The interactions of molecules such as surfactants with solid interfaces are not sufficiently understood since their study is challenging with standard spectroscopic methods. In this work, octanol-d 17 as a model system confined in the mesopores of SBA-15 is studied by variable temperature deuterium solid-state NMR, and the findings are compared to those of bulk octanol-d 17 . The magic angle spinning (MAS) as well as the static, nonspinning case, are investigated, showing that the described observations are independent of the applied NMR method. The 2 H NMR spectra of both the bulk and the confined octanol-d 17 show a large and a small quadrupolar Pake pattern below the melting point, suggesting a rigid conformation of the observed molecules with a 3-fold jump motion of the terminal CD 3 -group. Apart from the melting of the solid, no other phase transition is observed for either sample. The confined octanol-d 17 forms a pore solid, exhibiting a melting point 38 K lower than bulk octanol-d 17 . The interactions of the molecule with the mesoporous SBA-15 bring about a distribution of activation energies for the melting process, resulting in a gradual melting process.

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This review gives an overview of current trends in the investigation of confined molecules such as higher alcohols, ethylene glycol and polyethylene glycol as guest molecules in neat and functionalized mesoporous silica materials. All these molecules have both hydrophobic and hydrophilic parts. They are characteristic role-models for the investigation of confined surfactants. Their properties are studied by a combination of solid-state NMR and relaxometry with other physicochemical techniques and molecular dynamics techniques. It is shown that this combination delivers unique insights into the structure, arrangement, dynamical properties and the guest-host interactions inside the confinement.

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.