Surfactant Organization in MCM-41 Mesoporous Materials As Studied by ¹³C and ²⁹Si Solid-State NMR

Mesoporous silica materials (MCM-41) were characterized by 13 C and 29 Si magic angle spinning (MAS) NMR. 13 C chemical shifts reveal a high content of gauche conformations in the chain of cationic surfactant molecules within the nanopores. 13 C T 1 's and proton to carbon cross-polarization times ( T CH ) demonstrate that the surfactant inside the pores exhibits anisotropic motions progressively reduced from the chain end toward the polar head. Variable-temperature 13 C MAS NMR spectra show a strong thermal dependence of the motions and highlight a gellike aggregation of the surfactant. On the other hand, the transfer of magnetization from the surfactant protons to the silicons on the pore surface was found to be efficient and observed by cross-polarization dynamics following the nonprotonated silicon atoms. Silicons and carbons receive magnetization from the same proton system, because a unique T 1 ρ ( 1 H) is measured from the point of view of silicon and carbon nuclei. Our 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 aliphatic chains.