Elevation of melting temperature for confined palmitic acid inside cylindrical nanopores

High resolution 13C nuclear magnetic resonance was employed to study the phase behavior of the amphiphilic long-chain palmitic acid (PA) confined inside the cylindrical nanopores in the matrix of titanate nanotubes. For a series of mixtures of titanate nanotubes and palmitic acid at various mass ratios, it was shown that annealing at the bulk melting temperature (approximately 335.5 K) of PA induced fast chemisorption of PA on the nanotube surface followed by slow physical trapping of PA into the cylindrical nanopore. It was found that the trapped PA remained solidlike substantially above the bulk melting temperature. Contrary to the bulk neat PA, for the trapped PA, the isotropic molecular-chain reorientation was shown to remain arrested even above the bulk melting temperature. When destabilized at approximately 349 K, the trapped PA deserted the nanopore and formed bulk PA, which could be retrapped into the nanopore upon annealing at the bulk melting temperature. The entire process was shown as reversible.