Effect of nanopore confinement on the thermal and structural properties of heneicosan

The effect of nano-confinement on the phase transitions of heneicosane (n-C21H44, C-21) were investigated using differential scanning calorimetry (DSC) and variable temperature powder X-ray diffraction (XRD). When C-21 is confined within the pores of controlled pore glasses (CPGs), SBA-15 or other mesoporous materials, the solid-solid (T-s) and solid-liquid (T-m) transition temperatures are depressed. The depression of the phase transition temperatures (Delta T-s, Delta T-m) for C-21 in CPGs and SBA-15, the degree of supercooling and enthalpy change (Delta H-s, Delta H-m) for C-21 in CPGs, exhibited a linear dependence on the inverse pore size. The Delta T-m values were insensitive to the polarity pore wall but varied with the pore geometry. XRD analysis revealed that the lamellar ordering characteristic for the solid C-21 was disturbed in the nanopores, with the extent increasing as the pore size decreased. A new rotator phase 12. 51 is found for C-21 confined in CPG (12 nm) and SBA-15 (14.4 nm).