Solid-State NMR Studies of the Formation of Monomers and Dimers in Stearic Acid Confined in Titanate Nanotubes

Abstract

This work employs two-dimensional solid-state NMR methods to uncover the structure and molecular arrangement of physically trapped stearic acid (SA) inside a titanate nanotube (TiNT). Thermal annealing the mixture of SA and the water-washed TiNT is shown to induce slow physical trapping of SA into the TiNT. In contrast to that the solid-state bulk exhibits only one carbonyl 13C peak of NMR and consists solely of dimer, the physically trapped SA exhibits two carbonyl 13C peaks with different chemical shifts that are assigned respectively to dimer and monomer both by the 13C homonuclear double-quantum measurement and by the 13C chemical-shift-tensor measurement. The trapped SA monomer and dimer are shown to grow simultaneously during thermal annealing with a constant number ratio between them at ∼1:1. The 13C homonuclear double-quantum and the 1H−13C HETCOR spectra indicate that the trapped SA monomer and dimer do not form separated clusters but are neighbors to each other. As such, the trapped SA in the TiNT undertakes a novel molecular arrangement alternating with dimer and monomer. The carbonyl 13C chemical-shift-tensor data and the hydroxyl 1H isotropic-chemical-shift data suggest that the hydrogen bond of the dimer is stronger under nanoconfinement than in the solid bulk SA. The observed novel molecular arrangement for the fatty acid and the correlated variation of the carbonyl 13C and the hydroxyl 1H chemical shift between the different molecular arrangements are of theoretical interest.