Direct and Indirect DNP NMR Uncovers the Interplay of Surfactants with Their Mesoporous Host Material

Two different mesoporous silica materials (SBA-15 andMCM 41) wereimpregnated with four different, commercially available surfactants,namely, E-5, PEG 200, C10E6, and TritonX-100. Differential scanning calorimetry was employed to confirm theconfinement of the surfactants in the pores of their host materials.Dynamic nuclear polarization enhanced solid state C-13 magicangle spinning (MAS) nuclear magnetic resonance (NMR) spectra wererecorded for these materials, showing that both the direct as wellas the indirect polarization transfer pathways are active for thecarbons of the polyethylene glycol moieties of the surfactants. Thepresence of the indirect polarization pathway implies the presenceof molecular motion with correlation times faster than the inverseLarmor frequency of the observed signals. The intensities of the signalswere determined, and an approach based on relative intensities wasemployed to ensure comparability throughout the samples. From thesedata, the interactions of the surfactants with the pore walls couldbe determined. Additionally, a model describing the surfactants'arrangement in the pores was developed. It was concluded that allcarbons of the hydrophilic surfactants, E-5 and PEG 200,interact with the silica walls in a similar fashion, leading to similarpolarization transfer pathway patterns for all observed signals. Forthe amphiphilic surfactants C10E6 and TritonX-100, the terminal hydroxyl group mediates the majority of the interactionswith the pore walls and the polarizing agent.

Automated summary

Two different mesoporous silica materials (SBA-15 and MCM 41) were impregnated with four commercially available surfactants. Differential scanning calorimetry and solid-state NMR were used to confirm the confinement of the surfactants in the pores and determine their interactions with the pore walls. The study found that hydrophilic surfactants interact with the silica walls in a similar fashion, while for amphiphilic surfactants, the terminal hydroxyl group plays a major role in the interactions.