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

Abstract

Two different mesoporous silica materials (SBA-15 and MCM 41) were impregnated with four different, commercially available surfactants, namely, E5, PEG 200, C10E6, and Triton X-100. Differential scanning calorimetry was employed to confirm the confinement of the surfactants in the pores of their host materials. Dynamic nuclear polarization enhanced solid state 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded for these materials, showing that both the direct as well as the indirect polarization transfer pathways are active for the carbons of the polyethylene glycol moieties of the surfactants. The presence of the indirect polarization pathway implies the presence of molecular motion with correlation times faster than the inverse Larmor frequency of the observed signals. The intensities of the signals were determined, and an approach based on relative intensities was employed to ensure comparability throughout the samples. From these data, the interactions of the surfactants with the pore walls could be determined. Additionally, a model describing the surfactants’ arrangement in the pores was developed. It was concluded that all carbons of the hydrophilic surfactants, E5 and PEG 200, interact with the silica walls in a similar fashion, leading to similar polarization transfer pathway patterns for all observed signals. For the amphiphilic surfactants C10E6 and Triton X-100, the terminal hydroxyl group mediates the majority of the interactions with the pore walls and the polarizing agent.

Structures and Dynamics of Complex Guest Molecules in Confinement, Revealed by Solid-State NMR, Molecular Dynamics, and Calorimetry

“…In the next step, the EG polymers E5 and PEG200 were studied in confinement and compared to the commercially available surfactants C 10 E 6 and Triton X-100 [152]. Two different mesoporous silica materials (SBA-15 and MCM 41) were impregnated with these surfactants.…”