Direct and Indirect Dynamic Nuclear Polarization Transfer Observed in Mesoporous Materials Impregnated with Nonionic Surfactant Solutions of Polar Polarizing Agents

Mesoporous silica materials (SBA-15) with surfaces modified with aminopropyltriethoxysilane (APTES) of two different surface coverages were synthesized, and their structural pore characteristics were analyzed. These two mesoporous silica materials were impregnated with various solutions of radicals in a nonionic surfactant solvent. Differential scanning calorimetry (DSC) analysis of the impregnated mesoporous silica materials confirmed that the surfactant solutions were confined into the pores. Dynamic nuclear polarization (DNP)-enhanced solid-state ¹³C magic-angle spinning (MAS) NMR spectra recorded for these impregnated mesoporous silica materials showed the presence of superimposed spectra from direct and indirect channel polarization transfer processes not only for the confined surfactant solvent but also for the APTES surface modification. The observation of the indirect channel resonances implies that the surfactant solvents as well as the APTES exhibit molecular motions with correlation times on the order of or faster than the inverse Larmor frequency. Such motions are unexpected at the experimental temperature conditions of ∼120 K in particular for the immobilized APTES. Spectral line widths and intensities of the observed ¹³C MAS NMR spectra were sensitive to the specific combination of the radical, surfactant solvent, and APTES surface coverage. One particular combination showed identical widths and intensities for the direct and the oppositely phased indirect channel resonances, resulting in a blank spectrum. The differences in line widths and intensities are discussed with respect to the structural organization of the polarizing agent and surfactant within the pores and the complex interplay of intermolecular interactions between these constituents.