title: Studies on Confined Guest Molecules by ²H and DNP enhanced ¹³C solid-state NMR
Creators: name: Döller, Sonja Carina
Publisher: name: UNSPECIFIED
Year: 2024

description: The transformation of the chemical industry towards more sustainable methods of research and manufacturing is one of the largest undertakings of the current century. One key approach to enable this transformation is the utilization of sustainable and non-toxic solvent systems.

In the present work, the interactions of analyte molecules with surfaces were investigated by means of solid-state Nuclear Magnetic Resonance (ssNMR) spectroscopic methods. The focus was laid on polyethylene glycols and their amphiphilic derivatives. These amphiphilic derivatives can be utilized as surfactants and thus allow reactions to be conducted in aqueous media by forming small, lipophilic micro- or nanoreactors. Polyethylene glycols themselves are suitable as non-volatile and non-toxic substitutes for conventional organic solvents.

The polyethylene glycols as well as the surfactants n-octanol, Triton-X 100 and hexaethylene glycol monodecyl ether were investigated both in bulk and in contact with mesoporous silica materials. The interactions of molecules such as surfactants with solid interfaces are not sufficiently understood since their study is challenging with standard spectroscopic methods. Solid-state ²H NMR spectroscopy as well as solid-state ¹³C NMR spectroscopy enhanced by Dynamic Nuclear Polarization (DNP) were used to analyze the interactions of the molecules with each other as well as with the silica surface. In addition to regular spectroscopic methods enhanced by DNP, direct and indirect DNP NMR spectroscopy was also applied.

n-Octanol confined in SBA-15 was used as a model system for more complex surfactants as well as to emulate biological membranes. By analyzing the ²H NMR spectra obtained from the bulk as well as the confined system, the alignment of n-octanol in the mesoporous silica SBA-15 could be elucidated. In addition, the melting behavior of the octanol phase in the pores was documented by differential scanning calorimetry (DSC) measurements. This led to the determination that glass formation does not occur despite the confinement of the molecules within the pores and the resulting limited space available. Instead, crystallites are formed, which show various sizes and thus different melting temperatures due to the interactions with the pore walls, causing a gradual melting process.

n-Octanol was also used as a model system for the development of a new approach to analyze data obtained from NMR spectra enhanced by direct/indirect DNP. Four different commercially available radicals (AMUPol, TOTAPOL, bTbK, and AsymPol) were analyzed in n-octanol to elucidate how the polarization spreads through the sample. Using the newly developed approach, the spectra obtained could be evaluated quantitatively. This was previously possible only to a limited extent due to the necessary comparison of different samples with potentially variable radical concentrations or sample quantities. It could thus be shown that hydrophilicity is a key factor in how radicals used in DNP NMR spectroscopy interact with analyte molecules and how polarization is transferred. These findings illustrate that the choice of polarizing agent plays an essential role in ensuring an optimal polarization transfer and therefore the maximum amount of enhancement possible for DNP enhanced NMR measurements.

With the help of direct/indirect DNP and through applying the newly developed analytical method, the arrangements of polyethylene glycols as well as of surfactants in two different mesoporous silicates were subsequently investigated. The potential influence of the pore size on the behavior of the analyte molecules was to be tested as well. However, such an influence could not be observed. Nevertheless, by quantitative analysis, it was possible to document that the amphiphilic surfactants prefer different arrangements in the pores of the silica materials compared to the hydrophilic polyethylene glycols. The surfactants align themselves with their hydrophilic head groups in the direction of the pore walls. The hydrophilic radical AMUPol was localized along the pore wall as well. The polyethylene glycols, on the other hand, show a higher degree of mixing with the radical used and a tendency to coil due to intramolecular interactions, which leads to the uniform transfer of the polarization across all carbon atoms.

The findings of this work illustrate that the competition of the intra- and intermolecular interactions with those at the corresponding surface strongly influences the behavior of the confined molecules. This is of high relevance for a plethora of systems at the solid-liquid interface like heterogeneous catalysts, chromatographic applications, or drug delivery systems. In addition, the applicability of direct/indirect DNP NMR spectroscopy to systems at the solid-liquid interface is expanded, and the comparability of the obtained spectra is greatly improved through a novel approach to quantification of the data.