thus, N-substitution can be used to fine-tune the optical properties of perylene-based molecules.
In Chapter 5, the same methods are applied to study the vibronic absorption/emission and resonance Raman spectra of a newly synthesized donor-acceptor type molecule. The simulated absorption/emission spectra agree fairly well with experimental data, with discrepancies being attributed to solvent effects. Possible modes which may dominate the fine-structure in the vibronic spectra are proposed by analyzing the correlation function with the aid of Raman and resonance Raman spectra.
In the next two chapters, besides the above types of spectra, the methods are extended to study photoelectron spectra of several small diamondoid-related systems (molecules, radicals, and cations). Comparison of the photoelectron spectra with available experimental data suggests that the correlation function based approach can describe ionization processes reasonably well. Some of these systems, cationic species in particular, exhibit somewhat peculiar optical behavior, which presents them as possible candidates for functional devices.
Correlation function based methods in a more general sense can be very versatile. In fact, besides the above radiative processes, formulas for non-radiative processes such as internal conversion have been derived in literature. Further implementation of the available methods is among our next goals.
Thermoresponsive block copolymers of presumably highly biocompatible character exhibiting upper critical solution temperature (UCST) type phase behavior were developed. In particular, these polymers were designed to exhibit UCST-type cloud points (Tcp) in physiological saline solution (9 g/L) within the physiologically interesting window of 30-50°C. Further, their use as carrier for controlled release purposes was explored. Polyzwitterion-based block copolymers were synthesized by atom transfer radical polymerization (ATRP) via a macroinitiator approach with varied molar masses and co-monomer contents. These block copolymers can self-assemble in the amphiphilic state to form micelles, when the thermoresponsive block experiences a coil-to-globule transition upon cooling. Poly(ethylene glycol) methyl ether (mPEG) was used as the permanently hydrophilic block to stabilize the colloids formed, and polyzwitterions as the thermoresponsive block to promote the temperature-triggered assembly-disassembly of the micellear aggregates at low temperature.
Three zwitterionic monomers were used for this studies, namely 3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate (SPE), 4-((2-(methacryloyl- oxy)ethyl)dimethylammonio)butane-1-sulfonate (SBE), and 3-((2-(methacryloyloxy)ethyl)- dimethylammonio)propane-1-sulfate) (ZPE). Their (co)polymers were characterized with respect to their molecular structure by proton nuclear magnetic resonance (1H-NMR) and gel permeation chromatography (GPC). Their phase behaviors in pure water as well as in physiological saline were studied by turbidimetry and dynamic light scattering (DLS). These (co)polymers are thermoresponsive with UCST-type phase behavior in aqueous solution. Their phase transition temperatures depend strongly on the molar masses and the incorporation of co-monomers: phase transition temperatures increased with increasing molar masses and content of poorly water-soluble co-monomer. In addition, the presence of salt influenced the phase transition dramatically. The phase transition temperature decreased with increasing salt content in the solution. While the PSPE homopolymers show a phase transition only in pure water, the PZPE homopolymers are able to exhibit a phase transition only in high salinity, as in physiological saline. Although both polyzwitterions have similar chemical structures that differ only in the anionic group (sulfonate group in SPE and sulfate group in ZPE), the water solubility is very different. Therefore, the phase transition temperatures of targeted block copolymers were modulated by using statistical copolymer of SPE and ZPE as thermoresponsive block, and varying the ratio of SPE to ZPE. Indeed, the statistical copolymers of P(SPE-co-ZPE) show phase transitions both in pure water as well as in physiological saline. Surprisingly, it was found that mPEG-b-PSBE block copolymer can display “schizophrenic” behavior in pure water, with the UCST-type cloud point occurring at lower temperature than the LCST-type one.
The block copolymer, which satisfied best the boundary conditions, is block copolymer mPEG114-b-P(SPE43-co-ZPE39) with a cloud point of 45°C in physiological saline. Therefore, it was chosen for solubilization studies of several solvatochromic dyes as models of active agents, using the thermoresponsive block copolymer as “smart” carrier. The uptake and release of the dyes were explored by UV-Vis and fluorescence spectroscopy, following the shift of the wavelength of the absorbance or emission maxima at low and high temperature. These are representative for the loaded and released state, respectively. However, no UCST-transition triggered uptake and release of these dyes could be observed. Possibly, the poor affinity of the polybetaines to the dyes in aqueous environtments may be related to the widely reported antifouling properties of zwitterionic polymers.