Surfactant organization in MCM-41 mesoporous materials as studied by 13C and 29Si solid-state NMR

Mesoporous silica materials (MCM-41) were characterized by 13C and 29Si magic angle spinning (MAS) NMR. 13C chem. shifts reveal a high content of gauche conformations in the chain of cationic surfactant mols. within the nanopores. The 13C T1's and proton to carbon cross-polarization times (TCH) demonstrate that the surfactant inside the pores exhibits anisotropic motions progressively reduced from the chain end toward the polar head. Variable-temp. 13C MAS NMR spectra show a strong thermal dependence of the motions and highlight a gel-like aggregation of the surfactant. However, the transfer of magnetization from the surfactant protons to the silicons on the pore surface is efficient and obsd. by cross-polarization dynamics following the nonprotonated silicon atoms. Silicons and carbons receive magnetization from the same proton system, because a unique T1r(1H) is measured from the point of view of silicon and carbon nuclei. The authors' results demonstrate that there is such an interaction between the ammonium group of the surfactant and the silica surface which represents a strong limit to the motion of the aliph. chains.